Ian Walmsley

Oxford University

Light has the remarkable capacity to exhibit quantum features under ambient conditions, making exploration of the quantum world feasible in the laboratory and in the field. Further, the availability of high-quality integrated optical components makes it possible to conceive of large-scale photonics quantum states by bringing together the outputs of many different quantum light sources, coherently mixing them and counting the output photon number. We can envisage a scalable photonic quantum network that will facilitate the preparation of distributed quantum correlations among many light beams, thereby enabling a new regime of state complexity to be accessed - one for which it is impossible using classical computers to determine the structure and dynamics of the system. This is a new regime not only for scientific discovery, but also fro applications: it is possible to perform tasks that are impossible using known future information processing technologies. For instance, ideal universal quantum computers may be exponentially more efficiently than classical machines for certain classes of problems, and communications may be completely secure. Photonic quantum technologies will open new frontiers in quantum science and technology.
 

Filip Sośnicki, Maciej Gałka, Michał Mikołajczyk, Ali Golestani, Michał Karpiński

Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warszawa, Poland

The spectral-temporal degree of freedom of light is a promising platform for integrated photonic quantum information processing. The intrinsically deterministic electro-optic methods, allowing imprinting temporal phase profiles onto optical pulses in a noise-free manner, show great promise for spectral manipulation of quantum pulses.

To date, using single-tone electro-optic phase modulation we demonstrated 6-fold spectral compression of heralded single photon pulses [1], as well as deterministic spectral shifting of single-photon wavepackets [2]. Here we discuss spectral manipulation of light by more complex temporal phase modulation patterns.

We show that the use of phase modulation patterns akin to spatial holograms for diffractive shaping of transverse beam profiles may enable truly large-scale bandwidth compression. Our numerical simulations indicate the feasibility of reaching compression factors of the order of 103, enabling interfacing GHz- and MHz-bandwidth quantum systems.

Further we experimentally demonstrate an on-demand, non-periodic electro-optic temporal phase modulation system, based on direct driving of electro-optic phase modulators with output signals from photodiode receivers. The non-periodicity of the scheme is particularily relevant for non-deterministic events prevalent in photonic quantum information processing.

Our results demonstrate the added value for spectral manipulation of quantum light stemming from non-sinusoidal electro-optic temporal phase modulation patterns.

Kristoffer Joanesarson¹, Jake Iles-Smith^1,2, Mikkel Heuck¹, Yi Yu¹, Jesper Mørk¹

¹Department of Photonics Engineering, Technical University of Denmark, Ørsteds Plads, 2800 Kgs. Lyngby, Denmark
²School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom

Single photon switches have potential for future applications within optical quantum information processing and communication [1]. A potential realisation of a scalable on-chip single-photon switch is a single quantum scatterer with discrete energy levels inside a one-dimensional photonic crystal waveguide [2]. Incident photons on-resonance with the scatterer will completely reflect, whereas far-off resonant photons will transmit.
       Classical all-optical switching in photonic crystal structures have been shown to improve significantly by exploiting the Fano resonance which arises from breaking the Lorentzian lineshape symmetry [3, 4]. Motivated by these results we consider a partially transmitting element (PTE) embedded in the waveguide.
       In this work we take our quantum scatterer to be a Jaynes-Cummings (JC) system which we side-couple to our waveguide and calculate its one and two-photon switching properties. As our figure of merit we determine the JC emitter-cavity detuning necessary for the photon transmission spectra to shift from complete transmission to complete reflection. This detuning can be controlled dynamically for quantum dots in photonic crystal cavities by ultrafast electrical tuning using the Stark effect [5]. We show that by including a PTE with intermediate transmittance strength our figure of merit more than doubles.

Wolfram Pernice

University of Münster, Institute of Physics, Münster, Germany

Nanophotonic circuits employ waveguiding devices to route light across quasi-planar integrated optical chips in analogy to electrical wires in integrated electrical circuits. Using materials with high refractive index allows for confining light into sub-wavelength optical wires. Interaction with the environment is possible through near-field coupling to the evanescent tail of propagating optical modes. This approach is particularly interesting for designing highly sensitive detectors which are able to register individual photons and constitute fundamental building blocks for emerging quantum photonics. I will present recent progress on realizing waveguide integrated single photon detectors, with a focus on superconducting nanowire single photon counters (SNSPDs). SNSPDs provide high efficiency and good timing performance, as well as broad optical detection bandwidth. To move towards applications in high bandwidth quantum communication, ultrafast detectors with high efficiency are needed. We realize compact SNSPDs with sub-micrometer effective length by embedding them in photonic crystal cavities to recover high absorption efficiency. These detectors possess sub-nanosecond recovery times and ultralow noise equivalent power. Being made by scalable fabrication techniques, waveguide SNSPDs hold promise for photonic integrated quantum technologies.

Josef Hloušek, Ivo Straka, Michal Dudka, Martina Miková, Miroslav Ježek

Department of Optics, Palacký University, 17. listopadu 1192/12, 77146 Olomouc, Czech Republic

Accurate characterization of statistics of light is a crucial requirement of many applications in the field of quantum technology such as non-classical light preparation and characterization, quantum sensing and metrology, and quantum simulation. Photon-number-resolving measurements also represent enabling technology for many emerging biomedical imaging techniques. We present a photon-number-resolving detector consisting of a tunable free-space optical multiport network and a number of independent single-photon avalanche diodes. The spatial multiplexing scheme features precise balancing with no crosstalk between the individual detection ports and other systematic errors. Subsequent data processing is based on a real-time measurement of all the possible coincidence events between the ports. To estimate photon statistics of the input light, we apply a novel expectation-maximization-entropy algorithm based on entropy regularization of maximum-likelihood approach and expectation-maximization iteration technique [1]. We demonstrate high-fidelity photon statistics measurement of various sources of light, including laser, single- and few-mode thermal sources, photon-subtracted thermal light, and a set of several single-photon emitters [2]. The accurate photon-statistics measurement is also utilized for experimental verification of quantum-thermodynamics concepts [3].

Martin A. Wolff^1,2, Matvey Lyatti¹, Simone Ferrari^1,2, Wolfram H. P. Pernice^1,2, Carsten Schuck^1,2

¹University of Münster, Physics Institute, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²CeNTech - Center for NanoTechnology, Heisenbergstr. 11, 48149 Münster, Germany

The idea of exploiting the optical response of superconductors in bolometers, transition edge sensors and single-photon detectors is an active and rapidly growing field of research. In particular integrated quantum technology will benefit from embedding superconducting nanowire single-photon detectors (SNSPDs) with nanophotonic circuits. The vast majority of superconducting detectors however is rather demanding in terms of cryogenic environments requiring temperatures below 4 K. Hence, there is a growing interest in exploring the potential of high-critical temperature (high-Tc) superconductors for single-photon detection, which could allow for operation under significantly relaxed (liquid nitrogen) cooling requirements. Such high-Tc SNSPDs will further benefit from faster intrinsic electron-phonon interaction times thus resulting in detectors operating at higher speeds. Here we show progress towards the realization of high-Tc-SNSPDs fabricated from ultra-thin yttrium barium copper oxide (YBCO) films. We employ focused ion beam milling for the fabrication of high-quality YBCO nanowires on strontium titanate (STO) substrates, which is well suited as a wave-guiding material. First optical experiments at liquid nitrogen temperatures show a bolometric response and reveal complex vortex dynamics at 4 K, resulting in voltage switching, which is the basis for single-photon detection. In future work we will integrate YBCO nanowires with STO waveguides for integrated on-chip photodetection applications.

Naomi Holland, Dustin Stuart, Klara Theophilo, Axel Kuhn

University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK

A deterministic and coherent interface between atomic excitations and photonic states is a fundamental building block for hybrid quantum computation in a scalable quantum network. We aim to implement a novel single-atom single-photon interface via a high-finesse optical cavity [1]. To make the process deterministic, the atoms are trapped in microscopic dipole traps (optical tweezers), which may be dynamically reconfigured using a spatial light modulator (SLM) [2].

The optical tweezers are holographically generated - we discuss the physical setup used to realise this, along with a number of algorithms that may be used to calculate the holograms required to be displayed on the SLM [3]. We present our results in trapping arrays of single atoms in arbitrary configurations, and our current work towards moving these trapped single atoms to arbitrary locations. Finally, we discuss the possible integration of these techniques with the fibre-tip cavities also under construction in our group, and the significance of this to the field of quantum computation.

Ivo Degiovanni

I.N.RI.M. -Istituto Nazionale di Ricerca Metrologica-, Torino, Italy

For quantum physicist Quantum Metrology means high-resolution and highly sensitive measurements of physical parameters strictly exploiting “pure-quantum” features, such as e.g. quantum entanglement and/or quantum squeezing.

For metrologists Quantum Metrology represents the novel field of metrology developing measurement techniques necessary for the market success of quantum technologies. These measurement techniques are not limited to measurements of enhanced precision by exploitation of “pure quantum” features. Quantum metrology also contributes to the revised International System of Units (SI) based also on specific quantum effects allowing the “mise-en-pratique” of units directly from the fundamental constants.

This talk will discuss """quantum metrology""" from the perspective of metrologists, and from the one of quantum physicists, presenting some paradigmatic example of the two approaches based on photons.

Elena Losero^1,2, Alice Meda¹, Ivano Ruo-Berchera¹, Alessio Avella¹, Marco Genovese^1,3

¹INRIM, Strada delle Cacce 91, I-10135 Torino, Italy
²DISAT, Politecnico di Torino, I-10129 Torino, Italy
³INFN Sezione di Torino, via P. Giuria 1, I-10125, Torino, Italy

The use of quantum states of light can lead to significant improvements in absorption measurements respect to the limits imposed by their classical counterpart. Indeed, classical measurements are unavoidably limited by the intrinsic photon number fluctuations (i.e. shot noise) which scale as 1/ sqrt(n) and which can become the main source of uncertainty when low level of light is used.

After a brief introduction to quantum imaging and to the advantages of exploiting photon number correlation in spatially multimode twin beams (see Ref .[1,2]), we present recent results on the estimation of transmission/absorption coefficient with true and significant quantum enhancement. We investigate different estimators in terms of sensitivity, discussing on one side their relation with the best known strategy (i.e. using Fock states probe , see Ref. [3,4,5]), but also practical issues related to their implementation (e.g. the requirement on the stability of the system). It turns out that for very small absorption some of them are better suited to provide an unbiased and absolute estimation in an experimental measurement. We demonstrate that in ideal conditions twin beams can reach the ultimate sensitivity for all energy regimes, moreover, we propose a model to take into account for experimental imperfections and we perform the experiment reporting the best sensitivity per photon ever achieved in loss estimation experiments.

Marco Lopez

Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116, Braunschweig, Germany

Single-photon sources are considered being one of the key components in several quantum-based technologies, such as quantum communication and quantum key distribution. Furthermore, single-photon sources are of highest interest for application as standard sources also in radiometry. Here, they have the potential to be applied for the calibration of single-photon detectors or as noise-reduced sources. For the characterization of a single-photon source, besides emission wavelength and single photon purity (i.e. the g(2)(0)-value), the absolute emitted photon flux is an important parameter. At PTB, the spectral photon flux and the spectral radiant flux of a nitrogen-vacancy center based single-photon source was determined traceable to national standards, i.e. to the cryogenic radiometer (for the absolute photon flux) and to the blackbody radiator (for the spectral power distribution). A complete metrological characterization of the source was carried out, including a detailed measurement analysis for the spectral photon flux. The relative standard uncertainty is determined to be 4 %, dominated by the determination of the relative spectral power distribution. In addition, the angular emission pattern was measured and compared to calculations based on the model of Lukosz and Kunz.

We consider these investigations and the obtained results as a very important step towards the realization of a pure quantum-based radiation standard.

Ivo Straka, Jaromír Mika, Miroslav Ježek

Department of Optics, Faculty of Science, Palacký University, 17. listopadu 12, 77146 Olomouc, Czech Republic

We propose and experimentally demonstrate a device for generating light with arbitrary classical photon-number distribution. We use programmable acousto-optical modulation to control the intensity of light within the dynamic range of more than 30 dB and inter-level transitions faster than 500 ns with further speedup possible by employing electro-optical modulation. We propose a universal method that allows high-fidelity generation of user-defined photon statistics. Extremely high precision <0.001 can be reached for lower photon numbers, and faithful tail behavior can be reached for very high photon numbers. We demonstrate arbitrary statistics generation for up to 500 photons. For detection, we employ an avalanche diode that allows us sufficient photon-number resolution in the time domain. The proposed device can produce any classical light statistics with given parameters including Poissonian, super-Poissonian, thermal, and heavy-tailed distributions like log-normal. The presented methods can be used to simulate communication channels, calibrate the response of photon-number-resolving detectors, or probe physical phenomena sensitive to photon statistics.

Ronald Hanson

QuTech and Kavli Institute of Nanoscience, Delft University of Technology, The Netherlands

Entanglement – the property that particles can share a single quantum state - is arguably the most counterintuitive yet potentially most powerful element in quantum theory. The non-local features of quantum theory are highlighted by the conflict between entanglement and local causality discovered by John Bell. Decades of Bell inequality tests, culminating in a series of loophole-free tests in 2015, have confirmed the non-locality of Nature.

Future quantum networks may harness these unique features of entanglement in a range of exciting applications, such as distributed quantum computation, secure communication and enhanced metrology for astronomy and time-keeping. To fulfill these promises, a strong worldwide effort is ongoing to gain precise control over the full quantum dynamics of multi-particle nodes and to wire them up using quantum-photonic channels.

Diamond spins associated with NV centers are promising building blocks for such a network as they combine a coherent electron-optical interface with a local register of robust and well-controlled nuclear spin qubits.

Here I will introduce the field of quantum networks and discuss ongoing work with the specific target of realizing the first multi-node network wired by quantum entanglement.

Axel Kuhn¹, Thomas Barrett¹, Marwan Mohammed¹, Naomi Holland¹, Thomas Doherty¹, Klara Theophilo¹, Allison Rubenok², Jonathan Matthews², Oliver Barter¹, Dustin Stuart¹

¹University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK
²University of Bristol, H.H. Wills Physics Laboratory, Tyndall Ave, Bristol, BS8 1TL, UK

We demonstrate quantum logic and quantum feedback using narrow linewidth photons that are produced with an a-priori non-probabilistic scheme from a single rubidium atom strongly coupled to a high-finesse cavity [1]. We use a controlled-NOT gate integrated into a photonic chip to entangle these photons [2], and we observe non-classical correlations between photon detection events separated by periods exceeding the travel time across the chip by three orders of magnitude. Furthermore we apply a quantum-feedback scheme in a two-photon interference setting that allows deliberate switching between bosonic and fermionic photon behaviour, thus steering the 2nd photon in a Hong-Ou-Mandel two-photon interferometer to an arbitrarily chosen output port  [3]. Next steps to be taken to push the present state-of-the-art to a fully scalable quantum network will be briefly mentioned, see e.g. [4], and explored in detail by my co-workers with their own contributions.

K. Laiho¹, B. Pressl², A. Schlager², S. Auchter², H. Chen², T. Günthner², H. Suchomel³, J. Gessler³, M. Kamp³, S. Höfling^3,4, C. Schneider³, G. Weihs²

¹Technische Universität Berlin, Institut für Festkörperphysik, Hardenbergstr. 36, 10623 Berlin, Germany
²Institut für Experimentalphysik, Universität Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria
³Technische Physik, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
⁴School of Physics & Astronomy, University of St Andrews, St Andrews KY16 9SS, UK

Semiconductor Bragg-reflection waveguides (BRWs) emit correlated photon pairs called signal and idler via parametric down-conversion (PDC). In order to fulfill the necessary energy and momentum conservation in these highly dispersive platforms, one of the modes, in our case the pump, is a higher order spatial mode. To achieve this, BRWs are compounded of thin layers of AlGaAs with different thicknesses and aluminum concentrations and fabricated with standard semiconductor growth techniques. Several BRW characteristics such as their total nonlinearity and final PDC wavelength are highly sensitive to the fabrication tolerances in the structure's parameters. Our simulations show that, if not taken care of, one can easily result into samples with properties far away from the design parameters [1]. We then experimentally measure the group refractive indices of the interacting modes that dominate the joint spectral properties of signal and idler, produce broadband indistinguishable photon pairs and prepare polarization entangled states. By modifying the temporal delay between signal and idler we can flexibly vary the degree of entanglement directly at the source [2]. Our results show that BRWs are a versatile source for different quantum optics tasks and thus, can become truly practical in the integrated quantum photonics.

Thomas Barrett¹, Oliver Barter¹, Dustin Stuart¹, Allison Rubenok², Jonathan Matthews², Axel Kuhn¹

¹University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
²Centre for Quantum Photonics, H. H. Wills Physics Laboratory and Department of Electrical and Electronic Engineering, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol BS8 1UB, UK

Entanglement is a key resource for quantum information processing (QIP).  Mutually entangled cluster states are inherently more robust than pairwise chains of entangled qubits, however it is particularly difficult to create these in non-local networks where bringing together distant nodes is often impractical.  Instead quantum networks of interlinked stationary (typically single atoms or ions) and flying (photonic) qubits offer a scalable route to bridging these physical distances [1], but necessitate a reliable interface between these quantum elements.   A single atom strongly coupled to a single mode of the electric field, where the internal spin state of the atom is entangled to the emitted photon polarisation, is then an ideal architecture for realising such a system.  Probabilistic entanglement of distant atoms can be achieved by entanglement swapping acting on photons emitted from both atoms [2,3].

Here we discuss the essential first step, an a priori non-probabilistic source of polarised single-photons, that utilises the unparalleled control over the photonic states provided by the cavity-enhanced interaction with a single ⁸⁷Rb atom [4].  In particular we consider novel effects of non-linear Zeeman shifts on this system and the operation of a 4x4 multimode interferometer integrated onto a photonic chip [5] with pairs of cavity-photons.

David Gershoni

The Physics Department and the Solid State Institute, Technion – Israel Institute of Technology, Haifa, 32000, Israel

Photonic cluster states are a resource for quantum computation based solely on single-photon measurements. We use semiconductor quantum dots to deterministically generate long strings of polarization-entangled photons in a cluster state by periodic timed excitation of a precessing matter qubit. In each period, an entangled photon is added to the cluster state formed by the matter qubit and the previously emitted photons. In our prototype device, the qubit is the confined dark exciton, and it produces strings of hundreds of photons in which the entanglement persists over five sequential photons.

Tobias Heindel¹, Alexander Thoma¹, Martin von Helversen¹, Marco Schmidt^1,2, Alexander Schlehahn¹, Manuel Gschrey¹, Peter Schnauber¹, Jan-Hindrik Schulze¹, André Strittmatter^1,4, Jörn Beyer², Sven Rodt¹, Alexander Carmele³, Andreas Knorr³, Stephan Reitzenstein¹

¹Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany
²Physikalisch-Technische Bundesanstalt, Abbestraße 2-12, 10587 Berlin, Germany
³Institut für Theoretische Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany
⁴Present address: Institut für Experimentelle Physik, Otto-von-Guericke Universität Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany

The realization of integrated light sources capable of emitting non-classical multi-photon states, is a fascinating, yet equally challenging task at the heart of quantum optics [1].

In this work, we propose and experimentally demonstrate the triggered generation of photon twins by exploiting the energy-degenerate biexciton-exciton (XX-X) radiative cascade of semiconductor quantum dots (QDs) [2]. For this purpose, we select QDs featuring E_X^H = E_XX^H, which implies a matching between the fine structure splitting and the biexciton binding energy (ΔE_FSS = |E_bin^XX|). In this case, one decay channel of the XX-X cascade reveals the emission of photon-twins – a light state which is comprised of two temporally correlated photons with identical emission energy and polarization. Deterministically integrated within a microlens, this system emits highly-correlated photon pairs at a rate of up to (234 ± 4) kHz. Furthermore, we employ a photon-number-resolving detector to directly observe the twin-photon state, which enables a reconstruction of the photon number distribution of our source.

In conclusion, the twin-photon source presented in this work is an attractive type of integrated quantum light source, which might be exploited in emerging research fields, such as quantum-optical spectroscopy or quantum biology, which benefit from multi-photon excitation schemes.

Lukas Hanschke¹, Kevin A. Fischer², Jakob Wierzbowski¹, Tobias Simmet¹, Constantin Dory², Jonathan J. Finley¹, Jelena Vuckovic², Kai Müller¹

¹Walter Schottky Institut and Physik Department, Technische Universität München, 85748 Garching, Germany
²E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA

Resonantly excited two-level systems are poised to serve the pivotal role of on-demand single-photon sources. Here, we demonstrate that a two-level system can surprisingly also operate in a two-photon bundling regime.

Our system is an excitonic transition in a self-assembled quantum dot. For single-photon generation, it is typically excited with a resonant pulse of area π. This prepares the system in its excited state from where it spontaneously emits a single photon. However, emission that occurs during the presence of the laser pulse allows for re-excitation and, thus, multi-photon emission which spoils the single-photon character [1].

In contrast, when exciting with a pulse of area 2π, the system is expected to be returned to the ground state. However, emission during the presence of the pulse is most likely to occur when the system is in its excited state – exactly after an area of π was absorbed. This restarts the Rabi oscillation with a pulse area of π remaining in the pulse, leading to re-excitation with near-unity probability and the emission of a second photon within the excited state lifetime [2].

Jun HE^{1, 2}, Jiachao WANG¹, Rui SUN¹, Kong ZHANG¹, Junmin WANG ^{1, 2}

¹State Key Laboratory of Quantum Optics and Quantum Optics Devices,and Institute of Opto-Electronics, Shanxi University, P. R. China
² Collaborative Innovation Center of Extreme Optics, Shanxi University, P. R. China

Atoms trapped in the magic-wavelength optical tweezer will have the same light shift for the ground state and the excited state. Therefore the position-dependence differential light shift of the desired transition can be eliminated in the case of magic-wavelength optical tweezer. Based on the multi-level atomic model, we have calculated the magic wavelength for the Zeeman sub-levels of the ground state 6S_1/2 and the excited state 6P_3/2 of cesium atoms. For cesium 6S_1/2 |Fg = 4, mF = +4> - 6P_3/2 |Fe = 5, mF = +5> cycling transition, the magic wavelength was founded to be 937.7nm for a linearly polarized tweezer. And it has also been experimentally verified by using of the laser-induced fluorescence spectra of trapped single cesium atoms in a tweezer. Compared to the transition frequency in the free space, the differential light shift was measured to be about 0.7 MHz in the 937.7 nm linearly-polarized tweezer, which is about 1.2% of the trap depth. We have also investigated the effects of polarization of tweezer beam, bias magnetic field, and effective temperature of single cesium atoms upon the measurement results. We have demonstrated 852 nm triggered single-photon source based on single cesium atom trapped in a microscopic optical tweezer. And the two cases of 1064 nm and 937.7 nm (the magic wavelength) linearly-polarized tweezers are compared experimentally. The photon statistics show strong anti-bunching effect with typical g²(0) ~ 0.09, which clearly indicates the single-photon characters. The distinguishability of single photons is expected to be improved for the magic-wavelength optical tweezer. The Hong-Ou-Mandel two-photon interference measurement is employed to evaluate the photon distinguishability.

Andrew Shields

Toshiba Research Europe Ltd, UK

Applying quantum theory to information systems brings new functionalities that are not possible in conventional networks and computers. For example, the secrecy of encoded single photons transmitted along optical fibres can be tested directly and used to distribute cryptographic keys and digital signatures on communication networks. In this talk I will discuss recent work to realise practical systems for quantum key distribution (QKD) and their application to point-to-point and network-based encryption.

The past few years have seen rapid progress in the technology required to operate QKD in conventional data networks. I will discuss advances on increasing the secure key rate above 10 Mb/s, extending the range of a single link, enabling the co-existence of QKD with very high (Tb/s) data bandwidths on the same fibre and introducing QKD to multi-user access networks that can dramatically reduce the cost. This will be illustrated with examples of real world deployment of the technology on installed fibres and networks.

I will also discuss recent work on realizing next generation quantum networks based on distributed entanglement.

Chris Müller, Tim Kroh, Yanting Teng, Andreas Ahlrichs, Oliver Benson

Humboldt-Universität zu Berlin, Institut für Physik, Newtonstraße 15, 12489 Berlin, Germany

Quantum communication requires a suitable network consisting of quantum nodes and quantum channels [1]. For such a quantum network to be realized, it will be necessary to process, store, and send photons over long distances. It is unlikely that a single physical system can perform all these operations at the same time. Therefore, it will be required to connect different quantum systems within future quantum networks. This can be achieved via two-photon measurements and entanglement swapping [2].  A key requirement, however, is that the two different quantum systems emit indistinguishable photons. This can be checked by measuring Hong-Ou-Mandel (HOM) interference [3]. In our HOM experiment we interfere photons from two dissimilar sources: one pair-photon from cavity-enhanced spontaneous parametric down-conversion [4] and the other from a semiconductor quantum dot [5]. Active frequency-locking is mandatory to allow for data accumulation over a sufficiently long time and for further expanding the quantum network by additional units. We discuss achievable degrees of indistinguishability and estimate the success rate of future transfer of electronic states in quantum dots over long distances including frequency conversion to the telecom band [6].

Karolina Sedziak, Mikolaj Lasota, Piotr Kolenderski

Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland

We will present the problem of wavepacket shaping of a single photon which is heralded by the time-resolved detection of the other photon from an SPDC pair. The strength of the wavepacket narrowing depends on the parameters of the photon pair source and the width of the detection window for the heralding photon. Our theoretical predictions [1] were compared with the experimental results [2] showing very good agreement. We measured the reduction of the width of the heralded wavepacket to approximately 29% as compared to the case of non-heralding scenario. The results can be utilized to improve quantum communication and clock synchronization protocols.

Wolters¹, Buser¹, Béguin¹, Mottola¹, Jahn¹, Horsley¹, Benson², Warburton¹, Treutlein¹

¹Universität Basel, Department Physik, Klingelbergstr. 82 4056 Basel, Switzerland
²Humboldt-Universität Berlin, Institut für Physik, Newtonstr. 15, 12489 Berlin

Quantum networks promise a plethora of radically new applications and novel insights [1]: High-speed quantum cryptography networks will be used for unconditional secure communication in metropolitan areas, and memory enhanced quantum computers and simulators will allow for exponential speed-up in solving complex problems [2]. However, establishing the hardware for a quantum network is a challenging task. A source of indistinguishable single photons is required, along with means to store the single photons at each node [3]. We report on our concentrated efforts to build the basic element of quantum networks, i.e. a semiconductor quantum dot (QD) single photon source [4,5] coupled to a compatible atomic quantum memory [6].

On the one hand we demonstrate a rubidium quantum memory for broadband operation, matched to the QD exciton natural decay rate. The memory is demonstrated using attenuated laser pulses on the single photon level with a bandwidth of 0.66 GHz.  In the present memory, the readout noise is dominated by atomic fluorescence, and for input pulses containing on average μ1 = 0.27(4) photons the signal to noise level would be unity. On the other hand, we demonstrate deterministic single photon shaping with QDs, to match the narrow atomic transitions of Rubidium [7].

Christine Silberhorn

University of Paderborn, Germany

Recent achievements in the area of integrated quantum optics and quantum information processing have shown impressive progress for the implementation of linear circuits based on monolithic waveguide structrues. However, most experiments are based on χ⁽³⁾ -media, such as glas, silicon-on insulator or silica-on-silicon. In these platforms the implentation of highly efficient sources, frequency converters and fast active phase shifters and modulators pose severe challenges.The use of advanced waveguides structures, which harness a χ⁽²⁾ –non-linearity, allows for the realization various devices with multiple functionalites. These include single- and multi-channel sources with extraordinary brightness, quantum frequency conversion with tailored spectral-temporal properties, and complex circuitries comprising degenerate pair generation in orthogonal polarization, linear elements, and active elements such as polarization rotators or an electro-optically controllable time delay. Here we present several different examples of integrated devices based on χ⁽²⁾ –media for the implementation of advanced, integrated photon pair sources and quantum circuits.

Peter Schnauber¹, Johannes Schall¹, Samir Bounouar¹, Theresa Höhne², Suk-In Park³, Geun-Hwan Ryu³, Tobias Heindel¹, Sven Burger², Jin-Dong Song³, Sven Rodt¹, Stephan Reitzenstein¹

¹Technische Universität Berlin, Berlin, Germany
²Zuse Institut Berlin, Berlin, Germany
³Korea Institute of Science and Technology, Seoul, Korea

The deterministic integration of quantum emitters into on-chip photonic elements is crucial for the implementation of scalable on-chip quantum circuits. Recent activities in this field include hybrid QD-waveguides for enhanced photon in-coupling [1] and first, rather tedious steps towards the controlled integration of QDs using multistep-lithography [2] as well as AFM tip transfer [3]. Here we report on the deterministic integration of single quantum dots (QD) into on-chip beam splitters using in-situ electron beam lithography (EBL) [4]. In this single-step technique, photonic building blocks are patterned on top of chosen QDs immediately after spatially and spectrally pre-characterizing them through their cathodoluminescence signal [5]. To realize 50/50 coupling elements acting as central building blocks of on-chip quantum circuits we chose tapered multimode interference (MMI) splitters which feature relaxed fabrication tolerances and robust 50/50 splitting ratio. We demonstrate the functionality of the deterministic QD-waveguide structures by high-resolution µPL spectroscopy and by studying the photon cross-correlation between the two MMI output ports. The latter confirms single-photon emission and on-chip splitting associated with g(2)(0) < 0.5.

Ekkehart Schmidt¹, Mario Schwartz², Florian Hornung², Ulrich Rengstl², Stefan Hepp²Konstantin Ilin¹, Simone L. Portalupi²Michael Jetter², Peter Michler², Michael Siegel¹

¹Institute of Micro- und Nanoelectronic Systems (IMS), Karlsruhe Institute of Technology, Karlsruhe, Germany
²Institut für Halbleiteroptik und Funktionelle Grenzflächen (IHFG), Center for Integrated Quantum Science and Technology (IQST) and research center SCoPE, University of Stuttgart, Stuttgart, Germany

An on-chip quantum photonic device consists of quantum dots, waveguide based logic and SNSPD. The quantum dots are conveniently excited by a laser beam [1][2]. Stray flux of these exciting photons can be detected by the SNSPD and therefore causes the malfunction of the whole photonic circuit. We studied the efficiency of suppression of parasitic detector counts using an on-chip mirror. The SNSPDs were made from 6 nm thin NbN film deposited by reactive magnetron sputtering onto GaAs substrate with a 12 nm thick AlN buffer layer. Two identical SNSPDs were fabricated from the same NbN film at a distance of 50 µm from each other. The 120 nm wide nanowires are of a critical temperature of 9.9 K and a critical current density of 2.8 MA/cm² at 4.2 K. One of these SNSPDs was covered with a bi-layer of 20 nm thick AlN and 110 nm thick Al forming an on-chip mirror which significantly decreased the amount of parasitic counts seen at the detector. This technology could enable a fully on-chip Hanbury-Brown and Twiss experiment when combined with a single-mode waveguide based coupler and an integrated quantum dot single-photon source [2,3].

Jonathan Belhassen¹, Saverio Francesconi², Qifeng Yao³, Ivan Favero⁴, Aristide Lemaître⁵, Steve Kolthammer⁶, Ian Walmsley⁷, Maria Amanti⁸, Florent Baboux⁹, Sara Ducci¹⁰

¹jonathan.belhassen05@gmail.com
²saverio.francesconi@stud.unifi.it
³qifeng.yao@paris7.jussieu.fr
⁴ivan.favero@univ-paris-diderot.fr
⁵aristide.lemaitre@c2n.upsaclay.fr
⁶Steven.Kolthammer@physics.ox.ac.uk
⁷ian.walmsley@admin.ox.ac.uk
⁸maria.amanti@paris7.jussieu.fr
⁹florent.baboux@univ-paris-diderot.fr
¹⁰sara.ducci@univ-paris-diderot.fr

Photonic circuits provide a promising approach to achieving a wide range of quantum information tasks. Rapid progress has been made in recent years to develop circuits implementing the key ingredients of quantum information. A next important step towards large-scale applications will now be to develop active photonic circuits capable of generating and manipulating quantum states in an integrated manner.

Here we report the first realization of a monolithic III-V photonic circuit combining a parametric heralded single-photon source with a beam splitter [1]. Pulsed parametric down-conversion in an AlGaAs waveguide generates counterpropagating photons, one of which is used to herald the injection of its twin into the beam splitter, consisting of a multimode interferometer. This configuration allows us to realize an integrated Hanbury-Brown and Twiss experiment which confirms single-photon generation and manipulation, at room temperature and telecom wavelength.

After this first demonstration we are now working on the manipulation of more complex quantum states. In particular, we tailor biphoton frequency entanglement through amplitude and phase engineering of the pump beam with an SLM [2]. Original quantum states featuring non-Gaussian entanglement are generated, illustrating the potential of the III-V platform for the control of high-dimensional entanglement.

Ronald Walsworth

Harvard University, Cambridge, USA

Nitrogen vacancy (NV) quantum defects in diamond provide an unparalleled combination of magnetic field sensitivity and spatial resolution in a room-temperature solid, with wide-ranging applications in both the physical and life sciences.  NVs can be brought into few nanometer proximity of magnetic field sources of interest — such as single protons and electrons — while maintaining long NV spin coherence times, a large (~Bohr magneton) Zeeman shift of the NV spin states, and optical preparation and readout of the NV spin.  Recent applications include mapping magnetic signatures in >4 billion-year-old meteorites and early-Earth rocks that inform theories of solar system and Earth formation, noninvasive magnetic sensing of single neuron action potentials, measuring the spin chemical potential in magnetic devices, and NMR chemical fingerprinting at the scale of a single biological cell.  I will provide an overview of this technology and its diverse applications.

Alexander Stark^1,3, Nati Aharon², Thomas Unden³, Daniel Louzon^2,3, Alexander Huck¹, Alex Retzker², Ulrik Lund Andersen¹, Fedor Jelezko³

¹Department of Physics, Technical University of Denmark, Fysikvej, Kongens Lyngby 2800, Denmark
²Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
³Institute for Quantum Optics, Ulm University, Albert-Einstein-Allee 11, Ulm 89081

State-of-the-art methods for sensing weak AC fields are only efficient in the low frequency domain (<10 MHz). The inefficiency of sensing high-frequency signals is due to the lack of ability to use dynamical decoupling. In this work we show that dynamical decoupling can be incorporated into high-frequency sensing schemes and by this we demonstrate that the high sensitivity achieved for low frequency can be extended to the whole spectrum [1]. While our scheme is general and suitable to a variety of atomic and solid-state systems, we experimentally demonstrate it with the nitrogen-vacancy center in diamond. We achieve coherence times up to 1.43 ms resulting in a smallest detectable magnetic field strength of 4 nT at 1.6 GHz. Attributed to the inherent nature of our scheme, we observe an additional increase in coherence time due to the signal itself. In this talk I will also present a few other dynamical decoupling schemes [2-4], as well as our recent results [5], that could be utilized to further improve the resolution of sensing oscillating signals, and in particular, high frequency fields.

Wolfgang Elsäßer, Patrick Janassek, Sébastien Blumenstein

Institute of Applied Physics, Technische Universität Darmstadt, Schlossgartenstrasse 7, 64289 Darmstadt (Germany)

Ghost imaging (GI) is by far not a “spooky action” but rather a photon correlation imaging modality based on the fundamentals of quantum optics, either realized with entangled photons in the quantum GI version or with bunched photons from classical thermal sources.

Here, we introduce to the field of ghost modalities a novel, extremely compact ultra-miniaturized superluminescent diode source based on Amplified Spontaneous Emission (ASE). We demonstrate a GI experiment with classical thermal light based on the full in-coherence of light as requested for classical GI, namely in 1st order coherence being spectrally broad-band, in 2nd order coherence exhibiting Hanbury-Brown & Twiss photon bunching with a correlation coefficient of two and being spatially incoherent due to the dynamic mode filamentation.

We then extend the field of ghost modalities in analogy to this classical spatial GI principle with classical light to ghost spectroscopy. We propose and realize a first ghost spectroscopy (GS) experiment with classical thermal light by exploiting spectral correlations of light emitted by a broad-band semiconductor-based superluminescent diode (SLD) and demonstrate the applicability of this ghost modality in a real-world proof-of-principle experiment by measuring a ghost absorption spectrum a(l) of the characteristic absorption features of chloroform at 1214nm, i.e. a ghost spectrum.

Changhyoup Lee¹, Mark Tame^2,3, Kwang-Geol Lee⁴, Xifeng Ren^5,6, Carsten Rockstuhl^1,7

¹Institute of Theoretical Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe, Germany
²University of KwaZulu-Natal, Durban, South Africa
³National Institute for Theoretical Physics, KwaZulu-Natal, South Africa
⁴Department of Physics, Hanyang University, Seoul, Korea
⁵Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, China
⁶Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, China
⁷Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe, Germany

Plasmonic systems that support surface plasmon polaritons provide one of the most practical sensing platforms, where the extreme field-confinement below the diffraction limit greatly enhances sensitivity. The associated sensing uncertainty, however, is fundamentally limited by the statistical nature of classical light, known as the shot-noise limit. Recent experimental works have shown the possibility of reducing quantum noise in plasmonic sensing by employing quantum resources [1,2,3]. More theoretical investigations have offered better understanding for separate roles of quantum resources and implemented plasmonic features in the context of quantum sensing [4,5]. In this work, we experimentally studied sensing performances of two types of quantum plasmonic sensors: a plasmonic nanowire and an attenuated-total-reflection prism setup for which the illumination are a two-photon N00N state and two-mode squeezed state, respectively. Our work aims achieving a sensing uncertainty below the shot noise limit on scales below the diffraction limit. We show that quantum benefits such as super-resolution and quantum noise-reduction are achieved even in the presence of metallic losses. With our experimental studies of quantum plasmonic sensors, we envisage that progress in quantum metrology will reshape the field of plasmonic biosensing -- a field that has already developed into mature technology for a few decades.

A.M. Fox

University of Sheffield, Sheffield, U.K.

On-chip quantum photonics relies on the integration of efficient single-photon sources with advanced quantum-optical circuits. In this presentation, I will review progress at the University of Sheffield on a chip-compatible III-V semiconductor platform in which quantum-dot (QD) single-photon sources are integrated into GaAs photonic circuits. I will first describe work demonstrating single-photon emission and interference using a quantum dot source integrated with a monolithic on-chip Hanbury Brown-Twiss interferometer. I will then describe the development of an electrically pumped single-photon source integrated with a nano-beam waveguide, and a high-speed, high coherence source using an InGaAs QD coupled to an H1 photonic crystal nano-cavity. Under resonant π-pulse excitation, an on-chip, on-demand single-photon source exhibiting high purity and indistinguishability has been demonstrated without spectral filtering. Finally, I will discuss chiral coupling between QD single photon sources and nano-photonic waveguides, which is a manifestation of chiral quantum optics. Experiments demonstrating both chiral emission and exciton spin initialization will be described. These results rely on the precise positioning of dot within the nano-photonic structure, and lay the foundations for developing on-chip spin networks with spin qubits localized in different QDs.

Hamza Abudayyeh^1,2, Boaz Lubotzky^1,2, Jennifer Hollingsworth³, Ronen Rapaport^1,2,4

¹Racah Institute for Physics, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel
²Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel
³Materials Physics & Applications Division: Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
⁴Applied Physics Department, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel

Nanocrystal quantum dots (NQDs) are excellent candidates for room temperature single photon sources (SPSs) due to their relative ease of fabrication, emission spectrum tunability, and scalability. However recent efforts in stabilizing NQDs have resulted in a significant increase in biexciton emission compromising their integrity as possible SPSs and leading to a trade-off between single photon purity and stability. Temporal filtering was suggested to dispose of the biexciton emission altogether but this lead to a significant decrease in efficiency. In this work we suggest three heralding techniques that would break this three-way compromise between purity, efficiency and stability. Using these techniques we were able to theoretically show that stable NQDs (with high biexciton quantum yields) can be used as a source for heralded single photons with efficiencies and purities approaching unity. Furthermore we confirm our theoretical prediction with a proof of concept experiment on a single core/thick-shell NQD.

Anna Musiał¹, Łukasz Dusanowski¹, Paweł Holewa¹, Paweł Mrowiński¹, Aleksander Maryński¹, Krzysztof Gawarecki², Tobias Heuser³, Nicole Srocka³, David Quandt³, André Strittmater³, Sven Rodt³, Stephan Reitzenstein³, Grzegorz Sęk¹

¹OSN Laboratory, Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, Wrocław, Poland
²Department of Theoretical Physics, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, Wrocław, Poland
³Institute of Solid State Physics, Technical University of Berlin, Hardenbergstraße 36, Berlin, Germany

Epitaxial semiconductor quantum dots (QDs) have been proven powerful and versatile platform to study quantum optics phenomena and realize quantum optical devices. The next step is to bring them towards practical applications. In the case of QD-based single-photon sources the challenges are: enhance extraction efficiency to obtain high rate of single photons on demand, increase the operation temperature towards cryogenic-free cooling systems, realize single QD-nanophotonic structures for high purity single-photon generation in the deterministic technology and employ electrical excitation as well as compact designs compatible with fiber networks.  

We present our progress in realization of compact fiber-based single-photon source at telecom wavelengths based on strain-engineered InGaAs/GaAs MOCVD-grown QDs deterministically integrated into mesas utilizing in-situ low-temperature electron-beam lithography combined with cathodoluminescence, for increased extraction efficiency. Comprehensive combined experimental (microphotoluminescence, photoluminescence excitation and correlation spectroscopy) and theoretical (8-band k·p numerical calculations combined with configuration interaction method for excitonic states) study of band structure and optical properties, and in particular single-photon generation will be presented. The influence of excitation energy as well as effect of temperature on the single photon emission purity has been studied and as a result triggered high-purity single-photon emission has been realized under p-shell resonant excitation.

Varun Verma

National Institute of Standards and Technology, Boulder, CO

Single-photon detectors are an integral part of experiments in quantum optics, and have applications in quantum computing, quantum communications, and the characterization of single photon sources. In particular, superconducting nanowire single-photon detectors (SNSPDs) are excellent broadband detectors due to their fast recovery times, low jitter, and low dark count rates. Until recently however, the efficiency of SNSPDs in the telecommunications band was relatively poor in comparison to other detector technologies. The recent development of amorphous superconducting alloys such as WSi and MoSi has led to significant improvement in system detection efficiency (~90%) compared to the early NbN-based SNSPDs. Furthermore, device yield has improved from ~30% to 100%, enabling for the first time the fabrication of SNSPD arrays and low-resolution single-photon cameras. I will discuss how these improvements in efficiency and device yield are enabling new applications such as imaging from the UV to the mid-infrared, with potential applications in astronomy and deep-space optical communications. Finally, I will outline new approaches to building arrays of SNSPDs for imaging at the single photon level.

Ivan Prochazka, Josef Blazej

Czech Technical University in Prague, Brehova 7, 11519 Prague 1, Czech Republic

We are presenting the development and achievements of photon counting instrumentation for metrology applications. In listed measurements the photon counting approach was verified to provide ultimate precision and accuracy. Satellite Laser Ranging (SLR) is a technique in which a network of observing stations measures the round trip time of flight of ultrashort laser pulses to satellites equipped with retroreflectors. It is the most accurate technique to determine the distances in space with sub-millimeter precision and few millimeters accuracy. Laser time transfer enables to compare time scales on ground and in space by means of SLR type measurements.

For metrology applications new photon counting detectors and epoch timing systems were developed. They do provide ultimate timing resolution and extreme detection delay stability. Solid state photon counting detectors on silicon having an active area diameter of 0.2mm are providing timing resolution better than 20ps, the epoch timing systems provide sub-picosecond timing resolution, linearity and stability. An entire photon counting measurement chain exhibits unique long-term detection delay stability of the order of hundreds of femtoseconds. The detectors were qualified for space missions. Recently nine units are operational in space for laser time transfer three new missions are under preparation.

Stephan Krapick¹, Jan Philipp Höpker¹, Varun Verma², Adriana Lita², Viktor Quiring¹, Harald Herrmann¹, Christine Silberhorn¹, Tim Bartley¹

¹Department of Physics, University of Paderborn, Warburger Str. 100, 33098 Paderborn, Germany
²National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA

Cryogenic temperatures are required for many quantum technologies, in particular quantum dots for single photon sources [1-4], and high-efficiency, low noise superconducting detectors [5]. Reconciling these operating conditions with photonic processing through integrated optics is an ongoing challenge. Of the many platforms for integrated optics, titanium in-diffused waveguides in lithium niobite (Ti:LN) offer a range of advantages, including very high coupling efficiency and low-loss transmission of orthogonal polarisation modes, low power and high speed electro-optic control, and efficient second-order nonlinear optical processing [6]. I will report on progress towards adapting these techniques at cryogenic temperatures. Specifically, I will report on a low-power electro-optic switch which is fully functional at 0.8K, the operating temperature of a broad class of superconducting nanowire single-photon detectors (SNSPDs). This is demonstrated through the use of such detectors with the switch in the same cryostat simultaneously. The ultimate goal is to integrate both components on a single chip; to that end, I will also report on progress towards integrating SNSPDs on our Ti:LN waveguides, specifically methods to increase absorption in a travelling-wave configuration. 

Gourgues Ronan

ronan@singlequantum.com

Nowadays, superconducting nanowire single photon detectors (SNSPDs) are the most advanced single photon detectors from the visible to the infrared. This is because of their high detection efficiency, very low dark count rate, high count rate, and high time resolution¹. Furthermore, they are compatible with Si technology² making them very suited for quantum photonic circuits.

I will present an integrated photonic device that emits and detects single photons on-chip.

The detectors are fabricated from a NbTiN film which is deposited directly on an oxidized Si waver. On top, we sputter a SiN layer from which photonic structures such as waveguides and grating couplers are fabricated.

To develop the fabrication process, we estimate the optical absorption of the SNSPD by performing 3D Finite Differential Time Domain simulations. We show the feasibility of our approach by presenting optical and electrical measurements. Our operational photonic chip lays the basis for a future antibunching measurement.

Geng Chen^1,2, Nati Aharon³, Yong-Nan Sun^1,2, Zi-Huai Zhang^1,2, Wen-Hao Zhang^1,2, De-Yong He^1,2, Jian-Shun Tang^1,2, Yaron Kedem⁴, Chuan-Feng Li^1,2, Guang-Can Guo^1,2

¹CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, China
²Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China
³Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem, 91904, Givat Ram, Israel
⁴Department of Physics, AlbaNova University Center, Stockholm University, 106 91, Stockholm, Sweden

Improving the precision of measurements is a significant scientific challenge. Previous works suggest that in a photon-coupling scenario the quantum fisher information shows a quantum-enhanced scaling of N², which in theory allows a better-than-classical scaling in practical measurements. In this work, utilizing mixed states with a large uncertainty and a post-selection of an additional pure system, we present a scheme to extract this amount of quantum fisher information and experimentally attain a practical Heisenberg scaling [1]. We performed a Heisenberg limited measurement of the Kerr non-linearity of a single photon, where an ultra-small Kerr phase of 6 *10^-8 was observed with an unprecedented precision of 3.6*10^-10. Moreover, by using an imaginary weak-value the scheme is robust to noise originating from the self-phase modulation.

Valentin Averchenko, Gerhard Schunk, Michael Foertsch, Martin Fischer, Dmitry Strekalov, Gerd Leuchs, Christoph Marquardt

Max-Planck-Institut für die Physik des Lichts, Staudtstraße 2, 91058 Erlangen

Cavity-assisted spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM) in nonlinear optical materials are practical methods to generate narrowband time-energy entangled photon pairs, which are required for a number of quantum information protocols. Here, we study the generation of the photon pairs for the general case of off-resonant conversion, namely, when the frequencies of the generated photons can possess a mismatch from the cavity resonances. Such frequency mismatch is temperature dependent and requires an additional control in an experiment. We propose a generic model to describe a cavity-assisted SPDC and SFWM. We show that the mismatch reduces the generation rate of the photons, distorts the spectrum and the autocorrelation function of the generated fields, and affects the photon generation dynamics. We verify results experimentally using parametric generation of the photon pairs in a nonlinear whispering gallery mode resonator (WGMR) as a platform with controlled frequency mismatch. Our results reveal the role of the frequency mismatch on the photons generation process and importance to control it. Results also constitute a step to full control over the spectro-temporal properties of narrowband entangled photon pairs and will be useful for heralded generation of narrowband single-photon pulses with the tailored temporal shape.

F. Böhm¹, N. Nikolay¹, C. Pyrlik², J. Schlegel², A. Thies², B. Lubotzki³, A. Dohms¹, H. Abudayyeh³, N. Sadzak¹, B. Sontheimer¹, A. Wicht², R. Rapaport³, O. Benson¹

¹AG Nanooptik, Humboldt-Universität zu Berlin, Germany
²Ferdinand-Braun-Institut für Höchstfrequenztechnik, Berlin, Germany
³The Racah Institute of Physics, The Hebrew University of Jerusalem, Israel

Efficient single photon sources effectively coupled to a single optical mode pose a crucial building block for future applications in quantum information science [1]. So far quantum optics experiments with single photon emitters, e.g. nitrogen-vacancy (NV) centers in diamond are limited to large experimental setups with sophisticated detection systems including high NA objectives and precisely aligned free-space optics.

We report on two different approaches aiming at integrating nano-sized quantum emitters to photonic structures in order to enhance and facilitate the collection of their emission into a single optical mode.

One approach is the evanescent coupling of a single NV center to waveguide structures fabricated from ultra-pure silica exhibiting exceptionally low intrinsic fluorescence [2]. We will present results regarding the excitation of single quantum emitters and the detection of single photons via these novel monolithic photonic structures.

Another approach is the coupling of single NV emitters to hybrid dielectric/metallic bulls-eye antennas [3]. These integrated devices allow the collection of a large amount of the NV centers emission and redirection of this emission with a high directionality. We will present results on the pre-characterization and functionalization of these hybrid antennas.

Emil V. Denning¹, Andreas D. Østerkryger¹, Jake Iles-Smith^1,2, Niels Gregersen¹, Jesper Mørk¹

¹Department of Photonics Engineering, DTU Fotonik, Technical University of Denmark, Building 343, 2800 Kongens Lyngby, Denmark
²Photon Science Institute and School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom

Quantum dots in photonic nanocavities play an important role in quantum optics and optical quantum information technologies, both as single photon sources and more generally as light-matter interfaces. Such nanocavities are often based on a waveguide-like dielectric structure with longitudinal translation invariance and confining the light in the transverse direction. The translation invariance is then broken by adding a pair of mirrors in order to form a cavity, the optical properties of which depend on the mirror reflectivity as well as the underlying waveguide structure [1].

Light--matter interaction has been studied extensively for waveguides, where the optical density of states is largely frequency independent [2], and for cavities, where the density of states is sharply peaked around the cavity resonance [3]. We study the transition regime, where the cavity mirrors are weakly reflecting and the optical structure simultaneously features aspects that are both waveguide- and cavity-like. Importantly, these features are strongly related, which becomes important when optimizing single-photon sources for simultaneous high indistinguishability and high efficiency.

Thomas H Doherty, Marwan Mohammed, Naomi Holland, Klara Theophilo, Dustin Stuart, Axel Kuhn

University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK

A scalable quantum network will require an effective interface between light and matter. A high finesse optical resonator, which strongly couples a single atom to a photon, forms a key element. The application of these systems has been demonstrated in multiple studies [1,2], including as scalable source of bespoke single photons. However, the techniques used to fabricate optical cavities have remained largely unchanged since their introduction. A variety of engineering challenges have prevented the production of highly curved, perfectly reflecting, mirrors. This has resulted in insufficient access to persistently trap an emitter in a cavity, weak atomic coupling and perturbing polarization dependent effects.

However, with the continual development of modern manufacturing processes, such as laser ablation [3] and focussed ion beam milling [4], these challenges can now be overcome. A new generation of resonators are being constructed with stronger atomic coupling, a reduction to birefringence and greater access to the cavity mode. We review the state-of-the-art in the techniques which make this possible, the potential applications of these resonators and their position in a future quantum network.

Philipp Fuchs¹, Thomas Jung¹, Hossam Galal², Mario Agio², Xiao-Liu Chi³, Stephan Götzinger³, Christoph Becher¹

¹Universität des Saarlandes, Fakultät NT - FR Physik, Campus E2.6, 66123 Saarbrücken
²Universität Siegen, Laboratorium für Nano-Optik, Walter-Flex-Str. 3, 57072 Siegen
³Friedrich-Alexander-Universität Erlangen-Nürnberg, Department Physik, 91058 Erlangen

Color centers in diamond, e.g. the nitrogen (NV), silicon (SiV) or germanium (GeV) vacancy centers, have become very promising candidates for the implementation of stationary qubits or bright single photon sources. One of the most challenging problems when working with these defects is the low collection efficiency of photoluminescence photons out of unstructured diamond films. Because of total internal reflection at the diamond-airinterface, this problem cannot be solved simply by using high NA objectives and the collection efficiency is usually limited to a few percent. Here, we present some new approaches to increase the collection efficiency by precisely controlling the color centers' dielectric environment.
The considered structures are based on thin membranes, fabricated in commercially available, high purity diamond material via reactive ion etching. Combining the thin membrane with a planar antenna structures allows for creation of tailored radiation patterns, leading to a high directivity and thereby high collection efficiency for all emitters in the membrane at the same time. A radiating dipole in such structures can be calculated analytically, which allows for computer-aided optimization of the structure.

Andrzej Gajewski, Karolina Slowik

Nicolaus Copernicus University, Faculty of Physics, Astronomy and Informatics, ul. Grudziadzka 5, Torun 87 - 100, Poland

We present the result of broad analysis of the possibility of using nanoantennas - both dielectric and metallic - in order to enhance two-photon emission rate from a quantum dot in a realistic scenario. Quantum dots are a very promising source of non-classical light with a wide variety of possible application and a strong candidate for elements of Integrated photonic quantum circuits.  One way of controlling properties of emitted photons is by the use of nanoantennas. Nanoantennas are nanoscaled optical devices, usually made of noble metals or dielectric. In analogy to their macroscopic counterparts, nanoantennas can mediate between propagating radiation and localized electromagnetic fields. With their nanoscopic size, they sustain resonant response within the optical or near-infrared range. Nanoantennas are commonly used as a way to enhance emission or redirect it. We show a result of the study of the geometry of nanoantennas which could in practice lead to enhancement of two-photon entangled emission from a single quantum dot. 

Paul L. J. Helgers^1,2, Klaus Biermann¹, Haruki Sanada², Yoji Kunihashi², Paulo V. Santos¹

¹Paul-Drude-Institut Berlin, Germany
²NTT Basic Research Laboratories, NTT Corporation, Atsugi, Japan

We investigate a concept for acoustically driven single-photon-sources, based on planar GaAs quantum wires (QWRs) [1] embedded in optical microcavities. The QWR forms at sidewalls of patterned mesas on (Al,Ga)As templates, due to anisotropic MBE overgrowth of a 10-nm quantum well structure [2,3]. Spin-polarized carriers are optically injected in the QWR and acoustically transported to an embedded recombination center to emit single photons.

We observe charge transport over tens of microns and measured nanosecond timescale spin relaxation times in these QWRs. The latter are increased due to surface acoustic waves, promising spin transport lengths of tens of microns. We measure line-edge-roughness of the pattern edge up to 20 nm, complicating acoustic transport for narrow QWRs. Line-edge-roughness is mainly caused by photolithography. Using scanning transmission electron microcopy, we evaluate the thickness (26 nm) and width (200 nm) of the QWRs. We will evaluate the potential of these structures as efficient acoustically driven single-photon-sources.

This project has received funding from the European Union's Horizon program under grant agreement No 642688.

P. Holewa¹, A. Musiał¹, P. Mrowiński¹, K. Gawarecki², J. Misiewicz¹, N. Srocka³, D. Quandt³, A. Strittmatter^3,4, S. Rodt³, S. Reitzenstein³, G. Sęk¹

¹Laboratory for Optical Spectroscopy of Nanostructures, Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
²Department of Theoretical Physics, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
³Institute of Solid State Physics, Technical University of Berlin, Hardenbergstraße 36, D-10623 Berlin, Germany
⁴Currently: Institute of Experimental Physics, Otto-von-Guericke University Magdeburg, Magdeburg, Germany

Here we report on temperature-dependent photoluminescence (PL) studies and single-photon emission purity of MOCVD-grown In_0.75Ga_0.25As/In_0.2Ga_0.8As/GaAs quantum dots (QDs) emitting at 1.3 μm [1-2]. In order to enhance the extraction efficiency and spatial isolation of QDs, deterministic mesas were fabricated over individual pre-selected QDs using low-temperature in-situ electron-beam lithography [3].

Emission from single QDs was observed up till 80 K. The activation energies for the PL quenching process neither differ much between various excitonic complexes, nor depend on the ground state energy. They are in the range of (10-20) meV which indicates that the main quenching mechanism is the escape of holes to higher states supported by the electronic structure calculations within 8 band kp model. For the positively charged trion (X⁺) a pronounced intensity increase is observed in the range of (10-30) K. Corresponding process has an activation energy of 1.9 meV characteristic also for X^- PL intensity reduction indicating thermal activation of positive carrier traps in the QD vicinity [4].

High-purity single-photon emission from X⁺ complex was measured up to at least 30 K, which is important step towards low-cost quantum-dot-based single photon sources at telecom, employing cryogenic-free Stirling cooling [5].

Mikolaj Lasota, Karolina Sedziak, Piotr Kolenderski

Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, ul. Grudziadzka 5, 87-100 Torun, Poland

One of the most important problems of fiber-based long distance quantum communication (QC) is the temporal broadening of single-photon wavepackets, resulting from the propagation of those photons through dispersive media. Due to this effect the detection windows for such signals have to be made sufficiently long, increasing the amount of noise registered by the single-photon detectors and lowering the performance of QC protocols. However, the temporal wavepackets of photons emitted by means of spontaneous parametric down-conversion (SPDC) process highly depend on the properties of a utilized pump laser and nonlinear crystal.

Here we investigate the problem of optimizing a SPDC source for its use in a given quantum communication scheme. In particular, we derive analytical formula for optimal pump laser settings for a given nonlinear crystal. We also design an optimal SPDC source for a QC application assuming that one can freely choose both the properties of the pump laser and the crystal. Finally, we apply the obtained results to the security analysis of symmetric and asymmetric quantum key distribution schemes. We show that by optimizing the SPDC source according to our guidelines one can extend the maximal security distance of a non-optimized scheme by several tens of kilometers.

José Luis Velázquez, Helmuth Hofer, Beatrice Rodiek, Sefan Kück, Alicia Pons, Joaquín Campos and Marco López1,*

Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116, Braunschweig, Germany / Instituto de Óptica, Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain

The purity of single-photon sources often suffers from multiple emission lines in the emission spectrum of quantum dots based semiconductor systems. Spectral filtering, which selects only one emitting center, leads to a significant decrease in the transmitted photon flux. Here, we present the optical characterization of a compact and efficient setup for filtering the single-photon emission of an InGaAs quantum dot. The setup consists of two bandpass optical filters placed one after the other. Through a precise rotation of the filters, the convolution of their transmission windows allows to reach a spectral filtering with a full width at half maximum (FWHM) of less than 0.1 nm and a transmission of approx. 90 % in the wavelength range from 920 nm to 930 nm. These results are promising towards the development of a compact and high efficient single-photon source based on such quantum dots. Such sources can be used, especially, in the quantum radiometry, where a high photon flux rate is required for the efficiency calibration of single-photon detectors. Furthermore, the experimental results obtained when using this setup for filtering the single-photon emission of a state-of-the-art InGaAs quantum dot with embedded microlens will also be presented at this conference.

Idan Meirzada¹, Yonatan Hovav², Sigal Arielle Wolf^1,3, Nir Bar-Gill^1,2,3

¹The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
²Dept. of Applied Physics, Rachel and Selim School of Engineering, Hebrew University, Jerusalem 91904, Israel
³The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

In recent years, there has been increasing interest in understanding the dynamics of nitrogen vacancy (NV) centers under IR excitation, which has a significant impact on the NV centers emission and charge state. Many models have been suggested to explain parts of the experimental results, however a full mechanism describing all the experimental results is yet to be known. In this work [1] we suggest a generalized model for NV center spin and charge state dynamics under green and IR excitation. Using a rate equation approach, we show that this model is consistent with the new and previous experimental works [2,3,4], thus suggesting a complete understanding of the behavior of NV centers under green and IR excitations. This model, in turn, may be utilized in various NV-based experiments, such as spin readout enhancement and charge control. Moreover, we identify key differences between the photo-dynamics of bulk and shallow NVs and suggest a method to significantly enhance the initialization fidelity of shallow NVs to the useful negatively-charged state.

Marta Misiaszek, Andrzej Gajewski, Piotr Kolenderski

Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland

We show a technique for dispersion measurements in a nonlinear crystals by making use of phase matching in the process of parametric down conversion. The method can be applied for various types of crystals. It allows to determine the coefficients of Sellmeier equations with limited detection capabilities.

Here we present the method based on an exemplary PPKTP crystal phase-matched for 396 nm to 532 nm and 1550 nm, which can be tuned with temperature and pump wavelength. Using only one spectrometer for the UV-visible range, we show a procedure to determine the dispersion in the IR range.

Marwan Mohammed, Thomas Doherty, Naomi Holland, Klara Theophilo, Dustin Stuart, Axel Kuhn

University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK

Optical fibre-tip Fabry-Pérot cavities [1] can be used for strong coupling of an atom's electronic state and the cavity's photon state, allowing for a reversible and controllable quantum interface.  Besides the benefit of coupling the light directly to the fibre, the small fibre-tip diameter allows for optical access with numerical apertures as strong as 0.6, making possible the use of tightly focused dipole traps that hold single atoms at cavity standing-wave anti-nodes [2]. Our symmetric confocal fibre cavity is formed of two single-mode fibres, with a finesse of 100,000 and a predicted co-operativity of 29. Whilst there are constraints on the mode-matching efficiency and mirror parameters of these cavity types, we are developing novel designs and mirror ablation techniques that will overcome these. The deterministic nature and strength of the atom-photon interaction will be particularly useful for photonic quantum networks.

Richard Nelz, Michel Challier, Ettore Bernardi, Elke Neu

Universität des Saarlandes, Fakultät NT - Fachrichtung Physik, Campus E2.6, 66123 Saarbrücken

Individual nitrogen vacancy (NV) color centers in diamond are bright, photo-stable dipole emitters [1] and consequently optimal candidates for novel scanning near field microscopy techniques [2]. Here, NV centers form one member of a Förster Resonance Energy Transfer (FRET) pair. Due to their broadband emission (> 100 nm), NVs are versatile donors for FRET to systems absorbing in the near infrared spectral range. Promising applications include, e.g., nanoscale imaging of fluorescent molecules or nanomaterials like graphene [2].

Critical parameters for FRET are the NV’s quantum efficiency, charge state stability and NV-sample-distance. Previous experiments performed nanodiamond-based FRET [2], however NVs in this material might suffer from quenching, instability and bad control of surface termination. We here present first results towards FRET using color centers in single crystal diamond (SCD) via demonstrating quenching of NVs in SCD when applying graphene to the surface. While the FRET effect is present, the NVs retain their magnetic sensing capabilities.

To precisely control the NV-sample-distance, we aim to use shallowly implanted NVs in optimized cylindrical nanostructures as scanning probes in our homebuilt combination of confocal and atomic force microscope.

Jasper Rödiger^1,2, Nicolas Perlot¹, Ronald Freund¹, Oliver Benson²

¹Fraunhofer Heinrich Hertz Institute (HHI), Einsteinufer 37, 10587 Berlin, Germany
²Humboldt-Universität zu Berlin, Institut für Physik, AG Nanooptik, Newtonstraße 15, 12489 Berlin, Germany

We provide performance results on a QKD scheme based on the time-frequency uncertainty relation, referred to as time-frequency (TF‑) QKD. It is a BB84-like QKD protocol with the two bases being realized by modulations in time and frequency, namely pulse position modulation (PPM) and frequency shift keying (FSK), where the energy-time uncertainty relation ensures security.

TF-QKD can be implemented mostly with standard telecom components in the 1550 nm band and is well suited for free-space and fiber communication. In TF-QKD, polarization is not used, thus can be used for duplexing.

With PPM and FSK, it is possible to use an arbitrarily large alphabet and thus to transmit multiple bits per photon. This is especially beneficial when many photons reach and saturate the detector, or when there are other limits on the photon rate, e.g. an upper limit on the gating frequency for detectors, which are operated in gating mode (e.g. InGaAs avalanche photon-diodes).

We have implemented the TF-QKD protocol [1] and performed transmissions over free-space and fiber channels, have performed numerical simulations regarding pulse forms and number of symbols per basis [2] and performed an experiment using four symbols in each basis  as a first step toward large alphabets.

Yoav Romach¹, Andrii Lazariev³, Itay Avrahami², F. Kleißler³, Silvia Arroyo-Camejo³, Nir Bar-Gill^1,2

¹The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel
²Department of Applied Physics, Rachel and Selim School of Engineering, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel
³Department of NanoBiophotonics, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077, Göttingen, Germany

Understanding the physical origin of noise affecting quantum systems is important for nearly every quantum application. While quantum noise spectroscopy has been employed in various quantum systems, such as superconducting qubits and trapped ions, traditional spectroscopy methods are usually efficient in measuring noise spectra with mostly monotonically decaying contributions. However, there are important scenarios in which the noise spectrum is broadband and non-monotonous, thus posing a challenge to existing noise spectroscopy schemes.
Here, we compared several methods for noise spectroscopy; spectral decomposition based on the CPMG sequence, the recently presented DYSCO sequence and a modified DYSCO sequence with a Gaussian envelope (gDYSCO).
The performance of the sequences is quantified by analytic and numeric determination of the frequency resolution, bandwidth and sensitivity, revealing a supremacy of gDYSCO to reconstruct non-trivial features.
Utilizing an ensemble of nitrogen vacancy centers in diamond coupled to a high density 13C nuclear spin environment we experimentally confirm our findings.
The combination of the presented schemes offers potential to record high quality noise spectra as a prerequisite to generate quantum systems unlimited by their spin-bath environment. In particular, this method could be used in NMR/MRI measurements to improve the sensitivity, bandwidth and accuracy.

Bernd Sontheimer¹, Mehran Kianinia², Carlo Bradac², Merle Braun¹, Igor Aharonovich², Milos Toth², Oliver Benson¹

¹AG Nanooptik, Institut für Phsysik, Humboldt Universität zu Berlin, Newtonstrasse 15, 12489, Germany
²School of Mathematical and Physical Sciences, University of Technology Sydney, Ultimo, NSW, 2007, Australia

Single photon sources (SPSs) are prime candidates for a myriad of applications in integrated quantum optics and information processing. Local quantum emitters in hexagonal boron nitride (hBN), a wide-band-gap two-dimensional material, have recently emerged as promising SPSs. While the origin and atomic structure of these emitters are still under debate, they can exhibit remarkable properties including the ability of sub-band-gap excitation at room temperature, high brightness and short excited state lifetime.[1] To fully understand the origin of these characteristics and harness or even engineer them in the future, the underlying electronic level structure has to be revealed. Up to now, efforts based on density functional theory are not fully conclusive. Here, we present our experimental approach using statistical analysis of the single photon stream of a SPS to gain insights into the emitters nature. By means of a two-laser repumping scheme we identify a class of hBN quantum emitters with a fast-decaying intermediate and a long-lived metastable state accessible from the first excited electronic state. Based on those findings we propose a level scheme that matches our observations. To demonstrate the utility of the unique photo-physics of these quantum emitters, we realize a new modality of far-field super-resolution imaging.[2]

N. Srocka¹, P. Mrowinski², Ł. Duanowski^2,3, A. Musiał², G. Sęk², D. Quandt¹, A. Strittmatter^1,4, S. Rodt¹, S. Reitzenstein¹

¹Institute of Solid State Physics, Technische Universität Berlin, 10623 Berlin, Germany
²Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, 50-370 Wrocław, Poland
³Present address: Institute of Experimental Physics, Otto von Guericke University Magdeburg, 39106 Magdeburg, Germany
⁴Present address: Technische Physik, University of Würzburg, 97074 Würzburg, Germany

Advanced quantum communication applications require single-photon sources featuring i) high photon-extraction efficiency, ii) high flux rate, iii) high suppression of multi-photon emission and iv) high degree of photon indistinguishability. The concept of monolithic microlenses aligned to self-assembled semiconductor-quantum-dots has been proven to be an efficient approach to satisfy all of these four requirements in a single device operating at 900 to 950 nm [1].
We report on applying the microlens approach to In(Ga)As/GaAs quantum dots emitting in the telecom O-band. The quantum dots are grown by MOCVD whereby their emission wavelength is shifted towards the NIR due to an introduced strain reducing InGaAs layer in the growth design. Sample growth is followed by further processing to enhance the photon-extraction efficiency on the semiconductor surface. We will report on the deterministic fabrication of monolithic microlenses utilizing in situ three-dimensional electron-beam lithography at low temperatures and results of a detailed spectroscopic evaluation of the processed structures [2].

Yuuki Tokunaga¹, Hayato Goto², Shota Mizukami³, Takao Aoki³

¹NTT Secure Platform Laboratories, NTT Corporation, Musashino 180-8585, Japan
²Frontier Research Laboratory, Corporate Research & Development Center, Toshiba Corporation, Kawasaki, Kanagawa 212-8582, Japan
³Department of Applied Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan

Nano-fiber cavity-QED systems are promising candidates for quantum information processing because of the fiber-based transmission capability and the efficient coupling of an atom and light thanks to the tight transversal-mode confinement and the large evanescent fields of a nanofiber. Especially, the small mode area of the nano-fiber cavity-QED systems greatly contributes to achieve the strong coupling region [1] even with the current large cavity internal losses compared to the free-space system. We show that the success probabilities of cavity-QED-based single photon generation using the stimulated Raman adiabatic passage or the Purcell effect are both upper bounded by a single dimensionless quantity ``internal cooperativity,""" which is introduced here as the single-atom cooperativity parameter with respect to the cavity internal loss rate, instead of the cavity total loss rate. This is the consequence of the tradeoff with respect to the cavity internal loss and atomic spontaneous emission. The upper bound is achieved by optimizing cavity external loss rate, which is possible by designing or tuning the transmittance of the output coupler. This result indicates that the nanofiber cavity-QED systems have a great potential to generate single photons with high probability by reducing the current cavity internal losses.

Imants Bersons¹, Rita Veilande¹, Ojars Balcers²

¹Institute of Atomic Physics and Spectroscopy, University of Latvia
²Sociotechnical Systems Engineering Institute, Vidzeme University of Applied Sciences

A plane wave solution of the Maxwell equations describes all optical phenomenon. But the plane wave is exposed to diffraction and when it spreads in a space it has to disappear, therefore, it cannot describe a photon. The quantization procedure provides the correct energy and the creation and annihilation probabilities of photons, but the diffraction of photons is suppressed by the artificial quantization box.

Our viewpoint is that the photon can be described only by a nonlinear equation with a soliton type solution. We assume that light induces the polarization and magnetization of a vacuum only along the direction of its propagation. Based on the Maxwell equation we propose a nonlinear equation which is similar to the generalized nonlinear Schrödinger equation [1].  Its soliton type solution could outline a photon. The one- and two-soliton solutions are found in a vacuum and in dielectrics [2, 3].

If the photon is a soliton, then when two photons collide, they have to shift in space like solitons do. Would it be observed?

Ryan Warburton, Richard Walker, Jakub Nedbal

Photon Force, Alrick Building, Edinburgh, EH9 3BF, Scotland

Single-photon avalanche diode (SPAD) detectors have been the cornerstone of photon counting and timing applications for many years.  Whilst single-point detection has uses in many quantum applications, there are many areas that would benefit from single-photon detection and timing with the ability to form an image simultaneously.  Our PF32 camera combines 1024 SPADs in a 32x32 array, each with its own timing capabilities to precisely tag the arrival of a photon to an accuracy of 55 ps.  In terms of quantum measurements, this opens up new possibilities: measuring the full-field of the down-converted photons in an entangled system, for example.  Miniaturisation is another key challenge of developing usable quantum systems: normal TCSPC systems can be bulky, whereas using CMOS processing, these 1024 SPADs and all the necessary electronics can be contained within a package about the same size as a digital camera and addressed through a USB3 cable.  This also helps with the integration of such detectors into larger systems where power requirements and real-estate are key considerations.  We will present the latest work undertaken with the PF32 camera, and demonstrate how it can help with the progress of quantum science.

Sigal A. Wolf^1,2, Itamar Rosenberg^1,2, Ronen Rapaport^1,2,3, Nir Bar-Gill^1,2,3

¹The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
²The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
³Dept. of Applied Physics, Rachel and Selim School of Engineering, Hebrew University, Jerusalem 91904, Israel

Nitrogen-Vacancy (NV) color centers in diamond have emerged as promising quantum solid-state systems, with applications ranging from quantum information processing to magnetic sensing. One of the most useful properties of NVs is the ability to initialize and read their ground-state spin projection optically at room temperature.

A commonly encountered problem is the limited coupling efficiency between the quantum system and light. Optical antennas selectively enhance the optical coupling to a specific resonant mode, resulting in modified spontaneous emission, called Purcell Factor (PF), and in high directionality of this emission, leading to a better photon collection efficiency. The resulting enhancement could have a major impact on the ability to readout the NV center ground-state, potentially achieving single-shot readout.

In our work we study experimentally and theoretically Purcell enhancement of the NV optical coupling. We find theoretically that a combined increase in spontaneous emission and in optical excitation could significantly increase the readout Signal to Noise Ratio (SNR). Using plasmonic structures we experimentally demonstrate enhanced radiative emission in NV centers. By varying the structures dimensions, we control the PF and show preliminary results of the effect of increased PF on the readout SNR.

Junyi Wu, Holger F. Hofmann

Graduate School of Advanced Sciences of Matter, Hiroshima University, Kagamiyama 1-3-1, Higashi Hiroshima 739-8530, Japan

In quantum metrology, the optimum quantum fisher information (FI) of a quadrature component of light field can be obtained in photon number measurements implemented by perfect photon number resolving (PNR) detectors. However due to photon losses in PNR detectors, the FI obtained in real PNR measurements drops off. In this presentation, the effects of photon losses $eta$ in PNR detectors on quantum FI is studied. We derive an upper bound $I(eta)$  on the FI obtained in the PNR measurement under photon losses $eta$ and show that $I_{mathrm{F}}$ decreases monotonically with respect to $eta$. It is found that the FI obtained from displaced-squeezed states $I_{epsilon}$ drops faster than the one obtained from coherent states $I_{0}$, and upon a critical photon losses $eta_{c}$, it holds that $I_{epsilon}

Ben Yuen

University of Oxford, Clarendon Laboratory, Parks road, Oxford, OX1 3PU

The dressed atom picture [1,2] has proved highly successful at describing light matter interactions in an intuitive way, and accounts for the dynamics of both atom and field. We generalise this approach to describe the interaction of atoms and a field comprising of multiple frequency components. The natural extension of the single mode dressed atom picture gives rise to singularities when the system is expanded perturbatively, or erroneous level shifts and resonances when diagonalised numerically due to degeneracies in the dressed state energies. We find a non-degenerate basis for multimode dressed atoms which allows us to calculate the eigenstates and time evolution of the system. Our method gives a new perspective on nonlinear optical processes and produces accurate analytic approximations for the time evolution of an atom driven by a multi-mode field.

Steffen Zienert, Wolfgang Elsäßer

Technische Universität Darmstadt, IAP - AG Halbleiteroptik, Schlossgartenstr. 7, 64289 Darmstadt

The so-called pseudo-thermal light source or Martienssen lamp [1] based on a monochromatic laser beam and a rotating diffuser (ground glass) has proven its capability of emitting light with a second order correlation coefficient of two, thus photon-bunched classical thermal light. This led to a fruitful number of quantum optics experiments, such as ghost imaging. Here, we extend this experimental scheme by adding a subsequent second rotating diffusor finally resulting in light with super-thermal statistics, i.e. a second order correlation coefficient exceeding two [2].

For a comprehensive analysis of the second order correlation function we deployed a Hanbury Brown and Twiss (HBT) intensity interferometer based photon-counting set-up consisting of two silicon based single-photon avalanche diodes (SPADs) with a PicoHarp 300 time-correlated single photon counting analysis system. We present experimental results of the second order correlation coefficient for various system parameters including spatial dependencies.