A summary of our 3^rd International Symposium on
“Single Photon based Quantum Technologies”

Benito Alén

Instituto de Microelectrónica de Madrid, 8 PTM Tres Cantos, 28760 Madrid, Spain

Single photon and entangled photon pair sources are an essential component of QKD cryptographic systems. For unattended long-life operation in potentially harsh environments, these devices shall contain the minimum number of optomechanical elements and moving parts, thus eliminating the risk of misalignment due to vibrations and/or temperature changes. Thus, for the development of plug&play single photon sources, which are both alignment-free and vibration resistant, a good start would be to integrate the pumping source and the single photon source in a monolithic design.

In this work, we will present our design for such a plug&play device [1]. It is based on a vertical multijunction heterostructure comprising quantum dots and two separated electrical injection and electrical tuning regions in a bi-polar transistor configuration. The connection between them is purely optical and thus, it naturally avoids sheet resistance problems when applied to nanophotonic devices. We will show finite element simulations of different electrical and photonic designs together with results obtained in the first fabricated devices.

Hamza Abudayyeh, Boaz Lubotzky, Somak Majumder, Niko Nikolay, Jennifer Hollingsworth, Oliver Benson, Ronen Rapaport

ronenr@phys.huji.ac.il

Deterministic coupling between various photonic nodes in a quantum network is an essential aspect for various quantum technologies. Single photons have become essential resources for a growing number of such applications in which solid-state atom-like systems such as semiconductor quantum dots and color defects in crystals excel. A particular interest has been developed in nanocrystal quantum dots and color centers in diamond as potential compact room-temperature emitters. There are however several challenges that inhibit the use of such sources in current technologies including low photon extraction efficiency, low emission rates and relatively low single photon purities. In this talk I will review our efforts in overcoming these technical difficulties using several complementary methods including designing several nanoantenna devices that enhance the directionality and emission rate of the nanoemitter approaching record high collection efficiencies of over 80% [1][2][3][4] and Purcell factors of over 100 thus achieving an enhancement factor of over 1000 in the single photon brightness [2]. In addition, we developed several temporal heralding techniques to increase the single photon purity of nanocrystal quantum dots from <90% to over 99.5% [5]. These combined techniques show great promise for producing highly pure, bright and efficient single photon sources on-chip.

Matthias C. Löbl¹, Clemens Spinnler¹, Alisa Javadi¹, Liang Zhai¹, Giang N. Nguyen^1,2, Julian Ritzmann², Leonardo Midolo³, Peter Lodahl³, Andreas D. Wieck², Arne Ludwig², Richard J. Warburton¹

¹Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
²Lehrstuhl für Angewandte Festkörperphysik, Ruhr-Universität Bochum, DE-44780 Bochum, Germany
³Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, Copenhagen DK-2100, Denmark

In a multi-electron atom, an excited electron can decay by emitting a photon. In a radiative Auger process, the other electrons are in an excited state, and a red-shifted photon is created [1]. On a single quantum dot, radiative Auger has not been observed. Here, we report radiative Auger for trions in individual quantum dots [2]. For the trion, just one electron is left after the optical recombination. The radiative Auger process promotes this Auger electron to a higher shell of the quantum dot, and the emitted photon is red-shifted. We show that the energy splitting between this red-shifted photon and the resonance fluorescence directly measures the single-particle splittings of the quantum dot [2], which is otherwise difficult to acquire. We prove the radiative Auger mechanism by measuring the photon statistics and the magnetic field dispersion of the emission. Going beyond the original work in the X-ray spectrum of atoms [1], we apply quantum optics techniques to the radiative Auger photons. We show how quantum optics gives access to the single-electron dynamics, notably the relaxation and tunneling rates. All these properties of radiative Auger can be exploited on other semiconductor nanostructures and quantum emitters in the solid-state.

Lukas Hanschke¹, William Rauhaus¹, Kevin A. Fischer², Jakob Wierzbowski¹, Stefan Appel¹, Daniil Lukin², Shuo Sun², Rahul Trivedi², Malte Kremser¹, Tobias Simmet¹, Constantin Dory², Jelena Vuckovic², Jonathan J. Finley¹, Kai Müller¹

¹Walter Schottky Institut, Physik Department, Technische Universität München, 85748 Garching, Germany
²E. L. Ginzton Laboratory, Stanford University, Stanford, CA 94306, USA

Due to their excellent optical properties, quantum dots are promising for applications in photonic quantum technologies. For on-demand single-photon generation, a two-level system given by an excitonic transition is typically excited with a resonant laser pulse of area π. This prepares the two-level system in its excited state from where it spontaneously emits a single photon. However, emission that occurs already during the presence of the laser pulse allows for re-excitation and, thus, multi-photon emission, which limits the single-photon purity [1].

In contrast, when exciting the system with a pulse of area 2π, the system is expected to be returned to the ground state. However, in this case 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 has been absorbed. This restarts the Rabi oscillation with a pulse area of π remaining in the pulse which leads to re-excitation with near-unity probability and the emission of a second photon within the excited state lifetime [2, 3].

Finally, we present the generation of single photons with ultra-low multi-photon probability [4]. Using two-photon excitation of the bi-exciton suppresses re-excitation and improves the single photon purity by several orders of magnitude for short pulses.

Dorian A. Gangloff¹, Gabriel Ethier-Majcher¹, Jonathan H. Bodey¹, Daniel H. Jackson¹, Leon Zaporski¹, Mete Atature¹, Claire Le Gall¹, Emil V. Denning², Jesper Mork², Maxime Hugues†³, Edmund Clarke³

¹Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK
²Department of Photonics Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
³EPSRC National Epitaxy Facility, University of Sheffield, Broad Lane, Sheffield, S3 7HQ, UK

Interfaces between single photons and single spins underpin the promise of flexible quantum architectures and unconditional security for communication. Their versatility arises from our ability to measure the spin information deterministically and use photons to generate entanglement between spins over large distances. Using leading solid-state systems, semiconductor quantum dots (QDs) and Nitrogen Vacancies in diamond, important milestones have been reached [1]. In the case of QDs, their near-ideal optical properties have allowed to distribute entanglement at an unprecedented rate of 7.3 kHz. Thus far, fewer praises can be sung about their spin coherence. The electron couples to a mesoscopic ensemble of N~100,000 nuclei and gaining control over this many-body system to the point where nuclei are a resource is a frontier challenge in the field.

In this talk, I will present our latest advances in quantum control of collective nuclear states [2, 3]. In our experiments, we operate the electron both as a control and a probe of the total nuclear spin polarisation, I_z. For a thermal nuclear ensemble, fluctuations of this polarisation (~√N) broaden the electron spin linewidth to ~100MHz. Using an all-optical method to prepare the nuclei, we reduce the uncertainty on nuclear polarisation to well below the nuclear Zeeman energy (ω_z~20MHz), thus allowing to resolve the hyperfine levels of the electron-nuclear system and access nuclear spin-wave modes, appearing as weakly allowed sideband transitions. The overarching goal is to use these collective nuclear modes for quantum storage [4], a decisive step towards a scalable network.

Johannes Schall¹, Peter Schnauber¹, Samir Bounouar¹, Theresa Höhne², Anshuman Singh³, Suk In Park⁴, Geun-Hwan Ryu⁴, Tobias Heindel¹, Sven Burger², Jin-Dong Song⁴, Kartik Srinivasan³, Sven Rodt¹, Marcelo Davanco³, Stephan Reitzenstein¹

¹Institute of Semiconductor Physics, Technical University Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
²Zuse Institute Berlin, Takustr. 7, 14195 Berlin, Germany
³National Institute of Standards and Technology, 100 Bureau Dr, Gaithersburg, MD 20899, United States
⁴Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, 02792 Seoul, South Korea

The deterministic integration of quantum emitters into on-chip photonic elements is crucial for the implementation of scalable integrated quantum circuits. We report on the single-step deterministic integration of quantum dots (QDs) into on-chip waveguide structures using in-situ electron beam lithography (in-situ EBL) [1].

We use in-situ EBL to realize multimode interference (MMI) splitters acting as 50/50 coupling elements – a central building block of on-chip quantum circuits. The deterministically fabricated QD-waveguide structures are studied by micro-photoluminescence spectroscopy and photon cross-correlation measurements between the two MMI output ports. The latter confirms single-photon emission associated with g⁽²⁾(0) ∼ 0.1 [2].

Furthermore, we integrate QDs as single-photon emitters in heterogenous SiN waveguides (WGs). Here in-situ EBL guarantees the precise alignment of single QDs at the center of GaAs tapers, injecting single photons with g⁽²⁾(0)=0.11 into the SiN WGs underneath. Beyond that, we demonstrate the generation of indistinguishable photons with a post-selected Hong-Ou-Mandel visibility of 0.89 [3]. Our device fabrication approach paves the way for scalable, silicon-based quantum photonics using QDs as excellent single-photon emitters.

Filip Sośnicki, Michał Mikołajczyk, Ali Golestani, Adam Widomski, Michał Karpiński

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

The quantum internet, one of the ultimate goals of the quantum community, is envisioned to be created through a hybrid quantum network[1]. Such network consists of variety quantum devices, such as photon pairs sources, quantum gates or detectors, as nodes interconnected by photonic links. A wide range of possible quantum nodes creates a challenge to interconnect them efficiently due to different spectro-temporal properties. E.g., the spectral bandwidth of single photons ranges from single MHz to hundreds of GHz. We propose using an interface - a bandwidth converter - to change spectral bandwidth by many orders of magnitude. It combines dispersive propagation with applying a time-dependent phase by direct electro-optic phase modulation (EOPM)[2,3]. Using the concept of Fresnel time lens, i.e., a temporal quadratic modulo-2pi phase, one can achieve high compression factors[4].

We have experimentally implemented a stable Fresnel time lens by using an EOPM driven by an amplified signal from an arbitrary waveform generator. We demonstrate a spectral compression of heralded single photons by more than two orders of magnitude, from 200 GHz to sub-2 GHz spectral width. Results indicate that an EOPM-based interface will allow efficient interconnection of, e.g., single photons from broadband SPDC sources with atomic-based quantum repeaters.

Jake Southall, Rob Purdy, Almut Beige

The School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom

Photons with well-defined energy are highly non-localised and occupy all available space in one dimension. On the other hand, optical elements, like mirrors, are highly-localised objects. To simplify the modelling of the electromagnetic field in the presence of optical elements, we therefore introduce annihilation operators for highly-localised field excitations [1]. These arise naturally if we quantise the negative as well as the positive frequency solutions of Maxwell’s equations, and enable us to obtain locally-acting interaction Hamiltonians for two-sided semi-transparent mirrors with a wide range of applications in quantum optics. Overall, we learn that the standard description of the electromagnetic field is incomplete and that the dynamical Hamiltonian and the energy observable are not the same for all photons.  

Tanja E. Mehlstäubler

PTB Braunschweig, QUEST Institute for Experimental Quantum Metrology, Bundesallee 100, 38116 Braunschweig, Germany,

Single trapped and laser-cooled ions in Paul traps allow for a high degree of control of atomic quantum systems.
They are the basis for modern atomic clocks, quantum computers and quantum simulators. Our research aims to use ion Coulomb crystals, i.e. many-body systems with complex dynamics, for precision spectroscopy. This paves the way to novel optical frequency standards for applications such as relativistic geodesy and quantum simulators in which complex dynamics becomes accessible with atomic resolution.

Marta Gilaberte Basset, Josué R. León Torres, Markus Gräfe

Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Albert-Einstein-Straße 7 , 07745 Jena, Germany

We demonstrate quantum imaging based on induced coherence without induced emission in a compact single-crystal setup. Photons that form the image never interacted with the object and photons that interacted with the object are discarded. This way imaging in exotic spectral ranges while the actual detection is in the VIS becomes feasible. Besides its high stability our system features portability and imaging in video rate. Our results will stimulate further work towards extreme light imaging devices in particular in the field of life science.

Pietro Lombardi^1,2, Hardy Schauffert³, Maja Colautti^1,2, Sofia Pazzagli⁴, Marco López⁵, Stefan Kueck⁵, Costanza Toninelli^1,2

¹CNR-INO, U.O.S. Sesto Fiorentino, via N.Carrara 1, Sesto F.no, Italy
²LENS & Dip. di Fisica, Università di Firenze, via G. Sansone 1, Sesto Fiorentino
³Atominstitute, TUWien, Wien, Austria
⁴Humboldt Universität, Berlin, Germany
⁵PTB, Bundesalle 100, Braunschweig, Germany

In this contribution we will discuss the possible impact in quantum metrology and quantum communication of the single-photon source that we have both theoretically and experimentally demonstrated few years ago[1], based on a quantum emitter embedded in a planar optical antenna.

According to our theoretical study, < 20% non-radiative losses and ~ 50% coupling of the emission into a SM fiber are achievable with such design, considering Dibenzoterrylene molecules (DBT) hosted in a crystalline anthracene matrix (Ac). The DBT:Ac system provides stable and narrowband (< 100MHz at 3K temperature) emission[2], and is hence extremely appealing for applications involving quantum memories.

We will present the implementation of a simplified version of the antenna operated at cryogenic temperature, showing 5MHz photon flux in the Fourier-limited line at first lens[3]. The device is exploited for the calibration of a SPAD and allows conceiving a molecule-based low-photon-flux standard source for quantum radiometry with sub-Poissonian statistics.

Finally, we will discuss the implementation of such design directly on the facet of a SM fiber, for both room and cryogenic temperature operation, as well as the implementation and the performances of an open-air QKD system based on true single photons provided by our sources.

Santiago Tarrago Velez, Sakthi Priya Amirtharaj, Anna Pogrebna, Christophe Galland

Institue of Physics, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland

Raman scattering has long been a vital spectroscopy tool, allowing researchers to study atomic-scale phenomena using visible light. The inelastic scattering of light takes place when the incoming radiation creates or annihilates a quantum of vibration in the medium. In this process, the conservation of energy and momentum creates correlations between the scattered light field and the state of the system, which can be harnessed to study and manipulate the state of the quanta of vibration that was created.

In this talk, we show how to combine spontaneous Raman scattering from ultrafast laser pulses with single photon counting in order to prepare a high frequency vibrational mode in a quantum state – even at room temperature and ambient pressure [1,2]. In particular, we show how to prepare the vibrational mode – one involving billions of atoms in a crystal – in the n=1 Fock state, in a superposition state, and show that the entanglement between light and the collective vibration is strong enough to violate a Bell inequality [3].

Manipulating room-temperature mechanical oscillators enables new ways to process quantum information at ultrafast timescales, and opens a new ways to study quantum phenomena occurring in molecular and solid-state systems. 

Xiaolong Hu

Tianjin University, School of Precision Instrument and Opto-Electronic Engineering, Tianjin 300072, China

Superconducting nanowire single-photon detectors (SNSPDs) have shown unprecedented performance; and they have been used in many quantum and classical photonic applications. In this talk, we first review two mechanisms of device timing jitter of SNSPDs – vortex-crossing-induced timing jitter [1] and inhomogeneity-induced timing jitter [2] that we recently uncovered. The former sets the lower bound of the timing jitter whereas the latter is one of the major contributing factors to the measured timing jitter. We will then review our work on fractal SNSPDs that can detect incident photons in all polarization states with high performance. [3, 4] Finally, we introduce the concept of superconducting nanowire multi-photon detectors (SNMPDs) that can measure M-fold photon coincidences with electronic circuitry as simple as the circuitry for one individual SNSPD. We believe that these progresses advance the field of superconducting nanowire photon counters and anticipate expanded application spaces of these devices.

Matthias Häußler¹, Fabian Beutel¹, Wladick Hartmann¹, Helge Gehring¹, Robin Stegmüller¹, Nicolai Walter¹, Max Tillmann², Michael Wahl², Tino Röhlicke², Andreas Bülter², Doreen Wernicke³, Nicolas Perlot⁴, Jasper Rödiger⁴, Wolfram H. P. Pernice¹, Carsten Schuck¹

¹Institute of Physics, University of Münster, Heisenbergstraße 11, 48149 Münster, Germany
²PicoQuant GmbH, Rudower Chaussee 29, 12489 Berlin, Germany
³Entropy GmbH, Gmunder Straße 37a, 81379 München, Germany
⁴Fraunhofer Heinrich Hertz Institute, Einsteinufer 37, 10587 Berlin, Germany

Emerging quantum technologies increase the demand for reliable tools that enable single-photon generation, manipulation and sensing on an increasingly large scale. In the framework of integrated photonics, these needs can be fulfilled by patterning highly stable photonic devices on monolithic silicon chips in CMOS compatible processes. In this work we show how advanced single-photon detection capabilities are achieved on a silicon chip, realizing a 4x4 array of niobium-titanium nitride superconducting nanowire single-photon detectors [1]. Each of the detectors is integrated with low-loss silicon nitride photonic waveguides which are individually addressable via dedicated fiber-optic channels. Here, we employ sophisticated broadband 3D polymeric fiber-to-chip interfaces for maximizing the system detection efficiency of our 2D detector array [2]. Our results show superior detection efficiency and timing performance at telecom wavelengths for a detector array operated at 3 K. Such waveguide-integrated detector arrays not only pave the way for advanced multi-channel and thus ultra-fast quantum key distribution but also offer the possibility of realizing additional functionality through integrating nanophotonic devices with waveguide-coupled detectors.

Philipp Zolotov^1,2,3, Alexander Divochiy², Pavel Morozov², Yury Vahtomin^2,3, Gregory Goltsman^1,2,3

¹HSE Tikhonov Moscow Institute of Electronics and Mathematics. Tallinskaya Ulitsa, 34, 123592 Moscow, Russia
²SCONTEL. Rossolimo 5/22-1, 119021 Moscow, Russia
³Moscow State Pedagogical University, M. Pirogovskaya, 119435 Moscow, Russia

Modern single-photon detectors are required to have outstanding performance to meet the demand for ongoing research in quantum technologies. For fiber-based quantum communication systems, it is of great importance to achieve high detection efficiency along with a low dark count rate and high speed in the C-band of the telecom range. This spectral range puts an insuperable obstacle for conventional single-photon detectors [1] and creates a niche for superconducting single-photon detectors (SSPDs) that proved to have superiority in a wide spectral range in terms of typical values of detection efficiency above 80%, counting rates above 10 MHz and temporal resolution of the order of 50 ps [2, 3]. Due to undesired thermal radiation at the IR range, high system detection efficiency (SDE) values of the detectors come with moderate dark count rates above 100 Hz. In our work we present practical SSPDs that show SDE >75% and ≤0.6 Hz of dark counts obtained with cold fiber filters. Such a remarkable result was realized for the detectors operated at 2.2 K in a closed-cycle cryostat and showed long-term stability. Implemented approach allowed us to preserve the high performance of the detectors and additionally optimize them for quantum telecommunication applications.

Josef Hlousek, Ivo Straka, Miroslav Jezek

Department of Optics, Palacky University Olomouc, 17. listopadu 12, 77146 Olomouc, Czech Republic

A vast majority of radiometric, spectroscopic, imaging, and optical communication methods rely on comparing light intensity levels measured by a photonic detector. The measurement accuracy is impaired by any deviation from the linear response of the detector, which leads to systematic errors. We present a direct single-source method for absolute measurement of nonlinearity to characterize the response of an arbitrary single-photon detector, namely several actively and passively quenched single-photon avalanche diodes (SPADs) and superconducting nanowire single-photon detectors (SNSPDs). The presented method does not require a reference detector or calibrated attenuators. Neither does it employ time-resolved generation and detection. The method applies to any single-photon detector regardless of the detection technology. Contrary to general belief, dead time is not the only effect responsible for saturation of the SPADs; we discover supra-linear behavior of SPADs and show that it cannot be fully explained using known theoretical models. Furthermore, we explore the nonlinearity of SNSPDs for various values of bias current and identify super- and sub-linear behavior caused by the interplay of recovery processes, latching, and two-photon sensitivity. Because of single-photon detector complex working principles, the direct absolute measurement of nonlinearity is the preferred way how to analyze the detector response.

Ana Predojevic

Stockholm University, Department of Physics, Albanova University Center, Stockholm, Sweden

Single self-assembled quantum dots are established emitters of single photons and entangled photon pairs. To be used in quantum information experiments quantum dots need to be excited resonantly and coherently. The use of resonant excitation makes this system well suitable for generation of photon pairs with near-unity efficiency and high purity and also for entangling schemes such as time-bin entanglement. The entanglement of photons generated by quantum dot systems can be employed in free space-and fibre-based quantum communication. In addition to this, the versatility of entanglement can be more optimally used and explored if the photons are entangled simultaneously in more than one degree of freedom – hyperentangled, which was also recently shown to be possible using quantum dots. However, the achievable degree of entanglement and readiness of the source for use in quantum communication protocols, depend on several additional functionalities such as high collection efficiency and coherence of the emitted photon pairs. Here, we will address engineered photonic systems that promise a more efficient and better performing sources of entangled photon pairs.

Dominik Koutný, Miroslav Ježek

Department of Optics, Faculty of Science, Palacký University Olomouc, 17. listopadu 1192/12, 77900 Olomouc

Entanglement quantification is of paramount importance to fundamental research as well as to many cutting-edge applications. Various approaches of entanglement detection have been proposed, but they usually provide only a witness or require the interference of multiple copies of the system under test. It was shown that quantum tomography is necessary for the exact determination of the entanglement in an unknown quantum state. The tomography yields complete information with a drawback of unfeasible scaling with the complexity of the system. Recently, artificial neural networks were exploited for the tomography by approximating the state wavefunction, and for entanglement witnessing. Despite these achievements, the question of how precisely the entanglement can be estimated directly from the incomplete measured data remains open. We approach this problem using convolutional neural networks. We focus on the characterization of two-qubit entanglement sources with imminent applications in quantum communications. We demonstrate significantly lower errors of quantum concurrence estimation from heavily undersampled Pauli measurements compared to state-of-the-art quantum tomography. We work toward the quantification of mutual information in multi-qubit systems and aim for testing on experimental data, particularly quantum dot sources and parametric generators. Preliminary tests show excellent performance of our method and its resistance to experimental imperfections.

Timm Kupko¹, Martin von Helversen¹, Lucas Rickert¹, Jan-Hindrik Schulze¹, André Strittmatter^1,2, Manuel Gschrey¹, Sven Rodt¹, Stephan Reitzenstein¹, Tobias Heindel¹

¹Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany
²Present Address: Institut für Experimentelle Physik, Otto-von-Guericke Universität, 39106 Magdeburg, Germany

Solid-state quantum light sources [1] have the potential to boost the performance of quantum communication [2,3]. Here, we report on tools that can be used to optimize the performance of quantum key distribution (QKD) implemented with triggered single-photon sources (SPSs) [4]. To this end we analyze the performance of a basic QKD testbed comprising a deterministically-fabricated quantum dot SPSs and a receiver module designed for polarization coding via the BB84 protocol. Exploiting temporal filtering in a two-dimensional parameter space, we analyze the sifted key fraction and the quantum bit error ratio expected in full implementations of QKD as a function of the acceptance time-window. Moreover, we investigate the impact of the temporal filtering on the photon statistics as well as the possibility to monitor g⁽²⁾(0) inside the quantum channel in real time. The developed routines enable us to choose optimal filter settings to maximize the expected distilled secret key depending on the losses of the quantum channel. Our findings can be directly applied and extended for advanced schemes of quantum communication, including measurement-device-independent QKD and quantum repeaters.

Franco Wong

77 Massachusetts Ave., Cambridge, USA

Floodlight quantum key distribution is a multimode two-way protocol that is capable of achieving Gbps secret-key rates by breaking conventional rules of operation, including sending many photons per bit duration and use of an amplifier to mitigate channel loss. In a table-top experiment with 10-dB channel transmission loss, we obtained a secret-key rate of 1.3 Gbps, surpassing other quantum protocols by 3 orders of magnitude.

Sooryansh Asthana, V. Ravishankar

Department of Physics, IIT Delhi, New Delhi

We propose to establish a robust connection between different features of quantumness,  exhibited through notions such as
non-locality, entanglement, quantum discord and non-Boolean logic,  and the anomalous weak values,  as espoused in the concept of weak measurements. This is accomplished by employing  the recently introduced concept of pseudo projection [1], and  the associated pseudo probability.
Pseudo-projections are Weyl ordered product of projection operators and their expectations are termed as pseudo-probabilities. The crucial feature is that negative pseudo-probability for joint outcome of two events can be experimentally realised as anomalous value of a projection operator. Thus,  weak measurements provide an experimental avenue to observe negative pseudoprobability and pseudo-projections facilitate an operator description to anomalous weak values.  In short, this study ties non-classical probability, anomalous weak values and different features of quantumness and opens up new avenues for testing nonclassicality via weak measurements. Along the sidelines, our derivations provide schemes for direct measurement of Bell CHSH inequality and Svetlichny inequality.

Rajni Bala¹, Sooryansh Asthana², V Ravishankar³

¹Department of Physics, Indian Institute of Technology Delhi, New Delhi, India-110016
²Department of Physics, Indian Institute of Technology Delhi, New Delhi, India-110016
³Department of Physics, Indian Institute of Technology Delhi, New Delhi, India-110016

Quantum key distribution (QKD) is one of the first application of quantum physics which has entered the
commercial market. The security of QKD protocols relies on the quantum nature of the physical system used. In
this work, we present a QKD protocol, security of which depends upon the contextuality of mono-party system.
It has been shown[1] that Bell CHSH nonlocality inequality[2] probes contextuality in single four-level system.
We use this contextuality inequality in the proposed protocol as a security check.
The advantage of the proposed protocols as compared to Ekert is that it does not involve entangled states, but
provides the same level of security. Thus, the cost of generation of entangled states is saved. The experimental
key generation rate of proposed protocol will also be higher than the one with entangled states because one can
use weak coherent pulses instead of limited brightness of entangled photon pair sources[3]. With advancement
in generation and manipulation of higher dimensional OAM states, our protocols can be experimentally realised.
Thus, we also propose the setup for experimental implementation of the protocol using orbital angular momentum
states. Our work is generalisable to conference QKD as well.
 

Florian Böhm^1,2, Niko Nikolay^1,2, Sascha Neinert^1,2, Oliver Benson^1,2

¹Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D-12489, Berlin, Germany
²IRIS Adlershof, Humboldt-Universität zu Berlin, Zum Großen Windkanal 6, D-12489, Berlin, Germany

So far, experiments on single solid-state quantum emitters are mainly limited to scientific workgroups with fundamental know-how and experience in the field, and setting up single-photon experiments can become a tedious and lengthy task for non-experts, e.g. biologists interested in utilizing solid-state emitters for sensing. One prime candidate for a quantum sensor is the nitrogen-vacancy (NV) center, an solid-state quantum emitter utilized for nanoscale magnetometry [1] or thermometry [2].

To facilitate the entry to this field of research we integrated an experimental platform for investigations on single-quantum emitters combined with coherent single-spin manipulation capabilities on the NV center into a stand-alone 19-inch mobile server rack cabinet. The Q.Rack combines all required components for optical and spin-manipulation measurements on single NV centers.

In contrast to approaches of integrated NV sensors [3], the Q.Rack is designed to be compact and mobile, as well as having the full functionality of an experimental setup in the lab. It is built in a modular design, which allows for easy replacement or integration of further components. Therefore the Q.Rack is not only limited to investigations on NV centers, but could also be used to investigate other single emitters, e.g. novel fluorescent defects in 2D materials.

Rasmus Flaschmann¹, Fabian Flassig¹, Stefan Strohauer¹, Lucio Zugliani¹, Noah Ploch¹, Thomas Kainz¹, Matthias Althammer², Rudolf Gross², Jon Finley¹, Kai Müller¹

¹Walter Schottky Institut, Technische Universität München, 85748 Garching, Germany
²Walther-Meißner-Institut, Technische Universität München, 85748 Garching, Germany

In recent years, Superconducting Single Photon Detectors (SSPDs) have raised tremendous attention as a possible key technology for optical quantum information processing. With their ability to detect single photons with a high efficiency, low dark count rate, fast response time and low timing jitter further investigations are of high interest [1,2,3].

These days, there is already a variety of detectors commercially available, mostly fiber-coupled system which mainly rely either on a manual alignment of the detector to the fiber core or on a deep reactive ion etching (DRIE) process [4,5]. Here, we present an approach to a self-aligning fiber-to-detector coupling mechanism that eliminates the necessity of DRIE and is suitable for arbitrary substrate/detector material combinations by being glued itself on a ferule.

In addition, we present our recent progress on NbTiN SSPDs on SiO₂. A figure of merit for the quality of a detector is the so-called constriction factor, which is the ratio of the switching current and the depairing current (C = I_sw(T) / I_dep(T)), and by increasing this ratio an enhanced detector performance is expected [6]. To these ends we present how by improving the quality of the superconducting detector as well as the detector design we achieve detectors where the count rate saturates already at 4.5K.

Hristina Georgieva¹, Marco López¹, Helmuth Hofer¹, Justus Christinck^1,2, Beatrice Rodiek^1,2, Peter Schnauber³, Arsenty Kaganskiy³, Tobias Heindel³, Sven Rodt³, Stephan Reitzenstein³, Stefan Kück^1,2

¹Physikalisch-Technische Bundesanstalt, Braunschweig, Germany
²Laboratory for Emerging Nanometrology, Braunschweig, Germany
³Institut für Festkörperphysik, Technische Universität Berlin, Berlin, Germany

Single-photon sources find application in many fields of quantum information processing. Therefore, there is an increasing need to ensure high accuracy and metrological traceability of measurements involving small photon fluxes. We present an absolute characterization of a triggered single-photon source based on a semiconductor quantum dot with an emission bandwidth below 0.1 nm in the near infrared at 922.4 nm. High efficiency is reached by combining a high extraction efficiency for the emitting quantum dot through a monolithic microlens and a high transmissive setup for the emitted light. The source is implemented for the relative calibration of two single-photon avalanche detectors reaching a relative standard uncertainty as low as 0.7 %. The result is verified by a comparison with a standard calibration using attenuated laser light. Finally, an Allan deviation analysis was performed giving an optimal averaging time of 92 s for the photon flux.

Timm Kupko¹, Lucas Rickert¹, Martin von Helversen¹, André Strittmatter, Sven Rodt¹, Stephan Reitzenstein¹, and Tobias Heindel^1,*

¹Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany
^*e-mail: tobias.heindel@tu-berlin.de

Triggered non-classical light sources hold the potential to boost implementations of quantum communication. In this context, deterministically fabricated quantum-light sources based on semiconductor quantum dots (QDs) are particularly appealing [1].

Here, I report our recent progress on how to optimize the performance of quantum communication scenarios implemented with realistic QD-based single-photon sources (SPSs). Using a receiver module designed for polarization-encoded BB84-QKD, we exploit two-dimensional temporal filtering of single-photon pulses to maximize the expected secret key fraction [2]. We investigate the impact of temporal filtering on the photon statistics and investigate the possibility to monitor g⁽²⁾(0) inside the quantum channel in real time. The routines developed enable us to choose optimal settings for the applied temporal filter. Furthermore, in a second poster, our team member Lucas Rickert aims at the development of plug-and-play SPSs. Lucas presents numerically optimized designs of fiber-coupled quantum light sources based on hybrid circular Bragg gratings operating at telecom wavelengths [3].

Our findings represent important contributions towards the development of QKD-secured communication networks based on quantum light sources - a challenge which we tackle in our group at Technische Universität Berlin [4].

Josef Hlousek, Michal Dudka, Ivo Straka, Miroslav Jezek

Department of Optics, Palacky University Olomouc, 17. listopadu 12, 77146 Olomouc, Czech Republic

The probability distribution of the number of photons in optical mode carries a great deal of information about physical processes that generate or transform optical signals. The statistics measurement allows for distinguishing classical, nonclassical, and non-Gaussian sources [1], and provides information on the applicability of the source in secure communications and quantum simulations. We propose and experimentally verify a workflow for the detection of arbitrary photon statistics using a multiplexed photon-number-resolving detector and a novel retrieval method based on the expectation-maximization algorithm weakly regularized by the maximum-entropy principle [2]. We demonstrate unprecedentedly accurate photon statistics measurements of chaotic, classical, nonclassical, non-Gaussian, and negative-Wigner-function light sources. Despite uncorrected systematic errors and significant variability of the input signals, our approach shows superior fidelity across the board with typical values exceeding 99.8% for mean photon numbers up to 20. We also demonstrate the extraction of correlation functions and nonclassicality witnesses with record precision. Furthermore, we present a numerical analysis to demonstrate a favorable scaling with the number of detection channels and the number of detection runs. The presented results open new paths for optical technologies by providing full access to the photon-number information without the necessity of detector tomography.

Varun Raj Kaipalath, Carlos Sevilla, Fabian Steinlechner

Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Straße 7, 07745 Jena, Germany

The spatial structure of light is a versatile degree of freedom that holds great potential for encoding large amounts of information in both classical and quantum optical communication systems. Spatial modes such as Hermite Gauss, or Laguerre Gauss modes are embedded in an infinite dimensional Hilbert space, a feature that can significantly enhance quantum information processing protocols[1]. In particular, Laguerre Gauss modes, which are associated with discrete value of orbital angular momentum (OAM), have enabled numerous experimental demonstrations of high-dimensional quantum protocols that exhibit key advantages such as improved noise-tolerance or channel capacity in comparison to two-dimensional qubit state encoding. Quantum states exhibiting orbital momentum entanglement can be generated via spontaneous parametric down conversion (SPDC) in a nonlinear crystal as a consequence of momentum conservation in the process[2]. One of the major challenges in using these states in quantum information protocols is the implementation of spatial mode analysis. Here, we study the OAM correlations of photon pairs generated via SPDC using a mode sorting technique called multiplane light conversion[3]. We measure both the radial and azimuthal components of the Laguerre Gauss modes simultaneously with high visibilities. The scheme is intended to be integrated to a modular OAM entangled photon source.

Teodora Kirova

Institute of Atomic Physics and Spectroscopy, University of Latvia, Jelgavas street 3, LV-1004, Riga, Latvia

There is an ongoing effort to improve the performance of single-photon sources, components and detectors to meet the current demands in quantum information, light detection, quantum metrology and single-molecule spectroscopy [1].

Recently single-photon source [2] and detector [3] were demonstrated in entangled mesoscopic ensembles of Rydberg atoms in dipole blockade regime, while single-photon transistor gain was substantially increased to over 100 [4] via DC-Stark tuning of Förster resonances [5].

We propose to improve the performance of single-photon devices via microwave-field-tuning [6] of Förster resonances, which will give additional control over both magnitude and direction of the AC-shift, unlike in the DC-field case. Additional benefits to devices time performance will be provided by quickly turning the interactions on and off via microwave source modulation.

Our model is based on Optical Bloch Equations for the density matrix, describing excitation of Rydberg atoms by coherent radiation and also including dipole-dipole interactions. Homogeneous and inhomogeneous broadening are incorporated to account for realistic experimental conditions. Preliminary numerical simulations are performed using different laser and microwave field strengths and detunings, as well as dipole-dipole interaction magnitude in order to find the best parameters for improving the performance of single-photon devices.

Aris Koulas-Simos, Jan Große, Martin von Helversen, Arsenty Kaganskiy, Martin Hermann, and Stephan Reitzenstein

Institute for Solid State Physics, Technical University Berlin, Germany

Amidst the second quantum revolution, the quest of efficient single photon sources for the realization of scalable, on-chip quantum devices still remains crucial [1]. Semiconductor quantum dots (QDs) have been suggested as a suitable candidate due to the on-demand emission of spectrally sharp non-classical light. However, the fabrication of large-scale QD arrays with simultaneously high extraction efficiency, homogeneous emission features and pre-defined positions has proven to be challenging. Here, we report on the fabrication of 28x28 mesa arrays, each containing site-controlled ensembles of 1-4 QDs by using the buried stressor growth method [2]. Micro-photoluminescence (µPL) maps show that we control the position of the QDs with an accuracy of 500 nm and the emission wavelengths vary only up to 4.1 nm. In µPL experiments, we clearly see the exciton, biexciton and trion signatures. Beyond that, Hanbury Brown and Twiss and Hong-Ou-Mandel (HOM) measurements validate the high quality of single-photon emission in terms of high single-photon purity and indistinguishability with a g⁽²⁾(0)≈0.025 and a visibility of about 70%.

Petr Steindl¹, Henk Snijders¹, Konstantin Iakovlev¹, Gerard Westra¹, Edward Hissink¹, John Frey², Justin Norman³, Art Gossard³, John Bowers³, Dirk Bouwmeester^1,2, Wolfgang Löffler¹

¹Huygens-Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands
²Dept. of Physics, University of California, Santa Barbara, California 93106, USA
³Dept. of Electrical & Computer Engineering, University of California, Santa Barbara, California 93106, USA

Multi-photon entangled states, in particular, loss-resilient graph and cluster states are interesting in fields from universal quantum computing [1] to metrology [2], and lately even all-optical quantum repeaters [3,4]. Despite these promises, deterministic production of many-photon cluster states is still an open problem.

Here we show quasi-deterministic production of photonic cluster states using a true single- photon source, linear optical manipulation, and post-selection. We use a high-brightness and high-fidelity single-photon source based on InAs quantum dots in a micropillar cavity [5]. In principle, using charged-exciton transitions, such a system can directly produce linear cluster states in a fully deterministic way [6], we however use a neutral transition it to produce a stream of single photons. We operate the single-photon source in continuous mode and use a delay- loop based quantum interference scheme to entangle consecutive photons, similar to the proposal of Pilnyak et al. [7]. Only recently, the same scheme was used for sequential quasi-deterministic creation of 4- photon cluster states [8]. Here we show quantum correlations of the stream of entangled photons, compare them to theoretical predictions, and explore limitations regarding the number of photons, achievable rates, stability and purity.

Amur Margaryan for the "RF Timer" collaboration

Yerevan Physics Institute

The lifetime of hot carriers in materials upon photoexcitation dictates the practicality of the materials in many applications such as solar energy conversion, surface chemistry, photonics and optoelectronics. Because of many attractive properties arising from its unique band-structure, graphene has been considered as one of the most promising materials for optoelectronic devices. Theoretically, the lifetime of hot carriers in graphene should be quite long, hundreds of picoseconds to a few nanoseconds. However, numerous experimental measurements in the past employing various methods have always measured a few ps decay time. Recently, it was shown (Lin Fan et al., J. Phys. Chem. Lett., 2018) that at very low irradiation levels, there exists a significant slow-decay process in graphene. This phenomenon was attributed to the excitation of image potential states (IPS) with a long lifetime. The employed laser system was a mode-locked Ti: Sapphire laser. The center wavelength was 790 nm, the pulse duration was ~35 fs and the fluence was at  ~10 μJ/cm2. At such low fluence, due to the extremely low signal level, many experimental tools cannot be applied anymore. We are considering a new, picosecond resolution, single photoelectron detector. It is based on the GHz radio frequency circular scanning system of keV electrons and position sensitive delay line anodes. Coupled with the synchronized ultra-fast lasers, this device will allow to study the lifetime of photoelectron emission from materials in a picosecond time domain. It will allow to investigate how important are the IPS electrons at even lower laser fluencies and whether these carriers can be effectively extracted to implement optoelectronic devices.

Abel Martinez-Suarez^1,2, Matteo Savaresi³, Davide Tedeschi³, Oliver Iff⁴, Magdalena Moczała-Dusanowska⁴, Javier Taboada-Gutierrez^1,2, Antonio Rivera⁵, Ovidio Peña-Rodriguez⁵, Raquel Gonzalez⁵, Armando Rastelli⁶, Sven Höfling³, Christian Schneider³, Pablo Alonso-Gonzalez^1,2, Rinaldo Trotta³, Javier Martin-Sanchez^1,2

¹Department of Physics, University of Oviedo, Oviedo, Spain
²Center of Research on Nanomaterials and Nanotechnology, CINN (CSIC−Universidad de Oviedo), El Entrego 33940, Spain
³Department of Physics, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Rome, Italy
⁴Technische Physik and Wilhelm Conrad Röntgen Research Center for Complex Material Systems, Physikalisches Institut, Universitat Würzburg, Am Hubland, D-97074 Würzburg, Germany
⁵Instituto de Fusión Nuclear, Universidad Politécnica de Madrid, José Gutiérrez Abascal 2, E-28006 Madrid, Spain
⁶Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Altenbergerstraße 69, 4040 Linz, Austria

The future development of ultra-compact two-dimensional (2D) photonic technologies for quantum information processing relies on our ability to fabricate deterministically single photon sources (SPSs) in 2D materials with tailored optical properties. A promising fabrication strategy is based on the irradiation of the 2D crystals with ion beams to induce defect-related SPSs ideally exhibiting reproducible emission energy [1]. On the other hand, a deliberate control of the emission energy in a reversible manner by integrating the as fabricated SPSs onto piezoelectric actuators is highly desirable as it opens exciting new possibilities for energy-tunable quantum photonic technologies based on SPSs [2-5]. 

In this work, we tackle the above mentioned aspects by presenting: i) preliminary results about the fabrication of light emitters with reproducible emission energy in WSe2 thick crystals by ion irradiation with H+ ions; ii) strain-tunable emission energy of SPSs in WSe2by employing hybrid piezoelectric-2D-materials devices that allow the introduction of reversible compressive/tensile in-plane strain fields [6].

Adarsh S. Prasad¹, J. Hinney¹, K. Hammerer², S. Mahmoodian², S. Rind¹, P. Schneeweiss^1,3, A. Sørensen⁴, J. Volz^1,3, A. Rauschenbeutel^1,3

¹TU Wien, Atominstitut, Vienna, Austria
²Institute for Theoretical Physics, Institute for Gravitational Physics (Albert Einstein Institute), Leibniz University Hannover, Germany
³Institute for Theoretical Physics, Institute for Gravitational Physics (Albert Einstein Institute), Leibniz University Hannover, Germany
⁴Center for Hybrid Quantum Networks (Hy-Q), Niels Bohr Institute, University of Copenhagen, Denmark

Single quantum emitters are well-known sources for single photons. Obtaining a large photon rate in a given optical mode requires a high emitter coupling efficiency which can be realized, e.g., by using a photonic crystal or an optical resonator. However, these structures also introduce optical losses which reduce the single-photon rate and, more fundamentally, decohere multi-photon states. Here, we demonstrate a novel mechanism for generating single photons and correlated photon pair states. Surprisingly, this mechanism allows one to obtain large single-photon rates also for low emitter–waveguide coupling efficiencies. Moreover, in stark contrast to other schemes, it takes advantage of dissipation, i.e., optical losses. The generated photon states are Fourier-limited and very well-suited for efficient interfacing with optical quantum emitters. In  particular, the generated states are much more narrow than in typical down-conversion schemes. Our novel mechanism can be implemented with quantum emitters whose resonance frequencies can be in any part of the electromagnetic spectrum, potentially allowing to generate nonclassical light covering frequencies from the microwave to the X-ray regime.
 

Lucas Rickert, Johannes Schall, Jan Große, Timm Kupko, Sven Rodt, Stephan Reitzenstein, Tobias Heindel

Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany

Very recently, hybrid circular Bragg gratings (h-CBGs) with embedded semiconductor quantum dots (QDs) attracted much interest as nearly optimal quantum light sources [1,2]. However, all efforts on this type of structure were so far limited to wavelengths < 900 nm, lacking fiber-compatibility crucial for applications in long-distance quantum communication. Here, we present optimized designs for hybrid CBG devices operating at telecom O-band wavelengths using finite-element simulations [3]. The designs show Purcell factors up to 30 and photon extraction efficiencies exceeding 95%. We discuss how the optical properties are affected by the variation of structural parameters and investigate the designs’ performance if fabrication-related structural imperfections are introduced, including imperfect side-wall etching and non-ideal positioning of the emitter inside the device. For the latter, the devices are robust against emitter displacements well within reported deterministic fabrication uncertainties for structures with embedded QDs [4]. Furthermore, we present simulations showing the CBG devices’ compatibility to optical single mode fibers with coupling efficiencies of up to 80% with off-the-shelf fibers. Additionally, we report on our recent progress in achieving even higher fiber coupling efficiencies close to unity using specialty fibers and also address C-band compatible CBG designs.

Andreas W. Schell

Leibniz University Hannover, Appelstr. 2, Hannover
Pysikalisch Technische Bunesanstalt, Bundesallee 100, Braunschweig

Bringing quantum technology from the laboratory to real world applications is a complex, but very rewarding, task. It will enable society to exploit the new opportunities the laws of quantum mechanics offer compared to purely classical physics. However, before the new quantum technology can be deployed, platforms to implement such a technology need to be discovered and developed. Here, we will show our ongoing efforts to implement such a platform using the so called hybrid approach for the assembly of quantum photonic elements. This approach is highly flexible and can be adapted to many different material systems and structures. In particular, we will introduce techniques based on scanning probe microscopy and three-dimensional laser writing. The hybrid quantum photonic elements assembled with these approached include emitter coupled to on-chip resonators and waveguides, different kinds of fiber integrated cavities and incorporate a variety of emitter such as NV centers, quantum dots, and defects in two-dimensional materials, such as hexagonal boron nitride. From these examples it can be seen that photonics elements assembled using hybrid techniques might help to facilitate the transition of quantum photonic networks out of lab to real-world applications.

Marco Schmidt, Martin v. Helversen, Sarah Fischbach, Arsenty Kaganskiy, Ronny Schmidt, Andrei Schliwa, Tobias Heindel, Sven Rodt, Stephan Reitzenstein

Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany

Spectrally tunable quantum light sources are key building blocks of future quantum information networks. Semiconductor-based quantum dots (QDs) are among the most promising candidates to act as single photon emitters in such devices. This is explained by their excellent quantum nature of emission in terms of single-photon purity, indistinguishability and polarization entanglement and by their compatibility with advanced device concepts. We demonstrate a strain-tunable single-photon source based on a deterministically fabricated QD microlens which is attached to a piezo-actuator via a gold thermocompression bonding [1]. The combination of deterministic fabrication, spectral-tunability and high photon-extraction efficiency makes the QD-microlens single photon source an interesting building block for the realization of quantum communication networks. The functionality of the strain tunable system and spectroscopic investigations including PID controlled spectral stabilization will be presented.

Karolina Sedziak-Kacprowicz, Artur Czerwinski, Piotr Kolenderski

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

We present the framework for the multilevel quantum states encoding based on temporal modes of photon propagating in a dispersive medium. The unitary evolution which the photon experiences during a propagation in a fiber can be seen ad an evolution of qudit state. The time-resolved single photon detection provides measurements, which is an analogy to spatial encoding in transverse momentum of a photon [1]. We estimate the fidelity of quantum state reconstruction for qubits and qutrits in realistic experimental settings, with two methods: least squares and maximum likelihood. The model we consider includes the effect of detector jitter and shot noise [2].

Preeti Sharma, Bhaskar Kanseri

Experimental Quantum Interferometry and Polarization (EQUIP), Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India

Spontaneous Parametric Down-Conversion (SPDC) is a second-order non-linear quantum process in which a pump photon annihilates to produce two photons of less energy, namely signal and idler. The study of profiles of the biphotons provides information of their correlations which in turn is significant to quantify entanglement. It is reported that the spatial coherence of pump is directly transferred to biphotons [1]. The partially spatially coherent fields are less affected by atmospheric turbulence on propagation and produce less speckles [2]. The biphotons generated from partially coherent pump are thus potential candidates for free-space optical communications and imaging purposes.

We study the spatial asymmetry in SPDC ring and variation in spectral width of biphotons generated from partially coherent pump beam using non-linear crystal in non-collinear SPDC process [3].  These changes in profiles have been observed due to spatial walk-off effect of extraordinary pump. The polarization entanglement in biphotons generated in this process has also been observed with variation in spatial coherence of pump. The dependence of spatial correlations on pump coherence properties results in degradation of polarization entanglement in biphotons with decrease in size and coherence of pump. This study is expected to have significant advantage in quantum communication applications.

Cebastien Joel GUEMBOU SHOUOP

Atomic and Nuclear Spectroscopy, Archeometry, University of Liège, Bat. B15 Sart Tilman, 4000 Liege 1, Belgium
Fundamental Physics Laboratory, Mathematics, Applied Computer Sciences and Fundamental Physics, University of Douala, P.O. Box: 24157 Douala, Cameroon

Quantum information provides the hope of being able to perform communication and computation tasks that cannot be achieved by conventional IT. Using photon as the most prominent candidate for quantum information processing, different scientists and advanced laboratories have been developing single-photon sources and photon detectors for quantum communication. The present work reviews the prospects developed by Michael Varnava et al. (2008), N. Somaschi et al. (2016), Alán Aspuru-Guzik and Philip Walther (2012), and M. D. Eisaman et al. (2011)1–3. While reviewing these four studies, the question about the implementation of quantum information in developing countries and the prospect of such technology is discussed.4

Ivo Straka, Jan Grygar, Josef Hloušek, Miroslav Ježek

Department of Optics, Faculty of Science, Palacký University, 17. listopadu, 771 46 Olomouc, Czechia

We developed a statistical counting model of actively quenched SPADs under constant illumination [1]. The model covers recovery time, afterpulses with arbitrary statistics, twilight pulses, and their interaction. The results allow calculating mean detection rate as well as the probability distribution of the number of counts in a specific time window.

Each result is evaluated with various degrees of approximation. Consequently, the proposed methods range from Monte-Carlo simulations to explicit analytical formulas. The approximations are discussed, the methods compared among each other and set against experimental data. We measured 3 SPADs long enough to uncover the systematic errors in counting distributions that range from 10^-5 to 10^-2 in total variation distance. This makes the proposed model the most accurate counting model so far, but some surprising phenomena have been discovered as well - chiefly, recovery time seems to change with rate.

Our results improve the accuracy of everyday counting rate estimation, as well as other applications that rely on predictable detector response - transmission measurements and single-photon imaging. Counting statistics also offers a new way of characterizing non-Markovian phenomena, as it is affected by cumulative effects that cannot be fully distinguished in a start-stop histogram.

Cem Güney Torun¹, Tim Schröder^1,2

¹Department of Physics, Humboldt-Universität zu Berlin, Germany
²Ferdinand-Braun-Institut, Berlin, Germany

Single, optically active quantum systems, such as single photon sources and quantum memories are important building blocks for quantum technologies. Achieving sensitive quantum sensors or high-rate quantum communication devices requires efficient optical coupling to such quantum systems. In this work, a device that is simple in design and easy to fabricate but still enables photon extraction efficiencies as high as more complex designs is proposed. While previous work generally take the fraction of light that is collected by the first optical element (e.g. an objective lens) as the metric of efficiency, it is important to consider the total emission coupled to a fibre for integrated quantum applications. In particular, the coupling efficiency of a dipole emitter inside an inverted diamond nanocone to a single mode optical fibre is numerically optimized with this target figure of merit. Diamond nanocone structures can be fabricated by angled etching techniques [1]. JCMsuite, a finite element based software, was utilized to simulate far-field intensity distributions, fibre coupling, and optimization. From these simulations, nanostructure parameter sets were determined such that a dipole to fibre mode coupling efficiency of up to 65% or 87% collection efficiency to an objective lens can be achieved.

Rita Veilande¹, Imants Bersons¹, Ojars Balcers²

¹Institute of Atomic Physics and Spectroscopy, University of Latvia, 19, Raina Blvd. LV-1586, Latvia
²Vidzeme University of Applied Sciences, Valmiera, Latvia

A new nonlinear equation [1] for the description of the localised in space, 3D electromagnetic solitons, simpler than previously considered, is proposed. The obtained 3D solitons were proposed [2,3] as a model of the photons. The equation was constructed [1-3] based on the Maxwell equations for vector potential. The essential proposal was that a vacuum is a medium where light induces the polarisation and magnetisation components along the direction of the light propagation. The new 3D model has two advantages: firstly, instead of the integro-differential equations of the previously proposed models [2,3], the new equation [1] is a differential equation, which can be reduced to the traditional nonlinear Schrödinger equation; secondly, every point of the one- or N-soliton solution oscillates with the same frequency. We found the one and the degenerate two-soliton solution; both are a 3D compact, axially symmetric objects with the value of the field decreasing in all spatial directions. The interference–like phenomena of two overlapping solitons is seen.

The field of the solitons looks physically suitable, but the applicability of this model for the photons remains an open question.

Max Tillmann¹, Michael Wahl¹, Tino Röhlicke¹, Andreas Bülter¹, Doreen Wernicke², Martin Wolff³, Matthias Häußler³, Nicolai Walter³, Robin Stegmüller³, Fabian Beutel³, Wolfram Pernice³, Carsten Schuck³, Nicolas Perlot⁴

¹PicoQuant GmbH, Rudower Chaussee 29, 12489 Berlin, info@picoquant.com
²Entropy GmbH, Gmunder Str. 37a, 81379 München
³WWU Münster, Physikalisches Institut, CeNTech II, Heisenbergstr. 11, 48149 Münster
⁴Fraunhofer Heinrich-Hertz Institute, Einsteinufer 37, 10587 Berlin

Single photons are ideal information carriers in many classical and quantum applications. One of the key challenges in transferring these applications from a laboratory environment ‘into the field’ are the limited count rates achievable with today's hardware. Individual photon detection units – comprised of a single detector element and and its timing electronic – run against a physical limit if photons impinge too rapidly.

It is the scope of this project to develop key enabling components to push beyond the bandwidth-limit of single devices with a massively parallelized (x64) single-photon detection system. To this end, detector elements based on superconducting nanowires are optimized for lowest reset times and highest temporal resolution. Care is taken that the optical and electrical interfacing does not degrade the detection performance. To process the high data rates generated with such a system, the readout electronics need to be parallelized in a similar fashion. In particular, an on-chip (FPGA) data processing over all detector channels provides a viable solution to pre-process the initial data such that it can be transferred to a host computer via limited bandwidth links. Ultimately, the performance of this detection system will be evaluated in an applied scenario via a suitable QKD-scheme.

Liang Zhai¹, Matthias C. Löbl¹, Giang N. Nguyen^1,2, Julian Ritzmann², Alisa Javadi¹, Clemens Spinnler¹, Andreas D. Wieck², Arne Ludwig², Richard J. Warburton¹

¹Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
²Lehrstuhl für Angewandte Festkörperphysik, Ruhr-Universität Bochum, DE-44780 Bochum, Germany

A hybrid system consisting of a quantum dot (QD) based single-photon source and a rubidium-memory is a promising platform for implementing quantum networks [1]. For this purpose, GaAs QDs in AlGaAs are interesting since they can emit single photons frequency-matched to a rubidium-based quantum memory [2]. However, GaAs QDs suffered so far from strong telegraph noise due to an unstable charge environment [2]. Here, we present ultra-low noise and charge-tunable GaAs QDs [3]. We place the QDs inside a p-i-n diode enabling deterministic tuning of the QDs' charge-state and the emission frequency by applying an electric field to the diode. We achieve linewidths that are comparably close to the lifetime-limit than for the best InGaAs QDs [4]. For a typical QD, the resonance fluorescence has a linewidth of 640 MHz, just slightly above the lifetime-limit (590 MHz). The linewidth is measured by scanning over the QD resonance on minute time-scales. This result shows that the QDs are extremely stable. In resonance fluorescence, they do not show telegraph-noise (blinking) which would appear as bunching in a g2-measurement. On these QDs, we additionally demonstrate high fidelity optical initialization of an electron- as well as a hole-spin. We find that the spins have a long lifetime. Together with the excellent photonic properties, the new QDs form a promising platform for a spin-photon interface.

Kai Zou^1,2, Yun Meng^1,2, Nan Hu^1,2, Xiaojian Lan^1,2, Liang Xu^1,2, Zhao Wang^1,2, Xuhui Cao^1,2, Julien Zichi³, Stephan Steinhauer³, Val Zwiller³, Xiaolong Hu^1,2

¹School of Precision Instrument and Optoelectronic Engineering, Tianjin University, Tianjin 300072, China
²Key Laboratory of Optoelectronic Information Science and Technology, Ministry of Education, Tianjin 300072, China
³Department of Applied Physics, Royal Institute of Technology (KTH), SE-106 91 Stockholm, Sweden

Superconducting nanowire single-photon detectors^[1] (SNSPDs) have shown unprecedented performance including near unit system detection efficiency, low dark-count rate, fast operating speed, and low timing jitter. However, the meander-type designs of the SNSPDs make the detection efficiency polarization-dependent. How to achieve high detection efficiency for incident photons in all polarization states while preserving other merits of the detectors still remains a challenge. Here, we report on our design and demonstration of a NbTiN superconducting avalanche photodetector (SNAP) in fractal pattern^[2] simultaneously with 60±3% system detection efficiency at 1550 nm, 1.05 polarization sensitivity, 220 dark counts per second, 4-ns recovery time, and 45-ps timing jitter^[3]. 

In the family of SNSPDs based on polarization-insensitive designs, our current demonstration has shown the highest SDE among polycrystalline SNSPDs^[4-8] and low residual polarization sensitivity comparable with others’ work^[4-8]. The amorphous polarization-insensitive SNSPDs achieved even higher SDE [9]; however, their timing performances are not as good as the timing performances of polycrystalline SNSPDs. The concept of fractal design presented in this work can also be applied to amorphous materials including WSi and MoSi. We anticipate that the fractal SNAPs will find applications where both polarization-insensitive SDE and high timing resolution are crucial.