Quantum Communication

In a typical QKD set-up, the photons are generated by a single photon source, encoded into binary values (i.e., representing "0" and "1") and then transmitted to the receiver either via optical fibers or in free space. The receiver then decodes the state of photons and detects them using single photon sensitive detectors and time-tagging electronics. There are several methods for encoding and decoding the photons:

• via polarization: the binary information "1" or "0" is defined by the polarization of the single photons, e.g., binary "0" correlates with the horizontally polarized photon and binary "1" with vertically polarized photon
• via the phase, which requires the use of a interferometer system: the phase difference Δφ = φAlice - φBob of the two interferometers is then used for encoding the binary values, e.g., a phase difference Δφ=0 correlates with the binary "0" and the phase difference Δφ=π correlates with the binary "1"
• via entangled photons, which requires one sender of entangled photon pairs and two receivers (Alice and Bob) each equipped with a polarizer. Alice and Bob set the two angles at their respective polarization rotator randomly. If the angles of Alice and Bob match, both photons behave exactly the same at the beam splitter, i.e., they are either transmitted (binary "1") or reflected (binary "0").

PicoQuant offers several instruments such as time-tagging units and single photon sensitive detectors that can be used to build a quantum key distribution system:

Time-tagging Units

MultiHarp 150

High-Throughput Multichannel Event Timer & TCSPC Unit

• 4, 8, or 16 independent input channels and common sync channel (up to 1.2 GHz)
• High sustained data throughput (80 Mcps in time tagging mode, 180 Mcps in histogramming mode)
• Record-breaking dead time (650 ps) per channel
• No dead time across channels

HydraHarp 400

Multichannel Picosecond Event Timer & TCSPC Module

• Up to 8 independent input channels and common synch channel (up to 150 MHz)
• Time channel width of 1 ps
• Time tagging with sustained count rates up to 40 Mcps
• USB 3.0 connection

PicoHarp 300

Stand-alone Dual-channel Picosecond Event Timer & TCSPC Module with USB Interface

• Two identical synchronized but independent input channels
• Time channel width of 4 ps
• Time tagging with sustained count rates up to 5 Mcps
• USB 2.0 connection

TimeHarp 260

TCSPC and MCS board with PCIe interface

• One or two independent input channels and common synch channel (up to 84 MHz)
• Two models with either 25 ps (PICO model) or 1 ns (NANO model) base resolution
• Ultra short dead time (< 25 ns for PICO model, < 1 ns for NANO model)
• PCIe interface

Single Photon Detectors

PDM Series

Single Photon Avalanche Diodes

• Timing resolution down to < 50 ps (FWHM)
• Detection efficiency up to 49%
• Different active areas: 20, 50, and 100 µm
• Ultra stable at high count rates

Software

QuCoa

Quantum Correlation Analysis Software

• Antibunching (g⁽²⁾) measurements including fitting to several models
• Coincidence counting / event filtering, using AND, OR, NOT operators
• Preview of antibunching curve and correlation data during measurement
• Remote control via TCP/IP Interface

The following presentations are related to quantum communication and were recorded at our International Symposium on "Single Photon based Quantum Technologies" in September 2020.