Alexei Trifonov

Single photon counting at telecom wavelength and quantum key distribution

Abstract A brief overview is given of single photon detector performance requirements for quantum cryptography applications. The analysis is made with respect to restrictions necessary to secure the quantum key distribution channel. InGaAs/InP avalanche photodiode performance is analysed for single photon counting at 1550 nm. Quantum efficiency, dark current and afterpulsing probability (for times up to 100μs after an initial avalanche) are studied in a wider temperature range than previously reported (0deg; C to –80deg;C). We show that photon counting is a bottle-neck in current quantum key distribution systems and provides the source for future performance improvement.

Backscattering limitation for fiber-optic quantum key distribution systems

We characterized backscattering effects in optical fiber using a photon counting technique and considered its implications for quantum key distribution (QKD). We found that Rayleigh (elastic) backscattering can put strong limitations on a two-way QKD system’s performance. Raman (inelastic) scattering can restrict the ability of wavelength multiplexing of a quantum channel with strong classical data channel(s).

Practical single photon source for quantum communications

We describe a robust heralded single photon source based on parametric down conversion of CW 532-nm light in a periodically polled KTP waveguide. Low required pump power (sub-mW), reasonable operational temperature (43oC), high heralding efficiency (60%), and narrow spectral width of the heralded photons (sub-nm) make it an ideal light source for long-distance quantum communications.

Improving the performance of quantum key distribution apparatus

Results of active phase tracking system deployment for quantum communication are presented. The system allows high-visibility single photon interferometry over 100 km of standard optical fibre. Single photon detector optimization is performed and the estimates for long-distance quantum key distribution with heralded single photon source are presented.

Practical quantum cryptography

We report our recent results in development of the secure fiber-optics communication system based upon quantum key distribution (QKD). Emphasize is made on the limitation imposed by the state-of-the-art components crucial for the system performance. We discuss the problem of the interferometer design and highlight the possible security loopholes known. Together with single photon counting performance it places the main restriction on the distance range and the secure key rate of the QKD system based upon the weak coherent pulses. Finally we describe the result of the first test of the system using single photons produced by non-degenerate parametric down-conversion as a source.

Fiber‐Optics Quantum Cryptography with Single Photons

We report our results for quantum key distribution over 76 km of commercial optical fiber using one‐way interferometer and heralded photon source based on parametric down conversion.

Fast quantum key distribution with decoy number states

We investigate the use of photon number states to identify eavesdropping attacks on quantum key distribution (QKD) schemes. The technique is based on the fact that different photon numbers traverse a channel with different transmittivity. We then describe two QKD schemes that utilize this method, one of which overcomes the upper limit on the key generation rate imposed by the dead time of detectors when using a heralded source of photons.

Faraday Effect in Long Telecom Fibers with Randomly Varying Birefringence

We measure relative propagation delay between two orthogonally polarized pulses in long fibers in the presence of a weak axial magnetic field (~50μT). We find that the fiber birefringence modifies the Faraday polarization eigenstates, which became noncircular and wander in time.

Efficiency of entangled photon pair generation in bulk crystals and waveguides - : a comparison

The efficiency of spontaneous parametric down conversion is calculated and measured for several nonlinear crystals and waveguides in single spatial mode regime. Efficiency of waveguide sources is found to be far superior compared to the bulk crystal sources.