Quantum cryptography could well be the first application of quantum mechanics at the individual quanta level. The very fast progress in both theory and experiments over the recent years are reviewed, with emphasis on open questions and technological issues.

Quantum Cryptography

In information processing, as in physics, our classical world view provides an incomplete approximation to an underlying quantum reality. Quantum effects like interference and entanglement play no direct role in conventional information processing, but they can--in principle now, but probably eventually in practice--be harnessed to break codes, create unbreakable codes, and speed up otherwise intractable computations.

Quantum information and computation

Quantum key distribution (QKD) is the first quantum information task to reach the level of mature technology, already fit for commercialization. It aims at the creation of a secret key between authorized partners connected by a quantum channel and a classical authenticated channel. The security of the key can in principle be guaranteed without putting any restriction on an eavesdropper's power. This article provides a concise up-to-date review of QKD, biased toward the practical side. Essential theoretical tools that have been developed to assess the security of the main experimental platforms are presented (discrete-variable, continuous-variable, and distributed-phase-reference protocols).

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The security of practical quantum key distribution

Quantum key distribution is widely thought to offer unconditional security in communication between two users. Unfortunately, a widely accepted proof of its security in the presence of source, device, and channel noises has been missing. This long-standing problem is solved here by showing that, given fault-tolerant quantum computers, quantum key distribution over an arbitrarily long distance of a realistic noisy channel can be made unconditionally secure. The proof is reduced from a noisy quantum scheme to a noiseless quantum scheme and then from a noiseless quantum scheme to a noiseless classical scheme, which can then be tackled by classical probability theory.

https://science.sciencemag.org/content/sci/283/5410/2050.full.pdf

Unconditional Security Of Quantum Key Distribution Over Arbitrarily Long Distances

Quantum communication, and indeed quantum information in general, has changed the way we think about quantum physics In 1984 and 1991, the first protocol for quantum cryptography and the first application of quantum non-locality, respectively, attracted a diverse field of researchers in theoretical and experimental physics, mathematics and computer science Since then we have seen a fundamental shift in how we understand information when it is encoded in quantum systems We review the current state of research and future directions in this new field of science with special emphasis on quantum key distribution and quantum networks

https://www.arxiv.org/pdf/quant-ph/0703255

Quantum Communication

We present a time-multiplexed interferometer based on Faraday mirrors, and apply it to quantum key distribution The interfering pulses follow exactly the same spatial path, ensuring very high stability and self balancing The use of Faraday mirrors compensates automatically any birefringence effects and polarization dependent losses in the transmitting fiber First experimental results show a fringe visibility of 09984 for a 23-km-long interferometer, based on installed telecom fibers

“Plug and play” systems for quantum cryptography

System and method of communicating a key between two stations (1, 2) using an interferometric system for quantum cryptography. The method comprises the step of sending at least two light pulses over a quantum channel (3) and detecting the interference created by said pulses in one station (Bob). The interfering pulses run over the same arms of said interferometer, but in another sequence, so that they are delayed when they run over said quantum channel. The pulses are reflected by at least Faraday mirrors (14, 16, 22) at the ends of said quantum channel, so as to cancel polarization effects. Advantages: the system does not need alignment or balancing between the arms of the interferometers. Plug and play stations (1, 2).

Quantum cryptography device and method

Quantum cryptography over 23 km of installed telecommunications fibre using a novel interferometer with Faraday mirrors is presented. The interferometer needs no alignment nor polarisation control and features 99.8% fringe visibility. A secret key of 20 kbit length with an error rate of 1.35% for 0.1 photon per pulse was produced.

Interferometry with Faraday mirrors for quantum cryptography

Quantum Cryptography over 23km of installed Telecom fiber using a novel interferometer with Faraday mirrors is presented. The interferometer needs no alignment nor polarization control and features 99.8% fringe visibility. A secret key of 20kbit length with a error rate of 1.35% for 0.1 photon per pulse was produced.

/pdf/underwater-quantum-coding-2e9gg31gxa.pdf

Antoine Muller

Papers

“Plug and play” systems for quantum cryptography

Quantum cryptography device and method

Interferometry with Faraday mirrors for quantum cryptography

Interferometry with Faraday mirrors for quantum cryptography

Underwater quantum coding