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

We review the current status of single-photon-source and single-photon-detector technologies operating at wavelengths from the ultraviolet to the infrared. We discuss applications of these technologies to quantum communication, a field currently driving much of the development of single-photon sources and detectors.

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Invited review article: Single-photon sources and detectors

Microwave photonics with superconducting quantum circuits

We report the first quantum key distribution (QKD) experiment to enable the creation of secure keys over 42 dB channel loss and 200 km of optical fibre. We used the differential phase shift QKD (DPS-QKD) protocol, implemented with a 10-GHz clock frequency and superconducting single-photon detectors (SSPD) based on NbN nanowires. The SSPD offers a very low dark count rate (a few Hz) and small timing jitter (60 ps, full width at half maximum, FWHM). These characteristics allowed us to achieve a 12.1 bit s–1 secure key rate over 200 km of fibre, which is the longest terrestrial QKD over a fibre link yet demonstrated. Moreover, this is the first 10-GHz clock QKD system to enable secure key generation. The keys generated in our experiment are secure against both general collective attacks on individual photons and a specific collective attack on multiphotons, known as a sequential unambiguous state discrimination (USD) attack.

Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors

In the past 20 years, impressive progress has been made both experimentally and theoretically in superconducting quantum circuits, which provide a platform for manipulating microwave photons. This emerging field of superconducting quantum microwave circuits has been driven by many new interesting phenomena in microwave photonics and quantum information processing. For instance, the interaction between superconducting quantum circuits and single microwave photons can reach the regimes of strong, ultra-strong, and even deep-strong coupling. Many higher-order effects, unusual and less familiar in traditional cavity quantum electrodynamics with natural atoms, have been experimentally observed, e.g., giant Kerr effects, multi-photon processes, and single-atom induced bistability of microwave photons. These developments may lead to improved understanding of the counterintuitive properties of quantum mechanics, and speed up applications ranging from microwave photonics to superconducting quantum information processing. In this article, we review experimental and theoretical progress in microwave photonics with superconducting quantum circuits. We hope that this global review can provide a useful roadmap for this rapidly developing field.

Quantum key distribution with pulsed heralded single photon source was performed over 40 km of fiber for the first time to our knowledge. QBER was measured to be 4.23% suggesting security against unconditional attack.

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Quantum key distribution at 1550 nm using a pulse heralded single photon source.

Heralding of single photon at 1550 nm from pump pulsed nondegenerate spontaneous parametric downconversion is demonstrated. P(1) and P(2) of the source are 0.1871 and 2.4 × 10−3 respectively, with source triggering at 2.16 × 105 triggers per second. This source may be used in quantum key distribution systems.

http://cds.cern.ch/record/998476/files/0611112.pdf

Heralded single photon source at 1550 nm from pulsed parametric down conversion

In this paper, for the decoy state method using a finite number of decoy light intensities, we present improved upper and lower bounds for the asymptotic yield y{sub n} for n-photon states. In particular if all the light intensities are less than or equal to 1, they are not only a lower or upper bound, but in fact are the exact minimum or maximum.

Exact minimum and maximum of yield with a finite number of decoy light intensities

We have conducted a single-photon interference experiment over an 80-km optical fiber using a pulse-driven heralded single-photon source (HSPS). To the best of our knowledge, this is, thus far, the longest distance over which a single-photon interference experiment has been conducted using HSPSs (continuous-wave-pumped or pulse-driven). The effect of the 80-km transmission on the dispersion and fluctuation of polarization are more severe than those in our previous 40-km quantum key distribution (QKD) experiment. We have overcome the difficulties by some fine tunings and low-jitter controlling. By conducting ten consecutive transmission experiments over a total time of 30 min, an average quantum bit-error rate (QBER) of 7.9 plusmn 1.2% has been obtained. This QBER is lower than the threshold QBER of 10.55% which is considered as a limit for unconditional security for the QKD under negligible multiphoton emission.

Single-Photon Interference Experiment Over 80 km With a Pulse-Driven Heralded Single-Photon Source

The paper proposes on the use of non degenerate parametric downconversion PDC in BBO crystal to realize a heralded single photon source. Downconverted photons wavelength can be chosen using phase matching condition.

Alexandre Soujaeff

Papers

Quantum key distribution at 1550 nm using a pulse heralded single photon source.

Heralded single photon source at 1550 nm from pulsed parametric down conversion

Exact minimum and maximum of yield with a finite number of decoy light intensities

Single-Photon Interference Experiment Over 80 km With a Pulse-Driven Heralded Single-Photon Source

Heralded single photon source for quantum cryptography at 1550 nm