Realization Of Quantum Key Distribution Research Articles

Experimental multiplexing of quantum key distribution with classical optical communication

We demonstrate the realization of quantum key distribution (QKD) when combined with classical optical communication, and synchronous signals within a single optical fiber. In the experiment, the classical communication sources use Fabry-Pérot (FP) lasers, which are implemented extensively in optical access networks. To perform QKD, multistage band-stop filtering techniques are developed, and a wavelength-division multiplexing scheme is designed for the multi-longitudinal-mode FP lasers. We have managed to maintain sufficient isolation among the quantum channel, the synchronous channel and the classical channels to guarantee good QKD performance. Finally, the quantum bit error rate remains below a level of 2% across the entire practical application range. The proposed multiplexing scheme can ensure low classical light loss, and enables QKD over fiber lengths of up to 45 km simultaneously when the fibers are populated with bidirectional FP laser communications. Our demonstration paves the way for application of QKD to current optical access networks, where FP lasers are widely used by the end users.

Comment on “Quantum Key Distribution and Quantum Authentication Based on Entangled State”

Shi et al. (Phys. Lett. A 281:83–87, 2001) proposed a scheme which allows simultaneous realization of quantum key distribution and quantum authentication. However, this study points out a weakness in Shi et al.’s scheme, in which a malicious user can impersonate a legitimate participant without being detected. Furthermore, an improved scheme is proposed to avoid this weakness.

Passive sources for the Bennett-Brassard 1984 quantum-key-distribution protocol with practical signals

Most experimental realizations of quantum key distribution are based on the Bennett-Brassard 1984 (so-called BB84) protocol. In a typical optical implementation of this scheme, the sender uses an active source to produce the required BB84 signal states. While active state preparation of BB84 signals is a simple and elegant solution in principle, in practice passive state preparation might be desirable in some scenarios, for instance, in those experimental setups operating at high transmission rates. Passive schemes might also be more robust against side-channel attacks than active sources. Typical passive devices involve parametric down-conversion. In this paper, we show that both coherent light and practical single photon sources are also suitable for passive generation of BB84 signal states. Our method does not require any external-driven element, but only linear optical components and photodetectors. In the case of coherent light, the resulting key rate is similar to the one delivered by an active source. When the sender uses practical single photon sources, however, the distance covered by a passive transmitter might be longer than the one of an active configuration.

Integrated-optical realizations of quantum key distribution over maximally unbiased bases

Quantum key distribution (QKD) protocols based on higher order maximally unbiased bases (MUBs), potentially displaying improved range-security performance, are optically realized for higher (>2) dimensions and/or for multiple (>2) transmission and detection bases, inspired by analogies with the differential phase shift keying and pulse position modulation (PPM) formats of classical optical communication. In particular, we introduce optical realizations for: 1) a six-state protocol in qubit space, employing an extra phase modulated interferometer in the transmitter, detecting the photon in the PPM basis of three consecutive time slots and 2) a 16-state MUB protocol in four dimensions, based on differential phase encoding over four adjacent time slots. The transmitter (Alice) and receiver (Bob) for this system are realized using generalized Mach-Zehnder interferometers with four delay arms, and three phase modulators, further requiring a quantum Hadamard-4 gate, realized using four cross-connected directional couplers, to be implemented as an integrated-optical circuit. The conception and analysis of these optical realizations of abstract higher order QKD MUB-based protocols is inspired by a novel insight into the structure of the MUB set constructed by modulating a generator matrix (typically a Hadamard matrix) with a set of phasemasks. The operating characteristics of these generalized protocols are derived in terms of three simple figures of merit, the Eve detect (or Disturbance) rate, the Eve information rate, and the key creation rate.