Quantum Cryptography over Underground Optical Fibers

Field and long-term demonstration of a wide area quantum key distribution network

A wide area quantum key distribution (QKD) network deployed on communication infrastructures provided by China Mobile Ltd. is demonstrated. Three cities and two metropolitan area QKD networks were linked up to form the Hefei-Chaohu-Wuhu wide area QKD network with over 150 kilometers coverage area, in which Hefei metropolitan area QKD network was a typical full-mesh core network to offer all-to-all interconnections, and Wuhu metropolitan area QKD network was a representative quantum access network with point-to-multipoint configuration. The whole wide area QKD network ran for more than 5000 hours, from 21 December 2011 to 19 July 2012, and part of the network stopped until last December. To adapt to the complex and volatile field environment, the Faraday-Michelson QKD system with several stability measures was adopted when we designed QKD devices. Through standardized design of QKD devices, resolution of symmetry problem of QKD devices, and seamless switching in dynamic QKD network, we realized the effective integration between point-to-point QKD techniques and networking schemes.

Daylight quantum key distribution over 1.6 km

Quantum key distribution (QKD) has been demonstrated over a point-to-point 1.6-km atmospheric optical path in full daylight. This record transmission distance brings QKD a step closer to surface-to-satellite and other long-distance applications.

Quantum cryptography with entangled photons

By realizing a quantum cryptography system based on polarization entangled photon pairs we establish highly secure keys, because a single photon source is approximated and the inherent randomness of quantum measurements is exploited. We implement a novel key distribution scheme using Wigner's inequality to test the security of the quantum channel, and, alternatively, realize a variant of the BB84 protocol. Our system has two completely independent users separated by 360 m, and generates raw keys at rates of 400-800 bits/s with bit error rates around 3%.

Unconditional security of quantum key distribution over arbitrarily long distances

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.

Performance of InGaAs/InP Avalanche Photodiodes as Gated-Mode Photon Counters

We investigate the performance of separate absorption multiplication InGaAs/InP avalanche photodiodes as single-photon detectors for 1.3- and 1.55-mum wavelengths. First we study afterpulses and choose experimental conditions to limit this effect. Then we compare the InGaAs/InP detector with a germanium avalanche photodiode; the former shows a lower dark-count rate. The effect of operating temperature is studied for both wavelengths. At 173 K and with a dark-count probability per gate of 10(-4), detection efficiencies of 16% for 1.3 mum and 7% for 1.55 mum are obtained. Finally, a timing resolution of less than200 ps is demonstrated.

25 years of quantum cryptography

The fates of SIGACT News and Quantum Cryptography are inseparably entangled. The exact date of Stephen Wiesner's invention of "conjugate coding" is unknown but it cannot be far from April 1969, when the premier issue of SIGACT News ---or rather SICACT News as it was known at the time---came out. Much later, it was in SIGACT News that Wiesner's paper finally appeared [74] in the wake of the first author's early collaboration with Charles H. Bennett [7]. It was also in SIGACT News that the original experimental demonstration for quantum key distribution was announced for the first time [6] and that a thorough bibliography was published [19]. Finally, it was in SIGACT News that Doug Wiedemann chose to publish his discovery when he reinvented quantum key distribution in 1987, unaware of all previous work but Wiesner's [73, 5].Most of the first decade of the history of quantum cryptography consisted of this lone unpublished paper by Wiesner. Fortunately, Bennett was among the few initiates who knew of Wiesner's ideas directly from the horse's mouth. His meeting with the first author of this column in 1979 was the beginning of a most fruitful lifelong collaboration. It took us five more years to invent quantum key distribution [4], which is still today the best-known application of quantum mechanics to cryptography. The second author joined in slightly later, followed by a few others. But until the early 1990's, no more than a handful of people were involved in quantum cryptographic research. Since then, the field has taken off with a vengeance, starting with Artur K. Ekert's proposal to use quantum nonlocality for cryptographic purposes [33].The golden age started in earnest when Ekert organized the first international workshop on quantum cryptography in Broadway, England, in 1993. Since then, many conferences have been devoted at least partly to quantum cryptography, which has become a major international topic. The purpose of the aforementioned 1993 bibliography in SIGACT News was to cite as much as possible all papers ever written on the subject, including unpublished manuscripts: there were 57 entries in total. Today, such an undertaking would be nearly impossible owing to the explosion of new research in the field.The purpose of this column is to give an overview of the current research in quantum cryptography. It is not our intention to be exhaustive and we apologize in advance to any researcher whose work we may have omitted. Note that we do not necessarily agree with the claims in every paper mentioned here: this column should not be construed as a seal of approval!