Quantum cryptography using optical fibers

Efficient multiuser quantum cryptography network based on entanglement

We present an efficient quantum key distribution protocol with a certain entangled state to solve a special cryptographic task. Also, we provide a proof of security of this protocol by generalizing the proof of modified of Lo-Chau scheme. Based on this two-user scheme, a quantum cryptography network protocol is proposed without any quantum memory.

Enhanced autocompensating quantum cryptography system

We have improved the hardware and software of our autocompensating system for quantum key distribution by replacing bulk optical components at the end stations with fiber-optic equivalents and implementing software that synchronizes end-station activities, communicates basis choices, corrects errors, and performs privacy amplification over a local area network. The all-fiber-optic arrangement provides stable, efficient, and high-contrast routing of the photons. The low-bit error rate leads to high error-correction efficiency and minimizes data sacrifice during privacy amplification. Characterization measurements made on a number of commercial avalanche photodiodes are presented that highlight the need for improved devices tailored specifically for quantum information applications. A scheme for frequency shifting the photons returning from Alice's station to allow them to be distinguished from backscattered noise photons is also described.

Quantum information processing with atoms and photons

Quantum information processors exploit the quantum features of superposition and entanglement for applications not possible in classical devices, offering the potential for significant improvements in the communication and processing of information. Experimental realization of large-scale quantum information processors remains a long-term vision, as the required nearly pure quantum behaviour is observed only in exotic hardware such as individual laser-cooled atoms and isolated photons. But recent theoretical and experimental advances suggest that cold atoms and individual photons may lead the way towards bigger and better quantum information processors, effectively building mesoscopic versions of 'Schrödinger's cat' from the bottom up.

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 in free space

The range of quantum cryptography systems using optical fibers is limited to roughly 30 km because amplifiers cannot be used. A fully operational system for quantum cryptography based on the transmission of single photons in free space under daylight conditions has been demonstrated. The feasibility of a global system for quantum cryptography based on a network of ground stations and satellites is discussed.

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!