The theoretical existence of photon-number-splitting attacks creates a security loophole for most quantum key distribution (QKD) demonstrations that use a highly attenuated laser source. Using ultra-low-noise, highefficiency transition-edge sensor photo-detectors, we have implemented the first version of a decoy state protocol that incorporates finite statistics without the use of Gaussian approximations in a one-way QKD system, enabling the creation of secure keys immune to photon-number-splitting attacks and highly resistant to Trojan horse attacks over 107 km of optical fiber.PACS numbers: 03.67. Dd, 03.67.Hk, 85.25.Oj Quantum key distribution (QKD), which enables users to create a shared key with secrecy guaranteed by the laws of physics [1], is arguably the most advanced application in the growing field of quantum information science. Since the first demonstration in 1992 [2], the field has advanced sufficiently that commercial systems are now available. Most current QKD implementations use "prepare and measure" protocols that involve the sender (Alice) preparing a single photon in a quantum state and sending it to the receiver (Bob), who then measures the photon. Attempts by an eavesdropper (Eve) to obtain information about the state of the single photon will introduce an error rate in the transmission, which alerts the users to Eve's presence.For example, to implement the Bennett-Brassard 1984 (BB84) protocol [3], Alice randomly encodes a single photon with either a 0 or a 1 in one of two conjugate bases and sends the photon to Bob. Bob performs a measurement in one of the two bases, and communicates the time slots for which he obtained detection events. Alice and Bob then create a sifted key by only retaining events where they used the same basis. Ideally, Alice's sifted bits should be perfectly correlated with Bob's if Eve did not attack the transmission, but any real system has error rates due to experimental imperfections. Error correction [4] removes these errors, leaving Alice and Bob with a perfectly correlated key. However, this key is not yet completely secret because, in principle, the errors may have arisen from Eve attacking the system. Therefore, a final step of privacy amplification [5] is used to obtain a shorter, secret key about which Eve has negligible information.The lack of readily available single-photon sources, especially at telecom wavelengths where most fiber-based QKD systems operate, modifies the simple picture outlined above considerably. If the source emits more than one photon, Eve could remove one of the photons and store it until Bob announces his basis choice, at which time she would measure the photon in the correct basis and learn the bit value without introducing any errors. Therefore, in addition to assuming that all errors arise from Eve's interaction with single photons, it is also necessary to assume that Eve can gain full information about any sifted bits that arose from multi-photon events.To determine the number of sifted bits that were encoded in single photo...

Use of low-noise detectors can both increase the secret bit rate of long-distance quantum key distribution (QKD) and dramatically extend the length of a fibre optic link over which secure key can be distributed. Previous work has demonstrated use of ultra-low-noise transitionedge sensors (TESs) in a QKD system with transmission over 50 km. In this work, we demonstrate the potential of the TESs by successfully generating error-corrected, privacyamplified key over 148.7 km of dark optical fibre at a mean photon number μ = 0.1, or 184.6 km of dark optical fibre at a mean photon number of 0.5. We have also exchanged secret key over 67.5 km that is secure against powerful photon-number-splitting attacks.Many classical encryption schemes base their security on the perceived difficulty of efficiently performing certain computational tasks, such as the factoring of large numbers. Quantum key distribution (QKD), on the other hand, allows two users to create a shared, secret, random key for encrypting data, enabling communication that can be proven secure by the laws of physics [1]. Ideally, information is contained in the state of a single quantum, so an eavesdropper ("Eve") is unable to gain information without disturbing the system and revealing her actions. To implement QKD, it is necessary to have a source of single quanta, a method for encoding and decoding information onto and from these quanta, and a protocol for establishing a key. Photons are the obvious choice for sending information over large distances with little decoherence or loss. At present, there are no commercially-available single photon sources, but a heavily attenuated, pulsed laser source provides a practical alternative. Photon statistics from such a laser source follow a Poisson distribution, where the probability of a multi-photon signal is approximately μ 2 /2 for mean photon number μ < 1. The presence of these signals must be included in the secrecy analysis of the system, because an eavesdropper could gain information about multiphoton signals without being detected. Hypothetically, in the presence of channel loss an eavesdropper using a sophisticated (but presently unfeasible) photon-number-splitting (PNS) attack [2] could even gain complete knowledge of the key if the mean photon number, μ, exceeds a certain link-loss and therefore distance-dependent maximum value. Such upper limits on μ set a maximum QKD secret key transmission distance owing to the Contribution of an agency of the U.S. government; not subject to copyright.

The performance of selected, commercially available InGaAs/InP avalanche photodiodes operating in a photon-counting mode at an incident wavelength of 1.55 microm is described. A discussion on the optimum operating conditions and their relationship to the electric field distribution within the device is presented.

This article describes a time-of-flight sensor based on multiple pulsed laser sources which utilizes time-correlated single-photon counting. The sensor has demonstrated good performance at ranges of up to 17 km in daylight conditions. Analysis techniques were developed to examine the returns from targets containing more than one scattering surface.

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