We have demonstrated quantum key distribution (QKD) [1] over a 10-km, 1-airmass atmospheric range during daylight and at night. Secret random bit sequences of the quality required for the cryptographic keys used to initialize secure communications devices were transferred at practical rates with realistic security. By identifying the physical parameters LA-UR-02-449 using the "public channel" Alice reveals her basis choice for each bit of her raw key, but not the bit value. Bob communicates back the time slots of the bits in his raw key for which he used the same basis as Alice. In an ideal system Alice's transmitted bits and the results of Bob's measurements on this random, 50% portion of the raw key, known as the "sifted" key, are perfectly correlated; they discard the raw key bits for which Bob used the wrong basis. In practice Bob's sifted key contains errors. Fundamental quantum principles ensure that Eve is both limited in how much information she may obtain by eavesdropping on the quantum communications, and that she cannot do so without introducing a disturbance (errors) in Bob's sifted key from which Alice and Bob can deduce a rigorous upper bound on leaked information. Alice and Bob determine this bound after reconciling their sifted keys using post facto error correction [13] over their public channel, but at the price of leaking additional (side) information to Eve. From their partially-secret reconciled keys Alice and Bob extract the shorter, final secret key on which they agree with overwhelming probability and on which Eve's expected information is much less than one bit [14] after a final stage of "privacy amplification" [3] using further public channel communications. BB84 with ideal single-photon signals is unconditionally secure [15].

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...

Abstract. A working free-space quantum key distribution (QKD) system has been developed and tested over an outdoor optical path of ∼ 1 km at Los Alamos National Laboratory under nighttime conditions. Results show that QKD can provide secure real-time key distribution between parties who have a need to communicate secretly. Finally, we examine the feasibility of surface to satellite QKD.Quantum cryptography was introduced in the mid1980s [1] as a new method for generating the shared, secret random number sequences, known as cryptographic keys, that are used in crypto-systems to provide communications security. The appeal of quantum cryptography is that its security is based on laws of nature, in contrast to existing methods of key distribution that derive their security from the perceived intractability of certain problems in number theory, or from the physical security of the distribution process.Since the introduction of quantum cryptography, several groups have demonstrated quantum communications [2,3] and quantum key distribution [4-9] over multikilometer distances of optical fiber. Free-space QKD (over an optical path of ∼ 30 cm) was first introduced in 1991 [12], and recent advances have led to demonstrations of QKD over free-space indoor optical paths of 205 m [10], and outdoor optical paths of 75 m [11]. These demonstrations increase the utility of QKD by extending it to line-of-site laser communications systems. Indeed there are certain key distribution problems in this category for which free-space QKD would have definite practical advantages (for example, it is impractical to send a courier to a satellite). We are developing such QKD, and here we report our results of free-space QKD over outdoor optical paths of up to 950 m under nighttime conditions. The success of QKD over free-space optical paths depends on the transmission and detection of singlephotons against a high background through a turbulent medium. Although this problem is difficult, a combination of sub-nanosecond timing, narrow filters [13,14], spatial filtering [10] and adaptive optics [15] can render the transmission and detection problems tractable. Furthermore, the essentially non-birefringent nature of the atmosphere at optical wavelengths allows the faithful transmission of the single-photon polarization states used in the free-space QKD protocol.A QKD procedure starts with the sender, "Alice," gen-

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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.PACS Numbers: 03.65. Bz, 42.79.Sz Quantum cryptography was introduced in the mid1980s [1] as a new method for generating the shared, secret random number sequences, known as cryptographic keys, that are used in crypto-systems to provide communications security (for a review see [2]). The appeal of quantum cryptography (or more accurately, quantum key distribution, QKD) is that its security is based on laws of nature and information-theoretically secure techniques, in contrast to existing methods of key distribution that derive their security from the perceived intractability of certain problems in number theory, or from the physical security of the distribution process.Several groups have demonstrated QKD over multikilometer distances of optical fiber [3], but there are many key distribution problems for which QKD over lineof-sight atmospheric paths would be advantageous (for example, it is impractical to send a courier to a satellite). Free-space QKD was first demonstrated in 1990 [4,5] over a point-to-point 32-cm table top optical path, and recent work has produced atmospheric transmission distances of 75 m [6] (daytime) and 1 km [7] (nighttime) over outdoor folded paths (to a mirror and back). The close collocation of the QKD transmitter and receiver in folded-path experiments is not representative of practical applications and can result in some compensation of turbulence effects. We have recently performed the first point-to-point atmospheric QKD in full daylight, achieving a 0.5-km transmission range [8], and here we report a record 1.6-km point-to-point transmission in daylight, with a novel QKD system that has no active polarization switching elements.The success of QKD over atmospheric optical paths depends on the transmission and detection of singlephotons against a high background through a turbulent medium. Although this problem is difficult, a combination of temporal, spectral [9,10] and spatial filtering [11] can render the transmission and detection problems tractable [8]. The essentially non-birefringent nature of the atmosphere at optical wavelengths allows the faithful transmission of the single-photon polarization states used in the free-space QKD protocol.A QKD procedure starts with the sender, "Alice," generating a secret random binary number sequence. For each bit in the sequence, Alice prepares and transmits a single photon to the recipient, "Bob," who measures each arriving photon and attempts to identify the bit value Alice has transmitted. Alice's photon state preparations and Bob's measurements are chosen from sets of non-orthogonal possibilities. For example, using the B92 protocol [12] Alice agrees with Bob (through public discussion) that she will transmit a 45 • polarized photon state |45 , for each "0" in her sequence, and a vertical p...

Abstract. To move beyond dedicated links and networks, quantum communications signals must be integrated into networks carrying classical optical channels at power levels many orders of magnitude higher than the quantum signals themselves. We demonstrate the transmission of a 1550 nm quantum channel with up to two simultaneous 200 GHz spaced classical telecom channels, using reconfigurable optical add drop multiplexer (ROADM) technology for multiplexing and routing quantum and classical signals. The quantum channel is used to perform quantum key distribution (QKD) in the presence of noise generated as a by-product of the co-propagation of classical channels. We demonstrate that the dominant noise mechanism can arise from either four-wave mixing or spontaneous Raman scattering, depending on the optical path characteristics as well as the classical channel parameters. We quantify these impairments and discuss mitigation strategies.

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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.