Daylight Quantum Key Distribution over 1.6 km

Abstract

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...

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“…To date two groups have succeeded in exchanging keys over free-space ranges greater than 1 km [4]- [6] and ongoing experiments show that ranges of 10 [12] and 23 km [13] can easily be reached. In all experiments the first steps are being taken to reduce the size, mass and power consumption of the equipment primarily to allow portability in the short term and fully automated remote operation in the long term.…”

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