Practical Free-Space Quantum Key Distribution over 1 km

Abstract

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-

Daylight Quantum Key Distribution over 1.6 km

“…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.…”

Modified Wigner inequality for secure quantum-key distribution

“…Scientists are currently using one of two photon sources for practical QKD, faint lasers [5,6,7,8,9,10] or spontaneous parametric down-conversion to generate entangled photon states (SPDC) [11,12,13,14,15].…”

Squeezed light from microstructured fibres: towards free-space quantum cryptography

“…The first is short distance communication up to several kilometers [27,28], mainly in urban areas, where a fibre based connection is too expensive to deploy. The second is secure satellite communication, where a fibre link is not possible.…”