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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We have completed a new set of total cross section measurements of 31 elements and isotopes spanning the periodic table from Aϭ1 to 238. We employed the same technique as in Finley et al. ͓Phys. Rev. C 47, 237 ͑1993͔͒ with refinements intended to allow measurements on separated isotopes and improved systematic error control. The goal of the new measurement was 1% statistical accuracy in 1% energy bins with systematic errors less than 1%. This was achieved for all but the thinnest samples. Stringent checks of systematic errors in this measurement resulted in a reassignment of systematic uncertainties to the neutron total cross sections reported in Finley et al. Microscopic optical model calculations were carried out to interpret the results of the experiment. Two specific types of optical models were employed. The Jeukenne-Lejeune-Mahaux model was used in the range of 5-160 MeV, and a model based on the empirical effective interaction of Kelly was used from 135 to 650 MeV. These models are shown to be useful for predicting both neutron total cross sections and proton reaction cross sections. They are particularly important for light nuclei, for which standard global phenomenological parametrizations of the optical potential are insufficiently accurate.

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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