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-

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...

We describe a new error reconciliation protocol Winnow based on the exchange of parity and Hamming's "syndrome" for N −bit subunits of a large data set. Winnow was developed in the context of quantum key distribution and offers significant advantages and net higher efficiency compared to other widely used protocols within the quantum cryptography community. A detailed mathematical analysis of Winnow is presented in the context of practical implementations of quantum key distribution; in particular, the information overhead required for secure implementation is one of the most important criteria in the evaluation of a particular error reconciliation protocol. The increase in efficiency for Winnow is due largely to the reduction in authenticated public communication required for its implementation.

We present experimental results supporting physics-based ejecta model development, where our main assumption is that ejecta form as a special limiting case of a Richtmyer–Meshkov (RM) instability at a metal–vacuum interface. From this assumption, we test established theory of unstable spike and bubble growth rates, rates that link to the wavelength and amplitudes of surface perturbations. We evaluate the rate theory through novel application of modern laser Doppler velocimetry (LDV) techniques, where we coincidentally measure bubble and spike velocities from explosively shocked solid and liquid metals with a single LDV probe. We also explore the relationship of ejecta formation from a solid material to the plastic flow stress it experiences at high-strain rates ($1{0}^{7} ~{\mathrm{s} }^{\ensuremath{-} 1} $) and high strains (700 %) as the fundamental link to the onset of ejecta formation. Our experimental observations allow us to approximate the strength of Cu at high strains and strain rates, revealing a unique diagnostic method for use at these extreme conditions.

Using piezoelectric diagnostics, we have measured densities and velocities of ejected particulate as well as "free-surface velocities" of bulk tin targets shock loaded with high explosive. The targets had finely grooved, machined finishes ranging from 10 to 250 in. Two types of piezoelectric sensor ͑"piezopins"͒, lithium niobate and lead zirconate titanate, were compared for durability and repeatability; in addition, some piezopins were "shielded" with foam and metal foil in order to mitigate premature failure of the pins in high ejecta regimes. These experiments address questions about ejecta production at a given shock pressure as a function of surface finish; piezopin results are compared with those from complementary diagnostics such as x-ray radiography and time-resolved optical transmission techniques. The mass ejection shows a marked dependence on groove characteristics and cannot be described by a groove defect theory alone.

We use the Richtmyer-Meshkov instability (RMI) at a metal-gas interface to infer the metal's yield stress (Y) under shock loading and release. We first model how Y stabilizes the RMI using hydrodynamics simulations with a perfectly plastic constitutive relation for copper (Cu). The model is then tested with molecular dynamics (MD) of crystalline Cu by comparing the inferred Y from RMI simulations with direct stress-strain calculations, both with MD at the same conditions. Finally, new RMI experiments with solid Cu validate our simulation-based model and infer Y~0.47 GPa for a 36 GPa shock.

We have assembled together our ejecta measurements from explosively shocked tin acquired over a period of about ten years. The tin was cast at 0.99995 purity, and all of the tin targets or samples were shocked to loading pressures of about 27 GPa, allowing meaningful comparisons. The collected data are markedly consistent, and because the total ejected mass scales linearly with the perturbations amplitudes they can be used to estimate how much total Sn mass will be ejected from explosively shocked Sn, at similar loading pressures, based on the surface perturbation parameters of wavelength and amplitude. Most of the data were collected from periodic isosceles shapes that approximate sinusoidal perturbations. Importantly, however, we find that not all periodic perturbations behave similarly. For example, we observed that sawtooth (right triangular) perturbations eject more mass than an isosceles perturbation of similar depth and wavelength, demonstrating that masses ejected from irregular shaped perturbations cannot be normalized to the cross-sectional areas of the perturbations.

This effort investigates the underlying physics of ejecta production for high explosive (HE) shocked Sn surfaces prepared with finishes typical to those roughened by tool marks left from machining processes. To investigate the physical mechanisms of ejecta production, we compiled and re-examined ejecta data from two experimental campaigns [W. S. Vogan et al., J. Appl. Phys. 98, 113508 (1998); M. B. Zellner et al., ibid. 102, 013522 (2007)] to form a self-consistent data set spanning a large parameter space. In the first campaign, ejecta created upon shock release at the back side of HE shocked Sn samples were characterized for samples with varying surface finishes but at similar shock-breakout pressures PSB. In the second campaign, ejecta were characterized for HE shocked Sn samples with a constant surface finish but at varying PSB.