The Cassini Plasma Spectrometer (CAPS) will make comprehensive three-dimensional mass-resolved measurements of the full variety of plasma phenomena found in Saturn's magnetosphere. Our fundamental scientific goals are to understand the nature of saturnian plasmas primarily their sources of ionization, and the means by which they are accelerated, transported, and lost. In so doing the CAPS investigation will contribute to understanding Saturn's magnetosphere and its complex interactions with Titan, the icy satellites and rings, Saturn's ionosphere and aurora, and the solar wind. Our design approach meets these goals by emphasizing two complementary types of measurements: high-time resolution velocity distributions of electrons and all major ion species; and lower-time resolution, high-mass resolution spectra of all ion species. The CAPS instrument is made up of three sensors: the Electron Spectrometer (ELS), the Ion Beam Spectrometer (IBS), and the Ion Mass Spectrometer (IMS). The ELS measures the velocity distribution of electrons from 0.6 eV to 28,250 keV, a range that permits coverage of thermal electrons found at Titan and near the ring plane as well as more energetic trapped electrons and auroral particles. The IBS measures ion velocity distributions with very high angular and energy resolution from 1 eV to 49,800 keV. It is specially designed
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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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