



 "Circular type" quantum key distribution
Nishioka, T; Ishizuka, H; Hasegawa, T; Abe, J
IEEE Photonics Technology Letters (10411135), vol. 14, no. 4, Apr. 2002, p. 576578
A circular type interferometric system for quantum key distribution is proposed. The system, which adopts a Sagnac loop, is highly stable and simple. The stability derives from its selfalignment and compensation for birefringence and the systems simplicity allows it to achieve an extremely fast bit rate. Moreover, it is easily applicable to multiparty setup. Key creation with 0.1 photon per pulse at a rate of 1.2 kHz with a 5.4 percent quantum biterror rate over a 200 m fiber was realized. (Author)
 Experimental quantum cloning of single photons
LamasLinares, A; Simon, C; Howell, J C; Bouwmeester, D
Science (00368075), vol. 296, no. 5568, 26 Apr. 2002, p. 712714
Although perfect copying of unknown quantum systems is forbidden by the laws of quantum mechanics, approximate cloning is possible. A natural way of realizing quantum cloning of photons is by stimulated emission. In this context, the fundamental quantum limit to the quality of the clones is imposed by the unavoidable presence of spontaneous emission. In our experiment, a single input photon stimulates the emission of additional photons from a source on the basis of parametric downconversion. This leads to the production of quantum clones with nearoptimal fidelity. We also demonstrate universality of the copying procedure by showing that the same fidelity is achieved for arbitrary input states. (Author)

The effect of multipair signal states in quantum cryptography with entangled photons
Dusek, M; Bradler, K
Journal of Optics B: Quantum and Semiclassical Optics (14644266), vol. 4, no. 2, Apr. 2002, p. 109113
Real sources of entangled photon pairs (such as parametric down conversion) are not perfect. They produce quantum states that contain more than only one photon pair with some probability. Several aspects of the use of such states for the purpose of quantum key distribution are discussed. It is shown that the presence of 'multipair' signals (together with low detection efficiencies) causes errors in transmission even in the absence of an eavesdropper. Moreover, it is shown that even eavesdropping that draws information only from these 'multipair' signals increases the error rate. This fact represents the important advantage of entanglementbased quantum key distribution. Information that can be obtained by an eavesdropper from the 'multipair' signals is also calculated. (Author)
 Superpositions of the orbital angular momentum for applications in quantum experiments
Vaziri, A; Weihs, G ; Zeilinger, A
Journal of Optics B: Quantum and Semiclassical Optics (14644266), vol. 4, no. 2, Apr. 2002, p. S47S51
Two different experimental techniques for preparing and analyzing superpositions of Gaussian and LaguerreGaussian modes are presented. These involve exploiting an interferometric method in one case and using computergenerated holograms in the other. It is shown that by shifting a hologram with respect to an incoming Gaussian beam, different superpositions of the Gaussian and the LaguerreGaussian beam can be produced. An analytical expression connecting the relative phase, the amplitudes of the modes and the displacement of the hologram is given. The application of such orbital angular momenta superpositions in quantum experiments such as quantum cryptography is discussed. (Author)
 Generation of correlated photons via fourwave mixing in optical fibres
Wang, L J; Hong, C K; Friberg, S R
Journal of Optics B: Quantum and Semiclassical Optics (14644266), vol. 3, no. 5, Oct. 2001, p. 346352
Correlated photons have been used in quantum optics for the generation of squeezed light, demonstration of quantum interference effects, quantum cryptography, EinsteinPodolskyRosen experiments and most recently, quantum teleportation. Usually, they are generated using chisquared parametric processes to take advantage of large chisquared nonlinearities and to wavelength separate the correlated photons from the pump photons. Here, we examine the generation of correlated photons using chicubed parametric processes in optical fibers. We show that using such processes provides a simple and inexpensive correlated photon source readily compatible with communications technologies. (Author)
 Generation and manipulation of squeezed states of light in optical networks for quantum communication and computation
Raginsky, Maxim; Kumar, Prem
Journal of Optics B; Quantum and Semiclassical Optics (14644266), vol. 3, no. 4, Aug. 2001, p. L1L4
We analyze a fiberoptic component which could have multiple uses in novel information processing systems utilizing squeezed states of light. Our approach is based on the phenomenon of photonnumber squeezing of soliton noise after the soliton has propagated through a nonlinear optical fiber. Applications of this component in optical networks for quantum computation and quantum cryptography are discussed. (Author)
 A note on invariants and entanglements [for quantum communication]
Albeverio, Sergio; Fei, ShaoMing
Journal of Optics B; Quantum and Semiclassical Optics (14644266), vol. 3, no. 4, Aug. 2001, p. 223227
Quantum entanglements are studied in terms of the invariants under local unitary transformations. A generalized formula of concurrence for Ndimensional quantum systems is presented. This generalized concurrence has potential applications in studying separability and calculating the entanglement of formation for highdimensional mixed quantum states. (Author)
 Quantum key distribution over 1.1 km in an 850nm experimental allfiber system [for encryptions]
Liang, C; Fu, DH ; Liang, B ; Wu, LA ; Yao, DC ; Lu, SW
Acta Physica Sinica (10003290), vol. 50, no. 8, Aug. 2001, p. 14291433
A 1.1 km long allfiber quantum key distribution experimental setup has been realized for the first time at 850 nm. The system employs the BB84 protocol to establish a secret key between two parties, the security of which is guaranteed by Heisenberg's uncertainty relationship and the quantum noncloning principle. Phase modulated single photons are used to carry the key. The effective transmission rate is 3 bit/s, with a bit error rate of 9 pct. (Author)
 Entanglement of the orbital angular momentum states of photons
Mair, Alois; Vaziri, Alipasha; Weihs, Gregor; Zellinger, Anton
Nature (00280836), vol. 412, no. 6844, 19 July 2001, p. 313316
We demonstrate quantum entanglement involving the spatial modes of the electromagnetic field carrying orbital angular momentum. As these modes can be used to define an infinitely dimensional discrete Hilbert space, this approach provides a practical route to entanglement that involves many orthogonal quantum states, rather than just two multidimensional entangled states, and could be of considerable importance in the field of quantum information, enabling, for example, more efficient use of communication channels in quantum cryptography. (Author)
 Strange attractor in optical logic cells
GonzalezMarcos, A P; MartinPereda, J A
IN:Photonic Devices and Algorithms for Computing III; Proceedings of the Conference, San Diego, CA, July 2930, 2001, Bellingham, WA, Society of PhotoOptical Instrumentation Engineers, 2001, p. 6573
Optical logic cells, employed in such tasks as optical computing or optically controlled switches for photonic switching, display very particular behavior when the working conditions are slightly modified. One of the more striking changes occurs when some delayed feedback is applied between one of the possible output gates and a control input. A chaotic behavior results, and its possible applications range from communications to cryptography. But the main problem with this behavior is the binary character of the resulting signal. Most of the techniques employed today to analyze chaotic signals concern analog signals, where algebraic equations are possible. There are no specific tools to study digital chaotic signals. Some methods have been proposed. One is equivalent to the phase diagram used in studying analog chaos. The binary signal is converted to hexadecimal and then analyzed. This result provides more information than that obtained from previous methods. (Author)
 Entanglement purification for quantum communication
Pan, JianWei; Simon, Christoph; Brukner, Caslav; Zeilinger, Anton
Nature (00280836), vol. 410, no. 6832, 26 Apr. 2001, p. 10671070
The distribution of entangled states between distant locations will be essential for the future largescale realization of quantum communication schemes such as quantum cryptography and quantum teleportation. Existing general purification protocols are based on the quantum controlledNOT (CNOT) or similar quantum logic operations, which are very difficult to implement experimentally. Present realizations of CNOT gates are much too imperfect to be useful for longdistance quantum communication. Here we present a scheme for the entanglement purification of general mixed entangled states, which achieves 50 percent of the success probability of schemes based on the CNOT operation, but requires only simple linear optical elements. Because the perfection of such elements is very high, the local operations necessary for purification can be performed with the required precision. Our procedure is within the reach of current technology, and should significantly simplify the implementation of longdistance quantum communication. (Author)
 Undeniable cryptographic protocol for both sender and receiver and its applications
Li, Xianxian; Huai, Jinpeng
Beijing University of Aeronautics and Astronautics, Journal (10015965), vol. 27, no. 2, Apr. 2001, p. 182185
In data communications, digital signature schemes have been used to ensure the nonrepudiation of sent data and the integrity of the data. The nonrepudiation of received data is also very important in secure communication. In past years, this kind of cryptographic protocol was mainly implemented by the intervention of a trusted third party in the transmission and encryption of data. Thus, the dependability and security of the trusted third party was a bottleneck in these secure systems. To solve this problem, an undeniable cryptographic protocol for both sender and receiver is proposed. It is more efficient. Finally, its applications in electronic mail are discussed. (Author)
 Visual cryptography based on optical interference encryption technique
Seo, DH; Kim, JY; Lee, SS; Park, SJ; Cho, WH; Kim, SJ
IN:Photonic and quantum technologies for aerospace applications III; Proceedings of the Conference, Orlando, FL, Apr. 17, 18, 2001 (A0210251 0174), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 4386), 2001, p. 172180
We propose a new visual cryptography scheme based on optical interference that can improve the contrast and signal to noise ratio of reconstructed images when compared to conventional visual cryptography methods. The binary image being encrypted is divided into any number of n slides. For encryption, (n1) randomly independent keys are generated along with another random key based on an XOR process of random keys. The XOR process between each divided image and each random key produces the encryption of n encrypted images. These encrypted images are then used to make encrypted binary phase masks. For decryption, the phase masks are placed on the paths of a MachZehnder interferometer. (Author)
 Experimental entanglement distillation and 'hidden' nonlocality
Kwiat, Paul G; BarrazaLopez, Salvador; Stefanov, Andre; Gisin, Nicolas
Nature (00280836), vol. 409, no. 6823, 22 Feb. 2001, p. 10141017
Entangled states are central to quantum information processing, including quantum teleportation, efficient quantum computation, and quantum cryptography. We demonstrate experimentally the distillation of maximally entangled states from nonmaximally entangled inputs. Using partial polarizers, we perform a filtering process to maximize the entanglement of pure polarizationentangled photon pairs generated by spontaneous parametric downconversion. We have also applied our methods to initial states that are partially mixed. After filtering, the distilled states demonstrate certain nonlocal correlations, as evidenced by their violation of a form of Bell's inequality. Because the initial states do not have this property, they can be said to possess 'hidden' nonlocality. (Author)
 Photonic and quantum technologies for aerospace applications III; Proceedings of the Conference, Orlando, FL, Apr. 17, 18, 2001
Donkor, E, Ed; Pirich, A R, Ed; Taylor, E W, Ed
Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 4386), 2001
The present volume on photonic and quantum technologies for aerospace applications discusses quantum computer realization and design issues, quantum optics and optical quantum computing, field theory and electrodynamics in quantum computing, and cryptography, security, and encryption. Attention is given to measurement and analysis of chirp for fourwave mixing, highpower laser material for 944nm emission, a highspeed photonic analogtodigital converter, an optical subcarrier generation and multiplexing scheme for alloptical networks, and enabling photonic technologies based on electrochromic and photochromic tungsten oxide. Other topics addressed include highrepetitionrate spectrally synthesized modelocked laser pulses, a stepped conical zone plate antenna, fast quantum FourierWeylHeisenberg transforms, prospects of electricdipole forbidden transitions for qubit logic, visual cryptography based on an optical interference encryption technique, and the behavior of a persistent current qubit in a timedependent EM field. (CSA)
 Single photons on demand from a single molecule at room temperature
Lounis, B; Moerner, W E
Nature (00280836), vol. 407, no. 6803, 28 Sept. 2000, p. 491493
The generation of nonclassical states of light is of fundamental scientific and technological interest. For example, 'squeezed' states enable measurements to be performed at lower noise levels than possible using classical light. Deterministic (or triggered) singlephoton sources exhibit nonclassical behavior in that they emit, with a high degree of certainty, just one photon at a userspecified time. (In contrast, a classical source such as an attenuated pulsed laser emits photons according to Poisson statistics.) A deterministic source of single photons could find applications in quantum information processing, quantum cryptography, and certain quantum computation problems. Here we realize a controllable source of single photons using optical pumping of a single molecule in a solid. Triggered single photons are produced at a high rate, whereas the probability of simultaneous emission of two photons is nearly zero  a useful property for secure quantum cryptography. Our approach is characterized by simplicity, room temperature operation, and improved performance compared to other triggered sources of single photons. (Author)
 POVM inconclusive rate
Brandt, Howard E
IN:Quantum computing; Proceedings of the Conference, Orlando, FL, Apr. 26, 27, 2000 (A0043078 1259), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceeedings. Vol. 4047), 2000, p. 6988
The inconclusive rate is considered as a disturbance measure in key distribution in quantum cryptography. Bennett's twostate protocol is addressed for the case in which a positive operator valued measure is implemented by the legitimate receiver in the presence of individual attack by a general unitary disturbing eavesdropping probe. The maximum Renyi information gain by the disturbing probe is calculated for given receiver error and inconclusive rates. It is demonstrated explicitly that less information is available to an eavesdropper at fixed inconclusive rate and error rate than is available at fixed error rate only. (Author)
 Experimental entanglement of four particles
Sackett, C A; Kielpinski, D; King, B E; Langer, C; Meyer, V; Myatt, C J; Rowe, M; Turchette, Q A; Itano, W M; Wineland, D J
Nature (00280836), vol. 404, no. 6775, 16 Mar. 2000, p. 256259
Quantum mechanics allows for manyparticle wavefunctions that cannot be factorized into a product of singleparticle wavefunctions, even when the constituent particles are entirely distinct. Such 'entangled' states explicitly demonstrate the nonlocal character of quantum theory, having potential applications in highprecision spectroscopy, quantum communication, cryptography, and computation. In general, the more particles that can be entangled, the more clearly nonclassical effects are exhibited  and the more useful the states are for quantum applications. Here we implement a recently proposed entanglement technique to generate entangled states of two and four trapped ions. Coupling between the ions is provided through their collective motional degrees of freedom, but actual motional excitation is minimized. Entanglement is achieved using a single laser pulse, and the method can in principle be applied to any number of ions. (Author)
 Freespace quantum cryptography in daylight
Hughes, Richard J; Buttler, William T; Kwiat, Paul G; Lamoreaux, Steve K; Morgan, George L; Nordholt, Jane E; Peterson, C G
IN:Freespace laser communication technologies XII; Proceedings of the Conference, San Jose, CA, Jan. 24, 2000 (A0035451 0974), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 3932), 2000, p. 117126
Quantum cryptography is an emerging technology in which two parties may simultaneously generate shared, secret cryptographic key material using the transmission of quantum states of light. In this paper we describe the theory of quantum cryptography, and the most recent results from our experimental freespace system, with which we have demonstrated the feasibility of quantum key generation over a pointtopoint outdoor atmospheric path in daylight. We achieved a transmission distance of 0.5 km, which was limited only by the length of the test range. Our results provide strong evidence that cryptographic key material could be generated on demand between a ground station and a satellite (or between two satellites), allowing a satellite to be securely rekeyed on orbit. We present a feasibility analysis of surfacetosatellite quantum key generation. (Author)
 Quantum Cryptography for Secure Satellite Communications
Hughes, R J; Buttler, W T
RECON no. 20010104946.
Quantum cryptography is an emerging technology in which two parties may simultaneously generate shared, secret cryptographic key material using the transmission of quantum states of light. The security of these transmissions is based on the inviolability of the laws of quantum mechanics and informationtheoretically secure postprocessing methods. An adversary can neither successfully tap the quantum transmissions, nor evade detection, owing to Heisenberg's uncertainty principle. In this paper we have demonstrated the feasibility of quantum key generation over a pointtopoint outdoor atmospheric path in daylight.
 Strategies for Steganalysis of Bitmap Graphics Files
Fogle, Christopher J
NASA no. 19990036740
Steganography is the art and science of communicating through covert channels. The goal of steganography is to hide the fact that a message is even being transmitted. In the context of today's digital world, this ancient practice is enjoying resurgence due to the plethora of hiding places made possible by modern information media. Of particular concern is the use of graphics image files to conceal both legitimate and criminal communications.
 Some models of chaotic motion of particles and their application to cryptography
Szczepanski, J; Gorski, K; Kotulski, Z; Paszkiewicz, A; Zugaj, A
Archives of Mechanics  Archiwum Mechaniki Stosowanej (03732029), vol. 51, nos. 34, 1999, p. 509528
Reflection law models describing the motion of a free particle in a bounded domain are considered. Properties of such dynamical systems are strongly related to the boundary conditions, expressed by a map called a reflection law. We discuss recent results concerning the problem of transferring important properties like chaos, ergodicity, and mixing from the reflection law to the motion of the particle. Then we present in a consistent way a method of constructing block cryptosystems, using chaotic reflection law models with appropriate properties. We also propose an application of the mechanical particle model to constructing a pseudorandom number generator which can be applied in stream ciphers. The security of the cryptosystem based on particle motion is due to the property of the statistical independence of the actual location of the particle, after a number of reflections, from its initial location. (Author)
 Quantum cryptography on optical fiber networks
Townsend, Paul D
IN:Photonic quantum computing II; Proceedings of the Meeting, Orlando, FL, Apr. 15, 16, 1998 (A9836101 0974), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 3385), 1998, p. 213
Quantum cryptography exploits the fact that an unknown quantum state cannot be accurately copied (cloned) or measured without disturbance. By using such elementary quantum states to represent binary information, it is possible, therefore, to construct communication systems with verifiable levels of security that are 'guaranteed' by fundamental quantum mechanical laws. This paper describes recent progress at BT Laboratories in the development of practical optical fiberbased quantum cryptography systems. These developments include interferometric systems operating in the 1.3micron wavelength fiber transparency window over pointtopoint links up to 50 km in length and on multiuser passive optical networks. We describe how this technology performs on fiber links installed in BT's public network and discuss issues such as crosstalk with conventional data channels propagating at different wavelengths in the same fiber. (Author)
 Freespace quantum key distribution at night
Buttler, W T; Hughes, R J; Kwiat, P G; Lamoreaux, S K; Luther, G G; Morgan, G L; Nordholt, J E; Peterson, C G; Simmons, C M
IN:Photonic quantum computing II; Proceedings of the Meeting, Orlando, FL, Apr. 15, 16, 1998 (A9836101 0974), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 3385), 1998, p. 1422
An experimental freespace quantum key distribution (QKD) system has been tested over an outdoor optical path of 1 km under nighttime conditions at Los Alamos National Laboratory. This system employs the Bennett 92 protocol. Here, we give a brief overview of this protocol and describe our experimental implementation of it. An analysis of the system efficiency is presented, as well as a description of our error detection protocol, which employs a twodimensional parity check scheme. Finally, the susceptibility of this system to eavesdropping by various techniques is determined, and the effectiveness of privacy amplification procedures is discussed. Our conclusions are that freespace QKD is both effective and secure; possible applications include the rekeying of satellites in low Earth orbit. (Author)
 Positive operator valued measure in quantum information processing
Brandt, Howard E
IN:Photonic quantum computing II; Proceedings of the Meeting, Orlando, FL, Apr. 15, 16, 1998 (A9836101 0974), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 3385), 1998, p. 2335
The positive operator valued measure (POVM), also known as the probability operator valued measure, is useful in quantum information processing. The POVM consists of a set of nonnegative quantummechanical Hermitian operators that add up to the identity. The probability that a quantum system is in a particular state is given by the expectation value of the POVM operator corresponding to that state. Following a brief review of the mathematics and history of POVMs in quantum theory, and a pedagogical discussion of the quantum mechanics of photonic qubits, a particular implementation of a POVM for use in the measurement of photonic qubits is reviewed. (Author)
 Quantum information processing with cavity QED
Hood, C J; Lynn, T W; Mabuchi, H; Chapman, M S; Ye, J; Kimble, H J
IN:Photonic quantum computing II; Proceedings of the Meeting, Orlando, FL, Apr. 15, 16, 1998 (A9836101 0974), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 3385), 1998, p. 95100
Strongly coupled cavity QED systems show great promise for coherent processing of quantum information in the contexts of quantum computing, communication, and cryptography. We present here current progress in experiments for which single atoms are strongly coupled to the mode of a high finesse optical resonator. (Author)
 Photonic quantum computing II; Proceedings of the Meeting, Orlando, FL, Apr. 15, 16, 1998
Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 3385), 1998
The papers presented in this volume focus on quantum communications and cryptography, quantum computing, and quantum structures and algebras. Specific topics discussed include quantum cryptography on optical fiber networks; positive operator valued measure in quantum information processing; vibrational decoherence in iontrap quantum computers; and optical approach to quantum computing. Papers are also presented on quantum information processing with cavity QED; logic design for fieldeffect quantum transistors; and Qextension of the linear harmonic oscillator. (AIAA)
 Explorations in quantum computing [Book]
Williams, Colin P; Clearwater, Scott H
Santa Clara, CA, Springer TELOS, 1998
This book explains quantum computing in simple terms and describes the key technological hurdles that must be overcome in order to make quantum computers a reality. The book uses executable software simulations to help explain the material and is accompanied by a multiplatform CDROM containing Mathematica (Trademark) Version 2.2 and 3.0 notebooks that provide simulations and tutorials on most topics covered in the book. The topics addressed include: quantum mechanics and computers, simulating a simple quantum computer, the effects of imperfections, breaking unbreakable codes, true randomness, quantum cryptography, quantum teleportation, quantum error correction, and how to make a quantum computer. (AIAA)
 New Result in Quantum Cryptography
Brandt, Howard E
NASA no. 19990025973
In the entangled translucent eavesdropping scenario of key generation in quantum cryptography, I demonstrate that the unsafe error rate based on standard mutual information comparisons is equivalent to the maximum allowable error rate based on perfect mutual information for the eavesdropper. In this case, the unsafe error rate is not in fact overly conservative, as is commonly supposed.
 Secure communications using quantum cryptography
Hughes, Richard J; Buttler, William T; Kwiat, Paul G; Luther, Gabriel G; Morgan, George L; Nordholt, Jane E; Peterson, C G; Simmons, Charles M
IN:Photonic quantum computing; Proceedings of the Meeting, Orlando, FL, Apr. 23, 24, 1997 (A9735954 0970), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 3076), 1997, p. 211
The secure distribution of the secret random bit sequences known as 'key' material is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is an emerging technology for secure key distribution with singlephoton transmissions: Heisenberg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of nonorthogonal singlephoton states to generate shared key material over multikilometer optical fiber paths and over lineofsight links. In both cases, key material is built up using the transmission of a singlephoton per bit of an initial secret random sequence. A quantummechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. In our optical fiber experiment we have performed quantum key distribution over 24km of underground optical fiber using singlephoton interference states, demonstrating that secure, realtime key generation over 'open' multikm nodetonode optical fiber communications links is possible. (Author)
 New results on entangled translucent eavesdropping in quantum cryptography
Brandt, Howard E; Myers, John M; Lomonaco, Samuel J, Jr
IN:Photonic quantum computing; Proceedings of the Meeting, Orlando, FL, Apr. 23, 24, 1997 (A9735954 0970), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 3076), 1997, p. 1228
We present a mathematical physics analysis of entangled translucent eavesdropping in quantum cryptography, based on the recent work of Ekert et al. (1994). The key generation procedure involves the transmission, interception, and reception of two nonorthogonal photon polarization states. At the receiving end, a positive operator valued measure (POVM) is employed in the measurement process. The eavesdropping involves an informationmaximizing von Neumanntype projective measurement. We propose a new design for a receiver that is an alloptical realization of the POVM, using a Wollaston prism, a mirror, two beam splitters, a polarization rotator, and three photodetectors. We present a quantitative analysis of the receiver. We obtain closedform algebraic expressions for the error rates and mutual information, expressed in terms of the POVMreceiver error rate and the angle between the carrier polarization states. We also prove a significant result, namely, that in the entangled translucent eavesdropping approach, the unsafe error rate based on standard mutual information comparisons is equivalent to the maximum allowable error rate based on perfect mutual information for the eavesdropper. In this case, the above unsafe error rate is in fact not overly conservative. (Author)
 Relativistic corrections to the Ekert test for eavesdropping
Czachor, Marek
IN:Photonic quantum computing; Proceedings of the Meeting, Orlando, FL, Apr. 23, 24, 1997 (A9735954 0970), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 3076), 1997, p. 141145
A degree of violation of the Bell inequality depends on momenta of massive particles with respect to a laboratory if spin plays a role af a 'yesno' observable. For ultrarelativistic particles a standard Ekert test has to take into account this velocitydependent suppression of the degree of violation of the inequality. Otherwise 'Alice' and 'Bob' may 'discover' a nonexisting eavesdropper, where 'Alice' is the sender and 'Bob' is the recipient of cryptographic messages. (Author)
 Prospects for quantum computation with trapped ions
Hughes, R J; James, D F V
NASA no. 19980210966
Over the past decade information theory has been generalized to allow binary data to be represented by twostate quantum mechanical systems. (A single twolevel system has come to be known as a qubit in this context.) The additional freedom introduced into information physics with quantum systems has opened up a variety of capabilities that go well beyond those of conventional information. For example, quantum cryptography allows two parties to generate a secret key even in the presence of eavesdropping. But perhaps the most remarkable capabilities have been predicted in the field of quantum computation. Here, a brief survey of the requirements for quantum computational hardware, and an overview of the in trap quantum computation project at Los Alamos are presented. The physical limitations to quantum computation with trapped ions are discussed.
 Photonic Imaging Networks
Fainman, Y; Kellner, Albert
NASA no. 19980020946
We have selected as a prototype application diagnostic medical imaging and visualization. We have developed several radiological visualization station we have developed and evaluated methods for the loss less compression of images and imageformate data over a lossy packet network. We have studied noise mechanisms in transparent photonic networks. We have demonstrated paralleltoserial and serialtoparallel conversion using spectral domain fourwave mixing with 150 fsec laser pulses reaching serial data rates of over 1 Tbit/sec. we have constructed and demonstrated a nonlinear optical processor based on threewave mixing in nonlinear LBO crystals. We have employed the paralleltoserial and serialtoparallel processors for experimental demonstration of transmission of image information through an optical fiber channel. We have analyzed the secrecy capacity of a quantum cryptographic protocol for secret key generation and found that it primarily depends on estimates of information in eavesdropper's possession, and the expected fraction of inconclusive outcomes. We investigated experimentally a novel frequency division long distance interferometer for implementing quantum cryptographic protocol and found that signals are not affected by transmission over optical fiber, we have developed for the first time the rigorous definitions and the mathematical formalism for information leakage through possible eavesdropping on the quantum channel. We quantify effective defense frontiers against avesdroppers attacks.
 Twophoton geometric optical imaging and quantum 'cryptoFAX'
Sergienko, A V; Shih, Y H; Pittman, T B; Strekalov, D V; Klyshko, D N
IN:ICONO '95  Atomic and quantum optics: Highprecision measurements; Proceedings of the Conference, St. Petersburg, Russia, June 27July 1, 1995 (A9633587 0874), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 2799), 1996, p. 164171
A nonlocal twophoton quantum effect which surprisingly shows some features analogous to geometrical imaging optics is observed. The remote transfer of 2D analog information with a high degree of security (quantum 'cryptoFAX') is demonstrated. (Author)
 Fibreoptics quantum cryptography
Sochor, Vaclav
IN:ICONO '95  Atomic and quantum optics: Highprecision measurements; Proceedings of the Conference, St. Petersburg, Russia, June 27July 1, 1995 (A9633587 0874), Bellingham, WA, Society of PhotoOptical Instrumentation Engineers (SPIE Proceedings. Vol. 2799), 1996, p. 185187
Quantum cryptography is a new, multidisciplinary technique that distributes information to two or more parties in a way that can guarantees security against unauthorized eavesdroping. Three schemes of implementation of this goal by means of fiber optics, namely photon polarization, delayed interferometry, and quantum correlations of nonorthogonal states, are discussed. Realization of proposed schemes is discussed as well as the fundamental limits imposed on quantum cryptography by fiberoptics techniques. (Author)
 Quantum cryptography over underground optical fibers
HUGHES, R J; LUTHER, G G; MORGAN, G L; PETERSON, C G; SIMMONS, C; et al
Quantum cryptography is an emerging technology in which two parties may simultaneously generated shared, secret cryptographic key material using the transmission of quantum states of light whose security is based on the inviolability of the laws of quantum mechanics. An adversary can neither successfully tap the key transmissions, nor evade detection, owing to Heisenberg's uncertainty principle. In this paper the authors describe the theory of quantum cryptography, and the most recent results from their experimental system with which they are generating key material over 14km of underground optical fiber. These results show that opticalfiber based quantum cryptography could allow secure, realtime key generation over 'open' multikm nodetonode optical fiber communications links between secure 'islands.' (DOE)
 Generation of Antibunched Light by Excited Molecules in a Microcavity Trap
DEMARTINI, F; DIGIUSEPPE, G; MARROCCO, M; et al
In Rome Univ., Fourth International Conference on Squeezed States and Uncertainty Relations p 565573 (SEE N9627114 1074)
The active microcavity is adopted as an efficient source of nonclassical light. By this device, excited by a modelocked laser at a rate of 100 MHz, singlephotons are generated over a single field mode with a nonclassical subpoissonian distribution. The process of adiabatic recycling within a multistep FranckCondon molecular opticalpumping mechanism, characterized in our case by a quantum efficiency very close to one, implies a pump selfregularization process leading to a striking n squeezing effect. By a replication of the basic singleatom excitation process a beam of quantum photon (Fock states) can be created. The new process represents a significant advance in the modern fields of basic quantummechanical investigation, quantum communication and quantum cryptography. (Author)
 HighRate StrongSignal Quantum Cryptography
YUEN, HORACE P; et al
In Northwestern Univ., Fourth International Conference on Squeezed States and Uncertainty Relations p 363368 (SEE N9627114 1074)
Several quantum cryptosystems utilizing different kinds of nonclassical lights, which can accommodate high intensity fields and high data rate, are described. However, they are all sensitive to loss and both the high rate and the strongsignal character rapidly disappear. A squeezed light homodyne detection scheme is proposed which, with presentday technology, leads to more than two orders of magnitude data rate improvement over other current experimental systems for moderate loss. (Author)
 A Secure Key Distribution System of Quantum Cryptography Based on the Coherent State
GUO, GUANGCAN; ZHANG, XIAOYU; et al
In University of Science and Technology of China, Fourth International Conference on Squeezed States and Uncertainty Relations p 297300 (SEE N96 27114 1074)
The cryptographic communication has a lot of important applications, particularly in the magnificent prospects of private communication. As one knows, the security of cryptographic channel depends crucially on the secrecy of the key. The Vernam cipher is the only cipher system which has guaranteed security. In that system the key must be as long as the message and most be used only once. Quantum cryptography is a method whereby key secrecy can be guaranteed by a physical law. So it is impossible, even in principle, to eavesdrop on such channels. Quantum cryptography has been developed in recent years. Up to now, many schemes of quantum cryptography have been proposed. Now one of the main problems in this field is how to increase transmission distance. In order to use quantum nature of light, up to now proposed schemes all use very dim light pulses. The average photon number is about 0.1. Because of the loss of the optical fiber, it is difficult for the quantum cryptography based on one photon level or on dim light to realize quantum keydistribution over long distance. A quantum key distribution based on coherent state is introduced in this paper. Here we discuss the feasibility and security of this scheme. (Derived from text)
 Scaling of entanglement close to a quantum phase transition
Osterloh, A; Amico, L; Falci G,; Fazio, R
Nature (00280836), vol. 416, no. 6881, 11 Apr. 2002, p. 608610
Classical phase transitions occur when a physical system reaches a state below a critical temperature characterized by macroscopic order. Quantum phase transitions occur at absolute zero; they are induced by the change of an external parameter or coupling constant, and are driven by quantum fluctuations. Examples include transitions in quantum Hall systems, localization in SiMOSFETs (metal oxide silicon fieldeffect transistors, Kravchenko et al., 1994), and the superconductorinsulator transition in twodimensional systems. Both classical and quantum critical points are governed by a diverging correlation length, although quantum systems possess additional correlations that do not have a classical counterpart. This phenomenon, known as entanglement, is the resource that enables quantum computation and communication. The role of entanglement at a phase transition is not captured by statistical mechanics. A complete classification of the critical manybody state requires the introduction of concepts from quantum information theory. In this paper we connect the theory of critical phenomena with quantum information by exploring the entangling resources of a system close to its quantum critical point. We demonstrate, for a class of onedimensional magnetic systems, that entanglement shows scaling behavior in the vicinity of the transition point. (Author)
 Optimal signal detection in entanglementassisted quantum communication systems
Ban, M
Journal of Optics B: Quantum and Semiclassical Optics (14644266), vol. 4, no. 2, Apr. 2002, p. 143148
Minimization of error probability is considered in entanglementassisted quantum communication systems. It is shown that although quantum state signals being sent are not symmetric at a sender side, the square root measurement becomes optimum when they are made symmetric at the receiver side. For communication systems of coherent signals, where a twomode squeezedvacuum state is used as an entanglement resource, the quantum entanglement greatly reduces the average probability of error. The relation to the quantum dense coding of continuous variables is also discussed. (Author)
 Relativity, entanglement and the physical reality of the photon
Tiwari, S C
Journal of Optics B: Quantum and Semiclassical Optics (14644266), vol. 4, no. 2, Apr. 2002, p. S39S46
Recent experiments on the classic EinsteinPodolskyRosen (EPR) setting claim to test the compatibility between nonlocal quantum entanglement and the (special) theory of relativity. Confirmation of quantum theory has led to the interpretation that Einstein's image of physical reality for each photon in the EPR pair cannot be maintained. A detailed critique on two representative experiments is presented following the original EPR notion of local realism. It is argued that relativity does not enter into the picture; however, for the BellBohm version of local realism in terms of hidden variables such experiments are significant. Of the two alternatives, namely, incompleteness of quantum theory for describing an individual quantum system, and the ensemble view, it is only the former that has been ruled out by the experiments. An alternative approach gives a statistical ensemble interpretation of the observed data, and the significant conclusion that these experiments do not deny physical reality of the photon is obtained. After discussing the need for a photon model, a vortex structure is proposed based on the spacetime invariant propertyspin, and pure gauge fields. To test the prime role of spin for photons and the angularmomentum interpretation of electromagnetic fields, experimental schemes feasible in modern laboratories are suggested. (Author)
 Quantum information processing with atoms and photons
Monroe, C
Nature (00280836), vol. 416, no. 6877, 14 Mar. 2002, p. 238246
Quantum information processors exploit the quantum features of superposition and entanglement for applications not possible in classical devices, offering the potential for significant improvements in the communication and processing of information. Experimental realization of largescale quantum information processors remains a longterm vision, as the required nearly pure quantum behavior is observed only in exotic hardware such as individual lasercooled atoms and isolated photons. But recent theoretical and experimental advances suggest that cold atoms and individual photons may lead the way towards bigger and better quantum information processors, effectively building mesoscopic versions of 'Schroedinger's cat' from the bottom up. (Author)
 Quantum switch for continuous variable teleportation
Zhang, J; Xie, C; Peng, K
Journal of Optics B: Quantum and Semiclassical Optics (14644266), vol. 3, no. 5, Oct. 2001, p. 293297
We propose a quantum teleportation scheme in which a quantum state is teleported from the sending station (Alice) to either of two receiving stations (Bob1, Bob2). In this scheme, two pairs of EPR beams with identical frequency and constant phase relation are used to produce two pairs of conditional entangled beams by composing their modes on two beamsplitters. One output of a beamsplitter is sent to Alice and the two outputs of the other beamsplitter are sent to Bob1 and Bob2. Which receiving station actually receives the teleported state can be decided by correlating the inphase or outofphase quadrature components of two twomode squeezed vacuum states. The switch system manipulated by squeezed state light might be developed as a practical quantum switch device for the communication and teleportation of quantum information. (Author)
 Stimulated emission of polarizationentangled photons
LamasLinares, A; Howell, J C; Bouwmeester, D
Nature (00280836), vol. 412, no. 6850, 30 Aug. 2001, p. 887890
We use stimulated parametric downconversion to study entangled states of light that bridge the gap between discrete and macroscopic optical quantum correlations. We demonstrate experimentally the onset of laserlike action for entangled photons, through the creation and amplification of the spin1/2 and spin1 singlet states consisting of two and four photons, respectively. This entanglement structure holds great promise in quantum information science, where there is a strong demand for entangled states of increasing complexity. (Author)
 A method to protect quantum entanglement against certain kinds of phase and exchange errors
Yang, ChuiPing; GeaBanacloche, Julio
Journal of Optics B: Quantum and Semiclassical Optics (14644266), vol. 3, no. 1, Feb. 2001, p. S30S33
We present a method to protect the entangled states of distant particles against decoherence due to local (but collective) phase errors, and local exchangetype interactions, by pairing up the entangled particles. The method is based on a fourqubit code which forms a decoherencefree subspace for collective phase errors and exchange errors affecting the qubits in pairs. We also show how the scheme can be generalized to protect certain entangled states of more than two particles. (Author)






