Showing papers in "Journal of Modern Optics in 2000"
TL;DR: In this article, a mathematical characterization of these monotone magnitudes is presented, which are then related to optimal strategies of conversion of shared states, and more detailed results are presented for pure states of bipartite systems.
Abstract: In the context of quantifying entanglement we study those functions of a multipartite state which do not increase under the set of local transformations. A mathematical characterization of these monotone magnitudes is presented. They are then related to optimal strategies of conversion of shared states. More detailed results are presented for pure states of bipartite systems. It is show that more than one measure are required simultaneously in order to quantify completely the non-local resources contained in a bipartite pure state, while examining how this fact does not hold in the so-called asymptotic limit. Finally, monotonicity under local transformations is proposed as the only natural requirement for measures of entanglement.
567 citations
TL;DR: In this article, a physical random number generator based on the intrinsic randomness of quantum mechanics is described, where the random events are realized by the choice of single photons between the two outputs of a beam splitter.
Abstract: A physical random number generator based on the intrinsic randomness of quantum mechanics is described. The random events are realized by the choice of single photons between the two outputs of a beam splitter. We present a simple device, which minimizes the impact of the photon counters' noise, dead-time and after pulses.
300 citations
TL;DR: In this article, the authors describe a general strategy for implementing a conditional geometric phase between two spins, which is a universal gate for quantum computation, in that any unitary transformation can be implemented with arbitrary precision using only single-spin operations and conditional phase shifts.
Abstract: We describe in detail a general strategy for implementing a conditional geometric phase between two spins. Combined with single-spin operations, this simple operation is a universal gate for quantum computation, in that any unitary transformation can be implemented with arbitrary precision using only single-spin operations and conditional phase shifts. Thus quantum geometrical phases can form the basis of any quantum computation. Moreover, as the induced conditional phase depends only on the geometry of the paths executed by the spins it is resilient to certain types of errors and offers the potential of a naturally fault-tolerant way of performing quantum computation.
234 citations
TL;DR: In this article, the authors developed experimental quantum cryptography systems based on the transmission of non-orthogonal photon states to generate shared key material over multikilometre optical fibre paths and over line-of-sight links.
Abstract: 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 a new technique for secure key distribution with single-photon 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 non-orthogonal photon states to generate shared key material over multikilometre optical fibre paths and over line-of-sight links. In both cases, key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. Here we report the most re...
187 citations
TL;DR: In this article, the authors derived an experimentally testable criterion for the teleportation of quantum states of continuous variables, which was especially relevant to the recent experiment of Furusawa et al. where an input-output fidelity of 0.58 ± 0.02 was achieved for optical coherent stages.
Abstract: We derive an experimentally testable criterion for the teleportation of quantum states of continuous variables. This criterion is especially relevant to the recent experiment of Furusawa et al. where an input—output fidelity of 0.58 ± 0.02 was achieved for optical coherent stages. Our derivation demonstrates that fidelities greater than 1/2 could not have been achieved through the use of a classical channel alone; quantum entanglement was a crucial ingredient in the experiment.
176 citations
TL;DR: Some applications of quantum information theory to the analysis of quantum limits on measurement sensitivity are described, including superdense coding, fast database search, and the quantum Fourier transform.
Abstract: We describe some applications of quantum information theory to the analysis of quantum limits on measurement sensitivity. A measurement of a weak force acting on a quantum system is a determination of a classical parameter appearing in the master equation that governs the evolution of the system; limitations on measurement accuracy arise because it is not possible to distinguish perfectly among the different possible values of this parameter. Tools developed in the study of quantum information and computation can be exploited to improve the precision of physics experiments; examples include superdense coding, fast database search, and the quantum Fourier transform.
174 citations
TL;DR: This work presents a simple all-optical implementation of Grover's algorithm for efficient searching, in which a database of four elements is searched with a single query, and reduces the required number of optical elements from 24 to only 12.
Abstract: The essential operations of a quantum computer can be accomplished using solely optical elements, with different polarization or spatial modes representing the individual qubits. We present a simple all-optical implementation of Grover's algorithm for efficient searching, in which a database of four elements is searched with a single query. By `compiling' the actual setup, we have reduced the required number of optical elements from 24 to only 12. We discuss the extension to larger databases, and the limitations of these techniques.
174 citations
TL;DR: In this article, the authors consider the partial teleportation of entanglement in the noisy environment, employing the Werner-state representation of the noisy channel for the simplicity of calculation, and find that the fidelity becomes smaller as the initial state is entangled more for a given entenglement of the quantum channel.
Abstract: Partial teleportation of entanglement is to teleport one particle of an entangled pair through a quantum channel. This is conceptually equivalent to quantum swapping. We consider the partial teleportation of entanglement in the noisy environment, employing the Werner-state representation of the noisy channel for the simplicity of calculation. To have the insight of the many-body teleportation, we introduce the measure of correlation information and study the transfer of the correlation information and entanglement. We find that the fidelity becomes smaller as the initial state is entangled more for a given entanglement of the quantum channel. The entangled channel transfers at least some of the entanglement to the final state.
167 citations
TL;DR: In this paper, a family of asymmetric cloning machines for N-dimensional quantum states is introduced, and the trade-off between the quality of the copies imposed by quantum mechanics is shown to result from a complementarity akin to the Heisenberg uncertainty principle.
Abstract: A family of asymmetric cloning machines for N-dimensional quantum states is introduced. These machines produce two imperfect copies of a single state that emerge from non-identical Heisenberg channels. The trade-off between the quality of the copies imposed by quantum mechanics is shown to result from a complementarity akin to the Heisenberg uncertainty principle. More specifically, the probability distributions of the error operators affecting the two copies are the square modulus of two functions related by a Fourier transform. A no-cloning inequality is derived for the special case of isotropic cloners, quantifying the impossibility of perfect cloning: if π a and π b are the depolarizing fractions associated with the two copies, the domain in (π1/2 a , π1/2 b )-space located inside a particular ellipse representing close-to-perfect cloning is forbidden. More generally, an entropic no-cloning uncertainty relation is also discussed. Finally, the class of asymmetric cloning machines for quantum b...
157 citations
TL;DR: The spectrally selective optical properties of wavelength selective radiation emitters and filters based on periodically microstructured metal surfaces were investigated in this article, where the angle dependent emissivity of tungsten gratings with periods of 1.4 μm and 2.0 μm was measured.
Abstract: The spectrally selective optical properties of wavelength selective radiation emitters and filters based on periodically microstructured metal surfaces were investigated. Metal surfaces were structured by the use of a holographic mask and subsequent etching processes. Due to the microstructure, thermally excited surface plasmons couple to electromagnetic radiation. Therefore a structured tungsten surface can act as a selective radiation emitter. The calculation of the absorptance by a rigorous diffraction theory allows the prediction of the emissivity of such structures. The angle dependent emissivity of tungsten gratings with periods of 1.4 μm and 2.0 μm was measured. A peak emissivity of 70% at a wavelength of 1.6 μm was achieved. Band pass filters for the near infrared spectral range based on perforated metal films were investigated theoretically and experimentally. Filters with a grating period of 2.0 μm were produced. A peak transmittance of 80% at a wavelength 2.9 μm was achieved. The optic...
152 citations
TL;DR: In this article, a focused laser beam was used to stir a Bose-Einstein condensate of 87Rb atoms confined in a magnetic trap and observed the formation of a single vortex for a stirring frequency exceeding a critical value.
Abstract: We stir with a focused laser beam a Bose-Einstein condensate of 87Rb atoms confined in a magnetic trap. We observe the formation of a single vortex for a stirring frequency exceeding a critical value. At larger rotation frequencies we produce states of the condensate for which up to eleven vortices are simultaneously present. We present measurements of the decay of a vortex array once the stirring laser beam is removed.
TL;DR: In this paper, the authors use a computational model to investigate the different routes by which power is lost by an optical emitter placed in a microcavity environment, emphasizing the important role of the orientation of the dipole moment of the emitters.
Abstract: We use a computational model to investigate the different routes by which power is lost by an optical emitter placed in a microcavity environment. We make a quantitative investigation of the 1/2n 2 model used by many workers in evaluating the fraction of power radiated from a thin film of emitting material. We show the limitations of the 1/2n 2 model, emphasizing the important role of the orientation of the dipole moment of the emitters. Multi-layer systems, involving dielectric Bragg stack reflectors and metal mirrors, are compared for their efficiency in producing useful radiation. We consider both a standard Bragg reflector and the recently developed omni-directional Bragg stack. We show that metal mirrors, although lossy, may still be effective for producing useful radiation from microcavities. We focus our attention on parameters appropriate for organic microcavity light emitting diode structures.
TL;DR: In this paper, the authors consider some promising future directions for quantum information theory that could influence the development of 21st century physics and propose new strategies for improving the precision of quantum-limited measurements.
Abstract: I consider some promising future directions for quantum information theory that could influence the development of 21st century physics. Advances in the theory of the distinguishability of superoperators may lead to new strategies for improving the precision of quantum-limited measurements. A better grasp of the properties of multi-partite quantum entanglement may lead to deeper understanding of strongly-coupled dynamics in quantum many-body systems, quantum field theory, and quantum gravity.
TL;DR: In this article, the Hilbert-Schmidt distance between two arbitrary normalizable states is discussed as a measure of the similarity of the states and the connection to other definitions of the non-classicality of states are discussed.
Abstract: The Hilbert—Schmidt distance between two arbitrary normalizable states is discussed as a measure of the similarity of the states. Unitary transformations of both states with the same unitary operator (e.g. the displacement of both states in the phase plane by the same displacement vector and squeezing of both states) do not change this distance. The nearest distance of a given state to the whole set of coherent states is proposed as a quantitative measure of non-classicality of the state which is identical when considering the coherent states as the most classical ones among pure states and the deviations from them as non-classicality. The connection to other definitions of the non-classicality of states is discussed. The notion of distance can also be used for the definition of a neighbourhood of considered states. Inequalities for the distance of states to Fock states are derived. For given neighbourhoods, they restrict common characteristics of the state as the dispersion of the number operato...
TL;DR: In this article, a method for modelling the complicated soft tissue, based on the fractal approach, permitting numerical evaluation of the phase functions and four optical properties of tissue (scattering coefficient, reduced scattering coefficient, backscattering coefficient, and anisotropy factor) by the use of the Mie scattering theory.
Abstract: A knowledge of the local refractive index variations and size distribution of scatterers in biological tissue is required to understand the physical processes involved in light-tissue interaction. This paper describes a method for modelling the complicated soft tissue, based on the fractal approach, permitting numerical evaluation of the phase functions and four optical properties of tissue—scattering coefficient, reduced scattering coefficient, backscatter-ing coefficient, and anisotropy factor—by the use of the Mie scattering theory. A key assumption of the model is that refractive index variations caused by microscopic tissue elements can be treated as particles with size distribution according to the power law. The model parameters, such as refractive index, incident wavelength, and fractal dimension, that are likely to affect the predictions of optical properties are investigated. The results suggest that the fractal dimension used to describe how biological tissue can be approximated by par...
TL;DR: In this article, an alternative definition of squeezing is presented for this system, based on information entropy theory, which overcomes the disadvantages of the definition based on the Heisenberg uncertainty relation.
Abstract: The usual definition of squeezing, based on the Heisenberg uncertainty principle, measures uncertainty in terms of the standard deviation. It can run into difficulties when applied to squeezing in the two-level atom. An alternative definition of squeezing is presented for this system, based on information entropy theory, which overcomes the disadvantages of the definition based on the Heisenberg uncertainty relation. The utility of this definition is illustrated by examining squeezing in the information entropy of a two-level atom in the Jaynes-Cummings model, and in resonance fluorescence.
TL;DR: The performance of fault-tolerant quantum computation with concatenated codes using local gates in small numbers of spatial dimensions is discussed in this article, where it is shown that a threshold result still exists in three, two, or one spatial dimensions when next-to-nearest-neighbour gates are available, and explicit constructions are presented.
Abstract: The performance of fault-tolerant quantum computation with concatenated codes using local gates in small numbers of spatial dimensions is discussed. It is shown that a threshold result still exists in three, two, or one spatial dimensions when next-to-nearest-neighbour gates are available, and explicit constructions are presented. In two or three dimensions, it is also shown how nearest-neighbour gates can give a threshold result. In all cases, it is simply demonstrated that a threshold exists, and no attempt to optimize the error correction circuit or to determine the exact value of the threshold is made. The additional overhead due to the fault-tolerance in both space and time is polylogarithmic in the error rate per logical gate.
TL;DR: In this paper, the authors developed a method to entangle neutral atoms using cold controlled collisions. And they analyzed this method in two particular set-ups: optical lattices and magnetic microtraps, which offer the possibility of performing certain multi-particle operations in parallel.
Abstract: We develop a method to entangle neutral atoms using cold controlled collisions. We analyse this method in two particular set-ups: optical lattices and magnetic microtraps. Both offer the possibility of performing certain multi-particle operations in parallel. Using this fact, we show how to implement efficient quantum error correction and schemes for fault-tolerant computing.
TL;DR: In this paper, the spatial and temporal characterizations of phase fluctuations in non-Kolmogorov turbulence were analyzed and the generalized coherence length rho(0), the characteristic frequency f(R), and the characteristic time tau(R) were derived.
Abstract: Atmospheric turbulence severely limits the performance of ground-based imaging and laser propagation systems. Some observational results, showing atmospheric turbulence which does not obey Kolmogorov's theory, have prompted the study of optical propagation through non-Kolmogorov turbulence. This paper presents a theoretical approach to analyse the spatial and temporal characterizations of phase fluctuations in non-Kolmogorov turbulence. The spatial structure function, the temporal structure function and the temporal power spectrum of phase fluctuations are derived. The generalized coherence length rho(0), the characteristic frequency f(R) and the characteristic time tau(R) are expressed as functions of the index structure constant along the propagation path and the wind velocity. The long exposure MTF, the short exposure MTF and the imaging Strehl ratio are computed.
TL;DR: This work investigates the case when the prior entanglement is not maximal and the initial state of the entangled pair of qubits being used for the dense coding is a mixed state, and finds upper and lower bounds on the capability to do dense coding in terms of the various measures ofEntanglement.
Abstract: Ideal dense coding protocols allow one to use prior maximal entanglement to send two bits of classical information by the physical transfer of a single encoded qubit. We investigate the case when the prior entanglement is not maximal and the initial state of the entangled pair of qubits being used for the dense coding is a mixed state. We find upper and lower bounds on the capability to do dense coding in terms of the various measures of entanglement. Our results can also be reinterpreted as giving bounds on purification procedures in terms of dense coding capacities.
TL;DR: In this article, two observers (Alice and Bob) independently prepare two sets of singlets and send one particle of each singlet along an arbitrarily chosen direction and send the other particle to a third observer, Eve.
Abstract: Two observers (Alice and Bob) independently prepare two sets of singlets. They test one particle of each singlet along an arbitrarily chosen direction and send the other particle to a third observer, Eve. At a later time, Eve performs joint tests on pairs of particles (one from Alice and one from Bob). According to Eve's choice of test and to her results, Alice and Bob can sort into subsets the samples that they have already tested, and they can verify that each subset behaves as if it consisted of entangled pairs of distant particles that have never communicated in the past, even indirectly via other particles.
TL;DR: This article presents a primer on the current state of knowledge concerning two problems of quantum information theory, and discusses the relation of these problems to one of the most challenging questions of linear algebra: the classification and characterization of positive operator maps.
Abstract: Quantum mechanics is already 100 years old, but remains alive and full of challenging open problems On one hand, the problems encountered at the frontiers of modern theoretical physics like quantum gravity, string theories, etc concern quantum theory, and are at the same time related to open problems of modern mathematics But even within non-relativistic quantum mechanics itself there are fundamental unresolved problems that can be formulated in elementary terms These problems are also related to challenging open questions of modern mathematics; linear algebra and functional analysis in particular Two of these problems will be discussed in this article: (a) the separability problem, ie the question when the state of a composite quantum system does not contain any quantum correlations or entanglement; and (b) the distillability problem, ie the question when the state of a composite quantum system can be transformed to an entangled pure state using local operations (local refers here to co
TL;DR: In this article, the authors discuss ways in which entanglement can be established within such dissipative environments, and make use of a strong interaction of the system with its environment to produce entagglement in a controlled way.
Abstract: Quantum information processing rests on our ability to manipulate quantum superpositions through coherent unitary transformations, and to establish entanglement between constituent quantum components of the processor. The quantum information processor (a linear ion trap, or a cavity confining the radiation field for example) exists in a dissipative environment. We discuss ways in which entanglement can be established within such dissipative environments. We can even make use of a strong interaction of the system with its environment to produce entanglement in a controlled way.
TL;DR: An improved ‘plug & play’ interferometric system allowing fast key exchange is introduced and original electronics implementing the BB84 protocol and allowing user-friendly operation is presented.
Abstract: Some guidelines for the comparison of different quantum key distribution experiments are proposed. An improved ‘plug & play’ interferometric system allowing fast key exchange is introduced. Self-alignment and compensation of birefringence remain. Original electronics implementing the BB84 protocol and allowing user-friendly operation is presented. Key creation with 0.1 photon per pulse at a rate of 486 Hz with a 5.4% QBER, corresponding to a net rate of 210Hz, over a 23 Km installed cable was performed.
TL;DR: In this article, a simple expression for the retrodictive density operator is derived on the basis of Bayes' theorem for the premeasurement state associated with the result of any measurement.
Abstract: We derive on the basis of Bayes' theorem a simple but general expression for the retrodicted premeasurement state associated with the result of any measurement. The retrodictive density operator is the normalized probability operator measure element associated with the result. We examine applications to quantum optical cryptography and to the optical beam splitter.
TL;DR: It is shown that any bound entangled state can be used for the construction of a map corresponding to the binding entanglement channel, and this class of channels is provided.
Abstract: We define the binding entanglement channel as the quantum channel through which quantum information cannot be reliably transmitted, but which can be used to share bound entanglement. We provide a characterization of this class of channels. We also show that any bound entangled state can be used for the construction of a map corresponding to the binding entanglement channel.
TL;DR: The theory of interaction between an arbitrary electromagnetic shaped beam and a sphere with an eccentrically located spherical inclusion is presented in this article, which is built as a synthesis between two available theories (i) the generalized Lorenz-Mie theory for a homogeneous sphere (illuminated by an arbitrary shaped beam) and (ii) the theory for interaction between a plane wave and a dielectric sphere.
Abstract: The theory of interaction between an arbitrary electromagnetic shaped beam and a sphere with an eccentrically located spherical inclusion is presented. This theory is built as a synthesis between two available theories (i) the generalized Lorenz-Mie theory for a homogeneous sphere (illuminated by an arbitrary shaped beam) and (ii) the theory of interaction between a plane wave and an eccentrically stratified dielectric sphere.
TL;DR: In this paper, the authors demonstrate a method for 3D force calibration of optical tweezers by recording the trapping dynamics of polystyrene beads, which is realized by time-resolved detection of the horizontal and vertical position of a bead which is drawn to the focus of a laser beam.
Abstract: We demonstrate a method for 3-dimensional force calibration of optical tweezers by recording the trapping dynamics of polystyrene beads. This is realized by time-resolved detection of the horizontal and vertical position of a bead which is drawn to the focus of a laser beam. The method provides real time characterization of the force profile of an optical trap in all directions.
TL;DR: A protocol for quantum key distribution using three mutually non-orthogonal states that generates key bits most efficiently for three symmetric states is introduced.
Abstract: We introduce a protocol for quantum key distribution using three mutually non-orthogonal states. The protocol generates key bits most efficiently for three symmetric states. The generalized measurements which minimize the error probability and maximize the mutual information for such a system are known and can be implemented optically. We analyse eavesdropping strategies based on these optimal measurements.
TL;DR: In this paper, the role of entanglement for transmission of classical information through a memoryless depolarizing channel was analyzed and it was shown analytically that the mutual information cannot be increased by encoding classical bits into entangled states of two qubits.
Abstract: We analyse the role of entanglement for transmission of classical information through a memoryless depolarizing channel. Using the isotropic character of this channel we prove analytically that the mutual information cannot be increased by encoding classical bits into entangled states of two qubits.