976 publications from this institution
We consider the Bennett-Brassard cryptographic scheme, which uses two conjugate quantum bases. An eavesdropper who attempts to obtain information on qubits sent in one of the bases causes a disturbance to qubits sent in the other basis. We derive an upper bound to the accessible information in one basis, for a given error rate in the conjugate basis. Independently fixing the error rate in the conjugate bases, we show that both bounds can be attained simultaneously by an optimal eavesdropping probe, consisting of two qubits. The qubits' interaction and their subsequent measurement are described explicitly. These results are combined to give an expression for the optimal information an eavesdropper can obtain for a given average disturbance when her interaction and measurements are performed signal by signal. Finally, the relation between quantum cryptography and violations of Bell's inequalities is discussed.
A Reply to the Comment by T. Durt.Received 24 August 2000DOI:https://doi.org/10.1103/PhysRevLett.86.1393©2001 American Physical Society
A Reply to the Comment by Pavel Bóna.Received 6 February 2003DOI:https://doi.org/10.1103/PhysRevLett.90.208902©2003 American Physical Society
Quantum mechanics is well known for being counter-intuitive or even bizarre. Now, it can also be useful for practical applications. Quantum cryptography could be the first application of quantum mechanics at the individual quanta level. It takes advantage ofthe Heisenberg uncertainties to provide an absolutely secure communication scheme.
A family of two spin 1 2 states with the following properties is presented. These states are “local” in the sense that they do not violate any Bell—CHSH inequality. However, after each spin interacts with two independent local environments, the resulting states of the two spin 1 2 systems violate a Bell inequality. It is argued that: (1) The problem of classifying the nonlocal states of two spin 1 2 systems is still open. In particular, for mixed states the violation of Bell's inequality and the concept of nonlocality differ, (2) The fact that some dissipative environments can increase quantum correlations might be useful for quantum computation. (3) Careless application of generalized quantum measurements can violate Bell's inequality by more than 2√2, even for mixtures of product states.
No abstract is provided for this article.
Due to the statistical nature of the evolution of the polarization state in optical fibers and components, it is difficult to measure the global polarization dependent loss (PDL) of a series of concatenated components, each having a different PDL value. The global PDL of concatenated components with PDL cannot be obtained by a simple addition of the single PDL values, moreover, it requires a statistical description. In the present work the statistics of the PDL resulting from the concatenation of several components with PDL, connected by standard optical fibers or by elements having some polarization mode dispersion (PMD) is discussed. Simulations and experimental results supporting the existing theory are presented.
No abstract is provided for this article.
Characterization of single-mode optical fibers requires a basis for time, wavelength and polarization.Parametric downconversion in non-linear crystals naturally provide pairs of photons extremelyhighly correlated in time, energy (thus wavelength) and polarization. It is thus tempting to exploresuch photon pairs for fiber and fiber device characterization. Historically, entangled photon pairswhere first used in delicate tests of quantum mechanics [1]. Indeed, their correlation is higher thanclassically possible! However, nowaday photon pair sources can be made cheap and compact enoughto offer practical alternatives for many of the traditional measurement schemes. In addition photonpairs provide entirely new possibilities and open the door to new developments in metrology. Thepurpose of this contribution is first to present some concrete proposals and results along the abovelines, and next to draw the attention of the audience to these rather revolutionary new combinationsof quantum optics and communication.
After carrying out a protocol for quantum key agreement over a noisy quantum channel, the parties Alice and Bob must process the raw key in order to end up with identical keys about which the adversary has virtually no information. In principle, both classical and quantum protocols can be used for this processing. It is a natural question which type of protocols is more powerful. We prove for general states but under the assumption of incoherent eavesdropping that Alice and Bob share some so-called intrinsic information in their classical random variables, resulting from optimal measurements, if and only if the parties' quantum systems are entangled. In addition, we provide evidence that the potentials of classical and of quantum protocols are equal in every situation. Consequently, many techniques and results from quantum information theory directly apply to problems in classical information theory, and vice versa. For instance, it was previously believed that two parties can carry out unconditionally secure key agreement as long as they share some intrinsic information in the adversary's view. The analysis of this purely classical problem from the quantum information-theoretic viewpoint shows that this is true in the binary case, but false in general. More explicitly, bound entanglement, i.e., entanglement that cannot be purified by any quantum protocol, has a classical counterpart. This "bound intrinsic information" cannot be distilled to a secret key by any classical protocol. As another application we propose a measure for entanglement based on classical information-theoretic quantities.
