Sequential Quantum Teleportation of Optical Coherent States

We demonstrate a sequence of two quantum teleportations of optical coherent states, combining two high-fidelity teleporters for continuous variables. In our experiment, the individual teleportation fidelities are evaluated as F_1 = 0.70 \pm 0.02 and F_2 = 0.75 \pm 0.02, while the fidelity between the input and the sequentially teleported states is determined as F^{(2)} = 0.57 \pm 0.02. This still exceeds the optimal fidelity of one half for classical teleportation of arbitrary coherent states and almost attains the value of the first (unsequential) quantum teleportation experiment with optical coherent states.Comment: 5page, 4figure

Quantum versus classical domains for teleportation with continuous variables

By considering the utilization of a classical channel without quantum entanglement, fidelity Fclassical=1/2 has been established as setting the boundary between classical and quantum domains in the teleportation of coherent states of the electromagnetic field [S. L. Braunstein, C. A. Fuchs, and H. J. Kimble, J. Mod. Opt. 47, 267 (2000)]. We further examine the quantum-classical boundary by investigating questions of entanglement and Bell-inequality violations for the Einstein-Podolsky-Rosen states relevant to continuous variable teleportation. The threshold fidelity for employing entanglement as a quantum resource in teleportation of coherent states is again found to be Fclassical=1/2. Likewise, violations of local realism onset at this same threshold, with the added requirement of overall efficiency η>2/3 in the unconditional case. By contrast, recently proposed criteria adapted from the literature on quantum-nondemolition detection are shown to be largely unrelated to the questions of entanglement and Bell-inequality violations

Quantum Repeaters using Coherent-State Communication

We investigate quantum repeater protocols based upon atomic qubit-entanglement distribution through optical coherent-state communication. Various measurement schemes for an optical mode entangled with two spatially separated atomic qubits are considered in order to nonlocally prepare conditional two-qubit entangled states. In particular, generalized measurements for unambiguous state discrimination enable one to completely eliminate spin-flip errors in the resulting qubit states, as they would occur in a homodyne-based scheme due to the finite overlap of the optical states in phase space. As a result, by using weaker coherent states, high initial fidelities can still be achieved for larger repeater spacing, at the expense of lower entanglement generation rates. In this regime, the coherent-state-based protocols start resembling single-photon-based repeater schemes.Comment: 11 pages, 8 figure

Broadband teleportation

Quantum teleportation of an unknown broadband electromagnetic field is investigated. The continuous-variable teleportation protocol by Braunstein and Kimble [Phys. Rev. Lett. {\bf 80}, 869 (1998)] for teleporting the quantum state of a single mode of the electromagnetic field is generalized for the case of a multimode field with finite bandwith. We discuss criteria for continuous-variable teleportation with various sets of input states and apply them to the teleportation of broadband fields. We first consider as a set of input fields (from which an independent state preparer draws the inputs to be teleported) arbitrary pure Gaussian states with unknown coherent amplitude (squeezed or coherent states). This set of input states, further restricted to an alphabet of coherent states, was used in the experiment by Furusawa {\it et al.} [Science {\bf 282}, 706 (1998)]. It requires unit-gain teleportation for optimizing the teleportation fidelity. In our broadband scheme, the excess noise added through unit-gain teleportation due to the finite degree of the squeezed-state entanglement is just twice the (entanglement) source's squeezing spectrum for its ``quiet quadrature.'' The teleportation of one half of an entangled state (two-mode squeezed vacuum state), i.e., ``entanglement swapping,'' and its verification are optimized under a certain nonunit gain condition. We will also give a broadband description of this continuous-variable entanglement swapping based on the single-mode scheme by van Loock and Braunstein [Phys. Rev. A {\bf 61}, 10302 (2000)]Comment: 27 pages, 7 figures, revised version for publication, Physical Review A (August 2000); major changes, in parts rewritte

Quantum information with continuous variables

Quantum information is a rapidly advancing area of interdisciplinary research. It may lead to real-world applications for communication and computation unavailable without the exploitation of quantum properties such as nonorthogonality or entanglement. We review the progress in quantum information based on continuous quantum variables, with emphasis on quantum optical implementations in terms of the quadrature amplitudes of the electromagnetic field.Comment: accepted for publication in Reviews of Modern Physic

Criteria for Continuous-Variable Quantum Teleportation

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. [Science 282, 706-709 (1998)] where an input-output fidelity of $0.58 \pm 0.02$ was achieved for optical coherent states. 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.Comment: 12 pages, to appear in Journal of Modern Optic

Bichat guidelines for the clinical management of brucellosis and bioterrorism-related brucellosis

Interest in Brucella species as a biological weapon stems from the fact that airborne transmission of the agent is possible. It is highly contagious and enters through mucous membranes such as the conjunctiva, oropharynx, respiratory tract and skin abrasions. It has been estimated that 10-100 organisms only are sufficient to constitute an infectious aerosol dose for humans. Signs and symptoms are similar in patients whatever the route of transmission and are mostly non-specific. Symptoms of patients infected by aerosol are indistinguishable from those of patients infected by other routes. Regimens containing doxycycline plus streptomycin or doxycycline plus rifampin are effective for most forms of brucellosis. Isolation of patients is not necessary. Trimethoprim-sulfamethoxazole and fluoroquinolones also have good results against Brucella, but are associated with high relapse rates when used as monotherapy. The combination of ofloxacin plus rifampicin is associated with good results. Even if there is little evidence to support its utility for post-exposure prophylaxis, doxycycline plus rifampicin is recommended for 3 to 6 weeks

Quantum repeaters and quantum key distribution: analysis of secret key rates

We analyze various prominent quantum repeater protocols in the context of long-distance quantum key distribution. These protocols are the original quantum repeater proposal by Briegel, D\"ur, Cirac and Zoller, the so-called hybrid quantum repeater using optical coherent states dispersively interacting with atomic spin qubits, and the Duan-Lukin-Cirac-Zoller-type repeater using atomic ensembles together with linear optics and, in its most recent extension, heralded qubit amplifiers. For our analysis, we investigate the most important experimental parameters of every repeater component and find their minimally required values for obtaining a nonzero secret key. Additionally, we examine in detail the impact of device imperfections on the final secret key rate and on the optimal number of rounds of distillation when the entangled states are purified right after their initial distribution.Comment: Published versio

Bichat guidelines for the clinical management of brucellosis and bioterrorism-related brucellosis

Interest in Brucella species as a biological weapon stems from the fact that airborne transmission of the agent is possible. It is highly contagious and enters through mucous membranes such as the conjunctiva, oropharynx, respiratory tract and skin abrasions. It has been estimated that 10-100 organisms only are sufficient to constitute an infectious aerosol dose for humans. Signs and symptoms are similar in patients whatever the route of transmission and are mostly non-specific. Symptoms of patients infected by aerosol are indistinguishable from those of patients infected by other routes. Regimens containing doxycycline plus streptomycin or doxycycline plus rifampin are effective for most forms of brucellosis. Isolation of patients is not necessary. Trimethoprim-sulfamethoxazole and fluoroquinolones also have good results against Brucella, but are associated with high relapse rates when used as monotherapy. The combination of ofloxacin plus rifampicin is associated with good results. Even if there is little evidence to support its utility for post-exposure prophylaxis, doxycycline plus rifampicin is recommended for 3 to 6 weeks