Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B15: Quantum Entanglement I |
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Sponsoring Units: GQI Chair: Dagmar Bruss, Heinrich-Heine-Universitae Duesseldorf Room: Morial Convention Center 207 |
Monday, March 10, 2008 11:15AM - 11:27AM |
B15.00001: Hyperentanglement-assisted dense coding: Beating the channel-capacity ``limit'' Julio T. Barreiro, Tzu-Chieh Wei, Paul G. Kwiat Dense coding was the very first quantum information protocol to be proposed and experimentally realized more than a decade ago. Today, however, the achieved channel capacity (CC) remains fundamentally limited as conceived for photons using linear optics: Alice can only decode three of four potential messages sent by Bob, due to the impossibility to deterministically resolve all four Bell states using conventional techniques; the maximum CC is thus limited to $\log_2 3 \approx 1.585$ bits. However, when the particles encoding the Bell pair are entangled in an additional degree of freedom, the complete and deterministic discrimination of all Bell states is possible. Via the process of spontanteous parametric down conversion, we produce photon pairs simultaneously entangled in polarization and orbital angular momentum. Using the auxiliary entanglement and a robust intermode coupling scheme, our experiment achieves CC=1.624(7) bits, the first to surpass the CC ``limit'' for linear photonic superdense coding. Our encoding is suited for quantum communication without alignment and satellite to satellite communication. Additionally, our scheme also enables the remote preparation of single-photon highly-entangled states. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B15.00002: Testing Quantum Devices: Practical Entanglement Verification in Bipartite Optical Systems Hauke Haseler, Tobias Moroder, Norbert Lutkenhaus We present a method to test quantum behavior of quantum information processing devices, such as quantum memories, teleportation devices, channels and quantum key distribution protocols. The test of quantum behavior can be phrased as the verification of effective entanglement. Necessary separability criteria are formulated in terms of a matrix of expectation values in conjunction with the partial transposition map. Our method is designed to reduce the resources for entanglement verification. A particular protocol based on coherent states and homodyne detection is used to illustrate the method. A possible test for the quantum nature of memories using two non- orthogonal signal states arises naturally. Furthermore, closer inspection of the measurement process in terms of the Stokes operators reveals a security threat for quantum key distribution involving phase reference beams. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B15.00003: Transmission of photonic Bell states over a 2x32dB, 144km free-space link Alessandro Fedrizzi, R. Ursin, T. Herbst, M. Nespoli, R. Prevedel, T. Scheidl, F. Tiefenbacher, T. Jennewein, Anton Zeilinger We successfully transmitted both photons of various Bell states over a 144 km free-space link between the islands of Tenerife and La Palma. Creating and transmitting more than $6\times10^6$ highly entangled photon pairs/s over the $2\times32$ dB channel we received $0.07$ pairs/s at the receiver. We were able to distinguish between $\left|\psi^-\right\rangle$ and $\left|\psi^+\right\rangle$ states and verified the presence of entanglement by violating a CHSH Bell inequality to $S=2.61\pm0.11$, 5 standard deviations above the classical limit of 2. Using a small and compact photon source, we effectively emulate quantum communication in a loss regime comparable to a two-link satellite communication scenario. Furthermore, we convincingly demonstrate the feasibility of 2-photon quantum communication protocols like dense coding, teleportation or quantum cryptography without reference frame over long distance. Finally, with a flight time of 0.5 ms the transmitted Bell states are the longest lived photonic Bell states ever reported. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B15.00004: Correlated photon generation via four-wave mixing in a birefringent semiconductor waveguide Daniel Rogers, Julius Goldhar, Christopher Richardson, Alessandro Restelli, Joshua Bienfang, Charles Clark The next generation of quantum cryptography will benefit from a fast and practical source of entangled photon pairs. Current methods of generating entanglement, whether in bulk nonlinear crystals or microstructure optical fibers, pose significant challenges to integration into fieldable quantum communications systems. In order to meet the demands of speed and practicality, we investigate third-order nonlinearity in a semiconductor waveguide as a source of correlated and ultimately entangled photon pairs. This device offers the advantages of a fast nonlinear response based on the optical Kerr effect and the relative ease of coupling to standard optical fibers. It is potentially useful for free-space and fiber-optic quantum key distribution as well as a host of other applications such as correlated photon metrology and two-photon interferometry. We show the feasibility of using phase-matched four-wave mixing in a birefringent AlGaAs waveguide to generate correlated photon pairs at wavelengths compatible with silicon detectors. We demonstrate the operation of such a device, consider the effects of loss and two-photon absorption, and evaluate the implications of birefringent phase matching. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B15.00005: Macroscopic quantum memories get entangled from far away Zhen-Sheng Yuan, Yu-Ao Chen, Shuai Chen, J\"{o}rg Schmiedmayer, Jian-Wei Pan We report an experimental implementation of a fundamental unit for long-distance quantum communication. By means of entanglement swapping, entanglement is generated between two remote atomic ensembles connected with either 6 m or 300 m fibre-based optical channel, where the flying qubits--two emitted photons from the atomic ensembles are sent to an intermediate station for a joint Bell-state measurement. Afterwards, the measurement induced entanglement between the atomic ensembles are verified by the violation of Bell's inequality or by an entanglement witness. The striking features, phase-error insensitiveness and scalable flexibility, promise the present setup as a fundamental unit for future quantum information processing. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B15.00006: Long-distance atom-photon entanglement and its coherence properties H. Weinfurter, M. Weber, J. Volz, W. Rosenfeld, M. Krug, F. Hocke, F. Henkel The distribution of entanglement between quantum memories at remote locations is one major challenge for the first demonstration of a quantum repeater. Entanglement between matter and light [1] is crucial for achieving this task. Here we report the observation of entanglement between a single trapped atom and a single photon, separated 300 m via an optical fiber. The entanglement is verified by appropriate correlation measurements of the atom-photon pair after communicating the photon through the fiber. In addition we measured the temporal evolution of the atomic density matrix after projecting the atom-photon pair via a state measurement of the photon onto a well defined atomic spin state. We find that the atomic Zeeman qubit decoheres after 100 $\mu$s. Our results represent important steps towards the realization of entanglement between single neutral atoms at distances of several 100 m. \newline [1] J. Volz, M. Weber, D. Schlenk et al., Phys. Rev. Lett. {\bf 96}, 030404 (2006). [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B15.00007: Entanglement dynamics in a qubit-nanomechanical resonator system Dian Wahyu Utami, Aashish Clerk Over the recent years, finding signatures of entanglement in macroscopic systems has been a central goal in many aspects of mesoscopic physics. Here we present our study of non-equilibrium entanglement dynamics between a qubit and a nanomechanical resonator that is coupled to a phonon bath. Unlike previous studies, we specifically look at dispersively coupled qubit-oscillator system which has been realized in many different experiments. Using master equation expressed in gaussian wigner functions, we obtained an elegant and intuitive expression for the entanglement. Two ways of generating entanglement were found; entanglement of the qubit to the amplitude of the driven oscillator and to the phase of the oscillator. A full analytical result within the zero temperature limit of the two cases were derived. We also investigate the effect of finite temperature to the entanglement dynamics and found the phase entanglement to be more robust against temperature. The result indicates that an indirect method to measure the presence of entanglement in the system is possible. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B15.00008: Generation of Entanglement Outside of the Light Cone James Franson The probability amplitude to emit a photon at one location and then annihilate it at another location is proportional to the Feynman propagator, which has nonzero values outside of the forward light cone. This does not allow messages to be transmitted faster than light, but it does allow correlations, entanglement, and mutual information to be generated outside the light cone. These effects are illustrated by considering two distant atoms, one of which is initially in its excited state and the other in its ground state. The probability amplitude for the two atoms to exchange a photon and make a transition to the other state is calculated using perturbation theory and commutator techniques, which gives a result proportional to the Feynman propagator. These effects can be interpreted as being due to the propagation of virtual photons outside of the light cone or as a transfer of pre-existing entanglement from the quantum vacuum. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B15.00009: Towards single time-bin entangled photons from a quantum dot. Christophe Couteau, Gregor Weihs We present our results on photon statistics of quantum dots. We show evidence for photon antibunching and cross-correlation measurements within the biexciton-exciton cascade. We also discuss directions towards a demonstration of time-bin entangled photons from a dot. Recently, it has been showed theoretically that a quantum dot could provide pairs of time-bin entangled photons. The temporal entanglement is due to the fact that we know the twin photons will come in pairs but we don't know when: either both the photons are really or both are late. Under the right conditions and using a double excitation of the dot corresponding to the two time bins, Simon and Poizat showed that one could get deterministic emission of pairs for time-bin entanglement using a single quantum dot. We will present our latest results on photon statistics and our demonstration of photon antibunching as well as cross-correlations within the biexciton-exciton cascade using an original new set-up. Based on these measurements we are confident that we will be able to demonstrate time-bin entangled photons in the near future. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B15.00010: Teleportation and Broadcasting of continuous variable entanglement Archan S. Majumdar, Satyabrata Adhikari, N. Nayak We present the first example for broadcasting of the entanglement of a two-mode squeezed state of the electromagnetic field shared by two distant parties into two nonlocal bipartite entangled states. Using the technique of covariance matrices we demonstrate the entanglement between the nonlocal output modes and the separability of the local output modes. We find the range of values for the squeezing parameter and the amplifier phase for which broadcasting of continuous variable entanglement can be implemented for physical states. We next present a scheme for teleporting two-mode entangled states of continuous variables from Alice to Bob. Our protocol is operationalized through the creation of a four-mode entangled state shared by Alice and Bob using linear amplifiers and beam splitters. Teleportation of the entangled state proceeds with local operations and the classical communication of four bits. We compute the fidelity of teleportation and find that it exhibits a trade-off with the magnitude of entanglement of the resultant teleported state. [arXiv:0708.1869; arXiv:0710.2777] [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B15.00011: Entanglement structure of two-mode squeezed states in absorbing and amplifying environment Phoenix S.Y. Poon, C.K. Law We examine the structure of entanglement for two-mode squeezed states interacting with symmetric linear baths [1]. In Fock space, $\rho^{T_A}$ is observed to be maintaining a block diagonal form as the system evolves. We explicitly obtain the eigenvalues and eigenvectors of $\rho^{T_A}$ (the partial transposition of density matrix $\rho$) as a function of time. The decoherence induced by the baths are shown to destroy the degeneracy of $\rho^{T_A}$, leading to a set of eigenvectors for the construction of entanglement witness operators. We prove that the eigenvectors are time-independent, which is an indicator for the robustness of entanglement of two-mode squeezed states in the presence of noise. \newline \noindent [1] Phoenix S. Y. Poon and C. K. Law, Phys. Rev. A \textbf{76}, 012333 (2007). [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B15.00012: Quantum Second Law with Entanglement Ladislav Andrey It is proved rigorously that every quantum operation generating quantum entanglement states in the multipartite quantum system of qubits leads to the reduction of quantum entropy of whole system. By other words the quantum entanglement can be the source of quantum information. As the consequence the novel formulation of quantum second law is presented with possible applications to progressively developing quantum information science and quantum communications. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B15.00013: Entanglement and Standard Thermodynamic Relations Inti Sodemann, Alonso Botero We re-examine thermal properties of the standard (Boltzmann) canonical ensemble from the point of view of canonical typicality. In this approach, the thermodynamic system is described by the reduced density matrix of a random pure state from an energy-constrained total Hilbert space for the system and environment, with the entanglement entropy playing the role of thermodynamic entropy. We examine the average and variance of the entanglement entropy over all pure states of the restricted total Hilbert space, and show correspondence with the expected results of standard statistical mechanics in the limit of a large environment. We study the correlation between energy and entropy fluctuations, and show that the temperature can also be defined from a variational principle minimizing deviations from a microscopic version of the first law of thermodynamics, involving energy and entropy fluctuations. [Preview Abstract] |
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