Protocols for probabilistic entanglement-assisted quantum teleportation and for entanglement swapping of material qubits are presented. They are based on a protocol for postselective Bellstate projection which is capable of projecting two material qubits onto a Bell state with the help of ancillary coherent multiphoton states and postselection by balanced homodyne photodetection. Provided this photonic postselection is successful we explore the theoretical possibilities of realizing unit fidelity quantum teleportation and entanglement swapping with 25% success probability. This photon-assisted Bell projection is generated by coupling almost resonantly the two material qubits to single modes of the radiation field in two separate cavities in a Ramsey-type interaction sequence and by measuring the emerged field states in a balanced homodyne detection scenario. As these quantum protocols require basic tools of quantum state engineering of coherent multiphoton states and balanced homodyne photodetection they may offer interesting perspectives in particular for current quantum optical applications in quantum information processing.
We investigate the quantum electrodynamics of a single two-level atom located at the focus of a parabolic cavity. We first work out the modifications of the spontaneous emission induced by the presence of this boundary in the optical regime, where the dipole and the rotating-wave approximations apply. Furthermore, the single-photon state that leaves the cavity asymptotically is determined. The corresponding time-reversed single-photon quantum state is capable of exciting the atom in this extreme multimode scenario with near-unit probability. Using semiclassical methods, we derive a photon-path representation for the relevant transition amplitudes and show that it constitutes a satisfactory approximation for a wide range of wavelengths
We explore possibilities of entangling two distant material qubits with the help of an optical radiation field in the regime of strong quantum electrodynamical coupling with almost resonant interaction. For this purpose the optimum generalized field measurements are determined which are capable of preparing a two-qubit Bell state by postselection with minimum error. It is demonstrated that in the strong-coupling regime some of the recently found limitations of the non-resonant weakcoupling regime can be circumvented successfully due to characteristic quantum electrodynamical quantum interference effects. In particular, in the absence of photon loss it is possible to postselect two-qubit Bell states with fidelities close to unity by a proper choice of the relevant interaction time. Even in the presence of photon loss this strong-coupling regime offers interesting perspectives for creating spatially well-separated Bell pairs with high fidelities, high success probabilities, and high repetition rates which are relevant for future realizations of quantum repeaters.
The Born-Markov master equation analysis of the vibrating mirror and photon experiment proposed by Marshall, Simon, Penrose and Bouwmeester is completed by including the important issues of temperature and friction. We find that at the level of cooling available to date, visibility revivals are purely classical, and no quantum effect can be detected by the setup, no matter how strong the photon-mirror coupling is. Checking proposals of universal nonenvironmental decoherence is ruled out by dominating thermal decoherence; a conjectured coordinate-diffusion contribution to decoherence may become observable on reaching moderately low temperatures.PACS numbers: 03.65. Ta, 42.50.Xa, 03.65.Yz The nature of the quantum-classical border along the mass scale is still poorly defined. There remain some 10 orders of magnitude unexplored between the heaviest molecules for which c.o.m. interference has been observed [1], and the lightest nanomechanical objects, for which no quantum behavior has been seen [2]. In trying to close the gap top down, the primary experimental task is to find firm evidence, never seen so far, that the spatial motion of a mass as large as a nanomechanical object does follow the Schrödinger equation, notwithstanding environmental interactions, or noise, which would quickly decohere the wave function. Only having succeeded in suppressing that effect so that interference of a heavy object is detected beyond any doubt, can we turn to checking the presence of spontaneous (also called universal or intrinsic) decoherence [3] on top of the environmental one.An experimentally accessible system with potentialities to achieve the above goal is a photon in a high-quality resonating cavity, coupled by its radiation pressure to a nanomechanical oscillator, carrying one of the mirrors that close the cavity. After pioneering experiments [2] which did not detect any quantum effect on the mirror, as well as thoughtful theoretical analyses [4], a promising idea appeared for bridging the frequency gap and carrying out a genuine quantum test [5,6]. In that proposal, the vibrating mirror closes an optical cavity in arm A of a Michelson interferometer, arm B having another cavity with fixed mirrors. The vibrating mirror is expected to become entangled with a single photon traveling along both arms, the mirror being split into a kind of Schrödinger cat doublet. The interference of the photon is detected with the scope of extracting information about the quantum motion of the mirror. Since the vibrations of the mirror are much slower than the frequency of light, a shift of the interference pattern would be unobservable; the good chance is to record the visibility which is modulated by the motion of the mirror, creating revivals as the components of the superposition overlap again and again.Highly worth doing as it is, this is a very hard experiment, for various reasons. One thing is that high-quality optical resonators are needed to keep the photon alive for several, or at least one, return of the mirror; a less familiar ...
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