D.-G. Welsch

The van der Waals energy of atomic systems near absorbing and dispersing bodies

Within the frame of macroscopic quantum electrodynamics in causal media, a rigorous derivation of the van der Waals energy of an atomic system in the presence of an arbitrary arrangement of dispersing and absorbing dielectric bodies is given. As an application, the van der Waals potential of an atom in the presence of a dispersing and absorbing sphere is derived. Closed expressions for the long-distance (retardation) and short-distance (non-retardation) limits are given, and the effect of material absorption is discussed.

Lossy purification and detection of entangled coherent states

In work by Howell and Yeazell [Phys. Rev. A 62, 012102 (2000)], a proposal is made to generate entangled macroscopically distinguishable states of two spatially separated traveling optical modes. After considering a preparation with nonperfect photodetection, we discuss a random mode of entanglement purification without manual intervention using a number of preparations transmitted over a lossy optical transmission line. An efficiency greater than 1/2 of the purification scheme is required to increase entanglement at all. Furthermore, we study a nondemolition configuration to measure the purity of the state as contrast of interference fringes in a double-slit setup. As a consequence of the relationship between purity and entanglement of formation resulting from treating the entangled coherent states as a state of a bipartite quantum system, the contrast of interference fringes provides a direct means to measure entanglement.

OUANTUM-STATE TRANSFORMATION BY DISPERSIVE AND ABSORBING FOUR-PORT DEVICES

The recently derived input-output relations for the radiation field at a dispersive and absorbing four-port device [T. Gruner and D.-G. Welsch, Phys. Rev. A 54, 1661 (1996)] are used to derive the unitary transformation that relates the output quantum state to the input quantum state, including radiation and matter and without placing frequency restrictions. It is shown that for each frequency the transformation can be regarded as a well-behaved SU(4) group transformation that can be decomposed into a product of U(2) and SU(2) group transformations. Each of them may be thought of as being realized by a particular lossless four-port device. If for narrow-bandwidth radiation far from the medium resonances the absorption matrix of the four-port device can be disregarded, the well-known SU(2) group transformation for a lossless device is recognized. Explicit formulas for the transformation of Fock-states and coherent states are given.

Continuous-variable quantum teleportation through lossy channels

The ultimate limits of continuous-variable single-mode quantum teleportation due to absorption are studied, with special emphasis on (quasi-) monochromatic optical fields propagating through fibers. It is shown that even if an infinitely squeezed two-mode squeezed vacuum were used, the amount of information that would be transferred quantum mechanically over a finite distance is limited and effectively approaches zero on a length scale that is much shorter than the (classical) absorption length. The state-dependent teleportation fidelity can be close to unity only for short distances. To realize the largest possibly fidelity, asymmetrical equipment must be used, where the source of the two-mode squeezed vacuum is nearer to Alice than to Bob and as a consequence the coherent displacement performed by Bob cannot be chosen independently of the transmission lengths.

Femtosecond photon echoes via non-Markovian dephasing

Abstract The photon echo effect in dye molecules in solution has not been studied since effective mechanisms for inhomogeneous line broadening are not expected. We show that photon echoes may occur on a femtosecond time scale when the observation times are shorter than the memory time of the heat bath, and hence non-Markovian features of the molecular dephasing may become relevant. The properties and the dependence of the echo on the characteristics of the non-Markovian dephasing process are studied.

Squeezed vacuum state generation by means of interferometry

Abstract Squeezed vacuum states, which are useful for the interferometric detection of gravitational waves, are proposed to be generated by interference of squeezed coherent states.

Measuring quantum state overlaps of traveling optical fields

Abstract We propose a detection scheme for measuring the overlap of the quantum state of a traveling-field mode with a reference quantum state that is an arbitrary superposition of a finite (and under realistic conditions small) number N of Fock states, |Ψ〉= ∑ n=0 N c n |n〉 . In the scheme, an array of maximally N beam splitters is used in which the signal mode is successively mixed with modes prepared in coherent states and photon-number measurements are performed. To illustrate the method, we discuss the measurement of the quantum phase and the detection of Schrodinger-cat-like states.

Quantum state conversion by cross-Kerr interaction

A generalized Mach–Zehnder-type interferometer equipped with cross-Kerr elements is proposed to convert N-photon truncated single-mode quantum states into (N + 1)-mode single-photon states, which are suitable for further state manipulation by means of beamsplitter arrays and ON/OFF-detections, and vice versa. Applications to the realization of unitary and non-unitary transformations, quantum state reconstruction and quantum telemanipulation are studied.

Conditional quantum-state engineering in repeated 2-photon down-conversion

Abstract.The U(1,1) and U(2) transformations realized by three-mode interaction in the respective parametric approximations are studied in conditional measurement, and the corresponding non-unitary transformation operators are derived. As an application, the preparation of single-mode quantum states using an optical feedback loop is discussed, with special emphasis on Fock-state preparation. For that example, the influence of non-perfect detection and feedback is alsoconsidered.

Theory of resonance fluorescence from vibronic systems

The problem of resonant interaction between an electromagnetic radiation field and a vibronic system, such as a molecule or a localized electronphonon system, is considered. The vibronic coupling is assumed to result from displaced and distorted potential energy surfaces for the vibrational motion in the electronic quantum states involved in the resonant interaction with the radiation field. Furthermore, the vibrational energy relaxation is taken into account without assumptions about Markovian-type relaxation. In order to study correlation properties of higher than second order in the electric field strength of the light emitted a method of calculation of the expectation value of an arbitrary product combination of effective time evolution operators of the vibronic system is developed. The calculations are performed by utilizing Feynmans disentangling and operator ordering techniques and yield closed solutions in the low-temperature limit. The theory is used for studying the intensity and the normally ordered intensity correlation function of the resonance fluorescence light from a two-atom molecule excited by a relatively weak laser field, which undergoes phase fluctuations. It is shown that the vibronic coupling can give rise to an anticorrelation effect which is substantially stronger than in the case of resonance fluorescence from an atom. For the case of small vibrational damping a general formula which directly connects the asymptotic long-time behaviour of the normalized intensity correlation function with the vibrational relaxation in the molecule is deduced. It should be convenient for the determination of very small vibrational decay rates.