Quantum 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.Comment: 24 pages, RevTe

Entanglement transformation at absorbing and amplifying four-port devices

Dielectric four-port devices play an important role in optical quantum information processing. Since for causality reasons the permittivity is a complex function of frequency, dielectrics are typical examples of noisy quantum channels, which cannot preserve quantum coherence. To study the effects of quantum decoherence, we start from the quantized electromagnetic field in an arbitrary Kramers--Kronig dielectric of given complex permittivity and construct the transformation relating the output quantum state to the input quantum state, without placing restrictions on the frequency. We apply the formalism to some typical examples in quantum communication. In particular we show that for entangled qubits the Bell-basis states $|\Psi^\pm>$ are more robust against decoherence than the states $|\Phi^\pm>$.Comment: 12 pages, revtex, 10 eps figures, minor corrections in Appendi

Field quantization for open optical cavities

We study the quantum properties of the electromagnetic field in optical cavities coupled to an arbitrary number of escape channels. We consider both inhomogeneous dielectric resonators with a scalar dielectric constant $\epsilon({\bf r})$ and cavities defined by mirrors of arbitrary shape. Using the Feshbach projector technique we quantize the field in terms of a set of resonator and bath modes. We rigorously show that the field Hamiltonian reduces to the system--and--bath Hamiltonian of quantum optics. The field dynamics is investigated using the input--output theory of Gardiner and Collet. In the case of strong coupling to the external radiation field we find spectrally overlapping resonator modes. The mode dynamics is coupled due to the damping and noise inflicted by the external field. For wave chaotic resonators the mode dynamics is determined by a non--Hermitean random matrix. Upon including an amplifying medium, our dynamics of open-resonator modes may serve as a starting point for a quantum theory of random lasing.Comment: 16 pages, added references, corrected typo

On the equivalence of the Langevin and auxiliary field quantization methods for absorbing dielectrics

Recently two methods have been developed for the quantization of the electromagnetic field in general dispersing and absorbing linear dielectrics. The first is based upon the introduction of a quantum Langevin current in Maxwell's equations [T. Gruner and D.-G. Welsch, Phys. Rev. A 53, 1818 (1996); Ho Trung Dung, L. Kn\"{o}ll, and D.-G. Welsch, Phys. Rev. A 57, 3931 (1998); S. Scheel, L. Kn\"{o}ll, and D.-G. Welsch, Phys. Rev. A 58, 700 (1998)], whereas the second makes use of a set of auxiliary fields, followed by a canonical quantization procedure [A. Tip, Phys. Rev. A 57, 4818 (1998)]. We show that both approaches are equivalent.Comment: 7 pages, RevTeX, no figure

Spontaneous decay in the presence of dispersing and absorbing bodies: general theory and application to a spherical cavity

A formalism for studying spontaneous decay of an excited two-level atom in the presence of dispersing and absorbing dielectric bodies is developed. An integral equation, which is suitable for numerical solution, is derived for the atomic upper-state-probability amplitude. The emission pattern and the power spectrum of the emitted light are expressed in terms of the Green tensor of the dielectric-matter formation including absorption and dispersion. The theory is applied to the spontaneous decay of an excited atom at the center of a three-layered spherical cavity, with the cavity wall being modeled by a band-gap dielectric of Lorentz type. Both weak coupling and strong coupling are studied, the latter with special emphasis on the cases where the atomic transition is (i) in the normal-dispersion zone near the medium resonance and (ii) in the anomalous-dispersion zone associated with the band gap. In a single-resonance approximation, conditions of the appearance of Rabi oscillations and closed solutions to the evolution of the atomic state population are derived, which are in good agreement with the exact numerical results.Comment: 12 pages, 6 figures, typos fixed, 1 figure adde

Sterols sense swelling in lipid bilayers

In the mimetic membrane system of phosphatidylcholine bilayers, thickening (pre-critical behavior, anomalous swelling) of the bilayers is observed, in the vicinity of the main transition, which is non-linear with temperature. The sterols cholesterol and androsten are used as sensors in a time-resolved simultaneous small- and wide angle x-ray diffraction study to investigate the cause of the thickening. We observe precritical behavior in the pure lipid system, as well as with sterol concentrations less than 15%. To describe the precritical behavior we introduce a theory of precritical phenomena.The good temperature resolution of the data shows that a theory of the influence of fluctuations needs modification. The main cause of the critical behavior appears to be a changing hydration of the bilayer.Comment: 11 pages, 7 ps figures included, to appear in Phys.Rev.

Electromagnetic-field quantization and spontaneous decay in left-handed media

We present a quantization scheme for the electromagnetic field interacting with atomic systems in the presence of dispersing and absorbing magnetodielectric media, including left-handed material having negative real part of the refractive index. The theory is applied to the spontaneous decay of a two-level atom at the center of a spherical free-space cavity surrounded by magnetodielectric matter of overlapping band-gap zones. Results for both big and small cavities are presented, and the problem of local-field corrections within the real-cavity model is addressed.Comment: 15 pages, 5 figures, RevTe

Development and application of photosensitive device systems to studies of biological and organic materials

This report describes progress as of the third year of a 3-year DoE grant for 1/1/92 to 12/31/92. Because this is the last year of a 3- year grant cycle, this report will summarize progress over the entire 3-year period. The overall goals of the grant are to develop novel instrumentation and techniques for the performance of biological and materials research, and especially for the development of x-ray detectors suitable for use at storage ring sources. Research progress has been excellent and the overall goals, as well as most of the specific goals have been successfully met

Lyotropic effects of alkanes and headgroup composition on the lα -Hii lipid liquid crystal phase transition : hydrocarbon packing versus intrinsic curvature

The effects of mixed phospholipid headgroup composition and the addition of small amounts of alkane were examined with respect to the lamellar (L α) to inverse hexagonal (HII) phase transition in phospholipid-water liquid crystals. X-ray diffraction was used to probe the lattices and determine the phases. It has been postulated that competition between hydration repulsion, lipid monolayer curvature elasticity, and hydrocarbon packing determine the Lα-HIItransition, [Kirk, Gruner and Stein, Biochem. 23(1984) 1093]. Here, experiments which explored the effects of curvature and packing are described. It is shown that the radius of curvature of the lipid tubes of the HII phase could be adjusted by mixing high and low curvature lipids. However, large curvatures could not be expressed unless lipid hydrocarbon packing constraints were relieved, for instance, by the addition of alkane. Without alkane, the Lα phase extended to high temperatures; adding just 5 % alkane significantly reduced the temperature span of the Lα phase, by lowering the Lα to HII transition temperature, and dramatically expanded the HII lattice. Measurements of the internal dimensions of the HII lattice showed the lipid layer thickness to be nearly constant at a given temperature and that almost all of the lattice expansion was due to an increase in the radius of the water cores. Biological implications are discussed.Les effets liés à l'addition d'une petite quantité d'hydrocarbure et à la composition de groupements polaires de mélanges de phospholipides ont été examinés relativement à la transition de phase lamellaire (Lα ) hexagonale inverse (HII) dans des cristaux liquides eau-phospholipide. L'étude du réseau cristallin et la détermination des phases ont été faites aux rayons X par diffraction. Kirk, Gruner, Stein (Biochem. 23 (1984) 1093) ont postulé que la compétition entre la répulsion d'hydratation, l'élasticité due à la courbure d'une monocouche de lipide et l'empilement d'hydrocarbones détermine la transition Lα-H II. Dans cet article, nous décrivons les expériences qui permettent d'explorer les effets de courbure et d'empilement Nous montrons que le rayon de courbure des tubes de lipides de la phase HII peut être ajusté en changeant les proportions de lipides de haute et basse courbure. Cependant, pour de grandes courbures, les contraintes d'empilement lipide-hydrocarbone doivent être diminuées par l'adjonction d'hydrocarbure. Sans hydrocarbure, la phase Lα persiste à haute température; la simple adjonction de 5 % d'hydrocarbure réduit de façon spectaculaire l'intervalle de température à l'intérieur duquel la phase Lα existe; on observe une réduction de la température de transition Lα HII et une expansion spectaculaire du réseau HII. La mesure des dimensions internes du réseau HII montre que l'épaisseur de la couche du lipide est à peu près constante à une température donnée et que la presque totalité de l'expansion du réseau est due à une augmentation du rayon des groupes formés de molécules d'eau. Les implications biologiques sont discutées