acta physica slovaca

Abstract: We derive from first principles the phenomenological Hamiltonian for high-Q cavity damping that is often adopted in quantum optics and show that it is a far better approximation than was previously thought. We also obtain an explicit expression for the coupling strength between the discrete cavity quasimodes and the continuum that reveals the fast-time non-Markovian nature of cavity damping and the limit of validity of the independent reservoir assumption.

Abstract: We investigate the time development of a superposition of macroscopically distinct quantum states (Schrödinger cats) in an ensemble of two-level atoms. The system is interacting with a thermal environment of a macroscopic number of photon modes. The final equilibrium state of the atomic subsystem is diagonal in the energy eigenstates, and is determined by the Boltzmann distribution. The time scale of decoherence is, however, generally much shorter than that of dissipation. The initial fast regime of the time evolution associated with the decoherence is directed towards a classical state which is different from the thermal equilibrium. For general initial conditions the distance between the actual state of the system and this classical state is decreasing fast, suggesting an appropriate measure of decoherence.

Abstract: We discuss heating and decoherence in traps for ions and neutral particles close to metallic surfaces. We focus on simple trap geometries and compute noise spectra of thermally excited electromagnetic fields. If the trap is located in the near field of the substrate, the field fluctuations are largely increased compared to the level of the blackbody field, leading to much shorter coherence and life times of the trapped atoms. The corresponding time constants are computed for ion traps and magnetic traps. Analytical estimates for the size dependence of the noise spectrum are given. We finally discuss prospects for the coherent transport of matter waves in integrated surface waveguides.

Abstract: Quantum theory allows to measure incompatible observables sequentially in the course of repeated measurements. To get information about the observed system, all the observations must be synthetized. This is the main idea of quantum tomographical methods. As shown in this contribution, the maximum likelihood principle provides the best measure for relating the experimental data with predictions of quantum theory. Synthesis of incompatible observations appears to be a novel quantum measurement described by a positive operator-valued measure. Besides this the procedure finds the optimal state of the system, which fitts such a measurement in optimal way.

Abstract: We present a generalized master equation, in an operator form, for a two-level atom driven by a strong classical field and damped into a ``tailored'' reservoir with non-flat density of modes. The master equation is derived under the Born and Markov approximations. To derive the master equation the dressing transformation on the atomic operators is performed first and next the dressed operators are coupled to the reservoir and the corresponding damping rates are calculated. The modifications introduced by a strong field and/or by the reservoir with non-flat density of modes lead to non-standard terms in the master equation, some of which are reminiscent of terms known for squeezed vacuum reservoirs.

Abstract: We discuss a system comprising an anharmonic oscillator permanently excited by a series of ultra-short coherent pulses. Assuming that the system was initially in the vacuum state we investigate and compare its classical and quantum dynamics. Moreover, we compare the picture based on the single classical trajectory with the ``averaged'' one.

Abstract: The occurrence of a peculiar correlation effect in the quantum dynamics of an ion confined in a 2D Paul microtrap is reported. We analytically find that when the ion, prepared in a SU(2) vibrational coherent state, is irradiated by two orthogonal laser beams, the time evolution of the quantum covariance between two clearly interpretable bosonic observables displays an high sensitivity to the initial state parameters. Both the nonclassical nature of this effect and a simple proposal for its detection is briefly discussed.

Abstract: We show that the fidelity of teleportation of continuous quantum variables can be improved by conditional photon-number measurement of the entangled state. Further, we propose a teleportation scheme based on photon counting on the output fields of a squeezer that combines the mode whose quantum state is desired to be teleported and one mode of the two-mode squeezed vacuum playing the role of the entangled state.

Abstract: 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 prevent them from preserving 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 that relates at a four-port device the output quantum state to the input quantum state, without placing restrictions on the frequency. Basing on the relative entropy as an entanglement measure, we apply the formalism to the transformation of entanglement, with special emphasis on the entanglement degradation in absorbing optical fibers. In particular we show that the Bell basis states using Fock states are more robust against decoherence than the states .

Abstract: Recently we have proposed a method to create thin matter-wave bubbles from coherent atoms trapped in a magnetic potential. In this article we discuss in detail some properties of these states. In particular, we numerically and analytically investigate the Wigner function to demonstrate their non-classical nature. Furthermore, we study the energy and lifetime of the bubbles, which are long-lived resonances in a radio frequency-induced adiabatic potential, and illustrate how they can be transformed into excited eigenstates of the original harmonic trapping potential.

Abstract: The Hamiltonian formulation of pure Yang-Mills theory is analysed in the case when Gauss' law is satisfied identically by construction. It is shown that the theory has a Hamiltonian formulation in this case, provided one uses a special gauge condition, which is a natural generalisation of the Coulomb gauge condition in electrodynamics. The Hamiltonian formulation depends critically also on the boundary conditions used for the relevant field variables. Possible boundary conditions are analysed in detail. A comparison of the present formulation in the generalised Coulomb gauge with the well known Weyl gauge (A0 = 0) formulation is made. It appears that the Hamiltonians in these two formulations differ from one another in a non-trivial way. It is still an open question whether these differences give rise to truly different structures upon quantisation. An extension of the formalism to include coupling to fermionic fields is briefly discussed.

Abstract: An exact solution of the Einstein-Maxwell equations yields a general relativistic picture of the tachyonic phenomenon, suggesting a hypothesis on the tachyon creation. The hypothesis says that the tachyon is produced when a neutral and very heavy (over 75 GeV/c2) subatomic particle is placed in electric and magnetic fields that are perpendicular, very strong (over 6.9 x 1037 esu/cm2 or oersted), and the squared ratio of their strength lies in the interval (1,5]. Such conditions can occur when nonpositive subatomic particles of high energy strike atomic nuclei other than the proton. The kinematical relations for the produced tachyon are given. Previous searches for tachyons in air showers and some possible causes of their negative results are discussed. Experiments with the use of the strongest colliders and improvements in the air shower experiments are suggested. An unfortunate terminology is also discussed.