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Search Results: 1 - 10 of 200644 matches for " P. Torma "
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Laser probing of atomic Cooper pairs
P. Torma,P. Zoller
Physics , 2000, DOI: 10.1103/PhysRevLett.85.487
Abstract: We consider a gas of attractively interacting cold Fermionic atoms which are manipulated by laser light. The laser induces a transition from an internal state with large negative scattering length to one with almost no interactions. The process can be viewed as a tunneling of atomic population between the superconducting and the normal states of the gas. It can be used to detect the BCS-ground state and to measure the superconducting order parameter.
Beyond linear response spectroscopy of ultracold Fermi gases
J. Kinnunen,P. Torma
Physics , 2005, DOI: 10.1103/PhysRevLett.96.070402
Abstract: We study RF-spectroscopy of ultracold Fermi gas by going beyond the linear response in the field-matter interaction. Higher order perturbation theory allows virtual processes and energy conservation beyond the single particle level. We formulate an effective higher order theory which agrees quantitatively with experiments on the pairing gap, and is consistent with the absence of the mean-field shift in the spin-flip experiment.
Quantum transport of non-interacting Fermi gas in an optical lattice combined with harmonic trapping
V. Ruuska,P. Torma
Physics , 2004, DOI: 10.1088/1367-2630/6/1/059
Abstract: We consider a non-interacting Fermi gas in a combined harmonic and periodic potential. We calculate the energy spectrum and simulate the motion of the gas after sudden replacement of the trap center. For different parameter regimes, the system presents dipole oscillations, damped oscillations around the replaced center, and localization. The behaviour is explained by the change of the energy spectrum from linear to quadratic.
Polarization in Quantum Computations
P. Torma,S. Stenholm
Physics , 1996,
Abstract: We propose a realization of quantum computing using polarized photons. The information is coded in two polarization directions of the photons and two-qubit operations are done using conditional Faraday effect. We investigate the performance of the system as a computing device.
Effect of wavelength dependence of nonlinearity, gain, and dispersion in photonic crystal fiber amplifiers
A. Huttunen,P. Torma
Physics , 2005, DOI: 10.1364/OPEX.13.004286
Abstract: Photonic crystal fibers are used in fiber amplifiers and lasers because of the flexibility in the design of mode area and dispersion. However, these quantities depend strongly on the wavelength. The wavelength dependence of gain, nonlinearity and dispersion are investigated here by including the wavelength dependence explicitly in the nonlinear Schr\"odinger equation for photonic crystal fibers with varying periods and hole sizes. The effect of the wavelength dependence of each parameter is studied separately as well as combined. The wavelength dependence of the parameters is shown to create asymmetry to the spectrum and chirp, but to have a moderating effect on pulse broadening. The effect of including the wavelength dependence of nonlinearity in the simulations is demonstrated to be the most significant compared that of dispersion or gain.
Bloch oscillations in Fermi gases
M. Rodriguez,P. Torma
Physics , 2003, DOI: 10.1103/PhysRevA.69.041602
Abstract: The possibility of Bloch oscillations for a degenerate and superfluid Fermi gas of atoms in an optical lattice is considered. For a one-component degenerate gas the oscillations are suppressed for high temperatures and band fillings. For a two-component gas the Landau criterion is used for specifying the regime where Bloch oscillations of the superfluid may be observed. We show how the amplitude of Bloch oscillations varies along the BCS-BEC crossover.
Quasi two-dimensional superfluid Fermi gases
J. -P. Martikainen,Paivi Torma
Physics , 2005, DOI: 10.1103/PhysRevLett.95.170407
Abstract: We study a quasi two-dimensional superfluid Fermi gas where the confinement in the third direction is due to a strong harmonic trapping. We investigate the behavior of such a system when the chemical potential is varied and find strong modifications of the superfluid properties due to the discrete harmonic oscillator states. We show that such quasi two-dimensional behavior can be created and observed with current experimental capabilities.
Localization and diffusion in Ising-type quantum networks
P. Torma,I. Jex,W. P. Schleich
Physics , 2001, DOI: 10.1103/PhysRevA.65.052110
Abstract: We investigate the effect of phase randomness in Ising-type quantum networks. These networks model a large class of physical systems. They describe micro- and nanostructures or arrays of optical elements such as beam splitters (interferometers) or parameteric amplifiers. Most of these stuctures are promising candidates for quantum information processing networks. We demonstrate that such systems exhibit two very distinct types of behaviour. For certain network configurations (parameters), they show quantum localization similar to Anderson localization whereas classical stochastic behaviour is observed in other cases. We relate these findings to the standard theory of quantum localization.
Measuring charge based quantum bits by a superconducting single-electron transistor
J. Kinnunen,P. Torma,J. P. Pekola
Physics , 2002, DOI: 10.1103/PhysRevB.68.020506
Abstract: Single-electron transistors have been proposed to be used as a read-out device for Cooper pair charge qubits. Here we show that a coupled superconducting transistor at a threshold voltage is much more effective in measuring the state of a qubit than a normal-metal transistor at the same voltage range. The effect of the superconducting gap is to completely block the current through the transistor when the qubit is in the logical state 1, compared to the mere diminishment of the current in the normal-metal case. The time evolution of the system is solved when the measuring device is driven out of equilibrium and the setting is analysed numerically for parameters accessible by lithographic aluminium structures.
Co-existence and shell structures of several superfluids in trapped three-component Fermi mixtures
T. Paananen,P. Torma,J. -P. Martikainen
Physics , 2006, DOI: 10.1103/PhysRevA.75.023622
Abstract: We study the properties of a trapped interacting three component Fermi gas. We assume that one of the components can have a different mass from the other two. We calculate the different phases of the three component mixture and find a rich variety of different phases corresponding to different pairing channels, and simple ways of tuning the system from one phase to another. In particular, we predict co-existence of several different superfluids in the trap, forming a shell structure, and phase transitions from this mixture of superfluids to a single superfluid when the system parameters or temperature is varied. Such shell structures realize superfluids with a non-trivial spatial topology and leave clear observable signatures in the density profile of the gas.
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