| 1. |
- Clausen, J, et al.
(författare)
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Nonclassical dynamics induced by a quantum meter
- 2005
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 72:6, s. 062104-
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Tidskriftsartikel (refereegranskat)abstract
- Conventionally, the effect of measurements on a quantum system is assumed to introduce decoherence, which renders the system classical- like. We consider here a microscopic meter, that is, an auxiliary essentially quantum system whose state is measured repeatedly, and show that it can be employed to induce transitions from classical states into inherently quantumlike states. The meter state is assumed to be lost in the environment and we derive a non- Markovian master equation for the dynamic system in the case of nondemolition coupling to the meter; this equation can be cast in the form of an (N-a)th- order differential equation in time, where N-a is the dimension of the meter basis. We apply the approach to a harmonic oscillator coupled to a spin-1/2 2 meter and demonstrate how it can be used to engineer effective Hamiltonian evolution, subject to decoherence induced by the projective meter measurements.
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| 2. |
- Larson, Jonas, et al.
(författare)
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Effective mass in cavity QED
- 2005
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 72:1
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Tidskriftsartikel (refereegranskat)abstract
- We consider propagation of a two-level atom coupled to one electromagnetic mode of a high-Q cavity. The atomic center-of-mass motion is treated quantum mechanically and we use a standing wave shape for the mode. The periodicity of the Hamiltonian leads to a spectrum consisting of bands and gaps, which is studied from a Floquet point of view. Based on the band theory, we introduce a set of effective mass parameters that approximately describe the effect of the cavity on the atomic motion, with the emphasis on one associated with the group velocity and on another one that coincides with the conventional effective mass. Propagation of initially Gaussian wave packets is also studied using numerical simulations and the mass parameters extracted thereof are compared with those predicted by the Floquet theory. Scattering and transmission of the wave packet against the cavity are further analyzed, and the constraints for the effective mass approach to be valid are discussed in detail.
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| 3. |
- Salo, Janne, et al.
(författare)
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Non-Markovian decay and decoherence in open quantum systems
- 2005
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Ingår i: Decoherence, Entanglement and Information Protection in Complex Quantum Systems. - DORDRECHT : SPRINGER. - 1402032811 ; , s. 235-238
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Konferensbidrag (refereegranskat)abstract
- Interaction between a quantum system and its surroundings - be it another similar quantum system, a thermal reservoir, or a measurement device - breaks down the standard unitary evolution of the system alone and introduces open quantum system behaviour. Coupling to a fast-relaxing thermal reservoir is known to lead to an exponential decay of the quantum state, a process described by a Lindblad-type master equation. In modem quantum physics, however, near isolation of individual quantum objects, such as qubits, atoms, or ions, sometimes allow them only to interact with a slowly-relaxing near-environment, and the consequent decay of the atomic quantum state may become nonexponential and possibly even nonmonotonic. Here we consider different descriptions of non-Markovian evolutions and also hazards associated with them, as well as some physical situations in which the environment of a quantum system induces non-Markovian phenomena.
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| 4. |
- Salo, Janne, et al.
(författare)
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Non-Markovian thermalisation of a two-level atom
- 2006
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Ingår i: Optics Communications. - : Elsevier BV. - 0030-4018 .- 1873-0310. ; 259:2, s. 772-780
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Tidskriftsartikel (refereegranskat)abstract
- We consider thermalisation and spontaneous decay of a two-level atom beyond the Markovian approximation. While the standard elimination of the continuum of radiation modes results in exponential decay represented by a Lindblad equation of motion, we use a simple toy model that takes into account the finite relaxation rate of the environment and present an exact non-Markovian master equation of the Nakajima-Zwanzig form. Because the exact derivation of non-Markovian equations has proved very difficult for all more realistic (and hence much more complicated) models, we analyze the master equation obtained and also discuss difficulties that are likely to arise with non-Markovian evolution operators.
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| 5. |
- Salo, Janne, et al.
(författare)
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The varieties of Master Equations
- 2005
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Ingår i: Decoherence, Entanglement and Information Protection in Complex Quantum Systems. - DORDRECHT : SPRINGER. - 1402032811 ; , s. 239-280
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Konferensbidrag (refereegranskat)abstract
- Irreversible time evolution is in quantum systems described by Master Equations. These are usually derived by elimination of degrees of freedom belonging to an environment acting as a reservoir. When the reservoir is acting as a sink for information, it has got no memory, we obtain equations usually termed Markovian. In this limit it has been shown by semi-group arguments which form preserves all physically relevant properties of the quantum state. It has been noted that not all Markovian equations derived lead to physically meaningful conclusions. On the other hand, the derivations prove that exact equations can be obtained if the memory effects are retained. We have also observed that improper introduction of the memory features may lead to unphysical results. We pose the challenge to find the general criteria which guarantee that the time evolution described by a density matrix is physically acceptable. However, it is also possible to avoid memory problems by deriving an equation of motion with time dependent coefficients, a so called memory-less Master Equation. In order to apply the same considerations on more complicated models one runs into problems posed by the formal structure of the method. Thus we warn against ad hoc applications of irreversible Master Equations. We explain the properties and emergence of these features with the scope of simple models which can be analysed properly; thus we hope to provide some insight into the character of irreversible time evolution. First we consider an exactly solvable model in which a two-level atom is allowed to decay into a thermally relaxing reservoir, represented by another two-level atom (so-called faked continuum model). Memory equations are, however, highly dependent on the form of memory functions and even a slight inisparameterisation leads to non-positive. evolution. As an alternative to the foregoing approach, we outline the derivation of a generalised non-Markovian equation for a two-level system that is coupled to a finite-temperature bath and driven by an arbitrary time-dependent field. This derivation does not invoke the rotating-wave approximation and yields dynamically-modified rates for population decay and decoherence.
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