Decoherence and equilibration under nondestructive measurements

The evolution of observable quantities of finite quantum systems is analyzed when the latter are subject to nondestructive measurements. The type and number of measurements characterize the level of decoherence produced in the system. A finite number of instantaneous measurements leads to only a partial decoherence. But infinite number of such measurements yields complete decoherence and equilibration. Continuous measurements result in partial decoherence in finite time, but produce complete decoherence and equilibration as time tends to infinity. Resulting equilibrium states are characterized by representative statistical ensembles that, generally, retain information on initial conditions. Any system, to be observable, necessarily requires the presence of measurements, whose large number leads to the system equilibration and decoherence.     

Decoherence Induced by Discontinuous and Stochastic Unitary Evolution of Qubits in Quanturn Computers

A new kind of decoherence in quantum computer memory, called intrinsic decoherence, is investigated in some details, which is caused by discontinuous and stochastic unitary evolution of qubits in quantum computers on a sufficient short time scale. It is found that the intrinsic decoherence leads to quasi-periodic decaying oscillations of the state fidelity of qubits in the time evolution. Schemes to reduce the intrinsic decoherence are proposed.     

Entropy growth and degradation of entanglement due to phase decoherence in ion traps

We study how phase decoherence through intrinsic decoherence leads to growing entropy and a strong degradation of the maximum generated entanglement as a measure of information content of ionic state due to ion-laser interaction with a trapped ion. We calculate the partial entropy of the particle (atom or trapped ion) and field subsystems as well as the total entropy. The total entropy is shown to increase with time. Thus, the partial field or atomic entropy cannot be used as a direct measure of the particle–field entanglement. We find that, at least qualitatively, the difference between the total entropy and the sum of field and atom partial entropies can be used as an entanglement measure, when compared with an established entanglement measure based on the negativity of the eigenvalues of the partially transposed density matrix. We find a very strong sensitivity of the maximum generated entanglement on the decoherence and the chosen intrinsic decoherence parameter.     

Nonexponential decoherence and momentum subdiffusion in a quantum lévy kicked rotator

We investigate decoherence in the quantum kicked rotator (modeling cold atoms in a pulsed optical field) subjected to noise with power-law tail waiting-time distributions of variable exponent (Lévy noise). We demonstrate the existence of a regime of nonexponential decoherence where the notion of a decoherence rate is ill defined. In this regime, dynamical localization is never fully destroyed, indicating that the dynamics of the quantum system never reaches the classical limit. We show that this leads to quantum subdiffusion of the momentum, which should be observable in an experiment.     

Indirect decoherence in optical lattices and cold gases

The interaction of two–level atoms with a common heat bath leads to an effective interaction between the atoms, such that with time the internal degrees of the atoms become correlated or even entangled. If part of the atoms remain unobserved this creates additional indirect decoherence for the selected atoms, on top of the direct decoherence due to the interaction with the heat bath. I show that indirect decoherence can drastically increase and even dominate the decoherence for sufficiently large times. I investigate indirect decoherence through thermal black body radiation quantitatively for atoms trapped at regular positions in an optical lattice as well as for atoms at random positions in a cold gas, and show how indirect decoherence can be controlled or even suppressed through experimentally accessible parameters.     

Decoherence via the dynamical casimir effect

We derive a master equation for a mirror interacting with the vacuum field via radiation pressure. The dynamical Casimir effect leads to decoherence of a superposition state in a time scale that depends on the degree of "macroscopicity" of the state components, and which may be much shorter than the relaxation time scale. Coherent states are selected by the interaction as pointer states.     

Semi-Classical Limit and Minimum Decoherence in the Conditional Probability Interpretation of Quantum Mechanics

The Conditional Probability Interpretation of Quantum Mechanics replaces the abstract notion of time used in standard Quantum Mechanics by the time that can be read off from a physical clock. The use of physical clocks leads to apparent non-unitary and decoherence. Here we show that a close approximation to standard Quantum Mechanics can be recovered from conditional Quantum Mechanics for semi-classical clocks, and we use these clocks to compute the minimum decoherence predicted by the Conditional Probability Interpretation.     

Behavior of quantum coherence of Ξ-type four-level atom under bang-bang control

In this paper, we have studied the bang-bang (BB) decoupling scheme to suppress the phase decoherence, the amplitude decoherence and the general decoherence in a four-level Ξ-type atom system. The corresponding dynamical decoupling groups are given for designing the decoupling pulse sequences to suppress these three kinds of decoherence, respectively. Results show that in a proper time scale, the decoupling operations suppress the decoherence effectively. Especially in the ideal limits, it can suppress the decoherence completely. We also give the time scale in which the BB control works well. Numerical simulations show that, the larger cycle times N, the better effect of the BB decoupling operations under a fixed time scale.     

Non-Markovian dynamics of spin squeezing

We evaluate the spin squeezing dynamics of N independent spin-1/2 particles with exchange symmetry. Each particle couples to an individual and identical reservoir. We study the time evolution of spin squeezing under the influence of different decoherence. The spin squeezing property vanishes with evolution time under Markovian decoherence, while it collapses quickly and revives under non-Markovian decoherence. As spin squeezing can be regarded as a witness of multipartite entanglement, our scheme shows the collapses and revivals of multipartite entanglement under the influence of non-Markovian decoherence.     

Quantum decoherence of single-photon counters

The interaction of a quantum system with the environment leads to the so-called quantum decoherence. Beyond its fundamental significance, the understanding and the possible control of this dynamics in various scenarios is a key element for mastering quantum information processing. Here we report the quantitative probing of what can be called the quantum decoherence of detectors, a process reminiscent of the decoherence of quantum states in the presence of coupling with a reservoir. We demonstrate how the quantum features of two single-photon counters vanish under the influence of a noisy environment. We thereby experimentally witness the transition between the full-quantum operation of the measurement device to the "semi-classical regime", described by a positive Wigner function. The exact border between these two regimes is explicitely determined and measured experimentally.     

Dynamics of a Moving Two-Level Atom Under the Influence of Intrinsic Decoherence

We present a general and fascinating problem of quantum entanglement (QE) that is calculated with the help of quantum Fisher information (QFI) and von Neumann entropy (VNE) for moving two-level atomic systems. We calculate numerically the temporal evolution of the state vector of the entire system under the influence of intrinsic decoherence for a moving two-level atom. We demonstrate that the phase shifts of an estimator parameter, intrinsic decoherence, and the atomic motion play an important and prominent role during the time evolution of the atomic system. We observe that there is a monotonic relation between the atomic quantum Fisher information (QFI) and quantum entanglement (QE) in the absence of atomic motion. We also show that at the revival time the local maximum values of QFI decreases gradually. A periodic behavior of QFI is observed in the presence of atomic motion, which becomes more important and remarkable for two-level atomic systems. Moreover, the atomic quantum Fisher information and entanglement demonstrate an opposite response during the time evolution in the presence of atomic motion. We show that the evolution of entanglement is more susceptible to the intrinsic decoherence; a considerable change occurs in the degree of entanglement when the intrinsic decoherence parameter increases. Intrinsic decoherence in the atom–field interaction represses the nonclassical effects of the atomic systems. Both the entanglement and the quantum Fisher information saturate to their lower levels for longer time scales in the presence of intrinsic decoherence. For larger values of intrinsic decoherence, the sudden death of entanglement is observed.     

Detection of single spin decoherence in a quantum dot via charge currents

We consider a quantum dot attached to leads in the Coulomb blockade regime that has a spin 1 / 2 ground state. We show that, by applying an ESR field to the dot spin, the stationary current in the sequential tunneling regime exhibits a new resonance peak whose linewidth is determined by the single spin decoherence time T2. The Rabi oscillations of the dot spin are shown to induce coherent current oscillations from which T2 can be deduced in the time domain. We describe a spin inverter which can be used to pump current through a double dot via spin flips generated by ESR.     

Particle trapping induced by the interplay between coherence and decoherence

We propose a novel scheme to trap a particle based on a delicate interplay between coherence and decoherence. If the decoherence occurs as a particle is located in the scattering region and subsequently the appropriate destructive interference takes place, the particle can be trapped in the scattering area. We consider two possible experimental realizations of such trapping: a ring attached to a single lead and a ring attached to two leads. Our scheme has nothing to do with a quasi-bound state of the system, but has a close analogy with the weak localization phenomena in disordered conductors.     

Entropy Exchange and Entanglement in Superconducting Charge Qubit Inside a Resonant Cavity with Intrinsic Decoherence

By considering the intrinsic decoherence effect, we investigate the entropy exchange and entanglement in the interacting system of a superconducting charge qubit coupled to a single-mode optical cavity. We found that although the intrinsic decoherence leads to an irreversible evolution of the interacting system due to a suppression of coherent quantum features through the decay of off-diagonal matrix elements of the density operator, and has an apparently influence on the partial entropies of the two-component subsystems, it dose not destroy entropy exchange behavior. In addition, the lower bound of the concurrence, as the measure of entanglement of the coupling system, is calculated. It is shown that the evolution of entanglement is sensitive to the change of the intrinsic decoherence.     

Decoherence and energy loss in QCD cascades in nuclear collisions

The paper analyzes the consequences of the assumption that the effects of quantum coherence and the resulting angular ordering in QCD cascades are disrupted within the hot fireball created in ultrarelativistic heavy ion collisions. The effects of decoherence are taken into account by switching off angular ordering for the vertices of MC-generated cascade that lie within the fireball of some given size. We show that decoherence exerts a pronounced influence on the spatiotemporal pattern of a cascade. Related effects due to non-radiative losses of cascade gluons and those of final prehadrons are discussed. We show that decoherence leads to a substantial increase in the yield of cascade particles hadronizing inside the hot zone thus exerting a strong influence on the properties of final hadrons and amplifying the effects of collisional energy losses of cascade partons and final prehadrons that take place inside the fireball.     

Quantum decoherence in the theory of open systems

Within the framework of the Lindblad theory for open quantum systems, we determine the degree of quantum decoherence of a harmonic oscillator interacting with a thermal bath. It is found that the system manifests a quantum decoherence which is more and more significant in time. We also calculate the decoherence time scale and analyze the transition from quantum to classical behavior of the considered system. The text was submitted by the author in English.     

Quantum decoherence of the damped harmonic oscillator

In the framework of the Lindblad theory for open quantum systems, we determine the degree of quantum decoherence of a harmonic oscillator interacting with a thermal bath. It is found that the system manifests a quantum decoherence which is more and more significant in time. We also calculate the decoherence time and show that it has the same scale as the time after which thermal fluctuations become comparable with quantum fluctuations. The text was submitted by the author in English.     

Decoherence in quantum mechanics

We study decoherence in a simple quantum mechanical model using two approaches. Firstly, we follow the conventional approach to decoherence where one is interested in solving the reduced density matrix from the perturbative master equation. Secondly, we consider our novel correlator approach to decoherence where entropy is generated by neglecting observationally inaccessible correlators. We show that both methods can accurately predict decoherence time scales. However, the perturbative master equation generically suffers from instabilities which prevents us to reliably calculate the system’s total entropy increase. We also discuss the relevance of the results in our quantum mechanical model for interacting field theories.     

Continuous weak measurement and nonlinear dynamics in a cold spin ensemble

A weak continuous quantum measurement of an atomic spin ensemble can be implemented via Faraday rotation of an off-resonance probe beam, and may be used to create and probe nonclassical spin states and dynamics. We show that the probe light shift leads to nonlinearity in the spin dynamics and limits the useful Faraday measurement window. Removing the nonlinearity allows a nonperturbing measurement on the much longer time scale set by decoherence. The nonlinear spin Hamiltonian is of interest for studies of quantum chaos and real-time quantum state estimation.