Decoherence–fluctuation relation and measurement noise

We discuss fluctuations in the measurement process and how these fluctuations are related to the dissipational parameter characterizing quantum damping or decoherence. On the example of the measuring current of the variable-barrier or QPC problem we discuss the extra noise or fluctuation connected with the different possible outcomes of a measurement. This noise has an enhanced short time component which could be interpreted as due to “telegraph noise” or “wavefunction collapses”. Furthermore, the parameter giving the strength of this component is related to the parameter giving the rate of damping or decoherence.     

Quantum Correlation in Circuit QED Under Various Dissipative Modes

Dynamical evolutions of quantum correlations in circuit quantum electrodynamics (circuit-QED) are investigated under various dissipative modes. The influences of photon number, coupling strength, detuning and relative phase angle on quantum entanglement and quantum discord are compared as well. The results show that quantum discord may be less robust to decoherence than quantum entanglement since the death and revival also appears. Under certain dissipative mode, the decoherence subspace can be formed in circuit-QED due to the cooperative action of vacuum field. Whether a decoherence subspace can be formed not only depends on the form of quantum system but also relates closely to the dissipative mode of environment. One can manipulate decoherence through manipulating the correlation between environments, but the effect depends on the choice of initial quantum states and dissipative modes. Furthermore, we find that proper relative phase of initial quantum state provides one means of suppressing decoherence.     

Optimal quantum parameter estimation of two-qutrit Heisenberg XY chain under decoherence

Adopting the Milburn decoherence model, we investigate the performance of quantum Fisher information of the twoqutrit isotropic Heisenberg XY chain under decoherence. We find that the quantum Fisher information with respect to the decoherence rate and the magnetic field decreases exponentially in the long-time limit, which significantly reduces the precision of optimal quantum estimation. We also show that with the increase of the decoherence rate or the magnetic field,the QFIs go down considerably. Furthermore, we find that the precision of optimal quantum estimation can be enhanced by the entanglement in the input state.     

Quantum repeaters based on CNOT gate under decoherence

In this paper, we study single-qubit and single-user quantum repeaters based on CNOT gates under decoherence using the Kraus-operator representations of decoherence. We investigate the influence of decoherence on the information-disturbance trade-off of quantum repeaters. It is found that decoherence may lead to the appearance of three subspaces, called as the normal subspace, the anomalous subspace, and the decoherence-free subspace (DFS), respectively. It is indicated that in the normal subspace decoherence decreases the transmission and estimation fidelities, in the anomalous subspace decoherence enhances these fidelities, and in the DFS these fidelities do not change. The concept of the quality factor is introduced to evaluate the quality of the quantum repeater. It is indicated that the quality factor can be efficiently controlled and manipulated by changing the initial state of the probe qubit. It is found that under certain conditions the quantum repeater can be optimal even in the presence of decoherence.      

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity. We also find that the stationary quantum discord can be increased by applying a classical driving field. Furthermore, we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence. Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity.We also find that the stationary quantum discord can be increased by applying a classical driving field.Furthermore,we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence.Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

Decoherence induced by stochastic background of gravitational waves on matter-wave interferometers

Stochastic backgrounds of gravitational waves correspond to intrinsic fluctuations of spacetime leading to a universal decoherence mechanism. This mechanism defines an ultimate limit for matter-wave interferometry which sets a natural borderline between classical and quantum worlds. In this letter, we define figures which characterize the decoherence in terms of a coupling between the gravitational environment and the quadrupole of the interferometer. Using the ongoing progresses towards highly sensitive matter-wave interferometry in space, we then discuss the feasibility of experiments aimed at the observation of the decoherence border. PACS 03.75.-b; 04-30.-w; 04.62.+v     

Two-particle correlations and B^0\overline{B}^0 mixing

We study the EPR correlation implied by the entangled wavefunction of the pair created by the resonance. The analysis uses the basis provided by the mass eigenstates rather than the flavour states . Data on the inclusive dilepton charge ratio are close to the expectation of quantum mechanics, but nearly 8 standard deviations away from that of complete decoherence. Our results are compared with those obtained in the basis. Received: 24 November 1997 / Published online: 10 March 1998     

A Decoherence-Reduction Scheme by Waveguides in Quantum Information Processing

Based on the result of cavity quantum electrodynamics, we suggest a method, in which the Fabry-Perot cavity or the confocal cavity is replaced by a waveguide with the size comparable to the wavelength of the photon, to reduce decoherence caused by spontaneous emission in quantum information processing, especially in the realization of quantum computation. Since a waveguide has a lowest cutoff frequency while a Fabry-Perot cavity or a confocal cavity has none, the spontaneous emission of excited atoms will be forbidden in an ideal waveguide with an appropriate size. To avoid the influence of the non-ideal conducting walls on the atom in a realistic waveguide, which will lead to decoherence, we suggest that the waveguide should be coated by a thin film of transparent insulating medium. In our method, the quantum information is represented by a multi-level atom or molecule; any two of its levels can be used to represent a qubit in principle. Our method greatly extends the choice of the material to be used in the realization of quantum computation, and it can be used in most schemes to reduce the decoherence caused by spontaneous emission.     

Effects of Intrinsic Decoherence on Information Transport in a Spin Chain

Considering Milburn＇s intrinsic decoherence effect on quantum communication through a spin chain, we show that the transfer quality for quantum state and entanglement will obviously decrease with the increasing intrinsic decoherence rate. Some odd chains are much higher than even ones for the state transfer efficiency. The state transfer of a long chain is very sensitive to the intrinsic decoherence, which turns out to be an obstacle for information transport.     

Statistical properties of chaotic wavefunctions in two and more dimensions

Starting with Berry's hypothesis for fixed energy waves in a classically chaotic system, and casting it in a Green function form, we derive wavefunction correlations and density matrices for few or many particles. Universal features of fixed energy (microcanonical) random wavefunction correlation functions appear which reflect the emergence of the canonical ensemble as N↦∞. This arises through a little known asymptotic limit of Bessel functions. The Berry random wave hypothesis in many dimensions may be viewed as an alternative approach to quantum statistical mechanics, when extended to include constraints and potentials.     

Numerical modeling of the central spin problem using the spin-coherent-state representation

In this work, we consider decoherence of a central spin by a spin bath. In order to study the nonperturbative decoherence regimes, we develop an efficient mean-field-based method for modeling the spin-bath decoherence, based on the representation of the central spin density matrix. The method can be applied to longitudinal and transverse relaxation at different external fields. In particular, by modeling large-size quantum systems (up to 16 000 bath spins), we make controlled predictions for the slow long-time decoherence of the central spin.     

Amplitude-Damping Decoherence Suppression of Two-Qubit Entangled States by Weak Measurements

Taming decoherence is a critical issue in quantum information science. We here investigate amplitude-damping decoherence suppression of two-qubit entangled states by weak quantum measurements. It is shown that the weak measurements can effectively suppress the decoherence for different initial entangled states. More interestingly, we show that the weak measurements have different effects on the entanglement protection for two entangled states which are equivalent under a local unitary operation. This result implies that the entanglement protection effect could be modulated according to different demands.     

Realization of quantum walks with negligible decoherence in waveguide lattices

Quantum random walks are the quantum counterpart of classical random walks, and were recently studied in the context of quantum computation. Physical implementations of quantum walks have only been made in very small scale systems severely limited by decoherence. Here we show that the propagation of photons in waveguide lattices, which have been studied extensively in recent years, are essentially an implementation of quantum walks. Since waveguide lattices are easily constructed at large scales and display negligible decoherence, they can serve as an ideal and versatile experimental playground for the study of quantum walks and quantum algorithms. We experimentally observe quantum walks in large systems ( approximately 100 sites) and confirm quantum walks effects which were studied theoretically, including ballistic propagation, disorder, and boundary related effects.     

Realization of quantum controlled-NOT gate with resonant interaction

We propose a method for realizing the quantum controlled-NOT gate with a single resonant interaction in cavity QED. Our scheme only requires a single resonant interaction between two atoms and a cavity mode. Thus the scheme is very simple and the quantum dynamics operation can be realized at a high speed, which is important in view of decoherence. In addition, we also show that the gate can be realized in the ion trap system.     

Binding energy of a screened donor in a spherical quantum dot with a parabolic potential

A wavefunction is proposed for calculating the ground-state energy of a screened donor in a spherical quantum dot under a parabolic potential by a variational method. The donor is taken to be at the center of the quantum dot. Results are presented for four values of the screening parameter by the proposed wavefunction as well as for the hydrogenic donor case by a wavefunction used by Xiao et al.. To assess the accuracy of the results, ‘exact’ energies are also obtained by numerical integration of the Schrödinger equation. It is shown that the proposed wavefunction gives very good results in all cases including the hydrogenic donor case.     

Quantum Optics of Ultracold Molecular Fields

We apply concepts of quantum optical coherence to characterize the coherent generation of a molecular field from a quantum-degenerate atomic sample, and discuss the impact of the quantum statistics of the atoms on that field. For atoms initially in a BEC the resulting molecular field is to a good approximation coherent. This is in sharp contrast to the case of atoms in a normal Fermi gas, where we can made use of an analogy with the Tavis-Cummings model to show that the statistics of the resulting molecular field is similar to that of a single-mode chaotic light field. The BCS case interpolates between the two extremes, with an ＇incoherent＇ contribution from unpaired atoms superposed to a ＇coherent＇ contribution from atomic Cooper pairs. We also comment on the temporal fluctuations characteristic of the formation of molecular dimers from ultracold fermionic atoms.     

Tunneling control and localization for Bose-Einstein condensates in a frequency modulated optical lattice

The similarity between matter waves in periodic potential and solid-state physics processes has triggered the interest in quantum simulation using Bose-Fermi ultracold gases in optical lattices. The present work evidences the similarity between electrons moving under the application of oscillating electromagnetic fields and matter waves experiencing an optical lattice modulated by a frequency difference, equivalent to a spatially shaken periodic potential. We demonstrate that the tunneling properties of a Bose-Einstein condensate in shaken periodic potentials can be precisely controlled. We take additional crucial steps towards future applications of this method by proving that the strong shaking of the optical lattice preserves the coherence of the matter wavefunction and that the shaking parameters can be changed adiabatically, even in the presence of interactions. We induce reversibly the quantum phase transition to the Mott insulator in a driven periodic potential.