Destruction Of Quantum Coherence Research Articles

Dynamical Buildup of a Quantized Hall Response from Nontopological States

We consider a two-dimensional system initialized in a topologically trivial state before its Hamiltonian is ramped through a phase transition into a Chern insulator regime. This scenario is motivated by current experiments with ultracold atomic gases aimed at realizing time-dependent dynamics in topological insulators. Our main findings are twofold. First, considering coherent dynamics, the nonequilibrium Hall response is found to approach a topologically quantized time-averaged value in the limit of slow but nonadiabatic parameter ramps, even though the Chern number of the state remains trivial. Second, adding dephasing, the destruction of quantum coherence is found to stabilize this Hall response, while the Chern number generically becomes undefined. We provide a geometric picture of this phenomenology in terms of the time-dependent Berry curvature.

Ratcheting Up Energy by Means of Measurement

The destruction of quantum coherence can pump energy into a system. For our examples this is paradoxical because the destroyed correlations are ordinarily considered negligible. Mathematically the explanation is straightforward and physically one can identify the degrees of freedom supplying this energy. Nevertheless, the energy input can be calculated without specific reference to those degrees of freedom.

Quantum coherence and entanglement

We are in the midst of a renaissance in the study and interpretation of fundamental quantum mechanics, from quantum coherence and quantum entanglement to the quantum measurement process. This stems partly from the impressive advances in experimental techniques that are making yesterday's gedanken experiment today's reality. Another reason quantum coherence and entanglement have come to the forefront is the emerging and tantalizing field of quantum information science. The ability to store information in quantum systems may someday yield devices that process quantum superpositions of inputs in parallel or improve communication through the `wiring' implicit in quantum entanglement. Indeed, quantum computing has the remote prospect of revolutionizing the way we store and process information. This has become particularly important in the face of impending limits on the miniaturization of conventional computers due to quantum effects. Of course, all real systems face limits imposed by decoherence, or the environmentally-induced destruction of quantum coherence. Here, superpositions of quantum states suddenly become probabilistic mixtures of states when they interact with the environment or when the system parameters are fluctuating. Decoherence theory is commonly interpreted as a way to quantify the elusive boundary between quantum and classical worlds and almost always precludes the existence of complex quantum superpositions, except at extremely short time scales. In practice, it is very important to study how decoherence appears in particular quantum systems and try to fight against it. In this special issue of Journal of Optics B: Quantum and Semiclassical Optics, we highlight recent results in the exciting area of quantum coherence and entanglement, including generating interesting and complex quantum states, characterizing coherence and entanglement in these systems, and triumphing over relevant decoherence processes. We hope this issue will not only act as a bookmark in the ongoing study of fundamental quantum mechanics, but will also help guide the rapid developments in quantum information science.

Theoretical study of quantum dissipation and laser-noise effects on the atomic response

The nonlinear dynamics of dissipative quantum systems in incoherent laser fields is studied in the framework of a master equation with the random telegraph model describing the laser noise and the Markovian approximation dealing with the system-bath couplings. Floquet theory and time-dependent perturbation methods are used to facilitate both analytical and numerical solutions. We develop a theoretical formalism that provides a powerful tool for the detailed analysis of the dissipative quantum dynamics of multilevel systems driven by intense stochastic laser fields. It is found that the system relaxes to a steady state from the effect of the laser phase and frequency noise and the kinetics of this relaxation increases with the addition of dissipative terms, introduced by the coupling to the reservoir. Amplitude fluctuations show a different behavior. Other results concerning the destruction of quantum coherence and the dynamical localization will be established and further relaxation mechanisms such as spontaneous emission and the ionization process will also be considered.

Control of molecular chirality

We present a theoretical study of controlling molecular chirality in chemical reactions by circularly polarized fields. A theory of absolute asymmetric synthesis is formulated by standard perturbation theory. An estimation of the chiral polarization of the product is provided. We show that an unstable chiral configuration can be described as a spin one-half particle. The circularly polarized field with well adjusted parameters is applied to stabilize chiral molecules (destruction of quantum coherence in the two-level system).

Destruction of quantum coherence and stochastic ionization of Rydberg electrons by fluctuating laser fields

It is shown that diffusion and stochastic ionization of an optically excited Rydberg electron are generic long time phenomena which are consequences of the destruction of quantum coherence by laser fluctuations. Quantitatively these novel fluctuation-induced phenomena are characterized by non-exponential time evolutions whose power law dependences can be determined analytically. It is demonstrated that the competition between stochastic ionization and autoionization may lead to interesting new effects.

Crossover from coherent to incoherent dynamics in damped quantum systems

The destruction of quantum coherence by environmental influences is investigated by taking the damped harmonic oscillator and the dissipative two-state system as prototypical examples. It is shown that the location of the coherent-incoherent transition depends to a large degree on the dynamical quantity under consideration.

Nonlinear response of a periodically driven damped two-state system.

We study the nonlinear response of a periodically driven dissipative two-state system. The exact formal solution in the form of a series in the number of system transitions is derived. The series is summed in analytic form in the parameter region where incoherent transitions prevail. We also present the exact solution for the time evolution of the system in the entire region of temperatures and driving strength for a special value of the Ohmic viscosity. The destruction of quantum coherence by incoherent processes which are induced by the fluctuating and the driving forces is discussed on the basis of these solutions.

Destruction of quantum coherence in a nonlinear oscillator via attenuation and amplification.

The exact solution to the master equation of a nonlinear oscillator, subject to damping or amplification, is presented. The effect of such incoherent processes on the quantum-coherence properties and recurrences is obtained. It is shown that amplification destroys quantum coherence more rapidly than does attenuation.

Destruction of quantum coherence in a micromaser by finite detection efficiency

We discuss the relative effects of selective and nonselective atomic measurements on the “quantum purity” of a single mode field. This is a model for non-unit detection efficiency in a micromaser. We show that the quantum mixing associated with non- selective measurements imposes strong limitations on the detection efficiency that can be tolerated if quantum coherences are to be generated and detected.

Particle creation and destruction of quantum coherence by topological change

The possibility is considered that changes of spatial topology occur as tunneling events in quantum gravity. Creation of scalar and spinor particles during these tunneling transitions is studied. The relevant formalism based on the euclidean Schrödinger equation and coherent state representation is developed. This formalism is illustrated in a two-dimensional example. It is argued that the particle creation during the topological changes induces the loss of quantum coherence. The particle creation is calculated in the case of O(4)-invariant background euclidean four-dimensional metrics. This calculation is used for estimating the loss of quantum coherence. An upper limit on the rate of the topological changes, A⪅10 −17M Pl 4 , is derived from the observation of K 0− K 0 oscillations.