Search results for "quant-ph"
showing 10 items of 1378 documents
A quantum particle in a box with moving walls
2013
We analyze the non-relativistic problem of a quantum particle that bounces back and forth between two moving walls. We recast this problem into the equivalent one of a quantum particle in a fixed box whose dynamics is governed by an appropriate time-dependent Schroedinger operator.
Non-Markovianity and Coherence of a Moving Qubit inside a Leaky Cavity
2017
Non-Markovian features of a system evolution, stemming from memory effects, may be utilized to transfer, storage, and revive basic quantum properties of the system states. It is well known that an atom qubit undergoes non-Markovian dynamics in high quality cavities. We here consider the qubit-cavity interaction in the case when the qubit is in motion inside a leaky cavity. We show that, owing to the inhibition of the decay rate, the coherence of the traveling qubit remains closer to its initial value as time goes by compared to that of a qubit at rest. We also demonstrate that quantum coherence is preserved more efficiently for larger qubit velocities. This is true independently of the evol…
On quantumness in multi-parameter quantum estimation
2019
In this article we derive a measure of quantumness in quantum multi-parameter estimation problems. We can show that the ratio between the mean Uhlmann Curvature and the Fisher Information provides a figure of merit which estimates the amount of incompatibility arising from the quantum nature of the underlying physical system. This ratio accounts for the discrepancy between the attainable precision in the simultaneous estimation of multiple parameters and the precision predicted by the Cram\'er-Rao bound. As a testbed for this concept, we consider a quantum many-body system in thermal equilibrium, and explore the quantum compatibility of the model across its phase diagram.
A many-body approach to transport in quantum systems : From the transient regime to the stationary state
2022
We review one of the most versatile theoretical approaches to the study of time-dependent correlated quantum transport in nano-systems: the non-equilibrium Green's function (NEGF) formalism. Within this formalism, one can treat, on the same footing, inter-particle interactions, external drives and/or perturbations, and coupling to baths with a (piece-wise) continuum set of degrees of freedom. After a historical overview on the theory of transport in quantum systems, we present a modern introduction of the NEGF approach to quantum transport. We discuss the inclusion of inter-particle interactions using diagrammatic techniques, and the use of the so-called embedding and inbedding techniques w…
Domains of time-dependent density-potential mappings
2011
The key element in time-dependent density functional theory is the one-to-one correspondence between the one-particle density and the external potential. In most approaches this mapping is transformed into a certain type of Sturm-Liouville problem. Here we give conditions for existence and uniqueness of solutions and construct the weighted Sobolev space they lie in. As a result the class of v-representable densities is considerably widened with respect to previous work.
Newton algorithm for Hamiltonian characterization in quantum control
2014
We propose a Newton algorithm to characterize the Hamiltonian of a quantum system interacting with a given laser field. The algorithm is based on the assumption that the evolution operator of the system is perfectly known at a fixed time. The computational scheme uses the Crank-Nicholson approximation to explicitly determine the derivatives of the propagator with respect to the Hamiltonians of the system. In order to globalize this algorithm, we use a continuation method that improves its convergence properties. This technique is applied to a two-level quantum system and to a molecular one with a double-well potential. The numerical tests show that accurate estimates of the unknown paramete…
Brownian motion in trapping enclosures: Steep potential wells, bistable wells and false bistability of induced Feynman-Kac (well) potentials
2019
We investigate signatures of convergence for a sequence of diffusion processes on a line, in conservative force fields stemming from superharmonic potentials $U(x)\sim x^m$, $m=2n \geq 2$. This is paralleled by a transformation of each $m$-th diffusion generator $L = D\Delta + b(x)\nabla $, and likewise the related Fokker-Planck operator $L^*= D\Delta - \nabla [b(x)\, \cdot]$, into the affiliated Schr\"{o}dinger one $\hat{H}= - D\Delta + {\cal{V}}(x)$. Upon a proper adjustment of operator domains, the dynamics is set by semigroups $\exp(tL)$, $\exp(tL_*)$ and $\exp(-t\hat{H})$, with $t \geq 0$. The Feynman-Kac integral kernel of $\exp(-t\hat{H})$ is the major building block of the relaxatio…
Calculation of modification of alkali metal atomic transition probability in strong external magnetic field and its application
2010
International audience; Interaction of alkali atoms with external magnetic field induced a splitting and a shift of their energy levels. We have study this interaction for external field from 0 to 5000 Gauss when the alkali vapor is confined in submicron thin vapor cell with thickness L = λ/2. Rubidium and Sodium vapors have been studied. The Hamiltonian can be expressed as the sum of the unperturbated atomic Hamiltonian and the so-called Zeeman Hamiltonian. The probability of a transition, induced by the laser electric field is proportional to the square of the transfer coefficients modified by the presence of the magnetic field. We will show that the strong nonlinearity of the transition …
PECULIARITIES OF RESONANT ABSORPTION AND FLUORESCENCE IN EXTREMELY THIN CELL FILLED WITH Rb AND BUFFER GAS.
2010
Recently unique extremely thin cells (ETC) containing atomic vapour of alkali metals with the column thickness L of an order and much less than optical light wavelength λ have been developed. It has been demonstrated that these ETC are a very promising tool for a fundamental study of atom-light, atom-atom, atom-surface and atom-external magnetic field interactions. Particularly, a dramatically different behaviour of resonant absorption, fluorescence and resonant magneto-optical processes compared to that obtained with the help of cm-long ordinary cells has been demonstrated. Possible applications based on ETC are described, such as magnetometers with nanometric spatial resolution and tunabl…
QUANTITATIVE SPECTROSCOPY OF Rb ATOMS IN STRONG MAGNETIC FIELD BASED ON SUBMICRON THIN VAPOUR.
2010
The energy levels of atoms placed in an external magnetic field undergo frequency shifts and changes in their transition probabilities. It is demonstrated that using fluorescence spectra from a submicron thin vapour cell (STC) with the thickness L = λ/2, it is possible to efficiently study the above mentioned changes (“half-λ Zeeman technique” (HLZT)). The circularly polarized beam of extended-cavity diode laser (λ = 794 nm, laser bandwidth γL < 1 MHz) resonant with 87Rb D1 transition, after passing through Faraday isolator is directed onto the Rb STC with the thickness L = λ/2. The temperature of the STC is 120 °C, corresponding to N ~ 1013 atom/cm3. STC was provided by a special oven with…