Search results for "Quantum state"
showing 10 items of 149 documents
The coupling of the X1Σ+and a3Σ+states of the atom pair Na + Cs and modelling cold collisions
2006
The states X1?+ and a3?+ correlated to the ground-state asymptote of Na (3s) and Cs (6s) atoms have been experimentally investigated using high resolution Fourier-transform spectroscopy. The hyperfine splitting of the a3?+ state levels is partially resolved. Transitions to asymptotic vibrational levels of the a3?+ and X1?+ states were recorded simultaneously. The joint evaluation of the data of both the a3?+ and the X1?+ states allows us to determine accurate potential energy curves of both electronic states. Coupled-channels calculations are finally applied for deriving long range dispersion parameters and the exchange contribution of the molecular potentials, yielding a reliable descripti…
Competition of continuous and projective measurements in filtering processes
2016
A quantum system interacting with a repeatedly measured one turns out to be subjected to a non-unitary evolution which can force the former to a specific quantum state. It is shown that in the case where the repeatedly measured system is subjected to the action of its environment, the occurrence of a competition between the dissipation and the measurements can reduce the influence of the decay on the filtering process. Both theoretical predictions and numerical results are presented.
Classical Geometric Phases: Foucault and Euler
2020
In the last chapter we saw how a quantum system can give rise to a Berry phase, by studying the adiabatic round trip of its quantum state on a certain parameter space. Rather than considering what happens to states in Hilbert space, we now turn to classical mechanics, where we are concerned instead with the evolution of the system in configuration space. As a first example, we are interested in the geometric phase of an oscillator that is constrained to a plane that is transported over some surface which moves along a certain path in three-dimensional space. Contrary to determining the Berry phase, there is no adiabatic approximation of the motion along the curve involved. The Foucault phas…
Topological protection of highly entangled non-Gaussian two-photon states
2021
Abstract We study theoretically the evolution of entangled non-Gaussian two-photon states in disordered topological lattices. Specifically, we consider spatially entangled two-photon states, modulated by Laguerre polynomials up to the 3rd order, which feature ring-shaped spatial and spectral correlation patterns. Such states are discrete analogs of photon-subtracted squeezed states, which are ubiquitous in optical quantum information processing or sensing applications. We find that, in general, a higher degree of entanglement coincides with a loss of topological protection against disorder, this is in line with previous results for Gaussian two-photon states. However, we identify a particul…
Adiabatic approximation for quantum dissipative systems: formulation, topology and superadiabatic tracking
2010
A generalized adiabatic approximation is formulated for a two-state dissipative Hamiltonian which is valid beyond weak dissipation regimes. The history of the adiabatic passage is described by superadiabatic bases as in the nondissipative regime. The topology of the eigenvalue surfaces shows that the population transfer requires, in general, a strong coupling with respect to the dissipation rate. We present, furthermore, an extension of the Davis-Dykhne-Pechukas formula to the dissipative regime using the formalism of Stokes lines. Processes of population transfer by an external frequency-chirped pulse-shaped field are given as examples.
Hybrid optomechanics for Quantum Technologies
2014
We review the physics of hybrid optomechanical systems consisting of a mechanical oscillator interacting with both a radiation mode and an additional matter-like system. We concentrate on the cases embodied by either a single or a multi-atom system (a Bose-Einstein condensate, in particular) and discuss a wide range of physical effects, from passive mechanical cooling to the set-up of multipartite entanglement, from optomechanical non-locality to the achievement of non-classical states of a single mechanical mode. The reviewed material showcases the viability of hybridised cavity optomechanical systems as basic building blocks for quantum communication networks and quantum state-engineering…
Quantum Criticality of Spin Liquids in Novel Insulators and Magnets
2014
Strongly correlated Fermi systems are among the most intriguing and fundamental systems in physics, whose realization in some compounds is still under consideration. Quantum spin liquids are a promising new phases, where exotic quantum states of matter could be realized. Exotic quantum spin liquid (QSL) made of such hypothetic particles as fermionic spinons which carry spin \(1/2\) and no charge are considered in this chapter. Magnetic insulators with geometrical frustration produce important experimental facts shedding light on the nature of quantum spin liquid composed of spinons. We present a theory of the thermodynamic properties of quantum spin liquids, elucidating how their properties…
A Possible Time-Dependent Generalization of the Bipartite Quantum Marginal Problem
2018
In this work we study an inverse dynamical problem for a bipartite quantum system governed by the time local master equation: to find the class of generators which give rise to a certain time evolution with the constraint of fixed reduced states (marginals). The compatibility of such choice with a global unitary evolution is considered. For the non unitary case we propose a systematic method to reconstruct examples of master equations and address them to different physical scenarios.
Two-Qubit Pure Entanglement as Optimal Social Welfare Resource in Bayesian Game
2017
Entanglement is of paramount importance in quantum information theory. Its supremacy over classical correlations has been demonstrated in numerous information theoretic protocols. Here we study possible adequacy of quantum entanglement in Bayesian game theory, particularly in social welfare solution (SWS), a strategy which the players follow to maximize the sum of their payoffs. Given a multi-partite quantum state as an advice, players can come up with several correlated strategies by performing local measurements on their parts of the quantum state. A quantum strategy is called quantum-SWS if it is advantageous over a classical equilibrium (CE) strategy in the sense that none of the player…
Different operational meanings of continuous variable Gaussian entanglement criteria and Bell inequalities
2014
Entanglement, one of the most intriguing aspects of quantum mechanics, marks itself into different features of quantum states. For this reason different criteria can be used for verifying entanglement. In this paper we review some of the entanglement criteria casted for continuous variable states and link them to peculiar aspects of the original debate on the famous EPR paradox. Moreover, we give a handy expression for valuating Bell-type non-locality on Gaussian states. We also present the experimental measurement of a particular realization of the Bell operator over continuous variable entangled states produced by a sub-threshold type-II OPO.