arXiv subjects

L. Accardi

Publications and source records attributed to L. Accardi.

At least 19 recordsLinked to original sources

arXiv2014

Fermionic Meixner probability distributions, Lie algebras and quadratic Hamiltonians

We introduce the quadratic Fermi algebra, which is a Lie algebra, and show that the vacuum distributions of the associated Hamiltonians define the fermionic Meixner probability distributions. In order to emphasize the difference with the Bose case, we apply a modification of the method used in the above calculation to obtain a simple and straightforward classification of the 1--dimensional Meixner laws in terms of homogeneous quadratic expressions in the Bose creation and annihilation operators. There is a huge literature of the Meixner laws but this, purely quantum probabilistic, derivation seems to be new. Finally we briefly discuss the possible multi-dimensional extensions of the above results.

math-ph↗

arXiv2012

On the Stochastic Limit of Quantum Field Theory

The weak coupling limit for a quantum system, with discrete energy spectrum, coupled to a Bose reservoir with the most general linear interaction is considered: under this limit we have a quantum noise processes substituting for the field. We obtain a limiting evolution unitary on the system and noise space which, when reduced to the system's degrees of freedom, provide the master and Langevin equations that are postulated on heuristic grounds by physicists. In addition we give a concrete application of our results by deriving the evolution of an atomic system interacting with the electrodynamic field without recourse to either rotating wave or dipole approximations.

math-ph↗

arXiv2011

On classical and quantum liftings

We analyze the procedure of lifting in classical stochastic and quantum systems. It enables one to `lift' a state of a system into a state of `system+reservoir'. This procedure is important both in quantum information theory and the theory of open systems. We illustrate the general theory of liftings by a particular class related to so called circulant states.

quant-ph↗

arXiv2004

Coherent quantum control of $Λ$-atoms through the stochastic limit

We investigate, using the stochastic limit method, the coherent quantum control of a 3-level atom in $Λ$-configuration interacting with two laser fields. We prove that, in the generic situation, this interaction entangles the two lower energy levels of the atom into a single qubit, i.e. it drives at an exponentially fast rate the atom to a stationary state which is a coherent superposition of the two lower levels. By applying to the atom two laser fields with appropriately chosen intensities, one can create, in principle, any superposition of the two levels. Thus {\it relaxation} is not necessarily synonymous of {\it decoherence}.

quant-ph↗

arXiv2003

Coherent population trapping in the stochastic limit

A 2-level atom with degenerate ground state interacting with a quantum field is investigated. We show, that the field drives the state of the atom to a stationary state, which is non-unique, but depends on the initial state of the system through some conserved quantities. This non-uniqueness follows from the degeneracy of the ground state of the atom, and when the ground subspace is two-dimensional, the family of stationary states will depend on a one-dimensional parameter. Only one of the stationary states in this family is a pure state, and this state coincides with the known non-coupled population trapped state (zero population in the excited level. Another one stationary state corresponds to an equal weight mixture of the excited level and of the coupled state.

quant-ph↗

arXiv2002

KMS States on the Square of White Noise Algebra

It was shown in [AFS00] that there are only three types of irreducible unitary representations theta of sl_2. Using the Schurmann triple one can associate with each theta a number of representations of the square of white noise (SWN) algebra A. However, in analogy with the Boson, Fermion and q-deformed case, we expect that some interesting non irreducible representations of sl_2 may result in GNS representations of KMS states associated with some evolutions on A. In the present paper determine the structure of the *--endomorphisms of the SWN algebra, induced by linear maps in the 1--particle Hilbert algebra, we introduce the SWN analogue of the quasifree evolutions and find the explicit form of the KMS states associated with some of them.

quant-ph↗

arXiv2002

Quantum stochastic equation for the low density limit

A new derivation of quantum stochastic differential equation for the evolution operator in the low density limit is presented. We use the distribution approach and derive a new algebra for quadratic master fields in the low density limit by using the energy representation. We formulate the stochastic golden rule in the low density limit case for a system coupling with Bose field via quadratic interaction. In particular the vacuum expectation value of the evolution operator is computed and its exponential decay is shown.

quant-ph↗

arXiv2002

A stochastic golden rule and quantum Langevin equation for the low density limit

A rigorous derivation of quantum Langevin equation from microscopic dynamics in the low density limit is given. We consider a quantum model of a microscopic system (test particle) coupled with a reservoir (gas of light Bose particles) via interaction of scattering type. We formulate a mathematical procedure (the so-called stochastic golden rule) which allows us to determine the quantum Langevin equation in the limit of large time and small density of particles of the reservoir. The quantum Langevin equation describes not only dynamics of the system but also the reservoir. We show that the generator of the corresponding master equation has the Lindblad form of most general generators of completely positive semigroups.

math-ph↗

arXiv2001

The stochastic limit of the Fröhlich Hamiltonian: relations with the quantum Hall effect

We propose a model of an approximatively two--dimensional electron gas in a uniform electric and magnetic field and interacting with a positive background through the Fröhlich Hamiltonian. We consider the stochastic limit of this model and we find the quantum Langevin equation and the generator of the master equation. This allows us to calculate the explicit form of the conductivity and the resistivity tensors and to deduce a {\it fine tuning condition} (FTC) between the electric and the magnetic fields. This condition shows that the $x$--component of the current is zero unless a certain quotient, involving the physical parameters, takes values in a finite set of physically meaningful rational numbers. We argue that this behaviour is quite similar to that observed in the quantum Hall effect. We also show that, under some conditions on the form factors entering in the definition of the model, also the plateaux and the "almost" linear behaviour of the Hall resistivity can be recovered. Our FTC does not distinguish between fractional and integer values.

cond-mat.mes-hall↗

arXiv2001

Superfluidity in the stochastic limit

In the present paper we outline the stochastic limit approach to superfluidity. The Hamiltonian describing the interaction between the Bose condensate and the normal phase is introduced. Sufficient in the stochastic limit condition of superfluidity is proposed. Existence of superfluidity in the stochastic limit of this system is proved and the non-linear (quadratic) equation of motion describing the superfluid liquid is obtained.

quant-ph↗

arXiv2001

Interaction of 3-level atom with radiation

The interaction of 3-level system with a quantum field in a non-equilibrium state is considered. We describe a class of states of the quantum field for wich a stationary state drives the system to inverse populated state. We find that the quotient of the population of the energy levels in the simplest case is described by the double Einstein formula which involves products of two Einstein emission/absorption relations. Emission and absorption of radiation by 3-level atom in non-equlibrium stationary state is described.

quant-ph↗

arXiv2001

Stimulated emission with a non-equilibrium state of radiation

The stimulated emission from an atom interacting with radiation in non-equilibrium state is considered. The stochastic limit, applied to the non-relativistic Hamiltonian describing the interaction, shows that the state of atoms, driven by some non-equilibrium state of the field approaches a stationary state which can continuously emit photon, unlike the case with an equilibrium state.

quant-ph↗

arXiv2000

Quantum decoherence and the Glauber dynamics from the Stochastic limit

The effects of decoherence for quantum system coupled with a bosonic field are investigated. An application of the stochastic golden rule shows that in the stochastic limit the dynamics of such a system is described by a quantum stochastic differential equation. The corresponding master equation describes convergence of a system to equilibrium. In particular it predicts exponential damping for off-diagonal matrix elements of the system density matrix, moreover these elements for a generic system will decay at least as \exp(-tN{kT\over\hbar}), where N is a number of particles in the system. As an application of the described technique a derivation from first principles (i.e. starting from a Hamiltonian description) of a quantum extension of the Glauber dynamics for systems of spins is given.

quant-ph↗

arXiv2000

On the structure of Markov flows

A new infinitesimal characterization of completely positive but not necessarily homomorphic Markov flows from a C^*-algebra to bounded operators on the boson Fock space over L^2(R) is given. Contrarily to previous characterizations, based on stochastic differential equations, this characterization is universal, i.e. valid for arbitrary Markov flows. With this result the study of Markov flows is reduced to the study of four C_0-semigroups. This includes the classical case and even in this case it seems to be new. The result is applied to deduce a new existence theorem for Markov flows.

quant-ph↗

arXiv1999

The stochastic limit of quantum spin systems

The stochastic limit for the system of spins interacting with a boson field is investigated. In the finite volume an application of the stochastic golden rule shows that in the limit the dynamics of a quantum system is described by a quantum white noise equation that after taking of normal order is equivalent to quantum stochastic differential equation (QSDE). For the quantum Langevin equation the dynamics is well defined and is a quantum flow on the infinite lattice system.

quant-ph↗

arXiv1999

Non-Equilibrium Quantum Field Theory and Entangled Commutation Relations

Non-equilibrium quantum field theory studies time dependence of processes which are not available for the S-matrix description. One of the new methods of investigation in non-equilibrium quantum theory is the stochastic limit method. This method is an extension of the works by Bogoliubov, van Hove and Prigogine and it permits to study not only the system but also the reservoir degrees of freedom. We consider the stochastic limit of translation invariant Hamiltonians in quantum field theory and show that the master field satisfies a new type of commutation relations, the so called entangled (or interacting) commutation relations. These relations extend the interacting Fock relations established earlier in non-relativistic QED and the free (or Boltzmann) commutation relations which have been found in the large N limit of QCD. As an application of the stochastic limit method we consider the photon splitting cascades in magnetic field and show that photons in cascades form entangled states ("triphons") and they obey not Bose but a new type of statistics corresponding to the entangled commutation relations.

hep-th↗

arXiv1999

Non-Exponential Decay for Polaron Model

A model of particle interacting with quantum field is considered. The model includes as particular cases the polaron model and non-relativistic quantum electrodynamics. We compute matrix elements of the evolution operator in the stochastic approximation and show that depending on the state of the particle one can get the non-exponential decay with the rate t^{-3/2}. In the process of computation a new algebra of commutational relations that can be considered as an operator deformation of quantum Boltzmann commutation relations is used.

quant-ph↗

arXiv1998

Dynamical q-deformation in quantum theory and the stochastic limit

A model of particle interacting with quantum field is considered. The model includes as particular cases the polaron model and non-relativistic quantum electrodynamics. We show that the field operators obey q-commutation relations with q depending on time. After the stochastic (or van Hove) limit, due to the nonlinearity, the atomic and field degrees of freedom become entangled in the sense that the field and the atomic variables no longer commute but give rise to a new algebra with new commutation relations replacing the Boson ones. This new algebra allows to give a simple proof of the fact that the non crossing half-planar diagrams give the dominating contribution in a weak coupling regime and to calculate explicitly the correlations associated to the new algebra. The above results depend crucially on the fact that we do not introduce any dipole or multipole approximation.

math.QA↗