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Lajos Diosi

Publications and source records attributed to Lajos Diosi.

53 records · Page 3Linked to original sources
arXiv1997

Coherent States and the Measurement Problem

The convenience of coherent state representation is discussed from the viewpoint of what is in a broad sense called the measurement problem in quantum mechanics. Standard quantum theory in coherent state representation is intrinsically related to a number of earlier concepts conciliating quantum and classical processes. From a natural statistical interpretation, free of collapses or measurements, the usual von Neumann-Lüders collapse as well as its quantum state diffusion interpretation follow. In particular, a theory of coupled quantum and classical dynamics arises, containing the fluctuation corrections versus the fenomenological mean-field theories.

quant-ph↗
arXiv1995

A True Equation to Couple Classical and Quantum Dynamics

Starting from the Schrödinger-equation of a composite system, we derive unified dynamics of a classical harmonic system coupled to an arbitrary quantized system. The classical subsystem is described by random phase-space coordinates entangled with the quantized variables of the complementary subsystem. Our semiclassical equation is {\it true} in a sense that its predictions are identical to those of the fully quantized composite dynamics. This exact method applies to a broad class of theories, including e.g. the relativistic quantum-electrodynamics and the electron-fonon dynamics.

quant-ph↗
arXiv1995

Exact semiclassical wave equation for stochastic quantum optics

Semiclassical (stochastic) wave equations are proposed for the coupled dynamics of atomic quantum states and semiclassical radiation field. All relevant predictions of standard unitary quantum dynamics are exactly reproducible in the framework of stochastic wave equation model. We stress in such a way that the concept of stochastic wave equations is not to be restricted to the widely used Markovian approximation.

quant-ph↗
arXiv1995

Comment on 'Reduced System Dynamics from the N-Body Schrodinger Equation'

We argue that the "reduced wave function", proposed recently [Phys.Rev.Lett. {\bf 75}, 2255 (1995)], contains conditional and restricted information on the reduced system. The concept of "reduced wave function" can thus not represent a relevant alternative to the common reduced dynamics methods.

quant-ph↗
arXiv1995

All local quantum states are mixtures of direct products

According to Popescu's recent analysis [Phys. Rev. Lett. {\bf72}, 797 (1994)], {\it nonideal} measurements, rather than ideal ones, may be more sensitive to reveal nonlocal correlations between distant parts of composite quantum systems. The outcome statistics of joint nonideal measurements on local states should by definition admit local hidden variable models. We prove that the density operator of a local composite system must be convex mixture of the subsystems' density operators. This result depends essentially on a plausible consistency condition restricting the class of admissible local hidden variable models.

quant-ph↗
arXiv1995

Quantum dynamics with two Planck constants and the semiclassical limit

The mathematical possibility of coupling two quantum dynamic systems having two different Planck constants, respectively, is investigated. It turns out that such canonical dynamics are always irreversible. Semiclassical dynamics is obtained by letting one of the two Planck constants go to zero. This semiclassical dynamics will preserve positivity, as expected, so an improvement of the earlier proposals by Aleksandrov and by Boucher and Traschen is achieved. Coupling of quantized matter to gravity is illustrated by a simplistic example.

quant-ph↗
arXiv1994

Continuously Diagonalized Density Operatorof Open Systems

We showed several years ago that the density operator of Markovian open systems can be diagonalized continuously in time. The resulting pure state jump processes correspond to quantum trajectories proposed in recent quantum optics calculations or, at fundamental level, to exact consistent histories.

gr-qc↗
arXiv1994

On maximum number of decoherent histories

It is shown that $N^2$ is the upper limit for the number of histories in a decohering family of $N$-state quantum system. Simple criterion is found for a family of $N^2$ fine grained decohering histories of Gell-Mann and Hartle to be identical with a family of Griffiths' consistent quantum trajectories.

gr-qc↗
arXiv1994

Critique of weakly decohering and linearly positive histories

The usual composition rule of independent systems, as applied to decoherent histories or to linearly positive histories, requires (at least) medium decoherence and, consequently, a second constraint for the linearly positive histories of Goldstein and Page. Other plausible classical features, valid for medium decoherence, seem anyhow to be missed by linear positive histories.

gr-qc↗
arXiv1994

Quantum Master Equation of Particle in Gas Environment

The evolution of the reduced density operator $ρ$ of Brownian particle is discussed in single collision approach valid typically in low density gas environments. This is the first succesful derivation of quantum friction caused by {\it local} environmental interactions. We derive a Lindblad master equation for $ρ$, whose generators are calculated from differential cross section of a single collision between Brownian and gas particles, respectively. The existence of thermal equilibrium for $ρ$ is proved. Master equations proposed earlier are shown to be particular cases of our one.

gr-qc↗
arXiv1993

Unique Quantum Paths by Continuous Diagonalization of the Density Operator

In this short note we show that for a Markovian open quantum system it is always possible to construct a unique set of perfectly consistent Schmidt paths, supporting quasi-classicality. Our Schmidt process, elaborated several years ago, is the $Δt\to 0$ limit of the Schmidt chain constructed very recently by Paz and Zurek.

gr-qc↗
arXiv1993

Comment on Consistent Interpretation of Quantum Mechanics Using Quantum Trajectories

Recently, Griffiths presented a generalization of the consistent history approach to quantum mechanics. I can easily construct all possible complete families satisfying Griffiths' "noninterference conditions". Since only trivial families exist one may conclude that Griffiths' proposal has not got farther than the ordinary theory of quantum measurement.

gr-qc↗