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Lajos Diosi

Publications and source records attributed to Lajos Diosi.

At least 37 records · Page 2Linked to original sources
arXiv2002

Comment on "Stable Quantum Computation of Unstable Classical Chaos"

I think the title and content of the recent Letter by Georgeot and Shepelyanski [PRL 86, 5393 (2001), also quant-ph/0101004)] are not correct. As long as the classical Arnold map is considered, the classical computational algorithm can be made exactly equivalent with the quantum one. The claimed advantage of the Letter's quantum algorithm disappears if we correctly restrict the statistical analysis for the classical Arnold system.

quant-ph↗
arXiv2002

Single qubit estimation from repeated unsharp measurements

When estimating an unknown single pure qubit state, the optimum fidelity is 2/3. As it is well known, the value 2/3 can be achieved in one step, by a single ideal measurement of the polarization along a random direction. I analyze the opposite strategy which is the long sequence of unsharp polarization measurements. The evolution of the qubit under the influence of repeated measurements is quite complicated in the general case. Fortunately, in a certain limit of very unsharp measurements the qubit will obey simple stochastic evolution equations known for long under the name of time-continuous measurement theory. I discuss how the outcomes of the very unsharp measurements will asymptotically contribute to our knowledge of the original qubit. It is reassuring that the fidelity will achieve the optimum 2/3 for long enough sequences of the unsharp measurements.

quant-ph↗
arXiv2002

Advancement of estimation fidelity in continuous quantum measurement

We estimate an unknown qubit from the long sequence of n random polarization measurements of precision Delta. Using the standard Ito-stochastic equations of the aposteriori state in the continuous measurement limit we calculate the advancement of fidelity. We show that the standard optimum value 2/3 is achieved asymptotically for n >> Delta^2 / 96 >> 1. We append a brief derivation of novel Ito-equations for the estimate state.

quant-ph↗
arXiv2002

Exact positivity of the Wigner and P-functions of a Markovian open system

We discuss the case of a Markovian master equation for an open system, as it is frequently found from environmental decoherence. We prove two theorems for the evolution of the quantum state. The first one states that for a generic initial state the corresponding Wigner function becomes strictly positive after a finite time has elapsed. The second one states that also the P-function becomes exactly positive after a decoherence time of the same order. Therefore the density matrix becomes exactly decomposable into a mixture of Gaussian pointer states.

quant-ph↗
arXiv2002

Evolution of a qubit under the influence of a succession of unsharp measurements

We investigate the evolution of a single qubit subject to a continuous unitary dynamics and an additional interrupting influence which occurs periodically. One may imagine a dynamically evolving closed quantum system which becomes open at certain times. The interrupting influence is represented by an operation, which is assumed to equivalently describe a non-selective unsharp measurement. It may be decomposed into a positive operator, which in case of a measurement represents the pure measurement part, followed by an unitary back-action operator. Equations of motion for the state evolution are derived in the form of difference equations. It is shown that the 'free' Hamiltonian is completed by an averaged Hamiltonian, which goes back to the back-action. The positive operator specifies a decoherence rate and results in a decoherence term. The continuum limit to a master equation is performed. The selective evolution is discussed and correcting higher order terms are worked out in an Appendix.

quant-ph↗
arXiv2001

Comment on `Quantum-Anti-Zeno Paradox'

One can reduce the involved derivation of Balachandran and Roy of their `anti-Zeno' effect [Phys.Rev.Lett. 84, 4019 (2000)] to the derivation of standard Zeno effect. The mechanism of what the authors call `anti-Zeno' effect is a dynamic version of Zeno effect.

quant-ph↗
arXiv2001

Robustness and diffusion of pointer states

Classical properties of an open quantum system emerge through its interaction with other degrees of freedom (decoherence). We treat the case where this interaction produces a Markovian master equation for the system. We derive the corresponding distinguished local basis (pointer basis) by three methods. The first demands that the pointer states mimic as close as possible the local non-unitary evolution. The second demands that the local entropy production be minimal. The third imposes robustness on the inherent quantum and emerging classical uncertainties. All three methods lead to localized Gaussian pointer states, their formation and diffusion being governed by well-defined quantum Langevin equations.

quant-ph↗
arXiv2000

Calibration of Local Chiralities and Time-Arrows Based on Quantum Nonlocality

Unlike classical physics, quantum mechnanics is sensitive to mistaken choice of chiralities or time-arrows of local reference systems. Quantum correlations between distant electron spins, for instance, would reveal a mistaken local chirality. Local polarization measurements and classical communication enable the distant partners to compare their local chiralities. Local time-arrows can be calibrated in a similar way.

quant-ph↗
arXiv1999

On kinematics and dynamics of independent pion emission

Multiparticle boson states, proposed recently for 'independently' emitted pions in heavy ion collisions, are reconsidered in standard second quantized formalism and shown to emerge from a simplistic chaotic current dynamics. Compact equations relate the density operator, the generating functional of multiparticle counts, and the correlator of the external current to each other. 'Bose-Einstein-condensation' is related to the external pulse. A quantum master equation is advocated for future Monte-Carlo simulations.

hep-ph↗
arXiv1999

Royal Road to Coupling Classical and Quantum Dynamics

We present a consistent framework of coupled classical and quantum dynamics. Our result allows us to overcome severe limitations of previous phenomenological approaches, like evolutions that do not preserve the positivity of quantum states or that allow to activate quantum nonlocality for superluminal signaling. A `hybrid' quantum-classical density is introduced and its evolution equation derived. The implications and applications of our result are numerous: it incorporates the back-reaction of quantum on classical variables, it resolves fundamental problems encountered in standard mean field theories, and remarkably, also the quantum measurement process, i.e. the most controversial example of quantum-classical interaction is consistently described within our approach, leading to a theory of dynamical collapse.

quant-ph↗
arXiv1999

Quantum trajectories for Brownian motion

We present the stochastic Schroedinger equation for the dynamics of a quantum particle coupled to a high temperature environment and apply it the dynamics of a driven, damped, nonlinear quantum oscillator. Apart from an initial slip on the environmental memory time scale, in the mean, our result recovers the solution of the known non-Lindblad quantum Brownian motion master equation. A remarkable feature of our approach is its localization property: individual quantum trajectories remain localized wave packets for all times, even for the classically chaotic system considered here, the localization being stronger the smaller $\hbar$.

quant-ph↗
arXiv1999

Emergence of Classicality: From Collapse Phenomenologies to Hybrid Dynamics

In the past ten-fifteen years, stochastic models of continuous wave function collapse were being proposed to describe the continuous emergence of classicality from quantum. We advocate that the hybrid dynamics of canonically coupled quantum and classical systems is a more basic concept. Continuous collapse formalisms are obtained as special cases. To illustrate our claim we show how von Neumann collapse follows from hybrid dynamical equations.

quant-ph↗
arXiv1999

Post-Markov master equation for the dynamics of open quantum systems

A systematic first-order correction to the standard Markov master equation for open quantum systems interacting with a bosonic bath is presented. It extends the Markov Lindblad master equation to the more general case of non-Markovian evolution. The meaning and applications of our `post'-Markov master equation are illustrated with several examples, including a damped two-level atom, the spin-boson model and the quantum Brownian motion model. Limitations of the Markov approximation, the problem of positivity violation and initial slips are also discussed.

quant-ph↗
arXiv1999

On hybrid dynamics of the Copenhagen dichotomic world

In the Copenhagen viewpoint, part of the world is quantized and the complementary part remains classical. From a formal dynamic aspect, standard theory is incomplete since it does never account for the so-called 'back-reaction' of quantized systems on classical systems except for the highly idealized system-detector interaction. To resolve this formal issue, a certain 'hybrid dynamics' can be constructed to account for the generic interaction between classical and quantized parts. Hybrid dynamics incorporates standard quantum theory, including collapse of the wave function during system-detector interaction. Measurable predictions are robust against shifting the classical-quantum boundary (von Neumann-cut).

quant-ph↗
arXiv1998

Open system dynamics with non-Markovian quantum trajectories

A non-Markovian stochastic Schroedinger equation for a quantum system coupled to an environment of harmonic oscillators is presented. Its solutions, when averaged over the noise, reproduce the standard reduced density operator without any approximation. We illustrate the power of this approach with several examples, including exponentially decaying bath correlations and extreme non-Markovian cases, where the `environment' consists of only a single oscillator. The latter case shows the decay and revival of a `Schroedinger cat' state. For strong coupling to a dissipative environment with memory, the asymptotic state can be reached in a finite time. Our description of open systems is compatible with different positions of the `Heisenberg cut' between system and environment.

quant-ph↗
arXiv1998

Relativistic formulation of quantum state diffusion?

The recently reported relativistic formulation of the well-known non-relativistic quantum state diffusion is seriously mistaken. It predicts, for instance, inconsistent measurement outcomes for the same system when seen by two different inertial observers.

quant-ph↗
arXiv1997

The non-Markovian stochastic Schrodinger equation for open systems

We present the non-Markovian generalization of the widely used stochastic Schrodinger equation. Our result allows to describe open quantum systems in terms of stochastic state vectors rather than density operators, without approximation. Moreover, it unifies two recent independent attempts towards a stochastic description of non-Markovian open systems, based on path integrals on the one hand and coherent states on the other. The latter approach utilizes the analytical properties of coherent states and enables a microscopic interpretation of the stochastic states. The alternative first approach is based on the general description of open systems using path integrals as originated by Feynman and Vernon.

quant-ph↗