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Lajos Diosi

Publications and source records attributed to Lajos Diosi.

At least 19 recordsLinked to original sources
arXiv2023

Cancellation effects as a fingerprint of quantum collapse models at atomic scale

In this work the spontaneous electromagnetic radiation from atomic systems, induced by dynamical wave-function collapse, is investigated in the X-rays domain. Strong departures are evidenced with respect to the simple cases considered until now in the literature, in which the emission is either perfectly coherent (protons in the same nuclei) or incoherent (electrons). In this low-energy regime the spontaneous radiation rate strongly depends on the atomic species under investigation and, for the first time, is found to depend on the specific collapse model.

quant-ph↗
arXiv2022

Measurement-based Feedback Control of a Quantum System in a Harmonic Potential

We present a formulation of measurement-based feedback control of a single quantum particle in one spatial dimension. An arbitrary linear combination of the position and momentum of the particle is continuously monitored, and feedback proportional to the measured signal is used to control the system. We derive a feedback master equation and discuss a general approach to computing the steady-state solutions for arbitrary potentials. For a quantum harmonic oscillator or a free particle, we show that it is possible to cool and confine the system using feedback that simultaneously damps the measured observable and its conjugate momentum, as well as compensates for noise introduced by the measurement. In addition, we demonstrate that appropriate feedback adds a quadratic term in the measured observable to the Hamiltonian of the system. For the particular case of the harmonic potential, we describe the exact dynamics of the system that can be cooled to the ground state. Moreover, we provide an argument for the possibility to cool systems with arbitrary potentials provided that the measurement is strong enough to localise the particle on an interval smaller than the characteristic length scale of the potential.

quant-ph↗
arXiv2016

Quantum Flywheel

A quantum flywheel is studied with the purpose of storing useful work in quantum levels, while additional power is extracted continuously from the device. The flywheel gains its energy form a quantum heat engine. Generally, when a work repository is quantized the work exchange with the engine is accompanied with heat exchange, which may degrade the charging efficiency. In the particular realization of a quantum harmonic oscillator work repository, quantum and thermal fluctuations dominates the dynamics. Quantum monitoring and feedback control are applied to the flywheel, as it is shown to be an essential part of stabilizing and regulating its state of operation, and bringing the system to a steady state. A particular balance between information gained by measuring the system and the information fed back to the system is found to maximize the charging efficiency. The dynamics of the flywheel are described by a stochastic master equation that accounts for the engine, the external driving, the measurement, and the feedback operations.

quant-ph↗
arXiv2009

Quantum linear Boltzmann equation with finite intercollision time

Inconsistencies are pointed out in the usual quantum versions of the classical linear Boltzmann equation constructed for a quantized test particle in a gas. These are related to the incorrect formal treatment of momentum decoherence. We prove that ideal collisions would result in complete momentum decoherence, the persistence of coherence is only due to the finite intercollision time. A corresponding novel quantum linear Boltzmann equation is proposed.

quant-ph↗
arXiv2008

Test of weak measurement on a two- or three-qubit computer

Current quantum computer technology is sufficient to realize weak measurements and the corresponding concept of weak values. We demonstrate how the weak value anomaly can be tested, along with consistency and simultaneity of weak values, using only discrete degrees of freedom. All you need is a quantum computer with two--or better, three--qubits. We also give an interpretation of the weak value as an effective field strength in a postselected spin measurement.

quant-ph↗
arXiv2007

Simple theory of the measured current through quantum dots

A simple theory of the detected current I(t) flowing through charge qubits -- quantum dots -- is proposed in terms of standard continuous measurement theory. Applied to a double dot, our formalism easily confirms previous results on quantum Zeno effect, driven by growing ammeter performance gamma. Due to the transparent formalism, we can calculate the exact fluctuation spectrum S(omega) of the detected current, containing a significant Lorentzian peak near the Rabi frequency of the double dot.

cond-mat.mes-hall↗
arXiv2007

Unique family of consistent histories in the Caldeira-Leggett model

Based on the standard statistical interpretation of mixed quantum states, a unique family of consistent histories has been constructed for the quantum Brownian motion in the Caldeira-Leggett reservoir. Analytic solutions have been shown in the Markovian regime: they are uniquely defined coherent wave packets travelling near classical trajectories.

gr-qc↗
arXiv2006

Coupled Ito equations of continuous quantum state measurement, and estimation

We discuss a non-linear stochastic master equation that governs the time-evolution of the estimated quantum state. Its differential evolution corresponds to the infinitesimal updates that depend on the time-continuous measurement of the true quantum state. The new stochastic master equation couples to the two standard stochastic differential equations of time-continuous quantum measurement. For the first time, we can prove that the calculated estimate almost always converges to the true state, also at low-efficiency measurements. We show that our single-state theory can be adapted to weak continuous ensemble measurements as well.

quant-ph↗
arXiv2006

Continuous Wave Function Collapse in Quantum-Electrodynamics?

Time-continuous wavefunction collapse mechanisms n o t restricted to markovian approximation have been found only a few years ago, and have left many issues open. The results apply formally to the standard relativistic quantum-electrodynamics. I present a generalized Schrodinger equation driven by a certain complex stochastic field. The equation reproduces the e x a c t dynamics of the interacting fermions in QED. The state of the fermions appears to collapse continuously, due to their interaction with the photonic degrees of freedom. Even the formal study is instructive for the foundations of quantum mechanics and of field theory as well.

quant-ph↗
arXiv2005

Weak measurements in quantum mechanics

The article recapitulates the concept of weak measurement in its broader sense encapsulating the trade between asymptotically weak measurement precision and asymptotically large measurement statistics. Essential applications in time-continuous measurement and in postselected measurement are presented both in classical and in quantum contexts. We discuss the anomalous quantum weak value in postselected measurement. We concentrate on the general mathematical and physical aspects of weak measurements and we do not expand on their interpretation. Particular applications, even most familiar ones, are not subject of the article which was written for Elsevier's Encyclopedia of Mathematical Physics.

quant-ph↗
arXiv2005

Intrinsic time-uncertainties and decoherence: comparison of 4 models

Four models of energy decoherence are discussed from the common perspective of intrinsic time-uncertainty. The four authors -- Milburn, Adler, Penrose, and myself -- have four different approaches. The present work concentrates on their common divisors at the level of the proposed equations rather than at the level of the interpretations. General relationships between time-uncertainty and energy-decoherence are presented in both global and local sense. Global and local master equations are derived. (The local concept is favored.)

quant-ph↗
arXiv2003

Anomalies of weakened decoherence criteria for quantum histories

The theory of decoherent histories is checked for the requirement of statistical independence of subsystems. Strikingly, this is satisfied only when the decoherence functional is diagonal in both its real a n d imaginary parts. In particular, the condition of consistency (or weak decoherence) required for the assignment of probabilities appears to be ruled out. The same conclusion is obtained independently, by claiming a plausible dynamical robustness of decoherent histories.

quant-ph↗
arXiv2003

Tentative statistical interpretation of non-equilibrium entropy

We suggest a certain statistical interpretation for the entropy produced in driven thermodynamic processes. The exponential function of half irreversible entropy re-weights the probability of the standard Ornstein-Uhlenbeck-type thermodynamic fluctuations.(We add a proof of the standard Fluctuation Theorem which represents a more natural interpretation.)

physics.chem-ph↗
arXiv2003

Probability of intrinsic time-arrow from information loss

Time-arrow $s=+/-$, intrinsic to a concrete physical system, is associated with the direction of information loss $ΔI$ displayed by the random evolution of the given system. When the information loss tends to zero the intrinsic time-arrow becomes uncertain. We propose the heuristic relationship $1/[1+exp(-sΔI)]$ for the probability of the intrinsic time-arrow. The main parts of the present work are trying to confirm this heuristic equation. The probability of intrinsic time arrow is defined by Bayesian inference from the observed random process. From irreversible thermodynamic systems, the proposed heuristic probabilities follow via the Gallavotti-Cohen relations between time-reversed random processes. In order to explore the underlying microscopic mechanism, a trivial microscopic process is analyzed and an obvious discrepancy is identified. It can be resolved by quantum theory. The corresponding trivial quantum process will exactly confirm the proposed heuristic time-arrow probability.

quant-ph↗
arXiv2003

Progressive Decoherence and Total Environmental Disentanglement

The simple stationary decoherence of a two-state quantum system is discussed from a new viewpoint of environmental entanglement. My work emphasizes that an unconditional local state must totally be disentangled from the rest of the universe. It has been known for long that the loss of coherence within the given local system is gradual. Also the quantum correlations between the local system and the rest of the universe are being destroyed gradually. I show that, differently from local decoherence, the process of environmental disentanglement may terminate in finite time. The time of perfect disentanglement turns out to be on the decoherence time scale, and in a simple case we determine the exact value of it.

quant-ph↗
arXiv2002

Wigner Centennial: His Function, and Its Environmental Decoherence

In 1983, Wigner outlined a modified Schrödinger--von-Neumann equation of motion for macroobjects, to describe their typical coupling to the environment. This equation has become a principal model of environmental decoherence which is beleived responsible for the emergence ofclassicality in macroscopic quantum systems. Typically, this happens gradually and asymptotically after a certain characteristic decoherence time. For the Wigner-function, however, one can prove that it evolves perfectly into a classical (non-negative) phase space distribution after a finite time of decoherence.

quant-ph↗
arXiv2002

Shannon information increase and rescue in friction

On the standard microscopic model of friction we confirm the common belief that the irreversible entropy production originates from the increase of Shannon information. We reveal that the reversible microscopic dynamics would continuously violate the Gibbsian interchangeability of molecules. The spontaneous restoration of interchangeability constitutes the mechanism of irreversibility. This is the main suggestion of the Letter. We also point out that the irreversible entropy can partially be rescued if the size of the medium is finite. The rescueable entropy is found to be proportional to the square of the Fisher statistical (also called thermodynamic) distance.

physics.chem-ph↗