arXiv subjects

Lajos Diósi

Publications and source records attributed to Lajos Diósi.

At least 55 records · Page 3Linked to original sources

arXiv2016

New results unearthed from urtexts of quantum state diffusion

Thirty years ago, the present author discussed pure state unraveling (stochastic quantum trajectories) of Markovian open system dynamics. The fact that he considered all positive dynamics, not restricted for the Lindblad-Gorini-Kossakowski-Sudarshan complete-positive subclass, has remained unnoticed so far. Here we point out the importance of the transition rate operator and the merit of invariant (representation-independent) approach, with reference to invariant condition of positive dynamics and extension of quantum state diffusion beyond complete-positive dynamics. A new result, description of all diffusive unravelings of positive dynamics directly follows.

quant-ph↗

arXiv2016

Stochastic pure state representation for open quantum systems

We show that the usual master equation formalism of Markovian open quantum systems is completely equivalent to a certain state vector formalism. The state vector of the system satisfies a given frictional Schrödinger equation except for random instant transitions of discrete nature. Hasse's frictional Hamiltonian is recovered for the damped harmonic oscillator.

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arXiv2016

Sourcing semiclassical gravity from spontaneously localized quantum matter

The possibility that a classical space-time and quantum matter cohabit at the deepest level, i.e. the possibility of having a fundamental and not phenomenological semiclassical gravity, is often disregarded for lack of a good candidate theory. The standard semiclassical theory suffers from fundamental inconsistencies (e.g.: Schrödinger cat sources, faster-than-light communication and violation of the Born rule) which can only be ignored in simple typical situations. We harness the power of spontaneous localization models, historically constructed to solve the measurement problem in quantum mechanics, to build a consistent theory of (stochastic) semiclassical gravity in the Newtonian limit. Our model makes quantitative and potentially testable predictions: we recover the Newtonian pair potential up to a short distance cut-off (hence we predict no 1 particle self-interaction) and uncover an additional gravitational decoherence term which depends on the specifics of the underlying spontaneous localization model considered. We hint at a possible program to go past the Newtonian limit, towards a consistent general relativistic semiclassical gravity.

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arXiv2015

Process tomography via sequential measurements on a single quantum system

We utilize a discrete (sequential) measurement protocol to investigate quantum process tomography of a single two-level quantum system, with an unknown initial state, undergoing Rabi oscillations. The ignorance of the dynamical parameters is encoded into a continuous-variable classical system which is coupled to the two-level quantum system via a generalized Hamiltonian. This combined estimate of the quantum state and dynamical parameters is updated by using the information obtained from sequential measurements on the quantum system and, after a sufficient waiting period, faithful state monitoring and parameter determination is obtained. Numerical evidence is used to demonstrate the convergence of the state estimate to the true state of the hybrid system.

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arXiv2015

Is spontaneous wave function collapse testable at all?

Mainstream literature on spontaneous wave function collapse never reflects on or profit from the formal coincidence and conceptual relationship with standard collapse under time-continuous quantum measurement (monitoring). I propose some easy lessons of standard monitoring theory which would make spontaneous collapse models revise some of their claims. In particular, the objective detection of spontaneous collapse remains impossible as long as the correct identification of what corresponds to the signal in standard monitoring is missing from spontaneous collapse models, the physical detectability of the "signal" is not stated explicitly and, finally, the principles of physical detection are not revealed.

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arXiv2015

Orthogonal jumps of wavefunction in white-noise potentials

We investigate the evolution of the quantum state for a free particle placed into a random external potential of white-noise type. The master equation for the density matrix is derived by means of path integral method. We propose an equivalent stochastic process where the wavefunction satisfies a nonlinear Schrödinger equation except for random moments at which it shows orthogonal jumps. The relation of our work to the usual theory of quantum noise and damping is briefly discussed.

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arXiv2014

Unitary Equivalence of Quantum Walks

A simple coined quantum walk in one dimension can be characterized by a $SU(2)$ operator with three parameters which represents the coin toss. However, different such coin toss operators lead to equivalent dynamics of the quantum walker. In this manuscript we present the unitary equivalence classes of quantum walks and show that all the nonequivalent quantum walks can be distinguished by a single parameter. Moreover, we argue that the electric quantum walks are equivalent to quantum walks with time dependent coin toss operator.

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arXiv2014

Testing spontaneous wave-function collapse models on classical mechanical oscillators

We show that the heating effect of spontaneous wave-function collapse models implies an experimentally significant increment $ΔT$ of equilibrium temperature in a mechanical oscillator. The obtained form $ΔT$ is linear in the oscillator's relaxation time $τ$ and independent of the mass. The oscillator can be in a classical thermal state, the effect $ΔT$ is classical for a wide range of frequencies and quality factors. We note that the test of $ΔT$ does not necessitate quantum state monitoring but tomography. In both gravity-related (DP) and continuous spontaneous localization (CSL) models the strong-effect edge of their parameter range can be challenged in existing experiments on classical oscillators. For the CSL theory, the conjectured highest collapse rate parameter values become immediately constrained by evidences from current experiments on extreme slow-ring-down oscillators.

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arXiv2014

General Non-Markovian structure of Gaussian Master and Stochastic Schrödinger Equations

General open quantum systems display memory features, their master equations are non-Markovian. We show that the subclass of Gaussian non-Markovian open system dynamics is tractable in a depth similar to the Markovian class. The structure of master equations exhibits a transparent generalization of the Lindblad structure. We find and parametrize the class of stochastic Schrödinger equations that unravel a given master equation, such class was before known for Markovian systems only. We show that particular non-Markovian unravellings known in the literature are special cases of our class.

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arXiv2014

Equation and test of possible delay time of Newton force

Recently, a simple heuristic modification of the Newton potential with a non-zero delay-time $τ_G$ has been proposed. Our modification is largely suppressed for purely gravitational interactions, it becomes relevant under non-gravitational accelerations of the sources. We illustrate how the choice $τ_G\sim1$ms may already influence the 5th digit of G determined by Cavendish experiments. Re-evaluation of old Cavendish experiments and implementing slightly modified new ones may confirm the proposal or, at least, put a stronger upper limit on $τ_G$.

gr-qc↗

arXiv2014

Gravity-related spontaneous collapse in bulk matter

In the DP-model, gravity-related spontaneous wave function collapses suppress undesirable Schrödinger Cat states. We derive the equations of the model for the hydrodynamic-elastic (acoustic) modes in a bulk. Two particular features are discussed: the universal dominance of spontaneous collapses at large wavelengths, and the reduction of spontaneous heating by a slight refinement of the DP-model.

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arXiv2014

Determination of the stationary basis from protective measurement on a single system

We generalize protective measurement for protective joint measurement of several observables. The merit of joint protective measurement is the determination of the eigenstates of an unknown Hamiltonian rather than the determination of features of an unknown quantum state. As an example, we precisely determine the two eigenstates of an unknown Hamiltonian by a single joint protective measurement of the three Pauli matrices on a qubit state.

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arXiv2014

Comment on 'Uniqueness of the Equation for Quantum State Vector Collapse' [arXiv:1303.4284]

A diffusive stochastic Schrödinger equation (SSE) is shown for the first time, such that contributes to a non-completely positive dynamics. This contradicts to a recent Letter [arXiv:1303.4284] claiming that SSEs, under most general conditions, enforce complete-positivity. The general form and parametrization of the SSE in the Letter is different from an alternative simpler result, the difference is shown to be completely redundant because of the gauge-freedom of the state vector's phase.

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arXiv2014

Hybrid Quantum-Classical Master Equations

We discuss hybrid master equations of composite systems which are hybrids of classical and quantum subsystems. A fairly general form of hybrid master equations is suggested, its consistency is derived from the consistency of Lindblad quantum master equations. We emphasize that quantum measurement is a natural example of exact hybrid systems. We derive a heuristic hybrid master equation of time-continuous position measurement

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arXiv2013

Newton force from wave function collapse: speculation and test

The Diosi-Penrose model of quantum-classical boundary postulates gravity-related spontaneous wave function collapse of massive degrees of freedom. The decoherence effects of the collapses are in principle detectable if not masked by the overwhelming environmental decoherence. But the DP (or any other, like GRW, CSL) spontaneous collapses are not detectable themselves, they are merely the redundant formalism of spontaneous decoherence. To let DP collapses become testable physics, recently we extended the DP model and proposed that DP collapses are responsible for the emergence of the Newton gravitational force between massive objects. We identified the collapse rate, possibly of the order of 1/ms, with the rate of emergence of the Newton force. A simple heuristic emergence (delay) time was added to the Newton law of gravity. This non-relativistic delay is in peaceful coexistence with Einstein's relativistic theory of gravitation, at least no experimental evidence has so far surfaced against it. We derive new predictions of such a `lazy' Newton law that will enable decisive laboratory tests with available technologies. The simple equation of 'lazy' Newton law deserves theoretical and experimental studies in itself, independently of the underlying quantum foundational considerations.

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arXiv2013

Note on Possible Emergence Time of Newtonian Gravity

If gravity were an emergent phenomenon, some relativistic as well as non-relativistic speculations claim it is, then a certain emergence time scale tau_? would characterize it. We argue that the available experimental evidences have poor time resolution regarding how immediate the creation of Newton field of accelerated mass sources is. Although the concrete theoretical model of gravity's `laziness' is missing, the concept and the scale tau_? ~ 1ms, rooted in an extrapolation of spontaneous wave function collapse theory, might be tested directly in reachable experiments.

gr-qc↗

arXiv2013

Gravity-related wave function collapse: Is superfluid He exceptional?

The gravity-related model of spontaneous wave function collapse, a longtime hypothesis, damps the massive Schrödinger Cat states in quantum theory. We extend the hypothesis and assume that spontaneous wave function collapses are responsible for the emergence of Newton interaction. Superfluid helium would then show significant and testable gravitational anomalies.

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arXiv2013

Gravity-related wave function collapse: mass density resolution

Selected issues of the concept of spontaneous collapse are discussed, with the emphasis on the gravity-related model. We point out that without spontaneous collapses the Schrödinger cat states would macroscopically violate the standard conservation laws even in the presence of environmental noise. We prove that the collapse time of condensed matter c.o.m. superpositions is not sensitive to the natural uncertainty of the nuclear locations whereas we formulate the conjecture that superfluid He may show an anomalous low rate of spontaneous collapse compared to common condensed matter.

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