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Lajos Diósi

Publications and source records attributed to Lajos Diósi.

At least 73 records · Page 4Linked to original sources
arXiv2013

Non-Markovian Toy Quantum Chain

We propose a simple structure for stationary non-Markovian quantum chains in the framework of collisional dynamics of open quantum systems. To this end, we modify the microscopic Markovian system--reservoir model, consider multiple collisions with each of the molecules with an overlap between the collisional time intervals. We show how the equivalent Markovian quantum chain can be constructed with the addition of satellite quantum memory to the system. We distinguish quantum from classical non-Markovianity. Moreover, we define the counts of non-Markovianity by the required number of satellite qubits and bits, respectively. As the particular measure of quantum non-Markovianity the discord of the satellite w.r.t. the system is suggested. Simplest qubit realizations are discussed, and the significance for real system--environment dynamics is also pointed out.

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arXiv2012

Non-Markovian Open Quantum Systems: Lorentzian from Markovian

As a general mission, reduced dynamics and master equations are advocated as alternative method and philosophy instead of Green functions, Kubo theory and the like. A smart reduction of the Lorentzian open system to the Markovian one (Imamoglu, 1994) is presented in simple terms.

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arXiv2012

Non-Markovian Open Quantum Systems: Input-Output Fields, Memory, Monitoring

Principles of monitoring non-Markovian open quantum systems are analyzed. We use the field representation of the environment (Gardiner and Collet, 1985) for the separation of its memory and detector part, respectively. We claim the system-plus-memory compound becomes Markovian, the detector part is tractable by standard Markovian monitoring. Because of non-Markovianity, only the mixed state of the system can be predicted, the pure state of the system can be retrodicted. We present the corresponding non-Markovian stochastic Schrödinger equation.

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arXiv2012

Classical-Quantum Coexistence: a `Free Will' Test

Von Neumann's statistical theory of quantum measurement interprets the instantaneous quantum state and derives instantaneous classical variables. In realty, quantum states and classical variables coexist and can influence each other in a time-continuous way. This has been motivating investigations since longtime in quite different fields from quantum cosmology to optics as well as in foundations. Different theories (mean-field, Bohm, decoherence, dynamical collapse, continuous measurement, hybrid dynamics, e.t.c.) emerged for what I call `coexistence of classical continuum with quantum'. I apply to these theories a sort of `free will' test to distinguish `tangible' classical variables useful for causal control from useless ones.

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arXiv2011

Thermodynamic and quantum entropy gain of frame averaging

We are discussing a universal non-unitary map M subsequent to a generic unitary map U, whose von Neumann entropy gain coincides with the calculated thermodynamic entropy production. For many-body quantum reservoirs we prove that M can be the averaging over all translations of the spatial frame. Assuming the coincidence of microscopic and macroscopic entropy productions leads to a novel equation between entropy gain of frame averaging and relative entropy. Our map M turns out to coincide with the older one called twirl, used recently in the theory of quantum reference frames. Related results to ours have been obtained and we discuss some of them briefly. Possible relevance of frame averaging (twirling) for real world irreversibility is mentioned.

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arXiv2011

The gravity-related decoherence master equation from hybrid dynamics

Canonical coupling between classical and quantum systems cannot result in reversible equations, rather it leads to irreversible master equations. Coupling of quantized non-relativistic matter to gravity is illustrated by a simplistic example. The heuristic derivation yields the theory of gravity-related decoherence proposed longtime ago by Penrose and the author.

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arXiv2009

Schrodinger-Newton equation with complex Newton constant and induced gravity

In the reversible Schrodinger-Newton equation a complex Newton coupling G*exp(-i*alpha) is proposed in place of G. The equation becomes irreversible and all initial one-body states are expected to converge to solitonic stationary states. This feature is verified numerically. For two-body solutions we point out that an effective Newtonian interaction is induced by the imaginary mean-fields as if they were real. The effective strength of such induced gravity depends on the local wave functions of the participating distant bodies.

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arXiv2009

Monitoring the wave function by time continuous position measurement

We consider a single copy of a quantum particle moving in a potential and show that it is possible to monitor its complete wave function by only continuously measuring its position. While we assume that the potential is known, no information is available about its state initially. In order to monitor the wave function, an estimate of the wave function is propagated due to the influence of the potential and continuously updated according to the results of the position measurement. We demonstrate by numerical simulations that the estimation reaches arbitrary values of accuracy below 100 percent within a finite time period for the potentials we study. In this way our method grants, a certain time after the beginning of the measurement, an accurate real-time record of the state evolution including the influence of the continuous measurement. Moreover, it is robust against sudden perturbations of the system as for example random momentum kicks from environmental particles, provided they occur not too frequently.

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arXiv2009

Does wave function collapse cause gravity?

We give a twist to the assumption - discussed in various earlier works - that gravity plays a role in the collapse of the wave function. This time we discuss the contrary assumption that the collapse of the wave function plays a role in the emergence of the gravitational field. We start from the mathematical framework of a particular Newtonian gravitational collapse theory proposed by the author longtime ago, and we reconciliate it with the classical equivalence principle.

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arXiv2008

Non-Markovian continuous quantum measurement of retarded observables

We reconsider the non-Markovian time-continuous measurement of a Heisenberg observable x and show for the first time that it can be realized by an infinite set of entangled von Neumann detectors. The concept of continuous read-out is introduced and used to re-derive the non-Markovian stochastic Schrodinger equation. We can prove that, contrary to recent doubts, the resulting non-Markovian quantum trajectories are true single system trajectories and correspond to the continuous measurement of a retarded functional of x. However, the generic non-Markovian trajectories are mixed state trajectories. This version merges an Erratum [PRL, in print] with my Letter [PRL 100, 080401 (2008)], some corrections follow directly from the recent criticism by Wiseman and Gambetta, further corrections restore the validity of my Letter. Contrary to my suggestion there, the given continuous measurement schemes cannot yield pure state trajectories but mixed-state ones. Yet, it is possible to retain my claim that the non-Markovian stochastic Schrodinger equation describes true time-continuous measurement - with delay and retrodiction.

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arXiv2008

Remark on laser linewidth hazard in opto-mechanical cooling

I discuss the robustness of the pumped cavity dynamics against phase diffusion of the laser and conclude that opto-mechanical cooling has extreme sensitivity compared to laser cooling of atoms. Certain proposals of ground state opto-mechanical cooling by single cavity would require an unrealistic sharp laser linewidth or equivalently, a very low level of phase noise. A systematic way to cancel classical excess phase noise is the interferometric twin-cavity pumping, initiated for optically trapped macro-mirrors of future gravitational-wave detectors.

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arXiv2007

Shortnote on local hidden Grassmann variables vs. quantum correlations

Grassmannian local hidden variables are shown to generate all possible quantum correlations in a bipartite quantum system. Grassmann representation of fermions, common in field theory, opens a related perspective. Although Grassmann hidden variables can not challange Bell's locality theorem, they can become an interesting mathematical tool to investigate entanglement.

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arXiv2006

Quest for quantum superpositions of a mirror: high and moderately low temperatures

The Born-Markov master equation analysis of the vibrating mirror and photon experiment proposed by Marshall, Simon, Penrose and Bouwmeester is completed by including the important issues of temperature and friction. We find that at the level of cooling available to date, visibility revivals are purely classical, and no quantum effect can be detected by the setup, no matter how strong the photon-mirror coupling is. Checking proposals of universal nonenvironmental decoherence is ruled out by dominating thermal decoherence; a conjectured coordinate-diffusion contribution to decoherence may become observable on reaching moderately low temperatures.

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arXiv2006

Notes on Certain Newton Gravity Mechanisms of Wave Function Localisation and Decoherence

Both the additional non-linear term in the Schrödinger equation and the additional non-Hamiltonian term in the von Neumann equation, proposed to ensure localisation and decoherence of macro-objects, resp., contain the same Newtonian interaction potential formally. We discuss certain aspects that are common for both equations. In particular, we calculate the enhancement of the proposed localisation and/or decoherence effects, which would take place if one could lower the conventional length-cutoff and resolve the mass density on the interatomic scale.

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arXiv2006

On exact identity between thermodynamic and informatic entropies in a unitary model of friction

An elementary collision model of a molecular reservoir is considered upon which an external field is applied and the work is dissipated into heat. To realize macroscopic irreversibility at the microscopic level, we introduce a ``graceful'' irreversible map which randomly mixes the identities of the molecules. This map is expected to generate informatic entropy exactly equal to the independently calculable irreversible thermodynamic entropy.

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arXiv2006

On Conserved Current in Markovian Open Quantum Systems

We reexamine the markovian approximation of local current in open quantum systems, discussed recently by Gebauer and Car. Our derivation is more transparent, the proof of current conservation becomes explicit and easy.

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