About Me

I completed my PhD in 2013 under the supervision of Madalin Guta in the School of Mathematical Sciences at the University of Nottingham, resulting in my PhD thesis, Large Deviations and Dynamical Phase Transitions for Quantum Markov Processes. From 2014 to 2016 I was a postdoctoral researcher in the condensed matter theory group in the School of Physics and Astronomy at the University of Nottingham.

My research interests revolve around the study and classification of dynamical features of open quantum systems, from low-dimensional to complex many-body systems. In particular, I am interested in the mathematical description of such systems, which typically involves quantum probability and quantum dynamical semigroups; my goal is to use these theoretical frameworks in the study of concrete open quantum systems in order to explain and classify critical features such as dynamical heterogeneity and dynamical phase transitions.

(The above background image shows sample quantum jump trajectories in a particular many-body open quantum system.)

Contact Details

Merlijn van Horssen
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merlijnREMOVETHIS@gmail.com

Papers

Dynamics of many-body localization in a translation-invariant quantum glass model

van Horssen, M., Levi, E., Garrahan, J.P.Phys. Rev. B 92, 100305(R)2015

(Editors' Suggestion)
We study the real-time dynamics of a translationally invariant quantum spin chain, based on the East kinetically constrained glass model, in search for evidence of many-body localization in the absence of disorder. Numerical simulations indicate a change, controlled by a coupling parameter, from a regime of fast relaxation-corresponding to thermalization-to a regime of very slow relaxation. This slowly relaxing regime is characterized by dynamical features usually associated with nonergodicity and many-body localization (MBL): memory of initial conditions, logarithmic growth of entanglement entropy, and nonexponential decay of time correlators. We show that slow relaxation is a consequence of sensitivity to spatial fluctuations in the initial state. While numerical results and physical considerations indicate that relaxation time scales grow markedly with size, our finite size results are consistent both with an MBL transition, expected to only occur in disordered systems, and with a pronounced quasi-MBL crossover.

Open quantum reaction-diffusion dynamics: absorbing states and relaxation

van Horssen, M., Garrahan, J.P.Phys. Rev. E 91, 0321322015

We consider an extension of classical stochastic reaction-diffusion (RD) dynamics to open quantum systems. We study a class of models of hard core particles on a one-dimensional lattice whose dynamics is generated by a quantum master operator and where particle hopping is coherent while reactions, such as pair annihilation or pair coalescence, are dissipative. These are quantum open generalisations of the A+A→∅ and A+A→A classical RD models. We characterise the relaxation of the state towards the stationary regime via a decomposition of the system Hilbert space into transient and recurrent subspaces. We provide a complete classification of the structure of the recurrent subspace (and the non-equilibrium steady states) in terms of the dark states associated to the quantum master operator and its general spectral properties. We also show that, in one dimension, relaxation towards these absorbing dark states is slower than that predicted by a mean-field analysis due to fluctuation effects, in analogy with what occurs in classical RD systems. Numerical simulations of small systems suggest that the decay of the density in one dimension, in both the open quantum A+A→∅ and A+A→A cases, may go asymptotically as t^(-b) with 1/2 < b < 1.

Sanov and Central Limit Theorems for output statistics of quantum Markov chains

van Horssen, M., Guta, M.J. Math. Phys. 56, 0221092015

In this paper we consider the statistics of repeated measurements on the output of a quantum Markov chain. We establish a large deviations result analogous to Sanov's theorem for the empirical measure associated to finite sequences of consecutive outcomes of a classical stochastic process. Our result relies on the construction of an extended quantum transition operator (which keeps track of previous outcomes) in terms of which we compute moment generating functions, and whose spectral radius is related to the large deviations rate function. As a corollary to this we obtain a central limit theorem for the empirical measure. Such higher level statistics may be used to uncover critical behaviour such as dynamical phase transitions, which are not captured by lower level statistics such as the sample mean. As a step in this direction we give an example of a finite system whose level-one rate function is independent of a model parameter while the level-two rate is not.

Large Deviations, Central Limit and dynamical phase transitions in the atom maser

van Horssen, M., Guta, M.arXiv preprint, arXiv:1206.49562013

The theory of quantum jump trajectories provides a new framework for understanding dynamical phase transitions in open systems. A candidate for such transitions is the atom maser, which for certain parameters exhibits strong intermittency in the atom detection counts, and has a bistable stationary state. Although previous numerical results suggested that the "free energy" may not be a smooth function, we show that the atom detection counts satisfy a large deviations principle, and therefore we deal with a phase crossover rather than a genuine phase transition. We argue however that the latter occurs in the limit of infinite pumping rate. As a corollary, we obtain the Central Limit Theorem for the counting process.

The proof relies on the analysis of a certain deformed generator whose spectral bound is the limiting cumulant generating function. The latter is shown to be smooth, so that a large deviations principle holds by the Gärtner-Ellis Theorem. One of the main ingredients is the Krein-Rutman Theorem which extends the Perron-Frobenius theory to a general class of positive compact semigroups.

Characterization of Dynamical Phase Transitions in Quantum Jump Trajectories Beyond the Properties of the Stationary State

Lesanovsky, I., van Horssen, M., Guta, M., Garrahan, J.P.Phys. Rev. Lett. 110, 1504012013

We describe how to characterize dynamical phase transitions in open quantum systems from a purely dynamical perspective, namely, through the statistical behavior of quantum jump trajectories. This approach goes beyond considering only properties of the steady state. While in small quantum systems dynamical transitions can only occur trivially at limiting values of the controlling parameters, in many-body systems they arise as collective phenomena and within this perspective they are reminiscent of thermodynamic phase transitions. We illustrate this in open models of increasing complexity: a three-level system, a dissipative version of the quantum Ising model, and the micromaser. In these examples dynamical transitions are accompanied by clear changes in static behavior. This is however not always the case, and in general dynamical phases need to be uncovered by observables which are strictly dynamical, e.g. dynamical counting fields. We demonstrate this via the example of a class of models of dissipative quantum glasses, whose dynamics can vary widely despite having identical (and trivial) stationary states.

Asymptotic inference in system identification for the atom maser

Catana, C., van Horssen, M., Guta, M.Phil. Trans. R. Soc. A, Vol. 370, No. 1979, pp. 5308–53232012

System identification is closely related to control theory and plays an increasing role in quantum engineering. In the quantum set-up, system identification is usually equated to process tomography, i.e. estimating a channel by probing it repeatedly with different input states. However, for quantum dynamical systems such as quantum Markov processes, it is more natural to consider the estimation based on continuous measurements of the output, with a given input that may be stationary. We address this problem using asymptotic statistics tools, for the specific example of estimating the Rabi frequency of an atom maser. We compute the Fisher information of different measurement processes as well as the quantum Fisher information of the atom maser, and establish the local asymptotic normality of these statistical models. The statistical notions can be expressed in terms of spectral properties of certain deformed Markov generators, and the connection to large deviations is briefly discussed.

Thesis

Large deviations and dynamical phase transitions for quantum Markov processes

van Horssen, M.PhD thesis, The University of NottinghamDownload (PDF)2014

Supervisor: Dr. Madalin Guta

Conferences

7th Nottingham Symposium on Quantum Systems

University of Nottingham, Nottingham, United Kingdom 1 October, 2015

Third Nottingham Workshop on Quantum Non-equilibrium Dynamics

University of Nottingham, Nottingham, United Kingdom 22-23 June, 2015

Mathematical physics of non-equilibrium quantum systems

King's College, London, United Kingdom 18-19 December, 2014

EPSRC NetworkPlus Workshop on Non-equilibrium Quantum Systems

University of Nottingham, Nottingham, United Kingdom 29-30 May, 2014

Workshop on Quantum Probability, Statistics and Control

University of Nottingham, Nottingham, United Kingdom 4-5 July 2013

Student conference on mathematical and theoretical aspects of Quantum Mechanics

University of Nottingham, Nottingham, United Kingdom 7-9 November 2012

QuaCS Quantum Correlations Students Workshop

University of Nottingham, Nottingham, United Kingdom 1-3 July 2012.

Workshop on New Directions in Quantum Statistics

University of Nottingham, Nottingham, United Kingdom 25-26 June 2012

44 Symposium on Mathematical Physics "New Developments in the Theory of Open Quantum Systems"

Nicolaus Copernicus University, Torun, Poland 20-24 June 2012

Meeting on Analysis of Quantum Control and Open Systems

Aberystwyth University, Aberystwyth, United Kingdom 24-25 May 2012

Non-commutative Analysis Workshop

Imperial College London, London, United Kingdom 24-27 January 2012

Meeting on Quantum Probability, Noncommutative Geometry and Quantum Information

International Centre for Mathematical Sciences, Edinburgh, United Kingdom 16-18 January 2012

Principles and applications of quantum control engineering

The Kavli Royal Society International Centre, Chicheley, United Kingdom 12-13 December 2011

WIMCS Stochastic Cluster Meeting "Trends in Quantum Probability and Stochastics"

Aberystwyth University, Aberystwyth, United Kingdom 25-26 January 2011

Talks

Dynamics of many-body localisation in a translation invariant quantum glass model

Third Nottingham Workshop on Quantum Non-equilibrium Dynamics University of Nottingham, Nottingham, United Kingdom 22June 2015

Open quantum reaction-diffusion dynamics: absorbing states and relaxation (poster)

Mathematical physics of non-equilibrium quantum systems King's College, London, United Kingdom 18 December 2014

Phase purification in quantum Markov processes

Quantum information seminar University of Nottingham, Nottingham, United Kingdom 12 February 2014

Quantum Sanov Theorem

Quantum information seminar University of Nottingham, Nottingham, United Kingdom 18 March 2013

Dynamical phases and large deviations in open quantum systems

Student conference on mathematical and theoretical aspects of Quantum Mechanics University of Nottingham, Nottingham, United Kingdom 7 November 2012

The atom maser: large deviations and quantum phase transitions

QuaCS Quantum Correlations Students Workshop University of Nottingham, Nottingham, United Kingdom 3 July 2012

Large Deviations and Quantum Phase Transitions for the Atom Maser (poster)

Workshop on New Directions in Quantum Statistics University of Nottingham, Nottingham, United Kingdom 25 June 2012

Large Deviations and Quantum Phase Transitions for the Atom Maser (poster)

44 Symposium on Mathematical Physics Nicolaus Copernicus University, Torun , Poland 22 June 2012

Large deviations for the atom maser

Invited speaker at Analysis of Quantum Control and Open Systems Aberystwyth University, Aberystwyth, United Kingdom 24 May 2012

Large deviations for the atom maser

Postgraduate probability seminar University of Nottingham, Nottingham, United Kingdom 14 March 2012

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