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arXiv:1307.7027 · Physical Review B

Self-consistent projection operator theory for quantum many-body systems

Peter Degenfeld-Schonburg, Michael J. Hartmann

Published Jun 9, 2014• 55 citations
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Abstract

We derive an exact equation of motion for the reduced density matrices of individual subsystems of quantum many-body systems of any lattice dimension and arbitrary system size. Our projection operator based theory yields a highly efficient analytical and numerical approach. Besides its practical use it provides an interpretation and systematic extension of mean-field approaches and an adaption of open quantum systems theory to settings where a dynamically evolving environment has to be taken into account. We show its high accuracy for two significant classes of complex quantum many-body dynamics, unitary evolutions of nonequilibrium states in closed and stationary states in driven-dissipative systems.

DOI
10.1103/physrevb.89.245108

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We derive an exact equation of motion for the reduced density matrices of individual subsystems of quantum many-body systems of any lattice dimension and arbitrary system size. Our projection operator based theory yields a highly efficient analytical and numerical approach. Besides its practical use it provides an interpretation and systematic extension of mean-field approaches and an adaption of open quantum systems theory to settings where a dynamically evolving environment has to be taken into account. We show its high accuracy for two significant classes of complex quantum many-body dynamics, unitary evolutions of nonequilibrium states in closed and stationary states in driven-dissipative systems.
The description of quantum many-body systems is an exceedingly cumbersome challenge since the dimensions of their Hilbert spaces scale exponentially with the number of constituents. Whereas mean-field approaches are expected to provide accurate descriptions for high dimensional lattices, numerical approaches based on the density matrix renormalization group have very successfully been applied to one-dimensional chains. Yet for two-dimensional lattices there is still a pressing need for efficient descriptions as these are expected to host some very intriguing but still illusive phenomena in quantum many-body physics. Here we derive an exact equation of motion for the reduced density matrices of individual subsystems of quantum many-body systems of any lattice dimension and arbitrary system size. Our projection operator based approach is thus highly efficient and not restricted to unitary dynamics. We show its excellent accuracy for moderate size one-dimensional and small two-dimensional systems where very accurate or exact numerical solutions are available.
Exact and approximate master equations were derived by the projection operator method for the reduced statistical operator of a multi-level quantum system with finite number N of quantum eigenstates interacting with arbitrary external classical fields and dissipative environment simultaneously. It was shown that the structure of these equations can be simplified significantly if the free Hamiltonian driven dynamics of an arbitrary quantum multi-level system under the influence of the external driving fields as well as its Markovian and non-Markovian evolution, stipulated by the interaction with the environment, are described in terms of the SU(N) algebra representation. As a consequence, efficient numerical methods can be developed and employed to analyze these master equations for real problems in various fields of theoretical and applied physics. It was also shown that literally the same master equations hold not only for the reduced density operator but also for arbitrary nonequilibrium multi-time correlation functions as well under the only assumption that the system and the environment are uncorrelated at some initial moment of time. A calculational scheme was proposed to account for these lost correlations in a regular perturbative way, thus providing additional computable terms to the correspondent master equations for the correlation functions.
Exact and approximate master equations were derived by the projection operator method for the reduced statistical operator of a multi-level open quantum system with finite number NNN of quantum eigenstates interacting with arbitrary external deterministic fields and dissipative environment simultaneously. Unlike conventional master equations derived by the projection operator method for the quantum systems of this kind so far, these equations are homogeneous in the reduced statistical operator while, at the same time, taking into account initial correlations between the multi-level system and the environment. Consistent representation of the multi-level system evolution in terms of the SU(N)SU(N)SU(N) operator algebra facilitates efficient numerical analysis of these equations derived for various physically realistic quantum models.
The dynamics of a quantum system coupled to a classical environment and subject to constraints that drive it out of equilibrium are described. The evolution of the system is governed by the quantum-classical Liouville equation. Rather than evaluating the evolution of the mixed quantum-classical density operator, we derive exact equations of motion for the nonequilibrium average values of a set of operators or variables, along with correlation function expressions for the dissipative coefficients that enter these equations. These equations are obtained by requiring that the exact nonequilibrium averages are equal to local nonequilibrium averages that depend on auxiliary fields whose values satisfy evolution equations obtained using projection operator methods. The results are illustrated by deriving reaction-diffusion equations coupled to fluid hydrodynamic equations for a solution of quantum particles that can exist in two metastable states. Nonequilibrium steady states are discussed along with the reaction rate and diffusion correlation functions that characterize such states.
Open quantum many body systems describe several quantum simulators, ranging from arrays of superconducting circuits hosting correlated states of light, to ultracold atoms in optical lattices in presence of controlled dissipative processes. Their theoretical understanding is hampered by the exponential scaling of their Hilbert space and by their intrinsic nonequilibrium nature, limiting the applicability of many traditional approaches. In this work we extend Dynamical Mean Field Theory (DMFT), a powerful nonperturbative technique developed for strongly correlated quantum many-body systems, to Markovian open quantum systems. Within Open-DMFT, a master equation describing a lattice of bosonic particles is mapped, at large but finite connectivity, onto an impurity problem describing a single site coupled to Markovian and non-Markovian environments, where the latter accounts for quantum fluctuations beyond Gutzwiller mean-field theory. We introduce a new non-perturbative approach to solve the impurity problem, which dresses the impurity, featuring Markovian dissipative channels, with the non-Markovian bath, in a self-consistent scheme based on a resummation of non-crossing diagrams. Finally, we apply our Open-DMFT to solve a driven-dissipative Bose-Hubbard model, which is relevant to current experiments with dissipative ultracold bosons. We show that this model undergoes a dissipative phase transition towards a superfluid phase, which can be equally interpreted as a quantum many-body synchronization transition of an array of quantum van der Pol oscillators. We show how Open-DMFT captures crucial effects due to finite lattice connectivity, such as hopping-induced dissipation and heating, which are completely missed by mean field theories and which lead to a drastic reduction of the broken symmetry phase that we interpret as a desynchronization driven by quantum fluctuations.