Jarzynski Equality for Driven Quantum Field Theories
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PHYSICAL REVIEW X 8, 011033 (2018) Jarzynski Equality for Driven Quantum Field Theories † Anthony Bartolotta1,* and Sebastian Deffner2, 1Walter Burke Institute for Theoretical Physics, California Institute of Technology, Pasadena, California 91125, USA 2Department of Physics, University of Maryland Baltimore County, Baltimore, Maryland 21250, USA (Received 13 October 2017; revised manuscript received 8 December 2017; published 27 February 2018) The fluctuation theorems, and in particular, the Jarzynski equality, are the most important pillars of modern nonequilibrium statistical mechanics. We extend the quantum Jarzynski equality together with the two-time measurement formalism to their ultimate range of validity—to quantum field theories. To this end, we focus on a time-dependent version of scalar ϕ4. We find closed-form expressions for the resulting work distribution function, and we find that they are proper physical observables of the quantum field theory. Also, we show explicitly that the Jarzynski equality and Crooks fluctuation theorems hold at one- loop order independent of the renormalization scale. As a numerical case study, we compute the work distributions for an infinitely smooth protocol in the ultrarelativistic regime. In this case, it is found that work done through processes with pair creation is the dominant contribution. DOI: 10.1103/PhysRevX.8.011033 Subject Areas: Particles and Fields, Quantum Physics, Statistical Physics I. INTRODUCTION processes. About all real, finite-time processes the second law of thermodynamics asserts only that some amount of In physics there are two kinds of theories to describe entropy is dissipated into the environment, which can be motion: microscopic theories whose range of validity is expressed with the average, irreversible entropy production determined by a length scale and the amount of kinetic as hΣi ≥ 0 [9]. The (detailed) fluctuation theorem makes energy, such as classical mechanics or quantum mechanics; this statement more precise by expressing that negative and phenomenological theories, such as thermodynamics, fluctuations of the entropy production are exponentially which are valid as long as external observables remain close unlikely [5–8,10]: to some equilibrium value. Over the past two centuries, microscopic theories have P −Σ −Σ P Σ undergone a rapid development from classical mechanics ð Þ¼expð Þ ð Þ: ð1Þ over special relativity and quantum mechanics to quantum field theory. While quantum field theories were originally The most prominent (integral) fluctuation theorem [11] developed for particle physics and cosmology, this approach is the Jarzynski equality [12], which holds for all systems has also been shown to be powerful in the description of initially prepared in equilibrium and undergoing isothermal condensed matter systems. Examples include quasiparticle processes, excitations in graphene, cavity quantum electrodynamics, topological insulators, and many more [1–4]. hexpð−βWÞi ¼ expð−βΔFÞ; ð2Þ In contrast to the evolution of microscopic theories, the development of thermodynamics has been rather stagnant where β is the inverse temperature, W is the thermodynamic —until only two decades ago when the first fluctuation work, and ΔF is the free-energy difference between the theorems were discovered [5–8]. Conventional thermody- instantaneous equilibrium states at the initial and final namics can only fully describe infinitely slow, equilibrium times. In its original inception the Jarzynski equality Eq. (2) was formulated for classical systems with Hamiltonian [12] and Langevin dynamics [13]. Thus, W is essentially a *[email protected] notion from classical mechanics, where work is given by a † [email protected] force along a trajectory. The advent of modern fluctuation theorems for classical systems [5–8,10,12–14] has spurred Published by the American Physical Society under the terms of the development of a new field, which has been dubbed the Creative Commons Attribution 4.0 International license. – Further distribution of this work must maintain attribution to stochastic thermodynamics [15 20]. the author(s) and the published article’s title, journal citation, In stochastic thermodynamics one focuses on the fluc- and DOI. tuating properties of the central quantities such as work and 2160-3308=18=8(1)=011033(20) 011033-1 Published by the American Physical Society ANTHONY BARTOLOTTA and SEBASTIAN DEFFNER PHYS. REV. X 8, 011033 (2018) heat, which are defined for single realizations of processes arising from a quantum field theory in contrast to classical, operating far from equilibrium. Thus, in the study of thermal noise. nanoscale systems out of thermal equilibrium, it is natural In the following, we demonstrate that the two-time to ask in what regimes quantum effects become significant measurement formalism can be systematically used to and how fluctuation theorems apply to quantum systems investigate the work distribution functions of a restricted [21–34]. Nevertheless, it took another decade before it was class of quantum field theories, focusing on a time- clearly stated that in quantum mechanical systems W is not dependent version of λϕ4. Closed-form expressions for a quantum observable in the usual sense [35]. This means these work distributions are found at leading order, includ- that there is no Hermitian operator, whose eigenvalues ing loop corrections, through the use of a new diagram- are the classically observable values of W. This is the matic technique and a mapping between finite-time case because thermodynamic work is a path-dependent transition amplitudes and infinite-time scattering ampli- quantity—a nonexact differential. Hence, thermodynamic tudes. It is found that to the perturbative order considered, work is rather given by a time-ordered correlation function the work distribution function does not run with the [24,25,35]. renormalization scale, indicating that the distribution is To gain more insight into the underlying statistics of an observable of the quantum field theory. We verify that quantum work, the two-time measurement formalism the quantum Jarzynski and Crooks fluctuation theorems [36,37] has proven powerful: In this formulation, a quan- hold exactly and are independent of the renormalization tum system is prepared in contact with a heat bath of scale. Because of the form of the work distributions, it is inverse temperature β. The system is then decoupled from straightforward to show that the fluctuation theorems hold the environment and a projective measurement onto the if one removes the loop corrections (as would be the case initial energy eigenbasis is performed. Then, the system is for a classical field theory) and also in the nonrelativistic let to evolve before another projective measurement of the limit. energy is performed. As the system is isolated, the work These results demonstrate that quantum fluctuation performed on the system is identical to the change in theorems and stochastic thermodynamics can be extended energy. Despite its success, this formalism has several to include quantum field theories, our most fundamental limitations [38], including the lack of thermodynamic theory of nature. Thus, our results open the door for future accounting for the measurement process [39] and its application of fluctuation theorems to the study of problems inapplicability to coherently controlled quantum systems at the forefronts of physics—in condensed matter physics, [40]. Nevertheless, it is important to remark that in particle physics, and cosmology. complete analogy to how classical mechanics is contained This paper is organized as follows. In Sec. II, we review in quantum mechanics (in the appropriate limits), the two- the two-time measurement formalism and the quantum time measurement formalism produces work distribution Jarzynski equality. We define a restricted class of quantum functions which correspond to those of classical systems in field theories in Sec. III for which the work distribution semiclassical approximations [41–45]. function can be calculated. The energy projection operators To date another decade has gone by, yet quantum for a generic real scalar field theory are calculated in stochastic thermodynamics is still rather incomplete. How Sec. IV and a method for calculating finite-time transition to describe thermodynamic work and entropy production in amplitudes from infinite-time scattering amplitudes is open quantum systems is still hotly debated [46–56], and introduced. The mathematical details of this relationship with a few exceptions [57–66] most of the literature is between finite-time and infinite-time amplitudes are restricted to standard Schrödinger quantum mechanics. detailed in Appendix A. In Sec. V, we specialize to a λϕ4 The purpose of the present analysis is to significantly time-dependent version of and discuss its renormali- broaden the scope of stochastic thermodynamics and to zation. Then, Sec. VI discusses how closed-form expres- take the next, important step—extend quantum stochastic sions for the work distribution function can be calculated at thermodynamics to interacting quantum field theories. leading order using a graph theoretic technique. The details Conventional thermodynamics is a phenomenological of the derivation can be found in Appendix B while the theory that has no knowledge of the underlying micro- closed-form expressions for the work distribution function scopic dynamics. In small systems, the dynamics are are in Appendix C. We discuss the analytic