Work Fluctuations and Jarzynski Equality in Stochastic Resetting
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Work fluctuations and Jarzynski equality in stochastic resetting Deepak Gupta,1 Carlos A. Plata,1 and Arnab Pal2, 3, 4, ∗ 1Dipartimento di Fisica `G. Galilei', INFN, Universit´adi Padova, Via Marzolo 8, 35131 Padova, Italy 2School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 6997801, Israel 3Center for the Physics and Chemistry of Living Systems. Tel Aviv University, 6997801, Tel Aviv, Israel 4The Sackler Center for Computational Molecular and Materials Science, Tel Aviv University, 6997801, Tel Aviv, Israel (Dated: February 4, 2020) We consider the paradigm of an overdamped Brownian particle in a potential well, which is modu- lated through an external protocol, in the presence of stochastic resetting. Thus, in addition to the short range diffusive motion, the particle also experiences intermittent long jumps which reset the particle back at a preferred location. Due to the modulation of the trap, work is done on the system and we investigate the statistical properties of the work fluctuations. We find that the distribution function of the work typically, in asymptotic times, converges to a universal Gaussian form for any protocol as long as that is also renewed after each resetting event. When observed for a finite time, we show that the system does not generically obey the Jarzynski equality which connects the finite time work fluctuations to the difference in free energy, albeit a restricted set of protocols which we identify herein. In stark contrast, the Jarzynski equality is always fulfilled when the protocols continue to evolve without being reset. We present a set of exactly solvable models, demonstrate the validation of our theory and carry out numerical simulations to illustrate these findings. Finally, we have pointed out possible realistic implementations for resetting in experiments using the so-called engineered swift equilibration. Introduction.| Stochastic thermodynamics is a cor- of thermodynamics were interpreted by identifying the nerstone in non-equilibrium statistical physics [1{5]. Mi- contributions to the total entropy production [53], and croscopic systems satisfy stochastic laws of motion gov- furthermore it was shown to satisfy a universal integral erned by force fields and thermal fluctuations which arise fluctuation relation [54]. While these first studies focused due to the surrounding. The subject then teaches us that exclusively on the entropy production, efforts are yet to thermodynamic observables such as work, heat, entropy be made to understand other response functions. More- production etc. measured along the stochastic trajecto- over, not much is known about the distribution of these ries taken from ensembles of such dynamics will fluctuate observables. In particular, one important observable is too. Understanding the distribution and the statistical the work function which is produced due to external per- properties of these fluctuations is of great interest since turbations to the system. Work statistics encodes im- they hold a treasure trove of information about micro- portant features of an out-of-equilibrium thermodynamic scopic systems and how they respond to external pertur- process but its computation is usually quite daunting. bations. Indeed there has been a myriad of studies to un- Here, we set out to characterize work fluctuations in a derstand e.g., non-equilibrium dynamics of biopolymers stochastic system which is subjected to resetting. Our de- [6,7], colloidal particles [8{14], efficiency of molecular tailed analysis to this account then reveals emergence of bio-motors [15, 16] and microscopic engines [17], heat robust universal pattern in work-fluctuations: firstly re- conduction [18, 19], electronic transport in quantum sys- setting renders work-fluctuations Gaussian independent tems [20], trapped-ion systems [21] and many more [22]. of the nature of the external perturbation that produces Although we observe such diverse small systems with no it. Secondly, work fluctuations are found to obey the JE apparent similarity, it is remarkable to find that there under certain conditions which we identify through this exist some universal relations which are shared in com- comprehensive study. mon. One of the most celebrated ones is perhaps the General theory.| For the sake of generality, we put Jarzynski equality (JE) that relates the non-equilibrium forward our results in the paradigmatic framework of a fluctuations of the work to the equilibrium free energy one-dimensional overdamped Brownian particle in a po- difference [23{25]. Universalities of such kind have al- tential U(x; λ(t)), which is modulated externally through arXiv:1909.08512v2 [cond-mat.stat-mech] 2 Feb 2020 ways been considered as an important feature in physical the protocol λ(t). Motion of such a particle is governed sciences and in this paper we seek out for thermodynamic by the Langevin equation of the form invariant principles in stochastic resetting systems [26]. p x_(t) = −γ−1@ U(x; λ(t)) + 2Dη(t); (1) Dynamics with stochastic reset has drawn a lot of at- x tention recently because of its rich non-equilibrium prop- where γ and D are the friction and diffusion coefficients erties [26{41] and its broad applicability in first passage respectively that satisfy the fluctuation-dissipation rela- processes [42{52]. Nevertheless, thermodynamical per- tion, i.e., Dγ = kBT , with kB being the Boltzmann con- spective of resetting systems has been largely overlooked stant and T is the temperature of the medium. We as- so far. It was only recently when first and second laws sume hη(t)i = 0 and hη(t)η(t0)i = δ(t − t0). Moreover, let 2 us consider that the position of the particle at t = 0 is distributed according to the probability density function (PDF) pini(x0). At random times taken from an expo- nential distribution f(t) = re−rt, the particle in motion is stopped and teleported to the initial configuration. The external modulation of the potential performs work on the system which can be defined as [23] Z t 0 0 0 1 0 @U[x(t ); λ(t )] dλ(t ) W = dt 0 ; (2) kBT 0 @λ dt measured in units of kBT . In what follows, we will set kBT = D = 1 without any loss of generality. Since the reset process is instantaneous, we will as- sume that no work was done during this course (a finite FIG. 1: Schematic of a Brownian particle confined in a har- resetting-work will be discussed later). In order to quan- monic trap U(x; λ(t)) = κ(t)[x − y(t)]2=2, where λ(t) = tify the work fluctuations, it is convenient to first define fy(t); κ(t)g represents the set of time-dependent protocols the moment generating function (MGF) namely which are independently regulated. λ = y(t) and λ = κ(t) Z 1 indicate the center of the trap (panel A) and the stiffness −kW (panel B) respectively. The resetting mechanism acts both on Hr(k; t) ≡ dW e Pr(W; t) (3) −∞ the particle and the protocols as mentioned in the text. Here, we show the modulation of the protocol when it is renewed where Pr(W; t) is the PDF of the work at time t, averaged after each resetting (panel C) or remains unaltered (panel D). over the initial distribution pini(x0) and the underlying dynamics with stochastic resetting. To delve deeper, we make use of the renewal structure in resetting dynamics of W recursively from the knowledge of the moments of to construct a relation that connects the MGF for r > 0 the process without resetting. to that of r = 0 for any initial and subsequent resetting Universal work fluctuations.|The infinite set of positions moments given by Eq. (5) contains the same informa- tion as that of the full distributions Pr(W; t). However, ~ H0(k; s + r) physical intuition tells us that not all the moments con- H~r(k; s) = ; (4) 1 − rH~0(k; s + r) tribute significantly at long time. To see this, we con- sider a trajectory of time length t with multiple possible ~ R 1 −st where Hr(k; s) = 0 dt e Hr(k; t) and the subscript resetting events. The total work done along this long 0 indicates the observables with r = 0. We have added a trajectory can then be decomposed into the sum of the proof of Eq. (4) in [55]; but it is imperative to stress the partial works produced in each time interval between the following points here. Note that Eq. (4) holds for any resetting events. However, these intervals are statistically initial condition and naturally adheres to a fixed initial independent since the entire configuration of the system condition which was derived in [56, 57], but in the ab- (comprising the particle and the trap) is renewed after sence of any protocol λ(t). In the presence of protocol, each resetting event, and hence there are no correlations one needs to be meticulous since the structure of this between the intervals. Therefore, for a long enough ob- equation relies on the fact that λ(t) is also renewed after servation time t one would expect on an average ∼ rt each resetting. As we will see later, Eq. (4) does not hold number of resetting events and the total work W (t) can when the protocol is unaffected under resetting [55]. then be written as W ≈ W1 +W2 +W3 +···+W[rt]. Since The MGF, given by Eq. (4), can be inverted to obtain the intervals are disjoint, the Wi-s are also independent the full work statistics at a given time. Nonetheless, we and identically distributed. Moreover, if Wi-s are regular will show that it suffices to know the first and second (with finite mean and variance), one would expect that moment to predict the universal behavior of the work the distribution of W , according to the central limit the- fluctuations in the large time limit.