The Thermodynamic Uncertainty Theorem Kyle J. Ray1Alexander B. Boyd2 3 4yGiacomo Guarnieri5zand James P. Crutcheld6x 1Complexity Sciences Center and Physics Department

2025-05-06 0 0 880.12KB 22 页 10玖币
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The Thermodynamic Uncertainty Theorem
Kyle J. Ray,1, Alexander B. Boyd,2, 3, 4, Giacomo Guarnieri,5, and James P. Crutchfield6, §
1Complexity Sciences Center and Physics Department,
University of California at Davis, One Shields Avenue, Davis, CA 95616, USA
2Corresponding author
3The Division of Physics, Mathematics and Astronomy,
California Institute of Technology, Pasadena, CA 91125, USA
4School of Physics, Trinity College Dublin, College Green, Dublin 2, D02 PN40, Ireland
5Dahlem Center for Complex Quantum Systems,
Freie Universit¨at Berlin, 14195 Berlin, Germany
6Complexity Sciences Center and Physics Department,
University of California at Davis, One Shields Avenue, Davis, CA 95616
Thermodynamic uncertainty relations (TURs) express a fundamental tradeoff between the preci-
sion (inverse scaled variance) of any thermodynamic current by functionals of the average entropy
production. Relying on purely variational arguments, we significantly extend these inequalities by
incorporating and analyzing the impact of higher statistical cumulants of entropy production within
a general framework of time-symmetrically controlled computation. This allows us to derive an ex-
act expression for the current that achieves the minimum scaled variance, for which the TUR bound
tightens to an equality that we name Thermodynamic Uncertainty Theorem (TUT). Importantly,
both the minimum scaled variance current and the TUT are functionals of the stochastic entropy
production, thus retaining the impact of its higher moments. In particular, our results show that,
beyond the average, the entropy production distribution’s higher moments have a significant effect
on any current’s precision. This is made explicit via a thorough numerical analysis of swap and
reset computations that quantitatively compares the TUT against previous generalized TURs. Our
results demonstrate how to interpolate between previously-established bounds and how to identify
the most relevant TUR bounds in different nonequilibrium regimes.
Keywords: thermodynamic uncertainty relation, entropy production, nonequilibrium steady state, current
Introduction. The past few decades witnessed sub-
stantial technological advances in miniaturization that,
today, have culminated in experimental realizations of
nanoscale thermal machines [1–8]. These devices ex-
hibit three fundamental features. First, they operate un-
der nonequilibrium conditions, i.e., either by keeping the
system in a nonequilibrium steady-state by means of volt-
age or temperature biases or through the application of
an external time-dependent control protocol. This im-
plies that a certain amount of entropy production Σ—
which quantifies the amount of irreversible dissipation
associated with nonequilibrium processes—is always gen-
erated [9]. Crucially, the entropy production limits heat
engine and refrigerator performance, constrains the phys-
ical mechanisms underlying complex biological function-
ing [10], and is the central quantity in Landauer’s infor-
mation erasure—the keystone of the bridge between ther-
modynamics and communication theory. Second, due to
their microscopic nature, the fluctuations of all thermo-
dynamic quantities (such as heat, work, and so on) be-
come as significant as their average values. Last, but
not least, the laws of quantum mechanics have impor-
tant repercussions for fluctuations, which generally have
both thermal and quantal origins [11–14].
Since Onsager’s and Kubo’s pioneering discovery of the
fluctuation-dissipation theorem (FDT) [15–17], deter-
mining the universal properties of fluctuations in out-
of-equilibrium processes, as well as their role in dissipa-
tion, has been a cornerstone of stochastic thermodynam-
ics. In the ‘90s, Jarzynski and Crooks generalized the
FDT through the fluctuation relations (FRs) [18–29]. At
the microscopic scale, the FRs refine the famous Second
Law of Thermodynamics hΣi ≥ 0 by determining the
full distribution of stochastic thermodynamic quantities
and thus their fluctuations. That is, the FRs replaced
the familiar Second law inequality with an equality from
which the Second Law is easily derived through Jensen’s
inequality.
More recently, a third milestone was crossed by con-
necting thermodynamic fluctuations out of equilibrium
to dissipation. These broad results, called thermo-
dynamic uncertainty relations (TURs), were originally
discovered in nonequilibrium steady-states of classical
time-homogeoneous Markov jump-processes satisfying lo-
cal detailed balance [30, 31]. Today, though, TURs
have been generalized to finite-time processes [32–34],
periodically-driven systems [35–41], Markovian quantum
systems undergoing Lindblad dynamics [42–44], and au-
tonomous classical [33, 45] and quantum [45–50] systems
in steady-states close to linear response.
arXiv:2210.00914v3 [cond-mat.stat-mech] 25 Nov 2022
2
In all these, TURs bound the fluctuations of
any (time-reversal anti-symmetric stochastic) thermody-
namic quantity Jas a function of the average entropy
production hΣi:
2
Jvar(J)
hJi2f(hΣi).(1)
with hJiand var(J) = hJ2i − hJi2being the average
and variance of J, respectively. In this way, the scaled
variance 2
Jcan be seen as the inverse of current J’s
signal-to-noise ratio or precision. Since fis generally
a monotonically-decreasing function, TURs express the
trade-off that increased precision in Jinevitably comes
at the cost of more dissipation. Such a no-free-lunch
statement echoes that from the above-mentioned Second
Law. It differs critically, however, as it includes fluctua-
tions of many thermodynamic quantities of interest.
Our work significantly advances this line of inquiry
by analyzing the impact of higher statistical moments
of the entropy production on the signal-to-noise ratio of
thermodynamic currents Jin two state-of-the-art sce-
narios; namely, bit swap and reset protocols. We quan-
tify the impact of such higher moments onto the r.h.s.
of Eq. (1)—i.e. f(hΣi)—leading to replacing the latter
with:
2
Jg(hfmin(Σ)i),(2)
where g(x)x11. With this, the bound becomes
a functional of the stochastic entropy production Σ dis-
tribution. And so, critically, it accounts for all higher
moments.
Notably, these higher moments have become a focus of
attention [51] since they are particularly germane to dis-
sipation management in nanoscale devices—devices that
must be designed to tolerate large and potentially de-
structive fluctuations. For example, entropy production
variance and skewness determine the probability of ex-
periencing a trajectory (i.e., an experimental run) that
generates extreme dissipated heat flowing through the
system. This, naturally, can damage or disrupt the oper-
ation of these new classes of microscopic quantum hard-
ware [52].
In both examples, we compute Eq. (2), and quantita-
tively compare it against several different previously de-
rived TUR bounds. In particular, it is important to stress
that g(hfmin(Σ)i) appearing in our Eq. (2) agrees and
coincides with a recent result obtained in Ref. [53], al-
beit via a completely different derivation. Our approach,
moreover, focuses on realizing the bound by also finding
an explicit expression for the minimum-variance current
Jmin(~s) that saturates Eq. (2).
This minimum depends sensitively on the entropy pro-
duction’s higher-order fluctuations. In much the same
way that fluctuation theorems [19, 54, 55] reframe the
Second Law from an inequality to an equality, the TUT
replaces the bounds set by TUR with a saturable equal-
ity. Applying the TUT to thermodynamic simulations of
fundamental bit swap and reset computations, we demon-
strate that current fluctuations can depart substantially
from previous bounds set by TURs.
Minimum Scaled Variance Current. Currents Jare
observations of a system Sthat flip sign under time rever-
sal: When a movie of a flowing river is reversed, the pos-
itive currents become negative and visa versa. Formally,
a system trajectory is the sequence ~s s0sdt ···sτdtsτ,
where each st∈ S is the system’s state at time t. The
current associated with a reversed trajectory R(~s)
s
τs
τdt ···s
dts
0is minus the current of the forward tra-
jectory:
J(R(~s)) = J(~s).(3)
The system Smay be influenced by an external con-
trol parameter λtat every time t, thereby performing
a computation over the time interval t(0, τ). Under
time-symmetric control λ
τt=λtand conjugation of
the distribution under the operation Pr(s, τ) = Pr(s,0)
the probability of a reverse trajectory is exponentially
damped by the entropy production [56]:
Pr(R(~s),Σ) = eΣPr(~s, Σ).(4)
This is the Detailed Fluctuation Theorem (DFT) for a
Time-Symmetrically Controlled Computation (TSCC).
(Reference [57] details its derivation and scope.) This
DFT [54, 55, 58] includes NESS dynamics for which
the control parameter is constant λt=λt0. It can also
describe, as explored here, computations that begin in
equilibrium and are then allowed to relax after the appli-
cation of a time-symmetric control signal. These latter
symmetries are ubiquitous in computing [59].
The symmetry imposed by the TSCC imbues J’s
statistics with special properties in stochastic nonequi-
librium systems when compared with the entropy pro-
duction Σ. To address this, we derive the exact form of
the current Jmin that achieves minimum scaled variance
for any TSCC.
Given a TSCC operating over the time interval [0, τ],
described by probability distribution Pr(~s, Σ) over state
trajectories ~s and entropy productions Σ, our task is to
find a current function J(~s) = J(R(~s)) of the state
3
trajectories ~s that minimizes this scaled variance. The
proof that this can be done is found in Ref. [57]. We
provide a short overview here.
The demonstration develops over two steps. First, we
identify a special class of entropy-conditioned currents,
that can be expressed as functions of the entropy pro-
duction, containing a current with the minimum scaled
variance. Then, we use the TSCC detailed fluctuation
theorem Eq. (4) and the calculus of variations to de-
rive the primary result—the exact form of this minimum
current:
Jmin(~s) = hJ2
mini
hJminitanh (Σ(~s)/2) .(5)
While Eq. (5) may appear self-referential, it is worth
noting that any real constant may be chosen for the ra-
tio hJ2
mini
hJmini. The resulting current will achieve the min-
imum variance as dictated by the following Thermody-
namic Uncertainty Theorem.
Theorem 1 (Thermodynamic Uncertainty Theorem).
In a TSCC, the scaled variance of any current Jis
bounded below by the the scaled variance of Jmin , given
by:
2
Jmin =1
htanh(Σ/2)i1.(6)
Since this minimum is achievable by a well-defined cur-
rent Jmin, it sets the tightest possible bound that can
be determined using all moments of the entropy produc-
tion distribution. We note that this expression contrasts
with other past TURs [56, 60–62] in that it is an equality
rather than an inequality. It specifies the achievable min-
imum scaled variance since it depends on the underlying
physical system’s entropy production distribution Pr(Σ).
Equation (6) agrees and coincides with Eq. (16) of
Ref. [53], which was derived through a completely differ-
ent method; namely, by means of a “Hilbert uncertainty
relation” that leverages the reproducing element implied
by the Riesz representation theorem for any Hilbert space
equipped with an appropriate inner product. This uncer-
tainty relation was shown to reduce to the an equation
of the form of Eq. (2). Our approach is complementary
as it investigates how the relation stems from thermody-
namics through a detailed fluctuation theorem (DFT).
While reaching the same conclusion, our method has two
additional merits. First, it clearly highlights its relation
to other previously generalized TUR bounds, also ob-
tained from the requirement of a DFT. Thus, it gives
a quantitative comparisons, as shown shortly. Second,
it determines a physically realizable minimum-variance
currents Jmin(~s) that saturates Eq. (6).
Comparison to Past Uncertainty Relations. With a
blossoming variety uncertainty relations [30–47], it is nat-
ural to ask for direct comparisons. Reference [57] pro-
vides a thorough summary of previous TUR results, but
here we summarize the relevant comparisons. Barato and
Seifert [60] derived the first TUR, finding that precision
could not be maximized without a corresponding increase
in the average entropy production:
2
J2
hΣi2
BS ,(7)
where the subscripts on the latter refer to the original
authors—the Barato-Seifert (BS) bound on scaled vari-
ance.
Further exploration found that detailed fluctuation
theorems [54, 55] can be used to establish modified ther-
modynamic uncertainty relations. Hasegawa and Van Vu
[56] as well as Timparano, Guarnieri, Goold, and Landi
[62] used Eq. (4) to demonstrate that the scaled variance
is bounded below by:
2
J2
ehΣi12
HVV (8)
2
Jcsch2[g(hΣi/2)] 2
TGGL,(9)
where g(x) is the inverse of xtanh(x) and we have again
labeled the bounds for the authors. The 2
TGGL bound is
the tightest possible bound on scaled variance that can
be determined from the average entropy [62]. Comparing
these bounds independent of the TUT, one can see in
Figs. 1 and 2 that they are ordered:
2
BS(hΣi)> 2
TGGL(hΣi)> 2
HVV(hΣi).(10)
(See also Ref. [57] for a proof.) Note that, since 2
TGGL
and 2
HVV were derived from the TSCC DFT, which is
our starting point as well, the minimum scaled variance
is bounded below by these TURS, but not necessarily
2
BS.
With 2
Jmin’s exact form determined, though, a natu-
ral next question is how close the previous bounds, all
depending only on the average entropy production hΣi,
are to the actual minimum.
Fortunately, Timpanaro et al. [62] also showed that a
particular bimodal distribution:
Pr
min(Σ) δa) + eaδ(Σ + a)
achieves their lower bound 2
TGGL. This is the simplest
4
Figure 1. Thermodynamic Uncertainty Theorem and TURs:
Three dashed lines show previous TURS—2
BS,2
HVV, and
2
TGGL—all functions of average entropy production hΣi.
While they make nearly identical predictions for small en-
tropy production, they diverge as entropy increases, setting
very different bounds for average entropy production as low
as hΣi= 2kB. In contrast, the minimum scaled variance 2
Jmin
is not strictly a function of average entropy. The entropy dis-
tribution Pr(Σ|µ, σ2) depends on the variance parameter σ2
and is displayed on a sliding scale from high to low variance
(from dark to light). σ2ranges from 8×103to 8 ×103.
While µis adjusted to keep hΣifixed. This yields entropy dis-
tributions of the form shown on the right. The lowest values
of Σ2and var(Σ) closely match 2
TGGL. As σ2increases, the
variance of the entropy production increases and the curve
become lighter, achieving and surpassing the dashed line for
2
BS. Between these two extremes, there is a purple dashed
line that gives the minimum scaled variance of normal en-
tropy distributions.
distribution satisfying Pr(Σ) = eΣPr(Σ). That is, it
consists of a delta function at entropy production Σ = a
and then contains a mirror of that entropy production
at Σ = areduced by the exponential factor ea. Any
other NESS entropy distribution can be constructed from
a superposition of such distributions.
We investigate our new minimum scaled variance by
exploring a variety of possible distributions. We take a
similar strategy, breaking the entropy distribution into
the piecewise function:
Pr(Σ|µ, σ2) = n(µ, σ2)
eµ)2
2σ2if Σ 0
eΣe(Σµ)2
2σ2if Σ <0
.(11)
Here, n(µ, σ2) = R
0eµ)2
2σ2dΣ + R0
−∞ eΣe(Σµ)2
2σ2dΣ is
the normalization factor. In essence, our probability dis-
tribution is a normal distribution with average µand
variance σ2over the positive interval. And, the TSCC
DFT defines the distribution to be Pr(Σ) = eΣPr(Σ)
on the negative interval.
The variance σ2and average µof the positive en-
tropy portion of the distribution Pr(Σ|µ, σ2) define it.
In the limit σ2kBµ, the positive entropy distribu-
tion is nearly a delta function, and we recover roughly
the distribution Prmin(Σ) proposed by Timpanaro et al
[62]. We show this on the left side of Fig. 1 where
σ2103kBµ, corresponding to a two-peaked distri-
bution. In this case, 2
Jmin closely matches the bound
2
TGGL, as expected. However, Fig. 1 also shows that the
average entropy production is not the sole determinant
of the minimum scaled variance of the current.
As the variance of the positive normal distribution σ2
increases, Fig. 1 shows that the minimum scaled vari-
ance increases. Amidst that progression is a special dis-
tribution, where σ2= 2kBµ, for which Pr(Σ|µ, σ2) is a
normal distribution over the full range of entropy pro-
duction. It can be quickly shown that the variance for a
normal distribution that satisfies the TSCC DFT must
be σ2= 2kBµ. We highlight this special case with a pur-
ple dashed line in Fig. 1 and a blue dashed line in Fig.
2. NESSs, the most frequently studied subclass of TSCC
processes, approach a normal entropy production distri-
bution in the long time limit. Interestingly, the minimal
variance currents of the typical asymptotic behavior in
NESSs clearly violate the original TUR given by 2
BS in
the long-time limit.
If we continue beyond the normal distribution to higher
variances labeled in lighter colors in Fig. 1, the mini-
mum scaled variance 2
Jmin continues to increase, until
it surpasses the bound 2
BS [60]. Thus, by changing the
parameter σ2of the NESS distribution Pr(Σ|µ, σ2) and
its variance var(Σ) as well as other higher moments of
the entropy distribution Pr(Σ), the TUT interpolates be-
tween the TUR set by Timpanaro et al. and that set by
Barato and Seifert. Moreover, we can find entropy distri-
butions that far exceed even Barato and Seifert’s bound.
Thermodynamic Simulations. The entropy produc-
tion distribution Pr(Σ|µ, σ2) is convenient to examine,
but it is not obvious how it can be physically generated.
We now describe two computational protocols that are
able to show similar breadth of behavior, but are firmly
rooted in dynamical models of physical processes. Both
are TSCCs in that they are implemented with time sym-
metric control of a potential energy landscape, where the
thermal influence of a bath is applied through Langevin
dynamics.
First, consider a simple reset protocol. A system con-
sisting of a single positional variable xin asymmetric
double well potential Ustore is initially set up in equilib-
rium with a thermal environment at temperature T. If
5
swap
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reset
<latexit sha1_base64="JwtAjFsI4gjtO9pjvQilIyDLp2s=">AAAB83icbVBNS8NAEN3Ur1q/qh69LBbBU0mqoMeiF48V7Ac0oWy203bpZhN2J2IJ/RtePCji1T/jzX/jts1BWx8MPN6bYWZemEhh0HW/ncLa+sbmVnG7tLO7t39QPjxqmTjVHJo8lrHuhMyAFAqaKFBCJ9HAolBCOxzfzvz2I2gjYvWAkwSCiA2VGAjO0Eq+j/CEmQYDOO2VK27VnYOuEi8nFZKj0St/+f2YpxEo5JIZ0/XcBIOMaRRcwrTkpwYSxsdsCF1LFYvABNn85ik9s0qfDmJtSyGdq78nMhYZM4lC2xkxHJllbyb+53VTHFwHmVBJiqD4YtEglRRjOguA9oUGjnJiCeNa2FspHzHNONqYSjYEb/nlVdKqVb2Lau3+slK/yeMokhNySs6JR65IndyRBmkSThLyTF7Jm5M6L86787FoLTj5zDH5A+fzB9OUkjI=</latexit>
Figure 2. Bounds 2
BS > 2
TGGL > 2
HVV in solid lines (blue, red, and black respectively) and specific thermal processes with
dashed lines: The blue dashed line is the minimum scaled variance 2
Gaussian of any process that generates a Gaussian entropy
production distribution, which is achieved in the long-time limit of NESS processes. The red dashed line is the minimum
scaled variance 2
Discrete of an ideal discrete erasure. We compare two computational classes to the these bounds. (Left) we
plot 1366 different time-symmetric erasures. As expected (see Ref. [57] ) they are bounded by the scaled variance of the ideal
discrete erasure 2
Discrete, which lies well above the bounds 2
TGGL and 2
HVV. A number of erasure operations are well above the
minimum 2
TGGL set by Barato and Seifert. (Right) we plot the result 1193 different bit-flips. As with the erasure protocol,
many computations are above the Barato-Seifert bound. However, many computations achieve a minimum scaled variance well
below the discrete erasure bound 2
Discrete. Many computations are quite close to the strongest possible TUR 2
TGGL, indicating
that this theoretical bound is indeed achievable with TSCCs.
the two metastable states are Aand B, then take Aas
the one with a deeper well and higher initial probability.
Then, the energy landscape is tilted and the energy bar-
rier is removed. This computational potential Ucomp is
held so that probability mass flows from Ato B. This
is a “reset” in that it re-initializes the system to the B
state [59, 63]. Ref. [57] describes this protocol in detail.
The resulting entropy productions in minimum-
variance currents are shown in Fig. 2. Some computa-
tions are considerably less precise than specified by 2
BS,
the original TUR, but many lie well below this bound.
As expected, all computations are less precise than the
bounds 2
TGGL and 2
HVV, but none of them come very
close to the tightest theoretical bound given by 2
TGGL.
Instead, there is another curve that seems to bound
all of these time-symmetric erasures, shown in dashed
red. Reference [57] derives the bound 2
Discrete from
a discrete-state time-symmetric erasure, where the sys-
tem equilibrates to the tilted energy landscape before the
metastable information-storing potential is re-initialized
at the end of the computation.
Second, consider the same initial metastable states A
and B, which are stored in local equilibrium. Then, in-
stantaneously implement a harmonic potential and hold
the energy landscape for half a period of the oscillation.
If the coupling to the thermal environment is weak, then
this implements a reliable swap between Aand B, us-
ing momentum as memory to carry the distributions into
their new states. This is a slight modification of momen-
tum computing protocols described in Refs. [64, 65]. The
entropy and scaled variance of these swaps are shown on
the right-hand side of Fig. 2. They span the same space
of possibilities shown for Pr(Σ|µ, σ2) in Fig. 1: some
lying above 2
BS and some TSCCs sitting just above the
minimum set by 2
TGGL.
For both the reset and swap, we see that the mini-
mum scaled variance tends to increase as the variance
of the entropy-production distribution increases. Higher
moments of entropy production are critically important
in predicting the precision of computations.
Conclusion. We introduced two equalities that pro-
vide an explicit expression for the most accurate current
in any entropy production distribution that satisfies Eq.
(4) and for the minimal scaled variance it achieves. Anal-
ogous to the transition from the Second Law inequality to
the fluctuation relations of Crooks and Jarzynski, we de-
veloped a treatment of the entropy production that recog-
nizes it as a stochastic quantity with proper fluctuations.
This treatment yields a result—the Thermodynamic Un-
certainty Theorem—that is (i) an equality rather than
an inequality and (ii) depends on the stochastic entropy
production’s fluctuations rather than on only its average
摘要:

TheThermodynamicUncertaintyTheoremKyleJ.Ray,1,AlexanderB.Boyd,2,3,4,yGiacomoGuarnieri,5,zandJamesP.Crutch eld6,x1ComplexitySciencesCenterandPhysicsDepartment,UniversityofCaliforniaatDavis,OneShieldsAvenue,Davis,CA95616,USA2Correspondingauthor3TheDivisionofPhysics,MathematicsandAstronomy,CaliforniaI...

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