Efficiency fluctuations in quantum heat engines with non-quasi-static adiabatic strokes

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "Efficiency fluctuations in quantum heat engines with non-quasi-static adiabatic strokes".

Mira: The gist: The study analytically and numerically determines efficiency fluctuations in quantum Carnot and Otto engines operating under non-quasi-static adiabatic strokes with quasi-static isothermal or isochoric processes,

Kai: First, who's behind it and why it matters.

Paper summary: Kai: So we're looking at this paper, "Efficiency fluctuations in quantum heat engines with non-quasi-static adiabatic strokes," and what it claims is that when you run these engines, especially the Carnot and Otto types, with pistons moving faster than allowed by the slow process assumption, you get extra quantum noise that changes how efficient they are.

Mira: That’s right, Kai. The main idea here is looking at how the work output becomes random because of this non-quasi-static motion during those expansion and compression strokes—the adiabatic ones specifically—and how that randomness modifies the average efficiency we calculate.

Lev: From an error correction standpoint, if you were trying to run this on actual hardware, that means you're dealing with stochastic work outputs W1 and W2, which aren't just fixed numbers anymore, and that makes characterizing the noise a real headache for building stable systems forty-two <ref:2610.10647#pg2>.

Kai: Exactly. The paper models each piston as a single particle in a one-dimensional box with a moving wall, making the whole system's Hamiltonian time-dependent, which drives these quantum transitions during those non-equilibrium strokes forty-three <ref:2610.10647#pg2,a single particle in a>.

Mira: And they use this two-point measurement scheme just on the adiabatic strokes to find the efficiency distribution that includes both thermal and these new quantum fluctuations. The paper states that for an adiabatic stroke of finite duration, those quantum fluctuations actually dominate the thermal ones when you have a small piston size and low temperature <ref:2610.10647#pg3>.

Lev: That leads to some interesting numbers they show; they find that work done during these non-quasi-static strokes, W1 and W2, can actually take values outside the classical bounds of zero to one, meaning efficiencies eta n,m,j,k can be anything between zero and one <ref:2610.10647#pg2>.

Kai: So the key thing they find is that this randomness in W1 and W2 is what modifies the average efficiency calculation for both the Carnot engine and the Otto engine <ref:2610.10647#pg3>.

Mira: They define efficiency for these engines as a specific combination of work done during those strokes divided by the heat absorbed or rejected, like eta n,m,j,k = W¯ DA(Th) + W1 + W¯ BC (Tc) + W2 / Q¯ h <ref:2610.10647#pg3>.

Lev: The paper also points out that the probability distribution function for those work outputs P(W1, W2) is useful because it lets you figure out the distribution of the final efficiency for the Carnot engine <ref:2610.10647#pg3>.

Kai: When they look at the average efficiency in a quasi-static limit, say when piston speed goes to zero and time goes to infinity, they get a result that looks like eta¯qs = one - Q¯ c / Q¯ h <ref:2610.10647#pg3>.

Mira: That quasi-static average efficiency is what you'd expect classically, but the paper shows how the oscillations in that mean efficiency come specifically from those additional quantum fluctuations driven by the piston's motion that only disappear when you look at very large lengths <ref:2610.10647#pg3>.

Lev: If we take this to real hardware, it means your measured average efficiency won't just be one smooth number; it will have these quantum jitters that depend on the scale of the piston motion <ref:2610.10647#pg3>.

Kai: So what this means for us in the lab is that if we’re working at mesoscopic scales, this study gives us a way to predict how much noise we should expect when we run those non-quasi-static strokes <ref:2610.10647#pg1>.

Mira: It opens up the discussion for larger systems too, because the authors mention that these findings could be generalized if you consider many-particle systems as working substances, although they admit that this is an open problem <ref:2610.10647#pg3>.

Lev: And what’s missing, which they flag as a limitation, is the fact that they assumed quasi-static processes for those non-adiabatic strokes; if you introduce actual heat input fluctuations into those strokes, that's something we haven't fully settled yet <ref:2610.10647#pg3>.

Kai: So, to wrap up this segment, the paper "Efficiency fluctuations in quantum heat engines with non-quasi-static adiabatic strokes" shows that piston speed introduces extra quantum noise into the work done during expansion and compression <ref:2610.10647#pg1>.

Mira: It's showing us how these engines behave when you push them beyond the slow, quasi-static limits, revealing efficiency fluctuations that are driven by the underlying quantum mechanics of the piston's motion <ref:2610.10647#pg2>.

Lev: And for those trying to build stable engines, it highlights that you have to account for these stochastic outputs W1 and W2 because they aren't just classical inputs anymore <ref:2610.10647#pg3>.

Conclusion: Kai: So we’re looking at this paper, "Efficiency fluctuations in quantum heat engines with non-quasi-static adiabatic strokes," and what it really boils down to is that when you run these quantum Carnot and Otto engines, especially when the piston moves too fast for a slow process, you get extra noise.

Mira: Right. The core idea is that those rapid movements introduce stochastic work—random work output—which messes with our standard efficiency calculations. We're seeing how thermal effects mix with these new quantum fluctuations during the expansion and compression strokes.

Lev: From my side, that means if you try to build this on real hardware, those W1 and W2 values aren't fixed numbers anymore; they're probability distributions, which makes designing anything stable much harder.

Kai: So the paper says that by modeling each piston as a particle in a box with a moving wall, they can actually get these efficiency distributions for both engine types.

Mira: They show that for those non-quasi-static adiabatic strokes, quantum fluctuations end up dominating the thermal ones when you have small pistons and low temperatures. That means the efficiency isn't just one smooth value anymore; it has multiple sharp peaks at different possible values.

Lev: I saw that they found work outputs W1 and W2 can actually go outside the classical bounds of zero to one, which is a weird thing for an efficiency measure.

Kai: So what this changes for us in the lab is that if we’re working at mesoscopic scales, we have to account for these jitters when predicting how well our engines will actually perform.

Mira: It suggests that even in these relatively simple heat engines, the underlying quantum mechanics of piston motion has a measurable impact on the final efficiency statistics.

Lev: And while they use quasi-static assumptions for those strokes, the paper points out that if you actually include heat input fluctuations during those strokes, that’s something they haven't fully tackled yet.

Kai: It’s interesting because it shows how pushing a system beyond its slow limit reveals these hidden quantum effects in the performance of things we think are pretty well understood.