October 12, 2015 feature

# What is quantum in quantum thermodynamics?

(Phys.org)—A lot of attention has been given to the differences between the quantum and classical worlds. For example, quantum entanglement, superposition, and teleportation are purely quantum phenomena with no classical counterparts. However, when it comes to certain areas of thermodynamics—specifically, thermal engines and refrigerators—quantum and classical systems so far appear to be nearly identical. It seems that the same thermodynamic laws that govern the engines in our vehicles may also accurately describe the tiniest quantum engines consisting of just a single particle.

In a new study, physicists Raam Uzdin, Amikam Levy, and Ronnie Kosloff at the Hebrew University of Jerusalem have investigated whether there is anything distinctly quantum about thermodynamics at the quantum level, or if "quantum" thermodynamics is really the same as classical thermodynamics.

For the first time, they have shown a difference in the thermodynamics of heat machines on the quantum scale: in part of the quantum regime, the three main engine types (two-stroke, four-stroke, and continuous) are thermodynamically equivalent. This means that, despite operating in different ways, all three types of engines exhibit all of the same thermodynamic properties, including generating the same amounts of power and heat, and doing so at the same efficiency. This new "thermodynamical equivalence principle" is purely quantum, as it depends on quantum effects, and does not occur at the classical level.

The scientists also showed that, in this quantum regime where all engines are thermodynamically equivalent, it's possible to extract a quantum-thermodynamic signature that further confirms the presence of quantum effects. They did this by calculating an upper limit on the work output of a classical engine, so that any engine that surpasses this bound must be using a quantum effect—namely, quantum coherence—to generate the additional work. In this study, quantum coherence, which accounts for the wave-like properties of quantum particles, is shown to be critical for power generation at very fast engine cycles.

"To the best of my knowledge, this is the first time [that a difference between quantum and classical thermodynamics has been shown] in heat machines," Uzdin told *Phys.org*. "What has been surprising [in the past] is that the classical description has still held at the quantum level, as many authors have shown. The reasons are now understood, and in the face of this classicality, people have started to stray to other types of research, as it was believed that nothing quantum can pop up. Thus, it was very difficult to isolate a generic effect, not just a numerical simulation of a specific case, with a complementing theory that manages to avoid the classicality and demonstrate quantum effects in thermodynamic quantities, such as work and heat."

One important implication of the new results is that quantum effects may significantly increase the performance of engines at the quantum level. While the current work deals with single-particle engines, the researchers expect that quantum effects may also emerge in multi-particle engines, where quantum entanglement between particles may play a role similar to that of coherence.

Explore further

**More information:**Raam Uzdin, et al. "Equivalence of Quantum Heat Machines, and Quantum-Thermodynamic Signatures."

*Physical Review X*. DOI: 10.1103/PhysRevX.5.031044

© 2015 Phys.org

**Citation**: What is quantum in quantum thermodynamics? (2015, October 12) retrieved 20 October 2019 from https://phys.org/news/2015-10-quantum-thermodynamics.html

## User comments

HyperfuzzyHav3000inkosanaTime cannot be quantised just like time cannot dilate.

docileinkosanainkosanaOne must go back to Pythagoras. Minkowski did not understand the theorem of Pythagoras and thus assumed that one of the sides of a right-triangle can be the hypotenuse. All 20th century theoretical physics is based on this impossible mathematics and must thus be flawed: Physics that is modelled in terms of impossible mathematics can obviously not be correct..

derphysIt opens the possibility to obtain a "quantum signature" similar to Bell inequalities of perfect quantum coherent machines with reduced dissipation, like macroscopic superfluidity or superconductivity.

Torbjorn_Larsson_OM@ It is known that classical physics is a useful approximation, is all. So are heat and cold sinks.

@Hav3000: Time is not entropy, as seen by their different units. Is a pear a chair?

@inkosana: References, please! Minkowski didn't assume anything about triangles that wasn't implicit in the geometry of his spacetime, say. And that spacetime is simple and sound.

Torbjorn_Larsson_OMNit: Quantum mechanics and general relativity is consistent in the core theory (GR + SM). it is GR that breaks down at large energies/small scales. "It's often said that it is difficult to reconcile quantum mechanics (quantum field theory) and general relativity. That is wrong. We have what is, for many purposes, a perfectly good effective field theory description of quantum gravity. ... In other words, as an effective field theory, gravity is no worse, nor better, than any other of the effective field theories we know and love. The trouble is that all hell breaks loose for ε ~ 1. " [ https://golem.ph....639.html ]

docileinkosanaTo keep the discussion simple let us consider a 2 dimensional plane: If the coordinates are x and y, the square of the position vector is given by x^2+y^2 (NOTE THE PLUS). Minkowski claimed that when one of the coordinates is (ct) one can write that the position vector (hypotenuse) is given by (ct)^2 MINUS x^2.. This violates the theorem of Pythagoras which demands that the hypotenuse MUST ALWAYS be the POSITIVE SUM of the coordinates. Not even grade 8 pupil would be so stupid as Minkowski was.

Continue

inkosanaContinued

inkosanaThe correct derivations are given in my books: Why Galileo TRUMPS Einstein and in Why does E=mc2. They are available in Kindle format on Amazon.

inkosanajohnhewMust have gone to hunt bear in the far North.

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