The physicists, Tiago Batalhão at the Federal University of ABC, Brazil, and coauthors, have published their paper on the experimental demonstration of quantum thermodynamic irreversibility in a recent issue of *Physical Review Letters*.

Irreversibility at the quantum level may seem obvious to most people because it matches our observations of the everyday, macroscopic world. However, it is not as straightforward to physicists because the microscopic laws of physics, such as the Schrödinger equation, are "time-symmetric," or reversible. In theory, forward and backward microscopic processes are indistinguishable.

In reality, however, we only observe forward processes, not reversible ones like broken egg shells being put back together. It's clear that, at the macroscopic level, the laws run counter to what we observe. Now the new study shows that the laws don't match what happens at the quantum level, either.

Observing thermodynamic processes in a quantum system is very difficult and has not been done until now. In their experiment, the scientists measured the entropy change that occurs when applying an oscillating magnetic field to carbon-13 atoms in liquid chloroform. They first applied a magnetic field pulse that causes the atoms' nuclear spins to flip, and then applied the pulse in reverse to make the spins undergo the reversed dynamics.

If the procedure were reversible, the spins would have returned to their starting points—but they didn't. Basically, the forward and reverse magnetic pulses were applied so rapidly that the spins' flipping couldn't always keep up, so the spins were driven out of equilibrium. The measurements of the spins indicated that entropy was increasing in the isolated system, showing that the quantum thermodynamic process was irreversible.

By demonstrating that thermodynamic irreversibility occurs even at the quantum level, the results reveal that thermodynamic irreversibility emerges at a genuine microscopic scale. This finding makes the question of why the microscopic laws of physics don't match our observations even more pressing. If the laws really are reversible, then what are the physical origins of the time-asymmetric entropy production that we observe?

The physicists explain that the answer to this question lies in the choice of the initial conditions. The microscopic laws allow reversible processes only because they begin with "a genuine equilibrium process for which the entropy production vanishes at all times," the scientists write in their paper. Preparing such an ideal initial state in a physical system is extremely complex, and the initial states of all observed processes aren't at "genuine equilibrium," which is why they lead to irreversible processes.

"Our experiment shows the irreversible nature of quantum dynamics, but does not pinpoint, experimentally, what causes it at the microscopic level, what determines the onset of the arrow of time," coauthor Mauro Paternostro at Queen's University in Belfast, UK, told *Phys.org*. "Addressing it would clarify the ultimate reason for its emergence."

The researchers hope to apply the new understanding of thermodynamics at the quantum level to high-performance quantum technologies in the future.

"Any progress towards the management of finite-time thermodynamic processes at the quantum level is a step forward towards the realization of a fully fledged thermo-machine that can exploit the laws of quantum mechanics to overcome the performance limitations of classical devices," Paternostro said. "This work shows the implications for reversibility (or lack thereof) of non-equilibrium quantum dynamics. Once we characterize it, we can harness it at the technological level."

**Explore further:**
What is quantum in quantum thermodynamics?

**More information:**
T. B. Batalhão, et al. "Irreversibility and the Arrow of Time in a Quenched Quantum System." *Physical Review Letters*. DOI: 10.1103/PhysRevLett.115.190601

Also at arXiv:1502.06704 [quant-ph]

## katesisco

## johnksellers

## andrewbb

## andrewbb

Frankly, this should have been self-evident, but people will do any mental gymnastics required to believe that time is reversible.

## Hyperfuzzy

## Hyperfuzzy

May a simple hydrogen atom be dismantled in free space and re-assembled. Of course. The above is simply nonsensical and based upon a false experiment that has no pertinence to the premise as a proof of it. It may be an existence proof of non-reversibility under certain condition; but definitely not holistic. This is a known, axiomatic, not something that requires a proof, at least for me. Define it for all initial conditions, place the hydrogen in a jar without any external fields. Don't think all premise are provable! May only be definable with real real physics.

## Hyperfuzzy

## rebelclause

## Mimath224

On second thoughts, there would probably still be some kind of 'law' that would prevent our intervention.

Sorry Chaps just thinking out loud.

## howhot2

So let F & B represent the arrows of a dimension (forward and back). If they are not at 50/50 probability but a fraction off from forward or back, there will be a motion F or B that is along the dimension we call time. During the Big Bang all of the dimensions were created but what if the B wasn't fully made, if it was made at all? Only the constant 'c' of the speed of light sets the limit on the F/B ratio and defines the dimension of time.

Ponder that my quantum friends!

## Noumenon

I'm really perplexed as to why this is even a question.

The reason why it appears not possible to pinpoint what "causes it" at the microscopic level, is because the second law of thermodynamics is an epiphenomenon,...which is to say, ... it is NOT a fundamental law of nature,.... it is simply a mathematical result. Perhaps this enforces Bell's theorum by validating our notions of statistical theory at the microscopic level.

The "arrow of time" is likewise to conflate, what is in essence just mathematical probability, with substantive physical reality.

## richardnunziata

## CharlesRKiss

## Hyperfuzzy

## Ensign_nemo

This sort of idealized version of QM was found to be inadequate to explain certain observations, such as the Lamb shift, so the theory of quantum electrodynamics was created to incorporate the effects of these virtual particles.

I wonder if something similar is happening here, where there is a "hidden" interaction with virtual particles creating the QM equivalent of friction and making a unitary process into an irreversible process.

## Hyperfuzzy

We might want to recall that QM is NOT a theory but a tool, i.e. the wave equation between potential and kinetic. Note that this gives a statistical measure and ignores causality which allows all possibilities to exist at the same time. Totally unrealistic when one considers that all the particles may be defined as almost anything using superposition. Hence a single particle may be defined by a multiple set via superimposition or a multiple set may be defined as a single particle. Not to mention the error with causality. Because a "thing" that might exist is not an existence proof! hence, it is not QM, it's the interpretation of scientist without reason! The above is an attempt to experimentally display a null result. Think about it, bake a cake, null result. "maroons!"

## Bongstar420

So what happened when the frequency of the magnetic flipping was slowed?

and, doesn't the "entropy increase" mean an increase in temperature and therefore atomic velocity?

Wouldn't this mean that the magnetic field was being converted to momentum instead of spin due to a frequency differential between the field flux and the atomic field equilibrium flux?

## brodix

## AverageAlfred