Physicists confirm thermodynamic irreversibility in a quantum system

December 1, 2015 by Lisa Zyga report
In the experiment, a sample of liquid chloroform (CHCl3) is placed at the center of a superconducting magnet inside a nuclear magnetic resonance (NMR) magnetometer. Forward and reverse magnetic pulses are applied to the sample, which drives the carbon nuclear spins out of equilibrium and produces irreversible entropy. Credit: Batalhão, et al. ©2015 American Physical Society

(—For the first time, physicists have performed an experiment confirming that thermodynamic processes are irreversible in a quantum system—meaning that, even on the quantum level, you can't put a broken egg back into its shell. The results have implications for understanding thermodynamics in quantum systems and, in turn, designing quantum computers and other quantum information technologies.

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 "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 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]

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5 / 5 (8) Dec 01, 2015
The Second Law stands. Everywhere.
5 / 5 (2) Dec 01, 2015
It would be interesting to find that what is described as "genuine equilibrium" is not only difficult to achieve in practice, but actually impossible in principle. But I think you would have to reformulate quantum mechanics in some way in order to have that impossibility emerge as a natural consequence of some more fundamental principle.
4 / 5 (2) Dec 01, 2015
Given the time of year, think of neat packages: QM incorporating thermodynamic irreversibility changes the status of information conservation as information becomes inaccessible in principle due to thermodynamic evolution, and the revised quantum thermodynamics leads to a theory in which quantum gravity emerges as a by-product of quantum thermodynamics. A deep and unsuspected symmetry is revealed! Whee! Everybody celebrates.
3.5 / 5 (2) Dec 01, 2015
You would explain this new QM gravity theory to laymen by saying something like: "Gravity is actually a quantum thermodynamic gradient structure in space-time". And that's the best you'd be able to do, without the math behind it.
3.8 / 5 (4) Dec 01, 2015
I'm a little confused by this claim. I thought that what you "observe" is not the quantum effects, but the probability collapse of the quantum system. Isn't it possible that the quantum system is reversible but our observation of the aftermath of such a system is not?
4 / 5 (2) Dec 01, 2015
And others would point out that, while Relativity requires an invariant c, quantum gravity requires an invariant arrow of time. How weird is that?
5 / 5 (3) Dec 01, 2015
You can't undo time.
3.2 / 5 (5) Dec 01, 2015
A stick of dynamite can't be reconstructed after detonation, but equations show it can.

Frankly, this should have been self-evident, but people will do any mental gymnastics required to believe that time is reversible.
2.2 / 5 (5) Dec 01, 2015
Not sure this experiment proves anything. It only shows non-reversibility with this method. Consider each mode that alters the sample, say a phonon at a single site, ie. delta temperature, more available states, Boltzmann. Does relaxation exist, i.e. reversible for this state. Observing a phonon entering a sample and exiting a sample unchanged would be an example of what? In other words, what is the definition of reversibility and upon what states. Obviously not all states would be reversible, example; entering a more stable state might simply repeal the phonom as the nucleus or an electron bounces and returns exiting with the same phonom. The idea might be as silly as Einstein riding a wavelet at its peak, i.e. speed of light as zero. not constant. Therefore, reversibility may be common once we understand what reversibility is, spatially and temporally. This is rather amateurish. Seems more of a mind game but is definitely not well defined physics without a definition.
1 / 5 (3) Dec 01, 2015
A stick of dynamite can't be reconstructed after detonation, but equations show it can.

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

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.
1 / 5 (4) Dec 01, 2015
Many biological cells show this as nonsense. But we can always say, no, because...
1 / 5 (2) Dec 01, 2015
I agree with commenters who point out the method simply flips orbits. In the spaciotemporal continuum the experiment appaears also to ignore influence on the field pressure as disequilibriu at a given frequency results. To me, this back EMF, if you will, would speak to system symmetrical effects, but not to the magnitude of temporal symmetries derived from measuring the quantity of disequilibrium and the spacial displacement. I'd like to see a single carbon atom, measured dissipations, and field precessions, among other things, before I could agree the experiment demonstates what it claims.
2.3 / 5 (3) Dec 01, 2015
I think it right that 'one cannot un-scramble eggs' but that is with our current arrow of Time interpretation. But what if the arrow of time is just a manifestation of something far more complicated, that is Time having dimensions too. That is to say that what we perceive as 'time' moving forward is the equivalent of a Pythagorean, resultant vector. I wonder if it would then possible for each moment in time to be 'snapshot' so that going backwards would be possible.
On second thoughts, there would probably still be some kind of 'law' that would prevent our intervention.
Sorry Chaps just thinking out loud.
1 / 5 (2) Dec 01, 2015
Let say we are counting, 1 2 3 4 ... infinity. Now suppose we can count two directions and we label those F and B which are forward in time and backward in time. So if randomly placed F and B, such that random 1F 2B 3B 4F ... infinity[F,B] = 1. You see Infinity - infinity + 1 = 1. However infinity F = infinity B will be a 50/50 chance of F or B. So infinity[F or B] = 1 [F or B].
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!

3.7 / 5 (3) Dec 02, 2015
I don't think "dimensions" are physical. Given Noerther's theorum, they appear to be merely a mathematical means of coordinating symmetries and conservation laws. Saying that they represent much more than this is to enter into mathematical idealism.

Our experiment shows the irreversible nature of quantum dynamics, but does not pinpoint, experimentally, what causes it at the microscopic level, ... arrow of time

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.
5 / 5 (2) Dec 02, 2015
Is this proof that time travel is not possible.
1 / 5 (2) Dec 02, 2015
I always feel that intelligent life always contradicts these experiments, and the egg does indeed form inside a cracked shell, "provided you eat it for breakfast." How can such an experiment even be constructed without it?
1 / 5 (3) Dec 02, 2015
Anyway, I'm not certain an experiment defines a non-existence proof. Does anyone get this? How do you define "why" for all?
5 / 5 (1) Dec 05, 2015
Most QM "problems" have the hidden assumption that there are no "virtual" particles around to interfere with the system. It is similar to assuming a frictionless surface for a conservation of momentum problem in a basic mechanics course - it's convenient but not realistic.

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.
3 / 5 (2) Dec 05, 2015
Most QM "problems" have the hidden assumption ...

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!"
5 / 5 (1) Dec 05, 2015
..." 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."

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?
5 / 5 (2) Dec 05, 2015
We experience reality as flashes of cognition and so think of time as the point of the present moving from past to future and physics codifies this by treating it as scalar measures of duration, thus its symmetry, but the reality is change creating and dissolving these events, such that they go future to past. To wit, tomorrow becomes yesterday because the earth turns. This makes time an effect of activity, similar to temperature and what is being measured is essentially frequency. Duration is just the state of the present, as these events come and go. So the real reason time is asymmetric is because of the inertia of the activity being measured. The earth is turning one direction, not the other. The measurement is map, not territory.
5 / 5 (1) Dec 06, 2015
Does a forward only arrow in time not conflict with spooky action at a distance and entanglement in general?

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