Physicists investigate fundamental limits of quantum engines

August 4, 2017 by Lisa Zyga, feature

A quantum engine in which work is produced during the first and third strokes. Credit: Abah et al. ©2017 EPL
(—Quantum engines are known to operate differently than—and in some cases, outperform—their classical counterparts. However, previous research on the performance of quantum engines may be overestimating their advantages. In a new study, physicists have developed an improved method to compute the efficiency of quantum engines. They show that the ultimate efficiency of quantum systems is subject to tighter fundamental limits than those imposed by the second law of thermodynamics, which governs the efficiency of classical systems.

Physicists Obinna Abah and Eric Lutz at the Friedrich-Alexander University Erlangen-Nürnberg in Germany have published a paper on the -efficient quantum machines in a recent issue of EPL. Abah is currently a Royal Commission for the Exhibition of 1851 research fellow at Queen's University in Belfast, UK.

The performance of any kind of —quantum or classical—is largely determined by its (the ratio of to energy input) and its power (the rate of energy output in a given time). Conventional thermodynamics imposes a tradeoff between an engine's efficiency and its power—meaning when you increase one, the other decreases. For quantum engines, however, it's possible to increase both the efficiency and the power at the same time. This means that, with the proper methods, quantum engines can potentially produce more energy output from a given amount of energy input, and do so at a faster rate than before the improvement.

Some of the methods that allow for the simultaneous increase in efficiency and power are called "shortcut-to-adiabaticity" techniques. Adiabatic transformations are highly desirable because they dissipate little energy, which increases the efficiency of the system and speeds up the system's dynamics, which increases the system's power output. As their name implies, shortcuts to adiabaticity allow quantum machines to mimic adiabatic operation in a much shorter time than is possible using genuine adiabatic transformations, which are infinitely slow.

Although previous research has demonstrated the advantages of shortcuts to adiabaticity for enhancing the performance of heat engines, these methods typically do not account for the energy cost of the shortcut protocol when calculating the final efficiency of the system. As a result, the efficiency improvements due to shortcuts to adiabaticity appear to be for free, exaggerating their effects.

In the new study, Abah and Lutz developed a method for evaluating the performance of a system that accounts for the energy cost of these shortcuts. Their results show that shortcuts to adiabaticity enhance the performance of a system only if the shortcut is sufficiently fast, since faster shortcuts have lower energy costs. On the other hand, very slow shortcut protocols have higher energy costs that may exceed any potential energy gains.

"Our work shows that higher efficiency and higher power may be achieved at the same time with the help of shortcut-to-adiabaticty methods, even when the energetic cost of the shortcut is taken into account," Abah told

The physicists also showed that there is a fundamental limit to the efficiency of any quantum engine, no matter what kind of shortcuts to adiabaticity it uses. Surprisingly, the limits on a quantum engine are stricter than the limits imposed by the second law of thermodynamics, which sets the ultimate limits on the efficiency of classical engines.

As the physicists explain, the reason for the tighter bounds on quantum engines is because classical mechanics does not place restrictions on the speed of a process, whereas quantum mechanics does have speed restrictions, which are given by "quantum speed limits." The scientists plan to compare different shortcut methods in order to determine the one leading to the most energy-efficient machine. Understanding and their fundamental limitations on quantum systems is essential for designing future quantum engines.

"The advent of miniaturization will unavoidably lead to machines that are so tiny that their dynamics will generally obey the laws of quantum mechanics instead of those of classical mechanics," Abah said. "Their properties will then be governed by thermodynamics."

Explore further: Quantum shortcuts cannot bypass the laws of thermodynamics

More information: Obinna Abah and Eric Lutz. "Energy efficiency quantum machines." EPL. DOI: 10.1209/0295-5075/118/40005

Related Stories

Quantum shortcuts cannot bypass the laws of thermodynamics

March 16, 2017

(—Over the past several years, physicists have developed quantum shortcuts that speed up the operation of quantum systems. Surprisingly, some of these shortcuts theoretically appear to enable systems to operate ...

Physicists design zero-friction quantum engine

September 16, 2014

( —In real physical processes, some energy is always lost any time work is produced. The lost energy almost always occurs due to friction, especially in processes that involve mechanical motion. But in a new study, ...

What is quantum in quantum thermodynamics?

October 12, 2015

(—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 ...

Quantum effects lead to more powerful battery charging

May 1, 2017

(—Physicists have theoretically shown that, when multiple nanoscale batteries are coupled together, they can be charged faster than if each battery was charged individually. The improvement arises from collective ...

Maxwell's demon extracts work from quantum measurement

July 10, 2017

(—Physicists have proposed a new type of Maxwell's demon—the hypothetical agent that extracts work from a system by decreasing the system's entropy—in which the demon can extract work just by making a measurement, ...

Recommended for you

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

Physicists reveal why matter dominates universe

March 21, 2019

Physicists in the College of Arts and Sciences at Syracuse University have confirmed that matter and antimatter decay differently for elementary particles containing charmed quarks.

ATLAS experiment observes light scattering off light

March 20, 2019

Light-by-light scattering is a very rare phenomenon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of ...

How heavy elements come about in the universe

March 19, 2019

Heavy elements are produced during stellar explosion or on the surfaces of neutron stars through the capture of hydrogen nuclei (protons). This occurs at extremely high temperatures, but at relatively low energies. An international ...


Adjust slider to filter visible comments by rank

Display comments: newest first

Aug 04, 2017
This comment has been removed by a moderator.
Aug 04, 2017
This comment has been removed by a moderator.
3 / 5 (6) Aug 04, 2017
Actually the highest computational power would have hybrids of quantum and classical computers - which is also why the human brain (and also human hearing) operates in this regime.

Brains are neural networks.
Where and how does it do 'quantum computing'? (and why?)

I am aware of the highly speculative 'quantum mind' and 'quantum consciousness' hypotheses, but frankly they don't make much sense have no evidence and are really not required.
Neural networks can do all the processing needed.

Aug 05, 2017
This comment has been removed by a moderator.
Captain Stumpy
3 / 5 (2) Aug 05, 2017
@dingbat zeph
It's explained in the link given.
your link is to a known pseudoscience site that you personally run

that is not just spamming with pseudoscience, that is called phishing as you will have access to certain identifying information from anyone who visits without considering safety protocols

just because you can link your personal blog doesn't mean you have an explanation, either, especially as your basic premise of the site is it's foundation on a falsified belief

like almost all your other claims, you can't provide any reputable or scientific evidence for your claims
you can only link your beliefs about your claims

That is not an explanation, that is predatory proselytizing
Da Schneib
not rated yet Aug 05, 2017
What's really interesting is the finding that no matter how you try it, over the long haul even QM obeys the 2LOT.
Aug 06, 2017
This comment has been removed by a moderator.
Spaced out Engineer
not rated yet Aug 08, 2017
Isn't have an engine still assuming function. After all, all the information is all still there, if we are to uphold the second law. It's just a lot of correlates over some volume.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.