Tag Archives: thought experiment

Quantum back action and devil’s play

I always appreciate a reference to James Clerk Maxwell’s demon thought experiment (you can find out about it in the Maxwell’s demon Wikipedia entry). This time it comes from physicist  Kater Murch in a July 23, 2018 Washington University in St. Louis (WUSTL) news release (published July 25, 2018 on EurekAlert) written by Brandie Jefferson (offering a good explanation of the thought experiment and more),

Thermodynamics is one of the most human of scientific enterprises, according to Kater Murch, associate professor of physics in Arts & Sciences at Washington University in St. Louis.

“It has to do with our fascination of fire and our laziness,” he said. “How can we get fire” — or heat — “to do work for us?”

Now, Murch and colleagues have taken that most human enterprise down to the intangible quantum scale — that of ultra low temperatures and microscopic systems — and discovered that, as in the macroscopic world, it is possible to use information to extract work.

There is a catch, though: Some information may be lost in the process.

“We’ve experimentally confirmed the connection between information in the classical case and the quantum case,” Murch said, “and we’re seeing this new effect of information loss.”

The results were published in the July 20 [2018] issue of Physical Review Letters.

The international team included Eric Lutz of the University of Stuttgart; J. J. Alonzo of the University of Erlangen-Nuremberg; Alessandro Romito of Lancaster University; and Mahdi Naghiloo, a Washington University graduate research assistant in physics.

That we can get energy from information on a macroscopic scale was most famously illustrated in a thought experiment known as Maxwell’s Demon. [emphasis mine] The “demon” presides over a box filled with molecules. The box is divided in half by a wall with a door. If the demon knows the speed and direction of all of the molecules, it can open the door when a fast-moving molecule is moving from the left half of the box to the right side, allowing it to pass. It can do the same for slow particles moving in the opposite direction, opening the door when a slow-moving molecule is approaching from the right, headed left. ­

After a while, all of the quickly-moving molecules are on the right side of the box. Faster motion corresponds to higher temperature. In this way, the demon has created a temperature imbalance, where one side of the box is hotter. That temperature imbalance can be turned into work — to push on a piston as in a steam engine, for instance. At first the thought experiment seemed to show that it was possible create a temperature difference without doing any work, and since temperature differences allow you to extract work, one could build a perpetual motion machine — a violation of the second law of thermodynamics.

“Eventually, scientists realized that there’s something about the information that the demon has about the molecules,” Murch said. “It has a physical quality like heat and work and energy.”

His team wanted to know if it would be possible to use information to extract work in this way on a quantum scale, too, but not by sorting fast and slow molecules. If a particle is in an excited state, they could extract work by moving it to a ground state. (If it was in a ground state, they wouldn’t do anything and wouldn’t expend any work).

But they wanted to know what would happen if the quantum particles were in an excited state and a ground state at the same time, analogous to being fast and slow at the same time. In quantum physics, this is known as a superposition.

“Can you get work from information about a superposition of energy states?” Murch asked. “That’s what we wanted to find out.”

There’s a problem, though. On a quantum scale, getting information about particles can be a bit … tricky.

“Every time you measure the system, it changes that system,” Murch said. And if they measured the particle to find out exactly what state it was in, it would revert to one of two states: excited, or ground.

This effect is called quantum backaction. To get around it, when looking at the system, researchers (who were the “demons”) didn’t take a long, hard look at their particle. Instead, they took what was called a “weak observation.” It still influenced the state of the superposition, but not enough to move it all the way to an excited state or a ground state; it was still in a superposition of energy states. This observation was enough, though, to allow the researchers track with fairly high accuracy, exactly what superposition the particle was in — and this is important, because the way the work is extracted from the particle depends on what superposition state it is in.

To get information, even using the weak observation method, the researchers still had to take a peek at the particle, which meant they needed light. So they sent some photons in, and observed the photons that came back.

“But the demon misses some photons,” Murch said. “It only gets about half. The other half are lost.” But — and this is the key — even though the researchers didn’t see the other half of the photons, those photons still interacted with the system, which means they still had an effect on it. The researchers had no way of knowing what that effect was.

They took a weak measurement and got some information, but because of quantum backaction, they might end up knowing less than they did before the measurement. On the balance, that’s negative information.

And that’s weird.

“Do the rules of thermodynamics for a macroscopic, classical world still apply when we talk about quantum superposition?” Murch asked. “We found that yes, they hold, except there’s this weird thing. The information can be negative.

“I think this research highlights how difficult it is to build a quantum computer,” Murch said.

“For a normal computer, it just gets hot and we need to cool it. In the quantum computer you are always at risk of losing information.”

Here’s a link to and a citation for the paper,

Information Gain and Loss for a Quantum Maxwell’s Demon by M. Naghiloo, J. J. Alonso, A. Romito, E. Lutz, and K. W. Murch. Phys. Rev. Lett. 121, 030604 (Vol. 121, Iss. 3 — 20 July 2018) DOI:https://doi.org/10.1103/PhysRevLett.121.030604 Published 17 July 2018

© 2018 American Physical Society

This paper is behind a paywall.

What’s a science historian doing in the field of synthetic biology?

Dominic Berry’s essay on why he, a science historian, is involved in a synthetic biology project takes some interesting twists and turns, from a Sept. 2, 2016 news item on phys.org,

What are synthetic biologists doing to plants, and what are plants doing to synthetic biology? This question frames a series of laboratory observations that I am pursuing across the UK as part of the Engineering Life project, which is dedicated to exploring what it might mean to engineer biology. I contribute to the project through a focus on plant scientists and my training in the history and philosophy of science. For plant scientists the engineering of biology can take many forms not all of which are captured by the category ‘synthetic biology’. Scientists that aim to create modified organisms are more inclined to refer to themselves as the latter, while other plant scientists will emphasise an integration of biological work with methods or techniques from engineering without adopting the identity of synthetic biologist. Accordingly, different legacies in the biosciences (from molecular biology to biomimetics) can be drawn upon depending on the features of the project at hand. These category and naming problems are all part of a larger set of questions that social and natural scientists continue to explore together. For the purposes of this post the distinctions between synthetic biology and the broader engineering of biology do not matter greatly, so I will simply refer to synthetic biology throughout.

Berry’s piece was originally posted Sept. 1, 2016 by Stephen Burgess on the PLOS (Public Library of Science) Synbio (Synthetic Biology blog). In this next bit Berry notes briefly why science historians and scientists might find interaction and collaboration fruitful (Note: Links have been removed),

It might seem strange that a historian is focused so closely on the present. However, I am not alone, and one recent author has picked out projects that suggest it is becoming a trend. This is only of interest for readers of the PLOS Synbio blog because it flags up that there are historians of science available for collaboration (hello!), and plenty of historical scholarship to draw upon to see your work in a new light, or rediscover forgotten research programs, or reconsider current practices, precisely as a recent Nature editorial emphasised for all sciences.

The May 17, 2016 Nature editorial ‘Second Thoughts’, mentioned in Berry’s piece, opens provocatively and continues in that vein (Note: A link has been removed),

The thought experiment has a noble place in research, but some thoughts are deemed more noble than others. Darwin and Einstein could let their minds wander and imagine the consequences of certain actions or natural laws. But scientists and historians who try to estimate what might have happened if, say, Darwin had fallen off the Beagle and drowned, are often accused of playing parlour games.

What if Darwin had toppled overboard before he joined the evolutionary dots? That discussion seems useful, because it raises interesting questions about the state of knowledge, then and now, and how it is communicated and portrayed. In his 2013 book Darwin Deleted — in which the young Charles is, indeed, lost in a storm — the historian Peter Bowler argued that the theory of evolution would have emerged just so, but with the pieces perhaps placed in a different order, and therefore less antagonistic to religious society.

In this week’s World View, another historian offers an alternative pathway for science: what if the ideas of Gregor Mendel on the inheritance of traits had been challenged more robustly and more successfully by a rival interpretation by the scientist W. F. R. Weldon? Gregory Radick argues that a twentieth-century genetics driven more by Weldon’s emphasis on environmental context would have weakened the dominance of the current misleading impression that nature always trumps nurture.

Here is Berry on the importance of questions,

The historian can ask: What traditions and legacies are these practitioners either building on or reacting against? How do these ideas cohere (or remain incoherent) for individuals and laboratories? Is a new way of understanding and investigating biology being created, and if so, where can we find evidence of it? Have biologists become increasingly concerned with controlling biological phenomena rather than understanding them? How does the desire to integrate engineering with biology sit within the long history of the establishment of biological science over the course of the 19th and 20th centuries?

Berry is an academic and his piece reflects an academic writing style with its complicated sentence structures and muted conclusions. If you have the patience, it is a good read on a topic that isn’t discussed all that often.