Tag Archives: K. W. Murch

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.

Can the future influence the past? The answer is: mostly yes

The principles of quantum mechanics mystify me which, as it turns out, is the perfect place to start with the work featured in a Feb. 9, 2015 news item on ScienceDaily,

We’re so used to murder mysteries that we don’t even notice how mystery authors play with time. Typically the murder occurs well before the midpoint of the book, but there is an information blackout at that point and the reader learns what happened then only on the last page.

If the last page were ripped out of the book, physicist Kater Murch, PhD, said, would the reader be better off guessing what happened by reading only up to the fatal incident or by reading the entire book?

The answer, so obvious in the case of the murder mystery, is less so in world of quantum mechanics, where indeterminacy is fundamental rather than contrived for our reading pleasure.

A Feb. 13, 2015 Washington University at St. Louis (WUSTL) news release by Diana Lutz, which originated the news item, describes the research,

Even if you know everything quantum mechanics can tell you about a quantum particle, said Murch, an assistant professor of physics in Arts & Sciences at Washington University in St. Louis, you cannot predict with certainty the outcome of a simple experiment to measure its state. All quantum mechanics can offer are statistical probabilities for the possible results.

The orthodox view is that this indeterminacy is not a defect of the theory, but rather a fact of nature. The particle’s state is not merely unknown, but truly undefined before it is measured. The act of measurement itself that forces the particle to collapse to a definite state.

It’s as if what we did today, changed what we did yesterday. And as this analogy suggests, the experimental results have spooky implications for  time and causality—at least in microscopic world to which quantum mechanics applies.

Until recently physicists could explore the quantum mechanical properties of single particles only through thought experiments, because any attempt to observe them directly caused them to shed their mysterious quantum properties.

But in the 1980s and 1990s physicists invented devices that allowed them to measure these fragile quantum systems so gently that they don’t immediately collapse to a definite state.

The device Murch uses to explore quantum space is a simple superconducting circuit that enters quantum space when it is cooled to near absolute zero. Murch’s team uses the bottom two energy levels of this qubit, the ground state and an excited state, as their model quantum system. Between these two states, there are an infinite number of quantum states that are superpositions, or combinations, of the ground and excited states.

The quantum state of the circuit is detected by putting it inside a microwave box. A few microwave photons are sent into the box, where their quantum fields interact with the superconducting circuit. So when the photons exit the box they bear information about the quantum system.

Crucially, these “weak,” off-resonance measurements do not disturb the qubit, unlike “strong” measurements with photons that are resonant with the energy difference between the two states, which knock the circuit into one or the other state.

In Physical Review Letters, Murch describes a quantum guessing game played with the qubit.

“We start each run by putting the qubit in a superposition of the two states,” he said. “Then we do a strong measurement but hide the result, continuing to follow the system with weak measurements.”

They then try to guess the hidden result, which is their version of the missing page of the murder mystery.

“Calculating forward, using the Born equation that expresses the probability of finding the system in a particular state, your odds of guessing right are only 50-50,” Murch said. “But you can also calculate backward using something called an effect matrix. Just take all the equations and flip them around. They still work and you can just run the trajectory backward.

“So there’s a backward-going trajectory and a forward-going trajectory and if we look at them both together and weight the information in both equally, we get something we call a hindsight prediction, or “retrodiction.”

The shattering thing about the retrodiction is that it is 90 percent accurate. When the physicists check it against the stored measurement of the system’s earlier state it is right nine times out of 10.

Going from a 50% accuracy rate to 90% is quite amazing and according to the news release, this has many implications,

The quantum guessing game suggests ways to make both quantum computing and the quantum control of open systems, such as chemical reactions, more robust. But it also has implications for much deeper problems in physics.

For one thing, it suggests that in the quantum world time runs both backward and forward whereas in the classical world it only runs forward.

“I always thought the measurement would resolve the time symmetry in quantum mechanics,” Murch said. “If we measure a particle in a superposition of states and it collapses into one of two states, well, that sounds like a process that goes forward in time.”

But in the quantum guessing experiment, time symmetry has returned. The improved odds imply the measured quantum state somehow incorporates information from the future as well as the past. And that implies that time, notoriously an arrow in the classical world, is a double-headed arrow in the quantum world.

“It’s not clear why in the real world, the world made up of many particles, time only goes forward and entropy always increases,” Murch said. “But many people are working on that problem and I expect it will be solved in a few years,” he said.

In a world where time is symmetric, however, is there such a thing as cause and effect? To find out, Murch proposes to run a qubit experiment that would set up feedback loops (which are chains of cause and effect) and try to run them both forward and backward.

“It takes 20 or 30 minutes to run one of these experiments,” Murch said, “several weeks to process it, and a year to scratch our heads to see if we’re crazy or not.”

“At the end of the day,” he said, “I take solace in the fact that we have a real experiment and real data that we plot on real curves.”

Here are links to and citations for the Physical Review paper and an earlier version of the paper,

 Prediction and retrodiction for a continuously monitored superconducting qubit by D. Tan, S. Weber, I. Siddiqi, K. Mølmer, K. W. Murch. arXiv.org > quant-ph > arXiv:1409.0510 (Submitted on 1 Sep 2014 (v1), last revised 10 Nov 2014 (this version, v2))

I last mentioned Kater Murch and his work in a July 31, 2014 post titled: Paths of desire: quantum style.

Paths of desire: quantum style

Shortcuts are also called paths of desire (and other terms too) by those who loathe them. It turns that humans and other animals are not the only ones who use shortcuts. From a July 30, 2014 news item on ScienceDaily,

Groundskeepers and landscapers hate them, but there is no fighting them. Called desire paths, social trails or goat tracks, they are the unofficial shortcuts people create between two locations when the purpose-built path doesn’t take them where they want to go.

There’s a similar concept in classical physics called the “path of least action.” If you throw a softball to a friend, the ball traces a parabola through space. It doesn’t follow a serpentine path or loop the loop because those paths have higher “actions” than the true path.

A July 30, 2014 Washington University in St. Louis (Missouri, US) news release (also on EurekAlert) by Diana Lutz, which originated the news item, describes the issues associated with undertaking this research,

Quantum particles can exist in a superposition of states, yet as soon as quantum particles are “touched” by the outside world, they lose this quantum strangeness and collapse to a classically permitted state. Because of this evasiveness, it wasn’t possible until recently to observe them in their quantum state.

But in the past 20 years, physicists have devised devices that isolate quantum systems from the environment and allow them to be probed so gently that they don’t immediately collapse. With these devices, scientists can at long last follow quantum systems into quantum territory, or state space.

Kater Murch, PhD, an assistant professor of physics at Washington University in St. Louis, and collaborators Steven Weber and Irfan Siddiqui of the Quantum Nanoelectronics Laboratory at the University of California, Berkeley, have used a superconducting quantum device to continuously record the tremulous paths a quantum system took between a superposition of states to one of two classically permitted states.

Because even gentle probing makes each quantum trajectory noisy, Murch’s team repeated the experiment a million times and examined which paths were most common. The quantum equivalent of the classical “least action” path — or the quantum device’s path of desire — emerged from the resulting cobweb of many paths, just as pedestrian desire paths gradually emerge after new sod is laid.

The experiments, the first continuous measurements of the trajectories of a quantum system between two points, are described in the cover article of the July 31 [2014] issue of Nature.

“We are working with the simplest possible quantum system,” Murch said. “But the understanding of quantum interactions we are gaining might eventually be useful for the quantum control of biological and chemical systems.

“Chemistry at its most basic level is described by quantum mechanics,” he said. “In the past 20 years, chemists have developed a technique called quantum control, where shaped laser pulses are used to drive chemical reactions — that is, to drive them between two quantum states. The chemists control the quantum field from the laser, and that field controls the dynamics of a reaction,” he said.

“Eventually, we’ll be able to control the dynamics of chemical reactions with lasers instead of just mixing reactant 1 with reactant 2 and letting the reaction evolve on its own,” he said.

An artificial atom The device Murch uses to explore quantum space is a simple superconducting circuit. Because it has quantized energy levels, or states, like an atom, it is sometimes called an artificial atom. Murch’s team uses the bottom two energy levels, the ground state and an excited state, as their model quantum system.

Between these two states, there are an infinite number of quantum states that are superpositions, or combinations, of the ground and excited states. In the past, these states would have been invisible to physicists because attempts to measure them would have caused the system to immediately collapse.

But Murch’s device allows the system’s state to be probed many times before it becomes an effectively classical system. The quantum state of the circuit is detected by putting it inside a microwave box. A very small number of microwave photons are sent into the box where their quantum fields interact with the superconducting circuit.

The microwaves are so far off resonance with the circuit that they cannot drive it between its ground and its excited state. So instead of being absorbed, they leave the box bearing information about the quantum system in the form of a phase shift (the position of the troughs and peaks of the photons’ wavefunctions).

Although there is information about the quantum system in the exiting microwaves, it is only a small amount of information.

“Every time we nudge the system, something different happens,” Murch said. “That’s because the photons we use to measure the quantum system are quantum mechanical as well and exhibit quantum fluctuations. So it takes many of these measurements to distinguish the system’s signal from the quantum fluctuations of the photons probing it.” Or, as physicists put it, these are weak measurements.

Murch compares these experiments to soccer matches, which are ultimately experiments to determine which team is better. But because so few goals are scored in soccer, and these are often lucky shots, the less skilled team has a good chance of winning. Or as Murch might put it, one soccer match is such a weak measurement of a team’s skill that it can’t be used to draw a statistically reliable conclusion about which team is more skilled.

Each time a team scores a goal, it becomes somewhat more likely that that team is the better team, but the teams would have to play many games or play for a very long time to know for sure. These fluctuations are what make soccer matches so exciting.

Murch is in essence able to observe millions of these matches, and from all the matches where team B wins, he can determine the most likely way a game that ends with a victory for team B will develop.

Despite the difficulties, the team did establish a path of desire,

“Before we started this experiment,” Murch said, ” I asked everybody in the lab what they thought the most likely path between quantum states would be. I drew a couple of options on the board: a straight line, a convex curve, a concave curve, a squiggly line . . . I took a poll, and we all guessed different options. Here we were, a bunch of quantum experts, and we had absolutely no intuition about the most likely path.”

Andrew N. Jordan of the University of Rochester and his students Areeya Chantasri and Justin Dressel inspired the study by devising a theory to predict the likely path. Their theory predicted that a convex curve Murch had drawn on the white board would be the correct path.

“When we looked at the data, we saw that the theorists were right. Our very clever collaborators had devised a ‘principle of least action’ that works in the quantum case,” Murch said.

They had found the quantum system’s line of desire mathematically and by calculation before many microwave photons trampled out the path in Murch’s lab.

Here’s an illustrated quantum path of desire’s experimental data,

Caption: A path of desire emerging from many trajectories between two points in quantum state space. Credit: Murch Lab/WUSTL

Caption: A path of desire emerging from many trajectories between two points in quantum state space.
Credit: Murch Lab/WUSTL

The University of Rochester, a collaborating institution on this research, issued a July 30, 2014 news release (also on EurekAlert) featuring this poetic allusion from one of the theorists,

Jordan [Andrew N. Jordan, professor of physics at the University of Rochester] compares the experiment to watching butterflies make their way one by one from a cage to nearby trees. “Each butterfly’s path is like a single run of the experiment,” said Jordan. “They are all starting from the same cage, the initial state, and ending in one of the trees, each being a different end state.” By watching the quantum equivalent of a million butterflies make the journey from cage to tree, the researchers were in effect able to predict the most likely path a butterfly took by observing which tree it landed on (known as post-selection in quantum physics measurements), despite the presence of a wind, or any disturbance that affects how it flies (which is similar to the effect measuring has on the system).

The theorists provided this illustration of the theory,

Caption: Measurement data showing the comparison with the 'most likely' path (in red) between initial and final quantum states (black dots). The measurements are shown on a representation referred to as a Bloch sphere. Credit: Areeya Chantasri Courtesy: University of Rochester

Caption: Measurement data showing the comparison with the ‘most likely’ path (in red) between initial and final quantum states (black dots). The measurements are shown on a representation referred to as a Bloch sphere.
Credit: Areeya Chantasri Courtesy: University of Rochester

The research study can be found here,

Mapping the optimal route between two quantum states by S. J. Weber, A. Chantasri, J. Dressel, A. N. Jordan, K. W. Murch & I. Siddiqi. Nature 511, 570–573 (31 July 2014) doi:10.1038/nature13559 Published online 30 July 2014

This paper is behind a paywall but there is a free preview via ReadCube Access.