(upbeat music) - Hello, everyone, and welcome back to Conversations at the Perimeter. On this episode, Lauren and I chat with Nicole Yunger Halpern. She is a quantum thermodynamicist, which is just about as complicated as it sounds, but thankfully Nicole is gifted at explaining it, and she's done so in the context of steampunk, the science fiction genre that is sort of set in the Industrial Revolution.
And she's the author of the book by that title, "Quantum Steampunk: The Physics of Yesterday's Tomorrow." - I always really love talking to physicists who work at the intersection of different fields, so it was really fun to talk to Nicole and get to hear about her work at the intersection of quantum physics, information science, and thermodynamics.
Nicole joined us for this recording when she was visiting Perimeter Institute for a conference, but she also shared about her times at Perimeter back when she was a student within the Perimeter Scholars International master's program, which I also work in as a teaching faculty member. - So without further ado, let's dive into the world of "Quantum Steampunk" with Nicole Yunger Halpern. (upbeat music) Nicole, thank you so much for being here, and welcome back to Perimeter Institute.
- Thank you. It's really a pleasure to be here. Thanks for having me on the podcast. - I've just finished reading your book, "Quantum Steampunk: The Physics of Yesterday's Tomorrow," and we're so keen to talk to you about it because the ideas of quantum and steampunk I've never heard combined this way before. And I honestly have never read a book about thermodynamics before, certainly not quantum thermodynamics. So I'm hoping you can take our audience on a little bit of a backward journey.
And what's meant by the term "thermodynamics," and is it as scary as I always thought it was? - I would say it's not as scary. The thermodynamics is simply the study of energy, the different forms that energy can be in and the possible transformations amongst those forms. The theory of thermodynamics was developed during the 1800s. It was inspired by the Industrial Revolution.
For the first time, steam engines were being used on a large scale in industry, and people wanted to figure out how efficiently they could operate. So they gave thermodynamics this practical bent, thinking about powering factories and cooling systems down, and nowadays, also charging batteries. So thermodynamics has this practical viewpoint, but it's also become very foundational. This is a side of it that I really love.
In thermodynamics, we end up thinking about questions like why time flows in only one direction. And the thermodynamicists of the late 1800s also got to wrestle with fundamental questions about atomism. So the theory of atoms hadn't been entirely well accepted by the Victorian era. So people were debating about to what extent its useful or scientific to talk about tiny particles that no one can even see. Thermodynamics appeals to me in part because of this fundamental spirit that it has.
And as you mentioned, I also see its modern incarnation, quantum thermodynamics, as sharing its aesthetic with steampunk. Steampunk is a genre of literature, art, and film. Steampunk stories take place during the Victorian era. There are settings such as greedy, nighttime, dangerous London Streets, Sherlock Holmes in London, Meiji Japan, and the Wild wild West. In these settings are futuristic technologies, such as time machines, the dirigibles, and at automata.
There's this beautiful blend of old and new that creates both a sense of nostalgia and a sense of adventure and exploration and romance. In my field, which is at the intersection of quantum information and thermodynamics, we're taking the theory of thermodynamics from the 1800s, which was built for large, classical systems, such as steam engines. - Like steam engines. That is quite right. - Exactly. - I always think of the Industrial Revolution and thermodynamics, I think of steam trains.
- Exactly. Nowadays, experimentalists have great control over quantum systems, and the cutting edge technology of the day is not so much the steam engine, but cutting edge technologies include quantum devices, such as quantum computers and quantum sensors. But we need to take this theory of thermodynamics from the 1800s and extend it and really re-envision it for the 21st century. In the first place, how do the thermodynamic laws still hold for quantum systems?
How can we reformulate them for quantum systems and how they encode and interplay between energy and information, especially quantum information? Also, we know from the quantum information revolution that quantum phenomena, such as entanglements, can enhance information processing tasks, such as certain computations. Just as there are information processing tasks, there are thermodynamic tasks, such as charging batteries and powering cars.
So given that quantum phenomena can enhance information processing tasks, can they enhance thermodynamic tasks? In this intersection of quantum information and thermodynamics, we have, on the one hand, the thermodynamic theory of the Victorian era, and on the other hand, the futuristic technologies of quantum computing and the cutting edge science of quantum information theory. I see this fusion of old and new as sharing its aesthetic with steampunk.
- When I think of quantum thermodynamics, the very first simple image that popped into my head was a very, very small steam train. And I know that's not right. Can you tell us what's meant more broadly by the term "quantum?" What does quantum describe, and how does it connect to thermodynamics? - I think of quantum physics as very loosely being the physics of the small. What we mean by small can depend on context.
If you have a lot of particles that are crammed into space small enough that they interact very strongly, then that space might be as large as New York City. In which case, the particles that are crammed very closely together form a black hole. Quantum physics is relevant to black holes, even though black holes seem large in the sense that New York City seems large.
But the particles are crammed so close together that they interact very strongly, so we can't describe them with just the physics from before the 1920s. That's how I think of quantum. You also mentioned the possibility of quantum steam trains. Quantum engines have been developed. They've been designed by theorists and now realized experimentally. It turns out that quantum engine can be as small as a single atom.
That was the basis for the first quantum engine that was proposed in 1959, and then in more detail in 1967. - So I have to tell you, Nicole, I actually have some cousins who listen to this show, and they work as mechanics for aircraft and specialized vehicles. And I was particularly excited to talk to you today because I think that you could allow me to learn a bit more about something that they do. So can you actually tell us really what an engine is?
- We could focus on a heat engine since that's a simple and canonical example. And in particular, a heat engine that's some device that has access to two different environments at two different temperatures. Heat naturally flows from hot systems to cold systems. And an engine is a device that takes some of this heat that is flowing and turns it into work. Heats and work are the two different types of energy that can be transmitted between objects. Heat is random energy. It's uncoordinated.
It's the energy of particles jiggling all about. Work is organized, coordinated energy that can be directly harnessed to do something useful like push a rock up a hill. So a heat engine uses this difference between temperatures and takes the random heats and transforms it into useful work while not changing itself very much. - And what is the quantum version of that, the relationship between heat and work?
- What is quantum heat and what is quantum work are some of the fundamental and trickiest questions of quantum thermodynamics. I gave some intuition about what heats and work are in classical thermodynamics. We can take those intuitions and try to apply them to quantum systems, but here's a simple example of why defining or conceiving of quantum heat and work is tricky. Suppose that we want to measure how much heat a system absorbs.
We can measure its energy, then let the system absorb the heats, and then measure the energy again. The amount of energy it has at the end minus the amount of energy that it has at the beginning is the heat absorbed. But suppose that we try this process on a quantum system. If we measure a quantum system, we disturb it. And if we measure a quantum system's energy, we actually change its energy.
So by trying to measure the heat or work absorbed by a quantum system, we can change the amount of heat or work absorbed by the system. - And that doesn't happen in the classical sense. That's only when you're dealing with quantum particles. - Right. So we have to think about quantum work and heat totally differently. There's a whole spread of different definitions of quantum heat and quantum work that different people have proposed. I think of it as a menagerie.
So every time I find a new paper with a new definition of quantum heat and work, I add it to this file that I call menagerie of quantum heat and work. I think of the definitions as different species in the menagerie. I think that different definitions of quantum heat and work are useful in different contexts.
There is a very well-known trend in theoretical physics to unify, to put different definitions and theories and ideas together to make some one unified theory, especially at the Perimeter Institute.
However, I think that in this very operational theory of thermodynamics, where we're really thinking about agents who are given some resources, like environments at different temperatures, they're trying to perform tasks like a refrigerator system, it can be useful to define heat and work in terms of what sort of a system we have and, in particular, what we can do to it. How we can perform work on it, how we can poke it, how we can measure it.
What systems we have around, like batteries, that they can interact with. - I love that in your book you provide some really sort of straightforward examples, including beautiful illustrations, which I want to talk about too. But the examples, a lot of them involve a particle in a box. And it's so simple. I'd never thought of thermodynamics as being something that you could explain with particles in boxes.
Can you tell us why that's such a sort of standard explanation that you go back to frequently? - We very often think in thermodynamics about gas in a box. It's quite possible that many listeners learned in chemistry class in high school about ideal gases, their idealizations.
They have very simple properties, though they're described by pretty simple equations, in many cases, but they exhibit really interesting phenomena, such as they provide great examples of the second law of thermodynamics, which explains or helps us understand why time flows. They have properties like volume and pressure that are measurable. And by thinking about how the particles are acting on, say, the walls of their boxes, beating against the boxes' walls.
We can think of what pressure even means. This is a great playground for understanding thermodynamic quantities like pressure and volume and entropy. Also, in quantum theory we like thinking a lot about particles in boxes. Very often we think about single particles in boxes. So in the book, there are some examples of really, really even more idealized simple gases that consist even of single particles.
- And can you take us back to how you combined or blended these ideas of thermodynamics and quantum mechanics and steampunk? Like, where did that come from? How did you make those connections? - That's a good question. I encountered some steampunk works as I was growing up. For instance, I loved the book series the "Chronicles of Chrestomanci" by Diana Wynne Jones.
She was a wonderful and award-winning fiction writer, one of the best science fiction fantasy writers of the 20th century, not just according to me, but also authors that listeners might know, like Neil Gaiman. I encountered her works and Philip Pullman's works. I didn't realize, though, that steampunk was a genre. I didn't quite recognize what it was that I was reading. I didn't recognize that there was some unifying idea across these works.
Then, at the beginning of grad school, somehow I discovered that steampunk was a genre. By then, I had come to this intersection of quantum information and thermodynamics, and I suddenly just realized that it has the aesthetic and the spirit of steampunk. I was so delighted to find this shared connection between the hardcore physics that I was doing and the genre of literature, art, and film.
I wrote a blog post about it right at the beginning of my PhD. I blogged for Caltech's Quantum Institute. Then the idea started developing. It ended up the title of my PhD thesis, and then the name of my research group in Maryland, and then I wrote a book. - You mentioned grad school. That was here at Perimeter, right? - I earned my master's at the Perimeter Scholars International Program.
Then I was at Caltech for my PhD. - And I remember reading, I believe, in the book that it was at the Waterloo Public Library, which is just across the street, that you came across a book that had steampunk elements. Can you tell us what you found in that book that inspired you? - During most of my year in PSI, Perimeter Scholars International, I was in the Perimeter library working something like 12 hours a day. So I didn't have time to go to the Waterloo Public Library.
But in the spring, classes ended, and so I finally had a few free hours. And I explored the Waterloo Public Library. I found a novel by the Canadian poet Jay Ruzesky called "The Wolsenburg Clock." It is about an old, old clock in Austria. The author constructs this story showing how the clock is affected by and affects different people in this town throughout the centuries. One of the scenes takes place during the 1800s.
An inventor is about to clean up the clock and redesign it and really invest it with the spirit of what he does, which is build automata. So there's a scene in which he is standing on a balcony gazing down at a ballroom that he's converted into his workshop. His children and wife and so on are building clockwork-driven elephants and snakes and snake charmers. And he has this coat on that billows out behind him. - He sounds very steampunk. - Exactly. That scene stuck in my mind.
And shortly thereafter, I realized that that scene was steampunk. And shortly after that, I realized that my research was too. - So did you always know from the time you started your PhD that that's what you wanted to focus your thesis on? - Yes. During the end of my undergrad years, I increasingly found my way to quantum information theory, and I asked professors in my physics department who does the kind of quantum information theory that I'm interested in.
It was an abstract mathematical flavor. They pointed me to some webpages of some faculty members across the world. I increasingly honed in on phrases and ideas that really spoke to me. I found that they were at this intersection of quantum information theory and thermodynamics, where people are thinking about uncertainty and entropies and information and energy in a fundamental way. Shortly thereafter, I came to Perimeter.
I had the wonderful good fortune to work for my final project with someone who was a postdoc here at the time, Markus Muller, and Robert Spekkens, who's a faculty member here. We worked on a topic in quantum information, theoretic thermodynamics. At the end of the project, we said to each other, "Okay, so now we've gained this toolkit.
What shall we do with it?" I came up with a project, and that was the first project in my PhD. There was always another project and more collaborators to reach out to. - Since we're asking you about your PhD right now, as part of this show, we collect questions from some of our listeners. And a current PSI student here named Anna Knorr sent in a question about your thesis. - In your thesis, you write, "Steampunks dream it, Quantum information thermodynamicists live it."
Is that simply a cool slogan, or does it truly motivate you and drive you in your research? - Steampunk is a genre of science fiction and fantasy. It is seen as something that isn't true. However, I do believe that it is coming to life in the intersection of quantum information theory and thermodynamics. Thermodynamics was developed during the 1800s. We carry a bit of the Victorian era with us when we're doing thermodynamics. And quantum computers are futuristic technologies.
We're still building them. And quantum information theory is cutting edge science. So I do believe that quantum information thermodynamicists really are living what until now has been seen as a fiction, steampunk. - And in a lot of what you've been talking about, you've been saying your work is really at an intersection of several fields, one of which is information science. And I'm wondering if you can talk a little bit more about information.
I know in your book you say that we live in an information age. So can you tell us a little bit about why you said that and maybe first just what information is? - I've heard two definitions of information. One that's a bit easier to explain is information is the ability to distinguish between alternatives or a necessary ingredient for distinguishing between alternatives. For instance, when you get up in the morning and need to figure out what to wear, you need to know what the weather is like.
So you peer through the window, perhaps, and see that there's sun or there's rain. When you've peered through the window, you've gained information. You've been surprised. So information is that which gives us the ability to distinguish whether it's sunny or rainy. - And is information itself thermodynamic? Does information have an energy component to it, or does quantum information have an energy component to it? - Information plays a role in thermodynamics.
For instance, in my book, I walk through some examples that show that we can use information to turn heats into work. We said that heat is random, uncoordinated energy, and work is coordinated energy that's directly useful. If we have information, then we can run what's sometimes called an information engine to take those random heats and convert it into useful work. We can also run the engine backwards and perform work, such as by draining a battery, to gain information.
There's certainly an interplay between information and energy. - And Nicole, when I first got your book, the first thing I did was, of course, to flip through it and admire the really beautiful illustrations that Colin already talked about, but when I did that, there was one phrase that jumped out to me right from the beginning. And it's on page 19 where you have a section that's called The Liver of Information Theory. Can you tell us a little bit about this?
- Yes. When I was in high school, I had a biology teacher who said, "If you ever don't know the answer to a question on a test of mine, you should write down 'liver.'" (Colin laughs) The liver turns out to perform a ridiculous number of functions in the human body. So if you don't know the answer to a biology question and you answer liver, then you have an anomalously high probability of being correct.
Similarly, in information theory, if you're asked, "What is the optimal efficiency with which we can perform some information processing task?" and you answer, "It's given by an entropy," then you have an anomalously high probability being correct. So very often in information theory the answer to a question is entropy. So I think of entropy as the liver of information theory.
I should also mention, partially in response to Colin's recent question, that entropy is a manifestation of information in thermodynamics. Entropy is a measure of uncertainty, how little we know. And entropy comes up even in the second law of thermodynamics, which is a very, very fundamental statement. - Yeah, entropy is a concept you hear a lot in different branches of physics and science. And honestly, it's one that I have trouble wrapping my head around. I hope I'm not alone in that.
Can you give us a bit more of an idea? What does that mean to say that entropy sort of plays a role in information? - First, there are many different entropies, and entropy is a measure of uncertainty. At least that's how I think of it. For instance, I recently lived in the Boston area, and I came to learn that the weather in the Boston area is a very, very random variable.
On any given day, there's some probability that the weather will be mostly sunny, some probability that it'll be mostly rainy, some probability that it'll be mostly cloudy, some probability that it'll be mostly snowing. If on any given day you learn what the weather is, then you gain some amount of information, you are surprised by some amount, and a measure of how surprised you are is an entropic quantity. It depends on how probable that weather pattern was.
Also, suppose that you perform this process on many, many days. On each of many, many days, you learn what the weather is, and you average your uncertainty over many days. This is another measure of uncertainty. There are different measures of uncertainty that describe different contexts. So there are different entropies. I've described entropy in an information theoretic way. We can also see how it shows up in thermodynamics via your favorite example of a gas in a box. - Mm-hmm.
- Suppose that there is some gas in a box. It has some large-scale properties that characterize the gas as a whole, such as the energy of the gas, the volume of the gas, the number of particles in the gas. But we might have just this little bit of, or, I shouldn't say bits since that's a technical term here in information theory context. Suppose that we just know these few large-scale properties.
We could also zoom in on the gas particles and realize that, at any given instant, the gas particles have some positions, They have some momenta. So they have some masses, and they're moving with certain speeds in certain directions. There are many of these microscopic configurations that are consistent with one large-scale picture of energy and volume and particle number. If we know just those large-scale properties, then how ignorant are we of the microscopic configuration?
That's a thermodynamic entropy. - Lauren's last question reminded me that one of my favorite parts about your book is the chapter titles and the subtitles. They're are some of the most creative ones I've ever seen, including How to Insult a Quantum Information Theorist. so I'm hoping you can tell us, how do we insult a quantum information theorist? - Say, "Oh, quantum information theory.
Isn't that all just linear algebra?" - So if I were a quantum information theorist, why would I be insulted by that? - I can explain with a story. When I was an undergrad, I took a linear algebra course, and I was asked to explain what I was doing. I said, "I'm learning to solve basically the simplest equations." These are actually the kinds of equations that we encounter in middle school.
And in response I was told, "And for this you had to go to college?" (Colin laughs) So linear algebra is a somewhat straightforward extension of what we learn in middle school. Except in middle school, we don't deal with tens or hundreds of these equations at a time.
- And on the topic of titles and subtitles, I don't wanna risk giving away too many spoilers because I want people to read your book, but the subtitle of your entire book is The Physics of Yesterday's Tomorrow, and this is also the title of a chapter in your book. Can you tell us a little bit about this title? - I wish I could take credit for it, but my acquisitions editor at Johns Hopkins University Press, together with her team, came up with this subtitle.
And as soon as I saw it, I fell in love with it. I think of this subtitle as embodying the idea of quantum steampunk. It is a branch of physics, as well as chemistry, but it is the physics of yesterday's tomorrow in that quantum steampunk re-envisions the Victorian era's thermodynamics for the 21st century. - Because steampunk is such a visual aesthetic, you had an illustrator to create these beautiful diagrams in the book.
Can you tell us how that came to be and how... I assume there's not that many illustrators out there who just know quantum thermodynamics like the back of their hand. How did that relationship come to be? - The illustrator is Todd Cahill. He's a steampunk artist, and he actually had no experience with quantum physics whatsoever. So we had a lot of conversations. I encountered him through another steampunk artist. Couple of years ago, the steampunk artist Bruce Rosenbaum reached out to me.
He had watched a talk that I had given. I actually suspect that it was the first colloquium that I gave for the IQC, the Institute for Quantum Computing, near Perimeter. He said, "I love the spirit of this talk. Would you like to collaborate on a quantum steampunk sculpture?" Bruce creates large, as in human size, or even larger, interactive, kinetic steampunk sculptures for museums and restaurants and hotels.
I had no experience with anything like this ever before, but it sounded like fun, so I said, "Sure." We talked a lot about possible designs. We ended up collaborating with another artist to create a design of a quantum engine formed from a trapped ion with the classical counterpart of the engine on the outside. The sculpture as a whole looks like an armillary sphere hearkening back to a few centuries ago.
But also, this armillary sphere is a sphere, so if you show it to someone in quantum information, they'll say, "Oh, that looks like the Bloch sphere," which represents the state of a qubit, a basic unit of quantum information. After I started collaborating with Bruce, I wrote this book. And eventually, we needed to find an illustrator, so I asked Bruce if he could suggest anyone.
And he suggested Todd. Todd was great about learning about quantum physics and going back and forth with me about representing visually, in a quite beautiful way, images that I could only sketch in a very poor manner 'cause I have very little training in drawing. - I was gonna ask if these were images that you already had in your head, or if the collaborative process with him helped you form, you know, how do you visualize this stuff that is, most of it's invisible to us?
It's at the quantum level. - Yes. I sketched some diagrams, but again, I have very, very little training in drawing. So I gave him my poor little sketches and said, "Can you make these look steampunk? Then can you add some like flair here and there?" And that is what he does extremely well. So he turned my stick figure type drawings into beautiful illustrations. - And some of those illustrations sort of provide explanation of what's happening with some characters in your book.
It's a very fascinating book in the sense that it's a science book kinda with a steampunk novella woven into it, with characters, fictional characters. Can you tell us who those characters are and where they came from? - Yes. So the book is mostly nonfiction, but each chapter begins with a little snippet from a steampunk novel that resides in my imagination. There are characters in that story. The main characters are called Audrey, Baxter, and Caspian. They have this nemesis, Ewart.
I enjoyed playing with the tropes of steampunk in what is otherwise a very serious novel about hardcore science. I tried out this strategy when writing an article for Scientific American a couple of years ago. I was asked to write an article about quantum steampunk, and I thought that it would be fun to start with something quite different from the hardcore science that quantum steampunk is, to start with this playful snippet from an imaginary novel to illustrate what steampunk is.
So I partially wanted to illustrate what steampunk is for those unfamiliar with it. Partially, I wanted to have fun playing with these tropes, like the very spirited, vivacious, young girl in the Victorian era who refuses to be pinned down by the expectations of her society and corsets and so on and so forth. Also, dark, dangerous London streets. It was fun to play with these ideas. They did end up helpful later on in the chapters to illustrate scientific ideas.
Very often in an operational theory, like information theory, we speak in terms of agents. I think of an operational theory as one that can be phrased in terms of agents who have certain resources and need to perform certain tasks, so they try to figure out how to perform those tasks as efficiently as possible. Information theory is operational. In information theory, we think about the tasks of compressing data, communicating information over a noisy channel, and so on.
In thermodynamics, we think about refrigerating and charging batteries and powering cars and so on. Many of information theory's operational stories are phrased in terms of characters Alice and Bob. They have a friend, Caspian. Sometimes they are eavesdropped on by Eve. So I gave the characters in this imaginary steampunk novel names that begin with the same letters, A, B, C, and E. - It's funny, I didn't even put that together until just now.
We are actually sitting in the Alice room directly below the Bob room just to demonstrate how commonly used those terms are in science. - Yes, very relevant. - Sorry, go on. (chuckles) - I figured that some readers would see that very quickly, and other readers, say, who might come from the steampunk community, would maybe not make the connection between Audrey and Alice, but instead would recognize more of the steampunk tropes and smile at those.
I thought referring to characters such as Alice and Bob is really helpful for explaining our science and formalizing information theoretic and thermodynamic tasks. But we refer to Alice and Bob so much, it would be fun to have different characters. Hence Audrey and Baxter and so on. - I think this is just really what makes your book so unique, is that the steampunk is really infused all the way throughout. And we actually got another question sent in on a related topic.
This one is from Matt Duschenes, who's currently a PhD student at the Institute for Quantum Computing and the Perimeter Institute. - So you found a great connection, and we're really inspired by the genre of steampunk to drive your research directions. Do you think there are more serious opportunities for this kind of inspiration, and that physics as a field should look to draw more connections with art?
- I think that how quantum thermodynamics shares its spirit with steampunk is kind of a gift. It makes doing the physics even more fun because of this connection. I think that it would be wonderful to find more such connections between fields of science and genres of art and literature. I've always enjoyed studying everything, and I was drawn to physics in the manner of a natural philosopher from, say, the 1700s or early 1800s.
They studied, in a very rigorous sense, aesthetics, as well as geometry and astronomy and so on. I think that there's a lot of richness that we can add to our lives' interpretations and understandings by engaging in interdisciplinarity. That said, there's something that's unique to physics, and I think that to be a physicist, one really needs to focus very hard on the physics. But something like steampunk can provide extra energy and inspiration.
- Growing up, were you reading science fiction novels, or were you studying thermodynamics? How did sort of your formative years lead you in this direction? - I grew up reading just about everything just about all the time. (Colin laughs) I read while waiting to get picked up from school in the afternoon. I read while waiting for food to arrive at restaurants. I read on weekends. I read after school. This reading taught me to build worlds in my head.
I always had characters and plots and settings in my head. I think of my job now as building universes in my head for a living. - So we've already asked you about how you got interested in the specific field of quantum thermodynamics, but I'm wondering if you can also share with us how you came to find yourself as a physicist. - I did want to study everything, so I resisted choosing a major as long as possible. - That was at Dartmouth? - Yes, at Dartmouth College in New Hampshire.
I had always had philosophical inclinations. I always enjoyed engaging with abstract ideas. I had a philosophy teacher in high school who was fascinated by the paradoxes in quantum theory and relativity. He didn't have any background in physics, he would be the first to admit, but he passed on to me a curiosity about these fields. Meanwhile, I was absolutely adoring my calculus class and my physics class and so on. I absolutely wanted to keep studying those.
I found in the physics department a number of faculty members who were extremely good physicists. I've come to appreciate that more and more as I've become a colleague and been able to look at their works as a colleague. They also had philosophical inclinations. They also appreciated history. They helped me construct a major that was partway between the physics major and the create your own major.
I took a bunch of physics courses, and I took math, philosophy, and history courses related to physics. I got to call this major. My spirit was very much in the physics department, though. And by the end of my undergrad experience, I determined that it was physics that I wanted to burrow into very deeply. So after that, I tried out research as a research assistant in Lancaster University in the United Kingdom. Then I came to the Perimeter Institute.
And here, I had my first opportunity to do research on the intersection of quantum information theory and thermodynamics, and I absolutely adored it. - What did you adore about it so much? - I love the foundational perspective. I love the abstract ideas. Entropy is a strange idea and entropy- - Thank you. I thought so. - And entropy is a function. You can write down the mathematical form of a typical entropy. And it looks funny. It has multiple pieces that are kind of odd.
It has a negative sign. It has two copies of a probability. It has a logarithm. There are good reasons why this entropy has this mathematical form, and I go through such an argument in my book for why it makes sense, but if you just look at it, it looks like an odd duck. But on the other hand, entropy lies behind the second law of thermodynamics, which helps us understand why time flows. That is so very fundamental.
This tension between that funny-looking function and the very fundamental idea had drawn me for a long time. Also, as I mentioned, I've always had philosophical inclinations. I appreciated the fundamental nature of the laws of thermodynamics and the axioms of quantum theory. I appreciated how quantum information theory sits at the balance between the very fundamental, we get to think of some of the most entrancing paradoxes of our universe, and applications.
People are building quantum computers and quantum sensing that can be useful. I appreciated that balance. - So, Nicole, we've been talking to you a lot about your book, but I wanna make sure we also talk about your research contributions. And so I attended your colloquium here at Perimeter yesterday, and at one point, you had a slide and it was called Many-Body Localization Auto Cycles.
And I'm not gonna ask you about that, but at the bottom of the slide, you had some small text that said, "Ask me about my favorite symmetries." So since you said, I have to ask: Can you tell us about those favorite symmetries of yours? - One growing subfield that I've been dedicating a lot of time to is a quantum generalization of a very, very simple problem from undergraduate statistical physics or thermodynamics class.
Very often we think about a small system exchanging stuff with a big environment. One of the favorite examples in thermodynamics is a cup of coffee. A cup of coffee cools, it exchanges heats and particles with the air around it. We often think about this small system as exchanging energy or particles or maybe electric charge with the environment. These are properties that are measurable.
Quantum systems have properties that are measurable, but that you might not be able to measure simultaneously. They participate in uncertainty relations together. What's really interesting about quantum theory is what happens when you have these properties that can't be measured simultaneously, that participate in an uncertainty relation.
Very oddly, across the decades from the origins of this problem until a few years ago, people basically didn't think of the question, what happens to this simple setup that I've described that is in many an undergrad textbook if the properties that the small system exchanges with the big environment are these incompatible properties that we can't measure simultaneously and that participate in an uncertainty relation?
It's a really, really basic question because it takes a textbook problem and adds one quantum twist. But some very common arguments in thermodynamics rely on the assumption implicitly that we didn't realize until a few years ago that these properties are simultaneously measurable. So my group, as well as some other groups around the world, are exploring the rather quantum thermodynamic question of what happens if we take this simple setup and enable the properties to be incompatible?
It turns out it's not clear whether the small system can even thermalize. so come to be at the same temperature and so on as its environment. - A lot of the research that you're describing here and in the book seems very cutting edge and theoretical sort of blackboard work, but it's not entirely. There are connections to experiment and to application. Can you tell us where we are in that process between theory and experiment and application in quantum steampunk terms?
- Yes. Quantum thermodynamics has its roots in theory for quantum thermodynamics developed first during the 1930s. There was some work during the ensuing decades. There has really been a huge burst of activity over the past decade or so. The earlier quantum thermodynamic works were theoretical and even, to some extent, philosophically minded. That drew me into the field and drew me in part to Perimeter. Then I went to Caltech, which has a lot of experimental activity.
So I was increasingly exposed to experiment during the course of my PhD, increasingly came to interact with experimentalists. And as a postdoc, I ended up starting to collaborate a whole lot with lots of different experimental groups. That's also kind of the story of how quantum thermodynamics has progressed from theory to experiment. The past decade has seen the ability to perform quantum experiments that the founders of quantum theory thought would be impossible.
Experimentalists have amazing control over atoms, ions, photons, artificial atoms, and more. Quantum thermodynamicists have increasingly been taking advantage of that wonderful control achieved in labs. Labs have been realizing quantum engines that have been proposed since the late 1950s. They've been realizing refrigerators and quantum batteries and so on. - And some of your own research is currently being put to the experimental test. Is that right?
- Yes, I am currently working with four labs. One uses photons, one uses ions, and two use artificial atoms. - And what is the the goal of that research or the focus of it? - Different projects have different focuses. For instance, an experiment that was recently completed was in this subfield of quantum thermodynamics that I just discussed, that involves the exchange of thermodynamic properties that can be quantum incompatible.
It was not clear for a while that this little system analogous to the coffee cup could thermalize, come to a quiet state, in which there's no net flow of anything, such as energy or particles in and out, and it has the same temperature and some other properties as its environment. We've increasingly gained evidence that this small system, even in this particularly quantum setup, at least approaches thermalization, although we don't know exactly to what extent it does.
Together, with some theorist colleagues, proposed an experiment for observing whatever degree of thermalization we could. Christian Roos' group in Innsbruck, Austria used a set of trapped ions as their whole system, the small system and the environment. A couple of the ions formed the little system analogous to the cup of coffee, and the rest of the ions in a chain of ions formed the environment analogous to the air with which the coffee cup exchanges heats and particles.
So they set up these ions in a certain way. They let the ions evolve and exchange different properties. Then they measured the two ions and found that they did at least approach thermal equilibrium. - You mentioned quantum refrigerator. Can you explain what that is? Again, it's not a very small refrigerator. It's something else entirely, but we have this picture in our heads of what a refrigerator is and what it's for. Is that picture at all related to the quantum analogous refrigerator?
- I think of a refrigerator as anything that uses a resource to cool down a system. I'm working with Simone Gasparinetti's lab in Sweden at Chalmers University on building a quantum refrigerator. It consists of superconducting qubits. Superconductors are quantum systems. They're little circuits in which current can flow for all time without ever dissipating. Superconducting qubits are being used as the physical systems that encode basic units of quantum information in many quantum computers.
Chalmers University is building a quantum computer. A superconducting qubit quantum computer needs to be at low temperatures. If the quantum computer has just run a calculation, then it's effectively filled up its scrap paper. These superconducting qubits act like scrap paper that has been scribbled on. They need to be reset. They need to be, in a sense, erased like scrap paper for the next calculation. They are reset if they are cooled down even more.
This quantum refrigerator will be inside of the preexisting classical refrigerator that keeps all of the superconducting qubits at a low temperature. And the quantum refrigerator has the job of cooling down these used qubits even more. The experiment is supposed to be happening right now. We have numerical simulations. We'll see how well those are born out.
- I've heard you talk about the fact that the initial idea for one of your research projects, which I know has now been published with a team of your collaborators, first came up over an informal discussion over coffee. And I think this type of thing actually happens pretty often, maybe more often than we might think. It actually happened to me as well during my PhD that a project that I ended up spending a lot of time on came up over a discussion at lunch.
Although, I wasn't part of that discussion, but I ended up working on the project that resulted from that discussion. I'm just wondering if you can talk a little bit about that and maybe what you think is so special about those spontaneous discussions that between researchers. - This collaboration began during my PhD when I was at Caltech. There is a condensed matter theorist, Gil Refael, at Caltech. He has an office in a building called Bridge. But most often I saw him at the Red Door cafe.
After lunch one day, I was at the Red Door Cafe, he was at the Red Door Cafe, and he said, "Hey, you're interested in breaking the second law of thermodynamics, right?" Personally, I think that the second law of thermodynamics probably cannot be broken, but I am extremely enthusiastic about the second law of thermodynamics. So I asked, "What are you interested in discussing?" And he said, "There's this phase of matter, many-body localization." This phase was very, very popular.
It was undergoing a lot of study when I was in my PhD. Gil contributed a lot to that research. He said, "We've been studying many-body localization for a while now. And it's interesting from a physics perspective, but what is it good for?" So many-body localization is a phase of matter of quantum many-particle systems. The behavior of a many-body localized system contrasts with ordinary behavior that we might expect. Let's go back to the example of a classical gas in a box.
Suppose that we have a classical gas in a box, we measure its particles' positions, and we find that the particles are all clumped together in one corner of the box. Shortly thereafter, the particles will spread all over the box. They won't hang around in the same positions.
However, if we have a many-body localized system, which could consist of a bunch of cold atoms, and we measure the positions of these particles, then those particles will approximately hang around in the positions for a long time afterward, in contrast with the behavior that we would expect. A many-body localized system has some resistance to the second law of thermodynamics and the flow of time.
If we're thinking about the classical gas in a box, we know time is flowing if we're watching the gas because we see the gas expanding all across the box. That's why we think of many-body localization as, in some sense, resisting the second law of thermodynamics a little bit, although, eventually, the particles do spread out. Many-body localization had been proposed as a possible quantum memory for storing quantum information since things tend to stay put in it.
But Gil was thinking, just as there are information processing tasks, such as storing information, there are also thermodynamic tasks. So maybe I should ask a quantum thermodynamicist what we can do with this resource. We talked for a while, and we eventually brought into the project two more collaborators: Christopher White and Sarang Gopalakrishnan.
We came up with the idea of a quantum engine that can be changed between this many-body localized phase and a more thermalizing phase of matter in which particles and information spread out quickly. Many-body localization is a long name that has very many letters and syllables, so it's often called MBL. Gil came up with a wonderful name for the engine, the MBL mobile.
- We've been asking you a lot about your book and now about your research, but I also wanna ask you about something kind of at the intersection of those two. Can you tell us if writing the book helped you at all with your research? - Absolutely. It was extremely helpful. On the one hand, I had to extract the basic physics from a lot of different thermodynamic discoveries.
When I think of a highly competent theoretical physicist whom I admire, I think of someone who can explain a discovery in terms of just the basic physical story. That person knows what's really essential, what's really important, so they don't bog down the explanation with a lot of unnecessary details. I had to extract the basic physics from discoveries in this way, and that helped me understand a lot better what was really behind these thermodynamic settings and findings.
Also, I had to write at the end of my book what I thought was ahead for the field. I started thinking from quantum thermodynamics, we've gained wonderful fundamental insights. Can quantum thermodynamics also be practical? What would it take for quantum thermodynamics to be practical? The original theory of thermodynamics went hand in hand with the Industrial Revolution, which was extremely practical.
It would be wonderful for the quantum thermodynamic engines and refrigerators and so on that have been proposed to lead to utility. I mentioned that experimentalists have realized quantum thermodynamic engines. These experiments are proof of principle.
They show that if we work really hard, we can create and run quantum engines, but we tend to have to invest more work in cooling the engine down and in manipulating it than we can extract using the engine, which is quantum, so it's just a little bit of energy. I thought about what we would really need to make quantum thermodynamics practical. I started thinking about solar panels in Southern California. My PhD advisor, John Preskill, has solar panels on his house in Southern California.
He can use solar panels to great effect because he happens to be in an environment where they fit in and just do their own job very usefully. If we were in Buffalo instead, solar panels would not be so helpful. I think of the quantum engines that have been realized today as kind of similar to solar panels in Buffalo. We have to spend a lot of work on them, just as you would have to spend a lot of work scooping snow off your solar panels in Buffalo.
I started looking around for a quantum thermodynamic context for quantum thermodynamic technologies that is, frankly, like Southern California for solar panels. Shortly after writing that section of my book, I got an email from Simone Gasparinetti, the experimentalist that I mentioned who I'm working with in Sweden. I was not working with him at the time, but he said, "I'm starting up a lab.
How about we chat about what you think are great opportunities for quantum thermodynamic experiments?" And I said, "Recently, I've been thinking about this need for a quantum thermodynamic setting that's like Southern California for solar panels. I want to make quantum thermodynamics useful." I should also mention I'm not the only quantum thermodynamicist who would like to make quantum thermodynamic devices useful.
There are other people around the world thinking in this direction, but I'm just telling the story of how I came to this direction and this collaboration and this experiment. And Simone said, "Ah, I have such a setting." And so we embarked on this adventure of developing a quantum thermodynamic refrigerator that we wouldn't have to spend a lot of control on in operating, that would just do its own thing to reset qubits after a quantum computation in a superconducting qubit quantum refrigerator.
So the book has absolutely been useful for my physics research. - To follow up on that, are there any sort of big-picture breakthroughs or advances in your field? That, you know, you're still quite a young researcher with a long runway ahead in research. Are there breakthroughs that you hope you'll see or even make in your career? - I am very fascinated by this growing subfield that I mentioned before that involves incompatible properties of quantum thermodynamic systems.
There are a lot of really fundamental questions that haven't been thought about. For instance, to what extent does the small system reach thermal equilibrium? Also, I think there are really interesting discoveries to be made when we take this idea and bring into other fields. For instance, the last time I was in Santa Barbara, I went to a many-body physicist. There is toolkits in many-body physics called, it also has a horrendously long name, the eigenstate thermalization hypothesis.
It helps us understand why quantum systems thermalize. Why, in some sense, time flows for them in some ways similarly too for classical systems. This many-body physicist in Santa Barbara, Mark Srednicki, calls himself the high priest of the eigenstate thermalization hypothesis. He was one of the people who helped create this toolkit. It's very powerful. It has been transformative for quantum many-body physics. It's been used an enormous amount over the past few decades.
And I asked, "What if we try to apply it to a system that has these incompatible properties that are being exchanged?" And he said, "You know, I hadn't thought about that." And it turns out that this toolkit needs to be changed. This toolkit that has been around for many decades. I think that there are other such realizations waiting to be had in this subfield. - You've been so generous with your time, and you have a conference to get back to. Thank you so much for sitting down to chat with us.
It's just been a pleasure. - Thank you again for having me on the podcast. It's really been a pleasure. (upbeat music) - Thanks so much for listening. Perimeter Institute is a not-for-profit charitable organization that shares cutting edge ideas with the world thanks to the ongoing support of the governments of Ontario and Canada, and also thanks to donors like you. Thank you for being part of the equation. (upbeat music)