Author Archives: Nicole Yunger Halpern

About Nicole Yunger Halpern

I’m a theoretical physicist at the Joint Center for Quantum Information and Computer Science in Maryland. My research group re-envisions 19th-century thermodynamics for the 21st century, using the mathematical toolkit of quantum information theory. We then apply quantum thermodynamics as a lens through which to view the rest of science. I call this research “quantum steampunk,” after the steampunk genre of art and literature that juxtaposes Victorian settings (à la thermodynamics) with futuristic technologies (à la quantum information). For more information, check out my book for the general public, Quantum Steampunk: The Physics of Yesterday’s Tomorrow. I earned my PhD at Caltech under John Preskill’s auspices; one of my life goals is to be the subject of one of his famous (if not Pullitzer-worthy) poems. Follow me on Twitter @nicoleyh11.

Memories of things past

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My best friend—who’s held the title of best friend since kindergarten—calls me the keeper of her childhood memories. I recall which toys we played with, the first time I visited her house,1 and which beverages our classmates drank during snack time in kindergarten.2 She wouldn’t be surprised to learn that the first workshop I’ve co-organized centered on memory.

Memory—and the loss of memory—stars in thermodynamics. As an example, take what my husband will probably do this evening: bake tomorrow’s breakfast. I don’t know whether he’ll bake fruit-and-oat cookies, banana muffins, pear muffins, or pumpkin muffins. Whichever he chooses, his baking will create a scent. That scent will waft across the apartment, seep into air vents, and escape into the corridor—will disperse into the environment. By tomorrow evening, nobody will be able to tell by sniffing what my husband will have baked. 

That is, the kitchen’s environment lacks a memory. This lack contributes to our experience of time’s arrow: We sense that time passes partially by smelling less and less of breakfast. Physicists call memoryless systems and processes Markovian.

A snippet from Quantum Steampunk: The Physics of Yesterday’s Tomorrow

Our kitchen’s environment is Markovian because it’s large and particles churn through it randomly. But not all environments share these characteristics. Metaphorically speaking, a dispersed memory of breakfast may recollect, return to a kitchen, and influence the following week’s baking. For instance, imagine an atom in a quantum computer, rather than a kitchen in an apartment. A few other atoms may form our atom’s environment. Quantum information may leak from our atom into that environment, swish around in the environment for a time, and then return to haunt our atom. We’d call the atom’s evolution and environment non-Markovian.

I had the good fortune to co-organize a workshop about non-Markovianity—about memory—this February. The workshop took place at the Banff International Research Station, abbreviated BIRS, which you pronounce like the plural of what you say when shivering outdoors in Canada. BIRS operates in the Banff Centre for Arts and Creativity, high in the Rocky Mountains. The Banff Centre could accompany a dictionary entry for pristine, to my mind. The air feels crisp, the trees on nearby peaks stand out against the snow like evergreen fringes on white velvet, and the buildings balance a rustic-mountain-lodge style with the avant-garde. 

The workshop balanced styles, too, but skewed toward the theoretical and abstract. We learned about why the world behaves classically in our everyday experiences; about information-theoretic measures of the distances between quantum states; and how to simulate, on quantum computers, chemical systems that interact with environments. One talk, though, brought our theory back down to (the snow-dusted) Earth.

Gabriela Schlau-Cohen runs a chemistry lab at MIT. She wants to understand how plants transport energy. Energy arrives at a plant from the sun in the form of light. The light hits a pigment-and-protein complex. If the plant is lucky, the light transforms into a particle-like packet of energy called an exciton. The exciton traverses the receptor complex, then other complexes. Eventually, the exciton finds a spot where it can enable processes such as leaf growth. 

A high fraction of the impinging photons—85%—transform into excitons. How do plants convert and transport energy as efficiently as they do?

Gabriela’s group aims to find out—not by testing natural light-harvesting complexes, but by building complexes themselves. The experimentalists mimic the complex’s protein using DNA. You can fold DNA into almost any shape you want, by choosing the DNA’s base pairs (basic units) adroitly and by using “staples” formed from more DNA scraps. The sculpted molecules are called DNA origami.

Gabriela’s group engineers different DNA structures, analogous to complexes’ proteins, to have different properties. For instance, the experimentalists engineer rigid structures and flexible structures. Then, the group assesses how energy moves through each structure. Each structure forms an environment that influences excitons’ behaviors, similarly to how a memory-containing environment influences an atom.

Courtesy of Gabriela Schlau-Cohen

The Banff environment influenced me, stirring up memories like powder displaced by a skier on the slopes above us. I first participated in a BIRS workshop as a PhD student, and then I returned as a postdoc. Now, I was co-organizing a workshop to which I brought a PhD student of my own. Time flows, as we’re reminded while walking down the mountain from the Banff Centre into town: A cemetery borders part of the path. Time flows, but we belong to that thermodynamically remarkable class of systems that retain memories…memories and a few other treasures that resist change, such as friendships held since kindergarten.

1Plushy versions of Simba and Nala from The Lion King. I remain grateful to her for letting me play at being Nala.

2I’d request milk, another kid would request apple juice, and everyone else would request orange juice.

A (quantum) complex legacy: Part deux

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I didn’t fancy the research suggestion emailed by my PhD advisor.

A 2016 email from John Preskill led to my publishing a paper about quantum complexity in 2022, as I explained in last month’s blog post. But I didn’t explain what I thought of his email upon receiving it.

It didn’t float my boat. (Hence my not publishing on it until 2022.)

The suggestion contained ingredients that ordinarily would have caulked any cruise ship of mine: thermodynamics, black-hole-inspired quantum information, and the concept of resources. John had forwarded a paper drafted by Stanford physicists Adam Brown and Lenny Susskind. They act as grand dukes of the community sussing out what happens to information swallowed by black holes. 

From Rare-Gallery

We’re not sure how black holes work. However, physicists often model a black hole with a clump of particles squeezed close together and so forced to interact with each other strongly. The interactions entangle the particles. The clump’s quantum state—let’s call it | \psi(t) \rangle—grows not only complicated with time (t), but also complex in a technical sense: Imagine taking a fresh clump of particles and preparing it in the state | \psi(t) \rangle via a sequence of basic operations, such as quantum gates performable with a quantum computer. The number of basic operations needed is called the complexity of | \psi(t) \rangle. A black hole’s state has a complexity believed to grow in time—and grow and grow and grow—until plateauing. 

This growth echoes the second law of thermodynamics, which helps us understand why time flows in only one direction. According to the second law, every closed, isolated system’s entropy grows until plateauing.1 Adam and Lenny drew parallels between the second law and complexity’s growth.

The less complex a quantum state is, the better it can serve as a resource in quantum computations. Recall, as we did last month, performing calculations in math class. You needed clean scratch paper on which to write the calculations. So does a quantum computer. “Scratch paper,” to a quantum computer, consists of qubits—basic units of quantum information, realized in, for example, atoms or ions. The scratch paper is “clean” if the qubits are in a simple, unentangled quantum state—a low-complexity state. A state’s greatest possible complexity, minus the actual complexity, we can call the state’s uncomplexity. Uncomplexity—a quantum state’s blankness—serves as a resource in quantum computation.

Manny Knill and Ray Laflamme realized this point in 1998, while quantifying the “power of one clean qubit.” Lenny arrived at a similar conclusion while reasoning about black holes and firewalls. For an introduction to firewalls, see this blog post by John. Suppose that someone—let’s call her Audrey—falls into a black hole. If it contains a firewall, she’ll burn up. But suppose that someone tosses a qubit into the black hole before Audrey falls. The qubit kicks the firewall farther away from the event horizon, so Audrey will remain safe for longer. Also, the qubit increases the uncomplexity of the black hole’s quantum state. Uncomplexity serves as a resource also to Audrey.

A resource is something that’s scarce, valuable, and useful for accomplishing tasks. Different things qualify as resources in different settings. For instance, imagine wanting to communicate quantum information to a friend securely. Entanglement will serve as a resource. How can we quantify and manipulate entanglement? How much entanglement do we need to perform a given communicational or computational task? Quantum scientists answer such questions with a resource theory, a simple information-theoretic model. Theorists have defined resource theories for entanglement, randomness, and more. In many a blog post, I’ve eulogized resource theories for thermodynamic settings. Can anyone define, Adam and Lenny asked, a resource theory for quantum uncomplexity?

Resource thinking pervades our world.

By late 2016, I was a quantum thermodynamicist, I was a resource theorist, and I’d just debuted my first black-hole–inspired quantum information theory. Moreover, I’d coauthored a review about the already-extant resource theory that looked closest to what Adam and Lenny sought. Hence John’s email, I expect. Yet that debut had uncovered reams of questions—questions that, as a budding physicist heady with the discovery of discovery, I could own. Why would I answer a question of someone else’s instead?

So I thanked John, read the paper draft, and pondered it for a few days. Then, I built a research program around my questions and waited for someone else to answer Adam and Lenny.

Three and a half years later, I was still waiting. The notion of uncomplexity as a resource had enchanted the black-hole-information community, so I was preparing a resource-theory talk for a quantum-complexity workshop. The preparations set wheels churning in my mind, and inspiration struck during a long walk.2

After watching my workshop talk, Philippe Faist reached out about collaborating. Philippe is a coauthor, a friend, and a fellow quantum thermodynamicist and resource theorist. Caltech’s influence had sucked him, too, into the black-hole community. We Zoomed throughout the pandemic’s first spring, widening our circle to include Teja Kothakonda, Jonas Haferkamp, and Jens Eisert of Freie University Berlin. Then, Anthony Munson joined from my nascent group in Maryland. Physical Review A published our paper, “Resource theory of quantum uncomplexity,” in January.

The next four paragraphs, I’ve geared toward experts. An agent in the resource theory manipulates a set of n qubits. The agent can attempt to perform any gate U on any two qubits. Noise corrupts every real-world gate implementation, though. Hence the agent effects a gate chosen randomly from near U. Such fuzzy gates are free. The agent can’t append or discard any system for free: Appending even a maximally mixed qubit increases the state’s uncomplexity, as Knill and Laflamme showed. 

Fuzzy gates’ randomness prevents the agent from mapping complex states to uncomplex states for free (with any considerable probability). Complexity only grows or remains constant under fuzzy operations, under appropriate conditions. This growth echoes the second law of thermodynamics. 

We also defined operational tasks—uncomplexity extraction and expenditure analogous to work extraction and expenditure. Then, we bounded the efficiencies with which the agent can perform these tasks. The efficiencies depend on a complexity entropy that we defined—and that’ll star in part trois of this blog-post series.

Now, I want to know what purposes the resource theory of uncomplexity can serve. Can we recast black-hole problems in terms of the resource theory, then leverage resource-theory results to solve the black-hole problem? What about problems in condensed matter? Can our resource theory, which quantifies the difficulty of preparing quantum states, merge with the resource theory of magic, which quantifies that difficulty differently?

Unofficial mascot for fuzzy operations

I don’t regret having declined my PhD advisor’s recommendation six years ago. Doing so led me to explore probability theory and measurement theory, collaborate with two experimental labs, and write ten papers with 21 coauthors whom I esteem. But I take my hat off to Adam and Lenny for their question. And I remain grateful to the advisor who kept my goals and interests in mind while checking his email. I hope to serve Anthony and his fellow advisees as well.

1…en route to obtaining a marriage license. My husband and I married four months after the pandemic throttled government activities. Hours before the relevant office’s calendar filled up, I scored an appointment to obtain our license. Regarding the metro as off-limits, my then-fiancé and I walked from Cambridge, Massachusetts to downtown Boston for our appointment. I thank him for enduring my requests to stop so that I could write notes.

2At least, in the thermodynamic limit—if the system is infinitely large. If the system is finite-size, its entropy grows on average.

A (quantum) complex legacy

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Early in the fourth year of my PhD, I received a most John-ish email from John Preskill, my PhD advisor. The title read, “thermodynamics of complexity,” and the message was concise the way that the Amazon River is damp: “Might be an interesting subject for you.” 

Below the signature, I found a paper draft by Stanford physicists Adam Brown and Lenny Susskind. Adam is a Brit with an accent and a wit to match his Oxford degree. Lenny, known to the public for his books and lectures, is a New Yorker with an accent that reminds me of my grandfather. Before the physicists posted their paper online, Lenny sought feedback from John, who forwarded me the email.

The paper concerned a confluence of ideas that you’ve probably encountered in the media: string theory, black holes, and quantum information. String theory offers hope for unifying two physical theories: relativity, which describes large systems such as our universe, and quantum theory, which describes small systems such as atoms. A certain type of gravitational system and a certain type of quantum system participate in a duality, or equivalence, known since the 1990s. Our universe isn’t such a gravitational system, but never mind; the duality may still offer a toehold on a theory of quantum gravity. Properties of the gravitational system parallel properties of the quantum system and vice versa. Or so it seemed.

The gravitational system can have two black holes linked by a wormhole. The wormhole’s volume can grow linearly in time for a time exponentially long in the black holes’ entropy. Afterward, the volume hits a ceiling and approximately ceases changing. Which property of the quantum system does the wormhole’s volume parallel?

Envision the quantum system as many particles wedged close together, so that they interact with each other strongly. Initially uncorrelated particles will entangle with each other quickly. A quantum system has properties, such as average particle density, that experimentalists can measure relatively easily. Does such a measurable property—an observable of a small patch of the system—parallel the wormhole volume? No; such observables cease changing much sooner than the wormhole volume does. The same conclusion applies to the entanglement amongst the particles.

What about a more sophisticated property of the particles’ quantum state? Researchers proposed that the state’s complexity parallels the wormhole’s volume. To grasp complexity, imagine a quantum computer performing a computation. When performing computations in math class, you needed blank scratch paper on which to write your calculations. A quantum computer needs the quantum equivalent of blank scratch paper: qubits (basic units of quantum information, realized, for example, as atoms) in a simple, unentangled, “clean” state. The computer performs a sequence of basic operations—quantum logic gates—on the qubits. These operations resemble addition and subtraction but can entangle the qubits. What’s the minimal number of basic operations needed to prepare a desired quantum state (or to “uncompute” a given state to the blank state)? The state’s quantum complexity.1 

Quantum complexity has loomed large over multiple fields of physics recently: quantum computing, condensed matter, and quantum gravity. The latter, we established, entails a duality between a gravitational system and a quantum system. The quantum system begins in a simple quantum state that grows complicated as the particles interact. The state’s complexity parallels the volume of a wormhole in the gravitational system, according to a conjecture.2 

The conjecture would hold more water if the quantum state’s complexity grew similarly to the wormhole’s volume: linearly in time, for a time exponentially large in the quantum system’s size. Does the complexity grow so? The expectation that it does became the linear-growth conjecture.

Evidence supported the conjecture. For instance, quantum information theorists modeled the quantum particles as interacting randomly, as though undergoing a quantum circuit filled with random quantum gates. Leveraging probability theory,3 the researchers proved that the state’s complexity grows linearly at short times. Also, the complexity grows linearly for long times if each particle can store a great deal of quantum information. But what if the particles are qubits, the smallest and most ubiquitous unit of quantum information? The question lingered for years.

Jonas Haferkamp, a PhD student in Berlin, dreamed up an answer to an important version of the question.4 I had the good fortune to help formalize that answer with him and members of his research group: master’s student Teja Kothakonda, postdoc Philippe Faist, and supervisor Jens Eisert. Our paper, published in Nature Physics last year, marked step one in a research adventure catalyzed by John Preskill’s email 4.5 years earlier.

Imagine, again, qubits undergoing a circuit filled with random quantum gates. That circuit has some architecture, or arrangement of gates. Slotting different gates into the architecture effects different transformations5 on the qubits. Consider the set of all transformations implementable with one architecture. This set has some size, which we defined and analyzed.

What happens to the set’s size if you add more gates to the circuit—let the particles interact for longer? We can bound the size’s growth using the mathematical toolkits of algebraic geometry and differential topology. Upon bounding the size’s growth, we can bound the state’s complexity. The complexity, we concluded, grows linearly in time for a time exponentially long in the number of qubits.

Our result lends weight to the complexity-equals-volume hypothesis. The result also introduces algebraic geometry and differential topology into complexity as helpful mathematical toolkits. Finally, the set size that we bounded emerged as a useful concept that may elucidate circuit analyses and machine learning.

John didn’t have machine learning in mind when forwarding me an email in 2017. He didn’t even have in mind proving the linear-growth conjecture. The proof enables step two of the research adventure catalyzed by that email: thermodynamics of quantum complexity, as the email’s title stated. I’ll cover that thermodynamics in its own blog post. The simplest of messages can spin a complex legacy.

The links provided above scarcely scratch the surface of the quantum-complexity literature; for a more complete list, see our paper’s bibliography. For a seminar about the linear-growth paper, see this video hosted by Nima Lashkari’s research group.

1The term complexity has multiple meanings; forget the rest for the purposes of this article.

2According to another conjecture, the quantum state’s complexity parallels a certain space-time region’s action. (An action, in physics, isn’t a motion or a deed or something that Hamlet keeps avoiding. An action is a mathematical object that determines how a system can and can’t change in time.) The first two conjectures snowballed into a paper entitled “Does complexity equal anything?” Whatever it parallels, complexity plays an important role in the gravitational–quantum duality. 

3Experts: Such as unitary t-designs.

4Experts: Our work concerns quantum circuits, rather than evolutions under fixed Hamiltonians. Also, our work concerns exact circuit complexity, the minimal number of gates needed to prepare a state exactly. A natural but tricky extension eluded us: approximate circuit complexity, the minimal number of gates needed to approximate the state.

5Experts: Unitary operators.

Eight highlights from publishing a science book for the general public

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What’s it like to publish a book?

I’ve faced the question again and again this year, as my book Quantum Steampunk hit bookshelves in April. Two responses suggest themselves.

On the one hand, I channel the Beatles: It’s a hard day’s night. Throughout the publication process, I undertook physics research full-time. Media opportunities squeezed themselves into the corners of the week: podcast and radio-show recordings, public-lecture preparations, and interviews with journalists. After submitting physics papers to coauthors and journals, I drafted articles for Quanta Magazine, Literary Hub, the New Scientist newsletter, and other venues—then edited the articles, then edited them again, and then edited them again. Often, I apologized to editors about not having the freedom to respond to their comments till the weekend. Before public-lecture season hit, I catalogued all the questions that I imagined anyone might ask, and I drafted answers. The resulting document spans 16 pages, and I study it before every public lecture and interview.

Public lecture at the Institute for the Science of Origins at Case Western Reserve University

Answer number two: Publishing a book is like a cocktail of watching the sun rise over the Pacific from Mt. Fuji, taking off in an airplane for the first time, and conducting a symphony in Carnegie Hall.1 I can scarcely believe that I spoke in the Talks at Google lecture series—a series that’s hosted Tina Fey, Noam Chomsky, and Andy Weir! And I found my book mentioned in the Boston Globe! And in a Dutch science publication! If I were an automaton from a steampunk novel, the publication process would have wound me up for months.

Publishing a book has furnished my curiosity cabinet of memories with many a seashell, mineral, fossil, and stuffed crocodile. Since you’ve asked, I’ll share eight additions that stand out.

Breakfast on publication day. Because how else would one celebrate the publication of a steampunk book?

1) I guest-starred on a standup-comedy podcast. Upon moving into college, I received a poster entitled 101 Things to Do Before You Graduate from Dartmouth. My list of 101 Things I Never Expected to Do in a Physics Career include standup comedy.2 I stand corrected.

Comedian Anthony Jeannot bills his podcast Highbrow Drivel as consisting of “hilarious conversations with serious experts.” I joined him and guest comedienne Isabelle Farah in a discussion about film studies, lunch containers, and hippies, as well as quantum physics. Anthony expected me to act as the straight man, to my relief. That said, after my explanation of how quantum computers might help us improve fertilizer production and reduce global energy consumption, Anthony commented that, if I’d been holding a mic, I should have dropped it. I cherish the memory despite having had to look up the term mic drop when the recording ended.

At Words Worth Books in Waterloo, Canada

2) I met Queen Victoria. In mid-May, I arrived in Canada to present about my science and my book at the University of Toronto. En route to the physics department, I stumbled across the Legislative Assembly of Ontario. Her Majesty was enthroned in front of the intricate sandstone building constructed during her reign. She didn’t acknowledge me, of course. But I hope she would have approved of the public lecture I presented about physics that blossomed during her era. 

Her Majesty, Queen Victoria

3) You sent me your photos of Quantum Steampunk. They arrived through email, Facebook, Twitter, text, and LinkedIn. They showed you reading the book, your pets nosing it, steampunk artwork that you’d collected, and your desktops and kitchen counters. The photographs have tickled and surprised me, although I should have expected them, upon reflection: Quantum systems submit easily to observation by their surroundings.3 Furthermore, people say that art—under which I classify writing—fosters human connection. Little wonder, then, that quantum physics and writing intersect in shared book selfies.

Photos from readers

4) A great-grandson of Ludwig Boltzmann’s emailed. Boltzmann, a 19th-century Austrian physicist, helped mold thermodynamics and its partner discipline statistical mechanics. So I sat up straighter upon opening an email from a physicist descended from the giant. Said descendant turned out to have watched a webinar I’d presented for the magazine Physics Today. Although time machines remain in the domain of steampunk fiction, they felt closer to reality that day.

5) An experiment bore out a research goal inspired by the book. My editors and I entitled the book’s epilogue Where to next? The future of quantum steampunk. The epilogue spurred me to brainstorm about opportunities and desiderata—literally, things desired. Where did I want for quantum thermodynamics to head? I shared my brainstorming with an experimentalist later that year. We hatched a project, whose experiment concluded this month. I’ll leave the story for after the paper debuts, but I can say for now that the project gives me chills—in a good way.

6) I recited part of Edgar Allan Poe’s “The Raven” with a fellow physicist at a public lecture. The Harvard Science Book Talks form a lecture series produced by the eponymous university and bookstore. I presented a talk hosted by Jacob Barandes—a Harvard physics lecturer, the secret sauce behind the department’s graduate program, and an all-around exemplar of erudition. He asked how entropy relates to “The Raven.”

Image from the Harvard Gazette

For the full answer, see chapter 11 of my book. Briefly: Many entropies exist. They quantify the best efficiencies with which we can perform thermodynamic tasks such as running an engine. Different entropies can quantify different tasks’ efficiencies if the systems are quantum, otherwise small, or far from equilibrium—outside the purview of conventional 19th-century thermodynamics. Conventional thermodynamics describes many-particle systems, such as factory-scale steam engines. We can quantify conventional systems’ efficiencies using just one entropy: the thermodynamic entropy that you’ve probably encountered in connection with time’s arrow. How does this conventional entropy relate to the many quantum entropies? Imagine starting with a quantum system, then duplicating it again and again, until accruing infinitely many copies. The copies’ quantum entropies converge (loosely speaking), collapsing onto one conventional-looking entropy. The book likens this collapse to a collapse described in “The Raven”:

The speaker is a young man who’s startled, late one night, by a tapping sound. The tapping exacerbates his nerves, which are on edge due to the death of his love: “Deep into that darkness peering, long I stood there wondering, fearing, / Doubting, dreaming dreams no mortal ever dared to dream before.” The speaker realizes that the tapping comes from the window, whose shutter he throws open. His wonders, fears, doubts, and dreams collapse onto a bird’s form as a raven steps inside. So do the many entropies collapse onto one entropy as the system under consideration grows infinitely large. We could say, instead, that the entropies come to equal each other, but I’d rather picture “The Raven.” 

I’d memorized the poem in high school but never had an opportunity to recite it for anyone—and it’s a gem to declaim. So I couldn’t help reciting a few stanzas in response to Jacob. But he turned out to have memorized the poem, too, and responded with the next several lines! Even as a physicist, I rarely have the chance to reach such a pinnacle of nerdiness.

With Pittsburgh Quantum Institute head honchos Rob Cunningham and Adam Leibovich

7) I stumbled across a steam-driven train in Pittsburgh. Even before self-driving cars heightened the city’s futuristic vibe, Pittsburgh has been as steampunk as the Nautilus. Captains of industry (or robber barons, if you prefer) raised the city on steel that fed the Industrial Revolution.4 And no steampunk city would deserve the title without a Victorian botanical garden.

A Victorian botanical garden features in chapter 5 of my book. To see a real-life counterpart, visit the Phipps Conservatory. A poem in glass and aluminum, the Phipps opened in 1893 and even boasts a Victoria Room.

Yes, really.

I sneaked into the Phipps during the Pittsburgh Quantum Institute’s annual conference, where I was to present a public lecture about quantum steampunk. Upon reaching the sunken garden, I stopped in my tracks. Yards away stood a coal-black, 19th-century steam train. 

At least, an imitation train stood yards away. The conservatory had incorporated Monet paintings into its scenery during a temporary exhibition. Amongst the palms and ponds were arranged props inspired by the paintings. Monet painted The Gare Saint-Lazare: Arrival of a Train near a station, so a miniature train stood behind a copy of the artwork. The scene found its way into my public lecture—justifying my playing hooky from the conference for a couple of hours (I was doing research for my talk!).

My book’s botanical garden houses hummingbirds, wildebeests, and an artificial creature called a Yorkicockasheepapoo. I can’t promise that you’ll spy Yorkicockasheepapoos while wandering the Phipps, but send me a photo if you do.

8) My students and postdocs presented me with a copy of Quantum Steampunk that they’d signed. They surprised me one afternoon, shortly after publication day, as I was leaving my office. The gesture ranks as one of the most adorable things that’ve ever happened to me, and their book is now the copy that I keep on campus. 

Students…book-selfie photographers…readers halfway across the globe who drop a line…People have populated my curiosity cabinet of with some of the most extraordinary book-publication memories. Thanks for reading, and thanks for sharing.

Book signing after public lecture at Chapman University. Photo from Justin Dressel.

1Or so I imagine, never having watched the sun rise from Mt. Fuji or conducted any symphony, let alone one at Carnegie Hall, and having taken off in a plane for the first time while two months old.

2Other items include serve as an extra in a film, become stranded in Taiwan, and publish a PhD thesis whose title contains the word “steampunk.”

3This ease underlies the difficulty of quantum computing: Any stray particle near a quantum computer can “observe” the computer—interact with the computer and carry off a little of the information that the computer is supposed to store.

4The Pittsburgh Quantum Institute includes Carnegie Mellon University, which owes its name partially to captain of industry Andrew Carnegie.

The spirit of relativity

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One of the most immersive steampunk novels I’ve read winks at an experiment performed in a university I visited this month. The Watchmaker of Filigree Street, by Natasha Pulley, features a budding scientist named Grace Carrow. Grace attends Oxford as one of its few women students during the 1880s. To access the university’s Bodleian Library without an escort, she masquerades as male. The librarian grouses over her request.

“‘The American Journal of  Science – whatever do you want that for?’” As the novel points out, “The only books more difficult to get hold of than little American journals were first copies of [Isaac Newton’s masterpiece] Principia, which were chained to the desks.”

As a practitioner of quantum steampunk, I relish slipping back to this stage of intellectual history. The United States remained an infant, to centuries-old European countries. They looked down upon the US as an intellectual—as well as partially a literal—wilderness.1 Yet potential was budding, as Grace realized. She was studying an American experiment that paved the path for Einstein’s special theory of relativity.

How does light travel? Most influences propagate through media. For instance, ocean waves propagate in water. Sound propagates in air. The Victorians surmised that light similarly travels through a medium, which they called the luminiferous aether. Nobody, however, had detected the aether.

Albert A. Michelson and Edward W. Morley squared up to the task in 1887. Michelson, brought up in a Prussian immigrant family, worked as a professor at the Case School of Applied Science in Cleveland, Ohio. Morley taught chemistry at Western Reserve University, which shared its campus with the recent upstart Case. The two schools later merged to form Case Western Reserve University, which I visited this month.

We can intuit Michelson and Morley’s experiment by imagining two passengers on a (steam-driven, if you please) locomotive: Audrey and Baxter. Say that Audrey walks straight across the aisle, from one window to another. In the same time interval, and at the same speed relative to the train, Baxter walks down the aisle, from row to row of seats. The train carries both passengers in the direction in which Baxter walks.

The Audrey and Baxter drawings (not to scale) are by Todd Cahill.

Baxter travels farther than Audrey, as the figures below show. Covering a greater distance in the same time, he travels more quickly.

Relative lengths of Audrey’s and Baxter’s displacements (top and bottom, respectively)

Replace each passenger with a beam of light, and replace the train with the aether. (The aether, Michelson and Morley reasoned, was moving relative to their lab as a train moves relative to the countryside. The reason was, the aether filled space and the Earth was moving through space. The Earth was moving through the aether, so the lab was moving through the aether, so the aether was moving relative to the lab.)

The scientists measured how quickly the “Audrey” beam of light traveled relative to the “Baxter” beam. The measurement relied on an apparatus that now bears the name of one of the experimentalists: the Michelson interferometer. To the scientists’ surprise, the Audrey beam traveled just as quickly as the Baxter beam. The aether didn’t carry either beam along as a train carries a passenger. Light can travel in a vacuum, without any need for a medium.

Exhibit set up in Case Western Reserve’s physics department to illustrate the Michelson-Morley experiment rather more articulately than my sketch above does

The American Physical Society, among other sources, calls Michelson and Morley’s collaboration “what might be regarded as the most famous failed experiment to date.” The experiment provided the first rigorous evidence that the aether doesn’t exist and that, no matter how you measure light’s speed, you’ll only ever observe one value for it (if you measure it accurately). Einstein’s special theory of relativity provided a theoretical underpinning for these observations in 1905. The theory provides predictions about two observers—such as Audrey and Baxter—who are moving relative to each other. As long as they aren’t accelerating, they agree about all physical laws, including the speed of light.

Morley garnered accolades across the rest of his decades-long appointment at Western Reserve University. Michelson quarreled with his university’s administration and eventually resettled at the University of Chicago. In 1907, he received the first Nobel Prize awarded to any American for physics. The citation highlighted “his optical precision instruments and the spectroscopic and metrological investigations carried out with their aid.”

Today, both scientists enjoy renown across Case Western Reserve University. Their names grace the sit-down restaurant in the multipurpose center, as well as a dormitory and a chemistry building. A fountain on the quad salutes their experiment. And stories about a symposium held in 1987—the experiment’s centennial—echo through the physics building. 

But Michelson and Morley’s spirit most suffuses the population. During my visit, I had the privilege and pleasure of dining with members of WiPAC, the university’s Women in Physics and Astronomy Club. A more curious, energetic group, I’ve rarely seen. Grace Carrow would find kindred spirits there.

With thanks to Harsh Mathur (pictured above), Patricia Princehouse, and Glenn Starkman, for their hospitality, as well as to the Case Western Reserve Department of Physics, the Institute for the Science of Origins, and the Gundzik Endowment.

Aside: If you visit Cleveland, visit its art museum! As Quantum Frontiers regulars know, I have a soft spot for ancient near-Eastern and ancient Egyptian art. I was impressed by the Cleveland Museum of Art’s artifacts from the reign of pharaoh Amenhotep III and the museum’s reliefs of the Egyptian queen Nefertiti. Also, boasting a statue of Gudea (a ruler of the ancient city-state of Lagash) and a relief from the palace of Assyrian kind Ashurnasirpal II, the museum is worth its ancient-near-Eastern salt.

1Not that Oxford enjoyed scientific renown during the Victorian era. As Cecil Rhodes—creator of the Rhodes Scholarship—opined then, “Wherever you turn your eye—except in science—an Oxford man is at the top of the tree.”

Announcing the quantum-steampunk short-story contest!

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The year I started studying calculus, I took the helm of my high school’s literary magazine. Throughout the next two years, the editorial board flooded campus with poetry—and poetry contests. We papered the halls with flyers, built displays in the library, celebrated National Poetry Month, and jerked students awake at morning assembly (hitherto known as the quiet kid you’d consult if you didn’t understand the homework, I turned out to have a sense of humor and a stage presence suited to quoting from that venerated poet Dr. Seuss.1 Who’d’ve thought?). A record number of contest entries resulted.

That limb of my life atrophied in college. My college—a stereotypical liberal-arts affair complete with red bricks—boasted a literary magazine. But it also boasted English and comparative-literature majors. They didn’t need me, I reasoned. The sun ought to set on my days of engineering creative-writing contests.

I’m delighted to be eating my words, in announcing the Quantum-Steampunk Short-Story Contest.

From Pinterest

The Maryland Quantum-Thermodynamics Hub is running the contest this academic year. I’ve argued that quantum thermodynamics—my field of research—resembles the literary and artistic genre of steampunk. Steampunk stories combine Victorian settings and sensibilities with futuristic technologies, such as dirigibles and automata. Quantum technologies are cutting-edge and futuristic, whereas thermodynamics—the study of energy—developed during the 1800s. Inspired by the first steam engines, thermodynamics needs retooling for quantum settings. That retooling is quantum thermodynamics—or, if you’re feeling whimsical (as every physicist should), quantum steampunk.

The contest opens this October and closes on January 15, 2023. Everyone aged 13 or over may enter a story, written in English, of up to 3,000 words. Minimal knowledge of quantum theory is required; if you’ve heard of Schrödinger’s cat, superpositions, or quantum uncertainty, you can pull out your typewriter and start punching away. 

From NickTravers

Entries must satisfy two requirements: First, stories must be written in a steampunk style, including by taking place at least partially during the 1800s. Transport us to Meiji Japan; La Belle Époque in Paris; gritty, smoky Manchester; or a camp of immigrants unfurling a railroad across the American west. Feel free to set your story partially in the future; time machines are welcome.

Second, each entry must feature at least one quantum technology, real or imagined. Real and under-construction quantum technologies include quantum computers, communication networks, cryptographic systems, sensors, thermometers, and clocks. Experimentalists have realized quantum engines, batteries, refrigerators, and teleportation, too. Surprise us with your imagined quantum technologies (and inspire our next research-grant proposals).

From SteampunkEd

In an upgrade from my high-school days, we’ll be awarding $4,500 worth of Visa gift certificates. The grand prize entails $1,500. Entries can also win in categories to be finalized during the judging process; I anticipate labels such as Quantum Technology We’d Most Like to Have, Most Badass Steampunk Hero/ine, Best Student Submission, and People’s Choice Award.

Our judges run the gamut from writers to quantum physicists. Judge Ken Liu‘s latest novel peered out from a window of my local bookstore last month. He’s won Hugo, Nebula, and World Fantasy Awards—the topmost three prizes that pop up if you google “science-fiction awards.” Appropriately for a quantum-steampunk contest, Ken has pioneered the genre of silkpunk, “a technology aesthetic based on a science fictional elaboration of traditions of engineering in East Asia’s classical antiquity.” 

Emily Brandchaft Mitchell is an Associate Professor of English at the University of Maryland. She’s authored a novel and published short stories in numerous venues. Louisa Gilder wrote one of the New York Times 100 Notable Books of 2009, The Age of Entanglement. In it, she imagines conversations through which scientists came to understand the core of this year’s Nobel Prize in physics. Jeffrey Bub is a philosopher of physics and a Distinguished University Professor Emeritus at the University of Maryland. He’s also published graphic novels about special relativity and quantum physics with his artist daughter. 

Patrick Warfield, a musicologist, serves as the Associate Dean for Arts and Programming at the University of Maryland. (“Programming” as in “activities,” rather than as in “writing code,” the meaning I encounter more often.) Spiros Michalakis is a quantum mathematician and the director of Caltech’s quantum outreach program. You may know him as a scientific consultant for Marvel Comics films.

Walter E. Lawrence III is a theoretical quantum physicist and a Professor Emeritus at Dartmouth College. As department chair, he helped me carve out a niche for myself in physics as an undergrad. Jack Harris, an experimental quantum physicist, holds a professorship at Yale. His office there contains artwork that features dragons.

University of Maryland undergraduate Hannah Kim designed the ad above. She and Jade LeSchack, founder of the university’s Undergraduate Quantum Association, round out the contest’s leadership team. We’re standing by for your submissions through—until the quantum internet exists—the hub’s website. Send us something to dream on.

This contest was made possible through the support of Grant 62422 from the John Templeton Foundation.

1Come to think of it, Seuss helped me prepare for a career in physics. He coined the terms wumbus and nerd; my PhD advisor invented NISQ, the name for a category of quantum devices. NISQ now has its own Wikipedia page, as does nerd. 

We’re founding a quantum-thermodynamics hub!

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We’re building a factory in Maryland. 

It’ll tower over the University of Maryland campus, a behemoth of 19th-century brick and 21st-century glass across from the football field. Turbines will turn, and gears will grind, where students now sip lattes near the Stadium Drive parking lot. The factory’s fuel: steam, quantum physics, and ambition. Its goal: to create an epicenter for North American quantum thermodynamics.

The factory is metaphorical, of course. Collaborators and I are establishing a quantum-thermodynamics hub centered at the University of Maryland. The hub is an abstraction—a community that’ll collaborate on research, coordinate gatherings, host visitors, and raise the public’s awareness of quantum thermodynamics. But I’d rather envision the hub as a steampunk factory that pumps out discoveries and early-career scientists.

Image credit: https://soundcloud.com/user-299989503/steampunk-factory

Quantum thermodynamics has burgeoned over the past decade, especially in Europe. At the beginning of my PhD, I read paper after paper that acknowledged COST, a funding agency established by the European Union. COST dedicated a grant to thermodynamics guided by the mathematics and concepts of quantum information theory. The grant funded students, travel, and the first iterations of an annual conference that continues today. Visit Germany, Finland, France, Britain (which belonged to the European Union when I began my PhD), or elsewhere across the pond, and you’ll stumble across quantum-thermodynamics strongholds. Hotspots burn also in Brazil, Israel, Singapore, and elsewhere.

Inspired by our international colleagues, collaborators and I are banding together. Since I founded a research group last year, Maryland has achieved a critical mass of quantum thermodynamicists: Chris Jarzynski reigns as a king of the field of fluctuation relations, equalities that help us understand why time flows in only one direction. Sebastian Deffner, I regard as an academic older brother to look up to. And I run the Quantum-Steampunk Laboratory.

We’ve built railroads to research groups across the continent and steamers to cross the ocean. Other members of the hub include Kanu Sinha, a former Marylander who studies open systems in Arizona; Steve Campbell, a Dublin-based prover of fundamental bounds; and two experts on quantum many-body systems: former Marylander Amir Kalev and current Marylander Luis Pedro García-Pintos. We’re also planning collaborations with institutions from Canada to Vienna.

The hub will pursue a threefold mission of research, community building, and outreach. As detailed on our research webpage, “We aim to quantify how, thermodynamically, decoherence and the spread of information lead to emergent phenomena: classical objectivity and the flow of time.” To grow North America’s quantum-thermodynamics community, we’ll run annual symposia and an international conference. Our visitors’ program will create the atmosphere of a local watering hole. Outreach will include more posts on this blog—including by guest authors—a quantum-steampunk short-story contest (expect details this fall), and more.

Come visit us by dirigible, train, or gyropter. Air your most thought-provoking quantum-thermodynamics discoveries in a seminar with us, and solicit feedback. Find collaborators, and learn about the latest. The factory wheels are beginning to turn.

https://qtd-hub.umd.edu

With thanks to the John Templeton Foundation for the grant to establish the hub.

Rocks that roll

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In Terry Pratchett’s fantasy novel Soul Music, rock ’n roll arrives in Ankh-Morpork. Ankh-Morpork resembles the London of yesteryear—teeming with heroes and cutthroats, palaces and squalor—but also houses vampires, golems, wizards, and a sentient suitcase. Against this backdrop, a young harpist stumbles upon a mysterious guitar. He forms a band with a dwarf and with a troll who plays tuned rocks, after which the trio calls its style “Music with Rocks In.” The rest of the story consists of satire, drums, and rocks that roll. 

The topic of rolling rocks sounds like it should elicit more yawns than an Elvis concert elicited screams. But rocks’ rolling helped recent University of Maryland physics PhD student Zackery Benson win a National Research Council Fellowship. He and his advisor, Wolfgang Losert, converted me into a fan of granular flow.

What I’ve been studying recently. Kind of.

Grains make up materials throughout the galaxy, such as the substance of avalanches. Many granular materials undergo repeated forcing by their environments. For instance, the grains that form an asteroid suffer bombardment from particles flying through outer space. The gravel beneath train tracks is compressed whenever a train passes. 

Often, a pattern characterizes the forces in a granular system’s environment. For instance, trains in a particular weight class may traverse some patch of gravel, and the trains may arrive with a particular frequency. Some granular systems come to encode information about those patterns in their microscopic configurations and large-scale properties. So granular flow—little rocks that roll—can impact materials science, engineering, geophysics, and thermodynamics.

Granular flow sounds so elementary, you might expect us to have known everything about it since long before the Beatles’ time. But we didn’t even know until recently how to measure rolling in granular flows. 

Envision a grain as a tiny sphere, like a globe of the Earth. Scientists focused mostly on how far grains are translated through space in a flow, analogouslly to how far a globe travels across a desktop if flicked. Recently, scientists measured how far a grain rotates about one axis, like a globe fixed in a frame. Sans frame, though, a globe can spin about more than one axis—about three independent axes. Zack performed the first measurement of all the rotations and translations of all the particles in a granular flow.

Each grain was an acrylic bead about as wide as my pinky nail. Two holes were drilled into each bead, forming an X, for reasons I’ll explain. 

Image credit: Benson et al., Phys. Rev. Lett. 129, 048001 (2022).

Zack dumped over 10,000 beads into a rectangular container. Then, he poured in a fluid that filled the spaces between the grains. Placing a weight atop the grains, he exerted a constant pressure on them. Zack would push one of the container’s walls inward, compressing the grains similarly to how a train compresses gravel. Then, he’d decompress the beads. He repeated this compression cycle many times.

Image credit: Benson et al., Phys. Rev. E 103, 062906 (2021).

Each cycle consisted of many steps: Zack would compress the beads a tiny amount, pause, snap pictures, and then compress a tiny amount more. During each pause, the camera activated a fluorescent dye in the fluid, which looked clear in the photographs. Lacking the fluorescent dye, the beads showed up as dark patches. Clear X’s cut through the dark patches, as dye filled the cavities drilled into the beads. From the X’s, Zack inferred every grain’s orientation. He inferred how every grain rotated by comparing the orientation in one snapshot with the orientation in the next snapshot. 

Image credit: Benson et al., Phys. Rev. Lett. 129, 048001 (2022).

Wolfgang’s lab had been trying for fifteen years to measure all the motions in a granular flow. The feat required experimental and computational skill. I appreciated the chance to play a minor role, in analyzing the data. Physical Review Letters published our paper last month.

From Zack’s measurements, we learned about the unique roles played by rotations in granular flow. For instance, rotations dominate the motion in a granular system’s bulk, far from the container’s walls. Importantly, the bulk dissipates the most energy. Also, whereas translations are reversible—however far grains shift while compressed, they tend to shift oppositely while decompressed—rotations are not. Such irreversibility can contribute to materials’ aging.

In Soul Music, the spirit of rock ’n roll—conceived of as a force in its own right—offers the guitarist the opportunity to never age. He can live fast, die young, and enjoy immortality as a legend, for his guitar comes from a dusty little shop not entirely of Ankh-Morpork’s world. Such shops deal in fate and fortune, the author maintains. Doing so, he takes a dig at the River Ankh, which flows through the city of Ankh-Morpork. The Ankh’s waters hold so much garbage, excrement, midnight victims, and other muck that they scarcely count as waters:

And there was even an Ankh-Morpork legend, wasn’t there, about some old drum [ . . . ] that was supposed to bang itself if an enemy fleet was seen sailing up the Ankh? The legend had died out in recent centuries, partly because this was the Age of Reason and also because no enemy fleet could sail up the Ankh without a gang of men with shovels going in front.

Such a drum would qualify as magic easily, but don’t underestimate the sludge. As a granular-flow system, it’s more incredible than you might expect.

If I could do science like Spider-Man

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A few Saturdays ago, I traveled home from a summer school at which I’d been lecturing in Sweden. Around 8:30 AM, before the taxi arrived, I settled into an armchair in my hotel room and refereed a manuscript from a colleague. After reaching the airport, I read an experimental proposal for measuring a quantity that colleagues and I had defined. I drafted an article for New Scientist on my trans-Atlantic flight, composed several emails, and provided feedback about a student’s results (we’d need more data). Around 8 PM Swedish time, I felt satisfyingly exhausted—and about ten hours of travel remained. So I switched on Finnair’s entertainment system and navigated to Spider-Man: No Way Home.

I found much to delight. Actor Alfred Molina plays the supervillain Doc Ock with charisma and verve that I hadn’t expected from a tentacled murderer. Playing on our heartstrings, Willem Dafoe imbues the supervillain Norman Osborn with frailty and humanity. Three characters (I won’t say which, for the spoiler-sensitive) exhibit a playful chemistry. To the writers who thought to bring the trio together, I tip my hat. I tip my hat also to the special-effects coders who sweated over reconciling Spider-Man’s swoops and leaps with the laws of mechanics.

I’m not a physicist to pick bones with films for breaking physical laws. You want to imagine a Mirror Dimension controlled by a flying erstwhile surgeon? Go for it. Falling into a vat of electrical eels endows you with the power to control electricity? Why not. Films like Spider-Man’s aren’t intended to portray physical laws accurately; they’re intended to portray people and relationships meaningfully. So I raised nary an eyebrow at characters’ zipping between universes (although I had trouble buying teenage New Yorkers who called adults “sir” and “ma’am”).

Anyway, no hard feelings about the portrayal of scientific laws. The portrayal of the scientific process, though, entertained me even more than Dr. Strange’s trademark facetiousness. In one scene, twelfth grader Peter Parker (Spider-Man’s alter-ego) commandeers a high-school lab with two buddies. In a fraction of a night, the trio concocts cures for four supervillains whose evil stems from physical, chemical, and biological accidents (e.g., falling into the aforementioned vat of electric eels).1 And they succeed. In a few hours. Without test subjects or even, as far as we could see, samples of their would-be test subjects. Without undergoing several thousand iterations of trying out their cures, failing, and tweaking their formulae—or even undergoing one iteration.

I once collaborated with an experimentalist renowned for his facility with superconducting qubits. He’d worked with a panjandrum of physics years before—a panjandrum who later reminisced to me, “A theorist would propose an experiment, [this experimentalist would tackle the proposal,] and boom—the proposal would work.” Yet even this experimentalist’s team invested a year in an experiment that he’d predicted would take a month.

Worse, the observatory LIGO detected gravitational waves in 2016 after starting to take data in 2002…after beginning its life during the 1960s.2 

Recalling the toil I’d undertaken all day—and only as a theorist, not even as an experimentalist charged with taking data through the night—I thought, I want to be like Spider-Man. Specifically, I want to do science like Spider-Man. Never mind shooting webs out of my wrists or swooping through the air. Never mind buddies in the Avengers, a Greek-statue physique, or high-tech Spandex. I want to try out a radical new idea and have it work. On the first try. Four times in a row on the same day. 

Daydreaming in the next airport (and awake past my bedtime), I imagined what a theorist could accomplish with Spider-Man’s scientific superpowers. I could calculate any integral…write code free of bugs on the first try3…prove general theorems in a single appendix!

Too few hours later, I woke up at home, jet-lagged but free of bites from radioactive calculators. I got up, breakfasted, showered, and settled down to work. Because that’s what scientists do—work. Long and hard, including when those around us are dozing or bartering frequent-flyer miles, such that the satisfaction of discoveries is well-earned. I have to go edit a paper now, but, if you have the time, I recommend watching the latest Spider-Man movie. It’s a feast of fantasy.

1And from psychological disorders, but the therapy needed to cure those would doom any blockbuster.

2You might complain that comparing Peter Parker’s labwork with LIGO’s is unfair. LIGO required the construction of large, high-tech facilities; Parker had only to cure a lizard-man of his reptilian traits and so on. But Tony Stark built a particle accelerator in his basement within a few hours, in Iron Man; and superheroes are all of a piece, as far as their scientific exploits are concerned.

3Except for spiders?

Quantum connections

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We were seated in the open-air back of a boat, motoring around the Stockholm archipelago. The Swedish colors fluttered above our heads; the occasional speedboat zipped past, rocking us in its wake; and wildflowers dotted the bank on either side. Suddenly, a wood-trimmed boat glided by, and the captain waved from his perch.

The gesture surprised me. If I were in a vehicle of the sort most familiar to me—a car—I wouldn’t wave to other drivers. In a tram, I wouldn’t wave to passengers on a parallel track. Granted, trams and cars are closed, whereas boats can be open-air. But even as a pedestrian in a downtown crossing, I wouldn’t wave to everyone I passed. Yet, as boat after boat pulled alongside us, we received salutation after salutation.

The outing marked the midpoint of the Quantum Connections summer school. Physicists Frank Wilczek, Antti Niemi, and colleagues coordinate the school, which draws students and lecturers from across the globe. Although sponsored by Stockholm University, the school takes place at a century-old villa whose name I wish I could pronounce: Högberga Gård. The villa nestles atop a cliff on an island in the archipelago. We ventured off the island after a week of lectures.

Charlie Marcus lectured about materials formed from superconductors and semiconductors; John Martinis, about superconducting qubits; Jianwei Pan, about quantum advantages; and others, about symmetries, particle statistics, and more. Feeling like an ant among giants, I lectured about quantum thermodynamics. Two other lectures linked quantum physics with gravity—and in a way you might not expect. I appreciated the opportunity to reconnect with the lecturer: Igor Pikovski.

Cruising around Stockholm

Igor doesn’t know it, but he’s one of the reasons why I joined the Harvard-Smithsonian Institute for Theoretical Atomic, Molecular, and Optical Physics (ITAMP) as an ITAMP Postdoctoral Fellow in 2018. He’d held the fellowship beginning a few years before, and he’d earned a reputation for kindness and consideration. Also, his research struck me as some of the most fulfilling that one could undertake.

If you’ve heard about the intersection of quantum physics and gravity, you’ve probably heard of approaches other than Igor’s. For instance, physicists are trying to construct a theory of quantum gravity, which would describe black holes and the universe’s origin. Such a “theory of everything” would reduce to Einstein’s general theory of relativity when applied to planets and would reduce to quantum theory when applied to atoms. In another example, physicists leverage quantum technologies to observe properties of gravity. Such technologies enabled the observatory LIGO to register gravitational waves—ripples in space-time. 

Igor and his colleagues pursue a different goal: to observe phenomena whose explanations depend on quantum theory and on gravity.

In his lectures, Igor illustrated with an experiment first performed in 1975. The experiment relies on what happens if you jump: You gain energy associated with resisting the Earth’s gravitational pull—gravitational potential energy. A quantum object’s energy determines how the object’s quantum state changes in time. The experimentalists applied this fact to a beam of neutrons. 

They put the beam in a superposition of two locations: closer to the Earth’s surface and farther away. The closer component changed in time in one way, and the farther component changed another way. After a while, the scientists recombined the components. The two interfered with each other similarly to the waves created by two raindrops falling near each other on a puddle. The interference evidenced gravity’s effect on the neutrons’ quantum state.

Summer-school venue. I’d easily say it’s gorgeous but not easily pronounce its name.

The experimentalists approximated gravity as dominated by the Earth alone. But other masses can influence the gravitational field noticeably. What if you put a mass in a superposition of different locations? What would happen to space-time?

Or imagine two quantum particles too far apart to interact with each other significantly. Could a gravitational field entangle the particles by carrying quantum correlations from one to the other?

Physicists including Igor ponder these questions…and then ponder how experimentalists could test their predictions. The more an object influences gravity, the more massive the object tends to be, and the more easily the object tends to decohere—to spill the quantum information that it holds into its surroundings.

The “gravity-quantum interface,” as Igor entitled his lectures, epitomizes what I hoped to study in college, as a high-school student entranced by physics, math, and philosophy. What’s more curious and puzzling than superpositions, entanglement, and space-time? What’s more fundamental than quantum theory and gravity? Little wonder that connecting them inspires wonder.

But we humans are suckers for connections. I appreciated the opportunity to reconnect with a colleague during the summer school. Boaters on the Stockholm archipelago waved to our cohort as they passed. And who knows—gravitational influences may even have rippled between the boats, entangling us a little.

Requisite physicist-visiting-Stockholm photo

With thanks to the summer-school organizers, including Pouya Peighami and Elizabeth Yang, for their invitation and hospitality.