Monika Schleier-Smith’s journey to leading an experimental quantum physics lab at Stanford began with visions of peanut butter. Growing up, she was one of many students taught to picture an electron as a particle circling an atom’s nucleus. But her high school chemistry teacher approached the lesson differently, encouraging her to imagine the particle as a wave function: a cloud of possible places where the electron might be, smeared out in space. It was an illustration of “fundamental quantum uncertainty,” she recalled in a 2023 interview for the School of Humanities and Sciences’ Big Ideas program. “You can’t know where a thing is and how fast it is moving at the same time.”
“Those ideas were fascinating to me,” she said. She wanted to understand the math behind them—and perhaps take her curiosity into the lab.
Some two decades later, Schleier-Smith is an associate professor of physics widely known for her novel experimental designs. She supervises a lab of 15 students and postdocs working in three experimental groups, all focused on many-particle quantum systems—in this case, collections of atoms—that interact with one another and are controlled by fundamental forces of nature, such as electromagnetism. She and her team can assemble these systems and control their interactions and properties, engineering states in them known as entanglement, in which knowledge about one part of the system implies knowledge about the others. (It’s kind of like this: Say you’re shuffling two cards—one red and one blue—and flip one over. If you see red, then you know the other card is blue. At the quantum level, such relationships exist, except that each particle is in a probabilistic state of being either red or blue—“like a coin,” Schleier-Smith explains, “that has been flipped but has not landed yet.”) Scientists apply the concept of entanglement not only to improve sensors and high-precision clocks, but also to establish new paradigms for computation and long-distance information transmission. “The key idea,” Schleier-Smith says, “is that information does not have to be coded locally in individual particles,” as it does in conventional computers, “but can be stored in correlations between them.”
“Engineering quantum systems for research is nothing new,” says Brian Swingle, an associate professor of physics at Brandeis University who collaborates with Schleier-Smith. “There is a whole community out there using it to improve artificial magnets and superconductors, or to contribute to other areas of science.” But what Schleier-Smith understood very early and very clearly, he says, is that these systems could be used to probe fundamental questions about the nature of space and time and the interactions between them. “We used to think of water as being a continuous flow, and now we know it is made of distinct particles,” he says. “What if there are such particles for space-time?” Schleier-Smith is helping answer that question.
The scientific community has taken notice. In recent years, Schleier-Smith has earned a Sloan Research Fellowship, a National Science Foundation CAREER Award, a Presidential Early Career Award for Scientists and Engineers, and, in 2020, a MacArthur Fellowship. “Her work pushes beyond the ‘standard quantum limit,’ the highest degree of precision we can achieve with independent, uncorrelated atoms,” Debra Satz, dean of the School of Humanities and Sciences, told Stanford Report at the time. “It’s an exciting frontier with respect to measurement with implications for everything from GPS to continental drift.”
Insights on the fundamental questions of the universe, of course, do not come easy. But setting up an environment for creativity—that’s another place where Schleier-Smith shines.
Try, try again
On a typical midmorning, Schleier-Smith huddles in her office with a half-dozen grad students and postdocs for a lab meeting. This group uses lasers to cool atoms to a few millionths of a degree above zero, and to suspend them between two concave mirrors while bouncing photons between the mirrors. If a photon is just the right wavelength, it will induce an entangled state between clusters of these atoms and give rise to some peculiar properties. Avikar Periwal, PhD ’24, begins by discussing a graph of purple vertical bars with fuzzy orange edges. The bars represent clusters of atoms, and the fuzzy edges indicate entanglement oscillations, which Schleier-Smith says they are “excited to see.” Discussion is lively, with students describing results expected and unexpected, while Schleier-Smith probes their findings and suggests course corrections.
Say you’re shuffling two cards—one red and one blue—and flip one over. If you see red, then you know the other card is blue. At the quantum level, such relationships exist, except that each particle is in a probabilistic state of being either red or blue—“like a coin,” Schleier-Smith explains, “that has been flipped but has not landed yet.”
The lab itself is something like a cyberpunk abstract art exhibit: wires and fiber-optic cables weave and snake over a dizzying array of small mirrors and lenses stacked in tiers like a wedding cake and spread across multiple tables. It is high-precision work, with “a few hundred pieces that have to work together at once,” Schleier-Smith says. “And there’s usually one that doesn’t.” But that doesn’t faze her, says PhD candidate Michael Wahrman—the professor troubleshoots with “an enthusiasm and optimism entirely matched by her technical prowess.” One Sunday, former postdoc Shankari Rajagopal recalls, Schleier-Smith came into the lab to assess a broken laser and ordered a small part that could be replaced overnight, saving a potential monthlong interruption to return it to the manufacturer. “The challenge is always new,” Wahrman explains. “Rarely do you think, ‘I’ve seen this before.’ ” At the same time, he adds, “the best way to learn is to have something break down.”
Other times, the pause is what’s needed. Once, while Schleier-Smith was in graduate school, a piece of equipment broke, and while her peer repaired it, Schleier-Smith reviewed a paper from a pile on her desk that their adviser had given them “probably on the first day,” she recalls. The paper proposed a certain entangled state with improved measurements, but the authors did not know how to induce the interaction. It struck Schleier-Smith, however, as “a lot like something” she and her peer were doing in the lab. This revelation led to proof of concept on how entanglement could make atomic clocks even more precise, and several research groups are now working to apply the technique in real-world timekeeping—with uses in space travel, sensors, and GPS. (For context, a millisecond deviation on a satellite’s clock makes for a distance error of 300 kilometers— 185 miles—and a major challenge for tracking, say, a fast-moving spacecraft.)
Physics, a team sport
In the afternoon, Schleier-Smith and a doctoral student meet with Swingle and his advisee about using tabletop experiments to simulate theories of quantum gravity. The Standard Model of particle physics—the current best theory for describing the building blocks of matter and how they interact—includes three fundamental forces (electromagnetic, weak nuclear, and strong nuclear) but excludes gravity. That’s because gravity is not a force, according to Einstein, but a property of curved space-time difficult to describe on a quantum level. “We actually do have quantum theories of gravity,” explains Swingle, a theorist, but only for “toy universes.” These theoretical model universes are like our own but with a twist: They might have outward expansion restricted or a boundary imposed, neither of which our universe seems to have.
This is where Schleier-Smith’s vision stands out, Swingle says: Her methods enable them to find answers about these model universes even when calculations can’t be made directly. In the case of quantum gravity, this means engineering a small universe-like system of atoms in the lab. The atoms and photons are “trapped” between two curved mirrors and so essentially isolated from our world—a “universe in a bottle.” If information doesn’t enter or leak out, these bottled universes might behave like one of the toy models, leaving clues about the workings of our own universe.
On more relaxed afternoons, Schleier-Smith and her students begin in a spacious room with dry-erase boards, sofa chairs, and a wall-length kitchen counter. “The labs needed a place to hang out, share ideas, and just bump into each other,” Schleier-Smith says of the common room, which used to be a clump of office spaces. PhD candidate Gabriel Moreau is the group’s espresso mage. He measures out the beans carefully, cranks them through a hand grinder, then gently mists the packed-down grains with water to “dissipate the static.” The others chat unhurriedly about their weekends, workouts, sleepless nights, and, of course, physics. “If it’s fun every day,” Schleier-Smith says, “then it’s also productive.”
Learning through teaching
Two things stand out immediately about Schleier-Smith’s office: a dry-erase board smattered with green-and-orange diagrams and equations, and the bookcase. On the shelves, alongside slab-like textbooks such as Ultra-Cold Neutrons and Quantum Theory of Many-Particle Systems, are smaller titles like Tools for Teaching and Peer Instruction: A User’s Manual. “I actually might not have read all the education ones,” Schleier-Smith says, as she sips a cup of tea at a large circular table. “But it is something I care about.”
In the classroom, Schleier-Smith uses a methodology known as active learning. Instead of “lecturing at the students,” she says, she arranges time for them to work together on problems along the way. She learned the approach from Nobel laureate Carl Wieman, PhD ’77, a professor emeritus of physics and of education, and a former office neighbor who, after sitting in one of Schleier-Smith’s classes, showed up the next day with two pages of notes, front and back, and asked, “So, how good are you at taking feedback?” Teaching this way is a chance for Schleier-Smith to reflect on her own material, to figure out where people get stuck, and to clarify her own understanding and explanations of it in order to help them. It is also more fun. “I get to know more people, hear the questions they ask,” she says. “And students get to know each other.”
Time to think
Finally, there’s alone time. Schleier-Smith ran her first marathon as a senior physics major at Harvard, then continued the tradition every year as a graduate student at MIT. Running lets her mind wander, allowing things that seemed urgent but are not important—“like many emails,” she says with a smile—to fade away, letting more vital thoughts bubble up to the surface.
The professor admits to being intuition-based and curiosity-driven, open to choosing a direction and seeing where it leads. “If everybody is already working on something,” she says, “I don’t feel the need. I love to look for the underexplored.”
Nestor Walters, ’22, MS ’24, works in data science and writes essays, fiction, and poetry. Email him at stanford.magazine@stanford.edu.
