Monika leans against a sandstone wall smiling

Catalyzing Discovery:It’s like herding atoms

Monika Schleier-Smith is connecting the building blocks of reality in patterns that could revolutionize computing and reveal the structure of space and time.

By Derek Rosenfield
Associate Professor of Physics Monika Schleier-Smith

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If you can’t quite grasp the concept of entanglement, don’t feel discouraged. It didn’t make sense to Einstein either, even though his own work suggested it was possible—that two particles could become connected in such a way that information travels between them, across any distance, faster than the speed of light.

If scientists could harness such connections, they could build quantum computers that are totally different from and vastly more powerful than the devices we use today. But to get there, associate professor of physics Monika Schleier-Smith has to find out what even Einstein didn’t know for sure. 

In her Stanford lab, Schleier-Smith and her team have demonstrated how to manipulate atoms with an unprecedented level of control, synchronizing them in complex patterns that might not only form the basis of new computers, but reveal the means by which subatomic particles give rise to the force of gravity—a mystery so profound that physics currently provides no explanation.

Although it has potential applications, such efforts epitomize “basic” research, which takes aim at questions so essential there is no way of knowing where the work will ultimately lead. In fact, that uncertainty is important. It generates the powerful curiosity that drives people like Schleier-Smith.

A lens and metal parts in Monika’s lab

How to understand quantum uncertainty 

Uncertainty describes another strange property of the subatomic scale: You cannot know where a particle is and also how fast it’s moving at the same time. The Heisenberg uncertainty principle, “sounds really weird,” Schleier-Smith admits. After all, you can easily measure the speed of a large object, such as a car, without affecting its location. The highway patrol does this all the time.

But when you “look” at something as small as a particle, in effect you have to touch it. “If I want to precisely measure where something is, I have to bounce some light off of it,” Schleier-Smith explains. “And that photon—that particle of light that bounces off it—can push the thing and change how fast it’s moving.”

Once you understand what seeing does to subatomic particles, you begin to sympathize with quantum physicists. “You start to develop an intuition for why this uncertainty exists,” says Schleier-Smith.

But beyond this point, intuition will be harder to come by. Because if you have more than a couple of particles, it becomes incredibly difficult to see what all of them are doing, let alone control it.

Monika smiles at the camera next to machinery from her lab
A closeup of several lenses in Monika’s lab

One of Schleier-Smith’s favorites examples of “curiosity-driven” research is the work that led to the laser, which was co-invented by Stanford physics professor Arthur Schawlow. Another Stanford physicist, Steve Chu, later showed how lasers could be used to cool atoms—the starting point for Schleier-Smith’s work. (Both Schawlow and Chu went on to win Nobel Prizes. Today, Schleier-Smith works in the same building Schawlow once occupied.)

Metal machinery from the lab

How to herd atoms

Just to get started on this kind of fundamental research—to learn things that have always been true, but never been known—you need to build an apparatus that has never existed.

“There is no piece of equipment that we can just buy,” Schleier-Smith explains. “Everything is custom-built to do the science we want to do.”

(The ingredients do include one raw element: rubidium, symbol Rb, atomic number 37. Rubidium atoms, used in atomic clocks, are great for this kind of experiment.)

The work requires an extremely empty space, where nothing can interfere with your atoms: a vacuum. Emptying a vacuum chamber takes a couple of hours. “Baking” it to evaporate and pump out any residual substances takes a couple of weeks. But after that, once the chamber reaches the extremely low pressure required, it can stay like that for years. Which is helpful, because the next steps call for endless patience.

“In experimental physics, most of the time is spent problem solving, figuring out why the apparatus isn’t working the way it should, and fixing one thing after another,” Schleier-Smith says. “It requires a willingness to struggle, and sometimes things just don’t go as planned.”

One setup, for example, which the team has had under vacuum for several years, uses two mirrors to form an optical resonator, which bounces light back and forth repeatedly to generate the entanglement between atoms.

After their resonator was assembled, “we realized the mirrors weren’t as good as we had hoped,” Schleier-Smith recalls. “The light was supposed to bounce back and forth 100,000 times, but it only bounces back and forth 10,000 times. So the atomic interactions aren’t quite as strong.” Still, their lab’s resonator proved both unique and valuable in its own right. “It wasn’t exactly as we planned it, but it turned out to be really fruitful.”

To trap and manipulate the atoms requires a combination of magnetic fields and laser beams. Although light is energy, which you might think of as heat, what the lasers do is “essentially to bring the atoms to a standstill,” explains Schleier-Smith. That lack of motion translates into very low temperatures—a millionth of a degree above absolute zero.

Once the rubidium is inside the optical resonator—which is inside the magnetic field, which is inside the vacuum chamber—chilling the atoms with light takes only a few seconds.

Schleier-Smith’s team “looks” at measurements of subatomic propertiesor information shared across atomsto start to analyze what the atoms are really doing. Photo courtesy Monika Schleier-Smith

Congratulations! You have now reached the starting point for experiments in quantum information.

By adjusting everything very, very carefully, you can herd your rubidium gas into tiny clouds of about 10,000 atoms each, which you can actually see as little glowing balls. What we can see with the naked eye is not very useful, however. Even the finest intuition is no substitute for the computers on the receiving end of this entire apparatus, which analyze a mountain of data to discern what those thousands of atoms are really doing.

A closeup of several lenses in Monika’s lab

What is real?

What Schleier-Smith’s team “looks” at is measurements of subatomic properties such as angular momentum, or “spin.” The spin of atoms in one cloud can be synchronized with the spin of atoms in another cloud, in such a way that changing the spins in the first changes the spins in the second.

Think of these measurements as information shared across atoms. Atoms whose information is correlated in this way have a very special kind of connection: They don’t have to be next to each other, any more than two people have to be neighbors in order to be friends. By creating lots of connections between lots of atoms, you can generate all sorts of geometric patterns. Think of a tree, or a Möebius strip.

These patterns may not exist in space, but they do exist in reality. Sort of.

Again, it’s all right if these concepts feel slippery. Geometric relationships are mathematical relationships, and the connection between reality and the math we use to explain it can be mysterious. It’s one of the things that drew Schleier-Smith into physics.

“In high school chemistry classes, I learned that I should think of an electron not as sitting at one point in an atom, but smeared out on a cloud of quantum uncertainty,” she recalls. “I wanted to learn more about the elegant mathematical descriptions underlying that.”

The reason we think of an electron as being smeared is Heisenberg uncertainty: Since you can’t always know the electron’s exact location, you have to consider the set of points where it could possibly be. We represent that set of possibilities with math, and the set of points in space becomes a set of points on a graph.

When virtual shapes arise from real particles, the distinction between virtual and real begins to defy intuition, even for experts. Some physicists believe that the three-dimensional reality we know and love might actually emerge from entanglements between subatomic particles that generate the force of gravity. In other words, entanglements like the ones Schleier-Smith creates in her lab may give rise to space-time itself.

Schleier-Smiths team had to invent and create a piece of equipment that uses mirrors to form an optical resonator, which bounces light back and forth repeatedly to generate the entanglement between atoms. Photo courtesy Monika Schleier-Smith

Why do we need to know this?

Now you can see why Einstein resisted some of the predictions that grew out of his own theories. They’re very hard to think about, let alone to prove. Ordinary people can be forgiven for asking why this work matters.

The promise of incredibly powerful computers is one reason to believe that people like Schleier-Smith are onto something. After all, quantum mechanics have already given us incredibly powerful bombs. (She emphasizes that physicists will have to grapple ethically with technologies that emerge from quantum information.)

What drives Schleier-Smith’s work is a combination of possible outcomes and pure curiosity. “It turns out quantum uncertainty is something we can manipulate in the laboratory in a way that could enable extraordinarily precise measurements of time or gravity, which can teach us new things about the universe.”

Ironically, the subatomic systems in question are so complex you can’t simulate them on the computers we have today. That’s why Schleier-Smith’s team does what they do. “You can’t predict what will happen,” she says, “and so you have to build the thing and find out.”

Discovery vs. solutions

Outside of academia, almost nobody pursues basic research, without knowing what the payoff might be. The vast majority of corporate R&D is aimed at useful gadgets, new drugs, or other applications with commercial potential. 

Which kind of research should universities be doing: basic or applied? Schleier-Smith refuses to choose one over the other.

“I think we have to pursue both in parallel,” she says. “Often, the same tools that allow us to make fundamental discoveries feed into new technology. And those technologies enable new fundamental science as well.”

But if universities give faculty and students the freedom to follow their curiosity, how will we know we’re making progress?

“I think we should measure success by how surprised we are.”

“Our amazing graduate students are essential for every aspect of my research,” says Schleier-Smith. “I need grad students who can come into an empty lab, build up customized equipment to do something that’s never been done before, and also bring in their own ideas and make my ideas better.”  Photo courtesy Monika Schleier-Smith

Catalyzing Discovery: The impact

Why it matters

Controlling atomic interactions could lead to quantum computers vastly more powerful than current machines, with applications in artificial intelligence, drug development, climate forecasting, cybersecurity, and other fields. The same techniques could also be used to construct basic models of space-time itself, helping to solve one of the greatest mysteries in physics, the nature of gravity.

How well get there

Years of fundamental work lie ahead, probing the intricacies of subatomic interaction. Basic scientists need the freedom to let curiosity drive this work, and to engage in their own surprising interactions with Stanford’s experts in related fields.

Whats next

Schleier-Smith’s team is generating ever more complex patterns of atomic interaction, while perfecting techniques that will eventually be used by countless researchers.

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