Containing multitudes: SLAC scientist collaborates to create scalable qubits

Studying an object with zero dimensions takes serious creativity. Consider the singularity — a point packed with infinite energy that sparked the Big Bang. Or the humble mathematical point, an abstract but indispensable fixture in space-time. Grasping a thing that has no size or shape demands imagination and rigor.

Or take qubits. Manipulating these zero-dimensional, information-carrying ripples in quantum space requires mental and manual dexterity. Yet those who work on qubits seldom tout the rich set of skills they bring to bear on their research.

The scientific endeavor’s many creative dimensions are what drew Shannon Harvey to the work of finessing these dimensionless bits of information.

“What I love about working in quantum information is that we can use today’s technologies to play with nature’s quantum features, something that until recently would have seemed incredible,” said Harvey, a scientist at the U.S. Department of Energy’s (DOE) SLAC National Accelerator Laboratory. ​“I really thrive on the multifaceted nature of this research, solving and coming up with problems by embedding myself in the experimental details and trying to understand how they all fit together. For me, scientific exploration involves reading and writing papers, solving math problems, even soldering and welding. Often within the same day.”

Harvey brings her multifaceted set of skills to Q-NEXT, a DOE National Quantum Information Science Research Center led by DOE’s Argonne National Laboratory in partnership with SLAC. A national research hub, Q-NEXT aims to coax nature’s quantum features into sharing information over distances large and small. It’s a collaborative effort that’s helped by a knack for futzing with particles.

The particle of Harvey’s attention is a type of qubit called a quantum dot.

Picture an electron, a tiny ripple bopping around inside a tiny space. Now imagine fencing it in so tightly that it’s trapped in an even smaller space, smaller than its own wavelength — like putting up walls that hug someone so closely, they have no room to lift their arms. Hemmed in, the electron is compelled by the rules of physics to take on a set of specific energy values. Its wave transforms into a set of distinct wavelengths, like a chord separating into a series of pure tones. Those discrete energies let scientists fine-tune and control how the electron stores and shares information.

That’s a quantum dot: a particle that’s confined to a space smaller than its wavelength, transformed into an object with multiple energy values. (One might say that the particle is squeezed for information.)

The qubit is the basis of quantum technologies, which are expected to speed up drug discovery, make financial transactions more secure, provide eavesdrop-proof telecommunication and more. As a qubit species, the quantum dot has a lot going for it. For one, it’s tunable, like a radio, so it can share information over different frequencies depending on how it’s used. And, most relevant for Harvey’s research, quantum dots can be mass-produced.

“The real selling point of quantum dot qubits is that they’re scalable,” Harvey said. ​“You can put a ton of them on a chip and then build a quantum computer on that chip.”

That’s the dream: a chip that contains multitudes. Harvey and her colleagues are designing quantum dots so they can crowd millions — or even billions — onto a something the size of a drink coaster. That scalability is both a feature and a bug, Harvey said.

Scalability means quantum dots can be made affordably; perform consistently and reliably; and can compatibly work with larger systems and existing technologies.

The challenge — the bug — is that a chip chock-a-block with dots is noisy. The noise muddles the qubit’s signal.

“You want to be able to control the qubit’s energy. If there’s some noise that’s causing the energy to fluctuate in time, you’ll lose the knowledge of what your qubit is doing, lose control. And then the qubit stops being useful,” Harvey said.

The lower the noise, the more reliable and pliable the qubit.

But taming noise is only half of it. Harvey’s job is about more than shushing, like an usher at the symphony. She works to create a quiet environment in which a massive quantum dot brigade can perform harmoniously, sending and receiving data with no interference, no snags. What properties will smooth the information pathway? What’s the best way to connect quantum dots to surrounding structures, which are themselves noisy? At which temperature does the quantum dot perform best? How should quantum dots be spaced to prevent interference? What software capabilities are needed to keep everything under control?

The work is a mix of materials science, computer science, engineering and basic physics, not to mention patience, exploration and ingenuity. Harvey reaches across the disciplinary aisle at SLAC to connect with cosmologists building detectors for studying the outer universe. It’s a perk of working at the SLAC Millikelvin Facility, where researchers explore nature at both extremes of scale.

“It’s a really open environment. We lack walls literally. I’ve learned a lot from the other people in the building who have very different expertise than I have. I never knew how similar the things I think about are to the people who are doing experiments for cosmology,” she said. ​“It’s very different from what you see in academia. And even though it’s not this huge-scale facility, it’s really an example of what national labs can bring to the table. It’s a special experience.”

As a child, Harvey had ​“zero interest in science,” she said. ​“I just wanted to read novels all the time.” She enjoyed math, ​“but math was not quite enough connected to the real world. I have this wide-ranging curiosity where I want the answer to everything, and one of my main challenges is to focus down onto one thing instead of trying to work on everything.”

As an undergraduate at Cornell University, she saw that physics gave her a way both to connect with and answer many of the questions she had about the real world.

“I completely fell in love with experimental physics,” she said.

She earned her doctorate from Harvard and completed a postdoctoral fellowship under David Schuster, also a Q-NEXT collaborator, at Stanford University. A significant part of Q-NEXT research at SLAC takes place in partnership with Stanford University.

Her stint as a postdoc illuminated the lightning-fast progress that quantum information science had made in only a few years.

“I was amazed. All these pieces of equipment that I had spent painstaking hours in my Ph.D. building myself — now I could click and buy them. I thought, ​‘Wow. If I’d had this back then, I could have done my Ph.D. in two months,’” Harvey said. ​“That’s not exactly true, but it’s really exhilarating to be part of a community that’s moving quickly, propelling things forward. There’s so much intellectual vibrancy in quantum.”

The pace of advancements in quantum technology is not expected to let up.

“What’s great about quantum is that it’s where the action is right now,” she said. ​“Quantum computers have these far-off applications. But I think that, in a lot of ways, all these technologies that we are building are going to be the future of atomic physics and condensed matter physics no matter what. You can already see them having a big impact.”

For Harvey, the draw of quantum isn’t just its promise, but the joy of the pursuit.

“I made a lot of mistakes as a 21-year-old, but when I decided to do research in quantum — I really nailed that one,” she said. ​“I knew I would keep enjoying this for a very long time.”

This work was supported by the DOE Office of Science National Quantum Information Science Research Centers as part of the Q-NEXT center.