Quantum computing leaps into Science on Saturday

LLNL’s popular education outreach program, Science on Saturday, continued its “Computing the Future” lecture series on the last weekend of February with a presentation titled “Quantum Computing: A Cool Way to Compute the Impossible.” LLNL quantum physicists Kristin (Kristi) Beck and Sean O'Kelley (both PHYS) teamed up with veteran educator Stan Hitomi from Dougherty Valley High School (DVHS; San Ramon, CA) to give students, parents, and community members a peek into cutting-edge quantum research at LLNL. 

Everything is wave-y

Quantum mechanics can be challenging to explain, even to technical audiences, but Kristi and Sean made the concepts approachable without sacrificing accuracy. Kristi, director of the Livermore Center for Quantum Science, opened by outlining where traditional computing runs into limits. Sean, a staff scientist in the Quantum Computing Physics Group, then grounded the audience in the fundamental physical principles behind quantum mechanics. He used a simple mnemonic: “Everything is wave-y, and every wave is thing-y,” to describe wave–particle duality, the idea that matter and light can behave like waves and like particles.

He illustrated the concept with a depiction of electrons in an atom as a 3D wave. “The shape of the waves affects chemical reactions and how the particles interact with each other,” Sean said. “The waviness of electrons explains properties of matter at all scales—why something is hard or soft, what color it is, or how it works.”

Sean reinforced the idea that all objects have a “waviness” using an unlikely example: “If you take a hippo that's about one hippo mass, moving a typical hippo speed, the quantum waviness is about 10^-35 meters. In contrast, water molecules jiggling around at room temperature move only about 10^-11 meters at a time—that’s about a million, million, million times smaller.” Canonically known as the de Broglie wavelength, the quantum waviness of an object is inversely proportional to its momentum. For a swimming hippo, its mass, and thereby momentum, is so large that the quantum effects are vanishingly small. “Heavy things are classical, and very light things are quantum,” Sean summarized.

To demonstrate a unique property of waves called superposition, Sean and Kristi used audible tones. They first played a tone at 440 Hz—a standard concert A—then at 455 Hz—closer to the tuning of Beethoven’s orchestra and pianos in the late 1800s. When they played the two tones together, the sound waves added, creating constructive interference that listeners perceived as a rhythmic pulsing, with the loudness rising and falling as the waves’ relative phase shifted. Like classical bits—a quantum bit, or qubit—can be measured as 0 or 1, but before measurement can exist as a combination of both states. These states describe probabilities, and those probabilities can add or cancel the same way two musical tones do.

A human-scale Grover’s algorithm

In the second interactive demonstration, two teams of students from the Dougherty Valley High School physics club took to the stage to act out Grover’s algorithm, a famous quantum search method.

The first team of three students showed what classical computer search methods look like, with each student holding reversible 0 or 1 bit placards, stepping through the options one at a time.

The second team of students represented a quantum system. Their placards displayed the possible three-digit qubit combinations, representing a superposition of states. Each student simulated randomness by shaking a box of dice, then moving forward by the number rolled. Students whose numbers satisfied the rules received a few inches of extra distance.

Compared to the six steps required by the classical “computer,” the quantum team solved the problem in only two steps, highlighting the potential speedup that quantum algorithms could bring.

Cool enough to stay quantum

In the latter half of the talk, Kristi and Sean took the audience on a virtual tour of the dilution refrigerators in LLNL’s Quantum Design and Integration Testbed (QuDIT), LLNL’s quantum computing research laboratory. Suspended from metal struts and cocooned in a metal casing, these “golden chandeliers” cool the small chips down to a beyond-frigid 10 millikelvin so that the quantum nature of the superconducting circuits dominates.

To put the temperature gap in perspective, Sean compares it to some more familiar scales. “The surface of the sun is about 18 times hotter than a pleasant day outside. This room is about 30,000 times hotter than our qubits.”

Quantum promises, quantum challenges

From fundamentals to capability, Kristi and Sean framed the long-term promise of their work. “With three qubits, we can compute 8 states, with four we can compute 16,” Kristi said. “A quantum computer with 300 perfect qubits can simulate systems that would require an impossibly large, universe-sized classical computer.”

But both speakers agreed that many challenges still remain. Qubits can lose information, effectively “forgetting their instructions,” and performance can be disrupted by environmental noise that is difficult to eliminate.

At LLNL, researchers are exploring materials like tantalum and other fabrication improvements to make qubits more resilient, and trying to measure and understand the effects of cosmic rays that can scramble the delicate qubits.

Beyond hardware, Kristi stressed the need for more development. “We need new kinds of programming to be able to make use of quantum systems and their way of approaching all the problems that we’d like them to be able to solve,” she said.

LLNL physicists leave their mark

The talk concluded with an extended Q&A session, with students crowding around the speakers to ask about their work, and some even asking the physicists to autograph their shoes.

After the talk, Kristi reflected: “It’s really in talking about the ‘why’ behind the research I do, that I remember myself, why I'm motivated to do it, and I go back to my own work so much more energized by that.”

Sean, who patiently answered many student questions, shared his own advice. “Never be afraid to ask a question that makes you look like a fool. I think the world is more forgiving than you probably think it is, and even if it’s not—it’s worth it.”

Watch past lectures on the Science on Saturday playlist on LLNL’s YouTube channel. For more information about upcoming events, visit the Science on Saturday page.

—Grace Jeng