Robert Cava: Still Creating After All These Years

When Robert Cava, the Russell Wellman Moore Professor of Chemistry, goes on sabbatical next month, he will do so in the wake of an extraordinary, seemingly off-the-cuff suggestion for improving a quantum computing device that collaborators in the Department of Electrical and Computer Engineering are working on.

ECE Associate Professor Nathalie de Leon and Professor Andrew Houck were fabricating a transmon qubit—a type of superconducting qubit used in quantum computers—out of a material that had several limitations. Houck and everyone else in the field had been using niobium, a rare metal found in everything from wind turbines to the steel frames of skyscrapers. But it yielded a device that the engineers suspected could be improved.

So Cava, a solid state chemist, went one row down on the periodic table to another rare element, suggesting they try tantalum. This ductile, resilient metal always occurs geologically with niobium but it has its own distinct properties. Cava thought those might lead to a better device.  

As it happens, he was right.

When researchers fabricated the qubit using tantalum as a base material, they reported results wherein the least successful outcomes with tantalum were several times better than the best with niobium. Their investigation into the improvements was published in Advanced Science last spring, an advance that has already broadly impacted the field.

Cava chalks his successful “guess” up to his expertise in Group 5 transition metals like tantalum, niobium, and vanadium. But he also credits the skill of spontaneous association, even a bit of creative magic, which has allowed him to generate solutions like this one over his long and fruitful career.

“The way my brain works is, I’ll be just listening to some kind of talk and something will pop into my head,” said Cava. “I think I was listening to Nathalie give a presentation. She’s a good communicator and I really enjoy her talks. That’s when this idea occurred to me. I made a lucky guess with tantalum. It just came into my brain. And thankfully, Nathalie and Andrew listened.

“I have a lot of experience with the periodic table. That’s my playground. And that column is one I’ve worked on a lot. So there’s vanadium and niobium and tantalum, and they all have a certain number of electrons, and as you go down the table things get less reactive. So I thought this might be a good choice.

“By the way, Nathalie does a great impression of me saying, ‘Why the hell aren’t you using tantalum?’ ”

For her own part, de Leon credits Cava with having “a wonderful intuition for what is possible.”

Next Challenge, Quantum Computing

Cava regards the world with a wry detachment, so it’s no surprise that he views himself through the same lens. When he questions the value of his ideas in informal conversations—“some of my ideas are good, some are not”—he’s not exhibiting false modesty. He’s just assessing scientific process reflected in a 40-year career in materials science that began at Bell Labs back in 1979. He joined the faculty at Princeton Chemistry in 1996.

The transmon qubit being developed in ECE is just one piece in the vast landscape of technology that may eventually go into the world’s first working quantum computer. But it’s an essential one, influencing the computational coherence time, or the amount of time that a qubit is able to express its information.

Cava says that most of this is beyond his concern. He is celebrated for discovering dozens of new superconducting compounds and among the first topological insulators. But by his own admission, he is less interested in how things are made than in what they’re made of.

That approach led to a new chapter that underscores his ongoing creativity and contributions to fields both within his area of expertise and outside of it. He was appointed to the Co-Design Center for Quantum Advantage (C²QA) three years ago by the Department of Energy. C²QA is headquartered at Brookhaven National Laboratory in Upton, New York, and Houck is its director. De Leon leads the consortium’s materials science thrust, through which Cava contributes.

Cava was initially surprised at his designation as one of the principal investigators. But later, he allowed that even quantum computers have to be made out of something.

“We have to connect our little part to a bigger world,” Cava said at the time. “The good news for me is, I’m curious to learn about new things. Especially if it’s something I don’t completely understand.” He added that curiosity in what’s coming next and in new things plays a big role in his continuing collaborations and projects.

Cava and de Leon first began working on the fabrication of qubits through a National Science Foundation Research Advanced by Interdisciplinary Science and Engineering (RAISE) proposal on alternative materials for superconducting qubits. Their funding was awarded several years ago.

“We invited Andrew to our kickoff meeting and had him explain to us what was known about the state-of-the-art and what all the material interfaces are,” said de Leon of the RAISE proposal.

“I pointed out that no one has tried cleaning these devices after fabrication, and hydrocarbons are probably a big problem. Bob pointed out that the oxides of niobium are very complicated and difficult to control. So tantalum was a possible alternative because it has a well-behaved oxide. We discovered later that it was an especially good material.

“We think the main two advantages to tantalum are, one, it oxidizes quickly in a way that is self-limiting,” she added. “Just like how aluminum cans don’t rust, tantalum forms a very thin, high-quality oxide and then stops oxidizing. Two, it is very resilient in that you can put it through harsh chemical treatments without any problems, unlike aluminum.”

Cava also thinks it’s possible simply that no one had thought of tantalum before.

“My opinion is that talk is always cheap, and if I just had the idea that tantalum should work better, that’s one thing. It’s another thing altogether to actually do it. Andrew and Nathalie actually did it. And I was shocked when it worked,” he said.

“I think the interesting thing here is that they care about something else than I care about. They care about the design for this device and how you make it, and the process for making the device. I don’t know anything about that, but I do know that what you make it out of matters. Right now, they’re tantalizingly close to being able to make a good quantum computer with this stuff.

“Andrew gave a talk that I went to a few weeks ago,” he added. “After I left, I thought of four more things to try.”