March 17, 2022

Article at Sydney Quantum Academy

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The joy of knotty problems

Rachpon Kalra
Rachpon Kalra

Sometimes all it takes is just one person to set your career path in a different direction. That’s what happened to Rachpon Kalra.

By Wilson da Silva

“HONESTLY, I started electrical engineering at university because I enjoyed math and physics in high school, but I thought I’d end up getting an MBA and go into finance or consulting,” says Rachpon Kalra, 32. “But then I did an internship with Prof Andrea Morello, and he was just amazing. I guess you could say I got infected by the research bug.”

Morello, Scientia Professor of quantum engineering at UNSW Sydney, is not what you expect when you think of quantum computing. Fond of colourful shirts and sporting a ponytail, he talks with contagious energy and is widely known as an inspiring science communicator. And he’s also a global leader in quantum research, winning the first ever Landauer Bennett Award in Quantum Computing, a prestigious international award.

Rachpon, born and raised in Thailand to a family of Indian textile merchants, spent three months over summer in a research program at UNSW Sydney. In between his second and third year at UNSW, he joined Prof Morello in a program very similar to the Sydney Quantum Academy’s Undergraduate Research Summer Program. “He’s a really good communicator in the way he explains everything,” recalls Rachpon. “His enthusiasm is really infectious. And the lab is super cool, with incredible equipment you get to use and play with, which kind of drew me in.

“The thing I enjoyed most, and continue to enjoy, is not just what quantum computing will deliver in the future, which is super exciting. It’s the day-to-day challenges and puzzles, trying to figure things out. Andrea encouraged this: he had a very rigorous approach he wanted us to take, putting pen to paper and working things out. ‘There’s no black box,’ he’d say – ‘Write it out, make sense of it, really rigorously, from the bottom up.’ And that really appealed to me.”

From there, Rachpon went on to do a PhD at the ARC Centre of Excellence for Quantum Computation and Communication Technology (CQC²T). Working under Morello’s supervision, Rachpon focused on enhancing single qubit operations in silicon to a fidelity of 99.9% while, at the same time, achieving record coherence times; this helped solve crucial problems on the roadmap towards a full-scale quantum processor in silicon.

It was exciting stuff, and led to a postdoctoral research fellowship at the ARC Centre of Excellence for Engineered Quantum Systems (EQUS) in Brisbane in 2016. There, he explored hybrid quantum devices consisting of microwave, optical and mechanical resonators that could be used for precision sensing.

“The initial vision was that it might become a component of a quantum internet,” Rachpon recalls. “Qubit signals in the form of microwave photons would be converted into optical photons that could be sent down an optical fibre. We were working on the device in the middle, the transducer based on a mechanical resonator. Turned out it wasn’t quite suited for that, but we ended up doing other cool things with it.”

Now a senior hardware research engineer at Microsoft, Rachpon works at Station Q, a partnership between Microsoft and the University of Sydney, based at the university’s Nanoscience Hub. It is one of four research labs in the world – the others in the United States, the Netherlands and Denmark – leading Microsoft’s effort to create a scalable general-purpose topological quantum computer.

The approach relies on two-dimensional quasiparticles know as ‘anyons’, whose paths in four-dimensional spacetime pass around one another to form ‘braids’. These ‘quantum braids’ form the core ‘logic gates’ of a topological quantum computer. Topological qubits are more stable because natural quantum fluctuations – which often make quantum states collapse or introduce errors in other designs – do not alter the braids, which retain their topological properties.

“It’s really deep physics, based on a new state of matter,” Rachpon smiles. “Sort of like tying a knot. And whether we tie a knot this way or that way, it is robust, so that when we shake the rope, it doesn’t change the direction of the knot.”

These days, Rachpon is still absorbed by knotty problems, and inspired day-to-day by another charismatic physicist, University of Sydney’s Prof David Reilly, director of Microsoft Station Q Sydney, another member of Sydney Quantum Academy. “There’s a bucketload of challenges to actually building this thing. It’s such a huge project, and I really like working towards that meaningful goal.”