**Sam Roberts long had a passion for abstract maths, and thought he’d end up teaching it at university. Instead, he’s pushing the limits of linear algebra to encode data into particles of light. **

*By Wilson da Silva*

**THE AMBITION **of PsiQuantum, a Californian quantum computing company valued at US$3.15 billion, is to build a fault-tolerant quantum computer running one million qubits – a staggering 1,500 times more qubits than have ever been successfully assembled anywhere. And helping them get there is Sam Roberts, 29.

“The approach we’re taking is to use photons, and encoding information in particles of light,” he says. “We’re building integrated silicon photonic chips with optical circuits to manipulate that information and perform computations. This approach can solve a lot of headaches, particularly in scaling up the system.

“The problems people want quantum computers to solve really require this large number of qubits,” he added. “Rather than start with smaller qubit systems and building up, we’re really focused on going straight for a large-scale million-qubit machine, and the photonic platform gives us a faster way to scale up by leveraging proven semiconductor manufacturing processes.”

A qubit, or quantum bit, is the basic unit of quantum information. In modern computers, data is encoded as a bit, a binary code that is either 0 or 1. A quantum computer dramatically extends the vocabulary of binary code by using two spooky principles of quantum physics – ‘entanglement’ and ‘superposition’ – which allows qubits to be 0, 1, or any arbitrary combination of 0s and 1s *simultaneously*. This exponentially expands the number of calculations possible.

But to make qubits do their magic, you need to rely on a lot of obscure mathematics to help understand the computations, as well as guide them. And that’s where Sam comes in.

Originally from the New South Wales regional town of Bowral, Sam excelled in maths at high school but also had a deep interest in science, so he wanted to combine the two at university. He imagined that, eventually, he’d land an academic position, researching and teaching abstract maths using calculus, linear algebra, and differential equations.

Which is how he ended up completing an honours degree in advanced mathematics at the University of Sydney under Professor Stephen Bartlett, a theoretical physicist working on deep problems in quantum information theory and chief investigator at the Australian Research Council Centre of Excellence for Engineered Quantum Systems (EQUS) project on designer quantum materials.

That led to Sam to complete a maths-focused PhD in physics on many-body spin models – known as symmetry-protected topological phases – which could potentially be used in quantum data processing and storage. In it, he explored fault-tolerant properties of these phases, and proposed a novel cluster-state scheme to achieve computation on a large scale.

“I was looking at these exotic quantum systems, called topological phases of matter, and how you might use them in the design of a quantum computer,” recalls Sam. “Can these systems naturally suppress and dissipate errors? Turns out they can.”

As it turned out, this is exactly applicable to what PsiQuantum is trying to achieve. At a conference, he bumped into Terry Rudolph, a professor of quantum physics at Imperial College London and also a co-founder of PsiQuantum, who invited him to visit their labs.

“I’d never really considered industry; I had an academic bent and wanted to do a postdoc in research,” he says. “But their lab was an eye-opener – seeing the PsiQuantum team working on so many interesting problems – but with this one, singular goal that had a direct application. That was pretty amazing.”

He was offered a job, took it, and started working at PsiQuantum just before handing in his PhD thesis. And hasn’t looked back.

“Building a useful quantum computer requires a wide diversity of expertise – different types of maths, all sorts of physics, computer science and even chemistry – so you’re at the confluence of all these different fields, because there’s a bunch of different ways of solving it. There are enough problems to solve in quantum technology to keep you entertained for a lifetime. It’s been quite amazing so far, and I’m enjoying it thoroughly.”