A prototype cryogenic cabinet in which PsiQuantum is testing quantum computing using light in silicon chips.Credit: PsiQuantum
During his years in academia, Jeremy O’Brien says he had “precisely zero ambition to go into business”. Yet today, the Australian physicist runs a private quantum-computing firm that venture-capital funders and governments have rushed to invest in — and which is making some of the field’s boldest promises.
In a whirlwind nine years since O’Brien founded PsiQuantum with three academic colleagues, the company has quietly raised more than US$1 billion and values itself at more than $3 billion — meaning that its coffers probably rival those for internal quantum-computing efforts at Google or IBM. In the past year alone, PsiQuantum, which has 350 staff and is based in Palo Alto, California, has scored major investments from governments in Australia and the United States, adding to previous private funding rounds. “They have received one of the biggest venture-capital investments in the quantum community,” says Doug Finke, a computer scientist in Orange County, California, who works at the business-analysis firm Global Quantum Intelligence.
All that investment is chasing an audacious goal: using light in silicon chips to create a giant programmable quantum computer that can outperform classical machines — and to do it soon. By the end of 2027, the firm’s researchers told Nature, PsiQuantum aims to be operating a photonic quantum computer that can run commercially useful problems and is ‘fault-tolerant’: that is, it makes computations possible by correcting for the errors that are inherent in these fragile systems. If they succeed, this would put the firm ahead of its major rivals and leapfrog researchers doing toy problems on small-scale quantum computers.
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Yet compared with its competitors, PsiQuantum has shown very little. Rather than building up gradually, as others have done, by debuting systems of tens or hundreds of quantum bits, PsiQuantum is aiming to jump to a machine that will require something in the order of one million qubits. (PsiQuantum’s researchers haven’t published a specific number.) To do that, it will need to overcome technical challenges it has not proved it can solve, says Chao-Yang Lu, a physicist working on photonic quantum computing at the University of Science and Technology of China in Shanghai. He is one of several scientists who worry the firm is promising things it will struggle to deliver.
“My impression is there’s a lot of scepticism about how much progress PsiQuantum has made,” says Shimon Kolkowitz, a quantum physicist at the University of California, Berkeley. He calls a bet on them “extremely high risk”.
PsiQuantum researchers say the firm has achieved more than it has publicly shown, and that funders have scrutinized its plans. O’Brien himself talks about challenges in the past tense and insists that there is little doubt of success. And some independent researchers see its plans as being at least plausible. “I think it’s an amazing gamble,” says Pascale Senellart, a quantum optical physicist at the French National Centre for Scientific Research in Palaiseau. “It’s really worth exploring.”
Flying qubits
PsiQuantum’s approach is radically different to that of some major rivals (see ‘Comparing quantum computers’ at the end of this article), because of its choice of qubit — the basic unit of quantum information. Whereas the binary digits (bits) of classical computers encode either a 1 or a 0, qubits can be put into a ‘superposition’ — existing in two states at once, a combination of both 1 and 0, with a chance of being measured as either. Calculations come from ‘entangling’ these qubits, meaning that their quantum states become intrinsically linked and interdependent. To prevent errors from destroying the calculations, a quantum computer will need around 10,000 physical qubits working together to make each useful ‘logical’ qubit, O’Brien says. With a few hundred of these, researchers hope that quantum computers will be able to perform complex calculations, such as modelling chemical processes at the quantum level, that would be much too difficult for a classical machine.
Many firms in the field make their qubits from atoms, ions or tiny rings of a superconducting material — in each case, a physical object that has some mass and is often fixed in place. But PsiQuantum is one of a handful of companies that uses massless particles of light, or photons — sometimes known as flying qubits.
The idea to use light as a qubit isn’t new. In the early 2000s, optical quantum computing was one of the first platforms to be explored experimentally. Some of PsiQuantum’s founders were involved in the field’s birth, says Senellart, who is a co-founder of Quandela, a firm based near Paris that makes photonic quantum computers.
Making a quantum computer with light is “on paper, quite easy”, she says. PsiQuantum creates qubits by using an optical device called a beam splitter to send a single photon simultaneously down two routes (known as waveguides) etched into silicon. Because photons have no charge or mass, they are largely unaffected by their surroundings. This means that, even at room temperature, photon-based qubits are insensitive to many types of noise that plague rival hardware. This ability to maintain quantum information and travel long distances at speed makes it easy to build big and fast systems. “That’s a huge asset,” says Senellart.
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But photons also come with hurdles. It is hard to generate single, near-identical photons on demand. They are readily absorbed and lost. And getting the flighty particles to interact is a challenge. Although light waves do interfere with each other, that kind of interaction alone is not sufficient for constructing multi-qubit gates, in which qubits entangle to form basic logic operations.
To generate photons, the firm pumps laser light through silicon. These sources are probabilistic: they produce pairs of photons perhaps once in every 20 attempts. Having a pair is necessary because the spare, or ‘herald’, photon provides a heads-up that allows the computer to use the surviving photon.
With this strategy, each chip needs many such sources, as well as super-efficient waveguides and optical components that can handle photons without losing them.
To perform logic operations, PsiQuantum first builds up clusters of entangled photon qubits by bringing photons together so that their light waves interfere, then making measurements on some of them in ways that entangle the remaining qubits. Calculations then occur by performing a succession of such measurements on pairs of photons from different clusters. Those measurements destroy the pairs but entangle their clusters, a technique known as fusion-based quantum computing (see ‘Quantum computers with ‘flying qubits’’). Senellart says that PsiQuantum has an impressive team working on developing the theory behind such a computer. “They are coming up with a lot of smart schemes,” she says.
PsiQuantum’s approach continually generates and destroys photons, with each qubit needing to exist only for long enough to be entangled or measured with another, not for the duration of the calculation. And although each operation involves an element of chance and photons will get lost, failures are detectable as part of the measurement, says Mercedes Gimeno-Segovia, a physicist at PsiQuantum who is working on the computer’s architecture.
“It’s an incredibly forgiving way of doing quantum computing,” says Andrew White, an optical physicist at the University of Queensland in Brisbane, Australia, and a former academic colleague of O’Brien. “You can take very high error rates and still have it scale.”
