This billion-dollar firm plans to build giant quantum computers from light. Can it succeed?

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.”
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.”
PsiQuantum is not the only company pursuing quantum computing using light. Most firms plan to eventually scale up their systems by entangling devices using photonic qubits, so mastering them now makes sense, says Senellart. None is close to PsiQuantum’s billion-dollar backing. The company Xanadu, in Toronto, Canada, uses as its qubits ‘squeezed’ states, which encode information into the electromagnetic field of multiple photons. It has created a machine of more than 200 qubits and has raised a few hundred million dollars in funding. Quandela, which has raised $67.5 million, sells small machines that use on-demand sources of single photons, to boost efficiency and reduce a computer’s size. And ORCA Computing, based in London, is developing a way to store single photons as a short-term quantum memory; it has raised at least $15 million.
PsiQuantum’s physicists say they have the advantage of building on the foundations of two existing mega industries: semiconductor manufacturing for computers, and photonics, which creates the fibre optics used in telecommunications cables. The firm’s chips — made in partnership with US semiconductor giant Global Foundries — combine conventional technology such as beam splitters with components such as single-photon detectors and sources, which are rarely used outside laboratories. These are all etched onto silicon wafers, where they manipulate telecoms-frequency photons.
“In a way, we’re taking existing technologies and making them behave quantum mechanically, as opposed to inventing completely new technology, then trying to figure out how to scale it,” says Mark Thompson, one of PsiQuantum’s co-founders and its chief technologist.
For its system to work, the company needs millions of precision electronic components that must operate at unprecedented efficiency and can’t simply be taken off the shelf. Detectors, for example, need to be chilled to around 4 kelvin using liquid helium. The biggest challenge, says O’Brien, has been creating optical switches to divert photons into calculations. For this, the company built its own facility to grow high-purity wafers of barium titanate, a material that can steer light efficiently. The firm spent a vast amount of money and made a big bet, says O’Brien: this leap of faith is one of the things he is proudest of.
Outsiders must take O’Brien and his colleagues largely at their word that they have solved such challenges. The firm, which once had a reputation for being secretive, is starting to open up. In April, it published a preprint outlining its hardware platform, which White calls jaw-dropping (K. Alexander et al. Preprint at arXiv https://doi.org/gtzxqj; 2024). It shows efficiencies, such as in getting light off a chip into a fibre, that are “well ahead of what the best university labs could do”, he says.
Some of the demonstrations in the paper are “extremely impressive”, says Senellart. But others are missing, such as the rate of photons produced per second, she adds. What’s more, some of the numbers are not as good as they need to be, says Graeme Smith, a quantum physicist at the University of Waterloo in Canada. For example, he says, the likelihood that a heralded particle is detected is reported as 26%, when it needs to be more than 50%. “It is not encouraging that, after many years, they are still struggling with good single photon sources, since it is the most basic building block of their proposed architecture,” he says. Any photonic quantum computer will need to reduce photon losses to levels never previously demonstrated at scale, says Stephanie Simmons, a quantum physicist at Simon Fraser University in Burnaby, Canada.
In response, Thompson says the team’s papers are “just the tip of the iceberg” of what PsiQuantum has achieved. Commercial secrecy might be one reason that the team is not publishing its best work. “To publish in academic journals is very time-consuming and quite a distraction,” he says.
But PsiQuantum’s approach also feels like a gamble, says Senellart, because it has opted not to publicly show small demonstrations, often called noisy intermediate-scale quantum (NISQ) devices. “Personally, I would sleep better” with the incremental approach, she says. If the firm can manage without, “it’s just amazing”.
Missing out this stage means no working prototypes are available, and has irked academics who like to see concrete milestones. With no NISQ computer, the firm has been less active than some at recruiting end users or software partners, says Finke, adding to its air of mystery. White says: “Publicly, they haven’t demonstrated in their papers anything other than entanglement between a couple of photons.”
O’Brien says this is because proving a computer works with, say, 100 qubits, reveals nothing about whether it will scale to one million. Academic demonstrations tend to use shortcuts that wouldn’t fly at scale, adds Pete Shadbolt, one of the firm’s co-founders and its chief scientific officer. And the dearth of successful applications for small, noisy systems justifies the company’s strategy, says O’Brien. “We always knew that a useful quantum computer is going to be a big machine.”
Indeed, the US physicist who coined the term NISQ, John Preskill, said last year that even he thought that no useful applications had emerged from the NISQ era.
The company says it is building internal prototypes of increasing scale and complexity, but doesn’t market them as quantum computers. The goal is to find where the engineering needs to be improved, rather than running algorithms to solve small-scale problems, says Thompson. In the United Kingdom, the firm has constructed prototype cabinet-sized devices, about 2 metres tall, which include cryogenic cooling equipment and many of the necessary computing components; larger ones will come online in the United States in 2025, says Shadbolt. Ultimately, PsiQuantum’s computer would involve in the order of 100 such devices, says O’Brien — around the size of a warehouse.
In response to questions about PsiQuantum’s progress and plans, spokesperson Alex Mack said that they have been thoroughly scrutinized by funders. And the company has a lot of success to show in that arena. In the past year, it has raised more than $1 billion in loans, equity and grants from governments in Australia and the United States, in exchange for building its first two quantum computers there: the first in Brisbane by 2027 and the next in Chicago, Illinois. In convincing funders, O’Brien’s combination of technical savvy and communication skills is an asset, says White. “You talk to his detractors, and you talk to his fans alike, and the thing they all land on is that he is unstoppable.” Working among investors from California’s Silicon Valley — where $1 billion isn’t seen as a huge amount — also helps, says Senellart.
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However, PsiQuantum’s large public investments have raised eyebrows. In Australia, the National Audit Office is considering reviewing PsiQuantum’s US$620-million package from federal and state governments because of allegations that funders ran a system that lacked transparency and fair competition. Documents released under freedom of information requests made by Australian shadow science minister Paul Fletcher showed that PsiQuantum met at least twice with the Australian government to discuss an unsolicited bid. This occurred months before a handful of other firms, who were ultimately unsuccessful, were invited to pitch for funding.
This August, Fletcher told Sky News that the process was “essentially a backward engineered sham” to cover up that the government had already made the decision. (Australia’s federal government didn’t reply to Nature’s request for comment. Queensland’s state government — which changed political hands after an October election — has criticized the previous state administration for the deal, and says it is investigating.) Some researchers in Australia have also expressed concern about a country investing so much into a single company, although the government has funded other quantum-computing firms.
Mack confirms that PsiQuantum originally made an unsolicited pitch to the Australian government. “A lengthy diligence process followed,” he says, adding that Australia’s chief scientist led the process. He adds that much of the firm’s government funding is contingent on “successful execution” of milestones, such as its prototypes.
As one sign of support for PsiQuantum, it has withstood the scrutiny of around 50 specialists at the US Defense Advanced Research Projects Agency (DARPA). In 2023, DARPA selected PsiQuantum and Microsoft to advance in a programme investigating whether an underexplored approach to quantum computing can achieve “utility-scale operation”. The agency examined chips and analysed PsiQuantum’s plans, says Shadbolt. Under DARPA’s close inspection, “all the dirty laundry has to come out”, adds Simmons, who has experience of DARPA initiatives through her firm Photonic, based in Vancouver, Canada.
A DARPA document on its programme states that the projects are plausible but not guaranteed. Like anyone working on an evolving technology, PsiQuantum might find that some tasks are impossible or that a competitor can do something better or cheaper, says Finke. For White, even a failure would be a win, because, he says, the advances the firm is making will boost the wider photonics industry.
A final question is whether a completed machine will do what PsiQuantum promises. Lu worries that the firm, and some other quantum-computing start-ups, are making bold claims that are fanning inflated expectations. “There is a growing concern,” he says. Quantum computers offer potential advantages over classical machines, but “nobody really knows if quantum computers will help make better battery cells or design new pharmaceuticals”, adds Smith.
True to form, O’Brien remains confident. With enough logical error-corrected qubits carrying out enough operations, it will be possible, he says, to answer questions of profound value that would otherwise be unanswerable. “I think the utility of a quantum computer is unequivocal.”