PsiQuantum has a plan to make a massive quantum computer out of light

The Machine

The machine that could change the world will be housed in a room that looks like a data center crossed with an ice cream factory. Inside will be some 100 stainless-steel cabinets, each about six feet tall and connected to a supply of liquid helium that keeps them only a few degrees above absolute zero.

Inside those cabinets will be hundreds of chips, and on those, thousands of particles of light flying through a maze of optical switches and beam splitters. Each photon must be accounted for, because precisely measuring where it ends up will help answer questions that current computers might take millions of years to solve.

This computer, as described, does not exist. It's the brainchild of a company called PsiQuantum, founded in 2016 by four physicists from UK universities. In a crowded field of deep-pocketed competitors with similarly fantastical visions, the company aims to be first to fulfill its promise.

The Promise of Quantum Computing

In the years since the physicist Richard Feynman first envisioned them in 1981, quantum computers have promised to speed up everything from medical research to AI by harnessing the qualities of quantum particles. Unlike normal computer bits, which can be either a 1 or 0, quantum bits can exist in multiple states at once. And combining enough of those quantum bits together could produce a computer capable of tasks well beyond the reach of today's conventional machines.

But even today's best quantum prototypes are too small and error-prone to do anything useful. That makes PsiQuantum's promises for what its computers will ultimately do all the more bold.

Consider the company's hopes for predicting the effects of cytochrome P450 enzymes, which often break down drugs in the body. If pharma companies knew more precisely how they would work on a particular molecule, they could design more effective medications faster. Estimating this for a specific drug can take over 10 years with today's methods, says Philipp Ernst, vice president of quantum applications for PsiQuantum, but "we aim to get it down to four minutes."

Why PsiQuantum Stands Out

In a field full of such claims, PsiQuantum has attracted unusual investment and scrutiny for two reasons:

• It is one of the few companies aiming directly at building a large and useful machine
• It is already working with a major chip manufacturer to build its systems using existing semiconductor fabs

Its vision has attracted momentum: Last year, PsiQuantum raised $1 billion in funding and broke ground in Chicago on a site it's building in partnership with local governments. It also has a second site in the works in Australia, which it promises will be operational-meaning hardware-ready-in 2027. And it's one of just two companies (along with Microsoft) to reach the third stage of an intensive government evaluation program to see which quantum companies might succeed.

Evaluating whether PsiQuantum will do what it says is harder than, say, judging a drugmaker by its clinical trial results: Advances in quantum computing are incremental, opaque, and tough to verify from the outside. But the company is now approaching its prove-it moment, when years of closed-door work and hundreds of millions in investment will either culminate in a useful quantum computer or fall short. We could start to know which as soon as next year.

A New Kind of Machine

Terry Rudolph, one of PsiQuantum's four founders, is soft-spoken and shaggy-haired. He was born in Malawi and learned only after earning his first physics degree that he is a grandson of the famed physicist Erwin Schrödinger. He later self-published a 150-page book to explain quantum computing to teenagers (my PR contact gave me a signed copy with a wink that said "We never expect anyone to actually read this," but I can report that it is a funny and helpful book).

Around 2014, Rudolph and his cofounders became increasingly convinced that the quantum breakthroughs they were finding to be possible in theory might also be possible in a real machine. They eventually left their academic positions and divided the tasks before them:

• Terry Rudolph worked on theory
• Mark Thompson on engineering
• Pete Shadbolt on scaling the technology up
• Jeremy O'Brien on articulating the vision and finding investors (O'Brien served as CEO until February; he's been replaced by Victor Peng, a veteran of the semiconductor industry)

Why Quantum Matters

To understand why the quantum computer the company is building would be a big deal, consider how imprecise much of modern science remains. We cannot reliably predict, for example, which lithium-ion battery will catch fire or how quickly a critical aircraft component will corrode.

This isn't just because these systems are complex, though they are. It's that, at their core, they are governed by quantum mechanics. Subatomic particles don't have well-defined properties-this location and that velocity-but instead occupy quantum states spread across many possibilities. And that in turn influences a range of atomic and molecular behavior.

Schrödinger (Rudolph's grandfather, remember) showed how to describe this haziness mathematically a century ago this year, but precisely carrying out the calculations on real-world systems quickly becomes unfeasible even for the best computers. Scientists cope with this gap using approximations, imperfect simulations, or experiments on animals.

Feynman, David Deutsch, and other physicists in the 1980s wondered if we could do better. Maybe such complexity could instead be modeled using a new kind of machine. Rather than using transistors that are only ever on or off, this one would use particles held in quantum states, manipulate them to perform calculations, and then measure them at the end for an answer.

Using quantum systems to simulate quantum systems would for the first time allow a simulation of physics and chemistry that directly reflected reality. It would be an invaluable tool for designing new drugs, materials, or really anything affected by quantum mechanics. Revolutionary, in other words.

Humankind's leaps in understanding how nature works have often resulted in the invention of powerful new tools, Rudolph told me. "I don't think it's a coincidence that the Industrial Revolution coincided with our ability to calculate and simulate the laws of Newtonian mechanics, the laws of thermodynamics,...the laws of classical electromagnetism," he says. "Whenever we have more power to calculate and simulate and understand things, we build incredible machines that come from it." He sees something similar coming with quantum computers.

Chasing Photons

One mystery has always been which quantum thing-ions, atoms, or something entirely new engineered with quantum properties-could be made stable and controllable enough to use as a qubit, the basic unit in the quantum computing world.

Quantum systems are delicate, and observing any particular particle causes it to collapse into one state rather than a superposition of multiple states. If this happens during the computation rather than at the end, it produces an error that must be corrected for. Too many of these means the computer fails to produce a useful answer.

Just as engineers in the early days of aviation weren't sure whether airplane wings would be fixed or flap like a bird's, we're not yet sure which of these quantum things will work best:

• Google and IBM are betting on superconducting qubits, superconducting circuits made of aluminum or other metals
• Intel is using electrons
• PsiQuantum is using photons, the particles that make up light

"Photons have lots of nice things going for them," Rudolph says. They can maintain quantum states for a long time; indeed, the photons in the universe's cosmic microwave background may have done so for billions of years. But photons also move fast and scatter easily. More importantly, two photons are more likely to pass through one other than interact. That makes them a challenging candidate for quantum computation, in which qubits need ways to influence one another.

For a while, this last flaw seemed to doom the idea of quantum computing with light. But in 2001, researchers from the Los Alamos National Laboratory and the University of Queensland found a loophole. They discovered they could essentially fake interactions between photons by sending the light particles through a network of beam splitters and detectors. Their paper changed everything.

PsiQuantum was created to make the theory a reality. Size was the first problem; previous plans would have required a computer as large as California. Mercedes Gimeno-Segovia, who was a PhD student of Rudolph's in the early 2010s (after almost becoming a professional violinist instead), thought of a way for the machine to be smaller.

The basic process since then has been this:

1. First create photons with lasers and then "entangle" them, exploiting a quantum phenomenon in which the particles no longer have individual states but instead share one
2. Next, route them through a maze of gates that perform computations
3. Finally, read out details of their quantum state at the end, all while tracking and correcting for the errors that occur

Succeeding at each of these steps millions of times is not so much an engineering hurdle as a brick wall. And building the supply chain-like manufacturing new materials with the qualities to route individual photons around-is arduous.

Inside the Lab

To get a sense of it all, last year I joined Shadbolt at the SLAC National Accelerator Laboratory, in Menlo Park, California. The center has helped produce several Nobel Prizes and played a role in the 1968 discovery of quarks, fundamental building blocks of matter that make up protons and neutrons. But PsiQuantum set up shop there essentially to siphon liquid helium from SLAC's giant cryoplant.

This is what the company uses to cool its computing cabinets down to deep-space temperatures. Right now the cabinets operate at 2 K, or -456 °F, but the goal is to be able to run them slightly warmer-at a balmy -452 °F.

Most quantum approaches require the whole machine to be cooled to superconducting temperatures, so that much of the expense in running it will actually be spent on refrigeration. But photonic computers require only one piece to be this cold-the detectors that measure single photons at the end of the computation. And the required temperature can be a bit higher. (PsiQuantum said in May that it will spend some of the $100 million award in CHIPS Act funding it's slated to get on these detectors.)

The siphoning setup was a temporary solution; PsiQuantum now has its own cooling system at its testing facility in Milpitas, California, and is setting up a larger one at its production site in Australia next year. These helium systems represent some of the biggest capital expenditures for any quantum company and will consume a significant chunk of PsiQuantum's $1 billion funding round.

In the afternoon we drove to a lab in San Jose, where I donned a cleanroom suit-a head-to-toe covering that keeps dust at bay-to watch the manufacture of a blueish crystal called barium titanate. It's prized by PsiQuantum because it quickly and reliably routes light particles with very little electrical input, keeping the precious photons undisturbed as they move through the circuit.

But for all barium titanate's theoretical value to the company, its structure makes it a pain to manufacture, and the material wasn't available at scale when PsiQuantum got its start. The company, in what Rudolph told me was an agonizing decision, opted to make it in-house, requiring a massive investment.

I saw a technician-operating at what looked like a giant pressure cooker-adding the base elements to several hoppers; then I watched through a porthole as the elements got heated, vaporized, and finally crystallized into a thin layer on a wafer disc. At that time each disc took about 12 hours to make; the company now says several are produced each day.

The discs then get shipped to the chipmaker GlobalFoundries in Malta, New York, where PsiQuantum's chips are made.

The Manufacturing Bet

PsiQuantum's bet is that this entire supply chain, byzantine as it might sound, will make the company more efficient than its competitors. That's because, if you squint, it looks like a souped-up and high-precision version of the existing supply chain for silicon photonic chips, another type of technology that transmits information with light-one that's already used in data centers. If PsiQuantum produces its chips at scale, it can take advantage of tools and infrastructure that already exist.

But it's not a given that one working chip can easily be wired up to thousands more. That's why the company is testing in phases:

• Its Milpitas site has connected three cabinets together, with 250 chips in each
• The next step is to scale the systems up and see whether the company's techniques for correcting errors can keep up
• Once the cooling system arrives at the Australian site late next year, the company says, it aims to connect about 100 cabinets together

Then PsiQuantum will work up to running the world-changing algorithms it has promised.

The Timeline

The timeline for this, it's worth noting, is up for debate. News articles have said that 2027 is the year that PsiQuantum aims to have its first full-scale quantum computer come online at its Australian site, but the company insists the deadline has been misread, and that it only intends for its facility to be "operational" by the end of next year. That means cooling systems in place and hardware ready-but not necessarily a fully functional quantum computer running useful algorithms.