A Canadian startup called Xanadu has built a new quantum computer it says can be easily scaled up to achieve the computational power needed to tackle scientific challenges ranging from drug discovery to more energy-efficient machine learning.
Aurora is a “photonic” quantum computer, which means it crunches numbers using photonic qubits—information encoded in light. In practice, this means combining and recombining laser beams on multiple chips using lenses, fibers, and other optics according to an algorithm. Xanadu’s computer is designed in such a way that the answer to an algorithm it executes corresponds to the final number of photons in each laser beam. This approach differs from one used by Google and IBM, which involves encoding information in properties of superconducting circuits.
Aurora has a modular design that consists of four similar units, each installed in a standard server rack that is slightly taller and wider than the average human. To make a useful quantum computer, “you copy and paste a thousand of these things and network them together,” says Christian Weedbrook, the CEO and founder of the company.
Ultimately, Xanadu envisions a quantum computer as a specialized data center, consisting of rows upon rows of these servers. This contrasts with the industry’s earlier conception of a specialized chip within a supercomputer, much like a GPU.
But this work, which the company published last week in Nature, is just a first step toward that vision. Aurora used 35 chips to construct a total of 12 quantum bits, or qubits. Any useful applications of quantum computing proposed to date will require at least thousands of qubits, or possibly a million. By comparison, Google’s quantum computer Willow, which debuted last year, has 105 qubits (all built on a single chip), and IBM’s Condor has 1,121.
Devesh Tiwari, a quantum computing researcher at Northeastern University, describes Xanadu’s progress in an analogy with building a hotel. “They have built a room, and I’m sure they can build multiple rooms,” he says. “But I don’t know if they can build it floor by floor.”
Still, he says, the work is “very promising.”
Xanadu’s 12 qubits may seem like a paltry number next to IBM’s 1,121, but Tiwari says this doesn’t mean that quantum computers based on photonics are running behind. In his opinion, the number of qubits reflects the amount of investment more than it does the technology’s promise.
Photonic quantum computers offer several design advantages. The qubits are less sensitive to environmental noise, says Tiwari, which makes it easier to get them to retain information for longer. It is also relatively straightforward to connect photonic quantum computers via conventional fiber optics, because they already use light to encode information. Networking quantum computers together is key to the industry’s vision of a “quantum internet” where different quantum devices talk to each other. Aurora’s servers also don’t need to be kept as cool as superconducting quantum computers, says Weedbrook, so they don’t require as much cryogenic technology. The server racks operate at room temperature, although photon-counting detectors still need to be cryogenically cooled in another room.
Xanadu is not the only company pursuing photonic quantum computers; others include PsiQuantum in the US and Quandela in France. Other groups are using materials like neutral atoms and ions to construct their quantum systems.
From a technical standpoint, Tiwari suspects, no single qubit type will ever be the “winner,” but it’s likely that certain qubits will be better for specific applications. Photonic quantum computers, for example, are particularly well suited to Gaussian boson sampling, an algorithm that could be useful for quickly solving graph problems. “I really want more people to be looking at photonic quantum computers,” he says. He has studied quantum computers with multiple qubit types, including photons and superconducting qubits, and is not affiliated with a company.
Isaac Kim, a physicist at the University of California, Davis, points out that Xanadu has not demonstrated the error correction ability many experts think a quantum computer will need in order to do any useful task, given that information stored in a quantum computer is notoriously fragile.
Weedbrook, however, says Xanadu’s next goal is to improve the quality of the photons in the computer, which will ease the error correction requirements. “When you send lasers through a medium, whether it’s free space, chips, or fiber optics, not all the information makes it from the start to the finish,” he says. “So you’re actually losing light and therefore losing information.” The company is working to reduce this loss, which means fewer errors in the first place.
Xanadu aims to build a quantum data center, with thousands of servers containing a million qubits, in 2029.
