Scientists have linked two silicon quantum bits with photons over a relatively large distance. The new advance could end up being a watershed moment for a lesser-known quantum computing processor architecture, bringing silicon quantum computer a step closer to reality.
Quantum computers represent a nascent computing technology that could one day perform certain calculations like modelling the behaviour of molecules faster than a regular computer can. Today’s most popular quantum computer are constructed from superconducting wires held at extremely cold temperatures or atoms in a laser trap. But other scientists are developing devices made from other materials, like semiconductors.
Once they mature, quantum computers’ advantages over classical computers will come from their architecture. Unlike classical computers which compute by abstracting problems into two-state bits that communicate via the rules of logic, quantum computers’ basic component is a quantum bit or qubit. Qubits are highly-controlled atoms or artificial atoms that take on two states like bits do but interact with the richer mathematical rules of quantum mechanics. The two most popular qubit architectures today are the transmon qubit, a superconducting circuit that represents the qubit states based on different energies of an oscillating current, and the trapped ion qubit, where lasers manipulate the atomic states of ions in an array.
But a semiconductor-based “spin” qubit instead represents information based on the spin states of electrons trapped in a silicon semiconductor where an electrons’ spin states are analogous to a bar magnet pointing up or down. Hitting the electrons with microwaves manipulates those states. These devices have a few presumed advantages—mainly, that silicon is already a popular material in computing, so fabricating a silicon quantum computer might be cheaper. And a spin quantum computer might be able to operate at higher temperatures than superconducting quantum computers, which require a cryogenic environment.
Gizmodo reported back in 2018 that researchers working with Intel had successfully tested and performed algorithms on a two-spin qubit device. The 2018 paper foreshadowed what the researchers achieved in their newest research, published in Nature: they got two spin qubits to interact with each other over four millimetres, a relatively long distance by microchip standards, by exchanging photons through a small cavity.
This proof-of-principle experiment realises another important advantage of silicon quantum computers: spin qubits don’t just have to talk to their neighbours—a limitation of some superconducting qubits—but instead might be able to interact with qubits on the other side of the microchip.
“It adds substantial flexibility in how to wire those qubits and how to lay them out geometrically in future silicon-based ‘quantum microchips,’” Thaddeus Ladd, senior scientist at HRL Laboratories and a collaborator on the project, said in a Princeton press release. Jelena Vuckovic, professor of electrical engineering at Stanford University not involved in the study, said it’s an “important milestone” for this technology in the same release. But there’s more work on the horizon before this research will be useful.
“In the future, it will be very exciting to see if these results lead to coherent interactions and the transfer of quantum information between distant spin qubits,” John Nichol, assistant professor of physics at Purdue not involved in the study who works on spin qubits, told Gizmodo in an email.
Despite their promise, spin qubits still lag behind superconducting and trapped ion qubits in their development and adoption. But with more advances like these, we might see them grow in use and popularity soon.