First fully programmable quantum computer based on neutral atoms

Most quantum computers are based on superconductors or trapped ions, but an alternative approach using ordinary atoms may have advantages

A quantum computer that uses ordinary atoms to perform calculations could be a rival to more exotic devices, although one of its creators says there are still challenges ahead in scaling up the technology.

The most powerful quantum computers in use today rely on superconductors or trapped ions to form the basis of their qubits, or quantum bits. Both these systems have drawbacks: superconducting qubits, like those used by Google, require ultracold temperatures, while it is hard to arrange trapped-ion or superconducting qubits so that all of them can communicate with each other.

Now, Mark Saffman at the University of Wisconsin-Madison and his colleagues have built an alternative quantum computer using six qubits made from neutrally charged caesium atoms, as opposed to charged ions.

The atoms are trapped in a grid with lasers, spaced far enough away from each other that they don’t interact. But when individual atoms are excited by a laser shining at the right frequency, their orbiting electrons move so far from their parent atoms that they can quantum entangle with their neighbours – a key phenomenon for a quantum computer.

This two-dimensional structure offers an advantage compared with the set-up of trapped-ion machines, which are normally configured in a line to avoid unwanted interactions between the charged particles, limiting their ability to communicate.

“Because it’s all done with laser beams, you can actually reconfigure the positions of all your qubits,” says Charles Adams at Durham University, UK, who wasn’t involved in the work. “So if you decide you want to run a different algorithm with different connectivity between the qubits, you can just reprogram where the qubits are.”

Certain algorithms are difficult to run on trapped-ion or superconducting quantum computers because they require a high amount of connectivity between qubits. One example is phase-estimation algorithms used in quantum chemistry, which measure how the state of a quantum system evolves over time. Such algorithms might prove more feasible on neutral-atom machines.

The team’s device isn’t the first neutral-atom quantum computer, but previous attempts were designed to model specific physical problems or to run particular quantum algorithms. Saffman and his colleagues have built the first fully programmable neutral-atom quantum computer, meaning it can run any quantum algorithm and could theoretically be scaled up to rival other leading approaches.

Saffman also works for a company called ColdQuanta that is seeking to develop a commercial neutral-atom quantum computer. However, he says considerable obstacles still remain to building larger machines, such as introducing the ability for qubits to correct errors. “I absolutely don’t want to overhype where we are. As we progress on developing these machines, I think the road gets steeper, not the opposite,” he says.

The new device isn’t the only one demonstrating the promise of neutral-atom machines. French start-up Pasqal has developed a special-purpose neutral-atom computer with more than 100 qubits, designed for modelling complex chemistry problems. And Mikhail Lukin at Harvard University and his colleagues have built a neutral-atom machine that lets qubits entangle with qubits that are much further than neighbouring ones, though it isn’t fully programmable.

“These things are now moving into a space where, in the coming years, they can be serious competition to superconducting qubits and trapped ions,” says Andrew Daley at the University of Strathclyde, UK. “The rapid development in the last few years has been exciting.”


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