Physicists at Google have reached a second milestone on the way to a useful quantum computer. At a lab in Santa Barbara, California, they have shown that they can lower the error rate of calculations by making their quantum code larger.
The feat, reported in nature on February 22, it follows up on a celebrated 2019 experiment in which Google’s quantum computer achieved a ‘quantum advantage’ – by performing a calculation that would take a normal computer thousands of years.
Error correction is an unavoidable necessity if quantum computers are to fulfill their promise of solving problems beyond the reach of classical machines – for example factoring large integers into prime numbers, or understanding the detailed behavior of chemical catalysts.
“Google’s achievement is impressive, since it is very difficult to get better performance with a large code size,” says Barbara Terhal, a theoretical physicist specializing in quantum error correction at Delft University of Technology in the Netherlands. The improvement is still small, Google researchers admit, and the error rate needs to drop much more. “It came down a bit; we have to reduce it significantly,” Hartmut Neven – who oversees the quantum computing division at Google’s Mountain View, California headquarters – said during a press briefing.
All computers are subject to errors. A typical computer chip stores information in bits (which can represent 0 or 1) and copies some of the information into redundant ‘error corrected’ bits. When an error occurs — the result of stray electrons crossing an imperfectly insulated barrier, say, or a cosmic ray particle disrupting the circuit — the chip can automatically see the problem and fix it.
“In quantum information we can’t do that,” said Julian Kelly, Google’s director of quantum hardware, at the press briefing. Quantum computers are based on quantum states called qubits, which can exist in a mixture of ‘0’ and ‘1’ states. A qubit cannot be read out without its entire quantum state being irretrievably lost, meaning that its information cannot simply be copied onto redundant qubits.
But theorists have developed elaborate ‘quantum error correction’ schemes to address this problem. These typically rely on encoding a qubit of information — called a logical qubit — in a collection of physical qubits rather than a single one. The machine can then use some of the physical qubits to check the health of the logical qubit and correct any errors. The more physical qubits there are, the better they can suppress error. “The advantage of using multiple qubits to correct quantum errors is that it scales,” says Terhal.
But adding more physical qubits increases the chance of an error affecting two of them at the same time. To address this issue, Google researchers developed two versions of a quantum error correction procedure. One, using 17 qubits, was able to recover from one error at a time. A larger version used 49 qubits and could recover from two simultaneous errors, and with slightly better performance than the smaller version could achieve. “The improvement is very small right now, and it’s no guarantee that using more code will lead to even better performance,” says Terhal.
Joe Fitzsimons, a physicist at Horizon Quantum in Singapore, says that various labs have made great strides towards effective error correction, and Google’s latest result has many of the necessary features. But qubits need to store information long enough for the computer to perform calculations, and the Google team has yet to achieve that feat. “With a convincing demonstration of scalable error correction, we want to see an improvement in lifetimes”, as the system scales, says Fitzsimons.
Google has set itself a quantum computing roadmap with six important milestones. The Quantum advantage was the first, and the latest result was the second. Milestone six is a machine made of one million physical qubits, which encode 1,000 logical qubits. “At that stage, we can confidently promise commercial value,” says Neven.
Superconducting qubits are just one of several approaches to building a quantum computer, and Google still has the best chance of success, Neven says. “We’d be in a heartbeat if it became very clear that another approach would get us to a useful quantum computer faster.”
This article is reproduced with permission and was first published on February 22, 2023.