Physicists at Google have reached what they describe as the second milestone along the way to a useful quantum computer. At their lab in Santa Barbara, California, they demonstrated that making the quantum code larger can reduce the computational error rate.
feat reported in Nature On February 22nd, we’ll follow up on the famous 2019 experiment in which Google’s quantum computer achieved “quantum supremacy.” It does this by performing calculations that would take thousands of years on a normal computer.
To live up to the promise that quantum computers will solve problems that classical machines cannot solve, such as factoring large numbers of integers into primes or understanding the detailed behavior of chemical catalysts, error correction must be avoided. It is a requirement that cannot be
Barbara Terhal, a theoretical physicist specializing in quantum error correction at Delft University of Technology in the Netherlands, said: Improvements are still small, her Google researchers acknowledged, and the error rate needs to go down further. went down a little. We need to surrender in large numbers,” Hartmut Neven, who oversees the quantum computing division at Google’s Mountain View, Calif., headquarters, said at a press conference.
correction of mistakes
All computers are affected by the error. A typical computer chip stores information in bits (which can represent 0 or 1) and copies some of the information into redundant “error correction” bits. If an error is caused by a stray electron passing through an imperfect insulation barrier or a cosmic ray particle disrupting a circuit, the chip can automatically identify and correct the problem.
Julian Kelly, director of quantum hardware at Google, said at a press conference: Quantum computers are based on quantum states called qubits, which can have a mixture of ‘0’ and ‘1’ states. A qubit cannot be read unless its complete quantum state is irrevocably lost. That is, we cannot simply copy that information to the redundant qubits.
However, theorists have developed elaborate “quantum error correction” schemes to deal with this problem. These typically rely on collections of physical qubits, rather than single qubits, to encode qubits of information called logical qubits. A machine can then use some of the physical qubits to check the sanity of the logical qubits and correct any errors. The more physical qubits, the more error can be suppressed. “The advantage of using multiple qubits for quantum error correction is that it can be scaled,” he says Terhal.
However, adding more physical qubits also increases the chance that two of them will be subject to an error at the same time. To address this issue, a Google researcher performed two versions of his quantum error correction procedure. One of them used 17 qubits and he could recover from one error at a time. The larger version used 49 qubits, was able to recover from two simultaneous errors, and achieved slightly better performance than the smaller version. “Current improvements are very small, and there is still no guarantee that even larger code will lead to even better performance,” he says.
Joe Fitzsimons, a physicist at Horizon Quantum in Singapore, says that various laboratories have taken big steps toward effective error correction, and Google’s latest results include many of the needed features. says there is. But qubits also need to store information long enough for computers to perform computations, a feat Google’s team has yet to achieve. “For a compelling demonstration of scalable error correction, we expect lifetimes to improve as the system scales up,” he says.
Google has set its own quantum computing roadmap with six key milestones. Quantum Dominance was first, and the most recent result was second. Milestone 6 is a machine consisting of 1 million physical qubits that encode 1,000 logical qubits. “At that stage, we can confidently promise commercial value,” he says.
Superconducting qubits are just one of several approaches to building quantum computers, and Google still thinks it’s the most likely to succeed, says Neven. “If it becomes very clear that another approach will bring us to a useful quantum computer more quickly, we will quickly turn around.”
This article is reprinted with permission. first published February 22, 2023.
