by Google physicists have reached their second milestone on the way to a usable quantum computer. In a lab in Santa Barbara, California, they have shown that they can reduce the error rate of calculations by enlarging their quantum code.
The feat reported in Nature on February 22, follows on from an acclaimed 2019 experiment in which a Google quantum computer achieved a “quantum advantage” — performing a calculation that would have taken thousands of years on an ordinary computer.
Error correction is an inescapable requirement if quantum computers are to fulfill their promise of solving problems that are inaccessible to classical machines – such as the decomposition of large integers into prime numbers or the detailed behavior of chemical catalysts.
“Google’s performance is impressive because it’s very difficult to get better performance with large code size,” says Barbara Terhal, a theoretical physicist specializing in quantum error correction at Delft University of Technology in the Netherlands. The Google researchers admit that the improvement is still small, and the error rate still has to drop significantly. “It’s a bit run down; We need it to get down a lot,” Hartmut Neven – who oversees the quantum computing division at Google’s headquarters in Mountain View, California – said during a news conference.
correct mistakes
All computers are prone to errors. A common 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 occurs – for example as a result of stray electrons crossing an imperfectly insulating barrier or a cosmic ray particle disrupting the circuitry – the chip can automatically detect and fix the problem.
“We can’t do that with quantum information,” said Julian Kelly, Google’s director of quantum hardware, at the press conference. Quantum computers are based on quantum states, so-called qubits, which can exist in a mixture of “0” and “1” states. A qubit cannot be read without irretrievably losing its complete quantum state, i.e. its information cannot simply be copied to redundant qubits.
But theorists have developed sophisticated “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 state of the logical qubit and correct any errors. The more physical qubits there are, the better they can suppress an error. “The benefit of using multiple qubits for quantum error correction is that it scales,” says Terhal.
However, adding more physical qubits also increases the likelihood that two of them will be affected by an error at the same time. To address this problem, Google researchers performed two versions of a quantum error correction technique. One using 17 qubits was able to recover from one failure after another. A larger version used 49 qubits and was able to recover from two simultaneous errors with slightly better performance than the smaller version. “The improvement is very small right now, and there’s still no guarantee that using even larger code will result in even better performance,” says Terhal.
Joe Fitzsimons, a physicist at Horizon Quantum in Singapore, says various labs have made big strides towards effective error correction and that Google’s latest result has many of the features needed. But qubits also need to store information long enough for the computer to perform calculations, and Google’s team hasn’t succeeded in doing that yet. “For a convincing demonstration of scalable error correction, we would like to see an improvement in lifetime,” Fitzsimons says as the system scales.
Google has set itself a quantum computing roadmap with six important milestones. Quantum Advantage was first, and the latest result was second. Milestone six is a machine composed of one million physical qubits encoding 1,000 logical qubits. “At this 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 sees them as having the best chance of success, says Neven. “We would switch immediately if it becomes clear that a different approach will lead us to a useful quantum computer more quickly.”
This article is reproduced with permission and has been published for the first time on February 22, 2023.
