Nobody's perfect - this also applies in particular to Google's quantum computer “Sycamore”, which two years ago was able to solve a mathematical problem faster than a classic supercomputer.
Despite its efficiency, which is based on 53 superconducting quantum resonators that take on the role of quantum bits (qubits), the system is extremely susceptible to interference.
Employees of the Google quantum technology team have now developed a method to get this problem under control and published it in a technical article in the scientific journal "Nature".
Manfred Lindinger
Editor in the “Nature and Science” section.
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Like all existing quantum computers, Sycamore is particularly troubled by electrical stray fields and temperature fluctuations.
They destroy the fragile quantum states that are stored in frozen microwave resonators at Sycamore, for example.
Because important quantum information that the computer needs is lost due to interference, errors creep in quickly.
The classic error correction of a PC doesn't help.
The data encoded in the qubits cannot be copied like classic bits and therefore cannot be saved as a backup copy.
When copying, the original data is destroyed and lost, which reduces the performance of a quantum computer.
Other strategies to avoid errors are therefore required.
In the current issue of the journal Nature, the scientists from Google present a promising method that has succeeded in drastically reducing the rate of calculation errors.
"A big step towards practical application"
The builders of quantum computers have now learned to quickly identify errors in individual qubits and then correct them so that quantum information is retained over a longer period of time. Special algorithms and auxiliary qubits help with this. These immediately notice when a previously active quantum bit has been lost. This happens without contact, via the effect of quantum mechanical entanglement. A mechanism is then triggered that saves and restores the information in the lost quantum bit.
A successful strategy that the Google researchers at Sycamore are also using is not to store the quantum information in individual fragile superconducting quantum resonators, but to distribute and encode it over a large number of these physical objects. This creates logical quantum bits that are less sensitive to interference. The researchers recognize calculation errors by means of comparative measurements, whereupon a corresponding correction is made. This means that quantum information is retained longer. The error correction is carried out using a special algorithm. By repeatedly using the error correction codes, it was possible to reduce the error rate by a factor of a hundred, as Julian Kelly and his colleagues write.
Quantum physicist Tommaso Calarco from Forschungszentrum Jülich attaches similar importance to the current study by Google researchers as to the work on the quantum superiority of Sycamore two years ago.
"The presented methods for error correction are a big step towards practical application," says Calarco.
Without error correction, there could be no functioning quantum computer, according to Calarco.
The advantages of quantum information processing can only be used if the information is retained over the entire duration of the calculation.
It is not yet possible to say whether the factor hundred is enough or whether one needs even lower error rates so that Sycamore can show its full potential.