Advances in Quantum Error Correction

 Always heard that error correction is the key challenge of quantum computing,  here what appear to be advances by Google.   Implications, some suggested here.  For specific kinds of problems here.

Quantum Computing with Exponentially Fewer Errors By Arnout Jaspers, Commissioned by CACM Staff, August 19, 2021  

As long as a quantum computation is going on, one cannot actually check the values of the qubits, as that would destroy the complex quantum-state of the qubits, with the result being noise.

In an ideal world, a quantum computer with as few as 100 qubits would suffice to outperform all classical computers on certain classes of computation tasks. Google's Sycamore quantum processor, which has 53 qubits, is nowhere near accomplishing this.

The reason is that existing qubits, the computational building blocks of a quantum computer, are extremely sensitive to outside interference. Any magnetic or thermal disturbance will cause the qubit to change its internal state randomly; the classical analogy would be that all the bit values in the working memory of a microprocessor keep flipping from 0 to 1 and back, unpredictably.

As a result, quantum computing is still limited to computations taking no more than a few microseconds.

However, a team of Google researchers reported in Nature that they have successfully implemented quantum error correction on 21 qubits on the Sycamore chip, with huge potential for extending computation time.

Quantum error correction has been done before on smaller arrays of qubits, and on other types of qubit than the superconducting transmon qubits that Sycamore uses. 

"This is a strong answer to those sceptics who say a quantum computer will never work in real life," says computer scientist Koen Groenland of QuSoft, the Dutch research center for quantum software in Amsterdam, who was not involved in this research. In Groenland's view, the key achievement of the Google team was showing that the probability of errors decreased exponentially when using more qubits. That means even large qubit circuits can be made resilient to error by adding a modest number of extra qubits. Said Groenland, "That is a great perspective, because to keep a quantum computer working for hours, the error frequency must decrease from once per microseconds to a few per day, or even per week."  ... '