There are some obvious big picture issues that stand between us and useful quantum computing. Issues like whether we can make enough high-quality hardware qubits to connect into the error-corrected logical qubits we need, and how we generate the states needed to perform universal computation on those logical qubits. But there are also many less prominent challenges that will need to be solved before we can perform calculations.

One of these challenges is calibration. For devices like superconducting qubits, which Google relies on, there are always subtle variations among individual qubits. This hardware is put through a process called calibration, where different frequencies and amplitudes of the microwave pulses that control them are tested to find the combination that produces the lowest error rates.

However, you can’t perform this typical calibration process while performing calculations, which means drift becomes an issue for long and complicated algorithms. Google has figured out a solution: it’s possible to do calibration using the same data that’s used for error correction.

The hardware that companies like Google rely on are transmons—circuits consisting of a loop of superconducting wire connected to a resonator, controlled by pulses of microwave photons. These pulses are controlled by hardware outside the refrigeration, including classical computers and the microwave sources they control. This setup allows for detailed testing during calibration.

This innovation means that quantum processors can recalibrate themselves in real-time, making them more robust against drift and potentially paving the way for longer, more complex computations without interruptions.