Enlarge / The layout of Google’s qubits provide each internal qubit with connections to four of its neighbors.
But the layout isn’t the only issue that stands between Google and error-correcting qubits. Google Hardware Lead John Martinis said that you also need two-qubit operations to have an error rate of about 0.1% before error correction is realistically possible. Right now, that figure stands at roughly 0.3%. The team is confident it can be brought down, but they’re not there yet.
Another issue is wiring. Error correction requires multiple qubits to act as a single logical qubit, which means a lot more control wires for each logical qubit in use. And, right now, that wiring is physically large compared to the chip itself. That will absolutely have to change to add significant numbers of additional qubits to the chips, and Google knows it. The wiring problem “is boring—it’s not a very exciting thing,” quipped Martinis. “But it’s so important that I’ve been working on it.”
Enlarge / The chip’s packaging is dominated by the wiring needed to feed signals in and out of the chip.
Error correction also requires a fundamental change in the control hardware and software. At the moment, then reading out the results. But error corrections require more of a conversation, with constant sampling of the qubit state and corrective commands issued when needed. For this to work, Jeffrey noted, you’re going to really need to bring latency down.
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