Requires nearest-neighbor coupling

Chen also described how simply putting the hardware together isn’t the end of the challenge. While the individual qubits are designed to be identical, small flaws or impurities and the local environment can all alter the behavior of individual qubits. As a result, each qubit has its own frequency and error rate, and those have to be determined before a given chip can be used. Chen is working on automating this calibration process, which currently takes a day or so.What’s coming, hardware-wise

The processor that handled the quantum supremacy experiment is based on a hardware design called Sycamore, and it has 53 qubits (due to one non-functional device in a planned array of 54). That’s actually a step down from the company’s earlier Bristlecone design, which had 72 qubits. But Sycamore has more connections among its qubits, and that better fits with Google’s long-term design goals.

Google refers to the design goal as “surface code,” and its focus is on enabling fault-tolerant, error-correcting quantum computing. Surface code, as Google’s Marissa Giustina described it, requires nearest-neighbor coupling, and the Sycamore design lays out its qubits in a square grid. Everything but the edge qubits have connections to their four neighbors.

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