How do you build a quantum computer out of imperfect parts?

Today ’s prototype quantum computers are too error-prone to do anything useful.

In the 1990s , researchers worked out the theoretical foundations for a way to overcome these errors.

The key idea was to coax a cluster of physical qubits to work together as a single high-quality “logical qubit”.

The surface code is based on two overlapping grids of physical qubits.

The error-checking scheme is much simpler than those of competing quantum codes.

It only involves interactions between neighboring qubits, the feature that Preskill found so appealing.

In 2006 , two researchers showed that an optimized version of the code had an error threshold around 1% , 100 times higher than the thresholds of earlier quantum codes. But interest remained confined to a small community of theorists.

The Google Quantum AI team spent years improving their qubit design and fabrication procedures.

They knew they could build individual physical qubits with error rates below the surface-code threshold.

But they had to see if those qubits could work together to make a logical qubit that was better than the sum of its parts.