“Quantum computers promise exponential computational power by processing many possibilities simultaneously. But the machines we have today are still far from this ideal. For one, their bits, called qubits, are prone to frequent errors, and as a result, quantum computers are limited for now. To overcome errors, the machines need to tap into so-called magic states—and scientists at Google Research think their quantum chip could help get them there. In a new study published today in Science, the researchers show how to supply magic states more cheaply using a gardening-inspired technique called cultivation.

The standard recipe to make magic states is called distillation, and it creates fewer, more accurate quantum states from many imperfect ones. 'You can have up to 95 percent of your quantum computer being spent just generating these high-fidelity magic states,' Rosenfeld says. To bypass this bottleneck, in 2024 her colleagues at Google Research proposed an alternative technique called cultivation, which works on a single state.

Using Google’s Willow chip, Rosenfeld’s team encoded a rough magic state in a few qubits and then measured it. Measuring a quantum state typically collapses it, forcing it into one of two possible answers. So instead of asking the question, 'Are you 0 or 1?' which collapses the quantum state to either of those two options, the team asked, 'Are you the right magic state: yes or no?'

States that pass this test are then spread across more qubits for stronger protection, hence the gardening name: the state starts small, gets checked and then grows. Without cultivation, about four attempts in 1,000 produced a faulty state. With it, errors fell by roughly 40-fold.”

From Scientific American.