(ARS.Technica) Rsearchers testing error correction on Google’s quantum processor noted an odd phenomenon where the whole error-correction scheme would sporadically fail badly. They chalked this up to background radiation, a combination of cosmic rays and the occasional decay of a naturally occurring radioactive isotope.
To look at what’s going on, the Google team chose 26 of the least error-prone qubits on its processor and set them all in a single quantum state. Then, the researchers could let the processor idle for a short amount of time and see whether the qubits were still in that state.
Cosmic-ray hits were pretty easy to identify. After allowing the processor to idle for 100 microseconds, the typical background error rate was about four of the 26 qubits. When a cosmic ray happened to hit, about 24 of the qubits ended up in the error state—despite the fact that each qubit was about a millimeter apart from its neighbors.
osmic rays and radioactivity cause problems for classical computing hardware as well. That’s because classical computers rely on moving and storing charges, and cosmic rays can induce charges when they impact a material. Qubits, in contrast, store information in the form of the quantum state of an object—in the case of Google’s processor, a loop of superconducting wire linked to a resonator. Cosmic rays affect these, too, but the mechanism is completely different.
The impact of a cosmic ray also creates vibrational energy, which takes the form of what are called phonons. These phonons can also group together to form quasiparticles, in which small collections of phonons group together and start behaving like a single particle with distinct properties. It’s these quasiparticles that cause havoc, since they can exchange energy with the quantum computing hardware.
To look at what’s going on, the Google team chose 26 of the least error-prone qubits on its processor and set them all in a single quantum state. Then, the researchers could let the processor idle for a short amount of time and see whether the qubits were still in that state.
Cosmic-ray hits were pretty easy to identify. After allowing the processor to idle for 100 microseconds, the typical background error rate was about four of the 26 qubits. When a cosmic ray happened to hit, about 24 of the qubits ended up in the error state—despite the fact that each qubit was about a millimeter apart from its neighbors.
Because the quantum processor can sample the qubits’ states very quickly, the team could even track the spread of errors across the processor. Initially, errors are largely confined to the nearest qubits to the cosmic-ray impact. But, even as the error rate here begins to drop, qubits that are further from the point of impact start to see their error rates go up as the phonons spread out across the chip. Before things drop back to background, every qubit in the device typically sees its average error rate rise.
The key question then becomes whether these events happen often enough to affect computations; if they’re rare enough, we can just throw out those calculations that involve an error and start again. But the paper looked at this as well, and the news here is not good. On average, the (rather small) quantum processor used here experienced an error every 10 seconds. And most of the algorithms we’re interested in running on quantum computers are likely to take hours to complete. (That may seem like a long time, but remember these are calculations that can’t be done at all on traditional hardware.)
Cosmic Rays a potential hangup for quantum computers
