A potential hangup for quantum computing: Cosmic rays

The processor is called the Sycamore.

The whole error-correction scheme would fail badly when researchers were testing it on the quantum processor. They think it's due to background radiation, a combination of rays and decay of a radioactive isotope.

It seemed like a bit of a joke at the time, but it turned out that it was a mistake. The people behind the processor took the problem very seriously and are back with a new paper detailing how the radiation affects the qubits. Errors in quantum computations are kept from working unless we figure out a way to limit the impact of the rays.

The rays are a shame.

Classical computing hardware is affected by Cosmic rays and radioactivity. Classical computers rely on moving and storing charges, and cosmic rays can cause charges when they impact a material. Qubits store information in the form of the quantum state of an object, in the case of a loop of superconducting wire. The mechanism of Cosmic rays is completely different.

The impact of a Cosmic Ray creates a type of energy called phonons. In quasiparticles, small collections of phonons group together and start behaving like a single particle. The quasiparticles cause havoc because they can exchange energy with quantum computing hardware. The Cooper pairs of electrons form the foundation for superconductivity. The qubit can change its state and disrupt any entanglement.

If these phonons only affected a single qubit, the situation wouldn't be a problem, in fact, it would be exactly the sort of thing that quantum error correction is meant to handle. The hardware can identify when one of the qubits is malfunctioning by distributing quantum information across multiple entangled qubits.


The problem is that the quasiparticles won't end up being local, instead they should spread out around their site of origin and affect multiple qubits. That should be enough to stop error correction. The people who implemented the error correction in the earlier paper got together with physicists to see if it was actually happening in the quantum processing hardware.

In the chips.

To see what's going on, the team chose 26 of the least error-prone qubits on the processor and set them all in a single quantum state. The researchers could use the processor to shut down for a short time and see if the qubits were still in that state.

Cosmic-ray hits were easy to identify. The background error rate after the processor was allowed to run for 100 microseconds was about four qubits. Despite the fact that each qubit was about a millimeter away from its neighbors, about 24 of the qubits ended up in the error state.

The researchers looked for a state dependence to confirm that this was due to quasiparticles. The quasiparticles will lose energy quickly and won't be able to raise a qubit from its ground state to its excited state. They can absorb the energy from the qubit and let it go back to the ground state. If quasiparticles are involved in these interactions, you would expect more errors when all the qubits start out in the excited state than in the ground state. The research team saw that.

The quantum processor can sample the qubits' states very quickly and the team could even track the spread of errors. Errors are limited to the nearest qubits. 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. The device's average error rate rises before things drop back to normal.