When researchers at Google’s quantum computer encountered the strange phenomenon that a quantum processor’s error-correcting algorithm sometimes fails completely, they knew the problem lay in energetic particle action from background radiation, cosmic rays, or the natural decay of a stray radioactive isotope. They also had an inside joke about it, building a cosmic ray detector with years of work and huge money.
However, the researchers involved took the problem seriously and Nature PhysicsIn their dissertation published in. They came to the conclusion that only a packet detector could be really useful for an expensive quantum computer.
The universe is interfering
Radioactivity and cosmic rays also cause problems for conventional computers. This is because their action depends on the movement of electric charges, and cosmic particles generate a charge as they collide. The rays come in constantly, and if they hit the device at the right point, the bit will be one from zero, or vice versa, and you’re done with the problem.
For quantum bits, or qubits, the quantum state of a particle carries information. For a Google computer, the device is a superconducting wire loop attached to a resonator. Rays have a say on things here too, but in a slightly different way. The energy of the effect of cosmic rays generates vibrations in which quasiparticles called phonons appear at the atomic level. These can alter the energy of quantum devices, create quasiparticle pairs, or operate directly on the qubit itself.
It wouldn’t be a problem if the phonons only touched one qubit, the error-correction algorithm was designed to handle that problem exactly: it splits information into several entangled qubits, multiplies them, and determines which qubits are stuck out of the queue.
However, quasiparticles and quasiparticles are not at all static, but propagate and can act on a series of qubits.
Google experts put it to a simple test: they set the 26 most stable qubits on their devices to the same quantum state, then wait a bit and look at how much is left in the same state. In 100 microseconds, an average of 4 out of 26 qubits entered a false state. When a cosmic beam hits 24 qubits, it becomes defective even though it is spaced one millimeter apart.
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Quasi-particles lose their energy very quickly, so they are unable to tune qubits from the ground state to the excited state. However, they are able to do the opposite: they take energy from the exciting qubit and put it at rest. Accordingly, if quasiparticles cause the problem, it should detect more errors at excited 26 qubits than at rest 26 qubits. Measurements confirmed this assumption.
The speed of the machine allowed the researchers to monitor the spread of errors. They first appeared only in the immediate vicinity of the influence of cosmic rays, and then expanded and raised the average error rate per qubit.
One question remains: how often does this happen. If it is rare, it is enough to get rid of the accounts and start over. Here, too, the measurement was relatively bad news: on average, such an error occurred every 10 minutes. This is a problem because some computations can take hours for quantum computers – and here we’re talking about computations that classical computers can’t solve. Additionally, an average downtime of 10 minutes was measured with a relatively small processor, a situation that deteriorates directly with device size.
In connection with the solution of the problem, researchers formulate in the conditional. Astronomers already had the same problem with their imaging devices, which was solved by trying to prevent the propagation of phonons by modifying the physical structure of their detectors. Whether this problem can be solved is different for a quantum computer, but it’s definitely worth researching.
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