Natural radiation can mess with qubits

Natural radiation can limit the performance of superconducting quantum bits and thus potentially the operation of quantum computers, according to new research in the US.

Scientists may have to shield these “qubits”, perhaps by building computers underground or designing them so they are tolerant to radiation’s effects, a team from Pacific Northwest National Laboratory (PNNL) and Massachusetts Institute of Technology (MIT) writes in the journal Nature.

Computer engineers have known for at least a decade that cosmic rays or radiation emanating from materials like concrete can cause digital computers to malfunction – and quantum computers are even more sensitive.

“We found that practical quantum computing with these devices will not be possible unless we address the radiation issue,” says PNNL physicist Brent VanDevender.

The project began when MIT physicist Will Oliver was working with superconducting qubits and was perplexed by interference he says helped push them out of their prepared state, making them non-functional. After ruling out a number of possibilities, he considered the idea that natural radiation was pushing them into “decoherence”.

To test the idea, the researchers measured the performance of prototype superconducting qubits in two different experiments; they exposed the qubits to elevated radiation from copper metal activated in a reactor, and built a shield around the qubits that lowered the amount of natural radiation in their environment.

This clearly demonstrated, they say, the inverse relationship between radiation levels and length of time qubits remain in a coherent state.

Superconducting qubits are electrical circuits. They comprise multitudes of paired electrons, known as Cooper pairs, that flow through the circuit without resistance and work together to maintain the qubit’s tenuous superposition state.

If the circuit is heated or otherwise disrupted, electron pairs can split into “quasiparticles,” causing decoherence in the qubit that limits its operation.

“The radiation breaks apart matched pairs of electrons that typically carry electric current without resistance in a superconductor,” says VanDevender. “The resistance of those unpaired electrons destroys the delicately prepared state of a qubit.”

The researchers emphasise that factors other than radiation exposure are bigger impediments to qubit stability for the moment. Things like microscopic defects or impurities in the materials used to construct qubits are thought to be primarily responsible for the current performance limit of about one-tenth of a millisecond.

Once those limitations are overcome, however, radiation will begin to assert itself. And given the rate at which scientists have been improving qubits, a radiation-induced wall may be just a few years away.

“These decoherence mechanisms are like an onion, and we’ve been peeling back the layers for past 20 years, but there’s another layer that left unabated is going to limit us in a couple years, which is environmental radiation,” says Oliver.

“This is an exciting result, because it motivates us to think of other ways to design qubits to get around this problem.”