Needless to say, building fully functional quantum computers depends on, to a large extent, the integrity of qubits – the two-level logic elements of quantum computing that represent quantum information.
Unfortunately, a group of researchers from MIT has recently found that a qubit’s performance will soon hit a wall. In a paper published in the journal Nature, researchers argue that low-level, otherwise harmless, background radiation emitted by trace elements in concrete walls and cosmic rays are enough to make qubits decohere in just a few milliseconds.
“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,” said co-author William Oliver. “This is an exciting result, because it motivates us to think of other ways to design qubits to get around this problem.”
First, the researchers conducted a series of experiments involving irradiated copper and computer simulations to determine the effects of radiation on qubit’s coherence. Based on those experiments, the team was then able to infer the effects of naturally occurring environmental radiation. Results showed that, under such conditions, qubits would decohere in about 4 milliseconds.
Next, Oliver and his team built a 2-tonne wall of lead bricks around the dilution refrigerator housing superconducting qubits – the most popular qubit modality today – in Oliver’s lab on campus. For several weeks, students in the lab either lifted or lowered the wall every 10 minutes to measure the qubit’s integrity, thereby confirming the 4 millisecond prediction and demonstrating improved performance due to shielding.
Given these findings, one obvious way to address the problem would be to build quantum computers in deep basement facilities. However, it might be possible to avoid the deleterious effects of radiation altogether:
“If we want to build an industry, we’d likely prefer to mitigate the effects of radiation above ground,” Oliver said. “We can think about designing qubits in a way that makes them ‘rad-hard,’ and less sensitive to quasiparticles, or design traps for quasiparticles so that even if they’re constantly being generated by radiation, they can flow away from the qubit. So it’s definitely not game-over, it’s just the next layer of the onion we need to address.”