Doing calculations with a quantum computer is a race against time, thanks to the fragility of the quantum states at their heart. And new research suggests we may soon hit a wall in how long we can hold them together thanks to interference from natural background radiation.
While quantum computing could one day enable us to carry out calculations beyond even the most powerful supercomputer imaginable, we’re still a long way from that point. And a big reason for that is a phenomenon known as decoherence.
The superpowers of quantum computers rely on holding the qubits—quantum bits—that make them up in exotic quantum states like superposition and entanglement. Decoherence is the process by which interference from the environment causes them to gradually lose their quantum behavior and any information that was encoded in them.
It can be caused by heat, vibrations, magnetic fluctuations, or any host of environmental factors that are hard to control. Currently we can keep superconducting qubits (the technology favored by the field’s leaders like Google and IBM) stable for up to 200 microseconds in the best devices, which is still far too short to do any truly meaningful computations.
But new research from scientists at Massachusetts Institute of Technology (MIT) and Pacific Northwest National Laboratory (PNNL), published last week in Nature, suggests we may struggle to get much further. They found that background radiation from cosmic rays and more prosaic sources like trace elements in concrete walls is enough to put a hard four-millisecond limit on the coherence time of superconducting qubits.
“These decoherence mechanisms are like an onion, and we’ve been peeling back the layers for the past 20 years, but there’s another layer that left unabated is going to limit us in a couple years, which is environmental radiation,” William Oliver from MIT said in a press release. “This is an exciting result, because it motivates us to think of other ways to design qubits to get around this problem.”
Superconducting qubits rely on pairs of electrons flowing through a resistance-free circuit. But radiation can knock these pairs out of alignment, causing them to split apart, which is what eventually results in the qubit decohering.
To determine how significant of an impact background levels of radiation could have on qubits, the researchers first tried to work out the relationship between coherence times and radiation levels. They exposed qubits to irradiated copper whose emissions dropped over time in a predictable way, which showed them that coherence times rose as radiation levels fell up to a maximum of four milliseconds, after which background effects kicked in.
To check if this coherence time was really caused by the natural radiation, they built a giant shield out of lead brick that could block background radiation to see what happened when the qubits were isolated. The experiments clearly showed that blocking the background emissions could boost coherence times further.
At the minute, a host of other problems like material impurities and electronic disturbances cause qubits to decohere before these effects kick in, but given the rate at which the technology has been improving, we may hit this new wall in just a few years.
“Without mitigation, radiation will limit the coherence time of superconducting qubits to a few milliseconds, which is insufficient for practical quantum computing,” Brent VanDevender from PNNL said in a press release.
Potential solutions to the problem include building radiation shielding around quantum computers or locating them underground, where cosmic rays aren’t able to penetrate so easily. But if you need a few tons of lead or a large cavern in order to install a quantum computer, that’s going to make it considerably harder to roll them out widely.
It’s important to remember, though, that this problem has only been observed in superconducting qubits so far. In July, researchers showed they could get a spin-orbit qubit implemented in silicon to last for about 10 milliseconds, while trapped ion qubits can stay stable for as long as 10 minutes. And MIT’s Oliver says there’s still plenty of room for building more robust superconducting qubits.
“We can think about designing qubits in a way that makes them ‘rad-hard’,” he said. “So it’s definitely not game-over, it’s just the next layer of the onion we need to address.”
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