Quantum computers are advancing at an exciting pace, but unfortunately this progress may soon stall. Cosmic rays streaming down to Earth could interfere with the integrity of the information in these quantum computers, and now an MIT team has shown just how vulnerable they are and what it might take to protect them.
In traditional computers, information is represented in “bits” as either a zero or a one. But thanks to the spooky rules of quantum physics, the bits in quantum computers (called qubits) are able to exist in a superposition of both states at once. That means they could perform many operations in parallel, making them vastly more powerful than existing computer systems.
But there’s a major hurdle to making practical quantum computers. Qubits have fairly low coherence times, which refers to how long they can remain in this superposition state. That’s because they’re sensitive to outside interference like heat, magnetic and electric fields, or even the low-level radiation that surrounds us all the time.
Some of the worst offenders come from space. Cosmic rays and the cascade of secondary particles they create are constantly raining down on us, and while we personally might not notice them they can wreak havoc on electronics.
In a new study, researchers at MIT, Lincoln Laboratory and Pacific Northwest National Laboratory (PNNL) have now quantified just how much of a problem cosmic rays might be for quantum computers.
In experiments, the researchers placed disks of irradiated copper next to superconducting qubits to measure the effects of the radiation. The experiments were conducted inside a dilution refrigerator, minimizing other interference by cooling the environment down to around 200 times colder than the vacuum of space. A second irradiated copper disk was studied outside the fridge to measure how much radiation the quantum system was being exposed to.
Using this setup and other simulations, the team found that the qubit coherence time would be limited to around four milliseconds. Further experiments confirmed this figure, by placing or removing a radiation shield between the copper disks and the qubits. The shield did help, but it’s not the most practical of solutions – it was a two-ton wall of lead bricks.
The experiment shows that to get the most out of quantum computers, we’ll need to build them with adequate shielding. That could mean moving them deep underground, like neutrino-hunting experiments that also need protection from cosmic rays. But that might not be the only solution, the team says.
“If we want to build an industry, we’d likely prefer to mitigate the effects of radiation above ground,” says William Oliver, an author of the study. “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.”
The research was published in the journal Nature.
Source: MIT