Non-shielded quantum computers may only be able to run for a few milliseconds before background radiation completely destabilizes the systems, according to lab experiments described in a paper published in Nature on Wednesday.
In 1999, quantum computers could only operate for less than a nanosecond. Fast forward more than 20 years, and today's systems can now last for around 200 microseconds. However, experimenters at MIT say about four milliseconds is going to be the absolute limit for today's technology – when not sufficiently radiation shielded – due to cosmic rays and emissions from radioactive materials all around us affecting the computers' qubits. Specifically, the radiation will cause the qubits to lose coherence.
"Ionizing radiation is all around us all the time," Antti Vepsäläinen, first author of the study and a postdoctoral associate at MIT, told The Register. "It arises from the decay or trace radioactive elements in our building materials, or in the cosmic rays that constantly shower the Earth from outer space.
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"These types of highly energetic radiation have enough energy to create ionized electron-hole pairs and energetic phonons in the materials which they hit. Those excitations then further interact with other particles in the materials, creating a 'shockwave' of energy propagating through the material, ultimately hitting the superconductor where the qubits reside."
Vepsäläinen recommended protecting the devices in layers of lead shielding or rad-hardening the circuits themselves. "Less [radiation] is always better," he added. "With the current levels of ionizing radiation in our laboratory, we estimated that the qubit lifetime is limited to four milliseconds because of the radiation."
Background radiation won't have a noticeable impact yet on modern quantum computers. The qubits simply do not exist in a coherent state long enough, and are often destabilized by other factors before ionizing particles have a chance to strike.
"Ionizing radiation is not the dominating decoherence mechanism in today's superconducting qubits, but it is important to study this effect," Vepsäläinen told us.
However, as quantum computers continue to improve, in a few years or so radiation will become a limiting factor to how long they can run. "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," said William Oliver, co-author of the paper and an associate professor of electrical engineering at MIT.
There are multiple ways to protect quantum computers, from building them underground to fashioning the circuits from more radiation-resistant materials. "So it's definitely not game over, it's just the next layer of the onion we need to address," Oliver added. ®