Quantum computing promises to bring huge improvements over conventional computing, if it can be scaled up from the current, basically laboratory test, installations. It would be able to process much more complex problems than conventional computing and do so at far, far higher speeds. But scientists in the US have found a potential obstacle to such a scaling-up of quantum computing – cosmic rays and other very low level and harmless background radiation. Cosmic rays are high-energy particles, mainly protons but also atomic nuclei – protons and neutrons – stripped of their electrons, as well as those ‘freed’ electrons themselves. Cosmic rays are thus charged particles, positive or negative and they are all produced by stars, including our own Sun.

The researchers, at the Massachusetts Institute of Technology (MIT), the MIT Lincoln Laboratory and the Pacific Northwest National Laboratory (PNNL), highlighted that the key factor was the integrity of the qubit. Conventional computers store information in binary 0 and 1 form, and each binary digit is called a bit; these individual bits are manipulated by the computer. For a quantum computer, the basic unit of information is a qubit (from quantum bit); qubits are not binary digits but represent atoms, ions, photons or electrons, plus their various control devices, which operate together as memory systems and processing systems.

Quantum computers use the particle physics phenomena discovered through quantum mechanics, such as superposition and entanglement (as well as the long-known phenomenon of interference). Superposition is a combination of states that are normally described separately – if a musician plays two musical notes at once, that is a superposition of those notes. While a bit must be either a 0 or a 1, because of superposition, a qubit can simultaneously be 0 and 1, and every fraction in between.

The problem is that qubits cannot maintain superposition for very long before they ‘decohere’. The term ‘integrity’ in this context means the length of time that a qubit can operate before ‘decoherence’ causes it to lose superposition and its associated quantum information. Leading candidates for the development of quantum computers are superconducting qubits, and scientists and engineers have made huge progress in extending superconducting qubit integrity, from less than a nanosecond in 1999 to about 200 microseconds today. As a microsecond is 1 000 times larger than a nanosecond, over the past 21 years researchers have increased qubit integrity by some 200 000 times.

However, in an experiment using irradiated high-purity copper foil, the MIT and PNNL researchers have discovered that cosmic rays and otherwise harmless background radiation would cause qubit decoherence after only a few milliseconds. (A millisecond is 1 000 times larger than a microsecond and one-million times larger than a nanosecond.) At the rate that quantum computing researchers have been increasing qubit integrity, this barrier could be encountered in only a few years’ time.

“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 of years, which is environmental radiation,” observed MIT Lincoln Laboratory Fellow and electrical engineering and computer science associate professor William Oliver. “This is an exciting result, because it motivates us to design qubits to get around this problem.”

“It is fascinating how sensitive superconducting qubits are to the weak radiation,” noted MIT Research Laboratory of Electronics postdoctoral student and study lead author Antti Vepsäläinen. “Understanding these effects in our devices can also be helpful in other applications such as superconducting sensors used in astronomy.”

“Cosmic ray radiation is hard to get rid of,” pointed out MIT physics professor Joseph Formaggio. “It’s very penetrating, and goes right through everything like a jet stream. If you go underground, that gets less and less. It’s probably not necessary to build quantum computers deep underground, like neutrino experiments, but maybe deep basement facilities could probably get qubits operating at improved levels.”

“If we want to build an industry, we’d likely prefer to mitigate the effects of radiation above ground,” highlighted Oliver. “We can think about designing qubits in a way that makes them ‘rad-hard’, and less sensitive to quasiparticles [elementary excitations of solids which are treated as particles, although they are not composed of matter], 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.”