An MIT study reports that incoming cosmic rays can limit qubit’s performance, hindering advances in quantum computing. Credit: Christine Daniloff, MIT
Quantum computers building underground or designing radiation-resistant qubits may be necessary, researchers find.
The practices of quantum calculation depends on the integrity of the quantum bit, or qubit.
Qubits, the logical elements of quantum computers, are two-level cohesive systems that represent quantum information. Each qubit has the early possibility of being in a quantum superposition, carrying aspects of both states simultaneously, thus enabling a quantum version of parallel computation. Quantum computers, if they can be scaled to accommodate many qubits on a single processor, can be dizzyingly faster and can handle much more complex problems than today’s conventional computers.
But that all depends on the integrity of a qubit, or how long it can work before its superposition and quantum information are lost – a process called decoherence, which ultimately limits the computer’s runtime. Superconducting qubits – today a leading qubit modality – have achieved exponential improvement in this key metric, from less than one nanosecond in 1999 to about 200 microseconds today for the best performing devices.
But researchers at MIT, MIT Lincoln Laboratory, and Pacific Northwest National Laboratory (PNNL) have found that the performance of a qubit will soon hit a wall. In a paper published today in Nature, the team reports that the low, otherwise harmless background radiation emitted by trace elements in concrete walls and incoming cosmic rays are sufficient to cause decoherence in qubits. They found that this effect, if not edited, would limit the performance of qubits to only a few milliseconds.
Given the rate at which scientists are improving qubits, they can only reach these through radiation-induced wall in just a few years. To overcome this barrier, scientists need to find ways to protect qubits – and all practical quantum computers – from low-level radiation, often by building computers underground or designing qubits that are tolerant to the effects of radiation.
“These decoherence mechanisms are like an onion, and we have been peeling the layers back for the past 20 years, but there is another layer that has been left unforgettable that will limit us in a few years, that is environmental radiation,” says William. Oliver, University Professor of Electrical Engineering and Computer Science and Lincoln Laboratory Fellow at MIT. “This is an exciting result because it motivates us to think of other ways to design qubits to deal with this problem.”
The paper’s lead author is Antti Vepsäläinen, a postdoc at MIT’s Research Laboratory of Electronics.
“It’s fascinating how sensitive superconducting qubits are to the weak radiation. Understanding these effects in our devices can also be useful in other applications such as superconducting sensors used in astronomy, ”says Vepsäläinen.
Co-authors at MIT include Amir Karamlou, Akshunna Dogra, Francisca Vasconcelos, Simon Gustavsson, and Professor of Physics Joseph Formaggio, along with David Kim, Alexander Melville, Bethany Niedzielski, and Jonilyn Yoder at Lincoln Laboratory, and John Orrell, Ben Loer , and Brent VanDevender of PNNL.
A cosmic effect
Superconducting qubits are electrical circles made of superconducting materials. They comprise multitudes of coupled electrons, known as Cooper pairs, which flow through the circuit without resistance and work together to maintain the malignant superposition state of the qubit. If the circuit is heated or otherwise disturbed, electron pairs can be split into “quasi-particles”, causing decoherence in the qubit that limits the operation.
There are many sources of decoherence that can destabilize a qubit, such as fluctuating magnetic and electric fields, thermal energy, and even interference between qubits.
Scientists have long suspected that very low levels of radiation can have a similarly destabilizing effect in qubits.
“I’ve improved the quality of superconducting qubits much better over the last five years, and now we’re within a factor of 10 where the effects of radiation are important,” adds Kim, a technical staff member at MIT Lincoln Laboratory,
That Oliver and Formaggio teamed up to see how they can nail down the effect of low-level environmental radiation on qubits. As a neutrino physicist, Formaggio has expertise in designing experiments that protect against the smallest sources of radiation, to detect neutrinos and other hard-to-detect particles.
“Calibration is key”
The team, which worked with collaborators at Lincoln Laboratory and PNNL, first had to design an experiment to calibrate the impact of known levels of radiation on superconducting qubit performance. To do this, they needed a known radioactive source – one that slowly became less radioactive to assess the impact on essentially constant radiation levels, yet fast enough to assess a range of radiation levels within a few weeks, until the level of after-radiation,
The group chose to fight a foil of copper with high purity. When exposed to a high stream of neutrons, copper produces many amounts of copper-64, an unstable isotope with exactly the desired properties.
“Copper simply picks up neutrons as a sponge,” says Formaggio, who worked with operators at MIT’s Nuclear Reactor Laboratory to fight two small disks of copper for several minutes. They then place one of the disks next to the superconducting qubits in a dilution refrigerator in Oliver’s lab on campus. At temperatures about 200 times colder than the outer space, they measured the effect of the radioactivity of copper on the cohesion of qubits, while the radioactivity decreased – to the background levels of the environment.
The radioactivity of the second disk was measured at room temperature as a measure of the levels that hit the qubit. Through these measurements and related simulations, the team understood the relationship between radiation levels and qubit performance, one that could be used to deduce the effect of naturally occurring environmental radiation. Based on these measurements, the cobit’s coherence time would be limited to about 4 milliseconds.
“Do not play over”
The team then removed the radioactive source and went on to show that protecting the qubits from environmental radiation improves cohesion time. To do this, the researchers built a 2-ton wall of lead bricks that could be lifted and lowered onto a shear lift, to protect the refrigerator or expose it to ambient radiation.
“We built a small castle around this refrigerator,” Oliver says.
Every 10 minutes, and for several weeks, students in Oliver’s lab switch a knob to raise or lower the wall, because a detector detects the integrity of the qubits, or “relaxation rate”, a measure of how the environmental radiation affects has on the qubit. , with and without the shield. By comparing the two results, they effectively extracted the impact attributed to environmental radiation, confirming the 4 millisecond prediction and demonstrating that protection improves qubit performance.
“Cosmic radiation is difficult to remove,” says Formaggio. “It is very permeable, and goes through everything like a jet stream. As you go underground, that gets worse and worse. It is probably not necessary to build quantum computers deep underground, such as neutrino experiments, but perhaps deep basement facilities are likely to get qubits that work at improved levels. “
Going underground is not the only option, and Oliver has ideas for designing quantum computers that still work in the face of background radiation.
“If we want to build a sector, we probably want to reduce the effects of above ground radiation,” Oliver says. “We can think of designing qubits in a way that makes them ‘radhurd’, and less sensitive to quasi-particles, or designing traps for quasiparticles, so that even if they are constantly generated by radiation, they can flow away.” e qubit, That it’s definitely not a game-over, it’s just the next layer of onion we have to tackle. “
Reference: “Impact of Ionizing Radiation on Superconducting Qubit Coherence” by Antti P. Vepsäläinen, Amir H. Karamlou, John L. Orrell, Akshunna S. Dogra, Ben Loer, Francisca Vasconcelos, David K. Kim, Alexander J. Melville, Bethany M. Niedzielski, Jonilyn L. Yoder, Simon Gustavsson, Joseph A. Formaggio, Brent A. VanDevender and William D. Oliver, August 26, 2020, Nature.
DOI: 10.1038 / s41586-020-2619-8
This research was funded in part by the U.S. Department of Energy Office of Nuclear Physics, the U.S. Army Research Office, the U.S. Department of Defense, and the U.S. National Science Foundation.
