How “clean” should a quantum computing test facility be?

Now is the time to ban low-activity radioactive energy sources from facilities that house and conduct experiments with superconducting qubits, according to two recently published studies. Significantly improving the coherence times of quantum devices is a key step toward an era of practical quantum computing.

Two complementary articles, published in the journal Quantum PRX and the Instrumentation logdescribe which sources of interfering ionizing radiation are most problematic for superconducting quantum computers and how to remedy them. The results paved the way for a quantitative study of errors caused by radiation effects in protected underground facilities.

A research team led by physicists at the Department of Energy’s Pacific Northwest National Laboratory, in collaboration with colleagues at MIT’s Lincoln Laboratory, the National Institute of Standards and Technology, as well as several academic partners, has published its results to help the quantum computing community prepare. for the next generation of qubit development.

Electronic ‘noise’ undermines efforts to extend qubit lifespan

“The time is approaching when advances in design and materials will make qubits stable enough that environmental effects due to stray radiation become the limiting step toward quantum coherence,” said physicist Brent VanDevender, the one of the leaders of the research team. VanDevender was one of the first scientists to identify natural ionizing radiation as a source of instability for the operation of qubits, the basic unit of a quantum computer.

The slightest interference can cause errors that cause superconducting qubits to lose their quantum state, a process known as decoherence. The research team found that cosmic radiation and natural isotopes, which emit low levels of ionizing radiation and are found in common materials, are about equally responsible for decoherence.

“Once we established the effect of ionizing radiation on superconducting qubits, we knew we needed to systematically and quantitatively identify radiation sources in the environment,” said Ben Loer, senior experimental physicist.

“Our experience measuring ultra-low radiation levels in the laboratory has led us to include radiation sources in the very experimental units, cryostats, where these experimental qubits are studied.”

“We found that many electrical connectors are simply dirty in terms of their role as a radiation source,” VanDevender added.

Clean the basement

Together, the two studies highlight effective measures to protect sensitive experimental equipment from the effects of radiation exposure.

In the article of Instrumentation logThe research team describes precautions taken to significantly reduce the potential for exposure to atmospheric and isotopic radiation in a shielded underground qubit testbed on PNNL’s Richland, Washington, campus, called the Facility low-bottom cryogenic.

Built in an existing ultra-clean underground laboratory, the testbed includes a cryostat, also known as a dilution refrigerator, capable of cooling superconducting qubit devices to near absolute zero, a key to stabilize quantum computing devices of this design. The research team reports that this lead-shielded cryostat could reduce the error rate by 20 times compared to the error rate observed in a typical above-ground, unshielded installation.

Additionally, the team reports that some relatively simple precautions, such as eliminating natural sources of radiation in the materials inside the dilution refrigerator, go a long way toward making quantum computing devices viable. These sources include metal isotopes – natural variants of elements that spontaneously eject radiation in the form of alpha, beta and gamma rays – which can interfere with quantum devices.

In their exploration of these radiation sources within the laboratory, they used specialized ultrasensitive detection methods to identify contaminants in silicon, copper and ceramic electronic components such as circuit boards and cables used to collect the data from the instruments, and even the qubits themselves.

To reduce the impact of these devices, the team advises using materials like brass instead of the beryllium-copper alloys typically found in cables. Future goals of this research include testing the effectiveness of “radiation-hardened” qubits that are less sensitive to the impact of radiation and investigating materials with low background noise.

Transfer knowledge from sensitive detection technologies

In the complementary study, published in Quantum PRXThe research team directly measured the interactions of ionizing radiation on a superconducting sensor inside a cryocooler, a refrigerator capable of reaching ultra-cold cryogenic temperatures. They used simple radiation detection circuits printed on a piece of silicon similar to that used for qubits.

Here, they showed that stray radiation that interacts with a silicon circuit board and could potentially cause decoherence in a qubit or other “undesirable effects on circuit performance” corresponds well to the rate and spectrum of energy planned.

The research team drew on expertise developed during the design and construction of double beta decay detectors, neutrino detectors and dark matter detectors, which are also sensitive to low levels of radiation. They identified two complementary approaches to reducing the sensitivity of superconducting elements to stray radiation as first steps toward “radiation hardening”: isolating the superconducting elements on crystalline “islands” and simply making the crystalline substrate thinner.

“We have demonstrated which radiation sources are important and we look forward to seeing how the new devices will perform in our low-background facility,” Loer said.