Particle physics experiments

Experiments prove quantum computational errors are correlated, linked to cosmic rays


In experiments at the University of Wisconsin-Madison, researchers found that fluctuations in the electrical charge of several quantum bits, or “qubits,” can be strongly correlated, as opposed to completely random and independent. The team also linked tiny disturbances causing errors in the charge state of qubits to cosmic ray absorption. Credit: Illustration courtesy of UW-Madison

Research by a physicist at Lawrence Livermore National Laboratory (LLNL) and a host of collaborators sheds new light on one of the major challenges to realizing the promise and potential of quantum computing – correction of errors.

In a new article published in Nature and co-authored by LLNL physicist Jonathan DuBois, the scientists examined the stability of quantum computing, specifically the causes of errors and how quantum circuits respond to them. This must be understood in order to build a functioning quantum system. Other co-authors included researchers from the University of Wisconsin-Madison, the Fermi National Accelerator Laboratory, Google, Stanford University, and international universities.

In experiments at UW-Madison, the research team characterized a quantum test bench apparatus, finding that fluctuations in the electrical charge of several quantum bits, or “qubits” – the unit basis of a quantum computer – can be strongly correlated, as opposed to completely random and independent. When a disruptive event occurs, such as a burst of energy from outside the system, it can simultaneously affect every qubit in the vicinity of the event, resulting in correlated errors that can span the entire system. system, the researchers found. Additionally, the team linked tiny disturbances causing errors in the state of charge of qubits to cosmic ray absorption, a finding that is already having an impact on the design of quantum computers.

“For the most part, the schemes designed to correct errors in quantum computers assume that the errors between qubits are not correlated – they are random. Correlated errors are very difficult to correct, ”said co-author DuBois, who heads the Quantum Coherent Device Physics (QCDP) group at LLNL. “Basically what this article shows is that if a high energy cosmic ray hits the device somewhere, it has the potential to affect everything in the device at once. Unless you can prevent this from happening, you cannot perform error correction effectively, and you will never be able to create a functioning system without it.

Unlike the bits found in classical computers, which can only exist in binary states – zeros or ones – the qubits that make up a quantum computer can exist in overlays. For a few hundred microseconds, the data in a qubit can be at one or zero before being projected into a conventional binary state. While bits are sensitive to only one type of error, under their temporary excited state of charge, delicate qubits are susceptible to two types of errors, resulting from changes that may occur in the environment.

Even tiny charged pulses like those from cosmic rays absorbed by the system can create an explosion of (relatively) high energy electrons that can heat the substrate of the quantum device just long enough to disrupt the qubits and disrupt their quantum states, the researchers have found. When a particle impact occurs, it creates a wake of electrons in the device. These charged particles pass through the materials of the device, dispersing atoms and producing high-energy vibrations and heat. This changes the electric field as well as the thermal and vibrational environment around the qubits, causing errors, DuBois explained.

“We always knew it was possible and that it was a potential effect – one of many that can influence the behavior of a qubit,” added DuBois. “We even joked when we saw poor performance that maybe it was because of the cosmic rays. The importance of this research is that, given this type of architecture, it places quantitative limits on what you can expect in terms of performance for current device designs in the presence of environmental radiation.

To visualize the disturbances, the researchers sent radiofrequency signals through a four-qubit system and, by measuring their excitation spectrum and performing spectroscopy on them, were able to see the qubits “switch” from one quantum state to one. other, observing that they are all moving in energy at the same time, in response to changes in the charging environment.

“If our model of particle impacts is correct, we would expect most of the energy to be converted into vibrations in the chip that propagate over long distances,” said Chris Wilen, graduate student at the ‘UW-Madison, lead author of the article. “As the energy propagates, the disturbance would lead to qubit reversals which are correlated across the chip.”

Using this method, the researchers also looked at the lifespan of qubits – how long qubits can stay in their one and zero superposition – and the correlated changes in state of charge with reduction. the lifetime of all qubits in the system.

The team concluded that correcting quantum errors will require the development of mitigation strategies to protect quantum systems from correlated errors due to cosmic rays and other particle impacts.

“I think people have approached the problem of error correction in an overly optimistic manner, blindly assuming that errors are not correlated,” said Robert McDermott, professor of physics at UW-Madison, author principal of the study. “Our experiences absolutely show that errors are correlated, but as we identify the problems and develop a deeper physical understanding, we will find ways to work around them.”

Although long theorized, DuBois said the team’s findings had never been proven experimentally in a multi-qubit device before. The results will likely have an impact on the architecture of the future quantum system, for example by placing quantum computers behind lead shielding or underground, introducing heat sinks or dampers to quickly absorb energy and isolate qubits, and modify the types of materials used in quantum systems.

LLNL currently has a Quantum Computing Benchmark System, designed and built with funding from a Laboratory-Led Strategic Research and Development Initiative (LDRD) that began in 2016. It is being developed with the continued support for the National Nuclear Security Administration’s Advanced Simulation & Computing program and its Beyond Moore’s Law project.

As part of related follow-up work, DuBois and his team from the QCDP group are studying a quantum device that is significantly less sensitive to the load environment. At the extremely cold temperatures required by quantum computers (systems are kept at temperatures colder than outer space), DuBois said the researchers observed that the transport of thermal and coherent energy is qualitatively different from the ambient temperature. For example, instead of diffusing, thermal energy can bounce around the system like sound waves.

DuBois said he and his team are focused on understanding the dynamics of the “microscopic explosion” that occurs inside quantum computing devices when they interact with high-energy particles and on developing ways absorb the energy before it can disrupt the delicate quantum states stored in the device. .

“There are potentially ways to design the system to be as unresponsive as possible to these kinds of events, and in order to do that you have to really understand how it heats up, how it cools down and what exactly is going on. . throughout the process when exposed to background radiation, ”said DuBois. “The physics of what’s going on is quite interesting. It’s a frontier, even outside of quantum applications, due to the quirks of how energy is transported at these low temperatures. This makes it a physics challenge.

DuBois worked with the paper’s principal investigator, McDermott (UW-Madison) and his group to develop ways to use qubits as detectors to measure charge bias, the method the team used in the article to conduct their experiments.

Reference: “Correlated charge noise and relaxation error in superconducting qubits” by CD Wilen, S. Abdullah, NA Kurinsky, C. Stanford, L. Cardani, G. D’Imperio, C. Tomei, L. Faoro, LB Ioffe, CH Liu, A. Opremcak, BG Christensen, JL DuBois and R. McDermott, June 16, 2021, Nature.
DOI: 10.1038 / s41586-021-03557-5

The work presented, including DuBois’ contribution, was funded by a Collaborative Grant between LLNL and UW-Madison from the Office of Science, US Department of Energy.

The article included coauthors from UW-Madison, Fermi National Accelerator Laboratory, Kavli Institute for Cosmological Physics at University of Chicago, Stanford University, INFN Sezione di Roma, Theoretical Physics and High Energies Laboratory of the Sorbonne University and Google.