Physicists work to prevent information loss in quantum computing

Nothing exists in a vacuum, but physicists often wish it didn’t. If the systems studied by scientists could be completely isolated from the outside world, things would be much simpler.

Take quantum computing. It’s a field that’s already attracting billions of dollars in backing from tech investors and industry heavyweights including IBM, Google, and Microsoft. But if the smallest vibrations creep in from the outside world, they can cause a quantum system to lose information.

For example, even light can cause information leakage if it has enough energy to make the atoms oscillate inside a quantum processor chip.

“Everyone is really excited about building quantum computers to answer really hard and important questions,” said Joe Kitzman, a doctoral student at Michigan State University. “But vibrational excitations can really screw up a quantum processor.”

But, with new research published in the journal Nature communications, Kitzman and his colleagues are showing that these vibrations don’t have to be a hindrance. In fact, they could benefit from quantum technology.

“If we can understand how vibrations couple with our system, we can use it as a resource and tool to create and stabilize some kinds of quantum states,” Kitzman said.

That means researchers can use these findings to help mitigate the information lost by quantum bits, or qubits (pronounced “q bits”).

Conventional computers are based on clear binary logic. Bits encode information in one of two possible distinct states, often referred to as zero or one. Qubits, however, are more flexible and can exist in states that are both zero and one at the same time.

While it may seem like a cheat, it falls perfectly within the rules of quantum mechanics. However, this feature should give quantum computers valuable advantages over conventional computers for certain problems in a variety of areas, including science, finance and cybersecurity.

Beyond its implications for quantum technology, the report from the MSU-led team also helps set the stage for future experiments to better explore quantum systems in general.

“Ideally, you want to separate your system from the environment, but the environment is always there,” said Johannes Pollanen, Jerry Cowen Endowed Chair of Physics in MSU’s Department of Physics and Astronomy. “It’s almost like garbage that you don’t want to deal with, but you can learn all kinds of interesting things about the quantum world when you do.”

Pollanen also leads the Laboratory for Quantum Hybrid Systems, of which Kitzman is a fellow, at the College of Natural Science. For the experiments conducted by Pollanen and Kitzman, the team built a system consisting of a superconducting qubit and what are known as surface acoustic wave resonators.

These qubits are one of the most popular varieties among companies developing quantum computers. Mechanical resonators are used in many modern communication devices, including cell phones and garage door openers, and now groups like Pollanen’s are putting them to work in emerging quantum technology.

The team’s resonators allowed the researchers to tune the vibrations experienced by the qubits and understand how the mechanical interaction between the two affected the fidelity of quantum information.

“We are creating a paradigmatic system for understanding how this information is encrypted,” Pollanen said. “We have control over the environment, in this case, the mechanical vibrations in the resonator, as well as the qubit.”

“If you can understand how these environmental leaks affect the system, you can use that to your advantage,” Kitzman said. “The first step in solving a problem is understanding it.”

MSU is one of the few places equipped and staffed to perform experiments on these coupled qubit-mechanical resonator devices, Pollanen said, and the researchers are excited to use their system for further exploration. The team also included scientists from the Massachusetts Institute of Technology and Washington University in St. Louis.