Science Noise-canceling quantum bits achieve major breakthrough in error correction

In a new paper in the journal Science, researchers in Assistant Professor Hannes Bernien's lab at the University of Chicago's Pritzker School of Molecular Engineering describe a way to constantly monitor noise around quantum systems and adjust quantum bits in real time to reduce errors - they introduce the "spectator qubit" (spectator qubit).

Despite their great promise of solving a new kind of problem, today's quantum computers are inherently error-prone. A small perturbation in their surroundings (e.g., a change in temperature, pressure, or magnetic field) can destroy their fragile computational building blocks - i.e., quantum bits).

Now, new methods developed by researchers at the University of Chicago's Pritzker Institute for Molecular Engineering can constantly monitor the noise around quantum systems and adjust the quantum bits in real time to minimize errors.

The method, described online in Science, relies on "bystander quantum bits": a set of quantum bits embedded in a computer whose sole purpose is to measure external noise rather than to store data. The information gathered by such bystander quantum bits can then be used to offset important data in order to deal with the noise of the quantum bits.

Hannes Bernien

Assistant Professor Hannes Bernien, who led the research, likens the new system to a noise-cancelling headset: it constantly monitors surrounding noises and emits the opposite frequency to cancel them.

"With this approach, we can very robustly improve the quality of the quantum bits of data," Bernien said, "which I think is very important in quantum computing and quantum simulation."

Error correction has always been a daunting challenge in quantum systems.

As existing quantum computers scale up, the challenge of noise and error grows. The problem is twofold: qubits are prone to change with their environment: this can alter the information stored inside them, leading to high error rates; furthermore, if scientists measure a quantum bit and try to measure the noise it is exposed to, the state of the quantum bit can collapse and lose its data.

As a result, Professor Bernien says, "Trying to correct errors within a quantum system is a very difficult and daunting task."

Theoretical physicists have previously proposed a solution using bystander quantum bits, a set of quantum bits that do not store any necessary data, but which can be embedded in a quantum computer. The bystander quantum bits would track changes in the environment, acting like a microphone in a noise-canceling headset. Of course, the microphone would only detect sound waves, while the proposed bystander quantum bit would respond to any environmental perturbations that could alter the quantum bit.

Finally, Bernien's group set out to demonstrate that this theoretical concept could be used to cancel noise in a quantum array of neutral atoms.

The research team works in Hannes Bernien's lab at the University of Chicago's Pritzker Institute for Molecular Engineering. From left: postdoctoral scholar Conor Bradley, graduate student Vikram Ramesh, postdoctoral scholar Kevin Singh, and assistant professor Hannes Bernien.

In a neutral-atom quantum processor, atoms are suspended in place using laser beams called "optical tweezers" (which Bernien helped develop and for which he won, among other honors, the 2023 New Horizons in Physics Award from the Breakthrough Prize Foundation). In these large arrays of suspended atoms, each atom acts as a quantum bit, capable of storing and processing information in its superimposed state.

In 2022, Bernien and colleagues first reported the ability to fabricate a hybrid atomic quantum processor containing rubidium and cesium atoms. Now, they have adapted that processor so that rubidium atoms act as data quantum bits and cesium atoms are bystander quantum bits. The team designed a system that continuously reads real-time data from the rubidium atoms; and in response, tunes the cesium atoms with microwave oscillations.

The challenge, Bernien says, was to make sure the system was fast enough: Any adjustment the system made to the rubidium atoms had to be nearly instantaneous.

"What's really exciting is that it not only minimizes any noise in the quantum bits of data, but it's a case of actually interacting with the quantum system in real time."

-- To test their error-minimizing method, Bernien's group exposed the quantum array to magnetic field noise. They showed that the cesium atoms correctly picked up this noise; their system then eliminated it in real time in the rubidium atoms.

By combining bystander quantum bits (yellow) and data quantum bits (blue), the PME researchers can constantly monitor and correct noise and errors within the quantum computer.

However, the team says the initial prototype is just a starting point. Subsequently, they want to try to increase the amount of noise, change the type of perturbation and test whether this approach holds up.

"We have exciting ideas about how to make this system a whole lot more sensitive, but more work is needed to make it happen," Bernien said, "and this is a good starting point."

Ultimately, Bernien imagines a bystander quantum bit system that could run constantly in the background of any neutral-atom quantum computer, as well as in quantum computers of other architectures, minimizing errors as the computer stores data and performs calculations.

Reference links:

[1] https://www.science.org/doi/10.1126/science.ade5337

[2]https://news.uchicago.edu/story/noise-cancelling-qubits-can-minimize-errors-quantum-computers

[3]https://pme.uchicago.edu/news/noise-cancelling-qubits-developed-uchicago-minimize-errors-quantum-computers