Image: By combining viewer qubits (yellow) and data qubits (blue), PME researchers can constantly monitor and correct noise and errors in a quantum computer. see more
Photo credit: UChicago – Bernien Lab
Despite their immense promise to solve novel problems, today’s quantum computers are inherently error-prone. A small disturbance in their environment – for example a change in temperature, pressure or magnetic field – can destroy their fragile computing components, so-called qubits.
Now researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) have developed a new method to constantly monitor the noise around a quantum system and adjust the qubits in real time to minimize errors.
The approach described in Science is based on viewer qubits: a set of qubits embedded in the computer with the sole purpose of measuring outside noise, not storing data. The information gathered from such viewer qubits can then be used to suppress noise in vital data processing qubits.
assistant Prof Hannes Bernien, who led the research, compares the new system to noise-cancelling headphones that continuously monitor ambient noise and emit opposing frequencies to cancel it out.
“With this approach, we can improve the quality of the data qubits very significantly,” said Bernien. “I think that’s very important in the context of quantum computing and quantum simulation.”
A huge challenge
As existing quantum computers scale, the challenge of noise and errors has grown. The problem is twofold: qubits change easily in response to their environment, which can alter the information they store and lead to high error rates. Additionally, if a scientist measures a qubit to estimate the noise it has been subjected to, the qubit’s state will collapse and its data will be lost.
“It’s a very daunting and difficult task to correct the errors within a quantum system,” Bernien said.
Theoretical physicists had previously proposed a solution using spectator qubits, a set of qubits that don’t store any necessary data but could be embedded in a quantum computer. The viewer qubits would track changes in the environment, acting like the microphone in noise-cancelling headphones. A microphone, of course, only detects sound waves, while the proposed viewer qubits would respond to any environmental disturbances that might alter the qubits.
Two types of noise canceling qubits
Bernien’s group wanted to show that this theoretical concept could be used to suppress noise in a quantum array of neutral atoms – their favorite quantum computer.
In a quantum processor for neutral atoms, atoms are held in place using laser beams called optical tweezers. Bernien helped develop it, earning him awards such as the 2023 New Horizons in Physics Prize from the Breakthrough Prize Foundation. In large arrays of these floating atoms, each acts as a qubit, capable of storing and processing information in its superposition state.
In 2022, Bernien and colleagues first reported the possibility of fabricating a hybrid atomic quantum processor containing both rubidium and cesium atoms. Now they have adapted this processor so that the rubidium atoms act as data qubits while the cesium atoms act as viewer qubits. The team designed a system to continuously read real-time data from the rubidium atoms and, in response, optimize the cesium atoms with microwave oscillations.
The challenge, Bernien said, is making sure the system is fast enough — any adjustments to the rubidium atoms must be nearly instantaneous.
“What’s really exciting about this is that it not only minimizes the noise of the data qubits, but also is an example of actually interacting with a quantum system in real time,” said Bernien.
proof of principle
To test their error minimization approach, Bernien’s group exposed the quantum array to magnetic field noise. They showed that the cesium atoms correctly picked up this noise and their system then canceled it in the rubidium atoms in real time.
However, the research group says the first prototype is just a starting point. You want to try increasing the noise levels, varying the types of interference, and testing if the approach holds up.
“We have exciting ideas on how to improve the sensitivity of this system by a large factor, but it will require more work to implement it,” said Bernien. “That was a great starting point.”
Finally, Bernien envisions that a system of spectator qubits could run constantly in the background of every neutral-atom quantum computer, as well as quantum computers of other architectures, thereby minimizing error when the computer stores data and performs calculations.
Mid-circuit correction of correlated phase errors using a set of viewer qubits
Article publication date
May 25, 2023
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