For humans, background noise is usually only a minor irritant. But for quantum computers, which are very sensitive, this can spell the end of calculations. And as the “noise” of a quantum computer increases as the computer is tasked with more complex calculations, it can quickly become a major hindrance.
But because quantum computers could be so incredibly useful, researchers have been experimenting with ways around the noise problem. Typically, they try to measure the noise in order to fix it, with mixed success.
A group of scientists from the University of Chicago and Purdue University have collaborated on a new technique: Instead of trying to measure noise directly, they instead build a unique “fingerprint” of the noise on a computer quantum as seen by an executed program. on the computer.
This approach, they say, holds promise for mitigating the noise problem, as well as suggesting ways users can turn noise to their advantage.
“We wondered if there was a way to work with noise, instead of opposing it,” said David Mazziotti, a professor in the Department of Chemistry, James Franck Institute and Chicago Quantum Exchange and co-author of the study, published January 25 in Nature Communication Physical.
“A new approach”
Quantum computers are based on the laws of particle behavior at the atomic level. At this level, the particles obey a very strange set of rules; they can be in two different states at the same time or become entangled in space. Scientists hope to exploit these capabilities as the basis for computers.
In particular, many scientists want to use quantum computers to better understand the rules of the natural world, because molecules operate according to the laws of quantum mechanics, which should theoretically be easier to simulate using a quantum computer. .
But despite significant advances in quantum computing technology over the past decade, computing power has fallen short of scientists’ hopes. Many had assumed that increasing the number of computing bits — “qubits,” for quantum computers — would help alleviate the noise problem, but because noise limits precision, scientists still haven’t been able to figure it out. perform most of the calculations they would like.
“We thought it might be time for a new approach,” said co-author Saber Kais, professor of physics and chemistry at Purdue University.
To date, scientists have attempted to understand the effect of noise by directly measuring the noise in each qubit. But cataloging such subtle changes is difficult and, according to the band, may not always be the most efficient route.
“Quite often in physics, it’s actually easier to understand the overall behavior of a system than it is to know what each part is doing,” said co-author Zixuan Hu, a postdoctoral researcher at Purdue. “For example, it’s hard to simulate what each molecule is doing in a glass of water, but it’s much easier to predict how the whole thing will behave.”
So instead of trying to measure the actual noise precisely, the scientists decided to run a test to get an idea of the overall noise experienced by quantum computers.
They chose a particular calculation of a molecule displaying quantum behavior and ran it as a simulation on a quantum computer. Then they changed the parameters of the problem in several different directions and tracked the reaction of the noise. “Putting all of this together, we build a ‘fingerprint’ of the noise as perceived by the simulation we’re running,” Mazziotti said.
Hu explained that running a calculation of an already well-known molecule helped them unravel the specific effects of noise.
“We know very little about quantum computers and noise, but we know very well how this molecule behaves when excited,” Hu said. “So we’re using quantum computers, which we don’t know much about, to mimic a molecule that we are familiar with, and we see how it behaves. With these familiar patterns, we can derive some understanding.
This operation gives a more “bird’s eye view” of the noise that quantum computers simulate, said Scott Smart, who holds a Ph.D. student at the University of Chicago and first author of the article.
The authors hope that this information can help researchers think about how to design new ways to correct the noise. It might even suggest ways the noise might be helpful, Mazziotti said.
For example, if you try to simulate a quantum system such as a molecule in the real world, you know it will experience noise, because noise exists in the real world. In the previous approach, you use computing power to add a simulation of this noise.
“But instead of integrating noise as an additional operation on a quantum computer, perhaps we could use the noise intrinsic to a quantum computer to mimic the noise of a hard-to-solve quantum problem on a conventional computer,” said Mazziotti.
The authors believe this unique approach to the noise problem is useful as researchers continue to explore the young field of quantum computing.
“We still don’t know what kinds of problems quantum computers will be most useful for,” Mazziotti said. “We hope this will provide a different way of thinking about noise that will open up new avenues for simulating molecules with quantum devices.”
Quote: “Relaxation of steady states on a quantum computer gives a unique spectroscopic fingerprint of computer noise.” Smart, Hu, Kais and Mazziotti, Nature Communication PhysicalJanuary 25, 2022.
Funding: US Department of Energy Office of Basic Energy Sciences, National Science Foundation. We acknowledge the use of IBM Quantum services for this work.