Defining the Limits of Quantum Sensing

There are many methods to determine what the limits are for certain processes. Many of these methods look to reach the upper and lower bounds to identify them for making accurate measurements and calculations. In the growing field of quantum sensing, these limits have yet to be found.  That may change, thanks to research done by JILA Fellow Graeme Smith and his research team, with JILA and NIST Fellow James Thompson In a new study published in Physical Review Applied, the JILA and NIST researchers collaborated with scientists at the quantum company Quantinuum (previously Honeywell Quantum Solutions) to try and identify the upper limits of quantum sensing.

Quantum sensors are devices that can be used to measure gravitational waves, magnetic fields, and other physical properties. They can be part of global positioning systems or even satellites. To look at the limits of quantum sensors, the researchers studied how they behave in a type of magnetic field, called an AC magnetic field, that can be created by running an AC current through a coil. To understand various measurements of the magnetic field, the researchers used a quantity called the Quantum Fisher Information (QFI). According to first author and Smith research group graduate student, Anthony Polloreno: “The Quantum Fisher Information is a measure of how much information about a parameter you can extract from a quantum state. In this case, you have a quantum state, like the spin [of a particle], which is interacting with a magnetic field. You're interested in how much information you can extract about the magnetic field.” The researchers defined a new quantity, the integrated QFI (IQFI), a measurement of the upper bound, which they could then use in their mathematical calculations for the limitations of quantum sensing.

By simulating sensing experiments for various durations, the researchers found that the limitations of the quantum sensor were being affected by time. “The idea is that you have a quantum state in an AC magnetic field,” explained Polloreno. “The longer the thing sits in the field, in general, the more information you can learn about the magnetic field, which is why you would expect for instance, the IQFI to maybe increase as a function of time.” Looking at their data, the researchers developed a set of protocols for other scientists to use to test for a quantum sensor's upper bound, using the IQFI value. The researchers believe that these new protocols could be especially important, due to their narrower parameters, for a few applications, including axion detection and dynamical decoupling.

Axion Detection and Dynamic Decoupling

Axions are hypothetical particles that could be the source of dark matter. Other JILA Fellows are in the process of attempting to detect these axions using quantum sensors. “In axion detection, they do these kinds of broad frequency scans, where they're looking for a signal at many frequencies,” Polloreno stated. Having protocols that determine an upper bound for these broad frequency scans can help scientists save time and make more accurate measurements as the protocols start with narrow parameters.

The team's new protocols can also be used to understand dynamical decoupling. According to Polloreno: “Dynamical decoupling is this idea in quantum computation where you have noise that your qubits are experiencing. And you would rather there not be any noise, but of course, there is noise. So, dynamical decoupling tries to reduce some of this noise sensitivity by moving the susceptibility to the noise around to different frequency bands.” The researchers’ new protocols can help to zero in on the right frequency bands for lowering noise susceptibility, thereby assisting scientists with other quantum computing experiments. “From our work we've seen that our new protocols can be applied to both axion detection and dynamical decoupling in completely different ways,” Polloreno elaborated. “One is if you're trying to build sensors to detect things, and the other is if you're trying to isolate your system to not be susceptible to noise.”

Collaboration Leads to Results

This experiment not only illustrated successful collaboration between research teams at JILA, but also illustrated further cooperation with the Colorado-based quantum computing company, Quantinuum (previously Honeywell Quantum Solutions). Quantinuum was brought in via graduate student Joshua Levin, who has recently graduated from the University of Colorado Boulder with his PhD, and who had an internship at Quantinuum at the time of this study. Communicating with colleagues at JILA, Levin and the Quantinuum team were able to help create the theoretical setting of the experiment. “One of the examples we give in the paper is using a transverse field to estimate the amplitude of an AC magnetic field,” said Polloreno. “This example was provided by Josh and the scientists at Quantinuum.” Polloreno and other JILA researchers were grateful to have the benefits of the collaboration with an industrial team. “While at JILA, I've been grateful to work on projects that have been inspired by experimental collaborations,” Polloreno added. “This project was inspired by Graeme's group, James’ group, and Quantinuum. I think JILA’s commitment to connecting theory and experiment helped in this project coming about extremely naturally.”

Written by Kenna Hughes-Castleberry, JILA Science Communicator