A new signal among qubit sensor noise

Quantum sensors harness the power of qubits to sense even the tiniest changes in an environment, including variations in magnetic or electric fields. Researchers believe these sensors could ultimately be used in wide-ranging applications, including monitoring geologic activity and detecting early stages of cancer.

Aashish Clerk
Prof. Aashish Clerk

Theorists at the Pritzker School of Molecular Engineering (PME) at the University of Chicago have found a new way to use these sensors to measure a system’s temperature and energy levels: by understanding the signal that comes from a qubit as it is being prepared to sense a system.

The research, led by Prof. Aashish Clerk with graduate student Yu-Xin Wang, could have implications for understanding quantum materials, such as superconductors and quantum magnets. The results were published Nov. 11 in Nature Communications.

Finding a new signal

To be used as sensors, qubits must be placed in a superposition state. That means that unlike classical bits, which measure either 1 or a 0, qubits are placed in multiple states at the same time until they are measured.

But when researchers prepare qubits to be in a superposition state, the qubit can inadvertently “kick” the system they are trying to measure. That’s because preparing the qubits in this way causes a small, sudden change in the energy levels of the system being measured, something which physicists call a “quench.”

“That quench leads to a distinct signal that the qubit sensor can pick up,” Clerk said. “The signal has always been there, but now we have discovered a way to use it to learn about the system you are probing.”

For example, that signal can tell researchers the temperature of the system they are probing without further assumptions, as well as information about the system’s energy levels and thermal equilibration. That could be useful, for example, in determining when a material is about to undergo a phase transition. For certain materials, a small change in temperature causes the material to become magnetic or turn into a superconductor. Using this signal in a qubit sensor that has very fine spatial resolution could allow researchers to examine these material properties at a small scale.

Opening the door to new theory

Clerk and Wang calculated how researchers can make this signal even stronger and are working with experimentalists to test out their theory in quantum sensors made from nitrogen-vacancy centers in diamonds.

“This opens the door to a lot of new theory about what could happen if we probe complicated quantum systems using this technique,” Clerk said.

Citation: “Intrinsic and induced quantum quenches for enhancing qubit-based quantum noise spectroscopy,” Wang et al, Nov. 11, 2021, Nature Communications, DOI:

Funding: Department of Energy