Better precision from a quantum thermometer

CQT researchers and their collaborators describe a scheme that beats the classical precision limit for temperature measurement
25 November 2019

Stock photo of many classical thermometers CQT's Stella Seah and Valerio Scarani and collaborators have calculated that a stream of quantum bits can beat classical limits on temperature measurement. Image: The Faces/Shutterstock.com

Quantum computing hogs the headlines, but there are quantum advantages to be had for other technologies too. Researchers including CQT’s Stella Seah and Valerio Scarani describe in Physical Review Letters how quantum effects could bring extra precision to temperature measurements.

The team describes a ‘quantum thermometer’ that measures an environment by making repeated measurements of a separate system in contact with that environment. Each measurement is performed by a quantum bit interacting with the intermediary system for a short time.

What’s presented in the paper is a theoretical scheme. The approach might be realised in the real world with quantum bits, or qubits, encoded in objects such as Rydberg atoms interacting with a cavity.

“There are funny things going on,” says Valerio. The researchers calculate that a stream of qubits can extract more information about the temperature of the environment than is predicted by purely classical physics. That hits a limit known as the thermal Cramer-Rao bound.

Beyond the classical bound

Two types of advantages are reported. The first comes from the fact that each qubit encodes information on the temperature not only in the populations of its energy levels, but also in the coherence between them. Because of this, in some regime the quantum thermometer works best under surprising conditions.

Valerio explains, “You do better if you don’t let the system finish thermalising with the environment between measurements.” It’s as if you could get a better measurement of your own temperature with a set of old-style mercury bulb thermometers by reading out the results before the mercury stopped rising (although, of course, that does not work because those thermometers don’t exhibit quantum coherence).

The second advantage comes from the entanglement generated between successive qubits by their interaction with the probe. That information is accessed when successive qubits are measured as a group. This advantage becomes prominent in a regime of very small interaction, in other words when the probing is minimally disturbing. The researchers calculate the advantage numerically for groups of up to 12 qubits, finding that the precision could be improved by more than two decimal places.

More work ahead

There has been work before on the design of quantum thermometers, considering applications in experiments such as those using ultra-cold atoms or superconducting circuits. The new scheme in this paper is, for now, a toy model: more work is needed to assess whether it can be implemented in a platform that leads to practical advantages.

Stella and Valerio's collaborators in the research, and co-authors on the paper,  are CQT alumnus Stefan Nimmrichter, now at the Max Planck Institute for the Science of Light in Erlangen, Germany, Daniel Grimmer at the Institute for Quantum Computing at the University of Waterloo in Canada, and Jader Santos and Gabriel Landi of the University of Sao Paulo in Brazil.