For about a century, we have known that quantum systems are destroyed on measurement. This makes it extremely difficult to experiment with repeated measurements on the same quantum system because the system needs to be re-prepared for each subsequent measurement.

Elliott et al. achieved non-destructive quantum measurement using a single continuous-variable quantum mode, or qumode.

“We propose a probe made from a qumode, an infinite-dimensional quantum state that a laser beam can produce,” said author Nana Liu.

When an appropriately initialized qumode state interacts with a quantum system, the qumode state takes on the statistics and energy spectrum of the quantum system. Subsequent measurement of the qumode reveals the quantum system’s spectrum and the populations of the respective energy states.

“This enables a full characterization of the system to its observables, as though one had directly sampled the observable from the system,” said Liu. “Since we only directly measure the qumode, this leaves the quantum system of interest intact.”

The study experimentally validated this method by applying it to a spin-half system in a transverse field.

Applications for non-destructive quantum measurement abound. The qumodes can act as thermometers to measure the temperature of quantum systems in equilibrium, reconstruct partition functions, and measure out-of-equilibrium thermodynamical quantities.

“Previous work on using a qumode for computational problems inspired this study,” said Liu. “In the future, we want to explore deeper connections between sensing and computational problems. The next steps are experimental efforts to realize interactions for large-scale quantum systems.”

Source: “Non-destructively probing the thermodynamics of quantum systems with qumodes,” by Thomas J. Elliott, Mile Gu, Jayne Thompson, and Nana Liu, AVS Quantum Science (2023). The article can be accessed at

This paper is part of the Jonathan P. Dowling Memorial Special Issue: The Second Quantum Revolution Collection, learn more here.