Quantum states can only be created and observed under tightly controlled conditions. A research team from Innsbruck, Austria, has successfully created so-called hot Schrödinger cat states in a superconducting microwave resonator. The study, published recently in Science Advances, demonstrates that quantum processes can be detected and employed under less-than-ideal, warmer settings.
The physicist prepares the experiment in a helium-cooled cryostat. Image Credit: ©Dominik Pfeifer
Schrödinger's cat states are a fascinating quantum physics phenomenon in which a quantum item can be in two distinct states at the same time. In Erwin Schrödinger's thought experiment, a cat is simultaneously alive and dead. In real experiments, such simultaneity has been observed in the positions of atoms and molecules, as well as the oscillations of electromagnetic resonators.
Previously, similar parallels to Schrödinger’s thought experiment were constructed by first cooling the quantum object to its ground state, which has the least conceivable energy. Researchers led by Gerhard Kirchmair and Oriol Romero-Isart have shown for the first time that thermally excited states can be used to produce quantum superpositions.
Schrödinger also assumed a living, i.e. ‘hot’ cat in his thought experiment. We wanted to know whether these quantum effects can also be generated if we don't start from the ‘cold’ ground state.
Gerhard Kirchmair, Professor, Department of Experimental Physics, University of Innsbruck
In their study, the researchers generated cat states using a transmon qubit in a microwave resonator. They were successful in establishing quantum superpositions at temperatures as high as 1.8 Kelvin, which is sixty times hotter than the ambient temperature in the cavity.
Our results show that it is possible to generate highly mixed quantum states with distinct quantum properties.
Ian Yang, Postdoctoral Researcher, University of Innsbruck
To generate the hot Schrödinger cat states, the researchers used two specific techniques. Previously, these techniques were used to generate cat states beginning with the system's ground state.
It turned out that adapted protocols also work at higher temperatures, generating distinct quantum interferences. This opens up new opportunities for the creation and use of quantum superpositions, for example in nanomechanical oscillators, for which achieving the ground state can be technically challenging.
Oriol Romero-Isart, Professor, Theoretical Physics, University of Innsbruck
Romero-Isart is also a Research Group Leader at IQOQI Innsbruck and since 2024 Director of ICFO - the Institute of Photonic Sciences in Barcelona.
Thomas Agrenius, PhD Student, University of Innsbruck, added, “Many of our colleagues were surprised when we first told them about our results, because we usually think of temperature as something that destroys quantum effects. Our measurements confirm that quantum interference can persist even at high temperature.”
These study discoveries may aid in the development of quantum technologies.
“Our work reveals that it is possible to observe and use quantum phenomena even in less ideal, warmer environments,” “If we can create the necessary interactions in a system, the temperature ultimately doesn't matter,” emphasized Gerhard Kirchmair.
The study was supported by the Austrian Science Fund (FWF) and the European Union, among others.
Journal Reference:
Yang, I., et al. (2025) Hot Schrödinger cat states. Science Advances. doi.org/10.1126/sciadv.adr4492