Hot Schrödinger cat states created

Schrödinger cat states are a fascinating phenomenon in quantum physics in which a quantum object exists simultaneously in two different states. In Erwin Schrödinger’s thought experiment, it is a cat that is alive and dead at the same time. In real experiments, such simultaneity has been seen in the locations of atoms and molecules and in the oscillations of electromagnetic resonators. Previously, these analogues to Schrödinger’s thought experiment were created by first cooling the quantum object to its ground state, the state with the lowest possible energy. Now, researchers led by Gerhard Kirchmair and Oriol Romero-Isart have demonstrated for the first time that it is indeed possible to create quantum superpositions from thermally excited states. “Schrödinger also assumed a living, i.e. ‘hot’ cat in his thought experiment,” remarks Gerhard Kirchmair from the Department of Experimental Physics at the University of Innsbruck and the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences (ÖAW). “We wanted to know whether these quantum effects can also be generated if we don’t start from the ‘cold’ ground state,” says Kirchmair.

In their study published in the Science Advances, the researchers used a transmon qubit in a microwave resonator to generate the cat states. They succeeded in creating the quantum superpositions at temperatures of up to 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,” explains Ian Yang, who performed the experiments reported in the study.

The researchers used two special protocols to create the hot Schrödinger cat states. These protocols were previously used to produce cat states starting from the ground state of the system. “It turned out that adapted protocols also work at higher temperatures, generating distinct quantum interferences,” says Oriol Romero-Isart, until recently Professor of Theoretical Physics at the University of Innsbruck and research group leader at IQOQI Innsbruck and since 2024 Director of ICFO — the Institute of Photonic Sciences in Barcelona. “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.”

“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,” adds Thomas Agrenius, who helped develop the theoretical understanding of the experiment. “Our measurements confirm that quantum interference can persist even at high temperature.”

These research findings could benefit the development of quantum technologies. “Our work reveals that it is possible to observe and use quantum phenomena even in less ideal, warmer environments,” emphasizes Gerhard Kirchmair. “If we can create the necessary interactions in a system, the temperature ultimately doesn’t matter.”

The study was funded by the Austrian Research Fund FWF and the European Union, among others.