The quantum superposition principle allows us to prepare a system in a superposition of two arbitrary states. The paradigmatic example is the superposition of two coherent states. While the superposition of such states is typically called a Schrödinger cat state, in Erwin Schrödinger’s original thought experiment, the cat — a body-temperature and out-of-equilibrium system — is prepared in a superposition of two mixed states dominated by classical fluctuations. Physicists at the University of Innsbruck have now succeeded in creating hot Schrödinger cat states in a superconducting microwave resonator.
Yang et al. generated highly mixed quantum states with distinct quantum properties. Image credit: University of Innsbruck.
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.
In a new study, Dr. Gerhard Kirchmair and his colleagues at the University of Innsbruck demonstrated 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,” said Dr. Kirchmair, corresponding author of the study.
“We wanted to know whether these quantum effects can also be generated if we don’t start from the ‘cold’ ground state.”
To generate the Schrödinger cat states, the researchers used a transmon qubit in a microwave resonator.
They succeeded in creating the quantum superpositions at temperatures of up to 1.8 K — 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,” said Dr. Ian Yang, first author of the study.
The scientists 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,” said Professor Oriol Romero-Isart, co-author of the study.
“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”, said Thomas Agrenius, co-author of the study.
“Our measurements confirm that quantum interference can persist even at high temperature”.
The 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,” Dr. Kirchmair said.
“If we can create the necessary interactions in a system, the temperature ultimately doesn’t matter.”
A paper on the findings was published in the journal Science Advances.
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Ian Yang et al. 2025. Hot Schrödinger cat states. Science Advances 11 (14); doi: 10.1126/sciadv.adr4492
