The fundamental principle of quantum superposition enables the preparation of a quantum system in an arbitrary combination of two distinct states. A prime illustration of this lies in the superposition of two coherent states. While such superpositions are frequently referred to as Schrödinger cat states, Erwin Schrödinger’s seminal thought experiment envisioned a feline—a system at body temperature and out of thermal equilibrium—existing in a superposition of two mixed states heavily influenced by classical fluctuations. Researchers at the University of Innsbruck have now achieved the creation of hot Schrödinger cat states within a superconducting microwave resonator.
Yang et al. generated highly mixed quantum states with distinct quantum properties. Image credit: University of Innsbruck.
Schrödinger cat states represent a captivating phenomenon within quantum physics, wherein a quantum entity occupies multiple distinct states concurrently.
In Erwin Schrödinger’s conceptual framework, this manifested as a cat simultaneously alive and deceased.
Experimental realizations of such dual existence have been observed in the spatial positions of atoms and molecules, and in the oscillatory behavior of electromagnetic resonators.
Prior to this advancement, these experimental parallels to Schrödinger’s thought experiment were typically achieved by first cooling the quantum system to its energetic ground state, the lowest possible energy configuration.
In a novel investigation, Dr. Gerhard Kirchmair and his collaborators at the University of Innsbruck have conclusively demonstrated that quantum superpositions can indeed be constructed from thermally excited states.
“Schrödinger himself posited a living, or ‘hot,’ cat in his thought experiment,” remarked Dr. Kirchmair, the study’s corresponding author.
“Our aim was to ascertain whether these quantum effects could be elicited without commencing from the ‘cold’ ground state.”
For the generation of the Schrödinger cat states, the scientific team employed a transmon qubit integrated within a microwave resonator.
They successfully produced quantum superpositions at temperatures reaching 1.8 K, a thermal level sixty times greater than the ambient temperature of the resonator’s cavity.
“Our findings indicate the feasibility of generating highly mixed quantum states that exhibit discernible quantum characteristics,” stated Dr. Ian Yang, the study’s principal author.
The researchers utilized two specialized methodologies to bring about the hot Schrödinger cat states.
These methodologies had been previously applied to engineer cat states originating from the system’s ground state.
“It was found that modified protocols remain effective at elevated temperatures, yielding distinct quantum interference patterns,” explained Professor Oriol Romero-Isart, a co-author of the research.
“This breakthrough unlocks novel avenues for the creation and application of quantum superpositions, particularly in systems like nanomechanical oscillators, where attaining the ground state can present significant engineering challenges.”
“Many of our peers expressed surprise upon our initial announcement of these findings, as temperature is conventionally perceived as a factor that degrades quantum effects,” shared Thomas Agrenius, another co-author.
“Our measurements serve to validate that quantum interference can persist even under conditions of elevated temperature.”
The implications of this research could prove instrumental in the advancement of quantum technologies.
“Our investigation highlights the possibility of observing and harnessing quantum phenomena even within less-than-ideal, warmer environments,” Dr. Kirchmair commented.
“Provided that the requisite interactions can be established within a system, the definitive temperature becomes inconsequential.”
A treatise detailing these discoveries has been published in the esteemed 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
