Created by HotSchrödinger CatStates

Quantum states can only be prepared and observed under highly controlled conditions. Researchers at Innsbruck in Austria have now successfully created the state of a so-called Hot Schrodinger cat using a superconducting microwave resonator. This study, published in Science Advances, shows that quantum phenomena can also be observed and used in less complete warm conditions.

SchrödingerCat states are an attractive phenomenon in quantum physics, where quantum objects exist simultaneously in two different states. In Erwin Schrödinger’s thought experiment, it is a cat living and dead at the same time.

In real experiments, such simultaneity is seen in the positions of atoms and molecules, as well as the vibrations of electromagnetic resonators.

Previously, these analogues to Schrödinger’s thought experiments were first created by cooling a quantum object to its ground state.

Now, researchers led by Gerhard Kirchmair and Oriol Romero-Isart have demonstrated for the first time that it is indeed possible to create quantum layers from thermally excited states.

“Schrodinger also envisioned a living, namely “hot” cats in his thought experiments,” says Kirchmair, of the Department of Experimental Physics at the Inslum University of Austria Science (ÖAW) and the Institute for Quantum Optics and Quantum Information (IQOQI).

“We wanted to know if these quantum effects could also be produced if they didn’t start from a ‘cold’ ground state,” Kirchmair said.

In their study, researchers used transmon qubits in microwave resonators to generate cat states. They managed to create quantum layers at temperatures up to 1.8 Kelvin. This is 60 times the ambient temperature of the cavity.

“Our results show that it is possible to generate highly mixed quantum states with distinct quantum properties,” explains Ian Yang, who conducted the experiment reported in this study.

The researchers used two special protocols to create the HotSchrödinger CAT state. These protocols have been used previously to produce CAT states starting from the ground state of the system.

“It turns out that the adapted protocols also work at high temperatures, producing clear quantum interference,” says Professor Oriol Romero-Isart, a professor of theoretical physics at Innsbruck University and research group leader at IQOQI Innsbruck until recently, and director of ICFO since 2024.

“This opens up new opportunities for the creation and use of quantum superpositions in, for example, nanomechanical oscillators, and achieving ground state is technically challenging.”

“When I first talked about our results, many of our colleagues were surprised because we usually think of temperature as a disruption to quantum effects,” adds Thomas Agnius, who helped develop the theoretical understanding of the experiment. “Our measurements confirm that quantum interference can last even at high temperatures.”

These findings could benefit quantum technology development. “Our work reveals that quantum phenomena can be observed and used in warm, less ideal environments,” Kirchmair emphasizes.

“If the system can create the interactions it needs, temperature is ultimately irrelevant.”