In a surprising twist to one of quantum physics’ most famous thought experiments, researchers at the University of Innsbruck in Austria, have demonstrated that quantum states—those fragile, strange conditions where particles can exist in two opposing states at once—can survive, and even thrive, in warmer environments than previously thought.
Published recently in Science Advances, the study shows that “hot” Schrödinger cat states can be created in superconducting microwave resonators, tiny devices that trap and bounce microwaves around. Because they’re made of superconducting materials (which have no electrical resistance when very cold), they can hold and control energy precisely, making them perfect for studying delicate physical effects.
This marks a key step toward making quantum technologies more robust in real-world conditions where devices can’t reach the cold temperatures usually needed for physics processes.
“Our work reveals that it is possible to observe and use quantum phenomena even in less ideal, warmer environments,” lead researcher Gerhard Kirchmair from the University of Innsbruck and the Austrian Academy of Sciences said in a recent statement. “If we can create the necessary interactions in a system, the temperature ultimately doesn’t matter.”
A Schrödinger Quantum Cat State
If you’ve ever heard the phrase “Schrödinger’s cat,” you’ve encountered one of quantum physics’ most mind-bending concepts. In the original thought experiment, physicist Erwin Schrödinger imagined a cat sealed in a box with a device that had a 50/50 chance of killing it based on the decay of a radioactive atom. Until the box is opened, the cat is paradoxically both alive and dead—existing in a superposition of states.
Physicists have since recreated analogues of this scenario, not with actual cats but with particles, atoms, and even electromagnetic resonators. These so-called Schrödinger cat states are a hallmark of quantum mechanics, revealing the strange ways quantum objects can hold multiple realities simultaneously.
However, these effects are usually only observed under extremely cold and carefully controlled conditions. That’s because heat is typically the enemy of quantum coherence—it introduces noise that can collapse these delicate superpositions in a process known as decoherence.
Heating Up the Quantum World
In the new study, researchers challenged that conventional wisdom by asking a bold question: What if Schrödinger’s cat wasn’t cold and still—but warm and alive?
“Schrödinger also assumed a living, i.e. ‘hot’ cat in his thought experiment,” explained Kirchmair. “We wanted to know whether these quantum effects can also be generated if we don’t start from the ‘cold’ ground state.”
To find out, the team used a specific quantum state called a transmon qubit inside a microwave resonator, a device commonly used in quantum computing. Typically, experiments like this begin by cooling the system to nearly absolute zero, placing it in its “ground state”—the lowest possible energy condition. But this time, researchers allowed the system to stay warmer—up to 1.8 Kelvin (around -271.35°C), which is sixty times hotter than the resonator’s normal operating temperature.
“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, a researcher who helped develop the theory behind the work. “Our measurements confirm that quantum interference can persist even at high temperature.”
The team generated cat states from these higher-energy, thermally excited conditions using specially adapted experimental protocols.
“It turned out that adapted protocols also work at higher temperatures, generating distinct quantum interferences,” noted Oriol Romero-Isart, now Director of ICFO – the Institute of Photonic Sciences in Barcelona. “This opens up new opportunities for the creation and use of quantum superpositions.”
Making Better Quantum Systems
These findings could have major implications for the future of quantum computing and sensing technologies. One of the biggest challenges facing quantum engineers today is how to keep quantum systems isolated from heat and noise in practical settings. If quantum states can survive and even be created in warmer environments, that could make building quantum devices simpler, cheaper, and more scalable.
“Our results show that it is possible to generate highly mixed quantum states with distinct quantum properties,” said Ian Yang, the experimentalist behind much of the work.
The research could also aid the development of devices like nanomechanical oscillators, which are notoriously difficult to cool to their ground states. The team’s protocols show that starting from a “hot” state doesn’t necessarily rule out the possibility of achieving quantum superpositions.
“This opens up new opportunities,” said Romero-Isart. “For example, in systems where reaching the cold ground state is technically challenging.”
Kenna Hughes-Castleberry is a freelance science journalist and staff writer at The Debrief. Follow and connect with her on BlueSky or contact her via email at kenna@thedebrief.org