Physicists create hot Schrödinger cat states for the first time

Quantum phenomena usually demand extremely controlled, cold environments. Scientists traditionally cool systems to near absolute zero to observe clear quantum states. However, a recent experiment proves quantum effects can survive at much warmer temperatures.

Researchers have successfully created quantum states, known as Schrödinger cat states, in surprisingly hot conditions—up to 60 times warmer than previously possible. This breakthrough could reshape quantum technology by making it easier and cheaper to harness quantum mechanics.

Quantum mechanics is strange yet fascinating. Particles can exist in multiple states simultaneously, known as quantum superposition. A classic example is Schrödinger’s famous thought experiment involving a cat that is simultaneously alive and dead. This experiment highlights how quantum systems can defy classical intuition.

In real-world laboratories, scientists usually mimic this scenario by creating "Schrödinger cat states" using quantum particles. To achieve clear quantum effects, researchers traditionally chill these particles to extremely low temperatures. At these temperatures, particles reach their "ground state," the lowest energy state available, allowing scientists to observe pure quantum states without interference.

But what happens if scientists use hotter, less controlled conditions? Could quantum states still survive in warmer environments filled with disturbances? Recent research now shows that quantum superpositions—states where particles exist in multiple configurations—can indeed persist in warmer conditions.

A research team recently demonstrated that quantum states can form even in highly mixed and warmer environments. Instead of starting with a pure, cooled state, scientists began with mixed states, where thermal fluctuations are strong. They used a microwave resonator containing a quantum device called a transmon qubit to create these hot Schrödinger cat states.

Previously, scientists prepared quantum cat states by cooling a cavity to extremely low temperatures. The new study tested whether these quantum effects could occur at significantly warmer temperatures, specifically up to about 1.8 Kelvin—60 times hotter than typical experiments.

Remarkably, the experiment succeeded. Even under these challenging conditions, clear quantum superposition states emerged. The researchers confirmed their findings using detailed measurements called Wigner functions. These functions visually represent quantum interference patterns, revealing quantum features clearly, even in mixed states with very low purity.

“Our results show that it is possible to generate highly mixed quantum states with distinct quantum properties,” explained researcher Ian Yang, who led the experiments.

Creating Schrödinger cat states at higher temperatures wasn't just challenging; it also surprised many scientists. Normally, temperature is considered an enemy of quantum phenomena. Heat introduces chaos and disorder, typically destroying delicate quantum states.

“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, who contributed to developing the theory behind the experiment. “Our measurements confirm that quantum interference can persist even at high temperature.”

This discovery significantly expands scientists' understanding of quantum mechanics. Quantum interference—essentially, particles interacting with themselves—is a hallmark of quantum physics. Observing these patterns at higher temperatures suggests quantum phenomena could be more robust and widespread than previously thought.

Why does creating quantum states at higher temperatures matter? Simply put, cooling quantum systems is costly, complex, and technically challenging. Most quantum technology currently relies on ultra-cold conditions, limiting widespread use.

The success of this experiment suggests quantum technologies might not always require such demanding conditions. “Our work reveals that it is possible to observe and use quantum phenomena even in less ideal, warmer environments,” emphasized 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), and one of the leading researchers.

If quantum effects can reliably appear at higher temperatures, it could simplify developing quantum devices. Quantum computers, sensors, and communication systems might operate more easily and economically, accelerating the growth of practical quantum technologies.

“This opens up new opportunities for the creation and use of quantum superpositions, for example, in nanomechanical oscillators,” said researcher Oriol Romero-Isart.

Nanomechanical oscillators are tiny devices that currently struggle with the technical challenge of cooling to absolute zero. The ability to work at higher temperatures could transform these devices, making quantum technologies accessible in everyday applications.

Traditionally, Schrödinger cat states created in laboratories are "cold"—pure states produced under perfect conditions. However, this new study shows the possibility of creating "hot" cat states, superpositions emerging from noisy, warm, and mixed environments.

The research team carefully adapted existing techniques, previously successful only in cold conditions, to warmer initial states. Their achievement suggests quantum mechanics is more flexible than scientists initially believed.

By successfully generating quantum states with initial purity as low as 0.06, the team proved even highly mixed, messy quantum systems can exhibit clear quantum behavior. This low purity means the quantum states contained significant thermal energy—exactly what scientists usually avoid when creating quantum phenomena. Yet, despite these challenging conditions, unmistakable quantum interference patterns still emerged.

What does this mean for the future of quantum research? Primarily, it means quantum phenomena might be observed in a wider variety of conditions, including everyday environments. Quantum physics could eventually become a common feature of technology used in our daily lives. From quantum-enhanced sensors to more accessible quantum computers, the practical implications are enormous.

Kirchmair noted optimistically, “If we can create the necessary interactions in a system, the temperature ultimately doesn't matter.” This revolutionary idea could simplify future experiments and make quantum technology more accessible and practical.

This groundbreaking study provides a new perspective on quantum mechanics, challenging assumptions and opening exciting avenues for future research. Quantum phenomena might soon thrive outside controlled laboratories, potentially revolutionizing technology, computing, and engineering.

Research findings are available in the journal Science Advances.

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