Unlocking the secrets of phase transitions in quantum hardware

Phase transitions, like water freezing into ice, are a familiar part of our world. But in quantum systems, they can behave even more dramatically, with quantum properties such as Heisenberg  uncertainty  playing a central role. Furthermore, various spurious effects can cause the systems to lose, or dissipate, energy to the environment. When they happen, theses “dissipative phase transitions” (DPTs) push quantum systems into new states.

There are different types or “orders” of DPTs. First-order DPTs are like flipping a switch, causing abrupt jumps between states. Second-order DPTs are smoother but still transformative, changing one of the system’s global features, known as symmetry, in subtle yet profound ways.

DPTs are key to understanding how quantum systems behave in non-equilibrium conditions, where arguments based on thermodynamics often fails to provide answers. Beyond pure curiosity, this has practical implications for building more robust quantum computers and sensors. For example, second-order DPTs could enhance quantum information storage, while first-order DPTs reveal important mechanisms of system stability and control.

Theoretically, DPTs have been predicted to display specific properties, such as slowing down and bistability, occurring with specific power-law scales. So far, observing them has been a major scientific hurdle — especially the second-order ones.

But now, a team of researchers has done just that. Led by Professor Pasquale Scarlino at EPFL, they developed a superconducting Kerr resonator, a device with controllable quantum properties, and engineered it to experience a two-photon drive, which sends pairs of photons into the system to carefully control its quantum state and study how it transitions between different phases.

By systematically varying parameters like detuning and drive amplitude, they were able to study the system’s transitions from one quantum state to another. The approach allowed them to observe both a first-order and second-order DPT.

To ensure accuracy, the experiments were carried out at temperatures near absolute zero, reducing background noise to almost nothing. The Kerr resonator was pivotal because it can amplify quantum effects that are often too subtle to observe. Because it can respond to two-photon signals with extreme sensitivity, the researchers were able to use it to explore phase transitions with unprecedented precision—something traditional setups simply cannot achieve.

The setup allowed the team to monitor the behavior of photons emitted by the resonator with ultra-sensitive detectors. By using advanced mathematical techniques, like the connection with the spectral properties of the Liouvillian superoperator—a tool that models complex quantum processes—the scientists were able to precisely track and analyze the system’s phase transitions.

For the second-order DPT, the team observed a phenomenon called “squeezing,” where quantum fluctuations drop to levels lower than the natural background noise of empty space, signaling that the system has reached a highly sensitive and transformative state. Meanwhile, the first-order DPT showed distinct hysteresis cycles, where the system could exist in two states depending on how parameters were tuned.

Second, they found clear evidence of metastable states during the first-order DPT, where the system temporarily remained in one stable state before abruptly transitioning to another. This behavior, leading to a dependence of the system’s state on its previous history known as hysteresis, showcases how first-order DPTs involve competing phases.

Lastly, they observed "critical slowing down" in both types of transitions reproducing the expected scaling obtained from theoretical consideration. This ultimately demonstrates the validity of theoretical predictions based on the Liouvillian theory used by the authors. Near the critical points, the system’s response slowed significantly, highlighting a universal feature of phase transitions that could be harnessed for more precise quantum measurements.

Understanding DPTs opens new possibilities for engineering quantum systems that are both stable and responsive. This could revolutionize quantum information technologies, such as error correction in quantum computing or the development of ultra-sensitive quantum sensors.

More broadly, this research showcases the power of interdisciplinary collaboration—blending experimental physics, advanced theoretical models, and cutting-edge engineering to explore the frontiers of science.

“In fact, a very interesting aspect of this work is that it also demonstrates how close collaboration between theory and experiment can lead to results far greater than what either group could have achieved independently,” says Guillaume Beaulieu, the paper’s first author.

Other contributors

Sapienza University Aalto University University of Pavia

Reference

Guillaume Beaulieu, Fabrizio Minganti, Simone Frasca, Vincenzo Savona, Simone Felicetti, Roberto Di Candia, Pasquale Scarlino. Observation of first- and second-order dissipative phase transitions in a two-photon driven Kerr resonator. Nature Communications 10 March 2025. DOI: 10.1038/s41467-025-56830-w

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