A recent study published in Physical Review Letters reveals that quantum systems with long-range interactions exhibit different behaviours under various thermodynamic conditions. Specifically, these systems show distinct properties when placed in a heat bath (canonical ensemble) compared to when their energy is fixed (microcanonical ensemble), a phenomenon known as "ensemble inequivalence."
Traditionally, systems with short-range interactions behave identically under both conditions if the temperature and energy are appropriately related. Advancements in laboratory techniques now allow for the creation and study of systems with long-range interactions, making experimental verification of ensemble inequivalence possible.
"The mechanism regulating defect formation has long been recognised as universal, spanning a wide range of scales, from quantum magnets to galaxies. However, this traditional understanding is challenged by the inclusion of long-range interactions," explains Nicolò Defenu, former SISSA PhD student. "Through the analytical determination of full counting statistics in systems with such interactions, we reveal that universality persists but is indissolubly linked to the quantum nature of the system. This profound insight opens up new opportunities for experimental validation in platforms like Rydberg gases and trapped ions."
The research, conducted by Stefano Ruffo from SISSA, Nicolò Defenu from ETH Zurich, and David Mukamel from the Weizmann Institute of Science, includes a theoretical model predicting how atoms interact with light in a cavity at low temperatures. This model indicates that ensemble inequivalence is likely to occur during first-order phase transitions, characterised by abrupt changes in the system's properties.
This finding enhances our understanding of quantum systems and suggests new directions for experimental research in atomic, molecular, and optical physics.
Read the article in Physical Review Letters.