Research

Conceptual illustration: The researchers found that quantum phase transitions are abrupt and discontinuous when parameter N is below 7-8 (left) versus smooth and continuous above this threshold (right).

Researchers from the University of Hong Kong, led by Professor Zi Yang Meng, along with international collaborators, have made significant progress toward resolving a contentious puzzle in quantum physics by determining when deconfined quantum critical points represent genuine continuous phase transitions.

These unusual phenomena, where quantum systems transition between two different ordered states rather than between ordered and disordered phases, have challenged the traditional Landau framework since their theoretical prediction in the early 2000s.

The research team used quantum Monte Carlo simulations—a computational method for modelling quantum systems—to measure entanglement entropy, which quantifies how quantum particles share information across different regions of a system. Using square-lattice SU(N) spin models to study these critical points, they identified a crucial threshold.

The key finding centres on parameter N, which determines the quantum system's symmetry. For small N values (below 7-8), the entanglement entropy shows unexpected logarithmic behaviour inconsistent with smooth, continuous transitions. This suggests these systems 

undergo abrupt transitions rather than gradual continuous transitions.

When N exceeds this threshold, the systems behave as expected for genuine continuous phase transitions, matching predictions from established conformal field theory.

These findings provide important new insights into the fundamental nature of quantum phase transitions and could help resolve long-standing debates in quantum physics.

The research, published in Science Advances with Menghan Song as first author, involved collaboration with institutions including the Chinese University of Hong Kong, Yale University, University of California Santa Barbara, Ruhr University Bochum, and Dresden University of Technology.