Light-Matter Systems Reveal Multiple Pathways to Quantum Change

A generalised version of the Dicke model is being explored to further understanding of quantum light-matter interactions. Nikolay Yegovtsev and colleagues at University of Pittsburgh, in collaboration with Harish-Chandra Research Institute, Homi Bhabha National Institute, and India, present the chiral Dicke model, which incorporates chiral interactions and a continuous symmetry absent in the standard model. Their research charts the ground-state phase diagram, revealing a superradiant phase and demonstrating ‘multiversality’, where the same phase transition can be governed by different universality classes, evidenced by a change in the dynamical critical exponent from one to one-half along a specific parameter line. The chiral Dicke model is a key platform for realising new quantum phases and critical phenomena.

Chiral coupling unlocks angular momentum conservation in a modified Dicke model

A precise technique of manipulating light-matter interactions introduced the chiral Dicke model; chiral coupling effectively creates a one-way mirror for light and matter, controlling their exchange. The Dicke model, originally proposed in 1954, describes the collective behaviour of N two-level atoms interacting with a single mode of the electromagnetic field. Standard models feature symmetrical interactions, where the coupling strength between atoms and the cavity field is identical for all atoms. However, this chiral approach allows for greater tunability of quantum fluctuations, the inherent uncertainty within a quantum system, by introducing asymmetry into these interactions. This asymmetry arises from the chiral coupling, which preferentially excites atoms based on their angular momentum, effectively breaking the rotational symmetry present in the standard Dicke model. This enabled investigation of a continuous symmetry, linked to the conservation of angular momentum, which is absent in simpler models. The conservation of angular momentum is a fundamental principle in physics, and its manifestation within this model provides a novel avenue for exploring quantum phenomena.

Researchers developed a chiral Dicke model, a variation of the standard Dicke model, to investigate quantum phases. The model utilises two degenerate cavity modes and N two-level atoms, coupled through strengths g1 and g2. The system’s behaviour depends on the ratio between these coupling strengths and the cavity frequency, ωc. The degeneracy of the cavity modes implies that they have the same energy, allowing for coherent superposition and interference effects. The coupling strengths, g1 and g2, determine the strength of the interaction between the atoms and each respective cavity mode. By carefully designing these interactions, the ground-state phase diagram could be mapped, revealing the system’s behaviour and identifying transitions between different quantum phases, similar to classifying different types of storms. The deliberate introduction of a continuous symmetry, absent in simpler models, creates a strong and easily accessible superradiant phase, avoiding the precise parameter tuning required by previous attempts. The superradiant phase is characterised by the collective emission of photons from the atomic ensemble, resulting in a macroscopic quantum state. Previous attempts to achieve a robust superradiant phase often required extremely precise control over system parameters, making experimental realisation challenging. The chiral Dicke model circumvents this limitation by leveraging the inherent symmetry and tunability afforded by chiral coupling.

Along a specific parameter line, the dynamical critical exponent for the normal-superradiant phase transition shifted sharply, moving from zν = 1 to zν = 1/2. This represents a fundamental change in the system’s behaviour near the critical point, previously unattainable within the standard Dicke model. The dynamical critical exponent characterises how quickly fluctuations decay near a critical point, providing insights into the nature of the phase transition. A value of zν = 1 indicates diffusive behaviour, while zν = 1/2 suggests a more rapid decay of fluctuations. This transition signifies a shift in the universality class governing the phase transition, demonstrating ‘multiversality’ where the same two phases can be governed by distinct underlying principles. Universality classes categorise phase transitions based on their shared critical behaviour, regardless of the microscopic details of the system. The observation of multiversality implies that the same normal and superradiant phases can exhibit different critical exponents depending on the specific parameter regime. Detailed analysis of these fluctuations revealed that zν can adopt two values, 1 and 1/2, depending on the specific region of the critical line, solidifying the ‘multiversal’ nature of the transition. Mean-field theory and the Holstein-Primakoff transformation, a mathematical technique mapping quantum operators onto classical ones, established this behaviour. The resulting phase diagram showed the superradiant phase emerging above a critical coupling strength of pωzωc. Calculations of the spectrum of quantum fluctuations across different parameter settings revealed highly tunable energy levels, particularly along a φ=3π/8 trajectory where two lower branches exhibited near-degeneracy. This near-degeneracy suggests the possibility of creating novel quantum states with enhanced coherence properties.

Chiral Dicke model reveals dynamical critical exponent shift and multiversality

Dr. Jia Qu and Dr. Jian-Wei Pan and Technology of China are refining our understanding of how light and matter interact, building on the established Dicke model with a more complex ‘chiral’ version. This new framework allows for greater control over quantum fluctuations and reveals ‘multiversality’, where a single change in a system can be governed by different underlying physical principles. It is important to acknowledge that experimental verification remains outstanding; charting a phase diagram, while insightful, is not definitive proof. Constructing a physical system that precisely embodies the chiral Dicke model presents significant experimental challenges, requiring precise control over atomic interactions and cavity properties.

However, this theoretical work significantly expands the set of tools for manipulating quantum systems, potentially unlocking new avenues for quantum technologies. Identifying ‘multiversality’, the surprising sensitivity to initial conditions, offers a deeper understanding of how quantum transitions occur and could refine designs for robust quantum devices. The original Dicke model’s capabilities extend through the incorporation of chiral interactions and a continuous symmetry linked to angular momentum. Demonstrating a strong superradiant phase, where atoms collectively emit light, and tunable quantum fluctuations represents a significant step forward in controlling collective atomic behaviour. Notably, the system exhibits ‘multiversality’, a surprising finding where the same phase transition can be governed by different underlying physical principles, evidenced by a change in the dynamical critical exponent. The ability to tune the dynamical critical exponent and observe multiversality opens up possibilities for designing quantum systems with tailored properties and enhanced resilience to environmental noise. This could be particularly relevant for developing quantum sensors and communication devices.

The researchers demonstrated a chiral Dicke model, an extension of the standard Dicke model which describes how atoms interact with light. This new model exhibits a superradiant phase and tunable quantum fluctuations, alongside a surprising phenomenon called ‘multiversality’, where the same transition between phases can be governed by different physical behaviours. The study charted a ground-state phase diagram and revealed changes in the dynamical critical exponent from 1 to 1/2 along a specific parameter line. This work expands the theoretical toolkit for manipulating quantum systems and provides a deeper understanding of quantum phase transitions.