Non-equilibrium plasmon liquid in a Josephson junction chain.

Area of Science:

• Quantum statistical mechanics
• Condensed matter physics
• Quantum optics

Background:

• Equilibrium quantum systems typically feature weakly interacting normal modes.
• Driving systems far from equilibrium can lead to significant mode-to-mode interactions.
• Understanding these non-equilibrium dynamics is crucial for both fundamental theory and quantum device engineering.

Purpose of the Study:

• To investigate the non-equilibrium kinetics of one-dimensional plasmons in Josephson junction chains.
• To explore the transition from weak to strong interactions between plasma modes under varying driving conditions.
• To experimentally verify the emergence of a strongly interacting, non-equilibrium plasmon liquid.

Main Methods:

• Utilized multimode spectroscopy to drive and probe plasma modes in a long chain of Josephson junctions.
• Controlled the departure from equilibrium by varying the strength and number of excitation modes.
• Employed imaging techniques to resolve non-equilibrium plasmon populations and energy redistribution.

Main Results:

• Observed a transition from pairwise coupling at weak driving to high-order, cascaded couplings at strong driving.
• Stimulated interactions among hundreds of modes, leading to near-continuum internal dynamics.
• Resolved nonlocal energy redistribution in response to perturbations, confirming the liquid state.

Conclusions:

• Demonstrated that strong driving can induce a transition to a strongly interacting, non-equilibrium plasmon liquid.
• Validated the emergence of complex, high-order interactions in driven quantum systems.
• Provided experimental evidence for fundamental concepts in quantum statistical mechanics using engineered quantum devices.