Lightwave Terahertz Quantum Manipulation of Non-equilibrium Superconductor Phases and their Collective Modes

TL;DR

This paper develops a gauge-invariant density matrix framework to describe non-equilibrium superconductor states driven by intense terahertz light, revealing new quantum control mechanisms and experimental signatures of collective modes and nonlinear effects.

Contribution

It introduces a comprehensive, gauge-invariant model extending Anderson pseudo-spin theory to include center-of-mass motion and electromagnetic effects, enabling detailed analysis of THz-driven superconductor dynamics.

Findings

Observation of high harmonic generation at forbidden frequencies.

Identification of Rabi-Higgs collective modes.

Control of non-equilibrium condensate states with few-cycle THz fields.

Abstract

We present a gauge-invariant density matrix description of non-equilibrium superconductor (SC) states with spatial and temporal correlations driven by intense terahertz (THz) lightwaves. We derive superconductor Bloch--Maxwell equations of motion that extend Anderson pseudo-spin models to include the Cooper pair center-of-mass motion and electromagnetic propagation effects. We thus describe quantum control of dynamical phases, collective modes, quasi-particle coherence, and high nonlinearities during cycles of carrier wave oscillations, which relate to our recent experiments. Coherent photogeneration of a nonlinear supercurrent with dc component via condensate acceleration by an effective lightwave field dynamically breaks the equilibrium inversion symmetry. Experimental signatures include high harmonic light emission at equilibrium-symmetry-forbidden frequencies, Rabi--Higgs collective modes and quasi-particle coherence, and non-equilibrium moving condensate states tuned by few-cycle THz fields. We use such lightwaves as an oscillating accelerating force that drives strong nonlinearities and anisotropic quasi-particle populations to control and amplify different classes of collective modes, e.g., damped oscillations, persistent oscillations, and overdamped dynamics via Rabi flopping. Recent phase-coherent nonlinear spectroscopy experiments can be modeled by solving the full nonlinear quantum dynamics including self-consistent light--matter coupling.