arXiv subjects

Christoph Georges

Publications and source records attributed to Christoph Georges.

7 recordsLinked to original sources

arXiv2025

Observation of a zero-energy excitation mode in the open Dicke model

Approaching phase boundaries in many-body systems can give rise to intriguing signatures in their excitation spectra. Here, we explore the excitation spectrum of a Bose-Einstein condensate strongly coupled to an optical cavity and pumped by an optical standing wave, which simulates the famous Dicke-Hepp-Lieb phase transition of the open Dicke model with dissipation arising due to photon leakage from the cavity. For weak dissipation, the excitation spectrum displays two strongly polaritonic modes. Close to the phase boundary, we observe an intriguing regime where the lower-energetic of these modes, instead of showing the expected roton-type mode softening, is found to approach and persist at zero energy, well before the critical pump strength for the Dicke-Hepp-Lieb transition boundary is reached. Hence, a peculiar situation arises, where an excitation is possible at zero energy cost, but nevertheless no instability of the system is created.

cond-mat.quant-gas↗

arXiv2021

Observation of a dissipative time crystal

The formation of a phase of matter can be associated with the spontaneous breaking of a symmetry. For crystallization, this broken symmetry is the spatial translation symmetry, as the atoms spontaneously localize in a periodic fashion. In analogy to spatial crystals, the spontaneous breaking of temporal translation symmetry results in the formation of time crystals. While recent and on-going experiments on driven isolated systems aim to minimize dissipative processes, as it is an undesired source of decay, well-designed dissipation has been put forth as a constitutive ingredient in the formation of dissipative time crystals (DTCs). Here, we present the first experimental realisation of a DTC, implemented in an atom-cavity system. Its defining feature is a period doubled switching between distinct chequerboard density wave patterns, induced by controlled cavity-dissipation and cavity-mediated interactions. We demonstrate the robustness of this phase against system parameter changes and temporal perturbations of the driving. Our work provides a framework for realising phases of matter with spatiotemporal order in presence of dissipation. We note that this is the natural environment of matter, and therefore shapes its physical phenomena profoundly, making its study imperative.

cond-mat.quant-gas↗

arXiv2021

Mott transition in a cavity-boson system: A quantitative comparison between theory and experiment

The competition between short-range and cavity-mediated infinite-range interactions in a cavity-boson system leads to the existence of a superfluid phase and a Mott-insulator phase within the self-organized regime. In this work, we quantitatively compare the steady-state phase boundaries of this transition measured in experiments and simulated using the Multiconfigurational Time-Dependent Hartree Method for Indistinguishable Particles. To make the problem computationally feasible, we represent the full system by the exact many-body wave function of a two-dimensional four-well potential. We argue that the validity of this representation comes from the nature of both the cavity-atomic system and the Bose-Hubbard physics. Additionally we show that the chosen representation only induces small systematic errors, and that the experimentally measured and theoretically predicted phase boundaries agree reasonably. We thus demonstrate a new approach for the quantitative numerical determination of the superfluid--Mott-insulator phase boundary.

cond-mat.quant-gas↗

arXiv2021

Dynamical density wave order in an atom-cavity system

We theoretically and experimentally explore the emergence of a dynamical density wave order in a driven dissipative atom-cavity system. A Bose-Einstein condensate is placed inside a high finesse optical resonator and pumped sideways by an optical standing wave. The pump strength is chosen to induce a stationary superradiant checkerboard density wave order of the atoms stabilized by a strong intracavity light field. We show theoretically that, when the pump is modulated with sufficient strength at a frequency $ω_{d}$ close to a systemic resonance frequency $ω_{>}$, a dynamical density wave order emerges, which oscillates at the two frequencies $ω_{>}$ and $ω_{<} = ω_{d} - ω_{>}$. This order is associated with a characteristic momentum spectrum, also found in experiments in addition to remnants of the oscillatory dynamics presumably damped by on-site interaction and heating, not included in the calculations. The oscillating density grating, associated with this order, suppresses pump-induced light scattering into the cavity. Similar mechanisms might be conceivable in light-driven electronic matter.

cond-mat.quant-gas↗

arXiv2020

From a continuous to a discrete time crystal in a dissipative atom-cavity system

We propose the dynamical stabilization of a nonequilibrium order in a driven dissipative system comprised an atomic Bose-Einstein condensate inside a high finesse optical cavity, pumped with an optical standing wave operating in the regime of anomalous dispersion. When the amplitude of the pump field is modulated close to twice the characteristic limit-cycle frequency of the unmodulated system, a stable subharmonic response is found. The dynamical phase diagram shows that this subharmonic response occurs in a region expanded with respect to that where stable limit-cycle dynamics occurs for the unmodulated system. In turning on the modulation we tune the atom-cavity system from a continuous to a discrete time crystal.

cond-mat.quant-gas↗

arXiv2018

Light-induced coherence in an atom-cavity system

We demonstrate light-induced formation of coherence in a cold atomic gas system that utilizes the suppression of a competing density wave (DW) order. The condensed atoms are placed in an optical cavity and pumped by an external optical standing wave, which induces a long-range interaction mediated by photon scattering and a resulting DW order above a critical pump strength. We show that light-induced temporal modulation of the pump wave can suppress this DW order and restore coherence. This establishes a foundational principle of dynamical control of competing orders analogous to a hypothesized mechanism for light-induced superconductivity in high-$T_c$ cuprates.

cond-mat.quant-gas↗

arXiv2018

Dynamical Control of Order in a Cavity-BEC System

We demonstrate dynamical control of the superradiant transition of cavity-BEC system via periodic driving of the pump laser. We show that the dominant density wave order of the superradiant state can be suppressed, and that the subdominant competing order of Bose-Einstein condensation emerges in the steady state. Furthermore, we show that additional, non-equilibrium density wave orders, which do not exist in equilibrium, can be stabilized dynamically. Finally, for strong driving, chaotic dynamics emerges.

cond-mat.quant-gas↗