We demonstrate a 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 the 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.
We demonstrate a 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 the 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.
Abstract We theoretically and experimentally explore the emergence of a dynamical density wave (DW) 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 DW 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 DW 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.
This thesis reports on quantum simulation experiments conducted on strongly correlated fermionic atoms coupled to light in an optical cavity. To this day, the quantum-mechanical description of strongly correlated many-body systems remains a considerable theoretical challenge. Quantum simulation experiments with ultracold atoms contribute to the understanding of such systems through controlled studies of complex, engineered many-body states. Advances in measurement and control techniques continuously drive the development of this field, enabling the creation, detection, and manipulation of novel states of matter. The experiments presented in this thesis build upon the prior observation of cavity-induced density-wave ordering in a gas of strongly interacting 6Li atoms. In particular, we report on an extension of the cavity quantum electrodynamics setup through the implementation of a high-resolution imaging system. The imaging setup enables the in situ observation of the density waves through resonant absorption imaging within a single experimental realization. Using this technique, we investigate ordering both following adiabatic preparation and after a quench of the cavity-induced long-range interaction. Our observations support the expectation of a density modulation with a single spatial frequency and phase that extends across the entire atomic cloud. We further present a correlation study between atomic and photonic observables, confirming their correspondence and thereby directly testing the theoretical foundation of cavity-mediated interactions. By imaging density waves in the vicinity of the first higher-order transverse cavity mode, we demonstrate the capability of the imaging setup to reveal local order beyond global observables. Additionally, this thesis presents two studies of density-wave ordering based on measurements of the cavity photon leakage. The first study focuses on the dynamics of order formation following an instantaneous quench. We observe an exponential growth of the order parameter, with a growth rate predominantly governed by the quench strength, in agreement with a theoretical description based on a random phase approximation. In the second study, we realize a system of three mutually coupled orders, enabled by the simultaneous coupling of light to atoms and atom pairs via a photoassociation resonance. In combination with the strong correlations present in the gas, this gives rise to three coupled order parameters: the cavity photon field, a density wave, and a pair-density wave. The mutual competition and enhancement between these orders result in a measurable shift of the density-wave order phase boundary. We conclude the thesis by reporting on recent experimental advances in the implementation of a homogeneous trapping potential, leading to a uniform atomic density along two spatial directions. This represents work towards an ideal starting point for a range of exciting future experiments, including spectroscopic measurements, local potential patterning, and correlation studies in long-range interacting Fermi gases, leveraging the numerical aperture of the imaging system.
Recent quantum gas cavity-QED experiments demonstrated simultaneous coupling of photons to single atom transitions as well as correlated pairs of ultracold fermions trapped inside optical cavities. This enables simultaneous control over density ordering and pairing. Using extensive numerical simulations, we show that in a transversely driven, two-component degenerate Fermi gas, the interplay of cavity-induced and bare atom--atom interactions controls not only the power threshold for self-organization, but also the type of spatial ordering in the Z2-symmetry-broken superradiant state. In the repulsive interaction regime, unpaired or weakly paired fermions first self-organize through a charge-density-wave instability, and finite-momentum pairing only occurs at much stronger pump strengths. In contrast, an attractive superfluid undergoes a joint density--pairing instability, directly entering into intertwined phase with charge-density-wave and pair-density-wave orders. At strong pumping the photon-enhanced pair interaction generates localized density and pairing order even when the bare contact interaction is repulsive. In this cavity-dominated regime strong spatial localization suppresses the long range superfluid coherence. Our results identify the role of cavity-induced atomic interactions in supporting intertwined fermionic orders and pave the way for exploring exotic states with multiple orders in highly controlled hybrid light-matter systems.
Periodic driving of strongly correlated systems has led to outstanding discoveries from light-induced superconductivity to time crystals. To gain insights into the fundamental nature of these phenomena, we have recently studied the effects of an external periodic driving in a well-controlled yet simple many-body platform consisting of a Bose-Einstein condensate inside a high-finesse optical cavity. This system hosts a competition between a homogeneous condensate phase and a self-organized density wave phase, which manifests in a superradiant phase transition as the intensity of a transverse pump beam is increased. We demonstrate dynamical control of this phase transition by periodic driving of the pump laser. Furthermore, we discover the emergence of genuine dynamical phases, namely a discrete time crystalline phase and nonequilibrium density wave orders. Accompanying experimental results confirm our theoretical predictions about light-induced control in the atom-cavity system.
We demonstrate high-resolution in situ imaging of density-wave ordering induced by cavity-mediated interactions in a unitary Fermi gas. We observe long-range spatial correlations throughout the formation of density waves, both for adiabatic preparation and following a quench, with a pattern controlled by the cavity mode structure. Our single-shot microscopic images together with the real-time readout of the cavity photons provide access to atom-photon correlations. We use this capability to investigate order fluctuations as a function of time following a quench and to directly confirm the correspondence between optical and atomic observables. Our system opens rich perspectives, from local patterning to correlation measurements in long-range interacting quantum gases.
Abstract A density wave (DW) is a fundamental type of long-range order in quantum matter tied to self-organization into a crystalline structure. The interplay of DW order with superfluidity can lead to complex scenarios that pose a great challenge to theoretical analysis. In the past decades, tunable quantum Fermi gases have served as model systems for exploring the physics of strongly interacting fermions, including most notably magnetic ordering1, pairing and superfluidity2, and the crossover from a Bardeen–Cooper–Schrieffer superfluid to a Bose–Einstein condensate3. Here, we realize a Fermi gas featuring both strong, tunable contact interactions and photon-mediated, spatially structured long-range interactions in a transversely driven high-finesse optical cavity. Above a critical long-range interaction strength, DW order is stabilized in the system, which we identify via its superradiant light-scattering properties. We quantitatively measure the variation of the onset of DW order as the contact interaction is varied across the Bardeen–Cooper–Schrieffer superfluid and Bose–Einstein condensate crossover, in qualitative agreement with a mean-field theory. The atomic DW susceptibility varies over an order of magnitude upon tuning the strength and the sign of the long-range interactions below the self-ordering threshold, demonstrating independent and simultaneous control over the contact and long-range interactions. Therefore, our experimental setup provides a fully tunable and microscopically controllable platform for the experimental study of the interplay of superfluidity and DW order.