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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.
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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 $omega_{d}$ close to a systemic resonance frequency $omega_{>}$, a dynamical density wave order emerges, which oscillates at the two frequencies $omega_{>}$ and $omega_{<} = omega_{d} - omega_{>}$. 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.
We propose a scheme to probe quantum coherence in the state of a nano-cantilever based on its magnetic coupling (mediated by a magnetic tip) with a spinor Bose Einstein condensate (BEC). By mapping the BEC into a rotor, its coupling with the cantilever results in a gyroscopic motion whose properties depend on the state of the cantilever: the dynamics of one of the components of the rotor angular momentum turns out to be strictly related to the presence of quantum coherence in the state of the cantilever. We also suggest a detection scheme relying on Faraday rotation, which produces only a very small back-action on the BEC and it is thus suitable for a continuous detection of the cantilevers dynamics.
In this paper we explore the rich structure of macroscopic many-particle quantum states for Bose- Einstein condensate in an optical cavity with the tunable nonlinear atom-photon interaction [Nature (London) 464, 1301 (2010)]. Population inversion, bistable normal phases and the coexistence of normal{superradiant phases are revealed by adjusting of the experimentally realizable interaction strength and pump-laser frequency. For the negative (effective) cavity-frequency we observe remark- ably an inverted quantum phase transition (QPT) from the superradiant to normal phases with the increase of atom-field coupling, which is just opposite to the QPT in the normal Dicke model. The bistable macroscopic states are derived analytically in terms of the spin-coherent-state variational method by taking into account of both normal and inverted pseudospin states.
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.
The distributed quantum computation plays an important role in large-scale quantum information processing. In the atom-cavity-fiber system, we put forward two efficient proposals to prepare the steady entanglement of two distant atoms with dissipation. The atomic spontaneous emission and the loss of fiber are exploited actively as powerful resources, while the effect of cavity decay is inhibited by quantum Zeno dynamics and quantum-jump-based feedback control. These proposals do not require precisely tailored Rabi frequencies or coupling strength between cavity and fiber. Furthermore, we discuss the feasibility of extending the present schemes into the systems consisting of two atoms at the opposite ends of the $n$ cavities connected by $(n-1)$ fibers, and the corresponding numerical simulation reveals that a high fidelity remains achievable with current experimental parameters.
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