A promising route to novel quantum technologies are hybrid quantum systems, which combine the advantages of several individual quantum systems. We have realized a hybrid atomic-mechanical experiment consisting of a Si 3 N 4 membrane oscillator cryogenically precooled to 500 mK and optically coupled to a cloud of laser cooled 87 Rb atoms. Here, we demonstrate active feedback cooling of the oscillator to a minimum mode occupation of corresponding to a mode temperature of T min ≈ 200 μ K. Furthermore, we characterize in detail the coupling of the membrane to the atoms by means of sympathetic cooling. By simultaneously applying both cooling methods we demonstrate the possibility of preparing the oscillator near the motional ground state while it is coupled to the atoms. Realistic modifications of our setup will enable the creation of a ground state hybrid quantum system, which opens the door for coherent quantum state transfer, teleportation and entanglement as well as quantum enhanced sensing applications.
A promising route to novel quantum technologies are hybrid quantum systems, which combine the advantages of several individual quantum systems. We have realized a hybrid atomic-mechanical experiment consisting of a Si 3 N 4 membrane oscillator cryogenically precooled to 500 mK and optically coupled to a cloud of laser cooled 87 Rb atoms. Here, we demonstrate active feedback cooling of the oscillator to a minimum mode occupation of corresponding to a mode temperature of T min ≈ 200 μ K. Furthermore, we characterize in detail the coupling of the membrane to the atoms by means of sympathetic cooling. By simultaneously applying both cooling methods we demonstrate the possibility of preparing the oscillator near the motional ground state while it is coupled to the atoms. Realistic modifications of our setup will enable the creation of a ground state hybrid quantum system, which opens the door for coherent quantum state transfer, teleportation and entanglement as well as quantum enhanced sensing applications.
Hybrid systems combining mechanical oscillators and ultracold atoms provide novel \nopportunities for cooling, detection and quantum control of mechanical motion with \napplications in precision sensing, quantum-level signal transduction and for fundamental \ntests of quantum mechanics. \nIn this thesis I present experiments performed with a hybrid atom-membrane \nsystem, in which the vibrations of a Si_3N_4 membrane in an optical cavity are coupled \nto the motion of laser-cooled atoms in an optical lattice. The interactions are \nmediated by the lattice light over a macroscopic distance and enhanced by the cavity. \nVia the coupling to the cold atoms, the fundamental vibrational mode of the \nmembrane at 2π x 276 kHz is cooled sympathetically from room temperature to \n0.4(2) K, even though the mass of the mechanical oscillator exceeds that of the \natomic ensemble by a factor of 4 x 10^10. In other systems, sympathetic cooling \nof molecules with cold atoms or ions has been limited to mass ratios of up to 90. \nPrevious theoretical work has shown that our coupling mechanism is able to cool the \nmembrane vibration into the ground state and to perform coherent state transfers \nbetween atomic and membrane motion. \nUnder certain experimental conditions, the atom-membrane system shows self-oscillations, \nwhich arise from an effective delay in the backaction of the atoms onto \nthe light. This retardation drives the system into limit-cycle oscillations if the coupling \nis large. I study the dependence of this instability on several system parameters \nand find that a larger atom number and a smaller atom-light detuning make the system \nless stable. Further, the stability of the coupled system in presence of a delay is \ninvestigated theoretically and a modified expression for the sympathetic cooling rate \nis derived. This model allows to fit the measured atom number dependence with a \ndelay of τ = 88(1) ns. Moreover, direct measurements of the atomic backaction onto \nthe lattice light are presented. These show phase lags exceeding 180° in parameter \nregimes where the instability is observed, proving that the retardation arises within \nthe atomic ensemble. Finally, I present the results of numerical simulations, which \nshow that collective atomic effects within the atomic ensemble in an asymmetric \nlattice are able to induce the observed phase lag in the atomic backaction.
In this thesis, I report on the realization of a hybrid optomechanical system in which ultracold atoms are coupled to a micromechanical membrane. The atoms are trapped in the intensity maxima of an optical standing wave formed by retroreflection of a laser beam from the membrane surface. Vibrations of the membrane displace the standing wave, thus coupling to the center-of-mass motion of the atomic ensemble. Conversely, atoms imprint their motion onto the laser light, thereby modulating the radiation pressure force on the membrane. In this way, the laser light mediates a long-distance, coherent coupling between the two systems. \n \nWhen the trap frequency of the atoms is matched to the membrane frequency, we observe resonant energy transfer. In addition, by applying simultaneous laser cooling to the atoms, we can dissipate energy from the coupled system leading to sympathetic cooling of the membrane mode. The experimental data follows the theoretical estimations that predict the coupling to scale with the number of trapped atoms. Furthermore, by including the finite temperature of the atoms and their spatially inhomogeneous trapping potential in the theoretical model of the optomechanical coupling, we can accurately describe the width and shape of the resonance. \n \nIn an improved experimental setup, the membrane is enclosed in a cavity while the atoms are trapped in the standing wave lattice outside the cavity. The presence of the cavity results in a considerable enhancement of the coupling strength in proportion to the cavity finesse. So far we have observed sympathetic cooling of the membrane mode by a factor of 32 starting from room temperature. Theoretical estimates show that in such a setup ground-state cooling of the membrane mode should be possible, allowing one to access the quantum coherent coupling regime.
Coherent feedback stabilizes a system toward a target state without the need of a measurement, thus avoiding the quantum backaction inherent to measurements.Here, we employ optical coherent feedback to remotely cool a nanomechanical membrane using atomic spins as a controller.Direct manipulation of the atoms allows us to tune from strong coupling to an overdamped regime.Making use of the full coherent control offered by our system, we perform spin-membrane state swaps combined with stroboscopic spin pumping to cool the membrane in a room-temperature environment to T ¼ 216 mK ( nm ¼ 2.3 × 10 3 phonons) in 200 μs.We furthermore observe and study the effects of delayed feedback on the cooling performance.Starting from a cryogenically precooled membrane, this method would enable cooling of the mechanical oscillator close to its quantum mechanical ground state and the preparation of nonclassical states.
We report on the realization of a hybrid optomechanical system in which ultracold atoms are coupled to a micromechanical membrane. The atoms are trapped in the intensity maxima of an optical standing wave formed by retroreflection of a laser beam from the membrane surface. Vibrations of the membrane displace the standing wave, thus coupling to the center-of-mass motion of the atomic ensemble. Conversely, atoms imprint their motion onto the laser light, thereby modulating the radiation pressure force on the membrane. In this way, the laser light mediates a long-distance coherent coupling between the two systems. When the trap frequency of the atoms is matched to the membrane frequency, we observe resonant energy transfer. Moreover, we demonstrate sympathetic damping of the membrane motion by coupling it to laser-cooled atoms. Theoretical investigations show that the coupling strength can be considerably enhanced by placing the membrane inside an optical cavity. This could lead to quantum coherent coupling and ground-state cooling of the membrane via a distant atomic ensemble.
We discuss a hybrid quantum system where a dielectric membrane situated inside an optical cavity is coupled to a distant atomic ensemble trapped in an optical lattice. The coupling is mediated by the exchange of sideband photons of the lattice laser, and is enhanced by the cavity finesse as well as the square root of the number of atoms. In addition to observing coherent dynamics between the two systems, one can also switch on a tailored dissipation by laser cooling the atoms, thereby allowing for sympathetic cooling of the membrane. The resulting cooling scheme does not require resolved sideband conditions for the cavity, which relaxes a constraint present in standard optomechanical cavity cooling. We present a quantum mechanical treatment of this modular open system which takes into account the dominant imperfections, and identify optimal operation points for both coherent dynamics and sympathetic cooling. In particular, we find that ground state cooling of a cryogenically precooled membrane is possible for realistic parameters.
Hybrid quantum systems provide a promising route toward combining the complementary strengths of distinct physical platforms for quantum information processing. In this context, highly excited Rydberg atoms are particularly attractive due to their strong electric dipole moments, long lifetimes, and compatibility with both optical and microwave frequency regimes. Mechanical resonators, on the other hand, offer long coherence times and the ability to store quantum information in macroscopic degrees of freedom. In this thesis, the development of a novel hybrid platform interfacing ultracold Rydberg atoms with high-overtone bulk acoustic wave resonators (HBARs) in a cryogenic environment is presented. The core components of the experimental apparatus have been designed, assembled, and characterized, including a room-temperature realization featuring the complete cold-atom preparation chain. A first-generation superconducting atom chip for trapping ultracold atoms in a cryogenic environment has been designed and fabricated, establishing the foundation for future experiments under cryogenic conditions. The full experimental realization of the hybrid system, including the integration of the mechanical resonator, was delayed by the delivery of a custom-built cryostat. Nevertheless, the preparation of the cryogenic experiment has been completed, enabling rapid progress toward first experiments once cryogenic operation becomes available. At an operating temperature of 4 K, thermal phonon occupation of HBAR modes is significantly reduced compared to room temperature but remains above the quantum ground state, motivating the development of active cooling strategies. Complementing the experimental work, the feasibility of cooling a mechanical mode toward its quantum mechanical ground state using a cloud of Rydberg atoms is investigated theoretically. In this approach, Rydberg atoms act as a dissipative resource that extracts phononic excitations from the mechanical resonator and removes them via radiative decay. Furthermore, the potential of Rydberg atom-mediated interactions to generate entanglement between spatially separated mechanical oscillators is analyzed. Together, these results establish a foundation for future experimental investigations of Rydberg atom–mechanical oscillator hybrid systems.