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.
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.
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.
Optomechanical systems provide a versatile platform for precision measurements and investigations of fundamental physics, where bringing macroscopic resonators into the quantum regime is a widely pursued goal. Achieving such quantum behavior of solid-state mechanical resonators at room temperature would greatly broaden their applications by removing the need for cryogenic environments. Reaching this goal requires efficient cooling of mechanical motion, among various laser cooling methods, dynamical backaction cooling (DBC) is widely utilized in experiments but fundamentally limited when operating in the sideband-unresolved regime. Coherent feedback cooling (CFC) can overcome this limitation, while avoiding state collapse and the electronic restrictions inherent to measurement-based feedback. Here, we experimentally demonstrate CFC using an ultracoherent density phononic crystal membrane. By combining CFC with strong DBC in a relatively narrow cavity, we achieve a phonon occupation reduction from 5.5×106 to 166±7, corresponding to a cooling factor of 3.3×104 at room temperature, even with current experimental limitations. Our results show the potential of CFC for approaching the ground state of high-Q membranes at room temperature.
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.
We propose a hybrid optomechanical quantum system consisting of a moving membrane strongly coupled to an ensemble of N atoms with a Rydberg state. Due to the strong van-der-Waals interaction between the atoms, the ensemble forms an effective two-level system, a Rydberg superatom, with a collectively enhanced atom–light coupling. Using this superatom imposed collective enhancement strong coupling between membrane and superatom is feasible for parameters within the range of current experiments. The quantum interface to couple the membrane and the superatom can be a pumped single mode cavity, or a laser field in free space, where the Rydberg superatom and the membrane are spatially separated. In addition to the coherent dynamics, we study in detail the impact of the typical dissipation processes, in particular the radiative decay as a source for incoherent superpositions of atomic excitations. We identify the conditions to suppress these incoherent dynamics and thereby a parameter regime for strong coupling. The Rydberg superatom in this hybrid system serves as a toolbox for the nanomechanical resonator allowing for a wide range of applications such as state transfer, sympathetic cooling and non-classical state preparation. As an illustration, we show that a thermally occupied membrane can be prepared in a non-classical state without the necessity of ground state cooling.
This thesis presents theoretical and experimental work on light-mediated coupling between a collective atomic spin and a micromechanical membrane oscillator. With our work we address a fundamental question of quantum optics: Can a beam of light mediate coherent Hamiltonian interactions between two distant quantum systems? This is an intriguing question whose answer is not a priori clear, since the light carries away information about the systems and might be subject to losses, giving rise to intrinsic decoherence channels associated with the coupling. Our answer is affirmative and we derive a particularly simple sufficient condition for the interactions to be Hamiltonian: The light field needs to interact twice with the systems and the second interaction has to be the time reversal of the first. We demonstrate theoretically that, even in the presence of significant optical loss, coherent interactions can be realized and generate substantial amounts of entanglement between the systems. \n \nIn our experiments, we employ this approach to strongly couple a spin-polarized atomic ensemble and a micromechanical oscillator via a free-space laser beam across a distance of one meter in a room-temperature environment. The atomic ensemble consists of about ten million laser-cooled Rubidium atoms in an optical dipole trap that interact with the coupling laser via an off-resonant Faraday interaction. The mechanical oscillator is a silicon nitride membrane which is mounted in a single-sided optical cavity and couples to the laser field via radiation-pressure forces. In order to mediate a bidirectional Hamiltonian interaction between spin and membrane, the coupling beam is arranged in a loop such that it couples twice to the spin. This looped geometry enables destructive interference of quantum back-action by the light field on the spin. \n \nUsing this setup, we experimentally demonstrate for the first time strong Hamiltonian coupling between remote quantum systems and explore different dynamical regimes of cascaded light-mediated interactions: With the spin initialized in its ground state we observe normal-mode splitting and coherent energy exchange oscillations, both hallmarks of strong coupling. If we invert the spin to its highest energy state, we observe parametric-gain interactions, resulting in two-mode thermal noise squeezing. Furthermore, by shifting the phase of the light field between spin and membrane we can switch to non-Hamiltonian coupled dynamics, allowing us to observe level attraction and exceptional points. This high level of control in a strongly coupled modular system gives access to a unique toolbox for designing hybrid quantum systems and coherent optical feedback loops. Our approach to engineer coherent long-distance interactions with light makes it possible to couple very different systems in a modular way, opening up a range of new opportunities for quantum control. \n
The ground-state cooling of a macroscopic mechanical oscillator is a crucial prerequisite for implementing the quantum manipulation of the mechanical oscillator. Here we show that a degenerate optical parametric amplifier (OPA) can significantly improve the cooling of the mechanical membrane in the purely dissipative coupling system in the unresolved sideband regime. In the presence of the OPA with a parametric phase $\ensuremath{\pi}/2$, the mechanical membrane can be cooled from room temperature 293 K to 10 mK, a factor of 112 lower than that in the absence of the OPA. If the mechanical oscillator is precooled to 0.1 K, the mechanical membrane can be cooled into the microkelvin regime.