R. W. Peterson

Observation of Radiation Pressure Shot Noise on a Macroscopic Object

Macroscopic Uncertainty According to the Heisenberg uncertainty principle, it is impossible to know both the position and the momentum of a microscopic particle with absolute certainty; pinpointing the location introduces an uncertainty in the velocity, which translates into position uncertainty at later times. Now, Purdy et al. (p. 801; see the Perspective by Milburn) have measured the position of a macroscopic object (a small, but visible-to-the-naked-eye membrane suspended in an optical cavity) at cryogenic temperatures and observed the uncertainty in its position caused by the recoiling photons used for the measurement. A light, visible-to-the-naked-eye membrane is observed to fluctuate in step with the photons used to measure its position. [Also see Perspective by Milburn] The quantum mechanics of position measurement of a macroscopic object is typically inaccessible because of strong coupling to the environment and classical noise. In this work, we monitor a mechanical resonator subject to an increasingly strong continuous position measurement and observe a quantum mechanical back-action force that rises in accordance with the Heisenberg uncertainty limit. For our optically based position measurements, the back-action takes the form of a fluctuating radiation pressure from the Poisson-distributed photons in the coherent measurement field, termed radiation pressure shot noise. We demonstrate a back-action force that is comparable in magnitude to the thermal forces in our system. Additionally, we observe a temporal correlation between fluctuations in the radiation force and in the position of the resonator.

Strong Optomechanical Squeezing of Light

We create squeezed light by exploiting the quantum nature of the mechanical interaction between laser light and a membrane mechanical resonator embedded in an optical cavity. The radiation pressure shot noise (fluctuating optical force from quantum laser amplitude noise) induces resonator motion well above that of thermally driven motion. This motion imprints a phase shift on the laser light, hence correlating the amplitude and phase noise, a consequence of which is optical squeezing. We experimentally demonstrate strong and continuous optomechanical squeezing of 1.7 +/- 0.2 dB below the shot noise level. The peak level of squeezing measured near the mechanical resonance is well described by a model whose parameters are independently calibrated and that includes thermal motion of the membrane with no other classical noise sources.

Laser Cooling of a Micromechanical Membrane to the Quantum Backaction Limit.

The radiation pressure of light can act to damp and cool the vibrational motion of a mechanical resonator, but even if the light field has no thermal component, shot noise still sets a limit on the minimum phonon occupation. In optomechanical sideband cooling in a cavity, the finite off-resonant Stokes scattering defined by the cavity linewidth combined with shot noise fluctuations dictates a quantum backaction limit, analogous to the Doppler limit of atomic laser cooling. In our work, we sideband cool a micromechanical membrane resonator to the quantum backaction limit. Monitoring the optical sidebands allows us to directly observe the mechanical object come to thermal equilibrium with the optical bath. This level of optomechanical coupling that overwhelms the intrinsic thermal decoherence was not reached in previous ground-state cooling demonstrations.

Cavity optomechanics with Si3N4 membranes at cryogenic temperatures

We describe a cryogenic cavity-optomechanical system that combines Si3N4 membranes with a mechanically-rigid Fabry-Perot cavity. The extremely high quality-factor frequency products of the membranes allow us to cool a MHz mechanical mode to a phonon occupation of less than 10, starting at a bath temperature of 5 kelvin. We show that even at cold temperatures thermally-occupied mechanical modes of the cavity elements can be a limitation, and we discuss methods to reduce these effects sufficiently to achieve ground state cooling. This promising new platform should have versatile uses for hybrid devices and searches for radiation pressure shot noise.

Optomechanical Raman-ratio thermometry

The temperature dependence of the asymmetry between Stokes and anti-Stokes Raman scattering can be exploited for self-calibrating, optically-based thermometry. In the context of cavity optomechanics, we observe the cavity-enhanced scattering of light interacting with the standingwave drumhead modes of a Si3N4 membrane mechanical resonator. The ratio of the amplitude of Stokes to anti-Stokes scattered light is used to measure temperatures of optically-cooled mechanical modes down to the level of a few vibrational quanta. We demonstrate that the Raman-ratio technique is able to measure the physical temperature of our device over a range extending from cryogenic temperatures to within an order of magnitude of room temperature. Raman light scattering has proven to be a robust and powerful technique for in situ thermometry. Many material-specic properties governing Raman transitions, such as the Stokes shift, spectral linewidth, and scattering rate vary with temperature. However, for all Raman systems the ratio of spontaneously scattered Stokes versus anti-Stokes photons is a direct measure of the initial population of the motional state. For example, at zero temperature the process of anti-Stokes scattering, which attempts to lower the motional state below the ground state, is entirely suppressed, whereas the Stokes scattering, which raises the motional state, is allowed. For thermally occupied states, an absolute, self-calibrating temperature measurement is possible by measuring this asymmetry in Raman scattering. Distributed optical ber

Reconfigurable re-entrant cavity for wireless coupling to an electro-optomechanical device

An electro-optomechanical device capable of microwave-to-optics conversion has recently been demonstrated, with the vision of enabling optical networks of superconducting qubits. Here we present an improved converter design that uses a three-dimensional microwave cavity for coupling between the microwave transmission line and an integrated LC resonator on the converter chip. The new design simplifies the optical assembly and decouples it from the microwave part of the setup. Experimental demonstrations show that the modular device assembly allows us to flexibly tune the microwave coupling to the converter chip while maintaining small loss. We also find that electromechanical experiments are not impacted by the additional microwave cavity. Our design is compatible with a high-finesse optical cavity and will improve optical performance.

Harnessing electro-optic correlations in an efficient mechanical converter

An optical network of superconducting quantum bits (qubits) is an appealing platform for quantum communication and distributed quantum computing, but developing a quantum-compatible link between the microwave and optical domains remains an outstanding challenge. Operating at T < 100 mK temperatures, as required for quantum electrical circuits, we demonstrate a mechanically mediated microwave–optical converter with 47% conversion efficiency, and use a classical feed-forward protocol to reduce added noise to 38 photons. The feed-forward protocol harnesses our discovery that noise emitted from the two converter output ports is strongly correlated because both outputs record thermal motion of the same mechanical mode. We also discuss a quantum feed-forward protocol that, given high system efficiencies, would allow quantum information to be transferred even when thermal phonons enter the mechanical element faster than the electro-optic conversion rate.A mechanical-mediated quantum-compatible microwave–optical converter achieves high efficiency through a feed-forward protocol that harnesses correlations in the output noise.

Progress towards quantum state transfer between microwave and optical light using an electro-optomechanical resonator

We have constructed a bidirectional and efficient converter between microwave and optical light using a mechanically compliant membrane coupled via the optomechanical interaction. Ongoing work towards quantum state transfer is discussed.

Connecting microwave and optical frequencies with a vibrational degree of freedom

We describe the construction of a device that converts electromagnetic signals from microwave (7 GHz) to optical (282 THz) frequencies, and vice-versa. The frequency converter relies on a flexible silicon nitride membrane that is coupled via radiation pressure to both a microwave circuit and a Fabry-Perot cavity. The frequency converter achieves conversion efficiencies of ∼10%, and is potentially capable of frequency conversion of quantum signals.

A cryogenic cavity optomechanics system for membrane microresonators

We have constructed a monolithic Fabry-Perot cavity optomechanical system. From cryogenic temperatures we significantly damp a Si3N4-membrane and achieve conditions suitable for cooling MHz mechanical resonators from ~ 4 K into the quantum regime.