Session S08: FOCUS: Quantum Optomechanics

Live

Quantum sensing protocols using trapped-ions can enable detection of weak electric fields ($<$1 nV/m) by sensing displacements surpassing the Standard Quantum Limit (SQL) – the sensitivity achievable with a coherent state. We present experiments investigating the limits of electric field sensing via the excitation of the center-of-mass (COM) motion of 100s of ions in a 2D crystal. By coupling the mechanical motion of the ions to their spin states by way of an optical potential, the displacement of the ion crystal can be read out via the spin state. Recently, phase stabilization of this optical potential has improved the sensitivity by an order of magnitude. Probing on resonance with the COM mode provides the maximum sensitivity to electric fields. Using a scheme that cancels the thermal and zero-point motion ideally allows for detection of displacements $\sim$15 dB below the SQL. Currently, frequency fluctuations of the COM mode limit this sensitivity to $\sim$5 dB below the SQL. With future improvements, we predict electric field sensitivities of $\sim$1 nV/m. Electric fields of this size may be produced by dark matter candidates: axion and hidden photon dark matter in the neV (MHz) regime has not been experimentally explored at this level.   

Live

The integration of silicon-vacancy (SiV) color centers with diamond nanostructures has allowed demonstration of strong interfaces between photons and long-lived quantum memories. Leveraging the SiV's high strain susceptibility, recent work in the Loncar group has demonstrated coherent acoustic control of SiV spins in diamond using surface acoustic waves. Ongoing work in the Loncar group aims to show coherent interactions between single phonons and the SiV electron spin by fabricating optomechanical crystals with integrated SiV color centers. Engineering control of these interactions would pave the way towards the creation of hybrid quantum systems and phonon-mediated gates between SiV color centers.   

Live

The fundamental limitation in quantum measurement of any observable arises from the measurement added noises, one of the major contributions being from backaction noise. To improve force or impulse sensing beyond the standard quantum limit (SQL), we need to reduce or eliminate the backaction noise. Building on previous works by the gravitational wave community, here we present a continuous measurement protocol using a double-ring optomechanical cavity by coupling an optical field to the momentum of a small mirror. We demonstrate how this protocol with experimentally relevant parameters can lead to significant backaction noise evasion, yielding measurement noise below the standard quantum limit over many decades of frequency. For examples, we discuss the application of this protocol in measuring small impulse transfers through instantaneous and long range interactions, especially in the context of pressure calibration and detection of heavy dark matter particles respectively.   

Live

Experiments in matter wave interferometry and optomechanics are increasing the spatial extent of wavefunctions of massive quantum systems; this gives rise to new sources of decoherence that must be characterized. Here we calculate the position space decoherence of a quantum particle due to interaction with a fluctuating classical background gas for several different force laws. We begin with the calculation of this effect for the Newton potential. To our knowledge, this calculation has not been done before. We then solve the same problem in the case of a Yukawa interaction, which interpolates between our long-range force result and the well-studied formula for collisional decoherence from a contact interaction. Unlike the contact interaction case, where the decoherence rate becomes independent of distance for large quantum particle separations, we observe that a long-range interaction leads to quadratic scaling of the decoherence rate with distance even at large separations. This work is relevant to the generation of massive superposition in optomechanical and atom beam experiments, and to conclude we comment on the use of this decoherence signal for gravitational detection of dark matter.   

Live

We report the weighing of an optically trapped single silica microsphere in air, relying on our earlier work that resolved the instantaneous velocity of Brownian motion. Weighing experiments are typically limited by uncertainty in the microsphere’s density. We overcome this limitation by first deducing the radius of the sphere in a very weak dual-beam optical trap such that its power spectrum is independent of density. We then fix the radius and fit for the density of the sphere, detector calibration constant, and trap strength at arbitrary trapping laser powers. While effective, this method requires a large amount of data to sufficiently smooth the experimental power spectrum. However, once the calibration constant is found for a given trap strength, the equipartition theorem for short-time Brownian motion may be used to monitor mass changes on shorter timescales. We find agreement between the two methods. The equipartition method resolves our mass of 25 picograms in 100 milliseconds with a statistical uncertainty of $\sim$0.5 percent and a systematic uncertainty of $\sim$8.5 percent. Fast detection of the microsphere’s mass has applications in ice nucleation studies in which the formation of a thin layer of ice causes the mass to change with time.   

Quantum Control of Nanomechanical Structures

In this talk I will present the development and experiments with chip-scale quantum systems that allow us to control the state nanoscale mechanical resonators. In particular, I will focus on schemes for performing quantum gates between these resonators, and present approaches for scaling up to larger system sizes. Recent experimental results will be presented.   

Microwave to optics conversion using mechanical oscillators

Conversion between signals in the microwave and optical domains is of great interest, particularly for connecting future superconducting quantum computers into a global quantum network. The idea is to transduce microwaves that are usually lost after a mere few centimeters into an optical signal which does allow transmission of quantum information over tens or even hundreds of kilometers. Here we would like to discuss two recent important steps in realizing such a practical device by demonstrating coherent conversion between GHz signals and the optical telecom band with minimal thermal background noise, while also exploring a new and low-loss piezoelectric material for this process. \\ \\ In collaboration with: Moritz Forsch, Kavli Institute of Nanoscience and Robert Stockill, Kavli Institute of Nanoscience \\ \\ References \\ $[1]$ M. Forsch*, R. Stockill*, A. Wallucks, I. Marinkovi\'{c}, C. Gärtner, R. A. Norte, F. van Otten, A. Fiore, K. Srinivasan, and S. Gröblacher, Nature Phys. 16, 69-74 (2020) [2] R. Stockill*, M. Forsch*, G. Beaudoin, K. Pantzas, I. Sagnes, R. Braive, and S. Gröblacher, Phys. Rev. Lett. 123, 163602 (2019)