Session D33: Hybrid/Macroscopic Quantum Systems, Optomechanics, and AMO Systems III

Membrane-based Optomechanical Accelerometry

Optomechanical accelerometers promise quantum-limited readout, high detection bandwidth, self-calibration, and radiation pressure stabilization. We present a simple, scalable platform that enables these benefits with sub-micro-g sensitivity at acoustic frequencies, based on a pair of vertically integrated Si₃N₄ membranes with different stiffnesses, forming an optical cavity. As a demonstration, we integrate an ultra-high-Q (>10⁷), millimeter-scale Si₃N₄ trampoline (40 kHz fundamental resonance) above an unpatterned membrane (180 kHz) on the same Si chip, forming a finesse F≈2 cavity. Using direct photodetection in transmission, we resolve the relative displacement of the membranes with a shot-noise-limited imprecision of 7 fm/√Hz, yielding a thermal-noise-limited acceleration sensitivity of 600 nano-g/√Hz over a 1 kHz bandwidth centered on the trampoline resonance. We use radiation pressure to cold-damp the trampoline to an effective temperature of 4 mK and show that this method can be used to resolve stochastic accelerations as small as 50 ng/√Hz in integration times of minutes. In the future, we envision a F∼100 (using photonic crystal mirrors), centimeter-scale version of this device operating in a cryostat to search for fundamental weak forces such as vector dark matter.   

Optomechanical readout of an encapsulated micromechanical resonator

Cavity optomechanics is a mature field that has led to the development of ultra-sensitive detectors and has even allowed for the generation of mechanical quantum states at room temperature.  In these coupled systems mechanical motion can be read out from sidebands of the response of a coupled cavity, and a detuned drive can mediate the conversion of photons in a cavity mode and phonons in a coupled mechanical mode, modifying the mechanical mode noise temperature.  Applications of meso-scale resonators such as clocks and resonant sensors could benefit from the cooling achieved using this coupling, as reducing the noise temperature improves sensitivity in resonant sensors and phase noise in oscillators.  However, existing implementation of optomechanics rely on tools such as cryogenic systems or high-power lasers that are challenging to implement in an integrated system.  In this work, we couple an encapsulated micro-scale resonator to an on-chip lumped element resonator built using standard PCB processing. We demonstrate optomechanical readout of the encapsulated high-Q mechanical mode, as well as optomechanical heating and cooling of the mechanical mode that arises due to radiation pressure.   

Massive dissipation dilution of a nanomechanical torsion resonator

Strained nanomechanical resonators have been shown to exhibit ultrahigh quality

factors due to dissipation dilution. While the mechanism of dissipation dilution has been

extensively studied for transverse flexural modes of nanobeams and membranes, it is

commonly held that it does not apply to torsion modes. Here we show that torsion

modes can experience massive dissipation dilution due to tensile stress and draw a

connection to a century-old theory from the torsion balance community that suggests

that a torsion beam is naturally soft-clamped. Enabling this advance is the fabrication of

high-stress Si₃ N₄ nanobeams with width-to-thickness ratios of 10⁴ and the discovery that

their torsion modes have Q factors that scale as the width-to-thickness ratio squared,

yielding Q factors as high as 10⁸ and Q-frequency products as high as 10¹³ Hz. By

disrupting a commonly held belief in the nanomechanics community, our findings invite

a rethinking of strategies towards quantum experiments and precision measurement

with nanomechanical resonators.   

An ultra-low-loss torsion balance leveraging nanoscale dissipation dilution

Torsion balances play a key role in a diversity of pecision measurements. Recently there has been an effort to reduce their

dimensions to the micron scale, enabling chip-scale sensors and giving access to phenomena at the boundary of gravitational

and quantum physics. Here we describe a chip-scale torsion balance fashioned by suspending a rigid Si beam from a tensile-

strained Si3N4 nanoribbon. Stress in the nanoribbon produces a lossless torsion constant that dilutes the loss produced by

elastic deformation, yielding quality (Q) factors above 10^6 for the 10 Hz fundamental torsional mode of 0.1 mg prototype

devices, corresponding to an unprecedented (for this form factor) thermal torque sensitivity of 100 zNm/rt(Hz).  Inverting the

balance produces a resonance frequency shift of 10 Hz, pointing towards the possibility of a chip-scale micro-g gravimeter. Our

ultra-low-loss micromechanical torsion balance is easy to fabricate, lends itself to integration with on-chip light sources,

waveguides, patterned electrodes for detection and actuation schemes, and can be fabricated into arrays, hinting at a powerful

new platform for the quantum limited detection of forces below a piconewton.   

Optical lever measurements with an imprecision below that at theStandard Quantum Limit

An optical lever can be used to precisely monitor mechanical motion and is a natural choice for measuring tilt and rotation. It is relatively immune to technical noise sources such as laser intensity and frequency noise and does not require precise path length stability, unlike interferometric techniques. Here we report on optical lever readout of a strained Si3N4 nanobeam possessing ultra-high-Q torsional modes. We show that its sensitivity to angular displacement can be reduced more than an order of magnitude below the zero-point spectral

density of the beam's fundamental torsional mode (3 nrad/√Hz), in good agreement with a quantum noise model. To our knowledge, this constitutes the first non-interferometric displacement measurement with an imprecision below that at the Standard Quantum Limit, signaling the potential for a new class of cavity-free torsional quantum optomechanics.   

The Posner molecule: Ab initio investigations of a potential biomolecular qubit

The Posner molecule, Ca₉(PO₄)₆, is of central biochemical relevance. It has also gained recent attention for its hypothesized role as a biological quantum information processor. The molecule is thought to maintain long-lived nuclear spin coherences and entanglement among its ³¹P nuclei, which could potentially be used for liquid-state nuclear magnetic resonance quantum computing.

Central to the alleged favorable quantum characteristics of the molecule is the purported symmetrical arrangement of its ³¹P nuclei. An S₆ symmetry, that has been widely assumed, renders the six nuclei magnetically equivalent and simplifies the spin coupling network. In contrast, a lower point-group symmetry is likely to adversely affect the ability of the system to sustain long-lived spin coherences.

Using extensive ab initio molecular dynamics and structural relaxation calculations on over 10,000 molecular geometries, we establish that the Posner molecule predominantly assumes low symmetry structures (C_s and C_i) at room temperature, instead of higher symmetry configurations explored previously. We describe how this critical finding may affect the molecule's viability as a biomolecular qubit.   

Optically trapped nanospheres for scanning force sensing at sub-μm-distances from a surface

Optically levitated nanoparticles in vacuum are extremely well-decoupled from the environment, making them a powerful tool for precision measurement experiments. We trap a ~170 nm diameter silica nanoparticle in a single-beam tweezer trap and transfer it into a standing wave potential by retro-reflecting a laser beam from a gold-coated silicon mirror surface. In the transfer process, we reliably position the nanoparticle at distances of a few hundred nanometers to tens of microns from the conductive surface and use a piezo-driven mirror to scan the two dimensional space parallel to the mirror surface, achieving attonewton level force sensing at modestly low pressures. This method enables 3-D scanning force sensing near surfaces using optically trapped nanoparticles, promising for high-sensitivity scanning force microscopy, tests of the Casimir effect, and tests of the gravitational inverse square law at micron scales.   

Withdrawn

I will discuss a search for non-Newtonian forces that couple to mass, with a characteristic scale of 10 μm, using an optically levitated microsphere as a precision force sensor. A silica microsphere trapped in an upward-propagating, single-beam, optical tweezer is utilized to probe for interactions sourced from a nanofabricated attractor mass with a density modulation brought into close proximity to the microsphere and driven along the axis of periodic density in order to excite an oscillating response. We obtain force sensitivity of ≤10^-16 N/√Hz. Separately searching for attractive and repulsive forces results in the constraint on a new Yukawa interaction of |α| ≥ 10⁸ for λ > 10 μm. This is the first test of the inverse-square law using an optically levitated test mass of dimensions comparable to λ, a complementary method subject to a different set of system effects compared to more established techniques. Near-term improvements to the apparatus and experimental technique are expected to push the sensitivity into unexplored parameter space.   

Characterization of an on-chip rare-earth ion based microwave to optical transducer

Microwave to optical transduction can enable large-scale quantum networks and distributed quantum computing with superconducting qubits. Cavity coupled rare-earth ion (REI) ensembles offer a promising platform for converting between microwave and optical photons. Among REIs, erbium is of particular interest because of its optical transitions in the lowest-loss telecommunication band.

Here, we present an on-chip REI-based transducer utilizing a nanophotonic optical resonator and a superconducting microwave resonator on an erbium-doped yttrium orthovanadate substrate. We calibrate the microwave-to-optical transduction efficiency at dilution temperatures. Pulsed mode measurements will also be presented, where higher efficiency and lower transducer temperature are achieved.   

Hybrid systems based on electrons floating on superfluid helium

Piezoelectric surface acoustic waves (SAWs) are powerful for investigating and controlling elementary and collective excitations in condensed matter and quantum information science systems. In this talk I will describe our experiments on hybrid/synthetic quantum systems comprised of high-frequency SAW devices coupled to 1) electrons floating above the surface of superfluid helium and 2) superconducting qubit systems. Our experiments coupling electrons on helium to an evanescent piezoelectric SAW have allowed us to demonstrate precision acoustoelectric control of this unique trapped electron system for the first time. I will also discuss how these results open the door to an entirely new class of SAW-based experiments with electrons on helium in which the SAW wavelength can be made commensurate with the inter-electron distance. Additionally I will describe our recent experiments coupling GHz-frequency SAW resonators to superconducting qubits. In particular, I will describe a novel "quantum acoustic" architecture based on a purely capacitive coupling between a superconducting transmon qubit and a SAW resonator housed in a three-dimensional microwave cavity, which is able to achieve strong coupling (g ≅ 10 MHz) between the qubit excitations and SAW piezo-phonons.   

An ultra-high Q circuit optomechanical quantum memory with 2 milli-second lifetime

Harnessing the high coherency of mechanical oscillators combined with microwave superconducting resonators makes circuit optomechanics an ideal platform to implement long-life quantum memories as a core quantum technology.

These systems are implemented by LC superconducting resonators with a mechanically compliant capacitor. The conventional design to realize circuit optomechanics enabled observation of several quantum phenomena such as ground-state cooling and entanglement of mechanical objects.  However, the low controllability in this design severely limits their scalability and properties, specifically the mechanical quality factor, which largely determines the thermal coherence time.

Here we introduce a novel design to make a flat high-stress trampoline resonator with mechanical quality factors up to 25 million. We demonstrated ground-state cooling of this resonator down to 0.2 quanta and measured the thermal decoherence rate of 60 quanta/second, corresponding to the lifetime of T1=2.3 ms in the ground state.   

Optomechanics with Ultra-high-Q Perimeter Modes

Stressed mechanical resonators exhibit dissipation dilution, where the dissipation of their flexural modes is orders of magnitude lower than the intrinsic material loss. Structures with strong dissipation dilution typically utilize cascaded elements, requiring extreme aspect ratios which are difficult to fabricate and integrate with optical cavities. We demonstrate a new type of resonator: polygon-shaped resonators tethered at their vertices. Modes on the perimeter of the polygon exhibit strong dissipation dilution due to the periodicity of the structure, thereby allowing ultra-low loss in compact devices. We realize perimeter modes with Q of 3.6 billion at room temperature with spatial extent of only two acoustic wavelengths, exceeding state-of-the-art mechanical Q by a factor of four in ten times smaller devices. We demonstrate near-field optomechanical coupling between these resonators and photonic crystal micro-cavities in an integrated platform. Our optomechanical transducer has an optomechanical cooperativity above unity with mechanical Q exceeding 150 million at room temperature. Our system is ideally suited for room temperature quantum optomechanics experiments such as feedback cooling to the ground state and ponderomotive squeezing.   

Chip-based magnetic levitation of superconducting microparticles for macroscopic quantum experiments

Magnetic levitation has been proposed as a platform to greatly decouple the center-of-mass motion of a levitated mechanical resonator from its environment. As a result, this platform will enable novel, ultra-sensitive force and acceleration sensors, as well as quantum experiments with macroscopic objects of 10^13 atomic mass units. In our work, we demonstrate chip-based magnetic levitation of superconducting microparticles. Our integrated magnetic trap consists of a two-chip stack, with microfabricated niobium superconducting coils generating the magnetic trapping field. We trap near-spherical lead microparticles, which are fabricated in-house. We observe the motion of the levitated microparticle optically and via SQUID-based read-out at temperatures of 4K and 40mK. In the future, we aim to couple the levitated particle to superconducting circuits, in order to perform quantum control of its center-of-mass motion.