Mid-infrared supermirrors with finesse exceeding 400 000.

For trace gas sensing and precision spectroscopy, optical cavities incorporating low-loss mirrors are indispensable for path length and optical intensity enhancement. Optical interference coatings in the visible and near-infrared (NIR) spectral regions have achieved total optical losses below 2 parts per million (ppm), enabling a cavity finesse in excess of 1 million. However, such advancements have been lacking in the mid-infrared (MIR), despite substantial scientific interest. Here, we demonstrate a significant breakthrough in high-performance MIR mirrors, reporting substrate-transferred single-crystal interference coatings capable of cavity finesse values from 200 000 to 400 000 near 4.5 µm, with excess optical losses (scatter and absorption) below 5 ppm. In a first proof-of-concept demonstration, we achieve the lowest noise-equivalent absorption in a linear cavity ring-down spectrometer normalized by cavity length. This substantial improvement in performance will unlock a rich variety of MIR applications for atmospheric transport and environmental sciences, detection of fugitive emissions, process gas monitoring, breath-gas analysis, and verification of biogenic fuels and plastics.

High-Q Milligram-Scale Monolithic Pendulum for Quantum-Limited Gravity Measurements.

We present the development of a high-Q monolithic silica pendulum weighing 7 milligram. The measured Q value for the pendulum mode at 2.2 Hz was 2.0×10^{6}. To the best of our knowledge this is the lowest dissipative milligram-scale mechanical oscillator to date. By employing this suspension system, the optomechanical displacement sensor for gravity measurements we recently reported in Matsumoto et al. [Phys. Rev. Lett. 122, 071101 (2019)PRLTAO0031-900710.1103/PhysRevLett.122.071101] can be improved to realize quantum-noise-limited sensing at several hundred hertz. In combination with the optical spring effect, the amount of intrinsic dissipation measured in the pendulum mode is enough to satisfy requirements for measurement-based quantum control of a massive pendulum confined in an optical potential. This paves the way for not only testing dark matter via quantum-limited force sensors, but also Newtonian interaction in quantum regimes, namely, between two milligram-scale oscillators in quantum states, as well as improving the sensitivity of gravitational-wave detectors.

Demonstration of Displacement Sensing of a mg-Scale Pendulum for mm- and mg-Scale Gravity Measurements.

Gravity generated by large masses has been observed using a variety of probes from atomic interferometers to torsional balances. However, gravitational coupling between small masses has never been observed so far. Here, we demonstrate sensitive displacement sensing of the Brownian motion of an optically trapped 7 mg pendulum motion whose natural quality factor is increased to 10^{8} through dissipation dilution. The sensitivity for an integration time of one second corresponds to the displacement generated in a millimeter-scale gravitational experiment between the probe and a 100 mg source mass, whose position is modulated at the pendulum mechanical resonant frequency. Development of such a sensitive displacement sensor using a milligram-scale device will pave the way for a new class of experiments where gravitational coupling between small masses in quantum regimes can be achieved.