High frequency gravitational wave detection with optically levitated nanoparticles: an update on the LSD (levitated sensor detector)

Kilometer-scale ground-based gravitational-wave interferometers have generated a new field of astronomy by viewing the universe in gravitational wave (GW) radiation, operating at a peak sensitivity of frequencies ranging from 10s of Hz to a few kHz.¹ A great many discoveries have resulted from these detectors, such as the existence of binary black hole and neutron star systems.² It is of great scientific value to extend the GW search to other frequencies, just as has been the case for the exploration of the EM spectrum. The levitated sensor detector (LSD) is a 1 m tabletop scale high frequency (> 10 kHz) gravitational wave detection experiment currently under construction at Northwestern University.³ It will serve as a 1 m prototype for future generations of levitated mass based instruments. The LSD sensitivity has more favorable frequency scaling at these frequencies compared to laser interferometer detectors such as LIGO and VIRGO due to different limiting noise factors, the LIGO free spectral range, and the fact that the LSD is a resonant sensor. The LSD is sensitive to GWs from binary coalescence of sub-solar-mass primordial black holes and as-yet unexplored new physics in the high-frequency GW window, such as the annihiation of gravitationally bound states of the QCD axion by black hole superradiance. Many promising experiments and techniques exist for probing the GW spectrum below the LIGO frequency band; they include pulsar timing arrays,^4,5 atomic clocks and other interferometers,^6,7 LISA,^8,9 and DECIGO.¹⁰ There are also a number of proposals, experiments and initial bounds set above the LIGO frequency band, largely over 100 MHz.^11–18 Fewer established methods to systematically probe the kHz-MHz part of the GW spectrum, where a variety of interesting sources could exist. At Northwestern, we are constructing a compact Michelson interferometer configuration with Fabry-Pérot arms as shown in Figure 3, designed to work in the 10-100 kHz band. In the medium and long term, a multi index dielectric stack will be suspended at an anti-node of the standing wave inside each Fabry-Pérot arm. In the short term, this is likely to be a disc or disc like object (discussed in sections 2 and 2.2) — with which a degree of experimental success has already been had. A second laser is used to read out the position of the object as well as cool it along the cavity axes.¹⁹