A levitating dust grain in an optical trap is an ideal pendulum for measuring gravity. It hangs in vacuum, never touching the walls, and its motion depends only on external forces. The new work proposes an improved sensor: a mechanical 'squeezed' qubit based on a nonlinear oscillator. This is a system where the restoring force grows disproportionately to displacement, like a spring that becomes stiffer when stretched hard.
The main trick lies in a special driving regime. The particle is acted upon by two laser pulses shifted in frequency. This is similar to pushing a swing not just in rhythm but also squeezing the motion itself: the amplitude in one direction becomes larger, in the other smaller. Such a 'squeezed' pendulum responds much more strongly to a weak gravitational kick. Moreover, sensitivity grows with particle mass — the heavier the dust grain, the more noticeable its shift.
The price for amplification is noise. Ordinary friction from residual gas molecules (mechanical damping) in the squeezed state turns into anisotropic noise: it degrades the two oscillation components differently. This is like a directional microphone that makes the voice louder but also mixes in street noise unevenly. The authors found a balance between the gain in signal and the loss due to decoherence, opening the way to real quantum gravimeters based on levitating particles.
Levitating particles are already used in dark matter detectors: if a dark matter particle collides with a dust grain, the grain jerks, and that jerk can be detected.
In Stanislaw Lem's novel 'Solaris', the planet's ocean responds sensitively to gravitational disturbances — almost like a living gravimeter.