Quantum tricks with gravitational-wave detector mirrors

LIGO researchers tune motion of suspended interferometer mirrors to cool a 10-kg optomechanical oscillator to near its motional ground state

June 18, 2021

In the last few decades, physicists have devised new ways to cool objects close to absolute zero. At these low temperatures their atoms are near their quantum motional ground state. So far, this has been achieved only for small objects with masses in the nanogram range. Now members of an international team have managed to cool a human-scale “pseudo object” with a mass of 10 kilograms to close to its motional ground state. The pseudo object is the combined motion of four widely separated objects. The researchers controlled the relative motion of the four 40-kilogram test masses in the two arms of a LIGO detector such that they could observe this oscillator at an effective temperature of just 77 nanokelvin.

Paper abstract

The motion of a mechanical object, even a human-sized object, should be governed by the rules of quantum mechanics. Coaxing them into a quantum state is, however, difficult because the thermal environment masks any quantum signature of the object’s motion. The thermal environment also masks the effects of proposed modifications of quantum mechanics at large mass scales. We prepared the center-of-mass motion of a 10-kilogram mechanical oscillator in a state with an average phonon occupation of 10.8. The reduction in temperature, from room temperature to 77 nanokelvin, is commensurate with an 11 orders-of-magnitude suppression of quantum back-action by feedback and a 13 orders-of-magnitude increase in the mass of an object prepared close to its motional ground state. Our approach will enable the possibility of probing gravity on massive quantum systems.

Trapping and cooling of a 10 kg oscillator to 10 quanta. (a) Effective susceptibility of the oscillator for each setting of the damping filter, measured by exciting the feedback loop at each frequency and demodulating its response at the same frequency. The lines show fits to a model of the susceptibility of a damped harmonic oscillator with an additional delay, i.e. χeff[Ω]eiΩτ; fits to the phase response produce τ=0.9ms. (b) Displacement spectrum of the oscillator as the damping is increased. Solid lines show fits to a model of the observed spectrum Sobs (see text for details) where the effective susceptibility is determined by the response measurements in panel (a), and only the frequency-dependent imprecision noise and force noise are variable. Inset shows the inferred average phonon occupation for each of the curves in the main panel, as a function of the damping quality factor; also shown is a model (black dashed) with model uncertainties (gray band). (The disagreement between the simple model and data — both the transfer functions and spectra — around 150–155 Hz arises from a coupling between the motion of the pendulum and the upper intermediate mass of the suspension.)
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