Precision Optomechanical Platforms

LIGO’s test masses are 40 kg mirrors suspended as pendulums and read out by high-finesse optical cavities with attometer-level displacement sensitivity. These are among the most sensitive mechanical sensors ever built — and they are now being repurposed beyond gravitational-wave detection to probe regimes where quantum mechanics and gravity might intersect.

From detector to quantum laboratory

The key insight is that LIGO-scale optomechanical systems already operate in a regime where quantum effects are measurable. In 2020, the LIGO collaboration observed quantum correlations between light and the kilogram-mass mirrors — demonstrating that the motion of 40 kg objects is measurably influenced by quantum radiation pressure. This was the first observation of quantum correlations at this mass scale, opening the door to experiments that were previously the domain of microgram-scale optomechanics.

What these platforms enable

Quantum-gravity tests. Kilogram-scale masses with attometer readout sensitivity can search for exotic noise sources predicted by quantum-gravity models — spacetime granularity, gravitational decoherence, modified commutation relations. The Tabletop Tests of Quantum Gravity project uses these platforms as the experimental backbone.

Macroscopic quantum mechanics. Preparing a 40 kg mirror in a quantum state (squeezed, entangled, or cooled to its ground state) tests whether quantum mechanics holds at macroscopic scales. Any deviation would signal new physics — decoherence mechanisms, gravitational state reduction, or modifications to the Schrodinger equation.

Precision force sensing. The same displacement sensitivity that detects gravitational waves can measure any force that moves the test masses. Applications include searches for dark matter candidates that couple to ordinary matter, tests of Newtonian gravity at short distances, and measurements of Casimir forces.

EGG contributions

Our group develops the sensing and control technologies that make these platforms work at the quantum level:

• Quantum-noise-limited readout: Squeezed light injection and frequency-dependent squeezing (deployed in LIGO) reduce the measurement noise floor to the point where quantum back-action is observable
• Back-action evasion: Techniques like phase-sensitive optomechanical amplification that measure one quadrature without disturbing the other, enabling measurements below the standard quantum limit
• Correlation measurements: Cross-correlating the outputs of spatially separated optomechanical systems to search for correlated signals (e.g., from spacetime fluctuations) while rejecting uncorrelated noise

Open questions

• Mass scale: Current platforms use LIGO-scale (40 kg) and tabletop (gram to kilogram) masses. What is the optimal mass scale for different quantum-gravity tests — do heavier masses always help, or do decoherence mechanisms scale unfavorably?
• Environmental isolation: Quantum-gravity signals are expected to be extraordinarily weak. How do you distinguish a genuine quantum-gravity effect from residual environmental coupling (seismic, thermal, electromagnetic) at the required sensitivity levels?
• Entanglement verification: Proposed tests of gravity-mediated entanglement require verifying that two massive objects become entangled through their gravitational interaction alone. What witness measurements are sufficient, and how do you exclude electromagnetic or other non-gravitational coupling channels?
• Scalability: Can the techniques developed for LIGO-scale optics be adapted for next-generation platforms (e.g., larger masses, longer baselines, cryogenic operation)?