Classical gravimeters are fundamentally limited by standard quantum shot-noise when measuring local gravitational acceleration; spin-squeezed atom interferometers shatter the standard quantum limit to measure gravity with unprecedented precision.

Measuring microscopic fluctuations in Earth's gravitational field enables geophysicists to map subterranean aquifers, detect volcanic magma movement, and locate concealed underground tunnels.
Standard atomic gravimeters split and recombine cold atom wave packets, but their sensitivity has been fundamentally bottlenecked by the standard quantum limit (SQL)—the irreducible statistical shot-noise of unentangled atoms.
By preparing an ensemble of laser-cooled rubidium atoms in a quantum spin-squeezed entangled state via cavity-mediated optical interactions, this experimental gravimeter achieves sub-shot-noise phase sensitivity during free-fall gravity measurements.
Entanglement-enhanced atomic gravimeters unlock a new era of ultra-precise inertial navigation, quantum geodesy, and fundamental tests of the equivalence principle at the intersection of quantum mechanics and gravity.
Entanglement-Enhanced Atomic Gravimeter
Interferometers based on ultracold atoms enable an absolute measurement of inertial forces with unprecedented precision. However, their resolution is fundamentally restricted by quantum fluctuations. Improved resolutions with entangled or squeezed atoms were demonstrated in internal-state measurements for thermal and quantum-degenerate atoms and, recently, for momentum-state interferometers with laser-cooled atoms. Here, we present a gravimeter based on Bose-Einstein condensates with a sensitivity of −1.7−0.5+0.4 dB beyond the standard quantum limit. Interferometry with Bose-Einstein condensates combined with delta-kick collimation minimizes atom loss in and improves scalability of the interferometer to very-long-baseline atom interferometers.
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