Measurement noise 100 times lower than the quantum-projection limit using entangled atoms

Abstract

Quantum metrology uses quantum entanglement--correlations in the properties of microscopic systems--to improve the statistical precision of physical measurements. When measuring a signal, such as the phase shift of a light beam or an atomic state, a prominent limitation to achievable precision arises from the noise associated with the counting of uncorrelated probe particles. This noise, commonly referred to as shot noise or projection noise, gives rise to the standard quantum limit (SQL) to phase resolution. However, it can be mitigated down to the fundamental Heisenberg limit by entangling the probe particles. Despite considerable experimental progress in a variety of physical systems, a question that persists is whether these methods can achieve performance levels that compare favourably with optimized conventional (non-entangled) systems. Here we demonstrate an approach that achieves unprecedented levels of metrological improvement using half a million (87)Rb atoms in their 'clock' states. The ensemble is 20.1 ± 0.3 decibels (100-fold) spin-squeezed via an optical-cavity-based measurement. We directly resolve small microwave-induced rotations 18.5 ± 0.3 decibels (70-fold) beyond the SQL. The single-shot phase resolution of 147 microradians achieved by the apparatus is better than that achieved by the best engineered cold atom sensors despite lower atom numbers. We infer entanglement of more than 680 ± 35 particles in the atomic ensemble. Applications include atomic clocks, inertial sensors, and fundamental physics experiments such as tests of general relativity or searches for electron electric dipole moment. To this end, we demonstrate an atomic clock measurement with a quantum enhancement of 10.5 ± 0.3 decibels (11-fold), limited by the phase noise of our microwave source.

Spin precession experiments for light axionic dark matter

“…Current squeezing experiments have demonstrated squeeze factors of ffiffiffiffi N p ∼ 100, providing large signal boosts as well as relaxing the requirement on atom number [79]. These squeezing techniques have not yet been demonstrated in the context of atom interferometry, but in an optimistic scenario, we could expect at least an order of magnitude improvement from squeezing.…”

Lower bounds on the size of general Schrödinger-cat states from experimental data

“…Many experiments nowadays overcome 10 dB of squeezing, implying an effective size N eff ≥ 10. A recent highlight is a spin-squeezing experiment with half a million of cold rubidium atoms [4], where an effective size of 70 was achieved.…”

Two-axis-twisting spin squeezing by multipass quantum erasure

“…Also, spin squeezing can serve as a criterion to quantify many-body entanglement [8]. A widely used approach to create such states is to let atoms interact with a common mode of light, as demonstrated in a variety of systems including cavities [9][10][11], cold atoms [12][13][14][15], and vapor cells [3]. In the majority of current experiments, SSSs are conditioned on a measurement.…”