An Atomic Lens for Measuring Gravity express
A thrown stone falls with acceleration, and from the fall time you can calculate the force of gravity. Scientists repeat this experiment with a cloud of atoms cooled to near absolute zero. Such a cloud falls in a vacuum chamber, and laser pulses split its waves, like light in an interferometer. From the shift of the waves, gravity is measured with enormous precision.
Recently, it was proposed to squeeze the atomic cloud before measurement to reduce quantum noise — the jitter that limits accuracy. Squeezing was done, but the cloud quickly expanded, and the effect was lost. In the new work, the cloud is first sharply squeezed with a short pulse, like a photographer focusing with a lens. The density increases, atoms interact more strongly, and the squeezing is enhanced.
The result was verified by computer simulation. For an optimal pulse strength, precision is 20 times better than the standard quantum limit. This is four times better than the previous scheme without the pulse. The method paves the way for more accurate gravimeters — instruments for finding ore, underground voids, and for navigation without satellites.
Original abstract
as written by the authors arXiv:2604.10948
Free-fall atom interferometers offer a powerful platform for accurate, absolute gravitational sensing. Szigeti et al. [Phys. Rev. Lett. 125, 100402 (2020)] recently proposed a quantum-enhanced scheme that uses a spin-squeezed Bose-Einstein condensate as an input state to improve the phase sensitivity of the interferometer. The spin squeezing, generated via one-axis twisting interactions, was limited by condensate expansion. Here we present an improved state preparation in which a sudden trapping potential -- a delta kick -- is initially applied to focus the condensate. The resulting increase in density enhances the one-axis-twisting interactions and produces greater spin squeezing. Using multimode truncated-Wigner simulations, we quantify the performance of the interferometer and find that, for an optimal kick strength, the phase sensitivity surpasses the standard quantum limit by a factor of $$\sim 20$$. This represents a fourfold improvement over the original scheme without the delta kick and is well captured by a two-mode approximation.
The atomic cloud in the experiment is colder than the coldest place in space: its temperature is only a few millionths of a degree above absolute zero.