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.