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Extremely cold atoms have been used to measure gravity more precisely than we thought possible, beating a limit that stems from quantum weirdness.

Ultracold atoms are some of the most sensitive force sensors. They are useful for such work because at the coldest possible temperature 鈥� absolute zero 鈥� they take on quantum properties that are extremely susceptible to pushes and pulls in their environment. But that sensitivity can be muddled by the small fluctuations, or 鈥渜uantum noise鈥�, in the atoms鈥� states.

鈥淵ou can try to reduce the noise in your apparatus as much as possible, but at some point, you will hit a fundamental barrier, which is quantum noise, which you cannot surpass,鈥� says at the German Aerospace Center. However, he and his colleagues have now found a way to overcome this limit after all.

They built an interferometer, a device that traditionally detects the interference of light waves. The patterns created by the clashing light waves reveal the strength and nature of forces in their surroundings, including the pull of gravity. Instead of light, the researchers used about 6000 rubidium atoms cooled to absolute zero and measured their interference patterns. Due to quantum effects that are not prominent at warmer temperatures, the atoms behaved like waves.

The team then used lasers and electromagnetic fields to correlate the atoms to one another through quantum entanglement. This put the atoms in a more entangled, or so-called squeezed, quantum state, which minimised the noise they individually add to the environment, says Klempt. When he and his colleagues measured the pull of gravity with their interferometer, their measurement was less noisy and about 20 per cent more precise than the standard quantum limit usually allows for.

鈥淓ntanglement enhancement has been demonstrated in atomic systems since the early 2000s. However, these demonstrations were all in 鈥榯oy鈥� systems that were incapable of measuring anything useful,鈥� says at the Australian National University. While the new experiment still suffers from limitations and noise separate from the standard quantum limit, it is a proof of principle for implementing squeezing and reaping its benefits, he says.

at Leibniz University Hannover in Germany, who was not involved with the work, says the experiment 鈥渃arries quite some promises鈥� because atom-based interferometers are bound to become the best at measuring gravity, but making it work at a larger size or with more atoms may be technically challenging.

That could help with detecting gravity鈥檚 fingerprints on a cosmic scale. Klempt and his colleagues certainly want to go bigger in the future. They hope to use squeezing in the 10-metre-long , which is thousands of times larger than their experiment and may be able to detect new types of gravitational waves.