Physicists harness quantum “time reversal” to measure vibrating atoms. Side effect: most accurate atomic clock ever

Amazing stuff!

"... In a paper ... the team demonstrates that the technique, which they dubbed SATIN (for signal amplification through time reversal), is the most sensitive method for measuring quantum fluctuations developed to date. ...
The technique could improve the accuracy of current state-of-the-art atomic clocks by a factor of 15, making their timing so precise that over the entire age of the universe the clocks would be less than 20 milliseconds off.
The method could also be used to further focus quantum sensors that are designed to detect gravitational waves, dark matter, and other physical phenomena. ...

For their new study, the team studied 400 ultracold atoms of ytterbium ... They cooled the atoms to just a hair above absolute zero ...

The team used a system of lasers to trap the atoms, then sent in a blue-tinged “entangling” light, which coerced the atoms to oscillate in a correlated state. They let the entangled atoms evolve forward in time, then exposed them to a small magnetic field, which introduced a tiny quantum change, slightly shifting the atoms’ collective oscillations.

Such a shift would be impossible to detect with existing measurement tools. Instead, the team applied time reversal to boost this quantum signal. To do this, they sent in another, red-tinged laser that stimulated the atoms to disentangle, as if they were evolving backward in time.

They then measured the particles’ oscillations as they settled back into their unentangled states, and found that their final phase was markedly different from their initial phase — clear evidence that a quantum change had occurred somewhere in their forward evolution. ..."

From the abstract:

"Linear quantum measurements with independent particles are bounded by the standard quantum limit, which limits the precision achievable in estimating unknown phase parameters. The standard quantum limit can be overcome by entangling the particles, but the sensitivity is often limited by the final state readout, especially for complex entangled many-body states with non-Gaussian probability distributions. Here, by implementing an effective time-reversal protocol in an optically engineered many-body spin Hamiltonian, we demonstrate a quantum measurement with non-Gaussian states with performance beyond the limit of the readout scheme. This signal amplification through a time-reversed interaction achieves the greatest phase sensitivity improvement beyond the standard quantum limit demonstrated to date in any full Ramsey interferometer. These results open the field of robust time-reversal-based measurement protocols offering precision not too far from the Heisenberg limit. Potential applications include quantum sensors that operate at finite bandwidth, and the principle we demonstrate may also advance areas such as quantum engineering, quantum measurements and the search for new physics using optical-transition atomic clocks."

Physicists harness quantum “time reversal” to measure vibrating atoms | MIT News | Massachusetts Institute of Technology A new technique could improve the precision of atomic clocks and of quantum sensors for detecting dark matter or gravitational waves.

Time-reversal-based quantum metrology with many-body entangled states (no public access)