Can entanglement improve a clock already at the stability frontier?
In plain words
Squeezed atom states (correlated states with reduced noise in one quantity) have beaten the standard limit in clocks, but only in setups that were not the most stable clocks available. The question is whether entanglement can improve the very best clocks in absolute terms.
Precise statement
Demonstrate an optical lattice or tweezer clock whose absolute fractional frequency instability, with entangled or squeezed atomic states, is below that of the best unentangled clock with the same atom number and interrogation hardware, at instability levels of order $10^{-17}$ at 1 s or better.
What would settle it
A clock comparison showing an entanglement-enabled absolute stability improvement over the unentangled state of the art.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Spin squeezing in optical clocks gave gains of a few dB below the standard quantum limit in differential comparisons (2023-2024), not an absolute record.
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