Speaker
Description
Optical atomic clocks are our most precise tools to measure time and frequency. Their precision enables frequency comparisons between atoms in separate locations to probe the space-time variation of fundamental constants, the properties of dark matter, and for geodesy. Measurements on independent systems are limited by the standard quantum limit (SQL); measurements on entangled systems, in contrast, can surpass the SQL, to reach the ultimate precision allowed by quantum theory --- the so-called Heisenberg limit. While local entangling operations have been used to demonstrate this enhancement at microscopic distances, frequency comparisons between remote atomic clocks require the rapid generation of high-fidelity entanglement between separate systems that have no intrinsic interactions. We demonstrate the first quantum network of entangled optical clocks [1], using two 88Sr+ ions, separated by a macroscopic distance (≈2m), that are entangled using a photonic link. We characterise the entanglement enhancement for frequency comparisons between the ions. We find that, in the absence of decoherence due to the probe laser, entanglement improves the single-shot uncertainty by a factor close to √2, as predicted for the Heisenberg limit, thus halving the number of measurements required to reach a given precision. Practically, today's optical clocks are typically limited by laser decoherence; in this regime, we find that using entangled clocks confers an even greater benefit, yielding a factor 4 reduction in the number of measurements, compared to conventional correlation spectroscopy techniques [1].
As a proof of principle, we demonstrate this enhancement for measuring a frequency shift applied to one of the clocks. Our results show that quantum networks have now attained sufficient maturity for enhanced metrology. This two-node network could be extended to additional nodes, to other species of trapped particles, or to larger entangled systems via local operations.
References
[1] Nichol, B. C. et. al. (In preparation)
[2] Clements, E. R et al. Lifetime-Limited Interrogation of Two Independent 27Al+ Clocks Using Correlation Spectroscopy. Phys. Rev. Lett. 125, 243602 (2020)
