Quantum coherence between two atoms beyond Q=10^15

TL;DR

This paper demonstrates quantum coherence between two atoms with a Q-factor exceeding 10^15, using correlation techniques to observe phase evolution over billions of cycles, and applies this to high-precision frequency comparison.

Contribution

It introduces a novel method to measure extremely high Q-factors via quantum coherence and demonstrates quantum state detection in a multi-ion chain without spatial resolution.

Findings

Quantum coherence observed for 3.4x10^15 cycles.

Achieved fractional frequency uncertainty of 3.7x10^-16.

Reported Q-factors exceeding 10^16.

Abstract

We place two atoms in quantum superposition states and observe coherent phase evolution for 3.4x10^15 cycles. Correlation signals from the two atoms yield information about their relative phase even after the probe radiation has decohered. This technique was applied to a frequency comparison of two Al+ ions, where a fractional uncertainty of 3.7+1.0-0.8x10^-16/\sqrt{\tau/s} was observed. Two measures of the Q-factor are reported: The Q-factor derived from quantum coherence is 3.4+2.4-1.1x10^16, and the spectroscopic Q-factor for a Ramsey time of 3 s is 6.7x10^15. As part of this experiment, we demonstrate a method to detect the individual quantum states of two Al+ ions in a Mg+-Al+-Al+ linear ion chain without spatially resolving the ions.