Incoherent Measurement of Sub-10 kHz Optical Linewidths

TL;DR

This paper introduces an incoherent fluorescence-based method to measure narrow optical linewidths in quantum emitters, enabling more efficient characterization of materials for quantum technologies without requiring highly coherent lasers.

Contribution

It presents a novel fluorescence variance measurement technique for determining $T_2$ in quantum emitters, overcoming limitations of traditional photon echo methods.

Findings

Successful demonstration in erbium-doped crystal

Higher signal-to-noise ratio compared to coherent methods

No need for highly coherent laser sources

Abstract

Quantum state lifetimes $T_2$, or equivalently homogeneous linewidths $\Gamma_h = 1/\pi T_2$, are a key parameter for understanding decoherence processes in quantum systems and assessing their potential for applications in quantum technologies. The most common tool for measuring narrow optical homogeneous linewidths, i.e. long $T_2$, is the measurement of coherent photon echo emissions, which however gives very weak signal when the number of emitters is small. This strongly hampers the development of nano-materials, such as those based on rare earth ions, for quantum communication and processing. In this work we propose, and demonstrate in an erbium doped crystal, a measurement of photon echoes based on incoherent fluorescence detection and its variance analysis. It gives access to $T_2$ through a much larger signal than direct photon echo detection, and, importantly, without the need for a highly coherent laser. Our results thus open the way to efficiently assess the properties of a broad range of emitters and materials for applications in quantum nano-photonics.