Incoherent Measurement of Sub-10 kHz Optical Linewidths

Quantum state lifetimes T₂, or equivalently homogeneous linewidths Γ_h = 1/πT₂, 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 T₂, 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 T₂ 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.