Laser physics: In light of coherence

H. Hodara

Peter Franken

Hermann Haken

Yannik Brune, Marius Cizauskas, Marc Aßmann

Lasers serve as the fundamental workhorses of photonic quantum technologies, with perfectly coherent light fields being essential for many protocols that generate nonclassical light, implement coherent control schemes, and initialize qubits. However, no laser is absolutely ideal and the implications of deviations from perfect coherence in quantum technological tasks remain unclear. In this study, we theoretically and experimentally explore the quantum coherence properties of lasers from a resource theory perspective, establishing a significant connection between photonics, quantum optics, and quantum information science. We demonstrate that the maximum achievable quantum coherence for laser light is constrained by spontaneous emission and the purity of the dephased laser field state. As a critical example application in quantum information protocols, we show that the quantum coherence of a laser field with a given mean photon number directly governs the maximum purity attainable when initializing a qubit in a superposition state through resonant driving. Our findings are highly relevant for bridging applied physics and engineering with integrated photonic quantum technologies and resource theories, paving the way for reliable benchmarking of various coherent light sources for applications in photonics and quantum protocols.

William E. Hayenga, Mercedeh Khajavikhan

Second-order coherence measurements can unambiguously determine whether a light source is generating coherent radiation, that is, whether it is a laser. In a systematic study reported by Kreinberg et al. , this measurement is used to additionally reveal intriguing physics associated with ultra-small resonators, such as the identification of sub- or super-radiance emission.

O. Svelto