1999/03/25 by Howard M. Wiseman, H. M. Wiseman, Wiseman, H. M.
Computer Science · Physics and Astronomy · #Cold Atom Physics and Bose-Einstein Condensates #FOS: Physical sciences #Quantum Information and Cryptography #Quantum Mechanics and Applications #Quantum Physics (quant-ph) #quant-ph
paper · pdf · doi:10.48550/arxiv.quant-ph/9903082
10 pages of text plus one figure
arxiv created 1999/03/25 · openalex publication_date 1999/03/25 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
The standard quantum limit to the linewidth of a laser for which the gain medium can be adiabatically eliminated is ℓ0=κ/2n. Here κ is the intensity damping rate and n the mean photon number. This contains equal contributions from the loss and gain processes, so that simple arguments which attribute the linewidth wholly to phase noise from spontaneous gain are wrong. I show that an \em unstimulated gain process actually introduces no phase noise, so that the ultimate quantum limit to the laser linewidth comes from the loss alone and is equal to ℓult= κ/4n. I investigate a number of physical gain mechanisms which attempt to achieve gain without phase noise: a linear atom-field coupling with finite interaction time; a nonlinear atom-field coupling; and adiabatic photon transfer using a counterintuitive pulse sequence. The first at best reaches the standard limit ℓ0, the second reaches 3/4 ℓ0, while the third reaches the ultimate limit of ℓult = 1/2 ℓ0.