Collectively pumped superradiant laser with coherence scaling beyond the standard quantum limit

arXiv:2611.05976v1 Announce Type: new Abstract: For more than half a century, the standard quantum limit (SQL) was thought to limit the laser coherence $\mathfrak{C}$ -- the number of photons emitted from the laser into the beam in one coherence time -- to a scaling $\Theta(\mu^2)$, where $\mu$ is the mean number of optical-frequency excitations stored inside the laser. However, recently it has been shown [Baker et al., Nat. Phys. 17, 179 (2021)] that the Heisenberg limit -- an achievable ultimate limit set by quantum mechanics for the task of producing a beam with the standard properties of a laser beam -- is $\mathfrak{C}=\Theta(\mu^4)$, a quadratic enhancement.