Ultra‐broadband and passive stabilization of ultrafast light sources by quantum light injection

Nonlinear optical effects such as frequency conversion form the basis for many practical light sources. In a variety of settings, the performance of such sources is limited by quantum noise. In many nonlinear systems, this quantum noise gets strongly amplified, as a result of the large sensitivity of the nonlinear dynamics to changes in the initial conditions - a feature common to many nonlinear systems. Here, we develop a general theory of quantum noise resulting from nonlinear dynamics initiated by many-photon Gaussian quantum states. The theory provides guidelines to find the optimal quantum state to inject to maximally suppress the noise at the output. As a concrete example of the concept and theory, we consider the nonlinear optical phenomenon of supercontinuum generation by a femtosecond pulse, a famously noise-generating process, which is important in a range of applications in materials characterization and life science. By seeding supercontinuum generation with pulsed squeezed vacuum, one can achieve order-of-magnitude magnitude reduction of intensity and phase noise simultaneously, over a broad band of wavelengths, passively, and with no change in spectrum. The large magnitude and bandwidth of this effect is challenging to achieve by other means of stabilization, pointing to a promising approach for controlling quantum noise in a variety of nonlinear systems.