Quantum microwaves: Stabilizing squeezed light by phase locking

Bright sources of quantum microwave light are an important building block for various quantum technological applications. Josephson junctions coupled to microwave cavities are a particularly versatile and simple source for microwaves with quantum characteristics, such as different types of squeezing. Due to the inherent nonlinearity of the system, a pure dc-voltage bias can lead to the emission of correlated pairs of photons into a stripline resonator. However, a drawback of this method is that it suffers from bias voltage noise, which disturbs the phase of the junction and consequently destroys the coherence of the photons, severely limiting its applications. Here we describe how adding a small ac reference signal either to the dc bias or directly into the cavity can stabilize the system and counteract the sensitivity to noise. We first consider the injection locking of a single-mode device before turning to the more technologically relevant locking of two-mode squeezed states, where phase locking preserves the entanglement between photons. Finally, we describe locking by directly injecting a microwave into the cavity, which breaks the symmetry of the squeezing ellipse. In all cases, locking can mitigate the effects of voltage noise and enable the use of squeezed states in quantum technological applications.