Micro ring cavities with laser-machined mirrors for quantum-enhanced sensing

The fiber Fabry-Perot microcavities (FFPs) with laser-machined mirrors developed in our group have found many applications in atomic and solid-state cavity QED experiments and beyond. Here we show how laser-machined mirror technology can be leveraged to realize a micro-ring cavity. While maintaining key advantages such as small mode cross-section and high finesse, this novel design replaces the FPP's standing wave with a running-wave field that has no blind spots, and gives access to separated clockwise and anticlockwise modes. These are crucial advantages for applications such as atomic spin squeezing for quantum-enhanced clocks and sensors.