Chapter 4: Quantum noise

This chapter examines the fundamentals of quantum noise in laser interferometer type gravitational-wave detectors. After defining coherent and squeezed vacuum states, quantum noise in a simple Michelson interferometer is used to introduce shot noise, quantum back-action noise and the standard quantum limit. We discuss how quantum noise limits can be manipulated via internal interaction of the propagating field with mirror dynamics, changing the readout observable, or injecting a squeezed field into the interferometer. We finish with an overview of the current state of the art and outlook, circa 2018.