ABSTRACT
A new perspective for photonic instrumentation is offered by quantum sensors. In the streamline of quantum computing, quantum sensors are based on some counter-intuitive properties of quantum optics, like squeezed states offering a measurement uncertainty below the normal quantum limit, and entangled states or bi-photons, pairing the result of measurements on photons propagated even very far away from each other. In the chapter, we describe at some length the squeezing of photons and its application to improved-sensitivity interferometry, and the basics of entangled states that potentially can also achieve performance at sub-quantum limit, even though till now they are limited to a speculative stage, although very promising.
