We discuss the relationship between quantum coherence of finite dimensional systems and nonclassical quantum light. We demonstrate that quantifying the quantum coherence between coherent states always leads to a quantifier of nonclassicality that monotonically decreases under linear optical operations. This allows us to introduce a resource theory of nonclassicality based on linear optics that closely parallels the resource theory of coherence. Finally, we discuss the metrological power of a quantum state, which quantifies how sensitive a quantum state is to displacement operations. It is then shown that the metrological power leads to a nonclassicality monotone.
A "Schrödinger cat" state of free-propagating light can be defined as a quantum superposition of well separated
coherent states. 1, 2 We demonstrated, theoretically and experimentally, a protocol which allows to generate
arbitrarily large squeezed Schrödinger cat states, using a homodyne detection and photon number states as
resources. We implemented this protocol experimentally with light pulses containing two photons, producing a
squeezed Schrödinger cat state with a negative Wigner function. This state clearly presents several quantum
phase-space interference fringes between the "dead" and "alive" components, and it is large enough to become
useful for experimental tests of quantum theory and quantum information processing.