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In the point of view of quantum optics, an open system is treated as a generalized scattering center, the mediator of a transformation – commonly known as scattering – from input fields to output fields. The inputs and outputs are regarded as excitations of a reservoir or bath, and in the Markov case outputs may be measured with no disturbance to the unconditional open-system dynamics. Measuring, whether actual or virtual, provides then the basis upon which a stochastic conditional dynamics may be constructed [H. J. Carmichael, Open Quantum Systems, Ch. 4 in Strong Light-Matter Coupling: From Atoms to Solid-State Systems, World Scientific Publishing, 2014]. In this seminar, we will discuss the perspective offered by quantum trajectory theory, stemming from a formalized version of Bohr’s indivisible quantum
jump. By making photon counting records we effectively define an environment via a particular idealization of what might lie in the path of the scattered field at a distance (perhaps at a great distance) from the scattering center. Every scattered photon is then used up producing a record appropriate to this environment. Other idealized environments will produce different records and will disentangle the system and environment in different ways, all consistent with the validity of the master equation [H. J. Carmichael, Quantum Jumps Revisited: An Overview of Quantum Trajectory Theory, Conference paper (Springer, 2007)]. To assess the implications of complementarity, we will compare the master equation predictions against individual realizations with reference to a paradigmatic light-matter interaction formulation, the Jaynes-
Cummings (JC) model. We will focus on the correlations that rise between photons scattered by a two-level atom coupled to a driven cavity mode inside the strong coupling and low excitation regime. The system is known to display multi-photon blockade [H. J. Carmichael, Phys. Rev. X 5, 031028 (2015)] where the absorption of n photons blocks the absorption of an additional photon, as revealed in the transmission
spectrum and photon correlations between the scattered light; most notably in a beating of the correlation function that is a direct consequence of the nonlinear JC spectrum. Such a process plays a central role in the study of multi-photon quantum-nonlinear optics, a topic that has been subject of extensive research recently due to the exquisite control acquired over cavity and circuit QED architectures.