Bright and Dark States of Light: The Quantum Origin of Classical Interference

According to classical theory, the combined effect of several electromagnetic fields is described under the generic term of interference, where intensity patterns with maxima and minima emerge over space. On the other hand, quantum theory asserts that considering only the expectation value of the total field is insufficient to describe light-matter coupling, and the most widespread explanation highlights the role of quantum fluctuations to obtain correct predictions. We here connect the two worlds by showing that classical interference can be quantum-mechanically interpreted as a bosonic form of super- and subradiance, giving rise to bright and dark states for light modes. We revisit the double-slit experiment and the Mach-Zehnder interferometer, reinterpreting their predictions in terms of collective states of the radiation field. We also demonstrate that quantum fluctuations are not the key ingredient in describing the light-matter quantum dynamics when several vacuum modes are involved. In light of our approach, the unambiguous criterion for the coupling to occur is the presence of non-dark states when decomposing the collective state of light. We discuss how the results here discussed could be verified in trapped ion systems or in cross-cavity setups. Finally, we show how the bosonic bright and dark states can be employed to implement quantum gates, which paves the way for the engineering of multimode schemes for universal quantum computing.