Quantum dynamics of an electromagnetic mode that cannot contain N photons

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Science  15 May 2015:
Vol. 348, Issue 6236, pp. 776-779
DOI: 10.1126/science.1259345

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Tailoring the quantum dynamics of light

The energy levels of a quantum system are determined by the laws of quantum mechanics and the specifics of the physical setting. Light confined to a cavity has energy levels neatly arranged in a “ladder” of equidistant rungs, each rung corresponding to a fixed number of photons. Bretheau et al. devised a way to limit the dynamics to only the lowest few rungs by coupling the system to a qubit, which shifted the energy of one of the higher rungs. When they then drove the system at a frequency corresponding to the distance between the rungs, only the states lower in energy than the shifted state could participate.

Science, this issue p. 776


Electromagnetic modes are instrumental in building quantum machines. In this experiment, we introduce a method to manipulate these modes by effectively controlling their phase space. Preventing access to a single energy level, corresponding to a number of photons N, confined the dynamics of the field to levels 0 to N – 1. Under a resonant drive, the level occupation was found to oscillate in time, similarly to an N-level system. Performing a direct Wigner tomography of the field revealed its nonclassical features, including a Schrödinger cat–like state at half period in the evolution. This fine control of the field in its phase space may enable applications in quantum information and metrology.

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