Abstract
Josephson circuits provide a realistic physical setup where the light–matter fine structure constant can become of order one, allowing to reach a regime dominated by non-perturbative effects beyond standard quantum optics. Simple processes, such as spontaneous emission, thus acquire a many-body character, that can be tackled using a new description of the time-dependent state vector in terms of quantum-superposed coherent states. We find that spontaneous atomic decay at ultrastrong coupling leads to the emission of spectrally broad Schrödinger cats rather than of monochromatic single photons. These cats states remain partially entangled with the emitter at intermediate stages of the dynamics, even after emission, due to a large separation in time scales between fast energy relaxation and exponentially slow decoherence. Once decoherence of the qubit is finally established, quantum information is completely transfered to the state of the emitted cat.
Highlights
Photons describe the granular structure of the electromagnetic field radiated by single coherent sources, such as atoms and quantum dots. These quanta of light constitute well-defined monochromatic excitations because the spontaneous emission rate Γ is much smaller than the transition frequency ∆ of an emitter
This requires the use of superconducting waveguides with high impedance, since αQED = Zvac./2RK can be interpreted as the ratio of the vacuum impedance Zvac. = 1/ 0c 376 Ω to the quantum of resistance RK = e2/h 25812 Ω
As already mentioned above, the radiation is spectrally broad at ultrastrong coupling, and the resulting cat states are rather localized in the time domain rather than in frequency
Summary
Photons describe the granular structure of the electromagnetic field radiated by single coherent sources, such as atoms and quantum dots. As already mentioned above, the radiation is spectrally broad at ultrastrong coupling, and the resulting cat states are rather localized in the time domain rather than in frequency.
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