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Description
A Josephson junction in series with an impedance and voltage biased below the gap provides a simple quantum electrodynamics system in which the coupling constant between charge transfer across the junction and each mode of the impedance is determined by the ratio between the mode impedance and the relevant resistance quantum. Circuit engineering allows to implement interesting situations beyond reach of quantum optics in atomic physics, and noticeably to reach large values of the coupling constant.
In the simplest case of a single mode resonator, the transfer of a single Cooper pair only occurs when its energy 2eV can be transformed in 1,2,...,n excitations in the resonator. This inelastic tunneling phenomenon is the essence of Dynamical Coulomb Blockade. In the strong coupling regime, a single Cooper pair transfer can be associated to the emission of a few photons in the external circuit. The presence of a single excitation in the resonator can even block the creation of a second one, which then forces the resonator to emit a single photon in the external circuit before another Cooper pair can pass and re-excite it. This phenomenon provides a very simple single photon source.
In a two resonator circuit with different frequencies, the transfer of a single Cooper pair can simultaneously excite a single excitation in each resonator. The photons leaking out of the two resonators in the measurement lines are then entangled, which was probed. In the particular case of two resonators respectively with a high (low) quality factor, the stabilization of a single excitation Fock state in the high Q resonator could furthermore be achieved.
Applications are sought for these non-classical sources of radiation in the microwave domain that could be extended up to the THz frequency range, a quite unexplored domain of the electromagnetic radiation spectrum.
