Experiments with superconducting circuits; Dynamical Casimir effect and photon routing

We have been able to observe the Dynamical Casimir Effect (DCE) in a superconducting circuit consisting of a coplanar transmission line with a tunable electrical length [1]. The rate of change of the electrical length can be made very fast (a substantial fraction of the speed of light) by modulating the inductance of a superconducting quantum interference device at high frequencies (~10 GHz). In addition to observing the creation of real photons, we detect two-mode squeezing in the emitted radiation, which is a signature of the quantum character of the generation process. This phenomenon was predicted 40 years ago and has not been observed until now. We also study an artificial 3 level atom in the form of a transmon qubit coupled to superconducting 1D transmission line [2]. Strong interaction between the artificial atom and photons is revealed in the reflection of propagating microwaves and substantial extinction (99.6%) of the transmission has been observed. A strong control pulse, at the frequency corresponding to the transition between the two upper states, is used to route a weak probe tone at lower transition frequency. The maximum on-off ratio is 99% with a rise and fall time on the order of 10 ns. This fast controllable router provides a fundamental building block for quantum optics on chip.

1. C.M. Wilson et al. Nature, 479, 376 (2011).
2. I.-C. Hoi et al. Physical Review Letters, 107, 073601 (2011)