Quantum optics with microwave photons in superconducting circuits; dynamical Casimir effect and artificial atoms

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. The radiation reflected from the artificial atom can be shown to be non-classical, we observe clear anti bunching of the reflected signal [3].
1. C.M. Wilson et al., Nature 479, 376 (2011).
2. I.-C. Hoi et al., Physical Review Letters 107, 073601 (2011).
3. I.-C. Hoi et al., Physical Review Letters 108, 263601 (2012).