Three coupled qubits in a single superconducting quantum circuit

In 3-dimensional circuit Quantum Electrodynamics (cQED), an artificial atom or quantum bit (qubit) couples with a transverse electric mode in a microwave cavity.  Here, the qubit is a non-linear oscillator comprising of a Josephson junction SQUID (tunable non-linear inductor) with a shunting capacitor. The state of the qubit can be measured by scattering a microwave signal from the cavity. 
Recently, there has been significant interest in multi-mode quantum circuits as a new multi-qubit architecture. These circuits exploit geometrical symmetry to produce normal modes that couple in different ways to the electromagnetic cavity. One such circuit, which we call the Ring Transmon, has four Josephson junctions arranged in a Wheatstone-bridge-like fashion. This circuit has three normal modes each of which can be treated as a Transmon qubit. The first of these is a dipolar mode that is directly coupled to the cavity while the other two (one dipolar and one quadrupolar) are uncoupled and therefore protected from cavity-driven decoherence. This idea can be expanded to more complex rings with six or eight or more junctions. The interaction between the different qubits is in the form of a pairwise longitudinal coupling which enables the implementation of two-qubit gates. Therefore, these circuits hold the promise of forming a building block for multi-qubit interaction beyond nearest neighbor coupling and enabling the experimental realization of certain quantum error detection and correction codes.
In this talk, I will give an introduction to superconducting Transmon qubits, circuit QED architecture and describe our fabrication process.  I will talk about our studies on some Ring Transmon devices, in particular the coherence properties of the different modes, the implementation of a conditional NOT gate and the swapping of states between qubits. I will also present some ideas that we plan to implement in the near future.