High kinetic inductance superconducting cavity for strong coupling of Si/SiGe qubits

Achieving long range coherent interactions between qubits is a milestone of quantum information. In recent years the field of circuit quantum electrodynamics (cQED) has demonstrated long range coupling between superconducting qubits, quantum dots in various semiconductors systems, and even coupling between hybrid systems. Strong coupling between remote qubits requires that the coupling strength be larger than the decoherence rates of both the microwave cavity and the qubits. The coupling can be enhanced by increasing the impedance of the cavity. There are two possible ways to achieve such a goal: first, by using a superconductor with high kinetic inductance; second, by using an array of Josephson junctions, which constrains the application of an external magnetic field. We present a Si/SiGe quantum dot device that combines a high impedance cavity and low impedance leads to reduce the photon leakage through the DC lines. The resonator is made of NbN, a high kinetic inductance superconductor, while the leads are composed of a thin layer of NbN with a thicker layer of Nb on top to reduce the impedance of the leads. The third harmonic can be used to further enhance the coupling strength between the dots and the resonator, which is important for strong coupling.