We develop a compact four-port superconducting switch with a tunable operating frequency in the range of 4.8 GHz — 7.3 GHz. Isolation between channel exceeds 20~dB over a bandwidthof several hundred megahertz, exceeding 40 dB at some frequencies. The footprint of the device is 80×420 μm. The tunability requires only a global flux bias without either permanent magnets or micro-electromechanical structures. As the switch is superconducting, the heat dissipation during operation is negligible. The device can operate at up to -80~dBm, which is equal to 2.5×106 photons at 6 GHz per microsecond. The device show a possibility to be operated as a beamsplitter with tunable splitting ratio.
We address the scaling-up problem for superconducting quantum circuits by using lumped-element resonators based on a new fabrication method of aluminum — aluminum oxide —aluminum (Al/AlOx/Al) parallel-plate capacitors. The size of the resonators is only 0.04 mm2, which is more than one order smaller than the typical size of coplanar resonators (1 mm2). The fabrication method we developed easily fits into the standard superconducting qubits fabrication process. We have obtained capacitance per area 14 fF/μm2 and the internal quality factor 1×103−8×103 at the single-photon level. Our results show that such devices based on Al/AlOx/Al capacitors could be further applied to the qubit readout scheme, including resonators, filters, amplifiers, as well as microwave metamaterials and novel types of qubits, such as 0−π qubit.