Superconducting quantum devices

Abstract:

Hybrid circuit QED involves the study of coherent quantum physics in solid-state systems via their interactions with superconducting microwave circuits. Here we present a crucial step in the implementation of a hybrid superconducting qubit that employs a carbon nanotube as a Josephson junction. We realize the junction by contacting a carbon nanotube with a superconducting Pd/Al bilayer, and implement voltage tunability of the quantum circuit's frequency using a local electrostatic gate. We demonstrate a strong dispersive coupling to a coplanar waveguide resonator by investigating the gate-tunable resonator frequency. We extract qubit parameters from spectroscopy using dispersive readout and find qubit relaxation and coherence times in the range of 10-200ns.

Abstract:

Critical slowing down of the time it takes a system to reach equilibrium is a key signature of bistability in dissipative first-order phase transitions. Understanding and characterizing this process can shed light on the underlying many-body dynamics that occur close to such a transition. Here, we explore the rich quantum activation dynamics and the appearance of critical slowing down in an engineered superconducting quantum circuit. Specifically, we investigate the intermediate bistable regime of the generalized Jaynes-Cummings Hamiltonian (GJC), realized by a circuit quantum electrodynamics (cQED) system consisting of a transmon qubit coupled to a microwave cavity. We find a previously unidentified regime of quantum activation in which the critical slowing down reaches saturation and, by comparing our experimental results with a range of models, we shed light on the fundamental role played by the qubit in this regime.

Abstract:

This article presents a new filter design for arbitrary placement of real frequency transmission zeros in triple-mode filters, allowing for either asymmetric or symmetric responses. Triple-mode filters can be viewed, in general, as a series combination of coupled resonators with non-adjacent couplings, allowing for the realisation of transmission zeros. The strength and phase of couplings dictate the transmission zeros locations. The arbitrary placement of transmission zeros requires controlling all amplitude and phase rations, i.e., the ability to realise both negative and positive couplings. In general, this necessitates the use of capacitive and inductive probe structures complicating the construction and hence increasing the filter cost. The method presented in this article enables arbitrary placement of transmission zeros using only inductive or only capacitive couplings structures. In addition, broadband filtering characteristics can be realised using triple-mode filters. Examples of several narrow and broadband bandpass filters are given indicating that this method is valid for most triple-mode structures.