Strong Coupling Circuit QED with Semiconductor Quantum Dots

Abstract

Circuit quantum electrodynamics (QED) is a powerful approach to study excitations of and engineer and control interactions between superconducting qubits using microwave quantum fields. Recently, the potential of circuit QED has also been explored in the context of semiconductor quantum systems motivated by the possibility to study their excitations in new frequency regimes and the progress towards quantum information architectures based on semiconductor nanostructures. Two hybrid circuit QED architectures are explored in this thesis. They consist of gate-defined semiconductor double quantum dots acting as two-level quantum systems dipole coupled to single photonic modes of microwave cavities. In the first type of device, the double quantum dot charge qubit couples to a superconducting coplanar waveguide resonator at a rate lower than its decoherence rate. This weakly coupled system already provides an interesting platform to study the physics of the quantum dots at microwave frequencies. This thesis discusses experiments exploring microwave emission from a voltage-biased double quantum dot. We detect radiation emitted in inelastic electron tunneling processes between the dots and the leads and in interdot transitions resonant with the cavity. The dependence of the emission signal on the quantum dot level configuration provides a novel way to probe the hybridization and broadening of the electronic double dot states. In the second device architecture, the double quantum dot is coupled to a frequency-tunable high impedance resonator consisting of an array of superconducting quantum interference devices. Due to the high characteristic impedance of these resonators, the coupling strength is increased beyond the decay rates of qubit and resonator – the condition defining the strong coupling regime. Strong coupling is demonstrated in measurements of the vacuum Rabi mode splitting showing a coupling strength of 155 MHz, which is the highest coupling strength reported in comparable systems.The qubit linewidth of 40 MHz is independently extracted in spectroscopy measurements. Achieving strong coupling to microwave cavities poses a crucial step towards semiconductor-based quantum information architectures as it enables e.g. time-resolved measurements, quantum non-demolition readout or the coupling of distant qubits or different types of qubits via the resonator.