Hybrid Circuit QED with Spin Qubits

Abstract

Single photons and single electrons can be confined spatially in a solid-state device by using a millimeter-sized superconducting microwave resonator for the photons and nanoscale electrodes to form quantum dots for the electrons. The confined electrons realize a quantum mechanical two-level system (qubit), whose dipole interaction with single photons in the resonator is studied in the field of hybrid circuit quantum electrodynamics (hybrid circuit QED). These studies aim at investigating fundamental physics of quantum dots and light-matter interaction. They also work towards the realization of a scalable quantum dot based device for quantum information processing, where circuit QED is the main platform for state-of-the-art quantum information devices with superconducting qubits. So far, hybrid circuit QED studies were mainly focused on charge states in double quantum dots. In this thesis we explore hybrid circuit QED with a focus on spin states in gallium arsenide quantum dots, motivated by the potentially longer coherence time of qubits based on quantum dot spin states instead of charge states. These experiments are performed at millikelvin temperatures using an experimental setup that was in large parts designed in this work. We also advance the hybrid circuit QED device technology by developing a resonator that is magnetic field resilient and has a high characteristic impedance of the order of one kiloohm. This increases the qubit-photon coupling strength and allows for experiments in a magnetic field. In our first experiment we investigate spin states in a two-electron double quantum dot. There, the resonator acts as a spin-selective probe since it only couples with the spin-singlet states, which form a charge qubit, but is insensitive to the spin-triplet states. By probing the magnetic-field-dependent resonator transmission, we extract information about the singlet-triplet energy spectrum. In the presence of a double quantum dot voltage bias, we investigate a phenomenon called spin-blockade, which is based on a fundamental symmetry requirement for quantum states of electrons. While the qubit decoherence rate exceeds the qubit-photon coupling strength in the first experiment, the situation is reversed in our second study. There, we report strong coupling between single microwave photons and a three-electron spin-qubit, called resonant exchange (RX) qubit. We resolve the vacuum Rabi mode splitting, which is the experimental signature of strong coupling, with a coupling strength of 31MHz and a qubit decoherence rate of 20MHz. We tune both quantities electrostatically and obtain a minimal decoherence rate of 10MHz for 23MHz of coupling strength. The demonstration of strong spin-photon interaction is an important step towards long-range qubit-qubit interaction that involves spin qubits, which is realized in our third experiment. There, we implement a coherent link that controllably couples a RX qubit and a superconducting transmon qubit on the same device over a distance that is several orders of magnitude longer than the physical size of the spin qubit. We realize the link with a frequency-tunable high impedance resonator that is built of an array of superconducting quantum interference devices. The resonator couples strongly to both qubits, since the coupling rates of 52MHz and 180MHz extracted from the vacuum Rabi mode splitting exceed the corresponding decoherence rates of 11MHz and 1MHz for the RX qubit and the transmon, respectively. We spectroscopically observe coherent qubit-qubit interaction in the resonant and dispersive regime, where the interaction is mediated by real or virtual resonator photons, respectively. For the latter coupling scheme, we resolve an exchange splitting of 32MHz.