Strong Coupling Cavity QED with Gate-Defined Double Quantum Dots Enabled by a High Impedance Resonator

The strong coupling limit of cavity quantum electrodynamics (QED) implies the capability of a matterlike quantum system to coherently transform an individual excitation into a single photon within a resonant structure. This not only enables essential processes required for quantum information processing but also allows for fundamental studies of matter-light interaction. In this work, we demonstrate strong coupling between the charge degree of freedom in a gate-defined GaAs double quantum dot (DQD) and a frequency-tunable high impedance resonator realized using an array of superconducting quantum interference devices. In the resonant regime, we resolve the vacuum Rabi mode splitting of size 2g/2π=238 MHz at a resonator linewidth κ/2π=12 MHz and a DQD charge qubit decoherence rate of γ2/2π=40 MHz extracted independently from microwave spectroscopy in the dispersive regime. Our measurements indicate a viable path towards using circuit-based cavity QED for quantum information processing in semiconductor nanostructures.

All-Microwave Control and Dispersive Readout of Gate-Defined Quantum Dot Qubits in Circuit Quantum Electrodynamics

Pasquale Scarlino

Developing fast and accurate control and readout techniques is an important challenge in quantum information processing with semiconductor qubits. Here, we study the dynamics and the coherence properties of a GaAs/AlGaAs double quantum dot charge qubit strongly coupled to a frequency-tunable high-impedance resonator. We drive qubit transitions with synthesized microwave pulses and perform qubit readout through the state-dependent frequency shift imparted by the qubit on the dispersively coupled resonator. We perform Rabi oscillation, Ramsey fringe, energy relaxation, and Hahn-echo measurements and find significantly reduced decoherence rates down to γ2/2π∼3 MHz corresponding to coherence times of up to T2∼50 ns for charge states in gate-defined quantum dot qubits. We realize Rabi π pulses of width down to σ∼0.25 ns.

2019

Decoherence and excitation-transfer mechanisms in semiconductor quantum dots

Gaetano Parascandolo

The aim of this thesis is to supply a theoretical study of the interaction mechanisms between quantum dots beyond the simple picture of macroatoms. Coulomb interaction between excitons, exciton-phonon interaction as well as radiative interaction are, in particular, considered. These mechanisms can be exploited to coherently couple quantum dots, thus being the physical tool enabling quantum information processing using quantum-dot-based logic gates. On the other hand, the same mechanisms are responsible for the decoherence of the quantum-state that prevent the storing of the quantum information. Already if considered as simple two-level systems, quantum dots are subject to mutual interaction. Quantum dots in the excited state can be considered as dipoles, and are thus coupled with each other via the dipole-dipole electrostatic interaction. This results in excitation transfer between dots over distances of a few tens of nanometers. In Chapter 2 we show that taking into account the retarded nature of the electromagnetic field results in a correction to this effect, that become a leading contribution at large distances, effectively coupling quantum dots over distances of a few hundreds of nanometers. Strong exciton-phonon-coupling in quantum dots results in a very efficient decoherence mechanism. The strongly localized polarization in an excited quantum dot can induce virtual phonon emission and reabsorption processes which act as a phase-destroying mechanism. In quantum dot molecules the decay rate of the interband polarization is almost one order of magnitude larger than in the single quantum dot case, and depends on the interdot distance. The description of this coupling mechanism is possible only beyond the marcoatom picture. In Chapter 3 we develop a model that describes the phonon-mediated interaction between quantum dots in a dot molecule, explaining the strong distance dependence of the exciton dephasing rates in terms of a matching condition between the phonon wavelength and the interdot distance, which enhances the phonon-assisted scattering from bright to dark states. The heterodyne spectral interferometry is a novel implementation of transient nonlinear spectroscopy that enables to study the transient nonlinear polarization emitted from individual localized electronic transitions, in both intensity and phase. Two-dimensional spectra obtained by means of this technique display signals that can be associated to the coherent coupling between different resonances of the system under study. This technique is theoretically modeled for the first time in Chapter 4, where a very satisfactory description of the measured spectra is provided, showing that coherent coupling between different optical transitions of a quantum system result in off-diagonal peaks of the two-dimensional excitation spectrum. Furthermore, we show that in the low intensity regime, each spectral signal can be associated to a specific pair of coupled resonances as long as the level structure of the system under study is known.

EPFL2007

Decoherence and excitation-transfer mechanisms in semiconductor quantum dots

Gaetano Parascandolo

EPFL2007