Semiconductor qubits implemented on semiconductor platforms are important building blocks for future quantum communication and quantum computation systems, which promise unprecedented communication security and computation efficiency for certain tasks. One important enabler for such systems is the interconnection of distant semiconductor quantum bits (qubits) by flying qubits, quantum information encoded in photons, in an efficient and coherent way. The goal of this dissertation is the design and fabrication of an efficient optical interface between a single mode fiber and a gate-defined double quantum dot (GDQD) hosting a singlet-triplet spin qubit on a 220 nm GaAs/AlGaAs heterostructure membrane. This interface should enhance the extraction efficiency of the photons, to which the quantum information is transferred from the spin qubit and that are emitted by an optically active quantum dot (OAQD) defined by the same gate system, by means of optical nanostructures and coupling them into a free-space Gaussian mode that can be straightforwardly collected by a single mode fiber. In this dissertation, we successfully conceived and modeled a novel optical interface assisted by a photonic crystal cavity that provides substantial enhancement of the optical coupling efficiency. The photonic crystal cavity provides in-plane optical confinement, efficient out-coupling to an ideal free-space Gaussian beam, while accommodating the gate wiring and the electrical connectivity required by the GDQD and OAQD. To further increase the extraction efficiency, a bottom gold reflector is placed under the membrane to recycle the photons and further improve the beam shaping via coherent interaction. A noteworthy feature of this design is that all the essential components can be lithographically defined and deterministically fabricated on the GaAs/AlGaAs heterostructure membrane, which greatly increases the scalability of on-chip integration. According to our modeling results, the interface provides an overall coupling efficiency of 28.7% into a free-space Gaussian beam, assuming an SiO₂ interlayer fills the space between the reflector and the membrane, which is already an order of magnitude more than for the unpatterned membrane. The performance can be further increased by removing this SiO₂ interlayer below the photonic crystal. In this case, the overall efficiency is modeled to be 48.5%. Experimental characterization of the photonic crystal cavity was implemented by room temperature cross-polarization reflection measurements and millikelvin photoluminescence measurements. In reflection measurements, the photonic crystal cavities were illuminated by a tunable laser, and cross-polarized reflections were investigated for both intensity and beam profile. From the recorded far-field reflection patterns and the power spectra, we confirmed the desired Gaussian emission patterns. In photoluminescence experiments at millikelvin temperatures, the photonic crystal cavities were excited by a tunable laser above the bandgap of GaAs under applied gate voltages. From preliminary results, we reproduced the expected Stark-shifted emission spectra of the heterostructure membrane and observed promising emission peaks that might be the desired cavity enhancement.
Kui Wu (Thu,) studied this question.