Sebastian Kindel

Efficient interconnection between distant semiconductor spin qubits with the help of photonic qubits offers exciting new prospects for future quantum communication applications. In this paper, we optimize the extraction efficiency of a novel interface between a singlet-triplet-spin-qubit and a photonic-qubit. The interface is based on a 220-nm-thick $\mathrm{Ga}\mathrm{As}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructure membrane and consists of a gate-defined double quantum dot (GDQD) supporting a singlet-triplet qubit, an optically active quantum dot (OAQD) consisting of a gate-defined exciton trap, a photonic crystal cavity providing in-plane optical confinement, efficient outcoupling to an ideal free-space Gaussian beam while accommodating the gate wiring of the GDQD and OAQD, and a bottom gold reflector to recycle photons and increase the optical extraction efficiency. All the essential components can be lithographically defined and deterministically fabricated on the $\mathrm{Ga}\mathrm{As}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructure membrane, which greatly increases the scalability of on-chip integration. According to our simulations, the interface provides an overall coupling efficiency of 28.7% into a free-space Gaussian beam, assuming a ${\mathrm{Si}\mathrm{O}}_2$ interlayer fills the space between the reflector and the membrane. The performance can be further increased by undercutting this ${\mathrm{Si}\mathrm{O}}_2$ interlayer below the photonic crystal. In this case, the overall efficiency is calculated to be 48.5%.

Combining the capabilities of gate-defined quantum transport devices in $\mathrm{Ga}\mathrm{As}$-based heterostructures and of optically addressed self-assembled quantum dots could open up broad perspectives in quantum technologies. For example, interfacing stationary solid-state qubits with photonic quantum states would open up a pathway towards the realization of a quantum network with extended quantum processing capacity in each node. While gated devices allow very flexible confinement of electrons or holes, the confinement of excitons without some element of self-assembly is much harder. To address this limitation, we introduce a technique to realize exciton traps in quantum wells via local electric fields by thinning a heterostructure down to a 220-nm-thick membrane. We show that mobilities over $1\times {10}^6{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ can be retained and that quantum point contacts and Coulomb oscillations can be observed on this structure, which implies that the thinning does not compromise the heterostructure quality. Furthermore, the local lowering of the exciton energy via the quantum-confined Stark effect is confirmed, thus forming exciton traps. These results lay the technological foundations for devices like single-photon sources, spin-photon interfaces and eventually quantum network nodes in $\mathrm{Ga}\mathrm{As}$ quantum wells, realized entirely with a top-down fabrication process.