Integrated quantum optics using Site-controlled Quantum Dots and molecules – I-SQUAD

Over a 4-year period, I-SQUAD will provide the photonic structure designs, optimised sample fabrication and experimental demonstration of scalable optically interconnected spin qubit gates. To this aim, I-SQUAD is organized in three work packages (WPs) : growth, device design and fabrication, and quantum optics experiments. This structure reflects the strong interactions already established between the partners, that will ensure rapid feedback between the tasks needed to effectively develop the advanced QDM structures required in this project.

GaAs/AlGaAs slab photonic crystal (PhC) structures containing a single layer of low density, randomly positioned InAs QD layers emitting ~ 920 nm were grown to provide material for optimisation of the PhC design. Both doped and undoped structures were grown. In addition randomly positioned InAs QDM layers were grown and dot coupling was investigated by field-dependent photoluminescence measurements. Using the modelling results as an input, we fabricated H1 and L3 PhC cavities. We also fabricated PhC samples where the PhC cavity is coupled to the fundamental mode of a GaAs waveguide, for micro-photoluminescence measurements where the excitation is applied from the side. For these waveguide PhC micro-cavity samples we employed an additional step of sawing, in order to bring the tip of the waveguide as close as possible to the edge of the sample, to further facilitate optical excitation from the side. We are currently at the early stages of the project, carrying out optical characterization of single self-assembled InAs QDs embedded in PhC cavities in order to optimize the cavity parameters (mode wavelength, quality factor, enhancement of spontaneous emission, extraction efficiency).
In parallel, we have simulated and are currently testing the laser pulse sequences needed to initialize, control and read-out a single spin state in a QD and QDM using resonant fluorescence. The system we are currently using to test this protocol is a charged GaAs QD in a waveguide. Finally, by performing photoluminescence excitation (PLE) spectroscopy combined to an unconventional two-photon absorption process between s- and p-shell exciton states in a single QD, we have identified all the spin configurations of the excited states that could be used as cycling transitions for spin readout as in the protocol we propose in the initial project.

I-SQUAD will provide a scalable, integrated QD system using site-controlled QD/QDMs embedded in air-bridge photonic crystal structures, thus allowing the realisation of an ordered array of hosts for stationary qubits (e.g. single spins), using flying qubits (photons) confined in waveguides, to generate the entanglement between the remote, stationary qubits required for quantum computation. The achievement of such a challenging task will constitute a milestone toward a fully integrated quantum technology platform. Mastering the combination of site controlled dot growth, PhC cavity design and fabrication and coherent optical control of single spins to demonstrate spin-spin entanglement in a QD-PhC system will be a major breakthrough in demonstrating the viability of an optically driven semiconductor-QD-based platform for multiple qubit operation. This will have a great impact on the quantum optics and quantum computation communities where the ability to scale up the number of qubit operations remains a major challenge.

Electrical control of optically pumped electron spin in a single GaAs/AlAs quantum dot fabricated by nanohole infilling
S Germanis, P Atkinson, R Hostein, S Suffit, F Margaillan, V Voliotis, B. Eble, Physical Review B 102 (3), 035406 (2020)
journals.aps.org/prb/abstract/10.1103/PhysRevB.102.035406

1. Photocreation of a dark electron-hole pair in a quantum dot
SY Shiau, B Eble, V Voliotis, M Combescot
Physical Review B 101 (16), 161405 (2019) journals.aps.org/prb/abstract/10.1103/PhysRevB.101.161405

2. Non-lorentzian LDOS in coupled photonic crystal cavities probed by near-and far-field emission
D Pellegrino, D Balestri, N Granchi, M Ciardi, F Intonti, F Pagliano, A Yu Silov, F W Otten, T Wu, K Vynck, P Lalanne, A Fiore, M Gurioli
Physical Review Letters 124, 123902 (2020)
journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.123902

3. Nano-Cavity QED with Tunable Nano-Tip Interaction
MA May, D Fialkow, T Wu, KD Park, H Leng, JA Kropp, T Gougousi, P Lalanne, M Pelton, M B Raschke
Advanced Quantum Technologies, 1900087 (2020)
onlinelibrary.wiley.com/doi/full/10.1002/qute.201900087

Entanglement between stationary quantum memories and photonic qubits is crucial for future quantum communication networks. Motivated by this potential application, a promising research direction investigating quantum coherent optical manipulation and measurement of individual isolated spin qubits has emerged.
The discrete density of states of semiconductor quantum dots and their easy integration into conventional semiconductor device structures make them ideal for such applications. Coupled quantum dots, where carriers can tunnel coherently between the dots, have even greater potential: as storage units for spin qubits with long coherence times or as quantum gates needed for quantum computation. Self-assembled quantum dots have already demonstrated to be very efficient quantum light sources but their random position presents an obstacle for their integration in quantum photonic circuits and networks.
We propose to develop a new fabrication process for an integrated quantum optics platform based on site-controlled dots integrated in photonic crystal cavities linked by planar one-dimensional waveguide structures and demonstrate on-chip entanglement from remote spin qubits using this platform.
- The first objective of the project is to establish a growth strategy for strained quantum dots molecules (QDM) with unprecedented precision over the dots' position, energy and tunnel coupling rate. This strategy is based on the use of nanohole-patterned substrates to exert control over the migration of adatoms on the growing surface, leading to controllable nanoscale variations in thickness and composition during growth. Ex-situ nanohole patterned substrates will be used to demonstrate a robust, fully scalable process to create arrays of QDM. This a fundamental step in the creation of a quantum network of optically connected coupled quantum dot quantum gates.
- Photon extraction remains the principal challenge in realizing an efficient spin-photon interface. Embedding the emitters in photonic structures to take advantage of cavity quantum electrodynamics effects in a deterministic way will be our second objective. The QDM energy levels will be controlled with locally applied electric fields, perfectly matched to the optical mode of a photonic crystal (PhC) cavity coupled with highly efficient light injector/extractors.
- The goal of the I-SQUAD project is to establish the experimental conditions necessary for the realization of spin-spin entanglement on chip. Two demonstrators will be realized: a full spin control gate (initialization, coherent control and readout) with a QDM hole spin embedded in a PhC cavity and a demonstration of spin-photon entanglement. Together with on-chip two-photon interference from two different QDMs, these demonstrators will represent building blocks for future local quantum networks based on interconnected small-scale quantum information processors.