A. C. T. Thijssen

Polarization Engineering in Photonic Crystal Waveguides for Spin-Photon Entanglers.

By performing a full analysis of the projected local density of states (LDOS) in a photonic crystal waveguide, we show that phase plays a crucial role in the symmetry of the light-matter interaction. By considering a quantum dot (QD) spin coupled to a photonic crystal waveguide (PCW) mode, we demonstrate that the light-matter interaction can be asymmetric, leading to unidirectional emission and a deterministic entangled photon source. Further we show that understanding the phase associated with both the LDOS and the QD spin is essential for a range of devices that can be realized with a QD in a PCW. We also show how suppression of quantum interference prevents dipole induced reflection in the waveguide, and highlight a fundamental breakdown of the semiclassical dipole approximation for describing light-matter interactions in these spin dependent systems.

Interfacing Spins in an InGaAs Quantum Dot to a Semiconductor Waveguide Circuit Using Emitted Photons

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Mode structure of coupled L3 photonic crystal cavities

We investigate the energy splitting, quality factor and polarization of the fundamental modes of coupled L3 photonic crystal cavities. Four different geometries are evaluated theoretically, before experimentally investigating coupling in a direction at 30◦ to the line of the cavities. In this geometry, a smooth variation of the energy splitting with the cavity separation is predicted and observed, together with significant differences between the polarizations of the bonding and anti-bonding states. The controlled splitting of the coupled states is potentially useful for applications that require simultaneous resonant enhancement of two transitions.

Optical control of the emission direction of a quantum dot

Using the helicity of a non-resonant excitation laser, control over the emission direction of an InAs/GaAs quantum dot is demonstrated. The quantum dot is located off-center in a crossed-waveguide structure, such that photons of opposite circular polarization are emitted into opposite waveguide directions. By preferentially exciting spin-polarized excitons, the direction of emission can therefore be controlled. The directional control is quantified by using the ratio of the intensity of the light coupled into the two waveguides, which reaches a maximum of ±35%.

Putting the spin in photonic crystal waveguides(Conference Presentation)

By performing a full analysis of the projected local density of states (LDOS) in a photonic crystal waveguide, we show that phase plays a crucial role in the symmetry of the light-matter interaction. By considering a quantum dot (QD) spin coupled to a photonic crystal waveguide (PCW) mode, we demonstrate that the light-matter interaction can be asymmetric, leading to unidirectional emission and a deterministic entangled photon source. Further we show that understanding the phase associated with both the LDOS and the QD spin is essential for a range of devices that can be realized with a QD in a PCW. We also show how suppression of quantum interference prevents dipole induced reflection in the waveguide, and highlight a fundamental breakdown of the semiclassical dipole approximation for describing light-matter interactions in these spin dependent systems.

Artificial atoms for quantum information processing

Self-assembled quantum dots (QDs), nanosized semiconductors, are often known as artificial atoms due to their atomic-like spectra. For this reason they have long been proposed as a means to mediate interactions between single photons, a useful capability for photonic quantum information technology. I will describe the role of QDs in the latest developments in photonic quantum information technology (QIT), and highlight some of our progress in combining the atomic-like properties of QDs with photonic structures to perform a variety of functionalities.

An on-chip cross-waveguide QD spin-photon interface and its applications

Quantum dot (QD) systems containing electron spins may hold a role in a future photonic quantum circuit as a means of storing a quantum state in a spin superposition. In general, the spin superposition state maps directly to a photon emitted out of the plane (kz) with the photon in a polarization superposition. In waveguides, however, this is more difficult: one requires a waveguide mode that is able to transmit both TEx and TEy modes in coherent superposition. We have already demonstrated a cross-waveguide design, as shown in Fig 1(a), that converts spin superposition states to a path encoding when the QD is located close to the waveguide centre.

Planar waveguide architecture for the implementation of a network of optically controlled quantum dot spin qubits

We propose a device architecture for an in-plane network of optically connected quantum dots. At each node of the network, the dot resides at the intersection of two orthogonal waveguides which transmit the full polarization of an emitted photon to another node. A prototype device is presented.

Quantum Optics in Wavelength Scale Structures

We discuss interaction between light and matter in optical structures that are at the wavelength scale illustrating this with recent results from pillar microcavities containing single quantum dots.