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 that can be realised 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.

An in-plane spin-photon interface is essential for the integration of quantum dot spins with optical circuits. The optical dipole of a quantum dot lies in the plane and the spin is optically accessed via circularly polarized selection rules. Hence, a single waveguide, which can transport only one in-plane linear polarization component, cannot communicate the spin state between two points on a chip. To overcome this issue, we introduce a spin-photon interface based on two orthogonal waveguides, where the polarization emitted by a quantum dot is mapped to a path-encoded photon. We demonstrate operation by deducing the spin using the interference of in-plane photons. A second device directly maps right and left circular polarizations to antiparallel waveguides, surprising for a nonchiral structure but consistent with an off-center dot.

Abstract: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. This compares with V = 0.76(λ /n) 3 for an isolated cavity in the same lattice. 20. Three other modes exist between the + − 1 and − − 2 modes. Unfortunately, the close spacings and low quality factors of these other modes [18] make it impractical to identify their peaks unambiguosly in Fig. 3. It is, however, likely that the predicted 1.5 meV splitting of the + + 1 mode is responsible for the most prominent features; the predicted splittings of the other two modes are insufficient to explain the peak around 1.32 eV. 21. Note that the results for the FDTD simluations become inaccurate for the largest cavity separation, since the intensity above the center of the double cavity becomes very low. 22. E. Gallardo, L. J. Martínez, A. K. Nowak, H. P. van der Meulen, J. M. Calleja, C. Tejedor, I. Prieto, D. Granados, A. G. Taboada, J. M. García, and P. A. Postigo, "Emission polarization control in semiconductor quantum dots coupled to a photonic crystal microcavity," Opt. Express 18, 13301-13308 (2010).

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%.

We present a method to analyze the suitability of particular photonic cavity designs for information exchange between arbitrary superposition states of a quantum emitter and the near-field photonic cavity mode.As an illustrative example, we consider whether quantum dot emitters embedded in "L3" and "H1" photonic crystal cavities are able to transfer a spin superposition state to a confined photonic superposition state for use in quantum information transfer. Using an established dyadic Green's function (DGF) analysis, we describe methods to calculate coupling to arbitrary quantum emitter positions and orientations using the modified local density of states (LDOS) calculated using numerical finite-difference time-domain (FDTD) simulations. We find that while superposition states are not supported in L3 cavities, the double degeneracy of the H1 cavities supports superposition states of the two orthogonal modes that may be described as states on a Poincaré-like sphere. Methods are developed to comprehensively analyze the confined superposition state generated from an arbitrary emitter position and emitter dipole orientation.

We show the importance of polarization and phase engineering when designing quantum information devices. Using the example of a photonic-crystal waveguide we demonstrate, for the first time, designs for an integrated quantum dot spin-photon interface.OCIS codes: (270.5585) General; (050.5298) Integrated quantum photonic chips are a leading contender for future quantum technologies [1], which aim to use the entanglement and superposition properties of quantum physics to speed up the manipulation of data. Information may be stored and transmitted in photons, which make excellent flying qubits where quantum information can be transmitted along a channel. Photons suffer little from decoherence, and single qubit gates performed by changing photon phase, are straightforward. Less straightforward is the ability to create two qubit gates, where one photon is used to switch another's state. This is inherently difficult due to the fact that direct photon-photon interactions are extremely weak. In the medium-term, this may be overcome by using linear-optical gates in the so-called "KLM" scheme [2], where photons interfere non-classically, with post-selection of successful entanglement events. However, for a scalable quantum computer, deterministic two-qubit interactions are required, likely in a hybrid scheme with the photonic "flying" qubit interacting with a "static" matter qubit.One type of matter system which has potential to mediate photon-photon interactions is an artificial atom-like structure: a quantum dot (QD). QDs have been extensively investigated over the last decade, due to their large oscillator strength and strict atomic-like selection rules. Their solid-state nature means that it is relatively simple to enhance the light-matter interaction by incorporating them into microcavity structures, such as micropillars and photonic crystal (PhC) cavities, with much focus placed on achieving strong coupling in high quality factor, low volume cavities. Simultaneously, a sizeable research effort into using the electron spin state in quantum dots has shown much success. In particular, long spin coherence times (µs), optical initialization, coherent control and readout have all been demonstrated [3]. Thus the potential exists to use the QD spin as a static qubit, and the exciton transition to mediate the interaction by transfer of photon polarization state to QD spin state. Fig.1.(a) Layout of W1 PCWG with line of missing holes through the centre. Eigen modes of W1 waveguide for Ex (b) and Ey (c) with the phase between them (d).. If a charged QD is placed at a C-point (marked by cross) in a PCWG, we find that if the spin is |↑〉 the photons will be emitted in the forward direction along the waveguide (e) whereas if the spin is |↓〉 photons will propagate in the backwards direction (f).Deterministic photon-spin interactions are, however, only possible if we use the photonic structures described above. If future devices are to be part of a integrated quantum photonic chip then it appears that the leading technologies are ...

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