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.

Quantum dots (QDs) are semiconductor nanostructures in which a three-dimensional potential trap produces an electronic quantum confinement, thus mimicking the behavior of single atomic dipole-like transitions. However, unlike atoms, QDs can be incorporated into solid-state photonic devices such as cavities or waveguides that enhance the light-matter interaction. A near unit efficiency light-matter interaction is essential for deterministic, scalable quantum-information (QI) devices. In this limit, a single photon input into the device will undergo a large rotation of the polarization of the light field due to the strong interaction with the QD. In this paper we measure a macroscopic (∼6• ) phase shift of light as a result of the interaction with a negatively charged QD coupled to a low-quality-factor (Q ∼ 290) pillar microcavity. This unexpectedly large rotation angle demonstrates that this simple low-Q-factor design would enable near-deterministic light-matter interactions. DOI: 10.1103/PhysRevB.93.241409 The deterministic, lossless exchange of energy between charged QDs and single photons has been shown as the potential building block for a full range of components required for QI and quantum communication [1][2][3]. A deterministic light-matter interaction would give one the ability to both switch the photon state with a high fidelity as well as keep photon loss near zero (high efficiency). To achieve these simultaneously, it is essential that all the photon energy that couples to and from the quantum emitter must do so almost exclusively within a well-defined electromagnetic mode, where one can input/collect single photons. Input/output coupling efficiency is parametrized by the β factor, the ratio between the rate of coupling of the dipole to this well-defined mode compared to the total coupling rate of the dipole to all available electromagnetic modes, including leaky ones.Great success has been had in approaching this limit in several systems, including photonic crystal (PC) waveguides [4] and photonic nanowires [5]. For optical cavities, however, this limit has proven difficult to approach. Light-matter interaction strengths in the "strong coupling" regime have been achieved for high-Q-factor pillar microcavities [6] and in photonic crystal cavities [7], and could show high fidelity switching. However, the input/output mode is usually not well defined in high-Q-factor cavities: the escape rate to and from a well-defined input channel is similar to the escape rate to leaky cavity modes (CMs). These leaky modes arise either from the intrinsic design of the structure or from fabrication imperfections, putting a limit on the efficiency of high-Qfactor microcavities where the escape rate into the input/output mode is slow by design. However, in a low-Q-factor pillar the cavity lifetime is very short. Thus one may easily design a high-β-factor structure with a well-defined input/output mode, a crucial advantage [8].The β factor is directly linked to the competition between the rates of coherent and incohere...

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Solid-state quantum emitters have long been recognised as the ideal platform to realize integrated quantum photonic technologies. We demonstrate that a self-assembled negatively charged quantum dot (QD) in a low Q-factor photonic micropillar is a suitable design for deterministic polarisation switching and spin-photon entanglement. We show this by measuring a shift in phase of an input single photon of at least 2π/3. As we explain in the text, this is strong experimental proof

We investigate the effect of nuclear spins on the phase shift and polarisation rotation of photons scattered off a quantum dot-cavity system. We show that as the phase shift depends strongly on the resonance energy of an electronic transition in the quantum dot, it can provide a sensitive probe of the quantum state of nuclear spins that broaden this transition energy. By including the electron-nuclear spin coupling at a Hamiltonian level within an extended input-output formalism, we show how a photon scattering event acts as a nuclear spin measurement, which when rapidly applied leads to an inhibition of the nuclear dynamics via the quantum Zeno effect, and a corresponding stabilisation of the optical resonance. We show how such an effect manifests in the intensity autocorrelation g (2) (τ ) of scattered photons, whose long-time bunching behaviour changes from quadratic decay for low photon scattering rates (weak laser intensities), to ever slower exponential decay for increasing laser intensities as optical measurements impede the nuclear spin evolution.

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