Efficient deterministic giant photon phase shift from a single charged quantum dot

Abstract

Solid-state quantum emitters have long been recognised as the ideal platform to realize integrated quantum photonic technologies. We use a self-assembled negatively charged QD in a low Q-factor photonic micropillar to demonstrate for the first time a key figure of merit for deterministic switching and spin-photon entanglement: a shift in phase of an input single photon of > 90o with values of up to 2π/3 (120 o ) demonstrated. This > π/2 (90 o ) measured value represents an important threshold: above this value input photons interact with the emitter deterministically. A deterministic photon-emitter interaction is the only viable scalable means to achieve several vital functionalities not possible in linear optics such as quantum switches and entanglement gates. Our experimentally determined value is limited by mode mismatch between the input laser and the cavity, QD spectral fluctuations and spin relaxation. We determine that up to 80% of the collected photons have interacted with the QD and undergone a phase shift of π.The dramatic progress made in quantum dots (QD) has led to single photon sources with record efficiency and indistinguishability [1][2][3][4].However, QDs are not just exploited as sources; by maximising the interaction of the QD with light, one may use the QD transition to deterministically "switch" the phase, φ, of a single photon by up to π. Here we present the first solid-state implementation that achieves this key figure of merit. Using a low quality factor (Q-factor) micropillar in the "bad-cavity" limit we measure a phase shift of at least 2π/3 (120 • ). Accounting for background (20%), this corresponds to a π phase shift of all the photons reflected from the QD-cavity system. By combining this with the selection rules for QD spin transitions, one may unlock the ability to perform efficient, high fidelity quantum entanglement operations [5][6][7][8][9].There are two overarching requirements for designing a practical quantum photonic switch: firstly the passive photonic structure must possess well-defined and input and output modes facilitating efficient optical coupling. Secondly the quantum emitter should show perfect interaction (i.e. π phase shift for every interacting photon) and minimal photon scattering into leaky modes (γ) rather than the input/output mode (Γ), i.e. a high β-factor (β = Γ Γ+γ ). Such conditions have been satisfied using atom-cavity systems [10][11][12][13], but this has so far remained elusive for solid state quantum emitters, a more natural platform for integrable/scalable devices. In this manuscript we present a novel approach using a low Q-factor micropillar cavity often termed the "bad cavity" limit [14]. By doing this we ensure a well-defined, efficient input and output mode [15], in contrast to more traditional approaches using high Qfactor cavities [16,17] where limits in current fabrication tolerances lead to scattering into parasitic modes, limiting the phase shift to the order of ∼ π/10 [18-20]. Recent demonstrations have exploited a similar small photon p...

Photon Phase Shift at the Few-Photon Level and Optical Switching by a Quantum Dot in a Microcavity

“…5b). We attribute the non-zero polarization contrast to a certain degree of ellipticity of the intended circularly-polarized "Target" pulse [37]. To account for this, we normalize the calculated ratio Ξ in the absence of the "Control" pulse to the above value.…”