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arXiv:1609.02851 · ACS Photonics

Efficient Quantum Photonic Phase Shift in a Low Q-Factor Regime

Petros Androvitsaneas, A. B. Young, J. M. Lennon, C. Schneider, Stefan Alexander Maier, J.J. Hinchliff, George S. Atkinson, Edmund Harbord, Martin Kamp, Sven Höfling, John Rarity, Rupert F. Oulton

Published Jan 11, 2019• 30 citations

Abstract

Solid-state quantum emitters have long been recognized 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 polarization 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 that input photons can interact with the emitter deterministically. A deterministic photon-emitter interaction is a viable and scalable means to achieve several vital functionalities such as single photon 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. When on-resonance we estimate that up to ∼80% of the collected photons couple into the cavity mode and have interacted with the QD and undergone a phase shift of π.

DOI: 10.1021/acsphotonics.8b01380

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Quantum dots have long been considered systems that could deliver a significant advantage for practical quantum information applications. Their atomic dipolelike transitions and their compatibility to be integrated into a variety of photonic structures has highlighted this fact. Here, the authors measure an unexpectedly high single-photon level phase shift (~6${}^{\ensuremath{\circ}}$), induced upon reflection from a singly negatively charged quantum dot incorporated in a low-

Q

-factor cavity (

Q

~290). This measurement provides the first evidence for a system with unit fidelity and a high efficiency in its interactions. The result opens a new road to realistic spin-photon entanglement devices, in a simple and easy to fabricate photonic structure.

Quantum dots (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 devices [1]. 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 the past preliminary results have indicated that a low quality-factor (Q~290) pillar microcavity possesses a high ß-factor and that the instantaneous interactions should be deterministic and with high fidelity [2].

Realizing a sensitive photon-number-dependent phase shift on a light beam is required both in classical and quantum photonics. It may lead to new applications for classical and quantum photonics machine learning or pave the way for realizing photon-photon gate operations. Nonlinear phase-shifts require efficient light-matter interaction, and recently quantum dots coupled to nanophotonic devices have enabled near-deterministic single-photon coupling. We experimentally realize an optical phase shift of 0.19π ± 0.03 radians ( ≈ 34 degrees) using a weak coherent state interacting with a single quantum dot in a planar nanophotonic waveguide. The phase shift is probed by interferometric measurements of the light scattered from the quantum dot in the waveguide. The process is nonlinear in power, the saturation at the single-photon level and compatible with scalable photonic integrated circuitry. The work may open new prospects for realizing high-efficiency optical switching or be applied for proof-of-concept quantum machine learning or quantum simulation demonstrations.

Semiconductor quantum dots (QDs) are a type of solid-state quantum emitter that can act as a near-ideal quantum light-matter interface when integrated with high-quality nanophotonic systems. Though QDs have typically been used to create state-of-the-art, on-demand single photon sources, here we widen the perspective on QDs, showing how to design quantum photonic integrated circuits based on both linear and nonlinear QD phase shifters. Specifically, we find that linear QD phase shifters can be used to realize cryogenically-compatible, fast, low-loss, and high-fidelity reconfigurable linear circuits. When paired with QDs that mediate interactions between photonic qubits, generating nonlinear phase shifts, deterministic quantum photonic logic gates can be achieved. Thus, our work paves the way for the realization of on-chip, cryogenically-compatible linear and nonlinear quantum photonic circuits, including quantum photonic neural networks, which can form the foundation for scalable and efficient quantum photonic technologies.

The realization of quantum-photonic networks is a key challenge for optical quantum technologies. To scale such networks to many nodes and long distances, an efficient interface between stationary and flying qubits is necessary. One of the front-runners towards this goal is a spin-photon interface realised with a single charged QD with a localized electron spin that is strongly coupled to a micropillar cavity [1] . Through the strong spin-photon coupling, the polarization state of the optical field is controlled by the QD spin, thus modulating the cavity’s transmission properties. Controlling the polarization state over a wide angle is particularly useful for realizing high-fidelity polarization states and conditional phase quantum gates (e.g. CNOT) for photon-based quantum computing.