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Applied Physics Letters

Special topic on non-classical light emitters and single-photon detectors

Christoph Becher, Sven Höfling, Jin Liu, Peter Michler, Wolfram H. P. Pernice, Costanza Toninelli

Published Jan 3, 2022• 0 citations
Publisher

Abstract

The goal of this Special Topic is to provide new insights in the physics of single quantum emitters and single-photon detectors and their possible use for quantum photonic applications. The perspective article of Vural and others discusses the challenges for multi-source quantum implementations based on self-assembled InGaAs quantum dots. Decoherence and spectral diffusion processes in quantum dots, which limit the photon indistinguishability, are considered in their article. In their article, Anderson and others have investigated the noise sources that affect InAs/InP quantum dots emitting in the telecom C-band by comparing their behavior on a wetting layer for Stranski-Krastanov grown quantum dots with a nearly wetting layer-free environment achieved with the droplet epitaxy growth mode. They show that the droplet epitaxy growth mode is beneficial for a low-noise environment, which is a pre-condition for the generation of highly coherent photons.

DOI
10.1063/5.0078886

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The goal of this Special Topic is to provide new insights in the physics of single quantum emitters and single-photon detectors and their possible use for quantum photonic applications. The perspective article of Vural and others discusses the challenges for multi-source quantum implementations based on self-assembled InGaAs quantum dots. Decoherence and spectral diffusion processes in quantum dots, which limit the photon indistinguishability, are considered in their article. In their article, Anderson and others have investigated the noise sources that affect InAs/InP quantum dots emitting in the telecom C-band by comparing their behavior on a wetting layer for Stranski-Krastanov grown quantum dots with a nearly wetting layer-free environment achieved with the droplet epitaxy growth mode. They show that the droplet epitaxy growth mode is beneficial for a low-noise environment, which is a pre-condition for the generation of highly coherent photons.
The advent of quantum computation promises exciting advances, not only in fields like medicine and metrology, but many industries that rely on parameter-heavy calculations or simulation of molecular interaction. At the same time Shor's algorithm for quantum computers presents a threat to current asymmetric encryption protocols used in everyday communication. Flying qubits, i.e. single-photons, can help mitigate this problem via quantum key distribution, which is insusceptible to an increase in computational power. In addition, they can link quantum computers, forming a quantum network, so that quantum states can be transmitted between them. Sources of flying qubits need good performance in key metrics like single--photon purity, repetition rate, indistinguishability and brightness to become useful in these applications. They should ideally emit strongly entangled pairs of photons and be matched to other quantum technologies in bandwidth and emission energy. In this thesis the emission characteristics of single epitaxial quantum dots, the single-photon source of our choice, are investigated. Strongly entangled photon-pair emission is demonstrated for three different quantum-dot systems: InAsP quantum dots embedded in nanowire waveguides, suitable for integration into photonic circuits, show emission of single photons and entangled photon pairs under non-resonant and quasi-resonant excitation. Violation of Bell's inequality is demonstrated using the traditional set of polarization angles. GaAs quantum dots grown in droplet--etched nanoholes are tested with two resonant excitation methods: Using resonance fluorescence, near-unity indistinguishability and re-excitation limited single-photon purity, albeit not simultaneously with laser-inherited bandwidth, are measured. Using two-photon resonant excitation we set a new standard for single-photon purity, can generate pairs of entangled photons but suffer from reduced indistinguishability. In addition, nanofabrication of paraboloid shaped reflectors for enhanced extraction efficiency of photons and strain-tuning of the emission energy into resonance with the 87Rb D1-line are demonstrated. Strain-tunable InAs quantum dots emitting in the telecom C-band are investigated under above-band excitation and two different resonant two-photon excitation techniques, all of which cause pure single-photon emission. Using the robust phonon-assisted two-photon excitation technique, close-to ideal entangled photon-pair emission is demonstrated. For many of these findings photon arrival times were recorded over many hours with temporal precision on the order of 10 ps. We have developed a user-friendly, yet versatile piece of software in order to extract as much information as possible from this vast amount of data. These results will facilitate integration of quantum dot based single- and entangled-photon sources into future quantum networks and quantum key distribution systems.
The ability of two photons to interfere lies at the heart of many photonic quantum networking concepts and requires that the photons are indistinguishable with sufficient coherence times to resolve the interference signals. However, for solid-state quantum light sources, this can be challenging to achieve as they are in constant interaction with noise sources in their environment. Here, we investigate the noise sources that affect InAs/InP quantum dots emitting in the telecom C-band by comparing their behavior on a wetting layer for Stranski–Krastanov grown quantum dots with a nearly wetting layer-free environment achieved with the droplet epitaxy growth mode. We show that the droplet epitaxy growth mode is beneficial for a quiet environment, leading to 96% of exciton transitions having a coherence time longer than the typical detector resolution of 100 ps, even under non-resonant excitation. We also show that the decay profile indicates the presence of slow dephasing processes, which can be compensated for experimentally. We finally conduct Hong–Ou–Mandel interference measurements between subsequently emitted photons and find a corrected two-photon interference visibility of 98.6 ± 1.6% for droplet-epitaxy grown quantum dots. The understanding of the influence of their surroundings on the quantum optical properties of these emitters is important for their optimization and use in future quantum networking applications.
Nowadays, single-photon sources can be generated experimentally through a wide variety of platforms [1]. Semiconductor quantum dots [2] have demonstrated impressive performance in terms of photon purity and indistinguishability, particularly in Hong-Ou-Mandel experiments. Another widely used method involves nonlinear crystals, which can generate entangled photon pairs via nonlinear processes such as spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM). These are heralded single-photon sources, rather than deterministic ones, yet they remain essential tools in quantum optics [3]. Moreover, among the most commonly used single photon sources, there are NV centres in diamonds [4]. These single-photon sources emit quantum states of light, i.e., those that have no classical counterpart.
We investigate the spin properties of InAs/InGaAs/InP quantum dots grown by metalorganic vapor-phase epitaxy (MOVPE) deposition using droplet epitaxy, which emits in the telecom C band. Using pump-probe Faraday ellipticity measurements, we determine electron and hole g factors of | g e | = 0.93 and | g h | = 0.47 , with the electron g factor being nearly twice as low as typical molecular beam epitaxy Stranski-Krastanov (SK) grown samples. Most significantly, we measure a longitudinal spin relaxation time T 1 = 3.0 µ s , representing an order of magnitude improvement over comparable MBE SK grown samples. Despite significant electron g -factor anisotropy, we observed that it is reduced relative to similar material composition samples grown with MBE or MOVPE SK methods. We attribute this g -factor anisotropy and spin lifetime improvements to the enhanced structural symmetry achieved via MOVPE droplet epitaxy, which mitigates the inherent structural asymmetry in strain-driven growth approaches for InAs/InP quantum dots. These results demonstrate that MOVPE droplet epitaxy-grown InAs/InGaAs/InP quantum dots exhibit favorable spin properties for potential implementation in quantum information applications.