R. Mark Stevenson

Improved fidelity of triggered entangled photons from single quantum dots

We demonstrate the triggered emission of polarization-entangled photon pairs from the biexciton cascade of a single InAs quantum dot embedded in a GaAs/AlAs planar microcavity. Improvements in the sample design blue shifts the wetting layer to reduce the contribution of background light in the measurements. Results presented show that >70% of the detected photon pairs are entangled. The high fidelity of the (|HXXHX� + |VXXVX� )/ √ 2 state that we determine is sufficient to satisfy numerous tests for entanglement. The improved quality of entanglement represents a significant step towards the realization of a practical quantum dot source compatible with applications in quantum information.

Evolution of entanglement between distinguishable light states.

We investigate the evolution of quantum correlations over the lifetime of a multiphoton state. Measurements reveal time-dependent oscillations of the entanglement fidelity for photon pairs created by a single semiconductor quantum dot. The oscillations are attributed to the phase acquired in the intermediate, nondegenerate, exciton-photon state and are consistent with simulations. We conclude that emission of photon pairs by a typical quantum dot with finite polarization splitting is in fact entangled in a time-evolving state, and not classically correlated as previously regarded.

Single electron-spin memory with a semiconductor quantum dot

We show storage of the circular polarization of an optical field, transferring it to the spin-state of an individual electron confined in a single semiconductor quantum dot. The state is subsequently read out through the electronically-triggered emission of a single photon. The emitted photon shares the same polarization as the initial pulse but has a different energy, making the transfer of quantum information between different physical systems possible. With an applied magnetic field of 2 T, spin memory is preserved for at least 1000 times more than the excitons radiative lifetime.

Strong directional dependence of single-quantum-dot fine structure

By isolating quantum dots in microstructures with cleaved facets, we measure individual-quantum-dot photoluminescence emitted in the in-plane direction, in addition to the widely studied vertical direction. The emission is shown to be polarized in the plane, and the observed fine structure is found to be extremely directionally-dependent. These characteristics are attributed to exciton states with orthogonally aligned dipoles in the plane. The result suggests possibilities for single-quantum-dot devices, including side-emitting single-photon sources.

Multi-dimensional photonic states from a quantum dot

© 2018 IOP Publishing Ltd. Quantum states superposed across multiple particles or degrees of freedom offer an advantage in the development of quantum technologies. Creating these states deterministically and with high efficiency is an ongoing challenge. A promising approach is the repeated excitation of multi-level quantum emitters, which have been shown to naturally generate light with quantum statistics. Here we describe how to create one class of higher dimensional quantum state, a so called W-state, which is superposed across multiple time bins. We do this by repeated Raman scattering of photons from a charged quantum dot in a pillar microcavity. We show this method can be scaled to larger dimensions with no reduction in coherence or single-photon character. We explain how to extend this work to enable the deterministic creation of arbitrary time-bin encoded qudits.

Universal Growth Scheme for Quantum Dots with Low Fine-Structure Splitting at Various Emission Wavelengths

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fibre optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure-splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into 1D optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realise QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ‘dropletepitaxy’ of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ~900nm, and metal-organic vapour phase epitaxy on InP emitting at ~1550 nm, with mean FSS 4x smaller than for StranskiKrastanow QDs.

An entangled-LED-driven quantum relay over 1 km

Each datafile corresponds to a figure dataset in the paper. For figures 2c, 3, 4a & 4b the two time axes are given as the top row (X-values) and first column (Y-values). Data format for these is as follows: Y values accross columns and X values accross rows. In figure 4a, undefined fidelity points are indicated by value -0.5. For figures 3a, b, c & d, the two columns are separated as: e.g. figure3a_exp (for experimental data) and figure3a_sim (for simulated data).

Entangled Photon Generation by Quantum Dots

This book reviews recent advances in the exciting and rapidly growing field of semiconductor quantum dots via contributions from some of the most prominent researchers in the scientific community. Special focus is given to optical, quantum optical, and spin properties of single quantum dots due to their potential applications in devices operating with single electron spins and/or single photons. This includes single and coupled quantum dots in external fields, cavity-quantum electrodynamics, and single and entangled photon pair generation. Single Semiconductor Quantum Dots also addresses growth techniques to allow for a positioned nucleation of dots as well as applications of quantum dots in quantum information technologies.

Growth scheme for quantum dots with low fine structure splitting at telecom wavelengths

Quantum dots based on InAs/InP hold the promise to deliver entangled photons with wavelength suitable for the conventional telecom window around 1550 nm [1]. This makes them predestined to be used in future quantum networks applications based on existing fiber optics infrastructure. A prerequisite for the efficient generation of such entangled photons is a small fine structure splitting (FSS) in the quantum dot excitonic eigenstates [2], as well as the ability to integrate the dot into photonic structures to enhance and direct its emission. Using optical spectroscopy, we show that a growth strategy based on droplet epitaxy can simultaneously address both issues.

Growth scheme for quantum dots with low fine structure splitting at telecom wavelengths (Conference Presentation)

Quantum dots based on InAs/InP hold the promise to deliver entangled photons with wavelength suitable for the standard telecom window around 1550 nm, which makes them predestined to be used in future quantum networks applications based on existing fiber optics infrastructure. A prerequisite for the generation of such entangled photons is a small fine structure splitting (FSS) in the quantum dot excitonic eigenstates, as well as the ability to integrate the dot into photonic structures to enhance and direct its emission. Using optical spectroscopy, we show that a growth strategy based on droplet epitaxy can simultaneously address both issues. Contrary to the standard Stranski-Krastanow technique, droplet epitaxy dots do not rely on material strains during growth, which results in a drastic improvement in dot symmetry. As a consequence, the average exciton FSS is reduced by more than a factor 4, which in fact makes all the difference between easily finding a dot with the required FSS and not finding one at all. Furthermore, we demonstrate that droplet epitaxy dots can be grown on the necessary surface (001) for high quality optical microcavities, which increases dot emission count rates by more than a factor of five. Together, these properties make droplet epitaxy quantum dots readily suitable for the generation of entangled photons at telecom wavelengths.