Dynamics of excitonic complexes bound to isoelectronic centers - Toward the realization of optically addressable qubits

At a Glance

Metadata	Details
Publication Date	2016-09-01
Journal	PolyPublie (École Polytechnique de Montréal)
Authors	Philippe St-Jean
Analysis	Full AI Review Included

Technical Documentation & Analysis: Optically Addressable Qubits via Isoelectronic Centers (ICs)

Executive Summary

This study rigorously evaluates Isoelectronic Centers (ICs) in III-V and II-VI semiconductors (GaP, GaAs, ZnSe) as promising solid-state platforms for optically addressable qubits, demonstrating key advantages highly relevant to quantum networks:

Atomic Homogeneity & Scalability: ICs, as atomic-size defects, exhibit optical homogeneity comparable to Nitrogen-Vacancy (NV) centers in diamond, far superior to self-assembled Quantum Dots (QDs).
Strong Optical Coupling: N dyads in GaAs demonstrated fast radiative decay (0.2-0.7 ns), comparable to QDs and orders of magnitude faster than N ICs in GaP (100-500 ns).
Decoherence Challenge Identified: Exciton qubits suffer rapid spin randomization due to hyperfine interaction and LA phonon transfers, compromising coherence (T*₂ $\sim$ 115 ps in GaAs).
Hole-Spin Breakthrough: Demonstrated ultrafast (< 150 ps) and high-fidelity (F > 98.5 %) initialization of a hole-spin qubit bound to Te ICs in ZnSe.
Optimal Platform Identified: Hole-spins in nuclear spin-free hosts (like ZnSe or diamond) offer quenched hyperfine interaction, addressing the primary hurdle to long coherence times (T₂).

Technical Specifications

Parameter	Value	Unit	Context
Hole-Spin Initialization Fidelity	> 98.5	%	Te ICs in ZnSe (Tunneling scheme)
Hole-Spin Initialization Time	< 150	ps	Te ICs in ZnSe (Ultrafast initialization)
Trion Radiative Lifetime (ZnSe/Te Dyad Upper Bound)	< 50	ps	High efficiency allows picosecond initialization
N Exciton Radiative Lifetime (GaAs/N Dyad)	0.2 - 0.7	ns	Bright states (Comparable to epitaxial QDs)
N Exciton Radiative Lifetime (GaP/N Dyad)	100 - 500	ns	Limited coupling (Indirect band gap host)
Exciton Dephasing Time T*₂ (GaAs/N Dyad)	$\sim$ 115	ps	Primarily limited by Hyperfine interaction
Hole Escape Activation Energy (NN1, GaP)	44.2	meV	Non-radiative process limiting low-T operation
GaP:N Dyad Fine Structure Splitting (NN1)	2.1	meV	Indicates strong electron wave-function localization
ZnSe Strain-Induced HH-LH Splitting	12.6	meV	Shifts heavy-hole states below light-holes

Key Methodologies

The research utilizes highly precise material synthesis and advanced optical characterization combined with rigorous theoretical modeling:

Material Synthesis: Samples were grown using epitaxy (Molecular Beam Epitaxy, Metal-Organic Chemical Vapor Deposition) on GaAs or GaP substrates, incorporating ultra-thin $\delta$-doped layers of the isoelectronic impurity (N or Te).
Sample Density Control: Low impurity concentrations ($\sim 10^{11}$ cm^-2 sheet density for GaP:N, $2500$ µm^-2 for ZnSe:Te) were used to allow optical isolation and spatial resolution of single dyads via confocal microscopy.
Time-Resolved Photoluminescence (TRPL): Used to study exciton recombination dynamics across a temperature range (5 K to 60 K), analyzing decay times ($\tau_{\text{r}}$, $\tau_{\text{s}}$, $\tau_{\text{l}}$) and spontaneous emission rates ($\Gamma_{\text{rad}}$). Achieved temporal resolution was < 100 ps.
Polarization-Resolved Spectroscopy: Measured photoluminescence intensity as a function of linear and circular polarization, crucial for identifying the local symmetry (e.g., C_2v) and determining optical selection rules of excitonic states (excitons and trions).
Exciton Dynamics Modeling: Developed comprehensive balance of populations models involving eight excitonic states (heavy-hole and light-hole content) to quantify inter-level transfer mechanisms ($\gamma_{\text{hf}}$ via hyperfine interaction, $\Gamma_{\text{ap}}$ via LA phonons) and activation energies ($E_{\text{nr}}$) of non-radiative processes.
Hole-Spin Initialization Scheme: Demonstrated ultrafast initialization based on spin-preserving tunneling of a resonantly excited donor-bound exciton (D-X⁰) to a positive trion (X⁺) bound to the Te dyad.

6CCVD Solutions & Capabilities

The core challenge identified for developing scalable, long-coherence qubits using solid-state defects is the requirement for a host material that combines high crystal quality, low intrinsic nuclear spin, and amenability to nanophotonic integration. The superior performance observed in ZnSe (a II-VI semiconductor with low nuclear spin abundance) points directly toward the necessity of 6CCVD’s Single Crystal Diamond (SCD) as the ideal next-generation platform.

Research Requirement / Material Constraint	6CCVD Material Recommendation	Value Proposition for Quantum Engineering
Ultimate Spin Coherence (T₂): Need for nuclear spin-free hosts (Zn, Se, Te isotopes offer low nuclear spin).	Isotopically Purified SCD (99.999% C-12)	Diamond is the ultimate nuclear spin-free host, minimizing the dominant decoherence mechanism (hyperfine interaction) reported in both GaAs ICs and InAs QDs. Enables millisecond-scale coherence times for defects (NV, SiV) necessary for quantum repeaters.
High Optical Homogeneity / Scalability: Atomic defects provide sharp, reproducible lines.	Optical Grade SCD Wafers	Our MPCVD process produces defect-free bulk single crystal material, ensuring the atomic homogeneity and spectral stability required for generating identical photons across a quantum network.
Integration & High-Fidelity Optical Coupling: Requires interfacing with cavities/waveguides (ZnSe trion lifetime < 50 ps).	Ultra-Smooth Polishing (Ra < 1 nm for SCD)	Precision polishing is essential for creating low-loss optical interfaces (e.g., diamond photonics or solid immersion lenses) that leverage the Purcell effect to boost radiative emission rates and initialization speed well below 50 ps.
Advanced Qubit Architectures: Need precise control over defect orientation and thin-film strain (e.g., HH-LH mixing analysis).	Custom Diamond Dimensions and Thickness (SCD 0.1 µm - 500 µm)	We supply thin (sub-micron) films or thicker substrates up to 10 mm. Engineers can use precise layer thickness and crystallographic orientation to control strain, optimizing the energy level structure (Λ system) necessary for coherent spin control and single-shot readout.
Charge State Control / Electrical Addressing: BDD or controlled doping required for specific IC charge states.	Boron-Doped Diamond (BDD) Films	Can be tailored to manage the local Fermi level, stabilizing the charge state of implanted or inherent defects (like NV^- or the positive trions (X⁺) used in the ZnSe study), enabling electrical control interfaces.
Customization Potential: Study utilized specific dyad configurations requiring precise material modification.	Custom Metalization (Au, Pt, Ti, W, Cu)	6CCVD provides in-house metal contacts required for applying external electric fields, crucial for the exciton ionization initialization scheme mentioned in the analysis, or for localized field control in quantum structures.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

RÉSUMÉ: La réalisation de qubits pouvant être couplés efficacement à des photons optiques est nécessaire pour réaliser la transmission d’information quantique à longue distance, par exemple à l’intérieur d’un réseau quantique. Le principal obstacle empêchant la réalisation de ces qubits addressables optiquement vient de la grande difficulté de trouver une plateforme offrant à la fois une grande homogénéité et un fort couplage optique. Les centres isoélectroniques (CIs),qui sont des impuretés isovalentes à l’intérieur d’un matériau semi-conducteur, représentent une alternative fort intéressante aux systèmes de qubits adressables optiquement proposés dans la littérature, soit les boîtes quantiques auto-assemblées et les centres NV dans le diamant souffrant, respectivement, d’un fort élargissement inhomogène et d’un couplage optique moins grand que les CIs. En effet, la nature atomique des CIs leur assure une homogénéité comparable aux centres NV et leur capacité à lier des complexes excitoniques présentant de forts moments dipolaires permet d’obtenir un couplage avec les champs photoniques aussi fort que dans les boîtes quantiques. Le but du travail présenté dans cette thèse est d’évaluer le potentiel de différents complexes excitoniques liés à ces CIs pour fabriquer des qubits adressables optiquement. Cette thèse par articles est séparée en deux grandes sections. Dans la première section, correspondant aux articles 1 et 2, j’ai étudié les caractéristiques physiques de qubits excitoniques liés à des CIs d’azote dans le GaP (Article 1) et le GaAs (Article 2). Plus précisément, ces articles présentent une analyse approfondie combinant des mesures de photoluminescence résolue dans le temps et des modèles de balance de populations afin d’identifier et de quantifier les différents mécanismes impliqués dans la dynamique de recombinaison des excitons. Dans la seconde section, j’ai démontré l’initialisation d’un qubit de spin de trou lié à un centre isoélectronique de Te dans une matrice de ZnSe. Contrairement aux qubits excitoniques, la cohérence des qubits de spin n’est pas limitée par leur émission spontanée permettant ainsi d’atteindre des temps de cohérence beaucoup plus intéressants. Le premier article de cette thèse présente une étude de la dynamique de recombinaison des excitons liés à des CIs d’azote dans le GaP. Le principal avantage relié à l’étude de ce système vient du fait qu’une grande variété de centres sont accessibles : ils peuvent être formés de 1,2 ou 3 impuretés d’azote et présenter différentes symétries à l’intérieur du matériau hôte. ABSTRACT: The realization of qubits that can be efficiently coupled to optical fields is necessary for long distance transmission of quantum information, e.g. inside quantum networks. The principal hurdle preventing the realization of such optically addressable qubits arises from the challenging task of finding a platform that offers as well high optical homogeneity and strong light-matter coupling. In regard to this challenge, isoelectronic centers (ICs), which are isovalent impurities in a semiconductor host, represent a very promising alternative to the well-studied epitaxial quantum dots and NV centers in diamond which suffer, respectively,from a large inhomogeneous broadening and a less effective coupling to optical fields than ICs. Indeed, the atomic nature of ICs insures an optical homogeneity comparable to NV centers, and their ability to bind excitonic complexes with strong electric dipole moments allows them to offer an optical coupling similar to quantum dots. The aim of the work presented in this thesis is to evaluate the potential of different excitonic complexes bound to these ICs for building optically addressable qubits. This thesis by articles, is separated in two parts. In the first part, corresponding to Article 1 and 2, I study the physics of exciton qubits bound to N ICs in GaP (Article 1) and in GaAas (Article 2). More precisely, these articles present an analysis combining time-resolve PL measurements and balance of population models, allowing to identify and quantify the different mechanisms involved in the exciton recombination dynamics. In the second part, I demonstrate the initialization of a hole-spin qubit bound to a Te IC in ZnSe. Contrary to exciton qubits the coherence time of spin qubit is not limited by their spontaneous emission, allowing to preserve coherence on a much more significant timescale. The first article of this thesis presents a study of the recombination dynamics of excitons bound to N ICs in GaP. The principal advantage of studying this system comes from the large variety of atomic configurations that can be accessed: ICs can be formed from one, two or three impurities, and exhibit different local symmetries inside the host lattice. Although it appears very challenging to realize optically addressable qubits in this system due to its low coupling to optical fields, it has allowed for a better understanding of how the atomic configuration of the underlying IC influences the different mechanisms involved in exciton recombination dynamics (capture, inter-level transfers, and radiative and non-radiative recombination).

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Original Source

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