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Room-temperature solid state quantum emitters in the telecom range
Yu Zhou, Ziyu Wang, Abdullah Rasmita, Sejeong Kim, Amanuel Berhane, Zoltan Bodrog, Giorgio Adamo, Adam Gali, Igor Aharonovich, Wei-bo Gao
TL;DR
Telecom operation is preferred for quantum-network applications, but practical triggered sources face severe problems including blinking, bleaching, and low performance. This work reports photostable room-temperature SPEs embedded in a magnesium-doped GaN layer and operating at telecom wavelengths.
Problem
Telecom operation is preferred for quantum-network applications, while practical ultra-bright triggered SPEs remain lacking and existing emitters can suffer blinking, bleaching, and very low performance.
Method
The study examines SPEs in a 2 μm magnesium-doped GaN layer and considers optically active point defects near cubic inclusions in hexagonal material.
Results
The emitters are photostable, operate at room temperature in the telecom range, show linewidths from 3 nm to 50 nm, and follow a three-level model with slight long-timescale bunching.
Takeaways & Limitations
Photostable room-temperature telecom SPEs embedded in GaN are promising for on-chip quantum simulators and practical quantum communication.
Takeaways & Limitations
The luminescent impurity responsible for the emission remains unknown, and the available literature provides no evidence for a room-temperature telecom SPE.
Abstract
from arXiv · showhide
On demand single photon emitters (SPEs) play a key role across a broad range of quantum technologies, including quantum computation, quantum simulation, quantum metrology and quantum communications. In quantum networks and quantum key distribution protocols, where photons are employed as flying qubits, telecom wavelength operation is preferred due to the reduced fibre loss. However, despite the tremendous efforts to develop various triggered SPE platforms, a robust source of triggered SPEs operating at room temperature and the telecom wavelength is still missing. Here we report a triggered, optically stable, room temperature solid state SPE operating at telecom wavelengths. The emitters exhibit high photon purity (~ 5% multiphoton events) and a record-high brightness of ~ 1.5 MHz. The emission is attributed to localized defects in a gallium nitride (GaN) crystal. The high performance SPEs embedded in a technologically mature semiconductor are promising for on-chip quantum simulators and practical quantum communication technologies.
1 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences,
The supplied passage identifies affiliations at the University of Technology Sydney.
- The School of Mathematical and Physical Sciences is affiliated with the University of Technology Sydney in Ultimo, NSW.
- The Institute of Biomedical Materials and Devices belongs to the University of Technology Sydney’s Faculty of Science.
- The listed Australian location is Ultimo, NSW 2007.
6. Dept. of Atomic Physics, Budapest University of Technology and Economics, Budafokiút 8.,
The paper demonstrates room-temperature telecom single-photon emission from GaN defects, combining high purity, brightness, stability, and substrate-enhanced collection.
- The emitters are attributed to localized defects in GaN, although the luminescent impurity and absolute crystallographic structure remain unresolved.
- The emitters provide room-temperature telecom single-photon emission with g2(0) ~ 0.05 and brightness exceeding 10^6 counts/s.
- The GaN emitters span zero-phonon-line wavelengths from 1085 nm to 1340 nm, with room-temperature linewidths ranging from 3 nm to 50 nm.
- Pulsed excitation yields g2(0) = 0.14 ± 0.01, remaining below the 0.5 threshold for single-photon emission.
- At saturation, patterned-sapphire-substrate emitters reach 2.33×10^6 counts/s versus 1.13×10^6 counts/s for pristine GaN, an enhancement of ~2.
- The platform is presented as promising for on-chip quantum simulators and practical quantum communication because GaN nanofabrication is technologically mature.
Methods
The measurements combine confocal photoluminescence, photon counting, spectrum analysis, and correlation detection for GaN samples with distinct substrate structures. The setup also estimates extraction and overall quantum efficiencies from optical and detector losses.
- Sample preparation: The samples comprise a 2 μm Mg-doped GaN layer on 2 μm undoped GaN grown on planar sapphire, while figure 3 uses 6.5 μm GaN on patterned sapphire.The patterned substrate has cone structures with 2.5 μm width, 1.7 μm height, and 3 μm separation.
- Room-temperature measurements: Room-temperature measurements use a three-direction nanopositioner, fiber beam splitter, two SSPDs, and time-correlated photon counting.The correlation coincidence is recorded with a Picoharp PH300 card.
- Optical excitation and collection: Confocal excitation uses a 950 nm continuous-wave diode laser or a 1 ps Titanium:Sapphire laser operating at 80 MHz.The excitation beam is filtered, focused through an oil objective, and collected into a single-mode fiber serving as the confocal aperture.
- Detection: The collected signal is routed either to low-jitter superconducting single-photon detectors for counting or to a liquid-nitrogen-cooled InGaAs camera for spectrum analysis.The Hanbury-Brown and Twiss interferometer is part of the correlation-measurement arrangement.
- Cryogenic measurements: Cryogenic measurements use a closed-cycle cryostat with attocube steppers for rough scanning and galvo mirrors for fine scanning.This cryogenic configuration complements the room-temperature nanopositioner measurements.
- Efficiency estimation: A 12% overall quantum efficiency is estimated by combining extraction, fiber, objective, and detector efficiencies.The estimate uses an extraction efficiency of 0.13, fiber collection efficiency of 0.4, objective transmission of 0.3, and detector efficiency of 0.3.
Figures and Figure Legends
The figures characterize room-temperature GaN emitters through their spectra, photon correlations, stability, lifetime, and enhancement using patterned sapphire substrates.
- Spectral modeling: 1085 nm to 1340 nm spans the photoluminescence range of three emitters, with predicted and observed zero-phonon-line positions matching the modeled defect configurations.The model considers defects distributed around cubic inclusions and defects located at interfaces.
- Optical stability: No obvious blinking is observed over two minutes at excitation powers of 0.1, 0.8, and 1.5 mW.The stability measurement used 100 ms time resolution.
- Emitter dynamics: A fluorescence lifetime of 736 ± 4 ps is measured, while a three-level-system schematic is used to describe the emitter.The model includes a shelving state and transition-rate analysis.
- PSS enhancement: The far-field maximum intensity for the patterned-sapphire-substrate emitter is twice that of the pristine-GaN emitter.The patterns are compared for in-plane dipoles in pristine GaN and GaN grown on patterned sapphire.
- PSS enhancement: 2.33×106 counts/s from a patterned-sapphire-substrate emitter compared with 1.13×106 counts/s from pristine GaN shows enhanced saturated emission.The comparison uses saturation curves for emitters in the two substrate configurations.
1. Photoluminescence statistics
Room-temperature photoluminescence measurements summarize the zero-phonon-line positions and linewidths of infrared GaN emitters.
- Photoluminescence statistics: Zero-phonon-line positions and linewidths are summarized for 18 infrared emitters at room temperature.Photoluminescence spectra of ten emitters are shown separately, while the summary covers 18 emitters.
2. Second order correlation fitting procedure
The correlation-fitting procedure models the emitters with a three-level system containing a shelving state and extracts power-dependent transition parameters.
- 2. Second order correlation fitting procedure: A three-level system with a shelving state is required to explain the observed bunching in the second-order correlation curves.The model includes excitation, decay, and shelving-state transitions.
- 2. Second order correlation fitting procedure: The excitation rate k12 is proportional to excitation power, while other kxy values represent decay rates between energy levels.These rates define the three-level correlation model.
- 2. Second order correlation fitting procedure: The model expresses g2(τ) as a sum of two exponential terms determined by fitted correlation times and transition rates.The transition-rate parameters are obtained by fitting experimental values across excitation powers.
- 2. Second order correlation fitting procedure: The fitted excited-state lifetime is around 776 ± 39 ps, roughly matching the independently measured lifetime of 736 ± 4 ps.The agreement supports the three-level model used for the correlation data.
3. Model to compare SPEs with and without PSS
A numerical dipole model compares emission from pristine GaN with emission from GaN grown on a patterned sapphire substrate.
- 3. Model to compare SPEs with and without PSS: A single polarization component supports treating SPE1 as a dipole in the numerical model.The polarization is measured by rotating a half-waveplate before the collection polarizer.
- 3. Model to compare SPEs with and without PSS: The patterned-sapphire-substrate radiation pattern has a higher maximum intensity than the pristine-GaN pattern.The model compares far-field radiation patterns for the two structures.
- 3. Model to compare SPEs with and without PSS: The model treats the emitter as an electric dipole positioned within a geometry based on the patterned-sapphire-substrate structure.The dipole is placed 500 nm above the sapphire–GaN interface and between three substrate cones.
4. Comparison between SPEs in pristine GaN and SPEs in GaN grown on Patterned Sapphire Substrates (PSS)
SPEs in GaN grown on PSS show higher detected count rates than emitters in pristine GaN while retaining single-photon emission and similar lifetimes.
- More than 1×106 counts/s was reached by all five PSS emitters at 10.5 mW, exceeding the approximately 0.5 M cps observed for pristine GaN.The comparison uses saturation-curve measurements for emitters with similar spectra.
- Saturation curves were fitted with I(P)=I∞×P/(P+Ps), where P is excitation power, Ps is saturation power, and I∞ is the fitted intensity limit.
- All measured emitters in both pristine GaN and GaN on PSS had g2(0) below 0.5, confirming their single-emitter character.The PSS measurements are reported in Figure S7, while pristine-GaN measurements are reported in Figure S5.
- The lifetimes were similar between pristine GaN and PSS samples, whereas the PSS sample had higher average counts.Maximum counts were obtained from the fitted saturation curves and summarized with lifetimes in Table S1.
5. Modelling of a proposed defect structure
The model attributes the photoluminescence to exciton recombination involving a localized hole at a point defect and an electron shaped by nearby cubic GaN inclusions.
- Photoluminescence arises from exciton recombination with the hole tightly localized at the point defect and the electron loosely localized within the surrounding band potential.The electron distribution is influenced by the stacking sequence of cubic and hexagonal GaN bilayers.
- The exciton binding energy and photoluminescence wavelength depend on the point defect’s position relative to the cubic inclusion.The model includes changes in Coulomb energy, band-edge alignment, and electron-wavefunction shape.
- The proposed structure combines a point defect with three bilayers of cubic inclusion to reproduce the measured photoluminescence spectra.The model treats the point defect as setting the longest possible ZPL wavelength, while the cubic-inclusion width controls the ZPL wavelength interval.
- The model considers point defects distributed within and around the cubic inclusion, with material properties and a 2.9 MV/cm boundary field taken from prior references.