Low-dimensional solid-state single-photon emitters

Abstract

Solid-state single-photon emitters (SPEs) are attracting significant attention as fundamental components in quantum computing, communication, and sensing. Low-dimensional materials-based SPEs (LD-SPEs) have drawn particular interest due to their high photon extraction efficiency, ease of integration with photonic circuits, and strong coupling with external fields. The accessible surfaces of LD materials allow for deterministic control over quantum light emission, while enhanced quantum confinement and light-matter interactions improve photon emissive properties. This perspective examines recent progress in LD-SPEs across four key materials: zero-dimensional (0D) semiconductor quantum dots, one-dimensional (1D) nanotubes, two-dimensional (2D) materials, including hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDCs). We explore their structural and photophysical properties, along with techniques such as spectral tuning and cavity coupling, which enhance SPE performance. Finally, we address future challenges and suggest strategies for optimizing LD-SPEs for practical quantum applications.

Keywords: hexagonal boron nitride; low-dimensional materials; quantum dots; single photon sources; single-walled carbon nanotubes; transition metal dichalcogenides.

Figure 1:

Wavelength centric overview of low-dimensional materials based SPEs. (a) Electromagnetic spectrum showing spectral ranges and applications for SPEs across ultraviolet, visible, near-infrared, and telecommunication wavelengths, from left to right. The top portion highlights applications, while the bottom shows schematic illustrations of SPE materials: quantum dots, nanotubes, hBN, and TMDCs. Colored polygons indicate the spectral ranges covered by each material. (b)–(e) Different mechanisms for generating single photons. (b) Spontaneous decay of excited states, where an excitation laser promotes an electron to the excited state, and single photons are emitted during relaxation. (c) Spontaneous decay of localized excitons, where the laser creates excitons, which recombine to emit single photons. (d) Ambipolar emission in electroluminescent devices, where electron–hole recombination between the source and drain generates photons. (e) Unipolar emission mechanism via impact excitation in electroluminescent devices, where high-energy carriers excite electrons to emit photons during relaxation.

Figure 2:

Different excitation schemes in 2D SPEs. (a) Schematic of the optical excitation mechanism in WSe₂-based SPEs. An excitation laser excites electrons from the ground state to the excited state, forming excitons in the WSe₂ layer positioned on a nanopillar. The excitons drift, and the electron–hole recombination results in the emission of single photons via spontaneous emission. (b) A vertical electrical excitation device comprised of WSe₂ sandwiched between few-layer boron nitride and metal contacts. The device structure includes graphene as a conductive layer, with electrical excitation applied through metal contacts to generate excitons within the WSe₂ layer. (c) Resonant excitation. The excitation laser and emitted single photons are spectrally filtered based on their polarization. (d) Resonant excitation, spectrally filtering is applied to separate the propagation direction of the excitation laser and the emitted photons. (e) Nonresonant excitation. The excitation laser (ω′) and emitted single photons (ω) have different wavelengths. Spectral filtering is applied to isolate the single photons from the excitation laser based on their distinct wavelengths. (f) A kind of quasi-resonant excitation. Two near-resonant excitation laser pulses (ω ₊ and ω ₋) are used to predictably manipulate the emitter into its excited state, leading to the emission of single photons (ω). Adapted with permission from: (b), ref. [81] Copyright 2016, American Chemical Society.

Figure 3:

Criteria of SPEs, brightness, purity, and indistinguishability. (a) Experiment set up used to measure brightness B of an SPE under pulsed excitation conditions. The emitted photons are collected through a single-mode fiber and detected by a detector (D), with the brightness measured at different points (B ₁, B ₂, B ₃) depending on the collection efficiency. The logic count system is used to record photon detection events. (b) Brightness measurement results showing the integrated PL intensity of an SPE at two excitation powers, 600 μW (blue) and 1,200 μW (red), with emission peaks around 515 nm. The inset displays the pump-power-dependent PL intensity of a hBN SPE before and after coupled to a metallo-dielectric antenna. (c) HBT experiment setup used to measure purity of an SPE under pulsed excitation conditions. Emitted photons from the source are split by a beam splitter and directed to two detectors, D ₁ and D ₂, with the photon coincidences recorded by the logic count system. (d) Single-photon purity measured under continuous wave excitation. The data (points) and the fit (blue line) yield $g^{\left(2\right)}\left(0\right)=$ 0.33 ± 0.02. The inset shows the single-photon purity measured under pulsed excitation, demonstrating clear photon antibunching behavior with well-separated peaks in the coincidence counts. (e) HOM experiment setup used to measure indistinguishability of an SPE under pulsed excitation conditions. Two consecutive photons, separated by a time delay τ, are sent through a beam splitter, and their interference is measured at detectors D ₁ and D ₂. (f) Indistinguishability measurement results using the HOM effect, showing the $g_{\text{HOM}}^{(2)}\left(0\right)$ as a function of time delay for two interferometer settings: 0 ps (dark green) and 5 ps (orange). The reduction in photon correlations at zero-time delay demonstrates two-photon interference. Reproduced with permission from: (a), (c), (e), ref. [11], John Wiley and Sons; (f), ref. [103], Springer Nature.

Figure 4:

Single-photon emitters based on quantum dots. (a) The PL decay of a single PQD. The emission exhibits an initial fast decay (∼210–280 ps), followed by a slower mono-exponential decay. (b) Single-photon purity of GQD SPEs under nonresonant excitation at room temperature, yielding g ⁽²⁾(0) equals to 0.05 ± 0.05. (c) Summary plot showing emission wavelengths and operational temperatures of various EQD SPEs. (d) PL spectra of EQD SPEs measured at different temperatures: 3.9 K (left), 150 K (middle), and 300 K (right). At 3.9 K, the inset shows a power dependence plot with a fitted slope. At 150 K, the spectrum displays an acoustic phonon sideband. At 300 K, the PL peak exhibits a shift of 87 meV. (e) Indistinguishability M of InGaAs SPEs as a function of excitation power, measured at 4.2 K. The indistinguishability reaches a maximum value of 0.9956. Error bars are based on Poissonian statistics from detected events. (f) The purity of an InAs/InP QD in an optical horn at 8 K under quasi-resonant excitation with a $g^{\left(2\right)}\left(0\right)$ value of 4.4 × 10⁻⁴ and a background correction value of 2.2 × 10⁻⁴. Adapted with permission from: (a), ref. [112], The American Association for the Advancement of Science; (b), ref. [33], Springer Nature; (c), ref. [17], AIP Publishing; (d), ref. [16], Copyright 2014, American Chemical Society; (e), ref. [147], Springer Nature; (f), ref. [148], AIP Publishing.

Figure 5:

Single-photon emitters based on SWCNTs. (a) PL spectrum of a single SWCNT at room temperature (black line) and at 10 K (blue line) [176]. Inset: a polymer wrapped nanotube leading to reduced spectral diffusion and blinking [177] (left); PL polarization diagram (right). (b) Purity of electrically excited SWCNT SPEs at 1.6 K, yielding g ⁽²⁾(0) equals to 0.49 at 0.08 μW excitation power. (c) PL spectrum and purity for SPEs in (10, 3) SWCNTs functionalized with OCH₃-Dz. The PL spectrum shows an emission peak at 1.55 μm corresponding to the E ₁₁ ^∗ transition. The $g^{\left(2\right)}\left(0\right)$ equals to 0.01 measured at 220 K demonstrates high purity of the single-photon emission. (d) Schematic of nanotube defect (NTD) SPEs operating at room temperature and coupled to a tunable fiber cavity. The fiber cavity setup allows precise control of the cavity length (L _C) and enhances emission properties of the NTD SPEs. (e) The HOM second-order correlation function of an NTD SPE. HOM autocorrelation function of an NTD, measured in a copolarized interferometer configuration with interferometer delays of 0 ps (dark green) and 5 ps (orange). The zero-interferometer delay corresponds to a delay equal to the separation of one excitation pulse. The visibility is then measured to be v = 0.65 ± 0.24 at room temperature. Adapted with permission from: (a), ref. [176], Springer Nature; (b), ref. [178], Springer Nature; (c), ref. [74], Springer Nature; (d), (e), ref. [103], Springer Nature.

Figure 6:

Structure, defects, and performance of hBN SPEs. (a) Schematic of a hBN lattice structure highlighting various types of defects it can host. The lattice is composed of boron (B, yellow) and nitrogen (N, blue) atoms. Common defects include nitrogen vacancies (V_N), boron vacancies (V_B), oxygen substituting nitrogen or boron (O_N, O_B), carbon substituting nitrogen or boron (C_N, C_B), and complex vacancies with multiple atoms missing or substituted (e.g., V_B3H, V_B2O). (b) Schematic demonstrating the separation of excitation and emission light using a dichroic mirror (DM) during the optical characterization of hBN. The excitation (Exc.) light is directed onto the sample via an objective (Obj.), while the emitted (Emi.) light is reflected by the DM and collected for analysis. (c) Low-temperature spectra of the eight hBN SPEs, labeled 1 through 8. The ZPL for all emitters is reproducible, centered around 436 ± 0.7 nm. (d) Two photon interference between successively emitted photons from the same source with a delay of 12.5 ns, yielding a V _HOM of 0.56 ± 0.11. (e) PL intensity versus excitation power for hBN SPEs, comparing uncoupled (blue) and coupled (red) configurations using a metallo-dielectric antenna setup. The coupled system achieves a near-unity photon collection efficiency of 98 %, compared to 13 % for the uncoupled case. The inset shows the emitter intensity over time, demonstrating excellent temporal stability without blinking. (f) High purity hBN SPEs with $g^{\left(2\right)}\left(0\right)$ equals to 0.0064 under pulsed excitation. Adapted with permission from: (a), ref. [46], AIP Publishing; (b), ref. [67], Copyright 2016, American Chemical Society; (c), ref. [185], Springer Nature; (d), ref. [186], American Physical Society; (e), ref. [93], Copyright 2019, American Chemical Society; (f), ref. [107], American Physical Society.

Figure 7:

Quantum spin sensing based on hBN defects. (a) Wide-field imaging of magnetization of an exfoliated Fe₃GeTe₂ flake by V_B ⁻ spin defects in hBN. (b) Dependence of ODMR frequencies on the magnetic field. Experimental data (red) and fit (blue line) with parameters D/h = 3.48 GHz, E/h = 50 MHz and g = 2.000. (c) Simplified V_B ⁻ energy-level diagram and the transitions among the ground state (³A₂′), the excited state (³E′), and the metastable state (¹E′, ¹E′′). (d) $g^{\left(2\right)}\left(\tau \right)$ of carbon-related defects in hBN. Inset indicates the fitted $g^{\left(2\right)}\left(0\right)$ = 0.25 ± 0.02. (e) The ODMR spectrum of a single defect in hBN measured in the absence of magnetic field. The top-right inset shows a confocal image of the PL intensity of the hBN device under 532 nm laser illumination. The bottom-right inset shows the pulse sequences used in the measurement. (f) A schematic illustration of quantum microscopy with spin defects in hBN. The setup includes a quantum active hBN flake and a sample to be imaged. The laser is used for excitation, and PL is collected for imaging. The microwave (MW) input enables control of the spin defects in the hBN for quantum sensing applications. Adapted with permission from: (a), ref. [211], Springer Nature; (b), ref. [207], Springer Nature; (c), ref. [208], Copyright 2021, American Chemical Society; (d), ref. [213], Springer Nature; (e), ref. [214], Springer Nature; (f), ref. [215], Springer Nature.

Figure 8:

Structure, strain, and performance of layered TMDC SPEs. (a) Atomic structures of two crystallographic phases of TMDCs. The 2H phase (top) features a trigonal prismatic coordination of metal atoms (M) and chalcogen atoms (X), with an A-B-A stacking sequence. The 1T phase (bottom) is characterized by octahedral coordination and a C-B-A stacking sequence. The side and top views highlight the differences in atomic arrangements between the two phases. (b) Illustration of the WSe₂ monolayer transferred over gold nanorods. The strain induced by folds and wrinkles formed during the transfer process, particularly over the gaps between nanorods, leads to the localization of SPEs. (c) PL spectra from MoSe₂/WSe₂ heterobilayers with twist angles of 2° (green) and 20° (blue; intensity scaled by 130×). The twist angle significantly impacts the PL characteristics, with the 2° sample showing a strong peak near 1.3 eV and the 20° sample exhibiting multiple peaks around 1.6 eV. (d) PL intensity of WSe₂ SPEs grown via flux and CVT methods, both before and after coupling to an optical cavity. Flux-grown SPEs show a quantum yield of up to 65.2 % after cavity coupling (red), compared to 16.5 % without coupling (blue). CVT-grown SPEs achieve a quantum yield of 12.6 % with coupling (green) and 1.5 % without coupling (black). (e) Second-order autocorrelation function $g^{\left(2\right)}\left(\tau \right)$ for SPEs in a WSe₂ monolayer under pulsed quasi-resonant excitation. The blue data points represent experimental measurements, while the red curve is a fit. The pronounced antibunching at zero delay time, with $g^{\left(2\right)}\left(0\right)$ equals to 0.036 ± 0.004. (f) HOM interference visibility V _HOM as a function of the temporal postselection window size for SPEs in a WSe₂ monolayer coupled to a tunable open optical cavity. The blue line represents the measured visibility, with error bounds in gray. Visibility decreases with increasing integration window size, yielding V _HOM equals to 0.02. Adapted with permission from: (a), ref. [227], Licensee MDPI, Basel, Switzerland; (b), ref. [228], John Wiley and Sons; (c), ref. [229], Springer Nature; (d), ref. [230], Springer Nature; €, ref. [231], IOP Publishing; (f), ref. [232], American Chemical Society.

Figure 9:

Spectral tuning of SPEs. (a) Schematic representation of a 2D hBN flake under strain. The strain components ε ₁₁ and ε ₂₂ are applied along two principal axes of the lattice, demonstrating the uniaxial or biaxial strain induced in the material. The crystallographic orientation is indicated by the axes a ₁, a ₂, and a ₃. (b) PL spectra of SPEs in the hBN flake under varying pressures, from 0.58 GPa to 3.20 GPa. The PL peak shows a redshift of ∼5 nm as the applied pressure increases, indicating strain-dependent spectral tuning. (c) Schematic representation of the experimental setup, featuring a WSe₂ monolayer placed on a 200 µm piezoelectric substrate. Gold (Au) electrodes are used for electrical contact, and an external voltage is applied across the piezoelectric device to induce strain in the WSe₂ layer, enabling spectral tuning. (d) PL spectra of the SPEs in WSe₂ monolayer under different applied electric fields: +20 kV/cm (blue), 0 kV/cm (black), and −20 kV/cm (red). The shift in the PL peak energy with varying electric field demonstrates field-dependent control of emission properties. (e) Cross-sectional schematic of a heterostructure device comprising TMDC layers encapsulated by h-BN. The applied gate voltages (V _HS, V _TG, and V _BG) generate an electric field across the TMDC heterostructure, enabling electrical tuning of interlayer excitons (IX) via the DC Stark effect. A laser excites the system, creating interlayer excitons, which are influenced by both the electric field and potential strain (indicated by F _P ) applied to the device. (f) Illustration of the chemomechanical modification process for SPEs in monolayer WSe₂ using aryl diazonium chemistry. Treatment with 4-NBD results in the physisorption of a nitrophenyl (NPh) oligomer layer, consisting of 2-ring and 3-ring structures, onto the WSe₂ surface. This functionalization suppresses strain-induced defect emissions, enabling the formation of spectrally isolated SPEs. Nitrogen gas (N₂) is released as a by-product of the reaction. Adapted with permission from: (a), (b), ref. [261], Copyright 2018, American Chemical Society; (c), (d), ref. [262], Copyright 2019, American Chemical Society; (e), ref. [263], Springer Nature; (f), ref. [264], Springer Nature.

Figure 10:

SPEs couple with cavities. (a) Schematic of a two-level emitter coupled to an optical cavity mode. The emitter experiences dissipation (γ) and interacts with the cavity mode via coupling strength g. Photons escape the cavity with decay rate κ, while γ∗ represents additional dissipation channels. (b) Lifetime measurements of 32 InAs/InP QDs from 20 different cavities, showing the impact of detuning on the QD lifetime. The shaded region represents the cavity effect, with the red dashed line indicating the lifetime of a bulk dot. (c) Schematic of a SWCNT coupled to a tunable microcavity. The cavity length is adjustable (indicated by the red arrows), allowing control over the optical coupling with the SPEs in the SWCNT. (d) Schematic of a hBN flake placed on a plasmonic nanoparticle array, covered by a poly (methyl methacrylate) (PMMA) film. (e) Fabrication process for integrating CVD-grown hBN with one-dimensional photonic crystal cavities. The steps include CVD growth, transfer, resist coating, electron beam lithography (EBL), and reactive ion etching (RIE) with undercutting. (f) Schematic of the experimental setup used to measure PL intensity and spontaneous emission rate from strain-induced SPEs in a WSe₂ monolayer. Left: the emitters are positioned on gold pillars before the formation of the plasmonic nanocavity. Right: after the formation of the plasmonic nanocavity by flipping the material on a planar gold (Au) mirror, leading to enhanced emission properties. Adapted with permission from: (a), ref. [269], Copyright 2015 American Physical Society; (b), ref. [270], Copyright 2016 Optical Society of America; (c), ref. [271], Copyright 2017 American Chemical Society; (d), ref. [272], Copyright 2017, American Chemical Society; (e), ref. [221], Copyright 2020, American Chemical Society; (f), ref. [230], Springer Nature.