Optical quantum technologies with hexagonal boron nitride single photon sources

Single photon quantum emitters are important building blocks of optical quantum technologies. Hexagonal boron nitride (hBN), an atomically thin wide band gap two dimensional material, hosts robust, optically active luminescent point defects, which are known to reduce phonon lifetimes, promises as a stable single-photon source at room temperature. In this Review, we present the recent advances in hBN quantum light emission, comparisons with other 2D material based quantum sources and analyze the performance of hBN quantum emitters. We also discuss state-of-the-art stable single photon emitter’s fabrication in UV, visible and near IR regions, their activation, characterization techniques, photostability towards a wide range of operating temperatures and harsh environments, Density-functional theory predictions of possible hBN defect structures for single photon emission in UV to IR regions and applications of single photon sources in quantum communication and quantum photonic circuits with associated potential obstacles.

. Pictorial representation of absorption, excitation and spontaneous emission. (a, b) electron in ground state absorbing energy E = E 2 − E 1 and moved to excited state. (c) After few nanoseconds the electron decays back to ground state and releases the absorbed energy in the form of a photon. The spontaneous emission due to optical excitation is called photoluminescence and the spontaneous emission due to electrical excitation is called electroluminescence.
layer) as shown in Fig. 3a, b or AB stacking (B and N atoms of one layer are translated w.r.t another layer) configuration as shown in Fig. 3c, d.
These individual layers are packed together by weak van der Waals forces. The inter layer interactions affects the band structure of material and the variation in electronic band structure of monolayer and bulk hBN is shown in Fig. 3e, f. The monolayer hBN is a direct bandgap semiconductor material with 7.25 eV at high symmetry K point. As the number of layers increases, it becomes optically inactive (bulk) semiconductor with indirect bandgap of 5.79 eV with conduction band minimum at M and valence band maximum at K points.
The variation in bandgap of hBN and their corresponding Raman shifts essentially depends on interlayer interactions and number of individual layers stacked. Owing to larger bandgap, hBN exhibits high transparency of electromagnetic spectrum. hBN also exhibits relatively high refractive index and other electrical/optical/crystal properties of hBN were listed in Fig. 3g.
Single photon emitter. The single photon emitter is a non-classical light source, emits only one photon per excitation cycle, observed experimentally using a Hanbury, Brown and Twiss (HBT) interferometer 76 as shown in Fig. 4a. The elementary particle 'photon' cannot split further after passing through 50:50 beam splitter (as conventional light do), will be detected at any of the APD (avalanche photodiode) in HBT interferometer.
At zero delay time, the second order autocorrelation curve dips below 0.5 indicates the characteristics of a single photon emitter.
The single photon purity (represented by autocorrelation function g 2 (τ)) is the main characteristic of a quantum emitter and other important features of an ideal single photon source (quantum emitter) to be considered while developing for quantum applications is listed in Fig. 4b and experimentally observed some of the quantum emitter characteristics of luminescent point defects in hBN are listed in Fig. 4c.
Luminescent point defects in hBN acts as excellent single photons emitters, can be described as an artificial atom having ground and excited states within the host bandgap as shown in Fig. 4d.
The nature of luminescent point defects can be precisely understood by study of additional photophysical characteristics as listed in Table 2 and corresponding values observed in hBN quantum emitters were listed in Table 3.
Polarization measurements for 103 isolated luminescent point defects in hBN (Zero Phonon Line (ZPL) range of ∼550-740 nm), revealed that the absorption and emission dipoles are frequently misaligned. This dipole misalignment is framed as: The possible range of dipole misalignments, responsible mechanisms and their influence on light absorption are listed in Table 4.
Dipole misalignment �θ = ZPL energy −exciting light energy (�E). and nitrogen (grey) atoms, on Si/SiO 2 substrate. Three main quantum emitters (among various luminescent point defects predicted) namely C N defect (carbon (blue) replaces nitrogen atom) emits single photon in UV region, finds an application in free space quantum communication 67 . N B V N defect (nitrogen vacancy and boron replaces nitrogen) emits single photon in visible region, finds an application in quantum photonic circuits for quantum computing 68 and V B O 2 defect (boron vacancy with oxygen (yellow) atoms) emits single photon in near IR region, suitable for optical fiber quantum communication 69 .

Contrasting hBN quantum emission characteristics with other 2D materials and encapsulation of existing and emerging single photon sources
The comparison of some of the quantum emitter characteristics among hBN and 2D TMDs were listed in Table 5. Among all the atomically thin materials, hBN found to one step ahead in furnishing supreme quantum emitters close to ideal characteristics.
The advantages and drawbacks of existing, emerging single photon sources from single atom to very recent TMDs and 2D heterostructures are represented in Table 6.

Fabrication and activation processes of quantum emitters in hBN
Bulk hBN. In bulk hBN 96 , emitters are found to be activated by annealing the samples for 30 min at 850 °C under 0.5 or 1 torr of argon. The argon gas is selected to prevent oxidation. Annealing at different temperatures found that fluorescence intensity is maximum at 850 °C.
To date, no superior techniques were experimented to fabricate the new emitters in bulk hBN, as the emitters in bulk hBN faces the disadvantages as listed in Table 10.
CVD grown and exfoliated multi, single hBN layers. hBN multilayers grown using low-pressure chemical vapor deposition (LPCVD) technique 97 , found to host stable quantum emitters around 200 (per 10 × Table 1. Emission wavelength range of hBN quantum emitters. Among various luminescent point defects predicted in hBN flakes (as shown in Fig. 11), C N, N B V N and V B O 2 defects are consistent with recent experimental studies performed. In N B V N point defect, variation in emission energy is due to variations in local strain and dielectric environment. www.nature.com/scientificreports/ 10 µm 2 area) at room temperature. The growth technique was performed in quartz tube as schematic shown in Fig. 5. This LPCVD grown hBN multilayers are usually 1.4 nm (3 to 4 monolayers) thickness. More than 85% of emitters exhibit ZPL around 580 nm, with single photon purity g 2 (0) less than 0.5 and absolute photostability. No post growth annealing was performed for emitter activation or stabilization which is contrast to the emitter fabrication techniques observed in exfoliated multilayer hBN flakes 98 .
Other robust methodologies determined to fabricate quantum emitters in CVD grown multilayer and exfoliated multilayer hBN flakes are electron beam irradiation, laser irradiation, ion implantation 65,98 and Ar plasma etching 66 and their corresponding processing details were listed in Table 7. The stable emitters localize at flake  Table 2. Photophysical characteristics of quantum emitters. The photophysical characteristics of quantum emitters to characterize the nature of defect and their corresponding fitting equations where I = Fluorescence intensity, I ∞ = emission rate, P = laser excitation power , P sat = excitation power at saturation, θ i = angle between the initial polarization direction of light and transmission axis of the polarizer and DW = Debye-Waller. www.nature.com/scientificreports/ edges after ion implantation and subsequent annealing is shown in Fig. 6 and the comparison for photostability of emitters formed due to various ion implantations and activated through only annealing is shown in Table 8. Quantum emitters are engineered in CVD grown multilayer hBN and exfoliated multilayer hBN flakes, by performing stable emitter fabrication techniques as described in Table 8.
To date, no supreme emitter fabrication techniques were experimentally demonstrated for bulk (except annealing for emitter activation) and monolayer hBN and no annealing is performed to activate emitters in monolayer hBN as 64,99 surface states are often unstable and modifies upon annealing. An important analysis from emitter formation and activation process are listed in Table 9.
In exfoliated multilayer hBN flakes, Ar plasma etching and subsequent annealing in argon produces emitters with emission wavelength greater than 700 nm 66 and cathodoluminescence analysis of exfoliated multilayer flakes found single photon emission in the UV region 63 with a ZPL at around ~ 302 nm.

Photophysics of hBN quantum emitters
In traditional materials like diamond and silicon carbide, colour centres have similar spectral properties in both bulk and nano dimensional forms. But, in case of van der Waals crystals, the optical properties of 2D materials are different from their bulk 102 structures. Due to this reason, we present the quantum emission properties of hBN material in bulk, multi and mono layer form.
Optical characterization of hBN quantum emitters. Bulk hBN. Optical characterization of quantum emitters in bulk hBN 96 was performed by complete optical characterization setup (Confocal photoluminescence (PL) setup couple to HBT interferometer) as shown in Fig. 7a.
The PL map of bulk hBN shown in Fig. 7b represents an isolated single photon emitter (circled in red colour) along with other ensemble emissions. The corresponding PL spectrum and second order correlation measurements were shown in Fig. 7c, d.
The red dots in second order autocorrelation graph, shown in Fig. 7d, represent experimental data and the solid red line is a fit by using a three level model:  www.nature.com/scientificreports/ where τ 1 = lifetime of excited state, τ 2 = lifetime of metastable state and a = bunching factor. The emission from the defect bleaches after several minutes of excitation with 532 nm laser source. To obtain additional insight about luminescent point defect detection and its bleaching affect, the optical characterization was performed by replacing the 532 nm laser with another excitation laser of long wavelength (675 nm).
(1)   www.nature.com/scientificreports/ As similar to the emitter highlighted in Fig. 7b, an isolated emission with ZPL 760 nm, which satisfying the single photon emission characteristics is found. However, Bleaching is not observed for 675 nm excitation but another effect blinking is observed. This blinking behaviour of defect is clearly observed, for different excitation powers as shown in Fig. 7e-g.
Although, the emitters in bulk crystals are brightest and most stable [103][104][105] but has the disadvantages as listed in Table 10, which can be overcome by using atomically thin 2D hBN.  98 . Confocal PL maps of exfoliated multilayer hBN flakes in which stable emitters (white circles) are detected at flake edges after ion implantation followed by annealing. (a-c) boron implanted, (d-f) boron-nitrogen complex implanted, (g-i) oxygen implanted, (j-l) silicon implanted flakes. Large bright luminescence observed away from flake edges does not exhibit photon antibunching as in map (i). Table 8. Comparison table for emitter formation between various ion implantations and unimplanted (only initial annealing) flakes. The information in the table conforms that stable emitter formation probability is more in ion implanted samples than the reference sample (only annealed). Emitters formed by ion implantation followed by annealing exhibits absolute photostability whereas 40% of emitters formed through only initial annealing exhibit severe blinking and eventual bleaching. The number of stable emitters formation probability is more in ion implanted samples than the sample which has undergone only annealing 98  www.nature.com/scientificreports/ Specifically emitters reported in 2D hBN has PL spectra, photostability and photodynamic characteristics which are significantly different from their bulk counterpart 96,102 .
CVD grown and exfoliated multi, single hBN layers. Optical characterization of emitters from CVD grown hBN multi, single layer and exfoliated multilayer flakes were analyzed and it was noted that their PL spectrum exhibiting ZPL at ~ 580 nm to 623 nm.
The emission lines in multilayer are narrower than monolayer as observed in ref 64 and the emitter in hBN multilayer exhibits a stable fluorescence without any blinking or bleaching (at excitation power: 1mW, Time: 10 min). But in case of hBN monolayer the emitter blink and bleach for continuous excitation. Table 9. Analysis from emitter formation and emitter activation process. The analysis of emitter formation and emitter activation due to various emitter fabrication techniques, their hypothesis and important facts related to sequential annealing.

Analysis from emitter formation and activation process References
Electron beam irradiation, Laser irradiation, Ion implantation and Ar plasma etching generates new emitters 66,98 High temperature annealing only for emitter activation 98 Emitter formation probability is more in Laser irradiation as laser ablation breaks the samples into small fragments results in more number of emitters Samples are annealed after ion implantation and laser irradiation, which creates significant damage to hBN lattice that partly recovers during annealing Annealing of samples is not require after electron beam irradiation, as it is a subtler process reforms the lattice chemically with minimal damage Ion implantation has little influence on defect formation probability, indicates bombarding of ions introduces vacancies and activates intrinsic point defects already present, than introducing foreign fluorescent defects 98,100 Ion implantation increases stability of emitters and the hypothesis that ion implantation provides sufficient kinetic energy to eliminate some of the trapped species in the vicinity of emitter which leads to eliminate blinking of emitter 98 www.nature.com/scientificreports/ The creation and characterization of single photon emitters in a suspended, single crystal hBN film and free from substrate interactions were recently studied as in ref 106 . The emitters detected exhibits ZPL range from 550 to 700 nm with better single photon purity.
A significant difference in brightness was observed for the emitters detected in freely suspended hBN film compared to the emitters detected in substrate supported hBN, which provides a breakthrough advances in fabricating quantum photonic circuits.
Fluorescence measurements from traditional ensemble emitters were limited due to heterogeneity of emitters and individual defect-by defect studies are impractical. New techniques were proposed to quickly and accurately characterize the emission from ensemble emitters as experimented in ref 107 .
Optical stability of hBN quantum emitters towards wide range of temperatures and various annealing environments. Quantum emitters in hBN multilayer exhibits absolute thermal stability, optically stable even when operated upto 800 K 54 , capable to withstand aggressive annealing treatments in both oxidizing and reducing environments without any change in spectral properties 65 .
The emitters' stability towards temperature variation, annealing treatments in various gaseous environments is shown in Fig. 8a-f and in particular the variation in PL spectrum 55 of an emitter observed from cryogenic to room temperature is shown in Fig. 8g. An important analysis related to nature of emitters obtained from various annealing environments is summarized in Table 11.

Emission wavelength range of hBN quantum emitters. Quantum emitters in hBN multilayers
exhibit a broad range of UV to near IR single photon emission 63-66 as listed in Table 1, which is in contrast to colour centers and quantum dots (emission at a particular wavelength or over a narrow spectral range).
Emitters in hBN multilayer are classified into two groups based on Zero phonon line (ZPL) and phonon sideband (PSB) shapes. Emitters in group 1 have broad ZPLs along with PSBs and energies ranging from 576 nm (2.15 eV) to 652 nm (1.90 eV). Emitters in group 2 have narrow symmetric ZPLs with weak PSBs and energies ranging from 681 nm (1.82 eV) to 762 nm (1.63 eV).
In a survey of ~ 40 emitters, 70% belongs to group 1 and rest 30% of emitters belongs to group 2. The variation in ZPL of emitters in both the groups is due to variations in local strain and dielectric environment 65 .
The autocorrelation function measurements over a long time scale 65,96 of 0.1 s as shown in Fig. 9a provides an information about the presence of metastable states with decay times [110][111][112] . The presence of fewer metastable states with shorter life time makes the emitter brighter than the emitter having more metastable states with long lifetimes.

Modulation of emission characteristics of hBN quantum emitters (mechanical, optical, magnetic and electrical tuning)
However, robust, optically strong and thermally stable single photon emitters are detected in hBN, these emitters faces the limitations of moderate emitter brightness (fluorescence intensity), non-ideal single photon purity, more emitter lifetime, spectral diffusion at cryogenic temperatures and inhomogeneous emission distribution over a large spectral band. These limitations create a central problem for fabricating identical single photon emitters with ideal characteristics, which strengthen to fabricate the efficient quantum applications.
Many new techniques are proposed and proved to overcome these limitations and enhance the emission characteristics of hBN quantum emitters as explained below: www.nature.com/scientificreports/

Simultaneous excitation with two laser beams of different wavelengths. Excitation of hBN
quantum emitters with single laser (either with longer wavelength (675 nm) and high power (300 µW) or with mid-wavelength (532 nm) and low power (10 µW)), exhibits the photoluminescence measurements having moderate or less fluorescence intensity and photon emission rate. Autocorrelation measurements of quantum emitters (over a long time scale) revealed existence of a fast decaying intermediate state and a long-lived metastable state within the transition energy gap as shown in Fig. 9a and the electron decay from this long-lived metastable state leads to poor antibunching factor, low photon emission rate and needs high laser excitation power (to saturate emission intensity).
By employing simultaneous both red laser (675 nm and 300 µW) and green laser (532 nm and 10 µW) excitations 113 as shown in Fig. 9c, the enhancement in fluorescence intensity was observed due to certainty that the electrons are reverted from long-lived intermediate state to excited state, which leads to repopulation of excited state (terminating the electron decay from long-lived metastable state) as shown in Fig. 9b and the corresponding enhancement in emitter fluorescence intensity is shown in Fig. 9d.
The intensified photo physical characteristics of emitter due to two laser excitation in comparison with single laser excitation were listed in Table 13. It is noted that there is an enhancement in emitter fluorescence intensity and reduction of excitation saturation power, which are desirable, for two laser excitation in comparison to single laser excitation technique. (e, f) PL spectrum recorded for two different emitters after each annealing treatment in argon (initial annealing), hydrogen, oxygen, and ammonia respectively. There is no change in the PL spectrum after each annealing treatment. (g) The PL spectrum of hBN emitter observed from 4 to 300 K. The Width of the ZPL narrows with decrease in temperature and vice versa. Table 11. Analysis of nature of emitters from various annealing environments. The analysis of nature of emitters and its formation, crystallographic structure, charge states, location, stability of emitters and role of annealing temperature.

Analysis of nature of emitters from various annealing environments References
Luminescent defect might have a vacancy in its crystallographic structure, whose formation probability proportional to annealing temperature 65,108 Emitters are likely to be neutrally charged because annealing in hydrogen environment would expect to modify the negative charged state to neutral charge state 109 Emitters which are stable even after annealing might not be the surface states (observed in some TMDs) because surface states are unstable and can be easily modified by annealing in various reactive environments 65,99 Each annealing step creates some new emitters and quench some emitters and this quenching occurs for unstable emitters possibly located in topmost layers or edges of hBN flakes, sensitive to annealing www.nature.com/scientificreports/ Enhancing quantum emission due to coupling with plasmonic nanoparticle or via external magnetic field inducement. Coupling with plasmonic nanoparticle. Another superior technique to enhance the quantum emitter characteristics rather than simultaneous exciting with two laser beams is coupling the quantum emitters with plasmonic nanoparticles like gold, silver and platinum etc.
Overall photon emission rate enhancement is made possible by combination of both excitation enhancement and spontaneous enhancement rate. Coupling the gold nanospheres to the emitters in exfoliated flake edges as observed in ref 114 , enhances the excitation rate due to surface Plasmon resonance and spontaneous emission rate enhancement by Purcell effect (enhancement by environment).
Using AFM tip, the gold nanospheres are positioned to close proximity of emitters as shown in Fig. 9e, made to align with transition dipole angle of emitter in order to increase the plasmonic coupling effect and this transition dipole angle was deduced from flake orientation. The enhancement in quantum emitter characteristics due to single and double nanospheres coupling were listed in second row of the Table 13.
External magnetic field inducement. An effective alternate to plasmonic coupling enhancement is by inducing external magnetic field.
It is a known fact that electronic structure of defects in wide band semiconductors possess both electron un-occupied and occupied states in between the conduction and valence band edges of the energy gap. These occupied electrons exists in paramagnetic form (possibly with up ↑ or down ↓ spins). By inducing external magnetic field, these occupied energy states split up due to Zeeman effect (let's say one occupied state with a paramagnetic electron split up into two higher and lower energy states with energy difference (ΔE) and this paramagnetic electron prefers to sit in lower energy state).
When this lower energy paramagnetic electron absorbs a photon with energy = ΔE, then it jumps to higher energy state and emits the photon (with same or different energy) while relaxing back to lower energy state. This spin-dependent inter-system crossing (ISC) transition found to exhibit an anisotropic fluorescence response for applied magnetic field as experimented in ref 57 .
The schematic representation of inducing external magnetic field to the hBN film and simultaneous optical excitation is shown in Fig. 9f, where β represents angle made by horizontally applied external magnetic field w.r.t x plane and α (ε) represents absorption (emission) dipole w.r.t x − y plane.
The variation in PL spectrum with and without magnetic field is shown in Fig. 9g. The quantum emitter fluorescence emission found to increase monotonically for particular combination of applied magnetic field angle and absorption dipole angles as listed in Table 12.

Resonant excitation technique.
At cryogenic temperatures, the ZPL shape of emitters was influenced and broadened due to spectral diffusion 115 , which leads to erroneous ZPL measurements.
This spectral diffusion is observed in off-resonant excitation (electron is excited to higher vibronic energy levels of excited state and this excited electron should vibrationally relax to minimum energy level of excited state and followed by spontaneous emission to emit a single photon). The control of ZPL broadening (even when www.nature.com/scientificreports/ emitters experience spectral diffusion) is observed in on-resonant excitation 116 (electron is excited directly to minimum of excited state and then follows spontaneous emission). This off-resonant excitation occurs when excitation energy (hf) is more than luminescent point defect ZPL energy (ΔE ZPL ) and on-resonant excitation occurs when hf = ΔE ZPL . The controlled photophysical characteristics of emitters due to on resonant excitation is shown in third row of the Table 13 and the variation in PL maps due to Off and On resonant excitations is shown in Fig. 9h-j. Enhancement of single photon purity. Second order correlation values found to vary around 0.5 and this deviation from g 2 (0) = 0 (ideal value) is due to high residual background emission, which leads to decrease in single photon purity.
Emitters found in the He + ion implanted area with subsequent annealing with argon (shown as a dark region) exhibits correlation value g 2 (0) = 0.077 (indicated by blue arrow), reduced by five-fold 78 (indicating the reduction of background fluorescence) compared to the emitters found in non-irradiated area exhibiting g 2 (0) = 0.263 (indicated by white dashed arrow) as shown in Fig. 9k. The corresponding PL spectrum is shown in right to PL map.
Tuning the quantum emission through external strain inducement or external electric field inducement or through ionic liquid devices. Emitters in hBN multilayers exhibit different emission energies (ZPL) over a large spectral band (inhomogeneous spectral distribution) 65 due to variations in local Table 12. Higher fluorescence quantum emission observed for particular combination of applied magnetic field and light absorption dipole angles. Monotonical increase in fluorescence quantum emission is observed for β = 0° with applied constant magnetic field strength (B) = 890 Gauss and for both β = 45° and 90°, increase in fluorescence quantum emission is examined by varying magnetic field strength (B) from 0 to 400 Gauss.
Horizontally applied external magnetic field angle (β) w.r.t x plane Absorption dipole angle (α) w.r.t x − y plane Reference β = 0°α = 90°5 7 β = 45°α = 45° and 90°β = 90°α = 0° and 90°T able 13. Intensified photophysical characteristics of emitters due to three different enhancement techniques.Enhancement in fluorescence intensity and reduction of excitation saturation power for two laser excitationtechnique has shown in first row; increase in photon emission rate and controlled fluorescence lifetime due totwo nanospheres plasmonic coupling. Modulated ZPL (wavelength and width); enhanced single photon purityand controlled fluorescence lifetime due to on-resonant excitation is shown in third row. www.nature.com/scientificreports/ strain and dielectric environment, which creates a central problem for fabricating identical single photon emitters.
Tuning the quantum emitters (having uneven ZPL emission energies), through external strain inducement or through external electric field via stark effect or electrostatic tuning through ionic liquid devices, leads to increase the probability of multiple uneven emitters (different ZPL energies) to have the same emission energy (ZPL), which further tends to fabricate identical quantum emitters with necessary emission wavelength.
External strain inducement. Controllable external strain can be induced to the emitters in hBN film by placing on a 1.5 mm thick bendable polycarbonate (PC) beam 78 as shown in Fig. 9l and corresponding ZPL energy shifts of emitters due to variation in applied strain is shown in Fig. 9m. Tunability range observed for three different emitters (with different ZPL energies) are −3.1 meV/% (green), + 3.3 meV/% (yellow) and + 6 meV/% (red) respectively.
Similar experimental approach was observed in ref 117 for deterministic single photon emitter formation by a combination of nanoscale strain engineering and charge trapping.
External electric field inducement. Quantum emitters in hBN are expected to lie within the plane, which creates an in-plane dipole. By inducing an out-of-plane electric field through graphene gates, electrical tuning of quantum emission (via stark effect) from luminescent point defects hosts in hBN was observed in ref 118 . The schematic representation of multilayer hBN between top and bottom graphene electrodes (for electric field inducement) is shown in Fig. 9n.
Electrostatic tuning of quantum emission through ionic liquid devices. The quantum emission from luminescent point defects in hBN can also be tuned by placing the hBN sample in ionic liquid devices 97 as shown in Fig. 9o, in which poly (ethylene oxide) mixed with lithium perchlorate (PEO: Li-ClO4) is used as an electrolyte.
By applying positive (negative) gate voltages, the ZPL emission is found to be blue shifted (red shifted) by around 15 nm as shown in Fig. 9p. Tuned ZPL emissions are found to be stable and does not exhibit spectral diffusion.

Other recent advances in modulation of emission from 2D hBN quantum emitters.
Contemporary exploration on hBN integration with photonic microstructures like circular Bragg grating (CBG) was found to be eminent and rooted out the second harmonic generation 119 (absorbs two photons of same frequency and generates a single photon of twice the frequency absorbed) in exfoliated multilayer hBN as shown in Fig. 10. Some of the investigative explorations probe that this second harmonic generation (SHG) could be associated to defects 120 .
Novel techniques to enhance the quantum emission from hBN single photon sources is by coupling the quantum emitters with photonic crystal cavities from silicon nitride (Si 3 N 4 ) 121 and Al nano-antenna 122 , which revealed a 6-fold and 10 to 15 fold enhancement in photoluminescence measurements of a hBN quantum emitter at room temperature respectively.
Due to high transmittance and poor reflectance of monolayer hBN, it was found to be invisible under white light which leads to utmost difficulties in experimental works like transferring methods and fabrication of 2D heterostructures.
It was Serendipity, that hBN was found to enhance the contrast on transparent substrates 123 (polymer based interfacial layer on a polydimethylsiloxane (PDMS) substrate) and similar investigation was performed on other 2D TMDs and disseminate that TMDs exhibit significant difference in contrast on transparent substrates than opaque substrates.
The simulation results also reveal that contrast of hBN is high under shorter wavelength light than longer wavelength. Finally it was negotiated that selecting an appropriate substrate for optical experimentation is very crucial.
Tuning of quantum emission spectrum is found to feasible with externally applied tensile strain and present day a record tuning of 65 meV 124 was observed in hBN layers.

Prediction of emitter defect structures and applications of quantum emitters
Prediction of defect structures using first principle calculations. A list of 35 different possible luminescent point defects of hBN (based on most likely forming impurities incorporate during hBN growth and while annealing on substrate), as shown in Fig. 11, were examined by Density Functional Theory (DFT) calculations using Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) 125 and Vienna Ab initio Simulation Package (VASP) 126 on the basis of generalized gradient approximation by Perdew, Burke and Ernzerhof (PBE) functionals 127 .
Based on experimental observations, three main conditions, as shown in Table 14, were applied to authenticate a luminescent point defect as an effective potential quantum emitter.
Among the 35 luminescent point defects, N B V N , O B O B V N and C B V N are the defect structures found to satisfy the rules listed in Table 14 and their corresponding ZPL energies are 2.01 eV, 1.85 eV and 1.33 eV respectively (obtained through VASP calculations).
The Emitters detected in experimental studies (emission in visible region) whose ZPL is around ~ 2 eV, predicted to have N B V N 64 defect structure and simulated (using VASP) electronic structure of N B V N defect in hBN monolayer is shown in Fig. 12a www.nature.com/scientificreports/ The electronic structure of V B O 2 defect (obtained using VASP simulations) in hBN monolayer is shown in Fig. 12b. Among various oxygen related defects 66 examined by DFT using hybrid functionals (HSE06), found that B-vacancy with two oxygen atoms (V B O 2 ) defect structure as shown in Fig. 12c, is the most likely defect for emission in longer wavelength. The emission energy of V B O 2 defect (theoretically calculated) found to be consistent with experimental ZPL energy of near IR emitters fabricated using Ar plasma etching.
Recent studies reveal that boron and nitrogen dangling bonds as shown in Fig. 12d, e as another source of single-photon emission around 2 eV, which is described in ref 131 .  www.nature.com/scientificreports/ Numerous DFT calculations anticipate that single photon emission in UV region is due to carbon substitutional defects like C N defect (nitrogen atom is replaced with carbon) and C B defect 132 (boron atom is replaced with carbon). Their quantum emission (theoretical) was observed ~ 4.1 eV, which were consistent with experimental works as described in ref 63 .
By using Finite Difference Time Domain (FDTD) simulations as demonstrated in ref 114 , the enhancement of single photon source emission characteristics due to coupling with gold nanospheres was investigated as shown in Fig. 12f. The simulated spontaneous enhancement rate was consistent with experimental studies. X-polarized emitters which are perpendicular to the gold metal surface exhibits higher enhancement rates than y-polarized emitters.
Strain tunable quantum emission from N B V N defect structure as shown in Fig. 12g, was studied using DFT calculations by PBE approximation. Four strain directions were considered in simulations 78 to create similar effects of strain, induced using polycarbonate (PC) beam in experimental works. Table 14. Conditions to authenticate a luminescent point defect as an effective potential quantum emitter.

Three main conditions to authenticate a luminescent point defect as potential quantum emitter References
The energy levels of luminescent point defects must be located within the bandgap and none of the levels should be present within or near to bulk bands, which represents that these point defects are thermally stable against annealing and higher temperatures, which is the characteristic of an essential quantum emitter 128 Theoretically calculated emission energies of luminescent point defects must be consistent with observed ZPL energies of quantum emitters in experimental studies 129,130 Luminescent point defects must exhibit polarized excitation and emission in optical spectrum (simulated), as this polarization phenomena was observed in quantum emitters during experimental analysis www.nature.com/scientificreports/ The quantum emission from N B V N defect is observed ~ 2.01 eV (black peak), when no strain was applied (zero strain). When tensile strain is applied along ZZ 1 direction, the emission peaks were tunable towards lower wavelengths (blue tones) and similarly, when tensile strain applied along AC 2 direction, the emission peaks were tunable towards higher wavelengths (red tones).

Applications of quantum emitters. Many applications of quantum emitters have been proposed and
demonstrated in literature. Some major applications of quantum emitters are discussed here: i. Quantum computing, ii. Quantum cryptography (Quantum communication), iii. Quantum imaging and metrology and iv. Other fascinating applications Quantum computing. In classical computing, a conventional bit can be either a 0 or 1. Quantum computer provides a breakthrough revolution, in which a qubit can be both 0 and 1 at the same time on the basis of superposition principle.
In optical quantum computing, the single photons are considered as qubits. Initially, these single photons generated from single photon emitters can be either horizontally polarized (considered as logic 0) or vertically polarized (considered as logic 1). Then processing of qubits like creating superposition state (both logic 0 and 1 at same time) and flipping of qubits (logic 0→logic 1 or logic 1→logic 0) can be created using quantum gates like Hadamard (H) gate and Pauli-X gate etc. respectively. These quantum gates are developed using set of birefringent wave plates and the encrypted qubits finally accumulated using single photon detectors.
The schematic representation of a quantum photonic circuit in a quantum computer, constructed using single photon source, set of quantum gates and polarized beam splitters (PBS) and single photon detectors as shown in Fig. 13a, in this particular example quantum dot (QD) based single photon sources [133][134][135][136] are used for discussion.

Quantum cryptography (quantum communication).
Quantum cryptography technique is used to encrypt the key by using quantum effects (polarized photons) for secure communication and the complete quantum communication 137 setup is shown in Fig. 13b.
The quantum communication setup in which the single photon source (highlighted in red colour) is present at the transmitter, coupled to HBT setup for controlling the quality of source and EOM (electro optical modulator) to make photons polarized. The sender and receiver are separated by a distance of 50 m. The horizontal-vertical (H−V) and circular left-circular right (L−R) polarization basis photons were used for data encryption.
To date, Color centers in diamond 87 (free space) and Quantum dots 138 (optical fiber communication to a distance of 120 km) are used as single photon emitters in quantum communication systems.
Quantum imaging and metrology. Quantum imaging. Quantum imaging is a new sub-field of quantum optics, makes use of entangled photons to image the objects with higher resolution or other imaging criteria beyond the limitations of classical optics.
The simplified schematic of quantum imaging experiment is shown in Fig. 13c in which two entangled photons 139,140 are generated using BBO (beta barium borate) crystal.
Quantum metrology. Similar to quantum imaging application, quantum metrology 141,142 exploits entangled photons to make high sensitive measurements of assorted signal (weak emissions along with strong unwanted background) and physical parameters in order to avoid statistical errors. The schematic representation of quantum metrology circuit is shown in Fig. 13d.
Other fascination applications. Other fascinating applications of quantum emitters. Other fascinating quantum technology applications of single photon emitters (detected in 2D materials like hBN, WSe 2 , MoSe 2 and MoS 2 etc.) were listed in Table 15.
Surging the enhancement of emission characteristics in hBN like single photon purity, suppressing offresonant noise, reduction of excited state lifetime and elimination of photo blinking and bleaching using Purcell effect was perceived by creating an array of plasmonic nanocavities 149 .
Contemporarily, the enhancement without coupling to plasmonic nanoparticles was made feasible using photonic crystal cavities 143 and microcavities 144 , which enhances the implementation of scaled quantum photonic circuits. As similar to stark tuning of emission in hBN and graphene heterostructure, efficient spontaneous emission and enhancing photon extraction was realized through graphene-hBN hyperstructure 148 .
Coupling of quantum emitters in hBN to tapered optical fibers 152 escalates the application of on-demand quantum repeaters, but the collection efficiency was observed around 10%.
Deposition of TMDs like WSe2 material on plasmonic nanopillars 150 , originates simultaneous exciton trapping and enhancement in emission characteristics. Recently coupling the WSe2 material to silicon nitride 145,146 nanochip and waveguide found to increase photon extraction efficiency. Tunable quantum emission due to electric and magnetic fields and control of emitter lifetime in TMDs were also observed 31,147 as similar to spectral tuning observed in hBN 57,118 . Particularly, the graphene/hBN/TMDs heterostructure facilitated the controlled charge trapping of excitons 58 .   www.nature.com/scientificreports/ Implementation of qubits (single photons) using various 2D materials and heterostructures. The implementation of qubits using various 2D materials and heterostructures are listed in Table 16 with relevant discussion and the corresponding figures are shown in Fig. 14.
Typical qubits (single photons) can be implemented by 2D materials using various phenomena like spontaneous emission associated to intermediate energy state transition in hBN, spontaneous emission associated to localized excitons trapped and funneled due to point defects and crystal imperfections in TMDs.
Electrostatic potential traps due to moiré patterns and intentionally induced strain gradients can also effectively tap excitons for qubits implementation. IntenƟonally induced strain traps exciton at maximum strain point as shown in Figure 14(a, d).
External electric and magneƟc fields can control the excitonic emission.
InjecƟon of both electrons and holes into recombinaƟon layer as explained in Figure 14(f) leads to formaƟon of excitons.
The energy states and the periodicity of the superlaƫce can be controlled by the twist angle as shown in Figure 14(g).
External electric fields can control the excitonic emission.
[ www.nature.com/scientificreports/ Especially, in case of WSe 2 material the crystal imperfections due to intentionally induced strain gradients by transferring on dielectric nanopillars and folding the material using gapped golden rods as shown in Fig. 14a, d, which leads to funnel and trap excitons. Optically exciting these trapped excitons leads to single photon emission. In similar fashion, an array of quantum emitters can be created by trapping the excitons using set of dielectric nanopillars as shown in Fig. 14b, the corresponding dry transfer technique of layered material on array of nanopillars in shown in Fig. 14c.
Moiré super-lattice of 2D heterobilayers structure as shown in Fig. 14g with twist angle θ leads to formation of electrostatic potential traps for effective trapping of exciton as shown in Fig. 14h.
Qubits can also be implemented by electrical excitation of excitons using 2D heterostructures like graphene/ hBN/MoS 2 /hBN/graphene heterostructure as shown in Fig. 14e. The MoS 2 acts as recombination layer, hBN as tunneling barrier and graphene as transparent electrodes. To verify the effect of charge trapping complete structure is divided into two: one with MoS 2 layer (GBM) and without MoS 2 layer (GB).  www.nature.com/scientificreports/ Energy band structure of graphene/hBN/MoS 2 /hBN/graphene heterostructure is shown in Fig. 14f. By increase in applied voltage, quasi fermi level of graphene (left contact) reaches minimum of conduction band of MoS 2 , electrons tunnel through the hBN barrier into MoS 2 layer and quasi fermi level of graphene (right contact) reaches maximum of valence band of MoS 2 , holes tunnel through the hBN barrier into MoS 2 layer. Injection of electrons and holes in MoS 2 layer (recombination layer) leads to formation of exciton recombination followed by single photon emission.
Electrical manipulation of excitonic emission to generate entangled photons 59-61 using 2D heterostructures, as similar to shown in Fig. 14e, is also realized which facilitates the application of quantum imaging and metrology.  www.nature.com/scientificreports/ Practical challenges of implementing quantum emitters and their applications. Few major challenges are noticed in fabricating hBN single photon emitters and developing towards quantum applications. The challenges associated with emitters (luminescent point defects) hosts in hBN were listed in Table 17.
One of the major challenge to be resolved is moderate single photon purity (non-ideal) due to residual background emission. In most of the experiments it was noted that emission intensity of emitters spans over a large spectral band (inhomogeneous spectral distribution) which is a central problem for developing identical single photon sources.
High refractive index of the material which makes difficult to confine the light within the hBN structure is another important problem to be addressed and finally, indistinguishability of photons from the emitter is very poor.
The challenges associated with optical quantum applications were listed in Table 18. In quantum computing, the loss of quantum coherence due to temperature fluctuations is the one major challenge to be overcome for practical implementation and another problem is influence of other electromagnetic waves and undesirable interactions with outer environment leads to collapse of quantum properties of computer.
In case of quantum communication, Security loopholes or side channels leads to in-secure quantum communication. Currently, probabilistic single photon sources are used for entangle photon generation which are not reliable for on-demand applications like quantum imaging and metrology.
The complete summary of hBN materials used and their synthesis, stable emitter formation techniques, defects consistent with experimental studies, important photophysical characteristics of emitters and their emission enhancement techniques and major applications of quantum emitters were discussed in Table 19.

Conclusion and outlook
Single photon emission is observed from single atom in a cavity to recent 2D materials, however, each of these sources have their own constraints. However, the state-of-the-art quantum emitters in 2D materials found to be an effective alternate to traditional single photon emitters in three dimensional materials due to the disadvantage of photon extraction efficiency and external circuit coupling.
In the quest of A1 supreme quantum emitters, strong excitonic single photon emission is observed in monolayer TMDs  and in their Moiré patterns 49-53 at cryogenic temperatures. Despite, inducing a stronger strain in TMDs is found to improve their operating temperatures 22 and quantum yield 47,48 . It's an auspicious that emitters observed in multilayer hBN are found to be stable at a wide range of operating temperatures, various harsh environments and found to be localized at flake edges in mechanically exfoliated flakes which enhances the device fabrication and coupling to plasmonic cavity for higher integration quantum photonic circuits.
This solid quantum emitters in hBN are found to be generated by Ion implantation, laser ablation, Ar plasma etching followed by subsequent annealing and electron beam irradiation. But, the nature of emitters formed is still under debate. The cutting-edge research is fabricating stable quantum emitters in higher order (⁓ 100-200 SPEs per 10 × 10 μm 2 ) in large few-layer hBN fabricated with LPCVD technique, strongly publicized that more than 85% of the emitters have a ZPL at (580 ± 10) nm, which is the confined uneven spectral distribution reported to date 97 .
To enhance the performance of quantum photonic devices and converge to on-demand applications, fabricating the quantum emitters which covers the wide range of emission on a single platform is really worthy of attention. Significantly, hBN exhibits broad emission range from UV to near IR region and the first signature of Rabi oscillations and resonant fluorescence emission was observed from a resonantly drive hBN quantum emitter 153 . Contemporarily, an attractive phenomenon of second harmonic generation (SHG) 119,120 was discovered in hBN multilayer coupled to circular Bragg grating (CBG) photonic microstructures which, encourages the delightful applications.
Although the emission characteristic measurements in hBN were not up to the mark, quantification enhancement techniques like Two laser excitation technique, plasmonic coupling using gold nanospheres, external strain engineering, electrostatic tuning through ionic liquid devices 97 and very recently electric and magnetic field inducements 57 etc., techniques found to enhance and tune the quantum emitters emission properties and control the inhomogeneous spectral distribution. To date a record tuning of quantum emission around 65 meV 124 and higher rate of emission enhancement (around 6 to 15 fold) owing to coupling with photonic crystal cavities from silicon nitride (Si 3 N 4 ) 121 and Al nano-antenna 122 was perceived. However the quantum emitters in hBN experience spectral diffusion at cryogenic temperatures, Resonant and antistokes 154 excitation technique found to overcome the complication due to spectral diffusion.
Emitters in hBN are mostly stable under illumination at 532 nm. On the other facet, emitters exhibits fluorescence instability and spectral diffusion at blue laser excitation 155 . However employing different substrates like Al 2 O 3 156 or InGaP 157 found to reduce the random spectral fluctuations, layered hBN mounts a problem of invisibility on opaque substrates under white light illumination. Recent explorations make a Serendip that hBN was found to enhance the contrast on transparent substrates 123 , which makes experimental works feasible on hBN.
Efficient photon extraction and coupling to external photonic circuits and devices is at most important for practical implementation for quantum applications. Out of sort, only 10% efficiency in photon extraction using an optical fiber and coupling to an optical waveguide is achieved 146 at room temperature. A waveguide of proper design is needed to improve the coupling of quantum emitters and filter the propagating laser light, to enhance the feasible implementation of quantum repeaters.
As a quantum breakthrough, tunable excitonic emission in hybrid structures initiate the development of quantum memories 58 . Entanglement of photons for Quantum imaging and metrology was made possible by a back-gated WSe 2 monolayer, hBN and graphene heterostructure [59][60][61] , overcome the drawback of generating entangled photons at random process in nonlinear BBO crystal. www.nature.com/scientificreports/ Several computational evidences disclose that point defects responsible for single photon emission. Investigation of 35 different likely possible point defect structures by density-functional theory (DFT) provides a strong evidence that N B V N defect structure is more consistent with the emitter responsible for visible region emission, V B O 2 defect for near IR region and C N defect for UV emission. Recent theoretical studies reveal boron dangling bonds 125 are also the likely source of emission around 2 eV and C B defect 126  www.nature.com/scientificreports/