Optical quantum technologies with hexagonal boron nitride single photon sources

Abstract

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.

Figure 1

Pictorial representation of absorption, excitation and spontaneous emission. (a, b) electron in ground state absorbing energy E = E₂ − E₁ 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.

Figure 2

Schematic representation of three main quantum emitters hosts in hBN on Si/SiO₂ substrate, emitting single photons of different energies and their unique applications. hBN monolayer by alternating boron (pink) and nitrogen (grey) atoms, on Si/SiO₂ 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. N_BV_N defect (nitrogen vacancy and boron replaces nitrogen) emits single photon in visible region, finds an application in quantum photonic circuits for quantum computing and V_BO₂ defect (boron vacancy with oxygen (yellow) atoms) emits single photon in near IR region, suitable for optical fiber quantum communication.

Figure 3

Schematic representation of hBN stacking, electronic band structure of monolayer and Bulk hBN and electrical/optical/crystal properties of hBN material. (a, b) Top view and side view of AA’ stacking. (c, d) Top view and side view of AB stacking. (e, f) Electronic band structure of monolayer and bulk hBN with direct and indirect bandgaps respectively. (g) General properties of hBN material and similar Raman shifts (around values) can be observed for high quality crystals.

Figure 4

Schematic of HBT interferometer, important features of an ideal single photon source and experimentally observed quantum emitter characteristics, pictographic representation of atomic behaviour of defects within the host bandgap (a) Schematic representation of HBT interferometer working mechanism and resultant second order autocorrelation curve representing characteristics of a single photon emitter. (b) Important features of an ideal single photon source. (c) Experimentally observed some of the quantum emitter characteristics hosts in hBN, in which characteristic stability upto 800 K and single photon purity 0.01 makes a highest record among all the 2D materials (to date). (d) The energy band diagram of an hBN host with ~ 6 eV bandgap. A luminescent point defect in hBN (with energy range ~ 2.2–3 eV) exhibits an artificial atom kind of behaviour with ground and excited states.

Figure 5

Schematic representation of a LPCVD grown hBN multilayer in quartz tube. hBN multilayer growth on Cu foil in a quartz tube furnace at 1030 °C Temp. Ammonia borane (NH₃-BH₃) as a precursor and Ar/H₂ as carrier gas.

Figure 6

Localization of emitters at exfoliated multilayer hBN flake edges. 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).

Figure 7

Schematic of confocal photoluminescence (PL) set-up coupled to HBT for optical characterization, optical characterization of emitter in bulk hBN and its blinking behaviour at elevated excitation powers^,. (a) Complete optical characterization setup (Confocal PL setup coupled to HBT), photoluminescence setup for identifying and analyzing the emitters. HBT interferometer for second order autocorrelation measurements. (b) Confocal photoluminescence map of bulk hBN, obtained by 532 nm CW laser excitation and the scale bar indicates 10 μm. (c) Photoluminescence spectrum of isolated emitter (532 nm CW laser excitation) represented by solid red trace along with background spectrum obtained from the region adjacent to emitter represented by dotted grey trace. (d) Second order autocorrelation measurements $g^2\left(0\right)$ ~ 0.35, performed by using HBT interferometry at one of the ZPL. The obtained experimental data (red dots) is not background corrected and the dip of the curve below 0.5 indicates the single photon emission nature of defect. The fluorescence intensity plots of emitter (detected for 675 nm excitation) recorded as a function of time at excitation powers (e) 150 μW, (f) 600 μW and (g) 2000 μW. Blinking of defect is clearly visible for all the investigated powers.

Figure 8

Stability of emitters towards temperature variations and different annealing environments and PL spectrum of a hBN quantum emitter observed from cryogenic to room temperature^,,. Characterization of emitters during heating phase with temperature increments of 100 K during 300 − 800 K thermal cycle. (a–d) Second order correlation measurements and corresponding PL spectrum of emitter having ZPLs ~ 1.94 eV and ~ 1.75 eV respectively. (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.

Figure 9

Correlation measurements over a long time scale, pictographic representation of emitter level structure, schematics of two laser excitation, plasmonic coupling, external strain, electric and magnetic field inducements and ionic liquid devices, off and on-resonant excitation PL maps and corresponding PL spectrums of quantum emission enhancement techniques. (a) Second order autocorrelation measurements $g^2\left(0\right)$ performed over a long time scale, three exponential components best fits for autocorrelation curve which indicates the presence of metastable states between ground and excited states. Inset shows the presence of three metastable states with lifetimes of 5 µs, 31 ms and 480 ns. (b) Schematic representation of fast decaying intermediate and a long-lived metastable states and reversion of emitters from intermediate to excited state by repumping technique. (c) Schematic of two laser beam excitation of different wavelengths. (d) PL intensity of emitter for single lasers 532 nm, 675 nm and two lasers (532 nm + 675 nm) excitation. (e) A single gold nanosphere is coupled to emitter in flake and a second gold nanosphere is made to couple by using an AFM tip in order to form plasmonic cavity. (f) Schematic representation of horizontally induced magnetic field to the sample, which is under optical excitation. (g) Variation in fluorescence intensity of emitter due to applied magnetic field. (h–i) The PL maps of emitter under off and on-resonant excitations respectively, in which quantum emission is indicated by black arrow, the other white areas represent the emission from background (unwanted), which is not observed in on-resonant excitation PL map. (j) No PL signal is observed when resonant excitation was detuned (reduced) by 2 nm. (k) The PL map of irradiated area (He + ion implanted area, followed by annealing with argon) which exhibits a dark region indicating the reduction of background fluorescence compared to non-irradiated area and corresponding PL spectrum of white dashed area is shown in right panel. (l) Tensile (compressive) strain applied by a polycarbonate (PC) beam in which vertical force is applied to a free edge and colour represents the possible strain intensity.(d = distance between the fixed edge and hBN sample, L = distance between fixed edge and deflection point and $\delta$ = beam deflection). (m) ZPL energy shits as a function of applied strain for three different emitters and linear fit indicates overall strain induced ZPL energy shift ranges from − 3 to 6 meV/%. (n) Schematic representation of multilayer hBN sandwiched between two graphene electrodes (induce electric field) for stark tuning of emitters embedded in hBN. (o) Schematic representation of LPCVD grown hBN (transferred on gold electrode using PMMA assisted transfer) placed in an ionic liquid device. The gate voltage is applied to second electrode. (p) PL spectrum of emitter around 622 nm under no bias, blue and red-shifted by 15 nm for ± 6 V gate voltages respectively.

Figure 10

Schematic representation the SHG from a few-layered hBN film on a CBG. Schematic representation of multilayer hBN coupled to circular Bragg grating photonic microstructure and found to exhibit second harmonic generation by absorbing two photons of same frequency and generating a single photon of twice the frequency absorbed. Figure adapted with permission from ref. [Bernhardt, N et al., "Large few-layer hexagonal boron nitride flakes for nonlinear optics", Optics Letters, 46(3), p.564, 2021], The optical society.

Figure 11

Atomic structures of possible luminescent point defects for hBN quantum emitters. Atomic structures of 35 different possible hBN defects due to likely conditions. Defects group such as Si/C-defects, Stone–Wales defects (SWC_N), C-based defects, O-based defects, native defects, S-based defects, complex vacancy defects (VCompX) and few other defects were considered. Legend: white spheres represent nitrogen atoms, green spheres (boron), red spheres (oxygen), blue spheres (silicon), brown spheres (carbon), yellow spheres (sulphur), black spheres (fluorine), silver spheres (phosphorus),small white spheres (hydrogen).

Figure 12

Simulated electronic structures of N_BV_N and V_BO₂ defects, schematic representation of V_BO₂ defect in hBN monolayer, boron and nitrogen dangling bonds, coupling of gold nanosphere to emitter in hBN multilayer flake, Strain directions and spectral tuning of hBN quantum emitter and. (a, b) Electronic structures of N_BV_N and V_BO₂ defect, in which 1.95 eV transition (transition from a ground state at 1.95 eV to an excited state located at 3.90 eV) and 1.85 eV transition (transition from a ground state at 0.98 eV to an excited state located at 2.83 eV) were highlighted, consistent with experimental studies and Only spin-preserving transitions are assumed and occupied and unoccupied states are represented by black and grey arrows. (c–e) Schematic representation of V_BO₂ defect (Boron vacancy with two oxygen atoms (red colour)), Boron (green) and Nitrogen (grey) dangling bonds with hydrogen (white) passivated respectively. (f) Spontaneous emission enhancement (γ_sp/γ⁰_sp) due to defect coupling with gold nanospheres. (g) Effect of strain on N_BV_N defect structure.

Figure 13

Schematic representation of quantum photonic circuit in a quantum computer, complete quantum communication system for free space propagation, Schematic illustration of quantum imaging and quantum metrology circuits^,. (a) The quantum photonic circuit developed for quantum computing application, in which quantum light sources as quantum dots are employed. (b) The quantum communication setup in which the single photon source (highlighted in red colour) is present at the transmitter. (c, d) Circuit schematics for quantum imaging and metrology applications, in which necessary entangled photons are generated by nonlinear BBO (beta barium borate) crystal by spontaneous parametric down conversion technique and separated by a beam splitter, modified Mach–Zehnder interferometer (MZI) (background grey colour) shown in Fig. 13d for photon processing.

Figure 14

Techniques to implement qubits using other 2D materials and heterostructures^,,,,. (a) Intentionally induced strain gradient in WSe₂ layer to funnel a single exciton using dielectric nanopillar. (b, c) SEM image of an array of quantum emitters using nanopillars and schematic representation of dry transfer technique of layered material on nanopillars. (d) Monolayer WSe₂ folded around the gapped golden rods which leads to strain inducement and formation of potential wells to effectively trap and localize the excitons. (e) graphene/hBN/MoS₂/hBN/graphene heterostructure for excitonic emission using electrical excitation. (g) Illustration of moiré super-lattice due to 2D heterobilayers structure to form electrostatic potential traps. (h) Schematic representation of an exciton trapped in a moiré electrostatic potential site.