Tunable and high-purity room temperature single-photon emission from atomic defects in hexagonal boron nitride

Two-dimensional van der Waals materials have emerged as promising platforms for solid-state quantum information processing devices with unusual potential for heterogeneous assembly. Recently, bright and photostable single photon emitters were reported from atomic defects in layered hexagonal boron nitride (hBN), but controlling inhomogeneous spectral distribution and reducing multi-photon emission presented open challenges. Here, we demonstrate that strain control allows spectral tunability of hBN single photon emitters over 6 meV, and material processing sharply improves the single photon purity. We observe high single photon count rates exceeding 7 × 106 counts per second at saturation, after correcting for uncorrelated photon background. Furthermore, these emitters are stable to material transfer to other substrates. High-purity and photostable single photon emission at room temperature, together with spectral tunability and transferability, opens the door to scalable integration of high-quality quantum emitters in photonic quantum technologies.

collisions with He ions cause the breakup of covalent bonds that can show fluorescence activities.
Luminescence is observed only if the electrons free from covalent bonds have orbitals separated by an energy gap smaller than the exciting laser energy at 532 nm. Interestingly, the region with lower dose (A) shows the strongest fluorescence intensity. After annealing, the fluorescence from the irradiated areas is suppressed due to recrystallization and the effects of the treatment are visible only for the area irradiated with dose higher than 10 %) ions cm -2 . Supplementary Fig.1b shows a magnified cutout of these regions.
Several defects are visible inside region B and most of them are experimentally attributed to single photon emitters. Supplementary Fig.1c is an optical microscope image of the hBN flake after the full treatment.
With a magnification of 40X the effects of FIB are already visible for the region B. The FIB area presents slightly darker color, probably due to the increase of the flake thickness. Supplementary Fig.1d shows the dependence on the ion dose of the intensity ratio between the areas outside and inside the irradiated regions. The values of the PL intensity are extracted from the PL maps with integration over an area of approximately 25 μm 2 . This plot indicates an improvement in the intensity contrast between treated and untreated regions as the ion dose is increased.
Ion irradiation with a dose of 5×10 %) ions cm -2 has been tested on flakes with different thickness yielding similar results to the one shown in Fig.2 in the main text. Supplementary Fig.2  Continuous wave or pulsed laser beam passes through the image relay system with scanning mirrors to be focused on the sample surface. In the 4f configuration, the galvanometer mirror scanner is placed at the focal plane of the first lens such that the beam angle can be precisely controlled. An angle variation at optical plane of the galvanometer mirrors corresponds to a different spatial position on the sample, allowing a precise control on the spatial position of the excitation and detection of individual defects.
In the detection path, the fluorescence photons from the emitters can be selectively routed towards avalanche photodiodes (APDs) or to a spectrometer by simply rotating a half wave plate (HWP) in front of polarized beam splitter (PBS). Second-order autocorrelation functions ( (2) ( )) is measured by a Hanbury Brown and Twiss (HBT) interferometry setup using two APDs.
Supplementary Fig.5 shows a comparison between a single photon emitter inside and outside the irradiated region, similarly to the results already shown in Fig.3 in the main text. Supplementary Fig.5 is the PL intensity map of the region of interest. Several defects are visible inside the irradiated area, mainly at the cracks of the flakes. Two quantum emitters are analyzed and are highlighted by green (inside FIB area) and blue (outside FIB area) arrows. Supplementary Fig.5b is the autocorrelation measurement of the two emitters with CW excitation. Clearly the single photon purity ( (2) (0)) is enhanced for the emitter inside the irradiated area. Solid lines in Supplementary Fig.5c, d show the spectra of these two emitters. Dashed lines indicate the background emission measured just next to the emitters. The spectral weight of the background for the not irradiated region is sensibly greater than for the irradiated region. The emitter inside the FIB area ( Supplementary Fig.5c) provides a different example of spectral shape for the single photon emitters in layered hBN.
Background reduction due to He ion irradiation is observable also with Raman spectroscopy. This has been performed with low resolution grating, 100 lines mm -1 . Supplementary Fig.6 shows the spectra for the pristine (black line) and irradiated (blue line) hBN. Both spectra show a peak around 1000 cm -1 attributed to the SiO 2 substrate. Overall, the not irradiated regions have stronger broad band emission than the irradiated regions.
Spectroscopy on single photon emitters has been performed also at cryogenic temperature. A comparison of the spectra of the same emitter for T = 300 K and T = 10 K is shown in Supplementary Fig.7 with black and red colors, respectively. The minimum linewidth of the ZPL emission measured at cryogenic temperature is ~0.7 nm.
Second-order correlation function is measured for different pump powers yielding good antibunching with (2) (0) < 0.5 up to 6×10 2 counts s -1 . Supplementary Fig.8  The values of the autocorrelation functions at zero time delay are reported in Supplementary Fig.3c.
Measurements of autocorrelation function with CW excitation yield slightly higher values of (2) (0) than pulsed excitation. This mismatch is caused by the APD jitter that becomes negligible with short pulsed excitation.
Supplementary Fig.9 shows the saturation curve of the emitter reported in Fig.3  Dotted circles highlight a single photon emitter whose spectrum is shown in Supplementary Fig.11e, f.
Background is slightly increased after the transfer due to the fluorescence of the PMMA film and the PC substrate, but it is still negligible when compared to the peak intensity of the emitter.
After the transfer, the PC substrate is mounted in the experiment setup as shown in Supplementary Fig.12.
One side is clamped tightly and the other side is pushed downwards or pulled upwards to induce tensile or compressive strain, respectively. The amount of strain on the flake is calculated by the Euler-Bernoulli beam equation, which relates the beam deflection z(x) at position x to the applied load q(x): where E is Young's modulus and I is the second moment of area. In our rectangle-shaped substrate,  Supplementary Fig.13 shows another example of the effect of strain on the spectral emission. This emitter has a ZPL around 2.1 eV and blueshifts as the strain intensity is increased. This emitter has a tunability of +3.3 meV/%.
Simulations with various conditions of strain are shown in Supplementary Fig.13. The energy shift of the ZPL for different strain directions produce distinct behaviors, both in direction and magnitude of the energy shift. In particular, strain along ZZ1 and AC1 results a quadratic shift. Therefore, this simulation confirms our assumption about the non-monotonic behavior of the ZPL energy under the effect of strain. On the other hand, strain along ZZ2 and AC2 produces an almost linear shift. Biaxial strain is also investigated with homogeneous strain applied along the orthogonal directions AC2 and ZZ1. Biaxial strain also produces a quadratic energy shift. The effect of the high Poisson's ratio of the bendable substrate used for the experiments is taken into account and the simulation results in the curves labeled with ZZ1-P and