Resonance fluorescence and indistinguishable photons from a coherently driven B centre in hBN

TL;DR

This work demonstrates resonance fluorescence from coherently driven B-centres in hBN by integrating emitters into a hybrid metal-dielectric structure that enables crossed-polarisation laser rejection. By combining cw and pulsed resonant excitation with high-resolution spectral filtering, the authors observe the Mollow triplet and Hong–Ou–Mandel interference for ZPL photons, achieving post-selected HOM visibilities around $V_{ ext{HOM}} olinebreak = 0.93 \,\pm\, 0.21$ (SPE$_1$) and $0.92 \,\pm\, 0.26$ (SPE$_2$). The results establish near-ideal single-photon purity under pulsed resonant excitation and demonstrate high photon indistinguishability from B-centres in hBN, highlighting their potential for integrated quantum photonics. The study also discusses practical routes to improve signal-to-noise and rate, including spectral-diffusion suppression and Purcell-enhanced photonic structures, to scale up on-chip indistinguishable-photon sources based on identical emitters. $V_{ ext{HOM}}$ values and $g^{(2)}(0)$ metrics quantify the coherence and purity achievable with these emitters, signaling their viability for quantum information processing in two-dimensional materials.

Abstract

Optically active defects in hexagonal boron nitride (hBN) have become amongst the most attractive single-photon emitters in the solid state, owing to their high-quality photophysical properties, combined with the unlimited possibilities of integration offered by the host two-dimensional material. In particular, the B centres, with their narrow linewidth, low wavelength spread and controllable positioning, have raised a particular interest for integrated quantum photonics. However, to date, either their excitation or their detection has been performed non-resonantly due to the difficulty of rejecting the backreflected laser light at the same wavelength, thereby preventing to take full benefit from their high coherence in quantum protocols. Here, we make use of a narrow-linewidth emitter integrated in a hybrid metal-dielectric structure to implement crossed-polarisation laser rejection. This allows us to observe resonantly scattered photons, with associated experimental signatures of optical coherence in both continuous-wave (cw) and pulsed regimes, respectively the Mollow triplet and Hong-Ou-Mandel interference from zero-phonon-line emission. The measured two-photon interference visibility of 0.93 +/- 0.21 and 0.92 +/- 0.26 we measured for two emitters demonstrate the potential of B centres in hBN for applications to integrated quantum information.

Figures (9)

• Figure 1: Experimental setup and PLE characterisation of the emitter. (a) Experimental setup. (b) Dots: low-power (40 nW) laser scan to infer the emitter inhomogeneous linewidth. Plain line: Gaussian fit to the data. (c) Dots: medium-power (0.5 $\mu$W) $g^{(2)}(\tau)$ revealing Rabi oscillations. Plain line: fit to the data. (d) Dots: count rate as a function of the square root of the laser power in pulsed regime. Plain line: damped sine fit to the data. (e) Blue curve: $g^{(2)}(\tau)$ in pulsed regime. Dotted line: multi-peak fit to the data.
• Figure 2: Photon statistics of the ZPL signal. (a) Intensity time traces in the ZPL (blue) and PSB (turquoise) detection arms. (b) Blue dots: ZPL count rate as a function of the PSB count rate in the same time bin. Red line: linear fit to the data. (c) Extracted emitter count rate (blue dots) and laser background (orange dots) as a function of the laser power. (d) Dots: $g^{(2)}(\tau)$ measured in the ZPL arm at four different powers, demonstrating Rabi oscillations. Plain lines: fit to the data.
• Figure 3: Observation of the Mollow triplet. (a) Dots: count rate as a function of the detuning of the scanning FP filter at height different powers. Plain line: fit to the data. (b) Circles: Rabi frequency extracted from the Mollow triplet splitting. Plain line: fit to the data. Stars: Rabi frequency extracted from the ZPL $g^{(2)}$ oscillations.
• Figure 4: Two-photon interference in the resonant pulsed regime. (a) Blue line: histogram of the photon detections in the pulsed regime. Light from the laser is visible as the initial short peak, followed by a slower decay. The red line is a fit to the data, providing the relaxation time $T_1 = 1.70$ ns. Blue shading: events falling into the post-selection time window. Orange shading: unsuppressed laser, normalized to the same peak value. (b) Green dots: two-photon coincidences measured in the parallel configuration. Orange dots: coincidences measured in the orthogonal configuration. Plain lines: multi-peak fits to the data.
• Figure S1: PL spectrum of SPE$_1$.