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Optica

Rabi oscillations and resonance fluorescence from a single hexagonal boron nitride quantum emitter

Kumarasiri Konthasinghe, Chitraleema Chakraborty, Nikhil Mathur, Liangyu Qiu, Arunabh Mukherjee, Gregory D. Fuchs, A. Nick Vamivakas

Published Apr 25, 2019• 76 citations
Publisher

Abstract

Hexagonal boron nitride (hBN), a wide-bandgap 2D material, is rapidly emerging as a promising candidate for quantum optics experiments. In this work, we demonstrate, to the best of our knowledge, the first signature of Rabi oscillations, the time-domain analogue of the Mollow triplet, from a resonantly driven hBN quantum emitter. Resonant photoluminescence excitation measurements reveal that the emitter undergoes strong spectral diffusion with a time scale of 37±25  ms, resulting in a 0.6 GHz spectral diffusion broadened linewidth at the weak excitation limit. We further realize resonance fluorescence from the same emitter using a cross-polarized setup. The results shown here present an important step toward utilizing coherent optical control in 2D materials for the realization of scalable quantum information processing.

DOI
10.1364/optica.6.000542

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Hexagonal boron nitride (hBN), a wide-bandgap 2D material, is rapidly emerging as a promising candidate for quantum optics experiments. In this work, we demonstrate, to the best of our knowledge, the first signature of Rabi oscillations, the time-domain analogue of the Mollow triplet, from a resonantly driven hBN quantum emitter. Resonant photoluminescence excitation measurements reveal that the emitter undergoes strong spectral diffusion with a time scale of 37±25  ms, resulting in a 0.6 GHz spectral diffusion broadened linewidth at the weak excitation limit. We further realize resonance fluorescence from the same emitter using a cross-polarized setup. The results shown here present an important step toward utilizing coherent optical control in 2D materials for the realization of scalable quantum information processing.
Single-photon emitters in the solid state are subject to spectral broadening processes known as spectral diffusion and pure dephasing. Here, the authors show that a combination of quantum optics techniques, namely coherent laser drive and time-resolved photon correlations, can bring both processes to light in a single experimental protocol. This allows an extensive characterization of a large variety of solid-state quantum emitters. Their method is experimentally applied to a quantum emitter in the 2D material hexagonal boron nitride, further establishing this system as an appealing quantum photonics platform.
The potential of solid-state quantum emitters for applications critically depends on several key figures of merit. One of the most important is the quantum coherence of the emitted single photons, which can be compromised by fast dephasing and spectral diffusion. In hexagonal boron nitride (hBN), blue-emitting color centers (or B centers) are seen as favorable in this regard, in the light of prior studies mainly based on resonant excitation. Yet, their coherence properties in the more accessible regime of non-resonant excitation (or photoluminescence) has not been extensively characterized. Here, we investigate the coherence and spectral diffusion of the photoluminescence from a B center in the continuous wave regime using photon correlation Fourier spectroscopy. We determine that the emission lineshape consists in a homogeneous contribution, whose linewidth increases with the laser power, and which is broadened by spectral diffusion at a timescale of 10 to 100 microseconds. At low power and short time, the emission line is only a factor ~2 above the Fourier limit, while at long times, the inhomogeneous linewidth increases up to more than a gigahertz. Our work deepens the understanding of decoherence processes of this preeminent family of quantum emitters in hBN.
Defects in solid-state systems constitute a promising platform for the realization of deterministic quantum emitters. Among many candidate materials and emitters, point defects in hexagonal Boron Nitride (hBN) have recently emerged as particularly promising. In this work, we probe the coherence of an individual B center with a zero phonon line at 436 nm, under pulsed resonant excitation. We observe power-dependent Rabi oscillations up to 5π, demonstrating optical coherent control of the transition. We achieve an excellent single photon purity of 93% at π-pulse. Furthermore, we probe the coherence of the two-level system using Ramsey interferometry, revealing an inhomogeneous coherence time of T_2*=0.60 ns. These results establish B centers in hBN as viable candidates for triggered, coherent quantum emitters and represent an important step towards their integration into quantum photonic platforms.
Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional host for stable single-photon emission owing to its wide bandgap, high photostability, and compatibility with nanophotonic integration. We present a simulation-based study of temperature-dependent spectral dynamics and optical coherence in a mechanically decoupled quantum emitter in hBN. Employing a hybrid stochastic framework that combines Ornstein--Uhlenbeck detuning fluctuations with temperature-dependent, Gaussian-distributed discrete frequency jumps, motivated by experimentally observed spectral diffusion and blinking, we reproduce the measured evolution of inhomogeneous linewidth broadening and the progressive degradation of photon coherence across the relevant cryogenic range (5-30K). The model captures phonon-related spectral diffusion with a cubic temperature dependence and the onset of jump-like spectral instabilities at higher temperatures. By calibrating the hybrid diffusion, jump parameters to the experimentally measured full width at half maximum (FWHM) of the emission line and analyzing the second-order correlation function g(2)(τ)g^{(2)}(τ)g(2)(τ) under resonant driving, we establish a unified phenomenological description that links stochastic detuning dynamics to the decay of optical coherence in a resonantly driven emitter. Analysis of g(2)(τ)g^{(2)}(τ)g(2)(τ) under resonant driving reveals an additional dephasing rate γsd+jγ_{\mathrm{sd+j}}γsd+j​ that rises monotonically with temperature and drive strength, leading to a predicted critical crossover to overdamped dynamics at Tcrit≈25.91T_{\mathrm{crit}} \approx 25.91Tcrit​≈25.91~K. This hybrid framework provides a quantitative connection between accessible spectroscopic observables and the dominant noise mechanisms limiting coherent optical control in mechanically decoupled quantum emitters, exemplified in hBN and generalizable to similar emitters in other materials.
We experimentally investigate a solid-state quantum emitter---a B center in hexagonal boron nitride (hBN)---that has lifetime-limited coherence at short times, and experiences inhomogeneous broadening due to spectral diffusion at longer times. By making use of power broadening in resonant laser excitation, we explore the crossover between the inhomogeneous and the homogeneous broadening regimes. With the support of numerical simulations, we show that the line shape, count rate, second-order correlations, and long-time photon statistics evolve from a regime where they are dictated by spectral diffusion to a regime where they are simply given by the homogeneous response of the emitter, yielding restored Lorentzian shape and Poissonian photon statistics. Saturation of the count rate and line broadening occur not at the onset of the Rabi oscillations, but when the power-broadened homogeneous response becomes comparable with the inhomogeneous linewidth. Moreover, we identify specific signatures in both the second-order correlations and long-time photon statistics that are well explained by a microscopic spectral diffusion model based on discrete jumps at timescales of micro- to milliseconds. Our work provides an extensive description of the photophysics of B centers under resonant excitation, and can be readily extended to a wide variety of solid-state quantum emitters.
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 van der Waals 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 narrow-linewidth emitters integrated in a hybrid metal-dielectric structure to implement cross-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 two-photon interference visibilities of 0.93−0.21+0.070.9{3}_{-0.21}^{+0.07}0.93−0.21+0.07​ and 0.92−0.26+0.080.9{2}_{-0.26}^{+0.08}0.92−0.26+0.08​ we measured for two emitters demonstrate the potential of B centres in hBN for applications to integrated quantum information. Single-photon sources in van der Waals materials offer new perspectives of integration for quantum information applications. Here, the authors implement resonance fluorescence of a single colour centre in hBN, a quantum optics regime that simultaneously allows for emitter coherent control and generation of indistinguishable photons.