Photon correlation Fourier spectroscopy of a B center in hBN

TL;DR

This work uses photon-correlation Fourier spectroscopy (PCFS) to characterize the coherence of photoluminescence from a blue-emitting B center in hBN under non-resonant excitation. The authors separate two broadening mechanisms: a power-dependent homogeneous dephasing and a time-dependent inhomogeneous broadening from spectral diffusion, with diffusion timescales around 10 μs (and slower components at longer times). By fitting PCFS data with a Voigt model and comparing to Fourier-transform spectroscopy (FTS) results, they show that at low power the emission approaches the Fourier limit for short delays, while the linewidth broadens to beyond 1 GHz at longer delays due to SD. The analysis links microscopic SD processes to observable spectral features via the time-dependent effective spectrum S_eff(ω, τ), and demonstrates that both continuous diffusion (Ornstein-Uhlenbeck) and discrete jump models can explain the data, informing strategies to improve indistinguishability, including cavity-enhanced Purcell factors for long-delay photon interference.

Abstract

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.

Figures (9)

• Figure 1: (a) Experimental setup. A cw laser diode excites the sample, which is inserted in a cryostat. The collected light is channeled to a Mach-Zehnder interferometer with a variable arm. Light in the output arms is detected by APDs. DM: dichroic mirror; LCR: liquid crystal retarder; FPC: fiber polarization controler; TCSPC: time-correlated single photon counter. (b) PL spectrum measured using a grating spectrometer. (c) Time evolution of the PL emission during 180 s.
• Figure 2: (a) Dots: Visibility as a function of the optical path length difference. Solid lines: Voigt fit to the data. (b) Dots: 4-mW data. Blue (resp. green, resp. purple) plain line: Voigt (resp. exponential, resp. Gaussian) fit to the data. Dashed (resp dotted) line: Exponential (resp. Gaussian) component of the Voigt decay profile.
• Figure 3: (a) Homogeneous ($T_2$, blue dots) and inhomogeneous ($T_2^*$, orange dots) coherence times as a function of the laser power. (b) Homogeneous (blue dots), inhomogeneous (orange dots) and total (green dots) linewidth as a function of the laser power. The dashed gray line indicates the Fourier limit.
• Figure 4: Interferometric $g^{(2)}(\delta, \tau)$ measured at two different optical delays, $\delta = 0.1$ ns (a) and $\delta = 0.5$ ns. Blue (orange) curves: Parallel (orthogonal) polarization.
• Figure 5: Color dots: PCFS contrast $C(\delta, \tau)$ for three different values of $\tau$ measured at different laser powers: 0.3 mW (a), 1 mW (b), 2 mW (c) and 4 mW (d). The solid lines are fits to the data using Eq. \ref{['Voigt']}.