Temporal coherence of single photons emitted by hexagonal Boron Nitride defects at room temperature

TL;DR

The study probes whether room-temperature hBN defect centers can serve as practical single-photon sources with usable temporal coherence. Using Michelson interferometry alongside spectral and lifetime measurements, it extracts the pure dephasing time $T_2^*$ and analyzes the radiative lifetime $T_1$, the Debye-Waller factor, and spectral features of the ZPL around $1.746$ eV. The results show $T_1 = 2.54 \pm 0.04$ ns and $T_2^*\approx 382 \pm 11$ fs for the ZPL (and $T_2^*\approx 68 \pm 4$ fs for the full spectrum), indicating phonon-induced dephasing dominates ($\gamma^* \gg \gamma$) at room temperature and severely limits photon indistinguishability. Consequently, while RT hBN defects offer bright single-photon emission, achieving interference-based quantum photonics will require cryogenic temperatures or integration with optical cavities to suppress dephasing and enhance coherence for practical quantum protocols.

Abstract

Color centers in hexagonal boron nitride (hBN) emerge as promising quantum light sources at room temperature, with potential applications in quantum communications, among others. The temporal coherence of emitted photons (i.e. their capacity to interfere and distribute photonic entanglement) is essential for many of these applications. Hence, it is crucial to study and determine the temporal coherence of this emission under different experimental conditions. In this work, we report the coherence time of the single photons emitted by an hBN defect in a nanocrystal at room temperature, measured via Michelson interferometry. The visibility of this interference vanishes when the temporal delay between the interferometer arms is a few hundred femtoseconds, highlighting that the phonon dephasing processes are four orders of magnitude faster than the spontaneous decay time of the emitter. We also analyze the single photon characteristics of the emission via correlation measurements, defect blinking dynamics, and its Debye-Waller factor. Our room temperature results highlight the presence of a strong phonon-electron coupling, suggesting the need to work at cryogenic temperatures to enable quantum photonic applications based on photon interference.

Figures (4)

• Figure 1: Spectral and temporal characterization of the hBN emitter at room temperature. (a) PL spectrum in log-scale under CW, non-resonant (450 nm) laser excitation, and $1.85 P^{\text{cw}}_\textrm{Sat}$ excitation power. The ZPL (black Lorentian fit) is located at $1.747$ eV, while the rest of the emission (red Lorentzian peaks) comes from the PSB. The red arrows indicate the hBN defect spectrum FWHM. The vertical dashed lines indicate the filtered spectrum of the ZPL (grey) and the full spectrum (turquoise low-energy band pass) subsequently analyzed in the Michelson interferometer. (b) ZPL pump power dependence, recording the intensity with a single-photon detector, $P^{\text{cw}}_\textrm{Sat}= 0.54$ mW. (c) Spontaneous decay of the emitter showing a mono-exponential decay $T_1 = 2.54 \pm 0.04$ ns, measured under a pump power of $1.2 P^{p}_\textrm{Sat}$. The instrument response function of the detector is included in a gray-shaded area.
• Figure 2: Single photon character and blinking of the hBN emission. (a) Pulsed second-order correlation function under low pump power excitation, $1.2 P^{p}_\textrm{Sat}$ laser power and 40 MHz repetition rate. The measured antibunching is $g^{(2)}(0) = 0.11\pm 0.01$ (this result does not account for the two-detector jitter). The horizontal, dashed line marks the average height of uncorrelated peaks at long delays. (b) CW second-order correlation for different pumping powers. Similarly to panel (a), the histogram normalization is done with the uncorrelated coincidence peaks at long-delays. The correlation curves are vertically displaced for clarity (the horizontal black lines at 10 and 20 normalized coincidence levels mark the correlation baseline for the medium and high driving powers). The bunching times $\tau_2$ are specified in the left side of the panel.
• Figure 3: Temporal coherence via Michelson interference. (a) Fringe visibility in the output mode of the Michelson interferometer as a function of the temporal delay between the two arms for the filtered ZPL (dark blue trace) and full spectrum (turquoise). The portion of the spectrum used for each data set is indicated in Fig. \ref{['fig:Characterisation']} (a). Panels (b), (c) and (d) show the normalized intensity oscillations as a function of the piezo-tuned fine delay.
• Figure 4: Temporal coherence under different excitation energies. Left panels (a, c, e) show the PL spectra of three different emitters (that we call emitter I, II and III respectively) driven with two excitation energies for each case. Right panels (b, d, f) display the corresponding visibility decay for the Michelson interference performed for the defects and excitation lasers on the left. Blue, green and red colors on the figure correspond with excitation energies of 2.755, 2.330 and 1.907 eV respectively. All the $T_2^*$ have been calculated from the exponential fits shown in the figures. Spectra and visibility curves have been extracted with CW excitation and power of 1 mW.