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

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.

💡 Research Summary

In this work the authors investigate the temporal coherence of single photons emitted by a defect in hexagonal boron nitride (hBN) nanocrystals at room temperature. Using a home‑built confocal microscope they excite the emitters non‑resonantly with 450 nm continuous‑wave (CW) and pulsed lasers, collect the fluorescence through a high‑numerical‑aperture objective onto a distributed Bragg reflector (DBR) substrate, and analyse the emission with spectroscopy, time‑resolved photoluminescence, second‑order correlation, and Michelson interferometry.

The photoluminescence spectrum shows a zero‑phonon line (ZPL) at 1.746 eV (≈710 nm) with a full width at half maximum of 5 meV. By fitting the ZPL together with the phonon sidebands the Debye‑Waller factor is found to be 0.77 ± 0.02, indicating that a large fraction of the emitted photons resides in the ZPL. The radiative lifetime extracted from a mono‑exponential decay is T₁ = 2.54 ± 0.04 ns, which would correspond to a Fourier‑limited linewidth of about 62 MHz – far narrower than the observed ZPL width, confirming strong electron‑phonon coupling at room temperature.

Photon‑statistics measurements confirm single‑photon emission. Under pulsed excitation the second‑order correlation at zero delay yields g²(0) = 0.11 ± 0.01, while under CW excitation g²(0) = 0.46 ± 0.13. The CW data also reveal pronounced blinking: the bunching time constant shortens from 2.28 µs at low excitation (0.07 P_sat) to 0.08 µs at high excitation (4.76 P_sat), and the antibunching time τ₁ decreases with increasing pump power, reflecting a three‑level system with a dark state that modulates the brightness.

The central part of the study is the measurement of temporal coherence via a Michelson interferometer. By varying the path‑length difference up to tens of picoseconds (fine control of ~133 fs) the interference visibility V(τ) is recorded for both the full emission spectrum and the spectrally filtered ZPL. Fitting V(τ) with a Gaussian decay yields pure dephasing times T₂* of 382 ± 11 fs for the full spectrum and 68 ± 4 fs for the ZPL. These values are four orders of magnitude shorter than the radiative lifetime, indicating that phonon‑induced pure dephasing dominates the loss of coherence at room temperature. The authors also test three excitation wavelengths (450 nm, 532 nm, 640 nm) and find essentially no dependence of T₂* on the pump photon energy, suggesting that the dephasing mechanism is intrinsic to the defect‑phonon bath rather than driven by resonant absorption.

The authors discuss the implications of these findings for quantum‑photonic applications. Although hBN defects provide high brightness, a sizable Debye‑Waller factor, and nanosecond‑scale emission rates, the ultrafast dephasing (sub‑picosecond) precludes two‑photon interference, Hong‑Ou‑Mandel experiments, and the generation of indistinguishable photons at room temperature. To overcome this limitation, operation at cryogenic temperatures is required to suppress the phonon population and extend T₂* into the tens‑of‑picoseconds regime, as reported in previous low‑temperature studies. Moreover, integrating the emitters into high‑Q cavities (e.g., DBR or Fabry‑Pérot structures) could enhance the spontaneous emission rate via the Purcell effect, thereby reducing the relative impact of pure dephasing.

In summary, the paper provides a comprehensive experimental characterization of a single hBN defect emitter at ambient conditions, quantifying its spectral properties, photon statistics, blinking dynamics, and, most importantly, its temporal coherence. The results demonstrate that while hBN defects are promising room‑temperature single‑photon sources, strong electron‑phonon coupling leads to ultrafast loss of phase coherence, making low‑temperature operation or cavity‑QED engineering essential for any quantum‑information protocol that relies on photon indistinguishability.