🔬 mesoscale physics

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

This study demonstrates that while hexagonal boron nitride defects emit single photons at room temperature, their temporal coherence is severely limited by rapid phonon dephasing processes, indicating that cryogenic temperatures are necessary for viable quantum photonic applications relying on photon interference.

Original authors: J. -V. Vidal Martínez-Pons, S. -K. Kim, M. Behrens, A. Izquierdo-Molina, A. Menendez Rua, S. Paçal, S. Ateş, L. Viña, C. Antón-Solanas

Published 2026-10-19

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CC BY 4.0

Original authors: J. -V. Vidal Martínez-Pons, S. -K. Kim, M. Behrens, A. Izquierdo-Molina, A. Menendez Rua, S. Paçal, S. Ateş, L. Viña, C. Antón-Solanas

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest to build a new kind of technology based on the strange rules of quantum physics, scientists are searching for reliable sources of single particles of light. These sources are essential for tasks like ultra-secure communication and powerful computing, where information is carried by individual photons rather than streams of electricity. For these systems to work, the photons must not only be emitted one by one, but they must also be identical to one another, capable of overlapping and interfering like ripples in a pond. This ability to stay in step, known as temporal coherence, is the key that unlocks the potential of quantum networks. While some materials can produce these perfect photons, they often require extreme cold to function, making them impractical for everyday use. Hexagonal boron nitride, a material as thin as a single sheet of paper, has emerged as a promising candidate because it can emit light at room temperature, offering a path toward quantum devices that do not need bulky refrigeration.

A team of researchers set out to test whether these room-temperature emitters could truly meet the strict demands of quantum interference. They focused on tiny defects within crystals of hexagonal boron nitride, which act as natural sources of single photons. The scientists wanted to measure how long the light from these defects remains coherent, or in step with itself, before the chaotic vibrations of the material's atoms scramble the signal. To do this, they used a classic tool of optics called a Michelson interferometer, which splits a beam of light into two paths and then recombines them. By carefully adjusting the length of one path, they could introduce a tiny delay between the two halves of the light beam. If the photons are perfectly coherent, they will interfere with each other to create a visible pattern of bright and dark bands. If the coherence is lost, this pattern disappears.

The experiments revealed a stark reality about the behavior of these emitters at room temperature. When the researchers introduced a delay of just a few hundred femtoseconds—a timescale so short it is a quadrillionth of a second—the interference pattern vanished completely. This rapid loss of coherence meant that the vibrations of the atoms within the crystal, known as phonons, were disrupting the light almost instantly. The study found that these disruptive processes happen four orders of magnitude faster than the time it takes for the defect to naturally emit a photon. In simpler terms, the light loses its ability to interfere with itself long before the atom has even finished the job of releasing it. The researchers also examined how the color of the laser used to excite the defects affected this behavior. They found that while changing the laser color altered the brightness and the specific mix of light emitted, it did not significantly slow down the rapid loss of coherence caused by the heat and vibrations of the room-temperature environment.

The team also confirmed that these defects are indeed single-photon sources, meaning they release light particles one at a time, which is a fundamental requirement for quantum applications. However, the speed at which the coherence is lost suggests that the strong interaction between the electron and the vibrating crystal lattice is the dominant factor. This interaction is so powerful at room temperature that it effectively prevents the use of these specific emitters for protocols that rely on photon interference, such as creating entangled pairs of particles. The researchers noted that while the defects are bright and stable enough for certain types of quantum communication that do not require interference, the dream of using them for more advanced quantum computing tasks at room temperature remains out of reach.

Ultimately, the study concludes that to harness the full potential of these hexagonal boron nitride defects, the environment must be cooled down significantly. The rapid dephasing observed is a direct result of the thermal energy present at room temperature, which agitates the crystal lattice. The authors suggest that future work should focus on placing these defects in optical cavities at cryogenic temperatures, where the vibrations are frozen out, allowing the photons to maintain their coherence long enough to be useful for complex quantum operations. For now, the findings serve as a clear boundary marker: while hexagonal boron nitride is a fascinating material for room-temperature light emission, its current form is not yet a solution for the most demanding interference-based quantum technologies without the aid of extreme cold.

Technical Summary: Temporal Coherence of Single Photons Emitted by Hexagonal Boron Nitride Defects at Room Temperature

Problem Statement
Hexagonal boron nitride (hBN) defects are emerging as promising solid-state single-photon sources (SPS) for quantum technologies, including communications and computation. For these applications to be viable, the emitted photons must possess high temporal coherence, which determines their capacity to interfere and distribute entanglement. While state-of-the-art performance is often achieved with semiconductor quantum dots at cryogenic temperatures, there is a need to characterize hBN defects under room temperature conditions to assess their suitability for cryogenic-free applications. Specifically, the interplay between spontaneous decay and pure dephasing (driven by electron-phonon coupling) at room temperature remains a critical, yet under-characterized, factor limiting the indistinguishability of emitted photons.

Methodology
The authors investigated the emission properties of single defects within hBN nanocrystals at room temperature using a home-built confocal microscope. The experimental approach included:

Sample Preparation: Commercial hBN nanocrystals were drop-casted onto a distributed Bragg reflector (DBR) to enhance collection efficiency.
Spectral and Temporal Characterization: Photoluminescence (PL) spectra were recorded under non-resonant continuous wave (CW) and pulsed excitation (450 nm, 532 nm, and 640 nm lasers). The study analyzed the Zero Phonon Line (ZPL), phonon sidebands (PSB), and calculated the Debye-Waller (DW) factor.
Single-Photon Verification: Second-order correlation functions, $g^{(2)}(\tau)$ , were measured using Hanbury-Brown & Twiss setups to confirm single-photon emission and analyze blinking dynamics.
Temporal Coherence Measurement: The core of the study utilized Michelson interferometry to measure the first-order coherence function, $g^{(1)}(\tau)$ . By varying the temporal delay between interferometer arms (from femtoseconds to picoseconds), the authors determined the pure dephasing time ( $T_2^*$ ) and the total dephasing rate. Measurements were performed on both the filtered ZPL and the full emission spectrum.

Key Results

Emission Properties: The studied defect exhibited a ZPL at 1.746 eV with a full width at half maximum (FWHM) of 5 meV. The spontaneous decay lifetime ( $T_1$ ) was measured at $2.54 \pm 0.04$ ns. The source-to-detector efficiency was found to be low ( $\sim 0.005\% - 0.008\%$ ), and the emitter displayed significant blinking behavior, particularly under CW excitation.
Temporal Coherence: The Michelson interferometry revealed that the visibility of interference vanishes within a few hundred femtoseconds.
- For the filtered ZPL, the pure dephasing time was $T_2^* = 382 \pm 11$ fs.
- For the full spectrum, the coherence time was significantly shorter at $T_2^* = 68 \pm 4$ fs.
Dephasing Mechanisms: The pure dephasing rate is approximately four orders of magnitude faster than the spontaneous decay rate ( $\gamma^* \gg \gamma$ ). This indicates that phonon-induced dephasing is the dominant decoherence mechanism at room temperature, overwhelming other potential sources like spectral diffusion on these ultrafast timescales.
Excitation Energy Dependence: Experiments with different excitation wavelengths (450 nm, 532 nm, 640 nm) showed that while the DW factor and emission intensity varied, the coherence times remained in the femtosecond regime. Notably, for Emitter III, blue non-resonant driving resulted in a larger coherence time ( $90 \pm 4$ fs) compared to red laser driving ( $20 \pm 1$ fs), suggesting complex phonon-assisted excitation pathways or local photo-charge effects.
Indistinguishability: Based on the measured coherence times, the probability of emitting two consecutive, coherent single photons under saturation is estimated to be $\sim 0.015\%$ . For a Mach-Zehnder interferometer with a 25 ns delay, the two-photon interference probability is estimated at $\sim 0.0015\%$ .

Significance and Claims
The paper concludes that the strong electron-phonon coupling in hBN defects at room temperature severely limits the temporal coherence of emitted photons. The authors assert that these results highlight the necessity of operating at cryogenic temperatures to enable quantum photonic applications that rely on photon interference (such as entanglement distribution).

Consequently, the authors claim that room-temperature hBN emitters are currently constrained to quantum protocols that do not require photon interference, such as BB84, B92, and random number generation. The study provides a quantitative benchmark for the dephasing rates of hBN defects, contrasting them with other solid-state emitters (like NV centers in diamond) and emphasizing that while hBN is a viable source for certain quantum tasks, its utility for interference-based applications is currently restricted by thermal phonon interactions. Future work proposed by the authors involves studying these emitters at lower temperatures and coupling them to photonic cavities to mitigate dephasing and enhance coherence.

Technical Summary: Temporal Coherence of Single Photons Emitted by Hexagonal Boron Nitride Defects at Room Temperature

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