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Two photon interference using a single lead-vacancy center in diamond

E. Ota, K. Hirai, Y.Q.Chen, P. Wang, M. Miyakawa, T. Taniguchi, Onoda Shinobu, T. Makino, M. Hatano, T. Iwasaki

Published Mar 17, 2026• 0 citations

Abstract

The realization of quantum networks is based on the entanglement of stationary qubits via flying photons. Therefore, a fundamental requirement is a single photon source that can efficiently generate indistinguishable photons for the entanglement generation. The group-IV vacancy centers in diamond have emerged as promising candidates due to their spectral stability benefiting from inversion symmetry and long spin coherence time. In particular, the lead-vacancy (PbV) center exhibits fourier-limited linewidths even above 10 K and is predicted to have millisecond-scale spin coherence time near 9 K, making it suitable for scalable quantum networks. Here, we report the indistinguishability of photons emitted from a single PbV center in diamond through a two-photon interference experiment, known as the Hong-Ou-Mandel (HOM) effect. Fluorescence from the PbV center was split into two paths and recombined at a HOM interferometer. Photon correlations were measured for parallel and orthogonal polarized schemes. In parallel polarization, a clear interference dip below 0.5 at zero time delay was observed, whereas for orthogonal polarization, the correlation function exceeded 0.5. These results confirm the generation of indistinguishable photons from a single PbV center, validating their potential as the solid-state quantum emitter. This work paves the way for the PbV-based application for quantum information processing.

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A negatively charged lead-vacancy (PbV-) center in diamond is a novel quantum system which can be operated at a high temperature above 4 K owing to its large ground state splitting. To unlock the quantum properties of the PbV- center, the enhancement of the fluorescence intensity is a key issue. Here, we demonstrate enhanced emission from PbV- centers by a factor of approximately 10 using solid immersion lenses (SILs) fabricated on rough diamond surface caused by high-temperature anneal over 2000°C. Resonant excitation reveals the narrow emission close to the transform-limited linewidth. Furthermore, we demonstrate two-photon interference using one single PbV- center in an SIL. This enhanced emission and indistinguishability will lead to further development of the PbV- center towards quantum network node applications.

Quantum networks promise secure communication, distributed sensing and modular quantum computing by interconnecting distant quantum nodes through photonic links. Extending such networks beyond metropolitan distances requires quantum repeaters to overcome the exponential attenuation of photons in optical fiber. Across all architectures, a key requirement is the indistinguishability of single photons, which directly impacts the fidelity of photonic operations based on two-photon interference, such as Bell-state measurements and fusion gates. Here, we demonstrate generation of highly indistinguishable single photons from a coherently excited tin-vacancy center in diamond, achieving raw Hong-Ou-Mandel interference visibilities exceeding 0.95. By separating intrinsic emitter properties from technical imperfections, we show that decoherence plays a negligible role and that the remaining limitations are predominantly technical in nature, arriving at an intrinsic indistinguishability of up to 0.999. We further show that quantum frequency conversion to the telecom C-band preserves the photon indistinguishability. In combination with the long-lived electron and nuclear spin coherence times, these results establish tin-vacancy centers in diamond as a competitive platform for long-distance quantum networks and photonic quantum information processing. We further substantiate this potential through Monte Carlo simulations of a quantum-repeater link, demonstrating that the SnV-center platform surpasses the bound set by direct transmission.

Resonance fluorescence from a quantum emitter is an ideal source to extract indistinguishable photons. By using the cross-polarization to suppress the laser scattering, we observed resonance fluorescence from GeV color centers in diamond at cryogenic temperature. The Fourier-transform-limited line width emission with T 2 /2 T 1 ∼ 0.86 allows for two-photon interference based on single GeV color center. Under pulsed excitation, the separated photons exhibit a Hong–Ou–Mandel quantum interference above classical limit, whereas the continuous-wave excitation leads to a coalescence time window of 1.05 radiative lifetime. In addition, we demonstrated a single-shot readout of spin states with a fidelity of 74%. Our experiments lay down the foundation for building a quantum network with GeV color centers in diamond.

The generation of indistinguishable photons is a key requirement for solid-state quantum emitters as a viable source for applications in quantum technologies. Restricting the dimensions of the solid-state host to a size well below the wavelength of light emitted by a defect-center enables efficient external optical coupling, for example, for hybrid integration into photonic devices. However, stringent restrictions on the host dimensions result in severe limitations on the spectral properties reducing the indistinguishability of emitted photons. Here, we demonstrate two-photon interference from two negatively charged silicon-vacancy centers located in remote nanodiamonds. The Hong-Ou-Mandel interference efficiency reaches 61 % with a coalescence time window of 0.35 ns. We furthermore show a high yield of pairs of silicon-vacancy centers with indistinguishable optical transitions. Therefore, our work opens new paths in hybrid quantum technology based on indistinguishable single-photon emitters in nanodiamonds.