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Polarization-entangled photon-pair source with van der Waals 3R-WS2 crystal

Jiangang Feng, Yunkun Wu, Ruihuan Duan, Jun Wang, Weijin Chen, Jiazhang Qin, Zheng Liu, Guang-Can Guo, Xi‐Feng Ren, Cheng‐Wei Qiu

Published Aug 23, 2024• 44 citations
Publisher

Abstract

Abstract Ultracompact entangled photon sources are pivotal to miniaturized quantum photonic devices. Van der Waals (vdW) nonlinear crystals promise efficient photon-pair generation and on-chip monolithic integration with nanophotonic circuitry. However, it remains challenging to generate maximally entangled Bell states of photon pairs with high purity, generation rate, and fidelity required for practical applications. Here, we realize a polarization-entangled photon-pair source based on spontaneous parametric down conversion in an ultrathin rhombohedral tungsten disulfide (3R-WS 2 ) crystal. This vdW entangled photonic source exhibits a high photon-pair purity with a coincidence-to-accidental ratio of above 800, a generation rate of 31 Hz, and two maximally polarization-entangled Bell states with fidelities exceeding 0.93 and entanglement degree over 0.97. These results stem from scalable optical nonlinearity, enhanced second-order susceptibility by electronic transitions, and a well-defined symmetry-enabled selection rule inherent in 3R-WS 2 . Our polarization entangled photon source can be integrated with photonic structures for generating more complex entangled states, thus paving an avenue for advanced quantum photonic systems toward computation and metrology.

DOI
10.1186/s43593-024-00074-6

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Abstract Ultracompact entangled photon sources are pivotal to miniaturized quantum photonic devices. Van der Waals (vdW) nonlinear crystals promise efficient photon-pair generation and on-chip monolithic integration with nanophotonic circuitry. However, it remains challenging to generate maximally entangled Bell states of photon pairs with high purity, generation rate, and fidelity required for practical applications. Here, we realize a polarization-entangled photon-pair source based on spontaneous parametric down conversion in an ultrathin rhombohedral tungsten disulfide (3R-WS 2 ) crystal. This vdW entangled photonic source exhibits a high photon-pair purity with a coincidence-to-accidental ratio of above 800, a generation rate of 31 Hz, and two maximally polarization-entangled Bell states with fidelities exceeding 0.93 and entanglement degree over 0.97. These results stem from scalable optical nonlinearity, enhanced second-order susceptibility by electronic transitions, and a well-defined symmetry-enabled selection rule inherent in 3R-WS 2 . Our polarization entangled photon source can be integrated with photonic structures for generating more complex entangled states, thus paving an avenue for advanced quantum photonic systems toward computation and metrology.
Entangled photon-pair sources are pivotal in various quantum applications. Miniaturizing the quantum devices to meet the requirement in limited space applications drives the search for ultracompact entangled photon-pair sources. The rise of two-dimensional (2D) semiconductors has been demonstrated as ultracompact entangled photon-pair sources. However, the photon-pair generation rate and purity are relatively low, and the strong photoluminescence in 2D semiconductors also makes the operational wavelength range limited. Here, we use the spontaneous parametric down conversion (SPDC) of rhombohedral boron nitride (rBN) as a polarization entangled photon-pair source. We have achieved a generation rate of more than 120 hertz (a record-high SPDC coincidence rate with 2D materials) and a high-purity photon-pair generation with a coincidence-to-accidental ratio of above 200. Tunable Bell state generation is also demonstrated by simply rotating the pump polarization, with a fidelity up to 0.93. Our results suggest rBN as an ideal candidate for on-chip integrated quantum devices.
We present a tunable, single-mode-optical-fiber-based source of polarization entangled photon pairs for the near-infrared telecommunication band that is deployable in standard infrastructure. The photon pairs are generated via spontaneous parametric down-conversion (SPDC) in a submicron-scale thin film of the inversion-broken rhombohedral polytype of the transition metal dichalcogenide molybdenum disulfide (3R-MoS2_22​), located between two fiber connectors. By exploiting the intrinsic symmetries of the second-order nonlinear susceptibility tensor of 3R-MoS2_22​, this hybrid approach offers control over the generated two-photon polarization state through the incident pump polarization. Most notably, two of the four maximally entangled Bell states, as well as fully co-polarized pairs can be produced. This represents a substantial improvement in terms of tunability and simplicity over established fiber-integrated sources, which require additional optical elements, precise alignment, or careful engineering of design parameters. Additionally, a nonlinear drop in the background photoluminescence signal of 3R-MoS2_22​ is observed at low pump powers, allowing us to reach a coincidences-to-accidentals ratio (CAR) of (8.3±1.8)×103(8.3\pm1.8)\times10^{3}(8.3±1.8)×103, the highest value recorded for SPDC in van der Waals materials to date.
Bright entangled photon sources with on-demand quantum state control are essential for advancing quantum science and technologies. However, simultaneously achieving high brightness and state tunability remains challenging, as this necessitates a dual-phase control mechanism that facilitates both longitudinal phase matching and transverse polarization modulation. Here, we report a twin-twisted two-dimensional (2D) van der Waals (vdW) crystal architecture that enables spontaneous parametric down-conversion (SPDC) with both high generation efficiency and continuously tunable quantum states. The designed crystal comprises two sections of twisted 2D GaSe vdW flakes with opposite chiral handedness, in which interflake twist angle induces phase matching in SPDC, and intersection twisting provides an additional degree of freedom on tunability. We demonstrate that the twin-twisted GaSe crystals achieve a record-high photon-pair generation rate exceeding 13 200 Hz and a Bell state fidelity above 99% within 2D material family, and enable unprecedented tunability of quantum states from the maximally polarization entangled states to the fully separable states. This study opens a practical avenue toward twist-engineered quantum states, and paves the way for developing compact quantum photonic systems with tailored properties.
Nanoscale quantum light sources are essential building blocks for integrated quantum photonic systems. Here, we report a wavelength-scale entangled-photon source based on van der Waals-engineered NbOBr2 and benchmark its performance for telecom-wavelength quantum light generation. By exploiting the material's second-order nonlinearity, we generate quantum-correlated photon pairs via spontaneous parametric down-conversion. We then use a 90° twisted stacking to induce quantum interference in photon-pair generation, yielding polarization-entangled photons. This approach enables tunability of the quantum optical state via control of the excitation laser polarization. We experimentally obtain entanglement fidelities exceeding 95% for Bell states, along with a high coincidence-to-accidental ratio of ∼335 and a brightness approximately 1 order of magnitude higher than recently reported telecom sources based on transition metal dichalcogenide two-dimensional materials. These results establish twisted van der Waals engineering as a powerful platform for highly tunable, high-brightness quantum light sources at telecom wavelengths.
Nanoscale quantum light sources are essential building blocks for integrated quantum photonic systems. Here, we report a wavelength-scale entangled-photon source based on van der Waals-engineered NbOBr2_22​, and benchmark its performance for telecom-wavelength quantum light generation. By exploiting the material's second-order nonlinearity, we generate quantum-correlated photon pairs via spontaneous parametric down-conversion. We then use a 90∘^{\circ}∘ twisted stacking to induce quantum interference in photon-pair generation, yielding polarization-entangled photons. This approach enables tunability of the quantum optical state via control of the excitation laser polarization. We experimentally obtain entanglement fidelities exceeding 95% for Bell states, along with a high coincidence-to-accidental ratio of $\sim$335, and a brightness approximately one order of magnitude higher than recently reported telecom sources based on transition metal dichalcogenide (TMD) 2D materials. These results establish twisted van der Waals engineering as a powerful platform for highly tunable, high-brightness quantum light sources at telecom wavelengths.