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Nature

Continuous-variable multipartite entanglement in an integrated microcomb

Xinyu Jia, Chonghao Zhai, Xuezhi Zhu, Chang You, Yunyun Cao, Xuguang Zhang, Yun Zheng, Zhaorong Fu, Jun Mao, Tianxiang Dai, Lin Chang, Xiaolong Su, Qihuang Gong, Jianwei Wang

Published Feb 19, 2025• 81 citations
Publisher

Abstract

The generation of large-scale entangled states is crucial for quantum technologies, such as quantum computation1, communication2 and metrology3. Integrated quantum photonics that enables on-chip encoding, processing and detection of quantum light states offers a promising platform for the generation and manipulation of large-scale entangled states4,5. Generating entanglement between qubits encoded in discrete variables within single photons is challenging, owing to the difficulty of making single photons interact on photonic chips6–11. Devices that operate with continuous variables are more promising, as they enable the deterministic generation and entanglement of qumodes, in which information is encoded in light quadratures. Demonstrations so far have been limited to entanglement between two qumodes12–20. Here we report the deterministic generation of a continuous-variable eight-mode entanglement on an integrated optical chip. The chip delivers a quantum microcomb that produces multimode squeezed-vacuum optical frequency combs below the threshold. We verify the inseparability of our eight-mode state and demonstrate supermode multipartite entanglement over hundreds of megahertz sideband frequencies through violation of the van Loock–Furusawa criteria. By measuring the full matrices of nullifier correlations with sufficiently low off-diagonal noises, we characterize multipartite entanglement structures, which are approximate to the expected cluster-type structures for finite squeezing. This work shows the potential of continuous-variable integrated photonic quantum devices for facilitating quantum computing, networking and sensing. Deterministic generation of a continuous-variable eight-mode entanglement on an integrated optical chip is reported.

DOI
10.1038/s41586-025-08602-1

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The generation of large-scale entangled states is crucial for quantum technologies, such as quantum computation1, communication2 and metrology3. Integrated quantum photonics that enables on-chip encoding, processing and detection of quantum light states offers a promising platform for the generation and manipulation of large-scale entangled states4,5. Generating entanglement between qubits encoded in discrete variables within single photons is challenging, owing to the difficulty of making single photons interact on photonic chips6–11. Devices that operate with continuous variables are more promising, as they enable the deterministic generation and entanglement of qumodes, in which information is encoded in light quadratures. Demonstrations so far have been limited to entanglement between two qumodes12–20. Here we report the deterministic generation of a continuous-variable eight-mode entanglement on an integrated optical chip. The chip delivers a quantum microcomb that produces multimode squeezed-vacuum optical frequency combs below the threshold. We verify the inseparability of our eight-mode state and demonstrate supermode multipartite entanglement over hundreds of megahertz sideband frequencies through violation of the van Loock–Furusawa criteria. By measuring the full matrices of nullifier correlations with sufficiently low off-diagonal noises, we characterize multipartite entanglement structures, which are approximate to the expected cluster-type structures for finite squeezing. This work shows the potential of continuous-variable integrated photonic quantum devices for facilitating quantum computing, networking and sensing. Deterministic generation of a continuous-variable eight-mode entanglement on an integrated optical chip is reported.
The optical microresonator-based frequency comb (microcomb) provides a versatile platform for nonlinear physics studies and has wide applications ranging from metrology to spectroscopy. The deterministic quantum regime is an unexplored aspect of microcombs, in which unconditional entanglements among hundreds of equidistant frequency modes can serve as critical ingredients to scalable universal quantum computing and quantum networking. Here, we demonstrate a deterministic quantum microcomb in a silica microresonator on a silicon chip. 40 continuous-variable quantum modes, in the form of 20 simultaneously two-mode squeezed comb pairs, are observed within 1 THz optical span at telecommunication wavelengths. A maximum raw squeezing of 1.6 dB is attained. A high-resolution spectroscopy measurement is developed to characterize the frequency equidistance of quantum microcombs. Our demonstration offers the possibility to leverage deterministically generated, frequency multiplexed quantum states and integrated photonics to open up new avenues in fields of spectroscopy, quantum metrology, and scalable, continuous-variable-based quantum information processing.
Continuous-variable (CV) quantum microcombs generated in high-Q microresonators provide compact, frequency-multiplexed sources of entangled modes for integrated quantum information processing. Although deterministic Kerr-induced two-mode squeezing has been demonstrated on chip, achieving uniform squeezing across a large number of mode pairs remains a central challenge. Here we establish the conditions required for spectrally uniform squeezing and experimentally realize a vacuum-state CV quantum microcomb by combining a microresonator with an engineered single-family mode structure and optimized pump conditions. The device generates 14 independent two-mode squeezed states over a 0.7 THz bandwidth, each exhibiting more than 4 dB of raw squeezing, with a maximum of 4.3 dB. This uniform, high-performance quantum resource represents a key step toward scalable, integrated CV quantum technologies operating beyond classical limits.
An optical frequency comb comprises a cluster of equally spaced, phase-locked spectral lines. Replacing these classical components with correlated quantum light gives rise to cluster quantum frequency combs, providing abundant quantum resources for measurement-based quantum computation, and multi-user quantum networks. We propose and generate cluster quantum microcombs within an on-chip optical microresonator driven by multi-frequency lasers. Through resonantly enhanced four-wave mixing processes, continuous-variable cluster states with 60 qumodes are deterministically created. The graph structures can be programmed into one- and two-dimensional lattices by adjusting the configurations of the pump lines, which are confirmed inseparable based on the measured covariance matrices. Our work demonstrates the largest-scale cluster states with unprecedented raw squeezing levels from a photonic chip, offering a compact and scalable platform for computational and communicational tasks with quantum advantages.
Squeezed light underpins quantum-enhanced sensing and continuous-variable quantum information processing, and integrated photonics offers a route to producing it at scale. Universal to these applications are squeezed-light generation and measurement. Importantly, quantum measurements serve not only as readout but also as active operations in quantum-state evolution. However, integrating squeezed-light generation and photodetection on the same photonic chip has remained challenging because they impose fundamentally conflicting material requirements: low optical loss to preserve quantum correlations, but efficient photon absorption for photodetection. Here, we demonstrate squeezed-light generation, routing, and balanced homodyne detection integrated on a single photonic chip through heterogeneous integration. A two-mode squeezed quantum microcomb comprising 34 quantum modes is measured with approximately 3 dB squeezing. Our work establishes a scalable architecture for fully integrated squeezed-light quantum photonic systems, unifying quantum-state generation, processing, and detection on a single chip.
Continuous-variable (CV) multipartite entangled states, particularly Greenberger–Horne–Zeilinger (GHZ) states, are an indispensable resource for quantum networks and quantum information processing. Conventional approaches to generating CV GHZ states rely on combining multiple independently prepared squeezed lights through complex interferometric networks, which impose significant challenges in scalability and stability. Here, we experimentally demonstrate a compact and resource-efficient scheme for generating a tripartite CV GHZ state from a single quantum frequency comb produced by a synchronously pumped optical parametric oscillator. By employing multipixel homodyne detection, we simultaneously measure quadrature fluctuations across multiple frequency bands and reconstruct the full multimode covariance matrix in a single measurement. Through eigenmode decomposition, we extract a set of independent squeezed supermodes, which are subsequently transformed via a tailored linear unitary operation implemented in post-processing. Using this approach, we deterministically construct a tripartite GHZ state from three supermodes and verify its entanglement via the van Loock–Furusawa criteria, with the corresponding correlation variances measured to be 0.57, 0.75, and 0.57, all below the shot noise limit. This method effectively realizes a scalable entanglement generation scheme within a single optical beam, eliminating the need for complex interferometric networks. Our results provide a scalable and versatile platform for multimode quantum information processing.
Entangled continuous variable (CV) Gaussian states with different wavelengths plays a central role in recent CV-based approaches to quantum network, quantum information processing and quantum metrology. Typically, experiments demonstrating CV entanglement exploit the optical parametric frequency down conversion. Due to the probabilistic nature of photon pair generation, the entanglement involving the post-selection of photonic qubits is limited to at most three colors. Here We theoretically present a scheme for the deterministic generation of entanglement among bright multicolor CV Gaussian states from an optomechanical system using existing experimental technologies. In our scheme an optical frequency comb is input into an optomechanical resonator and then the amplified optomechanical coupling makes multipartite entanglement among them. Our scheme overcomes the limitation of usable frequency of entangled CVs in frequency conversion process. It can be extended to generate multipartite entanglement between orthogonal modes with a single frequency or between microwave and optical CVs, or even among a microwave frequency comb. This optomechanical device can be integrated on a chip.