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Physical Review Applied

Entangled-State Time Multiplexing for Multiphoton Entanglement Generation

Zhibo Hou, Jun-Feng Tang, Chang-Jiang Huang, Yun‐Feng Huang, Guo‐Yong Xiang, Chuan‐Feng Li, Guang‐Can Guo

Published Jan 24, 2023• 17 citations
Publisher

Abstract

Finding a path toward efficient multiphoton entanglement generation would boost both foundational studies and practical applications of quantum entanglement in e.g. quantum computation, metrology, and communication. The common approach has a notorious problem of exponentially decreasing success probability with increasing photon number, so another method is needed for scalability. This Letter presents a scheme for creating $2n$-photon entangled states with an exponential efficiency-enhancement factor of ${B}^{n\ensuremath{-}1}$ without spoiling the entanglement. Even with just the currently demonstrated multiplexing power, applying this method to state-of-the-art 12-photon states would yield an enhancement factor of 10${}^{4}$.

DOI
10.1103/physrevapplied.19.l011002

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Finding a path toward efficient multiphoton entanglement generation would boost both foundational studies and practical applications of quantum entanglement in e.g. quantum computation, metrology, and communication. The common approach has a notorious problem of exponentially decreasing success probability with increasing photon number, so another method is needed for scalability. This Letter presents a scheme for creating 2n2n2n-photon entangled states with an exponential efficiency-enhancement factor of Bn\ensuremath−1{B}^{n\ensuremath{-}1}Bn\ensuremath−1 without spoiling the entanglement. Even with just the currently demonstrated multiplexing power, applying this method to state-of-the-art 12-photon states would yield an enhancement factor of 104{}^{4}4.
Multiphoton entangled quantum states are key to advancing quantum technologies such as multiparty quantum communications, quantum sensing, or quantum computation. Their scalable generation, however, remains an experimental challenge. Current methods for generating these states rely on stitching together photons from probabilistic sources, and state generation rates drop exponentially in the number of photons. Here, we implement a system based on active feed-forward and multiplexing that addresses this challenge. We demonstrate the scalable generation of four-photon and six-photon Greenberger-Horne-Zeilinger states, increasing generation rates by factors of 9 and 35, respectively. This is consistent with the exponential enhancement compared to the standard nonmultiplexed approach that is predicted by our theory. These results facilitate the realization of practical multiphoton protocols for photonic quantum technologies.
Deterministic generation of single- and multiphoton states is a key requirement for large-scale optical quantum information and communication applications. While heralded single-photon sources (HSPSs) using nonlinear optical processes have enabled proof-of-principle demonstrations in this area of research, they are not scalable as their probabilistic nature severely limits their generation efficiency. We overcome this limitation by demonstrating a substantial improvement in HSPS efficiency via large-scale time multiplexing. Using an ultra-low loss, adjustable optical delay to multiplex 40 conventional HSPS photon generation processes into each operation cycle, we have observed a factor of 9.7(5) enhancement in efficiency, yielding a 66.7(24)% probability of collecting a single photon with high indistinguishability (90%) into a single-mode fiber per cycle. We also experimentally investigate the trade-off between a high single-photon probability and unwanted multiphoton emission. Upgrading our time-multiplexed source with state-of-the-art HSPS and single-photon detector technologies will enable the generation of >30 coincident photons with unprecedented efficiency.
Nonlocal quantum entanglement is a fundamental resource for future quantum networks. However, the efficiency of generating nonlocal entanglement between distant nodes is severely limited by the exponential loss incurred when locally generated entangled states are distributed through lossy quantum channels. This limitation becomes more pronounced in practical scenarios requiring the simultaneous distribution of multiple entangled pairs. Although classical multiplexing approaches, such as spatial, temporal, and frequency multiplexing, can increase the nonlocal entanglement generation rate, they do not improve the single-shot transmission efficiency. In contrast, quantum multiplexing, which can be generated by high-dimensional encoding of single photons, allows for the parallel generation of multiple nonlocal entangled pairs in a single transmission round, thereby enhancing the overall efficiency of nonlocal entanglement generation. Quantum multiplexing thus presents a promising route toward scalable quantum networks. This review introduces the mechanisms of generating nonlocal entanglement through quantum multiplexing, and focuses on two main methods: using high-dimensional single-photon encoding and high-dimensional biphoton entanglement distribution. Then it examines how quantum multiplexing can accelerate the generation of nonlocal quantum logical entanglement. Finally, it briefly explores the potential of quantum multiplexing for building large-scale quantum networks.
A scheme for the enhanced generation of higher photon-number states is realized, using an optical time-multiplexing setting that exploits a parametric down-conversion source for an iterative state generation. We use a quantum feedback mechanism for already generated photons to induce self-seeding of the consecutive nonlinear process, enabling us to coherently add photons to the light that propagates in the feedback loop. The addition can be carried out for any chosen number of round trips, resulting in a successive buildup of multiphoton states. Our system is only limited by loop losses. The looped design is rendered possible by a carefully engineered waveguide source that is compatible with and preserves the shape of the propagating mode. We compare the fidelities and success probabilities of our protocol with the common direct heralding of photon-number states. This comparison reveals that, for same the fidelity, our feedback-based setup significantly enhances success probabilities, being vital for an efficient utilization in quantum technologies. Moreover, quantum characteristics of the produced states are analyzed, and the flexibility of producing higher photon-number states with our setup beyond the common direct heralding is demonstrated.
Abstract In state-of-the-art quantum key distribution systems, the main limiting factor in increasing the key generation rate is the timing resolution in detecting photons. Here, we present and experimentally demonstrate a strategy to overcome this limitation, also for high-loss and long-distance implementations. We exploit the intrinsic wavelength correlations of entangled photons using wavelength multiplexing to generate a quantum secure key from polarization entanglement. The presented approach can be integrated into both fiber- and satellite-based quantum-communication schemes, without any changes to most types of entanglement sources. This technique features a huge scaling potential allowing to increase the secure key rate by several orders of magnitude as compared to non-multiplexed schemes.
Quantum entanglement represents a fundamental resource for quantum information processing, yet the dynamics of multi-photon entangled systems remain inadequately characterized for practical quantum computing applications. This research investigates the generation, manipulation, and characterization of entangled states involving two to eight photons using spontaneous parametric down-conversion techniques. Through theoretical analysis and numerical simulations, we developed a comprehensive framework that quantifies entanglement fidelity, decoherence rates, and Bell inequality violations across various multi-photon configurations. Our findings reveal that entanglement fidelity decreases exponentially with increasing photon number, exhibiting a decay constant of 0.087±0.012 per additional qubit in optical systems. The research demonstrates significant variations in decoherence times across different physical implementations, with ion trap systems maintaining coherence 1.7 times longer than photonic platforms for equivalent qubit numbers. We established mathematical relationships between system parameters and entanglement quality metrics, identifying critical thresholds for fault-tolerant quantum gate operations. The theoretical framework incorporates realistic noise models, environmental coupling mechanisms, and measurement-induced perturbations that affect practical implementations. Our analysis indicates that five-photon entangled states maintain sufficient fidelity (>83.5%) for error-corrected quantum algorithms when decoherence times exceed 75 microseconds. These results provide essential guidelines for designing scalable quantum processors and establishing benchmarks for experimental validation of multi-qubit entanglement protocols.