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Frontiers in Optics

Long-Range Transmission of Cluster States over 25 km of Optical Single-Mode Fiber via Multi-Level Time-Bin Encoding

Robert Johanning, Philip Rübeling, Jan Heine, Oleksandr V. Marchukov, Michael Kues

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We demonstrate the transmission of a cluster state over 25 km fiber using multi-level time-bin encoding and chirped pulse modulation. Our work paves the way to multipartite entangled long-distance quantum networking and distributed photonic computing.
DOI
10.1364/fio.2025.fth3c.6

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The next generation of telecommunication networks will rely on the transmission of complex quantum states to enable secure and transformative information processing, utilizing entanglement and superposition. Cluster states - multipartite entangled states that retain entanglement under local measurements - are a vital resource for quantum networking applications such as blind photonic quantum computing, quantum state teleportation and all-photonic quantum repeaters. However, the transmission of cluster states over optical fiber has remained elusive with previous approaches. Here, we demonstrate the first transmission of a four-qubit cluster state over 25 km of single-mode fiber by using a two-photon multi-level time-bin encoding. We directly generate the state by exploiting coherent control of a parametric generation process, rendering a resource-intensive controlled-phase gate obsolete. To enable efficient and reconfigurable projective measurements on the multi-level time-bin encoded state, we introduce chirped-pulse modulation and implement the first time-bin beam splitter, allowing us to certify genuine multipartite entanglement and to demonstrate one-way computing operations. Our approach enables the transmission of complex quantum states over long-distance fibers, permitting the implementation of multipartite protocols and laying the foundation for large-scale quantum resource networks.
The next generation of telecommunication networks will rely on the distribution of complex quantum resources to enable secure and transformative information processing, utilizing entanglement and superposition. Cluster states-multi-qubit entangled states that retain entanglement under local measurements-are the central resource for measurement-based quantum information processing and compelling for quantum networking applications including blind quantum computing and quantum key agreement. However, transmitting cluster states over optical fiber has remained elusive with previous approaches, as losses scale exponentially with the number of photons. Here, we report the first transmission of a four-qubit cluster state to two parties over a partially deployed single-mode fiber link. The qubits of this state are encoded in two photons with multi-level time-bins. We directly generate the cluster state by exploiting coherent control of a parametric generation process, rendering a resource-intensive controlled-phase gate obsolete. To enable processing of this state, we introduce beam splitting for time-bins via chirped pulse modulation-a novel, reconfigurable Fourier-domain pulse shaping technique. We certify genuine multi-qubit entanglement and demonstrate one-way computing primitives on the transmitted cluster state. Our approach achieves the transmission of cluster states over optical fiber, establishing complex quantum resources fully compatible with telecommunication infrastructure.
Efficient generation and high-quality distribution of entanglement is becoming increasingly more relevant in the field of quantum technologies, with important applications such as multiparty computation as well as quantum key distribution (QKD) on the rise. Quantum communication protocols based on entanglement offer an inherent quantum based randomness for key generation and provide in general higher security compared to prepare and measure implementations. Moreover, the future quantum internet will also be based on the distribution of entanglement for securely connecting quantum computers in a network. In this work we show the feasibility of using sequential time-bin entangled states for quantum key distribution in metropolitan networks using off-the-shelf components. The time-bin encoding ensures high fidelity distribution robust against random polarisation fluctuations occuring in optical fibers. Modulated laser pulses in the GHz frequency range are used to generate time-bin entangled photon pairs. The entangled photons are then sent over an about 30km long (9.5dB loss) fiber link within the Vienna fiber network, showing high degree of distributed entanglement with a measured 93\% quantum visibility.
Quantum networks that can perform user-defined protocols beyond quantum key distribution will require fully controllable entangled quantum states. To expand the available space of generated time-bin entangled states, we demonstrate a time-bin entangled photon source that produces qubit states $|{\psi}\rangle = \alpha |{00}\rangle + \beta |{11}\rangle $ with fully controllable phase and amplitudes. Eight different two-photon states have been selected and prepared from arbitrary states on the reduced two-qubit Bloch sphere. The photon pairs encoded in the time-bin scheme were generated at 2.4 MHz with a visibility of V=0.9475±0.0016V = 0.9475 \pm 0.0016V=0.9475±0.0016 , with a violation of the CHSH Bell’s inequality by 197 standard deviations. After entanglement distribution over 100 km of single-mode fibers, we obtained a visibility of V=0.9541±0.0113V = 0.9541 \pm 0.0113V=0.9541±0.0113 with a violation of the CHSH Bell’s inequality by 6 standard deviations. The prepared states had an average fidelity of 0.9540±0.00160.9540 \pm 0.00160.9540±0.0016 at the source and an average fidelity of 0.9353−0.0209+0.01000.9353 ^{+0.0100}_{-0.0209}0.9353−0.0209+0.0100​ after entanglement distribution, which shows that the quantum states generated by our time-bin entangled photon source can be fully controlled potentially to a level applicable to long-distance advanced quantum network systems.