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Physical Review Applied

Noiseless and efficient quantum information transmission for fiber-based continuous-variable quantum networks

Jiliang Qin, Jialin Cheng, Shaocong Liang, Zhihui Yan, Huadong Lu, Xiaojun Jia

Published Jun 11, 2024• 3 citations
Publisher

Abstract

Quantum state transmission and quantum information transmission (QIT) through fiber channels hold immense promise for advancing the scope of quantum information applications. However, besides unavoidable transmission loss, channel noise accelerates the decoherence of quantum states and severely limits the transmission distance in practical continuous-variable quantum state transmission. To address the issue of channel noise in metropolitan quantum fiber-optic links, we propose a scheme called dual-channel transferring after interference. This scheme exhibits immunity to additional channel noise while maintaining sufficient communication capacity. Based on this scheme, we experimentally achieve the transmission of a $\ensuremath{-}5.9$-dB squeezed state at 1.3 $\text{\ensuremath{\mu}}\mathrm{m}$ through 3 km of optical fiber. We obtain a $\ensuremath{-}3.2$-dB output squeezed state and demonstrate that this decrease is solely attributable to channel loss, independent of channel noise. Incidentally, we achieve an improved measurement sensitivity of guided acoustic-wave Brillouin scattering noise in a fiber channel exploiting squeezed light. In addition, beyond its simplicity of installation and ease of operation, this approach also offers the highest transmission capacity compared with the existing QIT technologies. Our scheme is suitable for almost all modulation-free continuous-variable quantum information protocols, such as quantum secret sharing, quantum entanglement swapping, and some quantum key distribution schemes with entangled states, as well as participating in the construction of future quantum networks.

DOI
10.1103/physrevapplied.21.064026

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Quantum state transmission and quantum information transmission (QIT) through fiber channels hold immense promise for advancing the scope of quantum information applications. However, besides unavoidable transmission loss, channel noise accelerates the decoherence of quantum states and severely limits the transmission distance in practical continuous-variable quantum state transmission. To address the issue of channel noise in metropolitan quantum fiber-optic links, we propose a scheme called dual-channel transferring after interference. This scheme exhibits immunity to additional channel noise while maintaining sufficient communication capacity. Based on this scheme, we experimentally achieve the transmission of a \ensuremath−5.9\ensuremath{-}5.9\ensuremath−5.9-dB squeezed state at 1.3 \ensuremath\mum\text{\ensuremath{\mu}}\mathrm{m}\ensuremath\mum through 3 km of optical fiber. We obtain a \ensuremath−3.2\ensuremath{-}3.2\ensuremath−3.2-dB output squeezed state and demonstrate that this decrease is solely attributable to channel loss, independent of channel noise. Incidentally, we achieve an improved measurement sensitivity of guided acoustic-wave Brillouin scattering noise in a fiber channel exploiting squeezed light. In addition, beyond its simplicity of installation and ease of operation, this approach also offers the highest transmission capacity compared with the existing QIT technologies. Our scheme is suitable for almost all modulation-free continuous-variable quantum information protocols, such as quantum secret sharing, quantum entanglement swapping, and some quantum key distribution schemes with entangled states, as well as participating in the construction of future quantum networks.
Einstein-Podolsky-Rosen (EPR)-entangled state light field at a telecommunication wavelength of 1.5 μm is an important quantum source for realizing the continuous variable quantum information processing and some quantum protocols over optical fiber channel. When the EPR-entangled state light field is distributed over the optical fiber channel, the disentanglement is always present because the the EPR entangled state interacts with the fiber channel. It affects the performance of quantum information processing. In this paper, we theoretically calculate the positive partial transposition (PPT) of the entangled state distributed over the optical fiber channel in the single-channel and dual-channel distribution scheme, respectively. Three types of initial entangled light field are considered and analyzed, they being an initial EPR entangled state, an EPR entangled state with asymmetric quadratures, and an EPR entangled state with asymmetric modes. Furthermore, the influence of the extra noise in the optical fiber on the transmission distance of EPR entangled state over the optical fiber channel is investigated. In the single-channel scheme or dual-channel scheme, the extra noise in the optical fiber channel leads the entangled state light field to be disentangled, and the transmission distance of EPR entangled state over the optical fiber channel to decrease rapidly with the increase of the extra noise. For maintaining the robustness of EPR entangled states in lossy optical fiber channels, the dual-channel scheme has more stringent requirements for the correlation quadrature symmetry and purity of the initial entangled state than the single-channel scheme. In the single fiber noise channel scheme, the maximum transmission distance and the robustness of the EPR entangled states with asymmetric modes are not sensitive to the asymmetry between modes. The change of asymmetry between modes does not lead to being disentangled. The maximum transmission distance does not change either. However, the decrease of asymmetry between modes results in the disentanglement in the double fiber noise channels’ scheme. The maximum transmission distance is reduced and the sudden death occurs to the entanglement. The present results will lay a foundation for continuous variables quantum information processing based on optical fiber, such as realizing continuous variables quantum communication over optical fiber and constructing metropolitan quantum network over optical fiber.
Transferring of a real quantum state in a long-distance channel is an important task in the development of quantum information networks. For greatly suppressing the relative phase fluctuations between the signal beam and the corresponding local oscillator beam, the usual method is to transfer them with time-division and polarization-division multiplexing through the same fiber. But the nonclassical states of light are very sensitive to the channel loss and extra noise, this multiplexing method must bring the extra loss to the quantum state, which may result in the vanishing of its quantum property. Here, we propose and realize a suitable time multiplexing method for the transferring and measurement of nonclassical states. Only the local oscillator beam is chopped into a sequence of light pulses and transmitted through fiber with continuous orthogonal-polarized signal beam. Finally, when the local oscillator pulses are properly time delayed compared to the signal beam, the quantum state can be measured in the time sequences without the influence of extra noise in the fiber. Our work provides a feasible scheme to transfer a quantum state in relative long distance and construct a practical quantum information network in metropolitan region.
Using an all-fiber source of pulsed twin beams, with the record intensity difference squeezing of 6.1±0.15 dB for fiber optical parametric amplifiers, we experimentally distribute the pulsed continuous variable (CV) quantum state over a long distance. After separating the signal and idler twin beams by 26 km single-mode fibers, the measured intensity difference noise is still below the shot noise limit by 2 dB. The result shows that the distribution process is only affected by the vacuum noise originated from the fiber transmission loss. The extra noise contributed by the acoustic wave Brillouin scattering and Raman scattering in 26 km transmission fibers has not been observed. The investigation is beneficial for studying long-distance CV quantum communication.
Quantum noise fundamentally limits the performance of fiber-optic systems beyond the standard quantum limit (SQL), restricting long-distance quantum key distribution, quantum communication, and precision quantum sensing. To overcome these limitations, quantum-squeezed states enable quadrature-dependent noise suppression, yet their benefits rapidly degrade under fiber attenuation, necessitating low-noise amplification. Since conventional phase-insensitive amplifiers (PIAs) impose a minimum 3 dB noise figure (NF) penalty and disrupt quantum correlations, phase-sensitive amplification (PSA) becomes essential. In this work, we propose a PSA based on dual-pump frequency-degenerate four-wave mixing (FWM) to amplify weak coherent squeezed states. Here, the PSA is seeded by an information-carrying single-mode squeezed state, where the information is encoded in the displacement degree of freedom, rather than in the squeezing itself. By optimizing the relative phases among the squeezed state, pump fields, and weak signal, the scheme maintains proper squeezing alignment and preserves the encoded quantum correlations during propagation. Under low-loss conditions, it is shown that the effective NF reaches −7.787 dB, demonstrating that the scheme enables quantum-limited amplification suitable for long-haul transmission and offering a viable path toward scalable fiber-based quantum technologies.
We report a continuous variable key distribution system that achieves a final secure key rate of 3.45 kilobits/s over a distance of 24.2 km of optical fiber. The protocol uses discrete signaling and post-selection to improve reconciliation speed and quantifies security by means of quantum state tomography. Polarization multiplexing and a frequency translation scheme permit transmission of a continuous wave local oscillator and suppression of noise from guided acoustic wave Brillouin scattering by more than 27 dB.
Quantum communications enable secure key transmission and entanglement distribution, but their rates and maximum transmission distances are limited due to the losses and noise introduced by quantum channels. Various techniques have been developed to address these limitations, tailored to the platform and the nature of the decoherence. In this thesis, we explore ways to enhance the performance of quantum communications, with a particular focus on increasing the key rates and transmission distances in quantum key distribution (QKD), a method that leverages quantum mechanics to securely share keys between trusted parties. Additionally, we aim to improve the purity of entangled states shared between two parties, contributing to broader advancements in quantum communications. We begin by introducing a software-enhanced continuous-variable QKD (CV-QKD) protocol, where keys are encoded onto the phase and amplitude degrees of the optical field. This protocol enhances both transmission distance and key rates through a post-selection method employing arbitrary filters, experimentally demonstrated on two different platforms. Its adaptability makes it highly promising, as it can be implemented on existing CV-QKD platforms and dynamically optimises key rates for rapidly changing channels. We then present a protocol for the pure-loss channel leveraging quantum repeaters to mitigate the channel losses by segmenting the total communication distance into smaller, more manageable intervals. This repeater-like protocol introduces a more efficient entanglement-swapping measurement at the repeater station than the state-of-the-art twin-field protocols. Instead of polarisation encoding, our protocol relies on encoding keys in the photon-number basis, optimising the states to achieve the highest key rates for each distance. Notably, this protocol shows strong potential for improving long-distance quantum communication by surpassing the capacity of the direct communication channel at a shorter distance than the existing repeater protocols such as twin-field QKD while achieving an overall higher transmission distance. Finally, in this thesis, we demonstrate that for the Pauli-dephasing channel, placing a node in the middle of the quantum channel to perform a Bell-state measurement as an entanglement-swapping operation does not result in improved key rates, even with reduced decoherence. As a result, we propose an entanglement purification protocol that refines a larger set of noisy entangled states into a smaller set of high-fidelity ones using local operations and classical communication. This purification protocol builds upon earlier purification methods, extending their application from two Bell pairs to any number of Bell pairs and thereby achieving the capacity of the dephasing channel. Through numerous recursive iterations, it produces near-perfect Bell pairs, making it well-suited for quantum repeaters and correcting dephasing errors in quantum computers.