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Frequency-bin state generation of entangled photons via Fourier optical synthesis

Tomoya Okita, Yuta Fujihashi, Masahiro Yabuno, Hirotaka Terai, Shigehito Miki, Rui‐Bo Jin, Ryōsuke Shimizu

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We present an alternative approach to creating frequency-bin states using Fourier optical synthesis with a nonlinear interferometer. The observed data demonstrate the control range of frequency spacing from 12.5 GHz to 3.2 THz.
DOI
10.1364/cleo_at.2025.jps200_199

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Frequency modes of light are one of the most promising platforms that provide access to high-dimensional quantum states amongst different photonic degrees of freedom capable of high-dimensionality, enabling robust, error-tolerant, and scalable quantum optical information systems. We demonstrate engineering of precisely controlled two-photon high-dimensional states entangled in frequency through time-domain Fourier optical synthesis. We generate and convert a continuous broadband frequency-entangled state into a large range of discrete frequency bins suitable for ITU standards, with spacings ranging from 12.5 GHz to 750 GHz, and observe spectral anticorrelations over 38 frequency bins, including intra-bin pure states at a 100 GHz bin spacing. We characterize the full quantum state dimensionality via Schmidt decomposition and observe lower bounds on the frequency-binned Hilbert-space dimensionalities of at least 289, formed by two entangled qudits with dimension 17. Furthermore, we demonstrate quantum nonlocality via frequency correlations in a transmission experiment over a campus-scale two-node fiber network. This work represents a crucial step towards building a versatile and relatively simple way of generating precisely controlled high-dimensional spectral qudits, with the potential of harnessing in wavelength-multiplexed quantum networks, high-dimensional information processing, and communication of quantum states specifically, and fiber-optic quantum remote sensing.
By using cascaded second-order nonlinear optical processes in a piece of periodically poled lithium niobate waveguide, we generate 1.5 μ\muμ m discrete frequency-bin entangled two-photon state with a spatial quantum beating visibility of 93.0 ± 5.2%.
Discrete frequency-bin entanglement is an essential resource for applications in quantum information processing. In this Letter, we propose and demonstrate a scheme to generate discrete frequency-bin entanglement with a single piece of periodically poled lithium niobate waveguide in a modified Sagnac interferometer. Correlated two-photon states in both directions of the Sagnac interferometer are generated through cascaded second-order optical nonlinear processes. A relative phase difference between the two states is introduced by changing the polarization state of pump light, thus manipulating the two-photon state at the output of the Sagnac interferometer. The generated two-photon state is sent into a fiber polarization splitter, and then a pure discrete frequency-bin entangled two-photon state is obtained by setting the pump light. The frequency entanglement property is measured by a spatial quantum beating with a visibility of 96.0±6.1%. The density matrix is further obtained with a fidelity of 98.0±3.0% to the ideal state. Our demonstration provides a promising method for the generation of pure discrete frequency-bin entanglement at the telecom band, which is desired in quantum photonics.
Time-bin entangled states are a promising paradigm for quantum communication between nodes of a quantum network. In addition, high-dimensional time-bin states are easy to generate and could offer significantly improved transmission fidelity compared to standard qubits. However, the overall rate of these transmissions is necessarily diminished because successive higher-dimensional time-bin states must be delayed such that they do not overlap in time. We propose to alleviate this concern by introducing an optical frequency shift on each time bin, taking advantage of quantum wavelength division multiplexing to greatly increase the rate of communication possible within a quantum channel. Here we report frequency shifts over a range of ∼ 2 nm (∼ 240 GHz) of telecom pulses in two time-bins separated by ∼ 250 ps, consistent with the requirements for multiplexing.
In this work, we propose and demonstrate the frequency-bin entangled two-photon state via cascaded second harmonic generation and spontaneous parametric down conversion (SHG-SPDC) processes in a single piece of PPLN waveguide. Our scheme is based on all fiber-pigtailed components at 1.5 μm telecom band. Two frequency-bins at 1531.34 nm and 1548.91 nm with bandwidth of 1.04 nm are prepared to be entangled. The frequency entanglement property of our generated two-photon states is measured by spatial two-photon quantum beating. A two-photon beating curve with a visibility of 87.92±0.47% is obtained, showing a good property of frequency-bin entanglement.