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Laser & Photonics Review

Mid‐Infrared and Telecom Band Two‐Color Entanglement Source for Quantum Communication

Wu‐Zhen Li, Chun Hua Zhou, Yang Wang, Li Chen, Xiao-Hua Frank Wang, Di‐Yuan Zheng, Meng Yu Xie, Yin‐Hai Li, Zhi‐Yuan Zhou, Wan‐Su Bao, Guang‐Can Guo, Bao‐Sen Shi

Published Jul 2, 2025• 5 citations
Publisher

Abstract

Abstract Due to the high noise caused by solar background radiation, the existing satellite‐based free‐space quantum communication experiments are mainly carried out at night, hindering the establishment of a practical, all‐day, real‐time global‐scale quantum network. Given that the 2–5 µm mid‐infrared (MIR) band has extremely low solar background radiation and strong scattering resistance, it is one of the ideal bands for free‐space quantum communication. Here, first, on the preparation of a high‐quality MIR (3370 nm) and telecom band (1555 nm) two‐color polarization‐entangled photon source is reported, and the effective detection of MIR photons through a frequency upconversion process. Then, the source is used to realize a principle quantum key distribution based on free‐space and fiber hybrid channels in a laboratory. This work represents a significant step toward developing all‐day global‐scale quantum communication networks.

DOI
10.1002/lpor.202500338

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Abstract Satellite quantum key distribution technology has developed rapidly using near-infrared wavelengths and is expected to enable global quantum communication. However, link availability is still hampered by detrimental effects in the free-space channel, such as background noise from solar radiation and attenuation from turbulence and weather such as haze and fog. One potential mitigation technique is to move to the mid-infrared atmospheric transmission window (3–5 µ ms) where background noise and turbulence effects are significantly reduced. While mid-infrared quantum technology is not as well developed, advancements in mid-infrared entangled photon pair generation and nonlinear upconversion single-photon detectors could be poised to enable daytime satellite downlinks with increased reliability. This review compares the state of the art for quantum transmitters and receivers in the mid-infrared to the more established near-infrared technology. The goal is to identify gaps in transmitter and/or receiver technology in the mid-infrared, and to determine if the mid-infrared can offer significant advantages over the near infrared for quantum communication.
The mid-infrared (MIR) band entangled photon source is vital for the next generation of quantum communication, quantum imaging, and quantum sensing. However, the current entangled states are mainly prepared in the visible or near-infrared bands. It is still lacking high-quality entangled photon sources in the MIR band. In this work, we optimize the poling sequence of lithium niobate to prepare two kinds of typical entangled states, the Hermit–Gaussian state and the comb-like entangled state at 3.2 µm. We have also calculated the photon pair rates and estimated the effect of fabrication resolution in the schemes. Our approach will provide entangled photon sources with excellent performance for the study of quantum information in the MIR band.
Quantum-enhanced optical systems operating within the 2-2.5 µm spectral region have the potential to revolutionize emerging applications in communications, sensing and metrology. However, until now, sources of entangled photons have been realized mainly in the near-infrared 700-1550 nm spectral window. Above 2 µm lies an atmospheric transparency window with nearly one-third of the solar blackbody radiation as what is typical at telecom wavelengths. There is, therefore, a growing interest to deliver sources and detectors operating in this wavelength range for free-space optical communications and in emerging fields of high-sensitivity metrology, specifically, gravitational wave detection [1,2]. Here, using a 130 fs pulsed laser at wavelength 1.05 μm to pump custom-designed non-linear crystals for spontaneous parametric down-conversion (SPDC) process and tailored superconducting nanowire single-photon detectors (SNSPD), we demonstrate for the first-time two-photon quantum interference and polarization entanglement in the mid-infrared, at ≃ 2.1 μm.