A single-photon qudit encoded in more than one degree of freedom (DOF) can effectively enlarge a single photon’s capacity and has great potential for applications in future high-capacity quantum communication fields. During the photon transmission in practical noisy channels, photon qudits may suffer from photon transmission loss, which largely limits their application. In this paper, we propose a practical noiseless linear amplification (NLA) protocol for protecting the single-photon qudit encoded in polarization and double-longitudinal momentum DOFs. This NLA adopts the imperfect entangled state generated from a practical spontaneous parametric down-conversion (SPDC) source as the auxiliary state and can be realized under current experimental conditions. After performing the NLA protocol, the fidelity of the target single-photon qudit can be increased and the encoded information in all DOFs can be well preserved. We show the adoption of the imperfect entangled state generated from SPDC source is superior to that of the imperfect single-photon state from practical heralded single-photon source. Our NLA protocol has application potential in current and future quantum communications.

一个多自由度（DOF）编码的单光子量子可以有效地扩大单个光子的容量，在未来大容量量子通信领域具有巨大的应用潜力。在实际噪声通道中的光子传输过程中，光子量子可能会遭受光子传输损失，这在很大程度上限制了它们的应用。在本文中，我们提出了一种实用的无噪声线性放大 (NLA) 协议，用于保护在极化和双纵向动量自由度中编码的单光子量子点。该 NLA 采用由实际自发参数下变频 (SPDC) 源产生的不完美纠缠态作为辅助状态，可以在当前实验条件下实现。执行 NLA 协议后，可以提高目标单光子量子的保真度，并且可以很好地保留所有自由度中的编码信息。我们表明，采用 SPDC 源产生的不完美纠缠态优于采用不完美单光子态的 SPDC 源。实用的单光子源。我们的 NLA 协议在当前和未来的量子通信中具有应用潜力。