We design a scheme for detecting a single photon loss from multi-modal quantum signals transmitted via a fiber or in free space. This consists of a special type of unitary coding transformation, the parity controlled-squeezing, applied prior to the transmission on the signal composed by information and ancilla modes. At the receiver, the inverse unitary transformation is applied—decoding, and the ancilla mode is measured via photon detection. The outcome reveals whether a photon loss has occurred. Distortion of the information part of the signal caused by an ancilla photon loss can be corrected via unitary transformation while loss of a photon from the information part of the signal can be detected with the probability exponentially close to unity but cannot be corrected. In contrast to the schemes of decoherence free subspaces and quantum error correction protocols, this method allows one to make use in principle of entire Hilbert space dimensionality. We discuss possible ways of synthesizing the required encoding–decoding transformation.

我们设计了一种方案，用于检测通过光纤或自由空间传输的多模态量子信号的单光子损耗。这包括一种特殊类型的单一编码变换，奇偶控制压缩，在传输之前应用于由信息和辅助模式组成的信号。在接收器处，应用逆幺正变换——解码，并通过光子检测测量辅助模式。结果显示是否发生了光子损失。由辅助光子损失引起的信号信息部分的失真可以通过幺正变换来校正，而来自信号信息部分的光子损失可以以指数接近于单位的概率被检测到，但不能被校正。与无退相干子空间和量子纠错协议的方案相比，这种方法允许人们在原则上利用整个希尔伯特空间维数。我们讨论了合成所需编码-解码转换的可能方法。