量子通信中的相对论性多普勒效应
Relativistic Doppler effect in quantum communication
摘要：
当电磁信号在真空中传播时，由于衍射效应，偏振检测器不能严格垂直于波矢量。即使探测器是完美的，真空也表现为一个嘈杂的通道。然而，通过观察者向声源的相对运动，"噪声"可以被减小，甚至几乎被消除。约化密度矩阵的标准定义不能用于光子偏振，因为横截性条件的行为类似于超选择规则。然而，我们可以定义一个有效的约化密度矩阵，它对应于一类受限制的正算子值测度。没有纯光子量子比特，也没有完全正交的量子比特状态。
英文摘要：
When an electromagnetic signal propagates in vacuo, a polarization detector cannot be rigorously perpendicular to the wave vector because of diffraction effects. The vacuum behaves as a noisy channel, even if the detectors are perfect. The ``noise'' can however be reduced and nearly cancelled by a relative motion of the observer toward the source. The standard definition of a reduced density matrix fails for photon polarization, because the transversality condition behaves like a superselection rule. We can however define an effective reduced density matrix which corresponds to a restricted class of positive operator-valued measures. There are no pure photon qubits, and no exactly orthogonal qubit states.
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