JILA, University of Colorado/NIST, 440 UCB, Boulder, Colorado 80309, USA.
Center for Theory of Quantum Matter, University of Colorado, Boulder, Colorado 80309, USA.
Phys Rev Lett. 2018 Oct 19;121(16):160501. doi: 10.1103/PhysRevLett.121.160501.
The quantum capacity of a quantum channel captures its capability for noiseless quantum communication. It lies at the heart of quantum information theory. Unfortunately, our poor understanding of nonadditivity of coherent information makes it hard to understand the quantum capacity of all but very special channels. In this Letter, we consider the dephrasure channel, which is the concatenation of a dephasing channel and an erasure channel. This very simple channel displays remarkably rich and exotic properties: we find nonadditivity of coherent information at the two-letter level, a substantial gap between the threshold for zero quantum capacity and zero single-letter coherent information, a big gap between single-letter coherent and private information, and positive quantum capacity for all complementary channels. Its clean form simplifies the evaluation of coherent information substantially and, as such, we hope that the dephrasure channel will provide a much-needed laboratory for the testing of new ideas about nonadditivity.
量子信道的量子容量捕获了其进行无噪声量子通信的能力。它是量子信息理论的核心。不幸的是，我们对相干信息不可加性的理解不足，使得我们难以理解除了非常特殊的信道之外的所有信道的量子容量。在这封信中，我们考虑了去相位通道，它是去相位通道和擦除通道的串联。这个非常简单的通道显示出了丰富而奇特的性质：我们在两个字母的水平上发现了相干信息的不可加性，零量子容量和零单字母相干信息之间的阈值之间存在很大的差距，单字母相干信息和私人信息之间存在很大的差距，以及所有互补通道的正量子容量。它的干净形式大大简化了相干信息的评估，因此，我们希望去相位通道将为测试关于不可加性的新想法提供一个急需的实验室。
