Física Teòrica: Informació i Fenòmens Quàntics, Departament de Física, Universitat Autònoma de Barcelona, 08193, Bellaterra, Spain.
NEST, Scuola Normale Superiore and Istituto Nanoscienze-CNR, 56127, Pisa, Italy.
Nat Commun. 2018 Oct 18;9(1):4339. doi: 10.1038/s41467-018-06848-0.
Thermal attenuator channels model the decoherence of quantum systems interacting with a thermal bath, e.g., a two-level system subject to thermal noise and an electromagnetic signal traveling through a fiber or in free-space. Hence determining the quantum capacity of these channels is an outstanding open problem for quantum computation and communication. Here we derive several upper bounds on the quantum capacity of qubit and bosonic thermal attenuators. We introduce an extended version of such channels which is degradable and hence has a single-letter quantum capacity, bounding that of the original thermal attenuators. Another bound for bosonic attenuators is given by the bottleneck inequality applied to a particular channel decomposition. With respect to previously known bounds we report better results in a broad range of attenuation and noise: we can now approximate the quantum capacity up to a negligible uncertainty for most practical applications, e.g., for low thermal noise.
热衰减器通道模型描述了与热浴相互作用的量子系统的退相干，例如，处于热噪声下的二能级系统以及在光纤或自由空间中传播的电磁信号。因此，确定这些通道的量子容量是量子计算和通信中一个悬而未决的问题。在这里，我们推导出了量子比特和玻色热衰减器的量子容量的几个上界。我们引入了这样的通道的扩展版本，它是可降级的，因此具有单字母量子容量，从而限制了原始热衰减器的量子容量。另一个玻色衰减器的界是通过瓶颈不等式应用于特定的信道分解得到的。与以前已知的界相比，我们在广泛的衰减和噪声范围内报告了更好的结果：我们现在可以在大多数实际应用中，例如在低热噪声的情况下，对量子容量进行近似，误差可以忽略不计。
