We consider the effect of loss on quantum-optical communication channels. The channel based on direct detection of number states, which for a lossless transmission line would achieve the ultimate quantum channel capacity, is easily degraded by loss. The same holds true for the channel based on homodyne detection of squeezed states, which also is very fragile to loss. On the contrary, the ``classical'' channel based on heterodyne detection of coherent states is loss-invariant. We optimize the a priori probability for the squeezed-state and the number-state channels, taking the effect of loss into account. In the low power regime we achieve a sizeable improvement of the mutual information, and both the squeezed-state and the number-state channels overcome the capacity of the coherent-state channel. In particular, the squeezed-state channel beats the classical channel for total average number of photons $N<8$. However, for sufficiently high power the classical channel always performs as the best one. For the number-state channel we show that with a loss $η\lesssim .6$ the optimized a priori probability departs from the usual thermal-like behavior, and develops gaps of zero probability, with a considerable improvement of the mutual information (up to 70 % of improvement at low power for attenuation $η=.15$).
Optimized Bose-Einstein-condensate production in a dipole trap based on a 1070-nm multifrequency laser: Influence of enhanced two-body loss on the evaporation process
An Optimized Bidirectional Quantum Teleportation Scheme with the use of Bell states
双向量子隐形传态方案是一种双方同时接收对方信息的双向量子通信过程。最近，在文献[1]（Zhou et al.，IEEE Access，7，44269（2019）），六量子比特簇态已被用于双向传送三量子比特纠缠态（Alice）和任意单量子比特未知量子态（Bob）。从实验的角度来看，团簇型纠缠态的制备和维持是非常困难和昂贵的。因此，在本文中，我们设计了一个双向量子隐形传态方案的最佳方案，并表明所提出的方案需要优化的量子资源量。
Optimized Quantum States for Sensing in the Presence of Loss and Phase Noise
Institute for Quantum Information and Matter, California Institute of Technology（加州理工学院量子信息研究所）；LIGO Laboratory, California Institute of Technology（加州理工学院激光干涉引力波天文台实验室）
