Quantum Channels Defy Entropy Additivity, University of Waterloo Finds

Researchers at the University of Waterloo have demonstrated that quantum channels deviate from behavior expected of their classical counterparts, specifically in how they handle entropy. For Rényi entropy values greater than 3/4 and between 0 and 1/4, the team proved the minimum output entropy of quantum channels does not follow the additivity rule guaranteed for classical stochastic channels. This finding utilizes and combines a product, conjugate Bell-state witness and a transpose-complement rank-defect witness to achieve the result, reducing the unresolved portion of the problem. Their estimates also improve the output dimension threshold for additivity violation of minimum output von Neumann entropy, first established in Belinschi, Collins and Nechida. Quantum channels defy classical expectations by failing to…