The authors develop an axiomatic approach to define the conditional entropy of bipartite quantum channels, which can reveal important features of the channel's causal structure that are not captured by the entropy of quantum channels or the conditional entropy of bipartite states.
The optimal error exponent for classical-quantum channel coding assisted by activated non-signaling correlations is equal to the well-known sphere packing bound, which can be written as a single-letter formula optimized over Petz-Rényi divergences. This characterization remains tight for arbitrarily low rates below the channel capacity.
The generalized quantum Stein's lemma characterizes the optimal performance of a variant of quantum hypothesis testing, which enables the formulation of a general framework for quantum resource theories with a second law analogous to that of thermodynamics.
The authors derive upper and lower bounds on the one-shot ε-error information transmission capacities (classical, quantum, entanglement-assisted classical, and private classical) of a discrete-time quantum Markov semigroup in terms of the structure of the peripheral space of the underlying quantum channel.
The optimal asymptotic error exponent for quantum entanglement distillation is given by the reverse relative entropy of entanglement, a single-letter quantity that can be evaluated using only a single copy of the quantum state.
The reliability function, which describes the optimal exponent of the decay of decoding error when the communication rate is below the capacity, has been determined for general classical-quantum channels.
The multiparameter quantum Cramér-Rao bound can be saturated at the single-copy level by a projective measurement if and only if the symmetric logarithmic derivatives of the quantum state commute and a unitary solution exists to a system of coupled nonlinear partial differential equations.
Quantum Doeblin coefficients provide an efficiently computable upper bound on a wide class of contraction coefficients, which quantify the decrease in information under quantum channels. The author also introduces reverse Doeblin coefficients that lower bound certain expansion coefficients.
양자 연성 덮개 보조정리를 이용하여 양자 채널 식별 문제에 대한 새로운 상한계를 제시하고, 동시 식별 용량과 무제한 식별 용량 사이의 분리를 증명하였다.
The quantum soft-covering problem aims to find the minimum rank of an input state needed to approximate a given quantum channel output. The authors prove a one-shot quantum soft-covering lemma and demonstrate its applications in rate-distortion coding, channel resolvability, and identification capacities of quantum channels.