Cheng H.-CHsieh M.-H.HAO-CHUNG CHENG2021-09-022021-09-02201800189448https://www.scopus.com/inward/record.uri?eid=2-s2.0-85038401753&doi=10.1109%2fTIT.2017.2781254&partnerID=40&md5=24741e837ea80199b278dc2c1ee41242https://scholars.lib.ntu.edu.tw/handle/123456789/580529In this paper, we study the tradeoffs between the error probabilities of classical-quantum channels and the blocklength n when the transmission rates approach the channel capacity at a rate lower than 1 {n} , a research topic known as moderate deviation analysis. We show that the optimal error probability vanishes under this rate convergence. Our main technical contributions are a tight quantum sphere-packing bound, obtained via Chaganty and Sethuraman's concentration inequality in strong large deviation theory, and asymptotic expansions of error-exponent functions. Moderate deviation analysis for quantum hypothesis testing is also established. The converse directly follows from our channel coding result, while the achievability relies on a martingale inequality. ? 1963-2012 IEEE.Channel capacity; Channel coding; Dispersion (waves); Entropy; Errors; Probability; Quantum entanglement; Quantum theory; Reliability; Testing; Asymptotic expansion; Classical-quantum channels; Concentration inequality; Error probabilities; Large deviation theory; Martingale inequalities; Sphere packing bound; Technical contribution; Communication channels (information theory)journal article10.1109/TIT.2017.27812542-s2.0-85038401753