Guha et al. [arXiv:1307.5368] have recently defined the locking capacity of a quantum channel as the largest rate of a uniformly distributed bit string, which Alice can share with Bob over the channel while an eavesdropper, Eve, who has access to the channel environment, can only gain negligible accessible information about the string. We show that this relaxation of the private capacity of a channel, can be much larger than the latter; in fact, if a sub-linear size key is shared between Alice and Bob, there are channels with zero private capacity but positive locking capacity. This gives a quantitative meaning to the information locking effect [DiVincenzo et al., PRL 92:067902, 2004], and answers an open question of Guha et al. [Based on arXiv:1403.6361, J Cryptol 2017].
Andreas Winter received the Diploma degree in mathematics from Freie Universität Berlin, Berlin, Germany, in 1997, and the Ph.D. degree from the Fakultät für Mathematik, Universität Bielefeld, Bielefeld, Germany, in 1999. He was a Research Associate with the University of Bielefeld until 2001, and then with the Department of Computer Science at the University of Bristol, Bristol, U.K. Since 2003, he has been with the University of Bristol, where he was appointed as a Lecturer of mathematics, and as a Professor of physics of information, in 2006. Since 2012, he has been the ICREA Research Professor with the Universitat Autònoma de Barcelona, Barcelona, Spain. His research interests include quantum and classical Shannon theory and discrete mathematics. He was a recipient, along with Bennett, Devetak, Harrow, and Shor of the 2017 Information Theory Society Paper Award.
