title: Inequalities and separations among assisted capacities of quantum channels.
abstract: We exhibit quantum channels whose classical and quantum capacities, when assisted by classical feedback, exceed their unassisted classical Holevo capacity. These channels are designed to be noisy in a way that can be corrected with the help of the output and a reference system entangled with part of the input. A similar construction yields quantum channels whose classical capacity, when assisted by two-way classical communication independent of the source, exceeds their classical capacity assisted by feedback alone. We give a hierarchy of capacity inequalities and open questions.
authors.affiliation: IBM T.J. Watson Research Center, Yorktown Heights, New York 10598, USA. bennetc@watson.ibm.com
authors.forename: Charles H
authors.initials: CH
authors.lastname: Bennett
authors.forename: Igor
authors.initials: I
authors.lastname: Devetak
authors.forename: Peter W
authors.initials: PW
authors.lastname: Shor
authors.forename: John A
authors.initials: JA
authors.lastname: Smolin