State protection by quantum control before and after noise

Hiroaki Wakamura, Ryûitirô Kawakubo, Tatsuhiko Koike

Research output: Contribution to journalArticlepeer-review


We discuss the possibility of protecting the state of a quantum system that goes through noise by measurements and operations before and after the noise process. We extend our previous result on nonexistence of “truly quantum” protocols that protect an unknown qubit state against the depolarizing noise better than “classical” ones [Phys. Rev. A, 95, 022321 (2017)] in two directions. First, we show that the statement is also true in any finite-dimensional Hilbert spaces, which was previously conjectured; the optimal protocol is either the do nothing protocol or the discriminate and reprepare protocol, depending on the strength of the noise. Second, in the case of a qubit, we show that essentially the same conclusion holds for any unital noise. These results describe the fundamental limitations in quantum mechanics from the viewpoint of control theory.

Original languageEnglish
JournalUnknown Journal
Publication statusPublished - 2017 Apr 25

ASJC Scopus subject areas

  • General

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