title: Ignorance is bliss: general and robust cancellation of decoherence via no-knowledge quantum feedback.
abstract: A "no-knowledge" measurement of an open quantum system yields no information about any system observable; it only returns noise input from the environment. Surprisingly, performing such a no-knowledge measurement can be advantageous. We prove that a system undergoing no-knowledge monitoring has reversible noise, which can be canceled by directly feeding back the measurement signal. We show how no-knowledge feedback control can be used to cancel decoherence in an arbitrary quantum system coupled to a Markovian reservoir that is being monitored. Since no-knowledge feedback does not depend on the system state or Hamiltonian, such decoherence cancellation is guaranteed to be general and robust, and can operate in conjunction with any other quantum control protocol. As an application, we show that no-knowledge feedback could be used to improve the performance of dissipative quantum computers subjected to local loss.
authors: Stuart S
authors: SS
authors: Szigeti
authors: Department of Quantum Science, Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 0200, Australia and ARC Centre for Quantum Computation and Communication Technology, The Australian National University, Canberra, Australian Capital Territory 0200, Australia.
authors: Andre R R
authors: AR
authors: Carvalho
authors: School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom.
authors: James G
authors: JG
authors: Morley
authors: School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom.
authors: Michael R
authors: MR
authors: Hush