title: Demonstration of a quantum nondemolition sum gate.
abstract: The sum gate is the canonical two-mode gate for universal quantum computation based on continuous quantum variables. It represents the natural analogue to a qubit C-NOT gate. In addition, the continuous-variable gate describes a quantum nondemolition (QND) interaction between the quadrature components of two light modes. We experimentally demonstrate a QND sum gate, employing the scheme by R. Filip, P. Marek, and U. L. Andersen [Phys. Rev. A 71, 042308 (2005)10.1103/PhysRevA.71.042308], solely based on off-line squeezed states, homodyne measurements, and feedforward. The results are verified by simultaneously satisfying the criteria for QND measurements in both conjugate quadratures.
authors.affiliation: Department of Applied Physics, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
authors.forename: Jun-Ichi
authors.initials: J
authors.lastname: Yoshikawa
authors.forename: Yoshichika
authors.initials: Y
authors.lastname: Miwa
authors.forename: Alexander
authors.initials: A
authors.lastname: Huck
authors.forename: Ulrik L
authors.initials: UL
authors.lastname: Andersen
authors.forename: Peter
authors.initials: P
authors.lastname: van Loock
authors.forename: Akira
authors.initials: A
authors.lastname: Furusawa