title: Quantum Delayed-Choice Experiment with a Beam Splitter in a Quantum Superposition.
abstract: A quantum system can behave as a wave or as a particle, depending on the experimental arrangement. When, for example, measuring a photon using a Mach-Zehnder interferometer, the photon acts as a wave if the second beam splitter is inserted, but as a particle if this beam splitter is omitted. The decision of whether or not to insert this beam splitter can be made after the photon has entered the interferometer, as in Wheeler's famous delayed-choice thought experiment. In recent quantum versions of this experiment, this decision is controlled by a quantum ancilla, while the beam splitter is itself still a classical object. Here, we propose and realize a variant of the quantum delayed-choice experiment. We configure a superconducting quantum circuit as a Ramsey interferometer, where the element that acts as the first beam splitter can be put in a quantum superposition of its active and inactive states, as verified by the negative values of its Wigner function. We show that this enables the wave and particle aspects of the system to be observed with a single setup, without involving an ancilla that is not itself a part of the interferometer. We also study the transition of this quantum beam splitter from a quantum to a classical object due to decoherence, as observed by monitoring the interferometer output.
authors.affiliation: Department of Physics, Fuzhou University, Fuzhou 350116, China.
authors.forename: Shi-Biao
authors.initials: SB
authors.lastname: Zheng
authors.affiliation: Department of Physics, Zhejiang University, Hangzhou 310027, China.
authors.forename: You-Peng
authors.initials: YP
authors.lastname: Zhong
authors.affiliation: Department of Physics, Zhejiang University, Hangzhou 310027, China.
authors.forename: Kai
authors.initials: K
authors.lastname: Xu
authors.affiliation: Department of Physics, Zhejiang University, Hangzhou 310027, China.
authors.forename: Qi-Jue
authors.initials: QJ
authors.lastname: Wang
authors.affiliation: Department of Physics, Zhejiang University, Hangzhou 310027, China.
authors.forename: H
authors.initials: H
authors.lastname: Wang
authors.affiliation: Department of Physics, Fuzhou University, Fuzhou 350116, China.
authors.forename: Li-Tuo
authors.initials: LT
authors.lastname: Shen
authors.affiliation: Department of Physics, Hangzhou Normal University, Hangzhou 310036, China.
authors.forename: Chui-Ping
authors.initials: CP
authors.lastname: Yang
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: John M
authors.initials: JM
authors.lastname: Martinis
authors.affiliation: Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: A N
authors.initials: AN
authors.lastname: Cleland
authors.affiliation: Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, USA.|Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Si-Yuan
authors.initials: SY
authors.lastname: Han