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A quantum system can behave as a wave or as a particle, depending on theexperimental arrangement. When for example measuring a photon using aMach-Zehnder interferometer, the photon acts as a wave if the secondbeam-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 afterthe photon has entered the interferometer, as in Wheeler's famousdelayed-choice thought experiment. In recent quantum versions of thisexperiment, this decision is controlled by a quantum ancilla, while the beamsplitter is itself still a classical object. Here we propose and realize avariant of the quantum delayed-choice experiment. We configure asuperconducting quantum circuit as a Ramsey interferometer, where the elementthat acts as the first beam-splitter can be put in a quantum superposition ofits active and inactive states, as verified by the negative values of itsWigner function. We show that this enables the wave and particle aspects of thesystem to be observed with a single setup, without involving an ancilla that isnot itself a part of the interferometer. We also study the transition of thisquantum beam-splitter from a quantum to a classical object due to decoherence,as observed by monitoring the interferometer output.

Quantum delayed-choice experiment with a beam splitter in a quantum superposition