Realization of quantum Wheeler's delayed-choice experiment

Light is believed to exhibit wave-particle duality(1) depending on the detecting devices, according to Bohr's complementarity principle(2), as has been demonstrated by the 'delayed-choice experiment' with classical detecting devices(3-9). A recent proposal(10) suggests that the detecting device can also occupy a quantum state, and a quantum version of the delayed-choice experiment can be performed. Here, we experimentally realize the quantum delayed-choice experiment and observe the wave-particle morphing phenomenon of a single photon. We also illustrate, for the first time, the behaviour of the quantum wave-particle superposition state of a single photon. We find that the quantum wave-particle superposition state is distinct from the classical mixture state because of quantum interference between the wave and particle states. Our work reveals the deep relationship between the complementarity principle and the superposition principle, and it may be helpful in furthering understanding of the behaviour of light.