image: By adding two ancilla qubits (represented by blue and red lines), the dual-selection of input and output beam splitters is realized. The first ancilla qubit (blue line) and the system qubit are prepared as a maximally entangled state after passing through the CNOT gate, described by the state |ψa1s⟩. H4 gate can be optionally placed, and whether H4 gate is applied determines the measurement basis of the first ancilla qubit, which realizes the selection of the input beam splitter. And the second ancilla qubit controls H3 gate, serving as the second controlled-Hadamard gate. The quantum state of ancilla qubit after passing through H2 gate is |ψa2⟩. A tunable phase shifter φ adds the relative phase eiφ on the state |1⟩. H1-H4: Hadamard gates.
Credit: ©Science China Press
In quantum world, particles like photons can behave like particles (localized points) or waves (spreading out, showing interference patterns). The classic double-slit experiment is a good demonstration of this wave-particle duality: If you fire photons through two slits without observing which slit they go through, they act like waves and form an interference pattern on the screen; otherwise, If you place detectors to observe which slit each photon goes through, they act like particles, and the interference pattern disappears. Thus observation collapses the wave behavior into particle behavior.
In 1978, physicist John Archibald Wheeler raised a curious question: What if delay the choice of whether to observe the photon until after it has passed the slits? He added a second double slits (or beam splitter) to the experiment apparatus, with which he can decide whether to detect which path the photon took (measure its particle nature) or allow it to interfere (measure its wave nature) AFTER the photon has passed through the first slits (or beam splitter). This is called ‘Wheeler’s delayed-choice experiment’. Experimental realizations of Wheeler’s idea have been demonstrated in various setups, and physicists found that the outcome still depends on what we decide after the photon has passed the slits, which challenges our notions of causality and realism.
Recently, a novel extension of Wheeler’s delayed-choice experiment has been proposed and realized by research groups from Ningbo University and University of Science and Technology of China. Traditional delayed-choice experiments typically involve only the presence or absence of the second beam splitter, but in this new work, the first beam splitter can also be inserted or removed. The selections of the two beam splitters are implemented in different ways. The first beam splitter is mimicked by preparing different quantum states for sensing the phase shift through measurements of different bases of an ancilla photon that is entangled with the photons entering the interferometer. By selecting one of two possible measurement bases for the ancilla photon, the first beam splitter is effectively inserted or removed. Meanwhile, the second beam splitter is randomly inserted or removed using the quantum delayed choice. The experimental results demonstrate the wave-particle duality of light in a more diverse way, providing new insights into the nature of quantum measurement and the complementarity of wave-particle duality.
This work opens up new possibilities for exploring the wave-particle duality of light and matter in a more diverse way. The novel interferometer structure proposed in this work allows for the observation of the wave-particle duality of light through entangled photons and quantum delayed choices. The experimental realization of this concept provides a new perspective on the nature of quantum measurement, especially the time order of quantum measurements. Future research in this direction may lead to further insights into the spatio-temporal correlation structure of reality and measurement in the quantum world.
See the article:
Experimental realization of Wheeler's delayed-choice experiment with dual selections, https://doi.org/10.1007/s11433-024-2587-y
Journal
Science China Physics Mechanics and Astronomy