A new version of Wheeler's set up

The particle-wave duality seem always under debate and ingenious experiments have been contrived from time to time to violate it. A famous example is the one proposed by John-Wheeler. It is called 'delayed-choice experiment'. In its existing version, a classical switch has been in use. Here comes a new design using quantum switches [http://physics.aps.org/articles/v4/102].

This so-called delayed-choice experiment was performed in 2007 using an interferometer [1]. In the normal setup, a beam splitter creates two separate light beams that later recombine in a second beam splitter. Detectors placed at the two outputs of this beam splitter both register an interference pattern. However, this wave detector can be turned into a particle detector by removing the second beam splitter, so that the two paths no longer interfere. In the experiment, the choice to add or remove the second beam splitter was made after an individual photon had already passed through the first beam splitter. The data showed that particle and wave behavior were unaffected by the delayed choice, as expected from standard quantum mechanics.

Radu Ionicioiu, now at the Institute for Quantum Computing in Waterloo, Canada, and Daniel Terno of Macquarie University in Sydney, Australia, wanted to see what happens in the thought experiment if the delayed choice is made through quantum means. They imagined that the interferometer contains a quantum device—perhaps an atom in a cavity or a micro-mirror placed on a cantilever—that can exist in two possible states. One state selects the particle experiment, and the other selects the wave experiment. This quantum control element can be placed in a combination, or superposition, of its two states, making the whole experiment participate in the wave-particle duality.

“We show you can do both wave and particle experiments at once,” Ionicioiu says. This means the choice of wave vs particle can be delayed indefinitely. The photon can be observed at one of the detectors and still not “know” if it is supposed to be a wave or a particle. It’s only when the observer decides to measure the state of the quantum control that the photon’s behavior can be identified as wavelike or particlelike.