The key to both experiments is the use of a quantum switch in the apparatus, which allows the interferometer to hover in a superposition of measuring wave or particle behavior. "In these traditional delayed choice experiments, somewhere in your apparatus you have a big, classical binary switch," says Peter Shadbolt, a co-author of the other study and a PhD student in quantum physics at the University of Bristol in England. "It has 'wave' written on one side and 'particle' written on the other side. What we do is replace that classical switch with a qubit, a quantum bit, which is a second photon in our experiment."
The quantum switch determines the nature of the apparatus—whether the two optical paths recombined to form a closed interferometer, which measures wavelike properties, or remained separate to form an open interferometer, which detects discrete particles. But in both cases, whether the interferometer was open or closed—and whether the photon zipped through the apparatus like a particle or a wave, respectively—was not determined until the physicists measured a second photon. The first photon's fate was linked to the state of the second photon via the phenomenon of quantum entanglement, through which quantum objects share correlated properties.
In the Bristol group's experiment, the state of the second photon determines whether the interferometer is open, closed or a superposition of both, which in turn determines the wave or particle identity of the first photon. "In our case that choice is more of a quantum choice," Shadbolt says. "Without this kind of approach, you wouldn't be able to see this morphing between wave and particle."
The device built by Tanzilli's group functions similarly—the interferometer is closed for vertically polarized photons (which therefore act as waves) and open for horizontally polarized photons (which behave as particles). Having sent a test photon through the apparatus, the researchers measured an entangled partner photon 20 nanoseconds later to determine the test photon's polarization, and hence on which side of the wave–particle divide it fell.
Because of the design of the experiment and the nature of entanglement, the test photon's wave or particle nature was not determined until the second photon was measured—in other words, until 20 nanoseconds after the fact. "The test photon makes its life in the interferometer and is detected, which means it is destroyed," Tanzilli says. "Afterward we determine its behavior." That order of operations takes the concept of delayed choice to the extreme. "It means that space and time seem to not play any role in this affair," Tanzilli adds.
Quantum information researcher Seth Lloyd of the Massachusetts Institute of Technology dubbed the phenomenon "quantum procrastination," or "proquastination" in a commentary for Science accompanying the two research papers. "In the presence of quantum entanglement (in which outcomes of measurements are tied together)," he wrote, "it is possible to hold off making a decision, even if events seem to have already made one."
The new experiments add another wrinkle to the warped world of quantum mechanics, where a photon can be seemingly whatever it wants, whenever it wants. "Feynman called it the one true mystery of quantum mechanics," Shadbolt says of wave–particle duality. "It's deeply, deeply strange. Quantum mechanics is deeply weird, completely without classical analogue, and we just have to accept it as such."