Quantum objects are notoriously shifty. Take the photon, for example. The quantum of light can act as a particle one moment, following a well-defined path like a tiny projectile, and a wave the next, overlapping with its ilk to produce interference patterns, much like a ripple on the water.
Wave–particle duality is a key feature of quantum mechanics, one not easily understood in the intuitive terms of everyday experience. But the dual nature of quantum entities gets stranger still. New experiments demonstrate that photons not only switch from wave to particle and back again but can actually harbor both wave and particle tendencies at the same time. In fact, a photon can run through a complex optical apparatus and disappear for good into a detector without having decided on an identity—assuming a wave or particle nature only after it has been destroyed.
Physicists have shown in recent years that a photon "chooses" whether to act as a wave or a particle only when forced. If, for instance, a photon is steered by a beam splitter (a kind of fork in the optical road) onto one of two paths, each leading to a photon detector, the photon will appear at one or the other detector with equal probability. In other words, the photon simply chooses one of the routes and follows it to the end, like a marble rolling through a tube. But if the split paths recombine before the detectors, allowing the contents of the two channels to interfere like waves flowing around a pillar to meet on the other side, the photon demonstrates wavelike interference effects, having essentially traversed both paths at once. In other words, measure a photon like a particle, and it behaves like a particle. Measure a photon like a wave, and it acts like one.
One might suspect that photons simply assume one of two behaviors—wave or particle—beforehand, or when they hit the beam splitter. But a 2007 "delayed choice" experiment ruled out that possibility. Physicists using an interferometer, an experimental device that includes the beam splitter, toggled between combining the paths and leaving them separate. But they made the choice only after the photon had passed through the beam splitter. The photons still demonstrated interference effects when recombined, even though (in a simpler world, at least), the particles should already have been forced to decide which path to take.
Now, two research groups have implemented an even more bizarre version of the delayed choice experiment. In two studies in the November 2 issue of Science, a team based in France and a group in England each reported using a quantum switch to toggle the experimental device. Except in this experiment, the switch was not flipped—thus forcing the photon to act like a wave or like a particle—until the physicists had identified the photon in one of the detectors.
By changing the settings on the device, both teams could not only force the experimental photon to behave as a particle or as a wave, but could explore intermediate states as well. "We can continuously morph the behavior of the test photon from wavelike to particlelike behavior," says Sébastien Tanzilli, a study co-author and a quantum optical physicist with the National Center for Scientific Research in Paris who is based at the University of Nice Sophia Antipolis. "Between the two extremes, we have states that come with reduced interference. So we have a superposition of wave and particle."
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."