Scientific Method / Science & Exploration

Measurements retroactively force photons to be both wave and particle

Temporal order and identity both get confused in the quantum world.

One of the devices in which photons will act as a particle and wave simultaneously.
Fernando Tarquin

One of the stranger features of the quantum world is that light—even individual photons—can behave as a wave or a particle, depending on how you measure it. But, according to papers released by Science today, the quantum weirdness doesn't end there. Researchers have now found a way to put a photon in a quantum superposition where it is both a wave and a particle at the same time. Worse still, one setup allows them to determine the photon's nature as a wave or particle after it has gone through an apparatus where it must act as one or the other.

Got that? Didn't think so, so let's go through it in more detail.

The two experiments use a similar design. Polarized photons are sent one at a time into a device, where they first encounter a beamsplitter, which has a 50/50 chance of sending them down one of two paths. On one of the paths, they will encounter a device that rotates the polarization a bit. Mirrors then send the photons toward an intersection flanked by two detectors.

If the intersection is empty, the photons will act as particles, meaning that they'll travel down one of the two paths, and the two detectors will click with equal frequencies. If, however, a second beamsplitter is present, the two photons can recombine and interfere with each other. In this case, the wave-like nature of the single photon takes over. It travels down both paths, interferes with itself, and, because of this interference, can only strike one of the two detectors. The presence of the second beamsplitter determines whether a photon will act as a wave or a particle.

One objection to this is that there could be some mechanism by which a photon could detect the sort of device it's about to enter and behave accordingly (a possibility that could occur through some hidden variables). The obvious way to deal with these objections is to decide whether or not to put the second beamsplitter in place only after the photon has passed through the first one, and must already be acting as either a wave or particle. But photons move very quickly, so switching a device in time would be exceedingly challenging.

This being quantum mechanics, it turned out to be easier to violate causality instead.

Using two entangled photons, it's possible for measurements of one photon to tell us about the fate of the second. If we send one photon through the device and then measure the second, it's possible to determine whether the first one should encounter the beamsplitter or not. Through the use of a delayed choice experiment, it's even possible to perform this measurement after the first photon has been through the entire apparatus and hit the detectors. In other words, the results of the measurement of photon two dictate what photon one must have already done.

So that's precisely what the authors of one paper did. By entangling the polarization of two photons and sending one through the device, then measuring the second, they could determine whether the beamsplitter was present after the first photon encountered it. Through a test of Bell's inequalities, which account for the presence of hidden variables, they were able to determine that the system was behaving in a truly quantum manner: the photons were both a wave and a particle at the same time.

The second paper did something substantially similar, but also did something a bit more complex with the second photon. Through a careful manipulation of its polarization, researchers were able to set the first photon (the one that went through the apparatus) into a superposition of wave and particle states, meaning it was an indeterminate mixture of the two. By changing how they handled the second photon, they could also control the probabilities of this superposition, making it more wave or particle-like on demand. As with the other experiment, they did this all after the first photon had been through the device and been measured by the two detectors.

Any one of these facts—being a wave and a particle at the same time, a measurement influencing the behavior of events that have already taken place, and a physical beamsplitter being in a quantum superposition of present and not present—are all pretty mind-bending. Putting them all together in a single experiment is especially so.

But an accompanying perspective suggests that the quantum mechanics aren't done yet. If we can get a quantum memory that is stable for extended periods of time, it should be possible to hold off measuring the polarization state of photon two for seconds, perhaps even minutes. In that case, the delay portion of these delayed choice experiments would actually be perceptible to the people doing the experiment.

Science, 2012. DOI: 10.1126/science.1226719, 10.1126/science.1226755  (About DOIs).

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