The subtlest experiment in quantum mechanics is also one of the simplest: send a stream of particles through two openings in a barrier, and you'll produce an interference pattern because the particles act as waves. Amazingly, this also works if you send the particles through one at a time—the interference pattern builds up slowly as more particles go through. The double-slit experiment has been replicated with photons, electrons, atoms, and even entire molecules.
Typically, the particle nature and the wave nature have to be observed separately; if you track the particles through a single slit, the interference pattern vanishes. However, Ralf Menzel, Dirk Puhlmann, Axel Heuer, and Wolfgang P. Schleich entangled two photons and allowed one to pass through a barrier with two slits. The entanglement enabled them to determine which opening the photon went through, but a detector on the other side still picked up an interference pattern, demonstrating light's wave- and particle-like characteristics simultaneously.
The key to the experiment is the particular state in which the photons were produced. The researchers started with a laser in a configuration known as TEM01 mode, which means the electric (E) and magnetic (M) fields are perpendicular (or transverse, T) to the direction the photons travel. The "01" means there are two distinct points of maximum intensity.
From a quantum mechanical point of view, a single photon from this source is a combination of two quantum states superposed, so a measurement will find a photon preferentially in one or the other of these intensity maxima with equal probability.
The beam of photons was directed onto a double slit so that each of the points of maximum intensity were more or less aligned with one of the openings. (The researchers tested this using a special detector, similar to the technology in digital cameras.) Thus, an individual photon should pass through either one or the other slit, based on which intensity maximum it is in.
But you wouldn't necessarily know which of the slits the photon went through—determining that required an additional step. Before reaching the openings, the laser was directed onto a crystal of beta barium borate (BBO), which reemits light in two complementary polarization states, leaving the pair of photons entangled. Both of these photons still had the double-maximum structure of the TEM01 mode and, since they were entangled, any measurement on one will tell us something about the other.
One photon (known as the signal) was sent through the double slits, while the second (the idler) passed to a photon counter, labeled D1. D1 revealed which slit the signal photon went through, thanks to the combination of entanglement and the TEM01 double maximum structure.
The researchers used a second counter, called D2, to reveal the final position of the signal photon. First, D2 was placed right next to the slits, so it was able to tell which the signal photon went through. This was used to confirm that the entanglement measurements matched up with the ones from the detector. Then D2 was pulled far enough away from the slits for the photons to interfere. In this position, it measured the complete interference pattern produced by the single photons.
It's the TEM01 beam structure that allowed the simultaneous measurement of the wave-like interference pattern and the particle-like path through one of the slits. When they used a laser with a single maximum, they lost the ability to determine which opening an individual photon passed through.
Ultimately, this experiment is an intriguing new addition to the literature on the wave-particle duality. In the traditional way of thinking, the particle and wave natures of photons cannot be accessed simultaneously. While these results seem to violate that principle, it's the TEM01 mode of the laser that allows the determination of which opening an individual photon passes through. Similarly, building up the interference pattern requires moving the D2 detector away from the slits, meaning that D1 provides the only information about the trajectory. Nevertheless, the researchers have provided an interesting new way to approach the wave-particle duality.