A key feature of quantum physics is the wave-particle duality: the tendency of physical systems to exhibit both wavelike and particle-like behaviors. One particularly striking example of the wave-particle duality is the quantum eraser. In a typical experiment, two photons are entangled in a particular way and sent along different paths. As is usual in entanglement, a measurement performed on one photon reveals the outcome of related measurements on the second. In the specific case of the quantum eraser, however, the measurement dictates whether the second photon will exhibit wave- or particle-like characteristics.
Updated version of the previous article.
Because quantum eraser experiments rely on entanglement, the impact of the measurement influences the second photon instantaneously. But to date, all the examples have been performed under circumstances that would technically allow communication between the devices that perform the measurements. New results by Xiao-Song Ma and colleagues definitively rule that possibility out: they placed the experimental apparatus on two of the Canary Islands, separated by 144 kilometers.
The quantum eraser experiment involves producing two sets of photons with correlated polarizations. One set, known as the system photons, are sent into a polarizing beam-splitter (PBS); as the name suggests, this directs light along different paths based on its polarization. The two possible paths for the system photon were then recombined, so they could either interfere (if the photon is behaving like a wave) or show up in one of two detectors (behaving like a particle).
In this case, the second group of photons—called the environment photons—was sent 144km across open air from La Palma to Tenerife. (The team had broken the previous record for entanglement across wide distances.) The lab on the second island used a telescope to collect the light (which dispersed significantly over the intervening distance) and send it to a device to measure its polarization. The orientation of this device was selected randomly using a "quantum random number generator."
In one orientation, the detector measured the circular polarization of the environment photons. Because they were entangled, the system photons also interacted as circularly polarized light, so the two paths produced by the PBS interfered with each other—meaning they behaved like waves. If the detector was set to the other orientation, it measured linear polarization of the environment photons. That meant the system photons also remained in linear polarization mode, so the PBS would simply act like a filter, sending the photon either along one path or another without interference. That selected the particle-like behavior for the system photons.
That's the nature of the quantum eraser: in the particle mode orientation, the interference pattern that would ordinarily occur was "erased." In wave mode, the specific path a photon might follow was erased. Since the distant detector on Tenerife was the one to select which mode the eraser would operate in, the result of the measurement on the system photons was known first—earlier in time than the "decision" was made. This is known as a delayed-choice measurement.
Partly due to the intrinsic difficulty of entanglement measurements, prior quantum erasure experiments were confined to a lab, meaning smaller distances between detectors. That meant experimenters could not rule out some kind of physical interaction between the detectors. The latest quantum eraser measurements, as with other long-distance entanglement experiments, handily eliminate the possibility of communication between the different detectors. This specific example just does so in a very intuitive manner.
This doesn't do violence to relativity by assuming instantaneous information transfer. Prior experiments ruled out the possibility of faster-than-light, yet non-instantaneous communication (see previous Ars coverage here). We've known for some years that entanglement precludes communication between apparatus separated by large distances.
Philosophical questions aside, the interesting aspect of quantum erasers (to this writer at least) is what they reveal about our preconceptions of particle and wave behavior.
PNAS. DOI: 10.1073/pnas.1213201110 (About DOIs).