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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.
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, 2012. DOI: 10.1073/pnas.1213201110 (About DOIs).
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