This is not something unexpected or shocking. It is how these particles are expected to behave, given the rules of quantum mechanics. I will try to
First of all, let's get rid of any faster-than-light ideas. Nothing that we know of can travel faster than light. Anything that did would have far
stranger properties than merely that of communicating information at implausible speeds. There is no communication between the measured particles, far
less between the measuring devices (or people) Alice, Bob and Victor. Please hold that thought.
Now then. These apparent temporal paradoxes are the result of
- thinking of the particles as objects, like marbles or bullets;
- considering the particles individually, as if they exist in isolation.
In actuality, what is happening is that a series of partial measurements is being made on a quantum system
over a period of time. The system
consists of all four particles and their histories, as well as the measuring devices and even, in some sense, the rest of the universe.
Each measurement partially 'collapses' the probability function (a mathematical relationship that fully describes the system), leading to a specific
result for the function at a certain time and place. This measurement can't tell us everything about the system; according to Heisenberg's famous
uncertainly principle, full information about the system can never be recovered. But we can still learn something about the particle and the system
from our measurements.
Now obviously these measurements, since they being made on the same system, must
correlate with one other. There is no choice in the matter.
This necessity lies at the heart of the Einstein-Podolsky-Rosen 'paradox', which suggests that information is flying between particles or measuring
devices faster than light. It isn't.
When Alice or Bob measure one of their particle pairs, this does not immediately give the other half of the pair any definite values at all; as long
as that second particle remains unmeasured, it is not a particle, a material entity occupying a single place at a particular time; it is merely a set
of probabilities. When Victor 'entangles' the two previously unmeasured members of the particle pairs produced by Alice and Bob, they become part of a
single system with a history that is the sum of their individual histories (this is the wave function of the system). This system is
with the particles previously measured by Alice and Bob. In fact, those particles are part of the new system, a four-particle system on which Victor
makes a fourth measurement.
Remember, a quantum state contains information about its own history. It is therefore no surprise that the histories of all four particles are
correlated. Really, it would be bizarre if they weren't.
What is this experiment telling us? Some of you are reading it as a signal sent backwards in time from a measurement made by Victor to the particles
measured by Alice and Bob, telling those particles what states to have when measured! On the contrary, it is far more reasonable to suppose that the
quantum state measured by Alice, Bob and Victor has correlated values at all points in space and time, as you would expect it to. Only those particles
that fulfil the conditions of entanglement are entangled. Remember, most particles don't make it all the way through the measurement process. Their
outcomes are not recorded; we don't know whether Victor entangled them or not. Perhaps these are the ones on which Victor did not make the correct
You mentioned that very few photons would make it through the entire apparatus. How do we know that Victor's sampling wasn't inherently biasing
the results to pairs of entangled photons?
rce: reader comment on OP linked article
Even the experimenters themselves do not rule out this possibility:
Note that in a conspiratorial fashion, Victor’s choice might not be free but always such that he chooses a separable-state measurement whenever
Alice and Bob’s pair is in a separable-state, and he chooses a Bell-state measurement whenever their pair is in an entangled state.
Original paper on Arxiv (PDF)
To me, the results of this experiment seem to confirm that the apparent randomness and indeterminacy that seem to affect physical systems at quantum
scales are something of an illusion. Reality, as we see from this experiment as well as the evidence that lies all around us, is deterministic in
spite of quantum uncertainty. The book of the future is already written.
edit on 24/4/12 by Astyanax because: editing was needed.