Chiao, Raymond Y. “Quantum Nonlocalities: Experimental Evidence."
The purpose of Raymond
Chiao’s essay is to show, by a careful discussion of specific experiments, that
the world possesses at least three kinds of nonlocal actionatadistance.
Chiao first defines actionatadistance in general as a correlation between
effects, events, or conditions separated by a spacelike interval. (If a light
signal cannot be sent between two events, their separation in space and time is
called a “spacelike interval.”) Quantum nonlocality, in particular, is a form
of actionatadistance which has no classical explanation. But does quantum
nonlocality violate special relativity? Not according to Chiao, for two
reasons: quantum nonlocality never reverses the order of cause and effect and
in any case, and quantum events cannot be used for signaling because of the
fundamentally probabilistic, uncontrollable nature of quantum events.
Chiao then interprets all
three kinds of quantum nonlocalities as resulting from the superposition
principle (i.e., quantum interference) in which the sum of allowable states is
also an allowable state. In the first and third examples, namely the
AharonovBohm effect and the tunnel effect, nonlocality arises out of
singleparticle interference. In the second case, the EinsteinPodolskyRosen
effect, nonlocality involves twoparticle interference, i.e., an entangled
state.
1. In the AharonovBohm
experiment, a beam of electrons is split, one beam passing through the hole of
a superconducting torus, the other around the torus. After being rejoined, the
beam displays a singleparticle interference pattern whose phase shift depends
on the magnetic flux contained by the torus. Chiao sees this kind of quantum
nonlocality as topological in nature: it is the global topology of the split
beams that lead to the local, interference, effect. The phenomenon arises from
the local gauge invariance of the electromagnetic interaction and it can be
used to explain the Lorentz force.
2. In the EPR experiment
with two particles, quantum nonlocality arises from the “nonfactorizability” of
the quantum states: since the twoparticle state of the system is the
superposition of the products of the two states of the individual particles,
the superposition cannot be factored mathematically into separate states for
each particle. Being so entangled, the state of the system, when measured,
depends on the states of both particles in the system regardless of the
distance separating them. In the 1960s, John Bell showed that EPR results
violate the philosophical assumptions Einstein and his colleagues made in
defending “local realism.” Bell then proposed that properties of particles,
such as position, momentum, spin, etc., do not exist until they are observed,
reminiscent of Berkeley’s idealism. Chiao presses this point further, claiming
that the nonfactorizability of the entangled states implies the
“nonseparability” of the quantum world.
Chiao then describes the EPR
experiment performed in his lab, in which pairs of photons are prepared in an
entangled state by spontaneous parametric downconversion in a nonlinear
crystal. Chiao used Franson’s modification where MachZehnder interferometers
replaced FabryPerot and Michelson interferometers in the detection process.
This leads to a modified Bell inequality in which the twin photons possess
neither definite energy (color) nor a definite time of emission prior to their
detection. Moreover, nonlocality is further demonstrated by the fact that a
change in the path length of one of the interferometer arms changes the
behavior of the photon which passes along the other, unchanged, interferometer
arm.
3. The third kind of nonlocality
occurs in quantum tunneling where certain kinds of superluminal velocities are
possible during tunneling. Here two photons are emitted simultaneously and
their arrival times at equal distances are measured. If a barrier is inserted
into one of the paths, the difference in the time of arrival constitutes a
precise definition of the tunneling time. But will the photon traversing the
tunneling path arrive before or after the photon following along the free path? In theory, a superluminal result is possible,
in which the tunneling photon arrives first. Chiao shows why this result does
not violate relativity: relativity allows for superluminal group velocities and
only forbids superluminal front velocities. Moreover, such superluminal effects
are governed by the uncertainty principle, and thus cannot constitute a
controllable signal.
He then describes in detail
the resulting experiment using a HongOuMandel interferometer. Chiao’s results
help decide between three conflicting theories about how to define tunneling
time, and they showed that the tunneling process is indeed superluminal in the
allowable sense. According to Chiao, such superluminal tunneling implies a
third kind of nonlocality: an observer moving past the barrier at close to the
speed of light would infer that the particle exists simultaneously at both the
entrance and the exit faces of the barrier!
Chiao concludes with some
additional philosophical and theological reflections in light of these results
and his Christian faith. He supports a “neoBerkeleyan” point of view in which
the free choices of observers lead to nonlocal correlations of properties of
quantum systems in time as well as in space, giving Berkeley’s dictum, esse est percipi, temporal as well as
spatial significance. Theologically he uses this generalized Berkeleyan point
of view to depict God as the Observer of the universe. Here God creates the
universe as a whole (ex nihilo)
and every event in time (creatio continua).
The quantum nonseparability of the universe is suggestive of the New
Testament’s view of the unity of creation. In the process Chiao discusses such
ideas as the quantum entanglement of all events in the universe given their
common origin in the Big Bang, and he responds to the challenge of the quantum
Zeno paradox.
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