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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 action-at-a-distance. Chiao first defines action-at-a-distance 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 “space-like interval.”) Quantum nonlocality, in particular, is a form of action-at-a-distance 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 Aharonov-Bohm effect and the tunnel effect, nonlocality arises out of single-particle interference. In the second case, the Einstein-Podolsky-Rosen effect, non-locality involves two-particle interference, i.e., an entangled state.

1. In the Aharonov-Bohm 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 single-particle interference pattern whose phase shift depends on the magnetic flux contained by the torus. Chiao sees this kind of quantum non-locality 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 two-particle 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 down-conversion in a nonlinear crystal. Chiao used Franson’s modification where Mach-Zehnder interferometers replaced Fabry-Perot 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 non-locality 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 Hong-Ou-Mandel 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 non-locality: 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 “neo-Berkeleyan” 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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