Experiments thought to prove non –
locality may be artifacts
Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D 07745 Jena
Entanglement, the instantaneous transfer of a quantum property through
space, is at the core of numerous philosophical discussions.
Usually experiments thought to prove entanglement,
are described theoretically with a single atom model.
However the experiments are performed in a completely different way:
Such experiments are essentially counting experiments!
Essentially, coincidence rates are measured. An excess of coincidences
compared to some theoretical expectation is seen as proof of nonlocality. Mathematically it is tested, whether the Bell inequalities are violated.
The present talk shows, that such an excess of coincidence rates
can be caused in a very classical way.
Experiments thought to prove non – locality may be artifacts
Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D 07745 Jena
The problem is,
that one has to do some dull calculations.
Exactly this will in the following be done for one
representative example (Gröblacher, Nature 446, 871-875 (2007).
It is dull, but the reward will be
perhaps a clearer view on non-locality.
The idealized version of the experiments
reproduced from Aspect, A. "Bell's inequality test: more ideal than ever" Nature 398, 189 - 190 (1999)
A single atom emits a pair of photons.
Polarization is analysed by a polariser on each side of the experiment.
classically, measuring on the right side does not affect the spin of the photon on the left
Probabilities that photons pass their polarisers are independent of each other
non locally, measurement on one side affects the spin of the photon on the other side.
either both polarisers simultaneously allow their photon to pass, or both stop them.
higher rate of identical results on both sides (surplus coincidences)
A real experiment
Gröblacher, S., Paterek, T., Kaltenbaek, R., Brukner, C., Zukowski, M., Aspelmeyer, M. and Zeilinger, A. "An
experimental test of non-local realism" Nature 446, 871-875 (2007)
Experiments thought to prove non – locality may be artifacts
Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D 07745 Jena
Single photon detectors
Single photon detectors are often believed
to be s i n g l e p h o t o n detectors
However, while they indeed can detect
a single isolated photon, they usually cannot
distinguish between 1, 2 or many photons.
From this follows a very critical problem
for measuring entanglement
It cannot be safely said whether a single photon pulse really
contains just one photon. This may become disastrous for
measurements of entanglement.
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When a laser-pumped down converting
crystal is used
The geometry in a real multi - atom or molecules light source. Always
billions of emitters are involved
Everbody who claims to have absolute control that exactly one photon
pair is emitted per time has to prove this thoroughly
One photon pair versus two or more pairs
classical (local):
all outcomes possible
non locality: A and B enriched
at the costs of C
i.e increase of coincidences
situation with two photons:
at least one photon has the correct
orientation to pass
classical increase of coincidences
Apparent non locality caused
by classical surplus coincidence rates
• Eik = (N+++ N-- - N+- - N-+ ) /
(N+++ N-- + N+- + N-+ )
• SNLHV = E11 + E23 + E22 + E23
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• If, according to the NonLocal HiddenVariable theory, the sum SNLHV turns out to
be smaller than 3.792 (= 4 – (4/ p) sin 9.4o) in the settings used here, this
would indicate classical locality. At larger values one would state nonlocality.
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• If the Eik = (N + N - N - N ) / (N + N + N + N ) are too large because of the just
decribed surplus coincidences, one states non – locality where the
situation is local – for simply classical experimental reasons
++
--
+-
-+
++
--
+-
-+
Experiments thought to prove non – locality may be artifacts
Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D 07745 Jena
Conclusion
• Present experiments thought to prove
non-locality can be interpreted
classically.
• Thus, care is needed when invoking
non-locality
Experiments thought to prove non –
locality may be artifacts
Karl Otto Greulich. Fritz Lipmann Institute Beutenbergstr. 11 D 07745 Jena
Detecting non-locality via entanglement, reflected by the instantaneous transfer
of quantum properties over a large distance, requires almost ideal
experimental conditions – ideal sources for entangled photon pairs and ideal
detectors. Deviations from this ideal situation may critically hamper
interpretations of experiments on non-locality or entanglement. Of two major
known loopholes, the detector loophole is not yet unequivocally closed. Now
another loophole emerges: When in such experiments downconverting
crystals with lasers as primary light sources are used, exceeding threshold
values in the Bell inequalities, i.e. their violation, can be caused by so far not
fully recognized problems in the generation and detection of photon pairs.
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Taken together, nonlocality is still not yet unequivocally proven. Thus, one of
the most serious experimental challenges of causality is on brittle ground.
References: K.O. Greulich , Another loophole for the Bell inequalities Proc. of SPIE Vol 7421 – 08, (2009) ;
K.O. Greulich Proc. of SPIE Vol 8121-15, (2011); for downloads see, http://www.fli-leibniz.de/www_kog/
then klick *Physics*
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KEYWORDS:
Non locality, Bell inequalities, single photon detectors, down conversion