Alladi Ramakrishnan Hall

Testing violation of local-realism (on Nobel Prize in Physics, 2022)

Prof. Sibasish Ghosh

IMSc

Bothered by (apparently?) intrinsic indeterminism of
measurement outcomes in Quantum Theory (QT) and also by the `spooky
action-at-a-distance' due to existence of quantum correlated far
apart systems, Einstein (together with Podolsky and Rosen in 1935) argued
for incompleteness of QT -- jointly using the principles of locality
and realism (known as local-realism). Almost after thirty years (in
1964), John S. Bell provided an experimentally verifiable scheme --
through the violation of a statistical inequality (known as Bell's
inequality (BI)) -- to test whether the measurement correlations in a
physical theory of Nature would be incompatible with the principle of
local-realism. Using the measurement correlations arising out of
projective measurements on the individual subsystems of the singlet
state of a combined system of two two-level quantum systems, Bell
also verified that incompatibility -- implying generically that QT
is incompatible with local-realism. Do measurement correlations in
Nature are also incompatible with local-realism, in general? Only
experimental results can answer that. The first consistent
experimental violation of a more general version of BI (known as the
CHSH inequality, named after its inventors (in 1969) Clauser, Horne,
Shimony, and Holt) -- although not very decisive, mainly due to the
existence of both `locality' as well as `detection' (also known as
`fare sampling') loopholes -- was provided by Clauser and his student
Freedman in 1972 using two-photon polarized entangled state, prepared
out of an atomic cascade process. Ten years later (in 1982), Aspect
and his collaborators designed a much better scheme for generating
two-photon polarized entangled states, again in atomic cascade, but
which helped to get rid of the locality loophole. Achieving detection
loophole-free experimental violation of the CHSH inequality using
entangled photon pairs was quite challenging until recently, mainly
because photo-detectors were not very efficient. But designing
experiment for showing violation of BI where both the loopholes can
be removed was quite challenging -- in which real success was made by
Zeilinger and his collaborators (and, independently by two other
groups) during 2014-2015. Using random measurement settings directed
by high-redshift quasars, Zeilinger and his collaborators also got
violation of CHSH inequality (in 2018) in which the
`freedom-of-choice' loophole has also been removed to a great extent.
Thus, we are now in a position to claim -- without (almost) any
ambiguity -- that measurement correlations in Nature are also
generically incompatible with local-realism. On the other hand, the
first experimental manifestation of quantum teleportation was
performed by Zeilinger and his collaborators in 1997 -- using
polarization entangled photon pairs. Loophole-free violation of BI
(or, more generally, CHSH inequality) has now paved the way for
designing device-independent secured quantum information processing
schemes (like quantum key distribution, quantum random number
generation, etc.).