LH12960
- When it comes to pitting quantum theory against classical notions of
the world, there’s one experiment that physicists say makes quantum
theory the clear winner: a test of Bell’s inequalities. We have found
that it’s possible to fake quantum results using classical physics in
such a test, reminding us to be cautious about the assumptions in
experiments. Bell inequalities measure the strength of correlations
between two particles, or how much their behaviors are coordinated.
Quantum physics allows for stronger correlations than classical physics,
violating a Bell inequality. In our experiment, a typical apparatus for
measuring Bell violations by photons was cheated using bright pulses of
light to manipulate the output of single-photon detectors. Researchers
thinking ahead to quantum devices for communication and computation have
proposed incorporating tests of Bell inequalities in such devices to act
as safeguards. Our work highlights the practical challenges of such
schemes. It also reminds us that no Bell tests so far have been without
seemingly reasonable ‘fine print’ assumptions about how the experiments
worked. In our case, we exploited the so-called detection loophole. Our
attack was an evil intervention. We have no reason to think the Universe
conspired to trick past experiments, but one would prefer to eliminate
the need for fine print. Various experimental groups worldwide are
working towards this goal.
