**Quantum experiments with human eyes as detectors and micro-macro entanglement**

It is interesting to think about ways of bringing quantum
phenomena into the realm of everyday experience. One promising
approach is via quantum optics. The human eye responds reliably to of
order one hundred photons. It is possible to create entangled states
of light of that order of magnitude by starting from a single
entangled photon pair, and amplifying one of the two initial photons
through a process known as quantum cloning by stimulated emission.
This is an example of micro-macro entanglement, i.e. entanglement
between a single microscopic quantum system and another system that is
in some sense macroscopic (similar to Schroedinger's famous cat
thought experiment). One can show that such micro-macro entanglement
allows one in principle to violate Bell inequalities, thus allowing
the demonstration of quantum non-locality using human eyes as
detectors. This leads to the question whether the same kind of
measurement also proves that the micro-macro state is entangled. A
careful analysis shows that the Bell inequality violation only proves
the original (micro-micro) entanglement. Unambiguously proving
micro-macro entanglement seems to require measurements with
single-photon resolution (which the human eye does not provide),
raising the question whether quantum-level resolution is a general
requirement for observing quantum effects in macroscopic systems.
References:
[1] P. Sekatski, N. Brunner, C. Branciard, N .Gisin, and C. Simon,
Quantum Experiments with Human Eyes as Detectors based on Cloning via
Stimulated Emission, Phys. Rev. Lett. 103, 113601 (2009).
[2] P. Sekatski, B. Sanguinetti, E. Pomarico, N. Gisin, and C. Simon,
Cloning Entangled Qubits to Scales One Can See, Phys. Rev. A 82,
053814 (2010).
Popular description of some of this research:
http://www.wired.com/wiredscience/2010/06/human-quantum-entanglement-detector/