1. Christian Huygens discovered in 1690 polarized light - this is the first quantum effect ever observed. The transformation behavior of rays of completely polarized light was first described by Etienne-Louis Malus 1809 (who coined the name ”polarization”), and that of partially polarized light by George Stokes 1852.
   In modern terminology, the behavior described by Malus (resp. Stokes) is identical to that of a qubit in a pure (resp. mixed) state. Stokes 1852 paper contains all modern quantum phenomena for a single qubit, discussed in classical terms. (For details, see my lecture http://www.mat.univie.ac.at/~neum/ms/optslides.pdf)
   The transverse nature of polarization was discovered by Augustin Fresnel 1866, and the description in terms of (what is now called) the Bloch sphere by Henri Poincare 1892. In modern terminology, polarization is a manifestation of the massless spin 1 nature of the unitary representation of the Poincare group defining photons.

2. The second oldest observed quantum effect are spectral lines, apparently first discussed in 1802 by William Hyde Wollaston. (For the history of spectroscopy, see http://www.spectroscopyonline.com/spectroscopy/article/articleDetail.jsp?id=381944)

Both phenomena require quantum physics for their explanation (though polarization can also be explained by a statistical version of classical electrodynamics).
But, of course, before 1900 nobody considered these to be quantum effects. Spectral lines were first described as a quantum effect in 1913 by Niels Bohr. Polarization was first described as a quantum effect in 1930 by Norbert Wiener.

1. Christian Huygens discovered in 1690 polarized light - this is the first quantum effect ever observed. The transformation behavior of rays of completely polarized light was first described by Etienne-Louis Malus 1809 (who coined the name ”polarization”), and that of partially polarized light by George Stokes 1852.
   In modern terminology, the behavior described by Malus (resp. Stokes) is identical to that of a qubit in a pure (resp. mixed) state. Stokes 1852 paper contains all modern quantum phenomena for a single qubit, discussed in classical terms. (For details, see my lecture http://arnold-neumaier.at/ms/optslides.pdf)
   The transverse nature of polarization was discovered by Augustin Fresnel 1866, and the description in terms of (what is now called) the Bloch sphere by Henri Poincare 1892. In modern terminology, polarization is a manifestation of the massless spin 1 nature of the unitary representation of the Poincare group defining photons.

2. The second oldest observed quantum effect are spectral lines, apparently first discussed in 1802 by William Hyde Wollaston. (For the history of spectroscopy, see http://www.spectroscopyonline.com/spectroscopy/article/articleDetail.jsp?id=381944)

Both phenomena require quantum physics for their explanation (though polarization can also be explained by a statistical version of classical electrodynamics).
But, of course, before 1900 nobody considered these to be quantum effects. Spectral lines were first described as a quantum effect in 1913 by Niels Bohr. Polarization was first described as a quantum effect in 1930 by Norbert Wiener.

1. Christian Huygens discovered in 1690 polarized light - this is the first quantum effect ever observed. The transformation behavior of rays of completely polarized light was first described by Etienne-Louis Malus 1809 (who coined the name ”polarization”), and that of partially polarized light by George Stokes 1852.
   In modern terminology, the behavior described by Malus (resp. Stokes) is identical to that of a qubit in a pure (resp. mixed) state. Stokes 1852 paper contains all modern quantum phenomena for qubits, explained in classical terms. (For details, see my lecture http://www.mat.univie.ac.at/~neum/ms/optslides.pdf)
   The transverse nature of polarization was discovered by Augustin Fresnel 1866, and the description in terms of (what is now called) the Bloch sphere by Henri Poincare 1892. In modern terminology, polarization is a manifestation of the massless spin 1 nature of the unitary representation of the Poincare group defining photons.

2. The second oldest observed quantum effect are spectral lines, apparently first discussed in 1802 by William Hyde Wollaston. (For the history of spectroscopy, see http://www.spectroscopyonline.com/spectroscopy/article/articleDetail.jsp?id=381944)

Both phenomena require quantum physics for their explanation (though polarization can also be explained by classical statistical electrodynamics).
But, of course, before 1900 nobody considered these to be quantum effects. Spectral lines were first described as a quantum effect in 1913 by Niels Bohr. Polarization was first described as a quantum effect in 1930 by Norbert Wiener.

1. Christian Huygens discovered in 1690 polarized light - this is the first quantum effect ever observed. The transformation behavior of rays of completely polarized light was first described by Etienne-Louis Malus 1809 (who coined the name ”polarization”), and that of partially polarized light by George Stokes 1852.
   In modern terminology, the behavior described by Malus (resp. Stokes) is identical to that of a qubit in a pure (resp. mixed) state. Stokes 1852 paper contains all modern quantum phenomena for a single qubit, discussed in classical terms. (For details, see my lecture http://www.mat.univie.ac.at/~neum/ms/optslides.pdf)
   The transverse nature of polarization was discovered by Augustin Fresnel 1866, and the description in terms of (what is now called) the Bloch sphere by Henri Poincare 1892. In modern terminology, polarization is a manifestation of the massless spin 1 nature of the unitary representation of the Poincare group defining photons.

2. The second oldest observed quantum effect are spectral lines, apparently first discussed in 1802 by William Hyde Wollaston. (For the history of spectroscopy, see http://www.spectroscopyonline.com/spectroscopy/article/articleDetail.jsp?id=381944)

Both phenomena require quantum physics for their explanation (though polarization can also be explained by a statistical version of classical electrodynamics).
But, of course, before 1900 nobody considered these to be quantum effects. Spectral lines were first described as a quantum effect in 1913 by Niels Bohr. Polarization was first described as a quantum effect in 1930 by Norbert Wiener.