Twisted photons can in principle carry a discrete unbounded
amount
of orbital angular momentum (OAM), which are of great significance
for quantum communication and fundamental tests of quantum theory.
However, the methods for characterization of the OAM quantum states
present a fundamental limit for miniaturization. Metasurfaces can
exploit new degrees of freedom to manipulate optical fields beyond
the capabilities of bulk optics, opening a broad range of novel and
superior applications in quantum photonics. Here we present a scheme
to reconstruct the density matrix of the OAM quantum states of single
photons with all-dielectric metasurfaces composed of birefringent
meta-atoms. We have also measured the Schmidt number of the OAM entanglement
by the multiplexing of multiple degrees of freedom. Our work represents
a step toward the practical application of quantum metadevices for
the measurement of OAM quantum states in free-space quantum imaging
and communications.
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