  Light beams having a vectorial field structure - or polarization - that
varies over the transverse profile and a central optical singularity are called
vector-vortex (VV) beams and may exhibit specific properties, such as focusing
into "light needles" or rotation invariance, with applications ranging from
microscopy and light trapping to communication and metrology. Individual
photons in such beams exhibit a form of single-particle quantum entanglement
between different degrees of freedom. On the other hand, the quantum states of
two photons can be also entangled with each other. Here we combine these two
concepts and demonstrate the generation of quantum entanglement between two
photons that are both in VV states - a new form of quantum "entangled
entanglement". This result may lead to quantum-enhanced applications of VV
beams as well as to quantum-information protocols fully exploiting the
vectorial features of light.
