(Phys.org)—For the first time, physicists have demonstrated that hyperentangled photons can be transmitted in free space, which they showed by sending many thousands of these photons between the rooftops of two buildings in Vienna. Hyperentanglement means that the photons are simultaneously entangled in at least two different properties—in this experiment, the researchers combined two two-dimensionally entangled properties to achieve four-dimensional hyperentanglement.