The more entanglement in the so-called Hilbert space—the realm where quantum information processing can take place—the better. Previous photonic approaches were able to reach 18 qubits encoded in six entangled photons in the Hilbert space. Purdue researchers maximized entanglement with a gate using four qudits—the equivalent of 20 qubits—encoded in only two photons. In quantum communication, less is more. "Photons are expensive in the quantum sense because they're hard to generate and control, so it's ideal to pack as much information as possible into each photon," said Poolad Imany, a postdoctoral researcher in Purdue's School of Electrical and Computer Engineering....