Standard Green Lasers Unlock Quantum CNOT Operations

Coherent photons possess two distinct forms of angular momentum: spin and orbital. When these orthogonal states exist in superposition, they can describe a qubit, the basic unit of quantum information. New research demonstrates that the internal structure of these photons can be accurately described using representation theory, specifically Lie algebra and Lie groups.

Applying this theory, scientists successfully performed a controlled-NOT (CNOT) operation using a standard green laser diode. They utilised beam splitters and waveplates to generate macroscopic entangled light, with amplitudes and phases controlled by rotating the waveplates. Through a process called Bell projection, the light reveals a dipole shape, allowing the team to visually identify the state from the far-field image.

To execute the logic gate, the team used a polarisation-dependent beam splitter to separate the spin state. They then applied a NOT operation—using a pair of cylindrical lenses—specifically to a vortex with a chosen polarisation before recombining the beams. This architecture proposes a promising platform for manipulating qubits made from macroscopically coherent photons.