Sorting Photons via Their Radial Quantum Number
Light beams can be more complex than simple plane waves, taking on intricate shapes with twisted wave fronts. Photons within these beams belong to modes that carry different momentum components, such as orbital angular momentum (OAM), and these differences can be exploited to separate photons into groups. Researchers have previously sorted photons based on their azimuthal OAM quantum number, which is related to the rate at which a mode’s wave front twists around the direction of light propagation. A new study demonstrates that photons can also be separated via their radial quantum number—another momentum-related quantum number that is linked to the radial structure of the wave front’s amplitude and phase. The team says that their method could be used to store and retrieve information in all of a photon’s spatial degrees of freedom. Such a possibility could increase the number of channels available for sending information, potentially boosting the speed of optical protocols used in technologies like quantum metrology or quantum communication.
The method developed by Robert Boyd from the University of Rochester, New York, and colleagues, relies on a key property of photons with different radial quantum numbers: they have slightly different phase velocities. Using a series of lenses and wave plates, the team constructed an optical path that exploited this phase-velocity difference to induce a phase shift on photons in different radial modes. After traversing the optical path, two photons with radial modes differing by one quantum number accumulated a phase shift of 180°. The photons were then guided into an interferometer that separated them via this amplified phase difference. The photons with different radial OAM quantum numbers were visualized on CCD screens as patterns with different numbers of rings.
This research is published in Physical Review Letters.
Katherine Wright is a Contributing Editor for Physics.