Researchers at Beijing Institute of Technology have developed a photonic TAM manipulator that effectively utilizes photon angular momentum, opening up new avenues for data transmission, encryption and quantum signal processing. The new technology enables efficient identification and real-time control of angular momentum patterns.

Rotating objects carry angular momentum, a property that extends to the tiniest particles, such as photons. Photons possess two different forms of angular momentum: spin angular momentum (SAM) and orbital angular momentum (OAM). Spin angular momentum dances between two eigenvalues, representing left and right circular polarization, while orbital angular momentum has an infinite number of eigenvalues, corresponding to the spiral phase. When SAM is combined with OAM, we witness the emergence of "total angular momentum" (TAM), a photonic toolbox with a wide range of applications, covering lidar, laser processing, optical communications, optical computing, quantum information, and more.

Just as OAM has brought revolutionary changes to the field, efficient identification and real-time control of TAM patterns also provide the key to TAM's breakthrough applications. However, existing methods for identifying photon TAM states have limitations, including limited dynamic range, low identification accuracy, and the inability to adjust filters on the fly. These constraints limit the development and application progress of TAM. Extracting the desired TAM pattern from a photon beam remains an unsolved mystery.

Conceptual structure of the total angular momentum manipulator: A beam carrying multiple angular momentum modes is filtered through the manipulator. Source: Li et al., doi10.1117/1.AP.5.5.056002.

According to the magazine Advanced Photonics, researchers at Beijing Institute of Technology have developed a photonic TAM manipulator that removes obstacles and enables on-demand manipulation of SAMs and OAMs. Their approach involves a symmetrical cascade of two similar units: a TAM splitter and a TAM inverter. These units consist of specialized optical elements called unpackers and correctors.

We can think of the photonic TAM manipulator as a conductor, leading a symphony orchestra of light. The TAM splitter converts the incoming beam into a spatially arranged combination of fringes, with each fringe representing a TAM pattern. The spatial filter begins to work, deciding which TAM patterns need to be retained and which need to be blocked. Finally, the TAM reverser brings the separated beam back to the spatial domain to complete the symphony. This conversion process maps the incident beam from the spatial domain to the "position-TAM domain", allowing for filtering before conversion to the spatial domain.

System performance in pass-through and selective blocking situations when multiple TAM states are incident. (a) Experimental results of the incident beam; (b) TAM spectra of the output beam in the above two cases. In the pass-through case, the output mode is the same as the input mode. For the selective blocking case, the spatial filter placed on the separation plane is Sp2. When blocked, the pattern of these beams changes from petal-shaped to donut-shaped. Source: Li et al., doi10.1117/1.AP.5.5.056002.

The experimental demonstration reported by the researchers supports the identification of up to 42 individual TAM patterns. The research results show that TAM has good state selection performance and is therefore particularly attractive for high-speed and large-capacity data transmission and high-security photonic encryption systems. It also provides new perspectives on high-fidelity photonic computing and quantum radar signal processing.