A research team has developed a new technique, called stimulated emission tomography, to measure qubits based on the orbital angular momentum (OAM) of photons. This methodology allows for the complete characterization of OAM quantum states, including those that are degenerate, meaning they share the same OAM magnitude but differ in their helicity sign. This advance is crucial for the development of quantum computing and quantum communication, as OAM qubits offer greater information capacity per photon compared to polarization qubits.

Traditionally, measuring OAM qubits has faced significant challenges, especially when dealing with degenerate states. Existing techniques often require complex optical setups or are unable to distinguish between states with the same absolute OAM value. Stimulated emission tomography overcomes these limitations by using a stimulated emission process to selectively amplify OAM states, enabling precise reconstruction of the qubit's density matrix. This method could simplify the characterization of quantum devices and improve the fidelity of operations.

The ability to accurately and efficiently measure degenerate OAM qubits opens new avenues for encoding information in quantum systems. Photons with OAM can carry a theoretically unlimited number of discrete states, making them promising candidates for creating high-dimensional qubits (qudits). This work represents a significant step towards realizing more robust and higher-capacity quantum systems, with direct implications for quantum cryptography and fault-tolerant quantum computing.