Expanding useful degrees of freedom for more powerful quantum cryptography

For millennia, people have studied how to send messages securely between two parties without those messages getting intercepted. The field of cryptography has developed many protocols over the last half-century to keep digital communications secure, but the advent of quantum computing risks invalidating all of them. To combat the decrypting potential of quantum computers, experts are increasingly turning to quantum cryptographic algorithms that are physically impossible to crack with any computer.

Most existing quantum cryptography encodes information using only two dimensions of polarization but the need for higher bitrates and more secure transmission has led researchers to examine approaches using all of a photon’s degrees of freedom. Forbes et al. examined recent advancements in this field, the potential these new approaches offer, and the existing challenges and directions for future research.

“Unlike our classical networks, quantum communications has not fully explored or used all of light’s degrees of freedom,” said author Andrew Forbes. “The time is right because we now know what can be done with simple two-dimensional states, and it works. With this done, we can lift our eyes to the future.”

By deliberately structuring photons, quantum communications can employ both polarization and orbital angular momentum, transmitting more information using fewer photons. However, a major challenge when using additional degrees of freedom is avoiding loss of information due to distortion. One potential solution involves encoding information using topology, such as quantum skyrmions, which remain unchanged in the face of distortion.

“But how to pack information into topology, and how to get it out again?” said Forbes. “This is both the challenge and opportunity.”

Source: “Quantum cryptography with structured photons,” by Andrew Forbes, Mostafa Youssef, Sachleen Singh, Isaac Nape, and Bora Ung, Applied Physics Letters (2024). The article can be accessed at https://doi.org/10.1063/5.0185281 .