The efficiency of quantum-cryptographic systems could be improved thanks to a new technique that uses "twisted light" to increase the amount of information carried per photon. Developed by an international team of researchers, the technique involves encoding 2.05 bits per photon by using the orbital angular momentum (OAM) of light instead of the more commonly used polarization of light, which only allows one bit per photon. The researchers say their new approach could be extended to achieve 4.17 bits per photon, and it could be used to make cryptographic systems more resilient to external eavesdroppers.

Unlike most other quantum technologies, quantum-cryptographic systems are already being used commercially by some banks and other organizations to ensure the secrecy of their communications. Such systems employ quantum-key distribution (QKD), which allows two parties – say Alice and Bob – to exchange an encryption key secure in the knowledge that it will not have been read by an eavesdropper (say, Eve). This guarantee is possible because the key is transmitted in terms of quantum bits (qubits) of information, which would be irreversibly changed if they were somehow intercepted and read, thereby revealing Eve.

One to two

Most current QKD schemes make use of the different polarization orientations of a photon – vertical, horizontal, diagonal, antidiagonal – but this only allows one qubit to be transferred per photon. In the new work, Mohammad Mirhosseini of the University of Rochester in the US and colleagues worldwide have doubled the amount to 2.05 qubits per photon by encoded their qubits using the OAM and the azimuthal angular position (ANG) of photons. OAM involves the wavefront of a beam of light spiralling around its propagation axis and is sometimes referred to as "twisted light".

These two properties provide what are known as "mutually unbiased bases" – an essential requirement for QKD. Using such bases means that a correct key is revealed only if Alice encodes the information using a particular basis and Bob measures in that same basis. As the OAM and ANG are mutually unbiased with respect to one another, an eavesdropper would not be able to detect a photon simultaneously in both bases, thereby boosting its security.

Twisted alphabet

Once Alice and Bob have generated their QKD, they publicly announce the basis they have used for each symbol in the key and compare what basis was used for sending and which basis was used for receiving. They only keep the part of the key in which they have used the same bases and this ultimately produces a secure key, which can then encrypt messages and indeed transmit them with regular encryption without the need for quantum cryptography.

Mirhosseini, who is part of Robert Boyd's group at Rochester's Institute of Optics, says that the team was able to encode a 7D "alphabet" – seven letters or symbols – using OAM and the ANG. "Our experiment shows that it is possible to use 'twisted light' for QKD and that it doubles the capacity compared with using polarization," he says, further explaining that "unlike with polarization, where it is impossible to encode more than one bit per photon, twisted light could make it possible to encode several bits, and every extra bit of information encoded in a photon means fewer photons to generate and measure".

The team demonstrated that its system can generate and detect information with 93% accuracy and at a rate of 4 kHz. In the future, the researchers hope to push the rate to the gigahertz level, which is desirable for telecommunication applications. In an earlier experiment that used a strong laser beam instead of single photons, Boyd's team was able to measure up to 25 modes or bases of OAM and ANG, rather than seven. If that method is applied to the new scheme, it could be used to transmit and measure 4.17 bits per photon using more sophisticated equipment.

The work is published in New Journal of Physics.