Individual photons have been put into a quantum superposition of two different colours by a team of physicists in the US and Germany. Such photons could be useful for connecting different parts of quantum-information networks that operate using differently coloured light.

Superposition is an important concept of quantum mechanics that allows a physical system to be in two or more quantum states at the same time – until a measurement on the system puts it into a specific state. A photon, for example, can be in a superposition of a horizontally polarized state and a vertically polarized state until it passes through a polarimeter.

Information can be encoded into quantum states and then processed in a quantum computer, which uses superposition and other features of quantum mechanics to process information much faster than is possible with conventional computers.

Two-colour states

Normally when physicists think of a photon, it is in a well-defined energy state having a specific colour. However, quantum mechanics allows the photon to be in a superposition of two or more energy states – or colours. In this latest work, Stéphane Clemmen and colleagues at Cornell University, Humboldt-University Berlin and Columbia University have created photons that are "bichromatic" by being in a superposition of two different colours.

The team made the bichromatic photons using a technique called "Bragg-scattering four-wave mixing". This takes place in a 100 m-long optical fibre that is pumped with two laser beams. When a "red" photon is shone into the fibre, it interacts with the laser light and is put into a bichromatic superposition of the initial red state and a second "blue" state.

The set-up can be adjusted so that the photon emerges from the opposite end of the fibre with an equal probability of being either red or blue when its colour is measured.

Phase proof

Clemmen and colleagues were also able to adjust the relative phase between the red and blue states in the quantum superposition. This allowed them to create photons that were all blue when detected, or all red, or a specific combination of red and blue. This ability to adjust the phase is proof that the photons were in a coherent quantum superposition. The team also showed that to a very high probability, the experiment detects one photon at a time – which means that the researchers are really seeing single photons in a superposition of two colours.

The technique could someday be used to connect quantum devices that operate using different colours of light. Two quantum memories, for example, could be put into a state of quantum entanglement by inputting a bichromatic photon. Such entangled memories would prove useful for a range of quantum-computing and quantum-communication applications. Other potential uses include spectroscopy measurements on living samples such as eyes, which must be done using very low levels of incident light.

The research is described in Physical Review Letters.