An international team of researchers has succeeded in measuring the shape of individual photons for the first time. The result could prove extremely useful for secure data transmission using light.
Pulses of light can have almost any shape in space and time, and these shapes depend on the amplitudes and phases of the pulse's frequency components. Data can be encoded in light pulses by modulating the amplitude or phase of the light. Single photons and other quantum light states can also be generated in a variety of complex shapes and encoding information in these different shapes could be an efficient way to securely transmit data. Indeed, a single photon shape could represent, for example, any letter in the alphabet, or even a quantum combination (or superposition) of several letters.
However, the problem is that once a photon has been sent through some apparatus – such as an optical fibre – its shape can become distorted and the information contained within becomes impossible to decipher. A team led by Marco Bellini of the Istituto Nazionale di Ottica in Florence, Italy, and colleagues have now managed to measure the precise shape of the mode of a quantum light state that appears at the receiving end by means of a "mode-selective" detector.
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