An ordinary envelope has a rectangular shape, and the letter inside does not change the envelope's properties. With photons, particles of light, the situation is different: their 'shape' in time can vary, and this shape itself carries information. In the new work, physicists created single microwave photons (which are light, but with a much longer wavelength than visible light) and gave them three different temporal shapes, as if three letters were placed in envelopes of different geometries.
These three shapes are mutually orthogonal: mathematically they do not overlap, like three different letters of the alphabet. The photons were sent through a 30-meter cable cooled to ultra-low temperatures, where thermal noise almost disappears. At the other end, a receiver could absorb one shape and reflect the other two, doing so with high selectivity—40 times better than for unwanted shapes. This is like a mailbox that only takes envelopes of a certain shape and returns the rest to the sender.
This trick expands the possibilities of quantum communication. Previously, only one standard photon shape was used to transmit quantum states; now information can be encoded in the shape itself, and at the receiver one can choose which shape to read. This is a step toward quantum networks where nodes exchange photons with different temporal profiles without interfering with each other.
The microwave photons used in the experiment have wavelengths on the order of centimeters, like the radiation in a typical microwave oven, but billions of times weaker in energy.
The idea of encoding information in the shape of a photon echoes the 'quantum envelopes' from Liu Cixin's novel 'The Three-Body Problem', where sophons change their properties to transmit messages.