It's hard to imagine a photon having a shape at all. For one thing, photons—pointlike, indivisible units of light—are massless, which is the whole essence of being a photon to begin with and what enables such particles to set the universe's maximum speed limit (the c in E = mc2). How does something without mass even occupy geometric space? Alternatively, how can space be not empty yet not contain any mass?
As it turns out, photons can take on different shapes and sizes and this winds up mattering a great deal when it comes to interactions between light and matter (such as atoms). To this end, researchers at the National University of Singapore have devised a method for shaping photons with extreme precision, allowing for an unprecedented look at these light-matter interactions at atomic scales.
Their work, which is described this week in Nature Communications, demonstrates that shape plays a key role in predicting whether or not an atom is likely to absorb an incoming photon, an insight likely to have consequences for the development of quantum information technologies, which hinge upon light-matter interactions.
This is among the most fundamental things in the electromagnetic world: Photons carry energy, and when a photon is absorbed by an atom, the atom takes up that energy. This might result in the atom emitting its own photons at new wavelengths (giving rise to the innate colors of objects) or otherwise displaying new properties. This interaction is what enables photosynthesis, as photons from the Sun convey energy to chloroplasts, which convert light energy into chemical energy.
Quantum technologies may similarly rely on this interaction, as information can be stored and retrieved by changing the state of an atom via photon absorption. We might just imagine the unenergized atom as a 0 and the energized version as a 1.
Specifically, what the Singapore group wanted to experimentally demonstrate is the fairly old prediction that the absorption of a photon by an atom should be the time-reversed mirror of the emission of of a photon by an atom. Geometrically, the events should look like the temporal inverse of each other and the probabilities of each event occurring should be related. Absorption is the opposite of emission.
In the words of Christian Kurtsiefer, lead investigator at the Centre for Quantum Technologies at the National University of Singapore, and colleagues, "the conditions for perfect absorption of an incident single photon by a single atom in free space can be found from the reversed process, the spontaneous emission of a photon from an atom prepared in an excited state."
To explore the relationship between photon shape and photon absorption, Kurtsiefer and his team built giant photons. So, imagine single photons that not only occupy space and have defined shapes, but are 4 meters in length. They were then able to take their superlong photons and apply different envelopes to them, resulting in two distinct shapes: one where the envelope sharply rises and one where it sharply falls. Imagine a teardrop shape pointed in different directions. In one version, the photon arrives dimly and ends brightly, and, in the other, it arrives brightly and ends dimly.
The physicists found that the photons that arrived dimly were about 50 percent more likely to be absorbed by an atom. This is what we would expect—when an atom spits a photon out, it's a bright or decaying photon (it leaves dimly), which is the inverse of the arriving photon's shape (or the arriving photon most likely to be absorbed).
We're left with an interesting demonstration featuring crazy huge light particles, but one also one that offers promise for future quantum technologies that might depend on manipulating light-matter interactions, such as sensors, memory architectures, communications networks, and other optoelectronic devices.