If you observe a beam of light closely enough, you will find that it is made up of a very great many individual photons, and it is the number of these photons that determine how bright the beam is. Normally bright is good because the brighter something is the easier it is to see, but sometimes you want just one photon, and researchers have been working to find a way to make that happen. Well some at MIT have recently succeeded in a pretty clever way.
The ability to create a stream of single photons from a larger current of them would be invaluable to quantum devices such as photonic gates that operate like logic gates in a computer. A single photon could be used to switch the gate and affect the light passing through it. To accomplish this, the researchers used a cloud of ultra-cold rubidium atoms and a couple lasers. One laser was shone on the cloud to stimulate electromagnetically induced transparency (EIT). Normally the cloud would be opaque, but the laser causes it to allow photons to pass through it, albeit slowly. However, only the first photon to enter the cloud sees it as transparent.
The atoms within the cloud enter what is called a Rydberg state in which an electron jumps to a very high orbit, making the atom quite large. This allows the atoms to interact quite strongly with each other, and thus prevents the other photons from passing through. Once that first photon leaves the cloud though, another photon will be able to pass through the cloud thanks to EIT, but only one photon.
The uses for this setup are quite diverse as it can enable new quantum devices, like quantum gates and more. Traditionally to observe and measure a photon would require destroying the photon to do so, but this example of non-resonant single photon physics may allow for observations to be made without destroying the photon.