Measurements are funny things sometimes. It is, on the whole, pretty easy to detect light. It is even pretty easy to detect single photons. But a single photon has very little energy in it, so a single photon detector can't easily distinguish between one photon or two photons. Instead, the experimenter needs to arrange things such that only a single photon is around at any one time. Likewise, in a large light field, it is very difficult to selectively remove just a single photon with any certainty.

Riding to the rescue comes the weirdness of quantum mechanics. A group of researchers has shown that by carefully coupling lasers together, an atomic gas can be set up to absorb just a single photon. And that absorption can be efficiently detected with a third laser.

Rydberg atoms and blockades

To make an exquisitely sensitive detector, the researchers suggest that we make use of something called a Rydberg atom. This is actually a normal atom in a very special state. Atoms are made up of a positive core of protons and neutrons surrounded by a cloud of electrons. Normally, these electrons occupy the lowest energy states—each state can contain just one electron, so they stack up, lowest to highest energy. Sitting above them, unoccupied, are an infinite number of states that the electrons could occupy. Some of these states have such high energy that the electron is only barely bound to the atom, and it spends most of its time quite some distance from the nucleus.

This great distance creates a very large electric field that affects the electrons in the surrounding atoms. The field changes their orbits and, as a result, changes the energy required to excite an electron up to a higher energy, creating a Rydberg atom.

Imagine that you have chosen your excitation energy exactly right, so that you will get precisely the Rydberg state that you desire. As soon as one atom enters that state, it modifies the other atoms, changing the energy required to excite them into the same state. The effect is that this one atom prevents the other atoms from entering the Rydberg state.

So what?

The researchers suggest we can use the creation of a Rydberg state to count photons in a light field. We can do this by sequentially subtracting single photons from the field and detect the fact that we have subtracted the photon. To do this, we choose an atom that will absorb our light. The light field excites the atoms from the ground state into an excited state, which is basically how a traditional detector works.

To create a Rydberg state, we shine a second light field on the atom. This light field will be absorbed only if the atom is in the excited state. When absorbed, it puts the atom into a highly excited Rydberg state.

This doesn't work very well with just a single atom, because the probability of a single atom absorbing a photon is very small, and the probability of absorbing two photons (even sequentially) is even smaller. To get around this, the researchers suggest that we use a gas of cold atoms. With a group of a million atoms, the light has near unity probability of being absorbed. And, if the atoms are all in close proximity to each other (say five micrometers), once one is in the Rydberg state, none of the others can enter the Rydberg state. A true single photon detector.

This can all be achieved using fairly standard equipment in any lab that plays with ultra cold atoms, so this bit of theory proposes something that is eminently testable.

What I like best, though, is the detection scheme. You could just wait for the Rydberg state to decay, but that's not very efficient—when the atom finally emits a photon, it could send it any direction, and we might miss it. It also means we would still be trying to detect single photons, for which the efficiency is low.

Instead, the researchers propose to use electromagnetically induced transparency (EIT). EIT requires a carefully selected set of atomic energy levels and laser beams. But as soon as one atom in the cloud enters the Rydberg state, it shifts all the energy levels and messes up the transparency. As a result, the probing light field (which is not at the level of single photons) experiences a large phase shift, which happens to be easy to detect. And, because of the blockade, you know that the phase shift is due to the absorption of a single photon.

Effectively, the absorption of a single photon is amplified into a macroscopic effect on a much brighter light beam.

The downside is that you need to have a match between the light you want to detect and the energy levels of an atom. In general, this is never satisfied, so this will never be a general detector. But in the lab we do a lot of basic quantum mechanics research on well-known atomic systems. Single photon detection and subtraction is one of the basic tools for this research, so an implementation of a better single photon counter will be a significant development. I expect that we will see the first experimental results in about a year.

Physical Review Letters, 2011, DOI:10.1103/PhysRevLett.107.093601