The raw material for many quantum computers and quantum key distribution systems are single photons. For a quantum computer, the current ideal implementation would allow users to perform computations using the quantum states of atoms or ions, and communicate between the atoms and ions using single photons. Likewise, most quantum key distribution is performed by encoding a quantum state in a single photon. The upshot of this reliance on photons is that network and computational flexibility requires some method to route photons to different locations.

This is not actually a huge problem, provided that the routing is not part of the computation—we define the paths through a computer or network before starting and then set everything in motion. But this is clearly not going to work if we want to implement a quantum "if" statement, which requires some form of dynamic routing. To overcome this problem, researchers at the California Institute of Technology have been working on single photon routers. These routers direct photons to different locations depending on the presence or absence of an atom. Since the presence of an atom at a certain location is itself a quantum property, this could be the "if" statement implementation researchers have been looking for.

The experiment makes use of ultra-cold cesium atoms, a micro-toroid, and some optical fiber—of course if it were this simple, implementations would be working in someone's garage. The optical fiber is stretched so that it has a thin section, where the fields associated with the single photons extend beyond the edges of the fiber. The stretched section is placed close to the micro-toroid, which sucks the passing photon into it.

Why does the photon get sucked in? Photons always occupy modes. The fiber, because it confines the photons, has only one spatial mode; likewise, the toroid has only a single spatial mode. But, because the toroid is such a great resonator (e.g., a photon, once in the toroid, will stay there for a long time), its mode has a very strong vacuum field associated with it (the vacuum field can be thought of as the probability of a passing photon choosing to occupy that mode). Surrounding this are the countless unconfined vacuum modes associated with free space, each of which has a very small vacuum field associated with it.

So the passing photon, confined in the fiber, finds its field overlapping with the immensely strong vacuum field of the toroid, and tunnels through the glass barrier, across the vacuum, and into the toroid. If set up correctly, this happens with a probability very close to one.

Now we've got the photon in the toroid, and its going to stay there for a long time. Eventually, however, it will depart—every time it goes past the fiber, it feels the vacuum field associated with the empty fiber. When it does leave, it will continue down the fiber as if nothing had happened. The researchers take advantage of the time it spends in the toroid, however, by literally raining cold cesium atoms down onto this space. As the atoms enter the fields associated with the photon, the photon responds as if the diameter of the toroid had suddenly increased.

To understand why, you have to know that a key property of an optical resonator is that a photon, on returning to its starting location, has exactly the same phase as when it started. The photons that will resonate in the toroid are those that see the total length of a complete circuit as a whole number product of their half wavelengths. This is what causes the constructive interference that allows a huge optical field to build up.

The presence of the cesium atom lengthens the circuit length and, as a result, the photon doesn't fit in the toroid anymore. Instead, it gets absorbed by the atom, which sits around for a while and then re-radiates the photon back into the toroid. But—and this is the cool part—the photon hits the atom from a particular direction and, as a result, sets the electrons around the atom vibrating at a particular orientation. Consequently, it cannot radiate the photon back into the toroid in the same direction that it was traveling previously. Instead, it radiates it in the reverse direction.

Since the photon is now circulating the reverse direction, when it finally tunnels back into the optical fiber, it will be going in the opposite direction. The presence of the atom has acted as a switch to change the direction the photon was going.

There are some other very cool things about this. As a single atom passes by the toroid, it has time, on average, to re-route a single photon. So although the photons heading into this device look like steady streams of photons with gaps (a phenomenon called bunching), the reflected photons are single photons with big gaps, called anti-bunched. Another important feature is that the router is robust in terms of physical alignment. As long as the fiber is close enough to the toroid, the exact distance doesn't matter that much, a property that holds true of the cesium atoms as well.

You might be asking how such a system can provide the proverbial quantum "if" statement. The answer to this lies in the absorption of the photon by the atom. If the atom cannot absorb the photon because it is already in an excited state that doesn't allow it, then the atom will not route the photon. I can envision setting up an experiment where there is a specific probability that the atom is in an excited state, called a superposition state. The photon would then be reflected or transmitted in a probabilistic manner that reflects the probabilities of the superposition state, meaning the router would display quantum behaviors.

I wonder if I can use this to get money for predicting the contents of Science six months in advance.

Physical Review Letters, 2009, DOI: 10.1103/PhysRevLett.102.083601

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