Optical connections are slowly replacing wires as a means of shuffling bits in between systems—there are already plans afoot to have different components within a single system communicate via an optical connection. But, so far at least, all the processing of those bits is taking place using electrons.

Yesterday's edition of Science includes a demonstration of an all-optical transistor that can be switched between its on and off states using a single photon. Although it's an impressive demonstration of physics, the work also indicates that we're likely to stick with electrons for a while, given that the transistor required two lasers and a cloud of a cold atomic gas.

The work relied on a cold gas of cesium atoms. These atoms have an extremely convenient property: two closely separated ground states, each with a corresponding excited state. All of these states are separated by an energy that corresponds to a specific wavelength of light, so using a laser of that wavelength allows you to shift the system into a different state.

The cloud of atoms was placed between two closely spaced, extremely efficient mirrors—a setup that's called an optical cavity. The optical cavity allows photons to bounce back and forth for seconds before they get lost, crossing the narrow distance many, many times.

What happens when you send in a photon that is at the right energy to correspond to the difference between one of the ground and excited states? Typically, it'll get absorbed by an atom, then emitted again, bouncing off the mirrors a few times before getting absorbed and re-emitted. As the process continues, quantum mechanics causes it to create what's called a collective excitation, where the entire gas cloud behaves like it's in an excited state.

Normally, the atoms start out in the lowest energy ground state, and a photon would place them in the corresponding excited state. But the authors used a separate laser to switch them from that excited state to the second ground state, shifting the collective excitation to the higher energy ground and excited states. Because the energy gap between these two is different, this changed the wavelength of the light involved ever so slightly, although the photon was still trapped in the optical cavity.

Let's focus on that second wavelength of light. Normally, the cesium gas is transparent to it, since it's all in the lowest energy ground state, while the light is specific to the higher energy ground state and its corresponding excited state. So, from a transistor perspective, the gate is open, and light flows through it. Things change when you trap a photon at the higher energy ground state. Now, the right energy states are available to it, but they're already occupied by the collective excitation of the atoms. The light can't even enter the gas and gets reflected back. In effect, the gate switches off.

The amazing thing is that a single photon is enough to create a collective excitation that blocks any light (of the appropriate wavelength) from crossing the cavity. "Remarkably," the authors write, "one stored gate photon can block more than ∼600 source photons." It's possible to store more photons, however, and the more you store in the device, the more efficient the blockage is. By three stored photons, almost nothing gets through the gate when it's in the off state.

To switch the gate back on, the authors can simply turn on their control laser again, shifting the system back to the low energy ground and excited states. The original photon will pop back out, and it's possible to detect it.

Overall, the system doesn't work all that efficiently—the combined storage and retrieval of photons in the gate only preserved about three percent of them. But they're also not using anything especially fancy (like single photon light sources) or chilling the atoms down as much as they could. So, their paper outlines a number of things that could be tried that would likely up the efficiency and stretch out the time that photons get stored within the gate.

The downside is that, even with the relatively relaxed approach, the amount of equipment required—two lasers to operate the gate, and a third to send photons through it, as well as the lasers needed to chill and trap the cesium atoms—probably involves a number of table tops. It's a far cry from miniature electronics.

Science, 2013. DOI: 10.1126/science.1238169 (About DOIs).