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All the early quantum computing work was done with light. Light is very easy to manipulate: a few mirrors, crystals, and light detectors and you to can have your very own quantum computer. Over the last two decades, though, that's changed. Almost all the major developments have used things like ions, rings of superconducting current, or defects in crystals.

This was, in some sense, a reasonable progression. To perform logical operations, you have to modify one quantum state based on the state of another. Light beams, however, tend to pass right through each other without even waving, let alone stopping to chat. Contrast that with ions. Two ions, being charged, cannot avoid talking to each other. That means the quantum state of one ion can strongly influence the state of the other. This makes logical operations much easier.

The flip side is that quantum states that are easily modified are also easily destroyed by the environment. The quantum state of light, on the other hand, is remarkably robust. This has been demonstrated rather spectacularly by performing quantum key distribution between two locations via a satellite.

So light-based quantum states are certainly not out of the picture for quantum computing, though they are mostly considered for information carriers between locations. At each location, the light's quantum state is transferred to something else to perform the computation. But that transfer may be unnecessary now that researchers have developed material structures that allows light to strongly modify light.

Two photons walk into a glass bar, neither of them notice

So, why are photons so aloof? The problem is that they have to talk to each other via an intermediary: the material in which they travel. When light travels through glass, it slows down because the light fields make all the electrons in the glass jiggle about. This slow down produces the material's refractive index, which we tend to think of as independent of the light's brightness. If we ramp up the light intensity, the electrons jiggle a little more vigorously, but it doesn't change the way the light moves through the medium, so the refractive index hasn't changed.

For very bright light beams, however, the electrons have to move much further than they are comfortable with—the electrons are bound to atoms, so there is a limit to how far they can move. Once the light makes the electrons uncomfortable, the intensity of the light changes the refractive index. The way the light moves through the material changes, and all sorts of strange things happen. New colors can be generated, the light can focus, or a light pulse might steepen and becomes even shorter and more intense.

If the glass is between two mirrors, it can make these weird effects more apparent. Starting with a very dim light, the distance between the two mirrors tells us what color of light will enter into the space between them (if the front mirror lets light leak in). Light with the right color will reflect back and forth between the mirrors, and build up in brightness as more light leaks into the gap. At the same time, light leaks out of the gap via the second mirror.

After a while, the flow in equals the flow out, and we have reached equilibrium. Remarkably, all of the light that is shone on the front mirror, apparently, passes through the mirror into the gap—none is reflected. The light leaking out of the second mirror has the same brightness as the light shining on the front mirror. And, the light between the mirrors is extremely bright; the more reflective the mirrors, the brighter the light between the mirrors. Effectively, the gap between the mirrors acts as light storage.

If the light is bright enough, it will change the refractive index of the material in between the mirrors. That changes the color of light that can be accepted into the gap between the mirrors. As a result, we never reach the equilibrium described above. Instead, light is, initially, not reflected by the front mirror. As the brightness in the gap increases, however, the front mirror suddenly starts reflecting light. Effectively, light has switched the direction of the flow of light.

This is exactly what we need for our optical quantum computer: light changing the state of light.

The common theme running through these stories is brightness. You need high brightness, and that means lots and lots of photons. But quantum states are stored in single photons, which are the very opposite of bright. That is why optical quantum computers are languishing.

Two photons walk into a pillar, only one walks out

This is where new materials research comes into play. The goal is to create structures that are so sensitive that a single photon can change their properties. This works with a single atom. Let's imagine that we have a single atom pinned in space, and a squirt gun that fires single photons. We have excellent aim, so every photon will hit the atom. And, because we are clever, we choose the color of our photon so that it matches the energy separation required to excite the atom from its ground state to an excited state.

If we fire a single photon at the atom, it absorbs the photon, and, some time later, spits out a similar photon in a random direction. But, if we fire two photons, one right after the other, then the atom absorbs the first photon and lets the second pass through. One photon controls the passage of another. In contrast to the example above, this only needs a single photon to have an effect. It is exactly what we need.

But atoms don't stay put, unless you surround them with other atoms. And atoms don't absorb any old color of light, but only those colors that nature has chosen. Even worse, atoms are very small: the chance of a photon actually hitting an atom and being absorbed is very small. The Universe has let us down, and we need, to put it bluntly, a better class of atoms.