Once a year, everyone at the MESA+ Institute, where I work, gets together to celebrate the achievements of the past 365 days. Everyone listens to talks by students, post docs, and learned professors. If something catches my interest, I grab the publications and have a closer look. This year was no different.

In one of the optics sessions, a soon-to-be-minted PhD presented one of his key findings: a funny kind of optical hardware that offers unique opportunities for researchers doing quantum experiments. Although simple and boring on the surface (it's a beamsplitter, nothing more than a partially reflective mirror), his simple component is exactly what makes optical quantum computing possible. I promise his results are exciting and unexpected.

An ode to the beamsplitter

A beamsplitter is just a partially reflective mirror. In a standard optics text-book, a beamsplitter is a plate of glass that reflects exactly half the light that strikes it and allows the other half to pass unimpeded (so no light is absorbed). But a light wave has more than just an amplitude (how bright it is)—it also has a phase. The phase of the transmitted and reflected light beams are not the same. Essentially, when light crosses and/or reflects from a surface, the electric field has to obey certain rules of continuity (just like movie fans, nature abhors discontinuities). So, for instance, the electric field is not allowed to suddenly jump from one value to another as it crosses the interface. The only way that this can be satisfied is if the reflected light and the transmitted light have a phase difference of 180 degrees.

This means that if we were to lay the reflected and transmitted waves next to each other, the peaks of the electric field would not line up. Instead, the peaks and the troughs would line up. Normally, this wouldn't matter, but it has strange consequences once you look deeper.

Let's imagine we have a beamsplitter—a partially reflective plate of glass—set up at a diagonal to a beam of light. The light beam comes in from the left. The light is partially transmitted to exit stage right and partially reflected to vanish through the floor. I can add a second beam, though, that approaches the beamsplitter from above. This beam will be partially reflected to exit stage right and partially transmitted to vanish through the floor. Now, the light exiting to the right will be a mix of some light that has been transmitted by the beamsplitter and light that has been reflected. So one beam (the reflected beam) has undergone a phase shift, while the other has not.

Now let's turn down the brightness of the light so that there is only one photon from each beam hitting the beamsplitter at a time. If the two photons arrive separately, then each can be reflected or transmitted and nothing special happens. But, if they arrive together, that phase change matters. If one photon tries to go right and one tries to go down, then their electric fields will add to zero at the interface, so no light will exit the beamsplitter. That means the beamsplitter somehow absorbed both photons—which it can't do, because it is made from a non-absorbing material. Since we need to conserve energy, the photons never take different paths.

Instead, both photons have to go right, or both photons have to go down. The chance of going in either direction is still 50/50, but whatever the dice roll, both photons stick together. This is called photon bunching, and all ordinary beamsplitters do this. Indeed, photon bunching is used in certain quantum computing operations.

But sometimes doing the opposite would also be good: to anti-bunch photons and to have control over the degree of bunching/anti-bunching. Until now, this was difficult.

That beamsplitter is... not right

What do you need to create a beamsplitter that allows both bunching and anti-bunching? White paint. Oh, and a few other small pieces of technical equipment.

White paint is white because it scatters light. Think of it like sugar. If you examine a single crystal of sugar, it appears transparent, but it glints. The glint comes from a small amount of light that is reflected from the sugar's crystal facets. A pile of sugar appears white because all those tiny reflections add up to all the light being reflected in a very disordered way.

The same is true for a thin layer of paint, except that some light makes it through. The light that goes through is typically reflected along the way, so you basically get a halo with randomly positioned bright and dark spots. The bright spots correspond to where different paths through the paint add up in phase.

That knowledge is what drives the bit of research that we're looking at. We can control which paths the light takes through the paint by controlling the phase of the light at each point where it enters the paint. The phase determines which path the light takes, and the phase determines whether the different paths add up at a location on the other side.

By controlling the phase of the light in a spatially dependent manner, we can choose to focus light to a point through a layer of paint. But this technique has far more flexibility. Instead of focusing to a point, you can also focus to two points with equal brightness. The paint acts like a beamsplitter.

To achieve this trick, the light goes through an LCD screen. The liquid crystal in an LCD display changes the apparent distance through which light must pass, depending on the voltage applied to the pixel. This allows the researchers to tune the relative phase of the light beam in space so each little patch of light has a slightly different phase from the patches that surround it.