Once a year, everyone at the MESA+ Institute, where I work, comes together to celebrate the achievements of the past 365 days. Everyone listens to lectures by students, postdocs and learned professors. If something piques my interest, I grab the publications and take a closer look. This year was no different.
In one of the optics sessions, a soon-to-be published doctorate presented one of his most important findings: a fun 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 reflecting mirror), its simple component is exactly what makes optical quantum computing possible. I promise you that his results are exciting and unexpected.
An ode to the beam splitter
A beamsplitter is just a partially reflecting mirror. In a standard textbook on optics, a beamsplitter is a sheet of glass that reflects exactly half of the light falling on it and lets the other half pass unobstructed (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 emitted and reflected light rays are not the same. Essentially, when light crosses and/or bounces off a surface, the electric field must obey certain rules of continuity (like movie buffs, nature abhors discontinuities). For example, the electric field should not suddenly jump from one value to another as it passes through the interface. The only way this can be met is if the reflected light and the transmitted light have a phase difference of 180 degrees.
This means that if we put the reflected and emitted waves side by side, the peaks of the electric field wouldn’t line up. Instead, the peaks and troughs would line up. Normally this wouldn’t matter, but it has strange consequences if you look deeper.
Let’s imagine we have a beamsplitter – a partially reflective sheet of glass – arranged diagonally to a beam of light. The light beam enters from the left. The light is partly transmitted to exit the stage on the right and partly reflected to disappear through the floor. However, I can add a second beam that approaches the beam splitter from above. This beam is partially reflected to leave the stage on the right and partially disappear through the floor. Now the light exiting to the right will be a mix of some light that has passed through the beamsplitter and light that has been reflected. So one beam (the reflected beam) has undergone a phase shift, the other one has not.
Now let’s lower the brightness of the light so that only one photon from each beam hits the beamsplitter at a time. If the two photons arrive separately, each can be reflected or transmitted and nothing out of the ordinary happens. But when they arrive together, that phase change matters. If one photon tries to go right and one tries to go down, then their electric fields at the interface will add to zero, so no light will leave the beamsplitter. That means the beamsplitter somehow absorbed both photons – which it can’t do, because it’s made of a non-absorbing material. Because we need to conserve energy, the photons never take different paths.
Instead, both photons must go to the right, or both photons must go down. The odds of going either way are still 50/50, but no matter what the die rolls, both photons stick together. This is called photon bundling and all common beam splitters do this. Photon bundling is indeed used in certain quantum computing operations.
But sometimes it’s also good to do the opposite: photons against bundling and control the amount of bundling/anti-bundling. So far this has been difficult.
That beam splitter is wrong
What do you need to make a beamsplitter that allows both bundling and anti-bundling? White paint. Oh, and a few other little technical gear.
White paint is white because it scatters light. Think of it like sugar. If you examine a single sugar crystal, it appears transparent, but it sparkles. The shimmer comes from a small amount of light reflected from the crystal facets of the sugar. A pile of sugar appears white because all those little reflections cause all the light to be reflected in a very disordered way.
The same goes for a thin layer of paint, only some light gets through. The light that passes through it mostly gets reflected along the way, so you basically end up with a halo of randomly positioned bright and dark spots. The bright spots correspond to where different paths through the paint add up in phase.
That knowledge is the driving force behind the piece of research we are looking at. We can control the 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 travels, and the phase determines whether the different paths add up at a location on the other side.
By spatially controlling the phase of the light, we can choose to focus light through a paint layer to a point. But this technique has much more flexibility. Instead of focusing on one point, you can also focus on two points with the same brightness. The paint acts like a beam splitter.
To achieve this trick, light passes through an LCD screen. The liquid crystal in an LCD screen changes the apparent distance light has to 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 that each small patch of light has a slightly different phase than the spots around it.