Enlarge / This is a more traditional beamsplitter. Note the lack of white paint. (credit: Colorado State)

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.

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