Immaculate quantum light source created at the edge of the silicon chip

The smallest amount of light you can have is a photon, so dim that it is virtually invisible to humans. Although imperceptible, these small energy dots are useful for carrying quantum information. Ideally, each quantum carrier would be the same, but there is no simple way to produce a flow of identical photons. This is particularly difficult when individual photons come from fabricated chips.

Now, Joint Quantum Institute (JQI) researchers have demonstrated a new approach that allows different devices to emit unique photons that are almost identical. The team, led by Mohammad Hafezi, a JQI associate, has fabricated a silicon chip that guides the light around the edge of the device, where it is inherently protected from disturbance. Previously, Hafezi and his colleagues have shown that this design can reduce the likelihood of degradation of the optical signal. In an article published online on September 10 in Nature, the team explains that the same physics that protects the light along the edge of the chip also ensures reliable photon production.

Unique photons, which are an example of quantum light, are more than just lights. This distinction has a lot to do with where the light comes from. "Almost all the light we encounter in our daily lives is filled with photons," says Elizabeth Goldschmidt, a researcher at the US Army Research Laboratory and co-author of the study. "But unlike a light bulb, some sources actually emit light, one photon at a time, and that can only be described by quantum physics," says Goldschmidt.

Many researchers are working to build reliable quantum light emitters so that they can isolate and control the quantum properties of single photons. Goldschmidt explains that such light sources are likely to be important for future quantum information devices, as well as to better understand the mysteries of quantum physics. "Modern communications rely heavily on non-quantum light," says Goldschmidt. "Similarly, many of us believe that single photons will be needed for any type of quantum communication application."

Scientists can generate quantum light by using a natural process of color change that occurs when a beam of light passes through certain materials. In this experiment, the team used silicon, a common industrial choice to guide light, to convert infrared laser light into single-color pairs of different colors.

They injected light into a chip containing a network of tiny silicon loops. Under the microscope, the loops look like glassy racetracks connected to each other. Light circulates around each loop thousands of times before moving to a neighboring loop. Lying, the path of the light would be several inches, but the loops can store the path in a space about 500 times smaller. The relatively long path is necessary to obtain several pairs of single photons from the silicon chip.

Such loop networks are commonly used as single photon sources, but small differences between the chips will vary the colors of the photons from one device to another. Even in a single device, random defects in the material can reduce the average photon quality. This is a problem for quantum information applications where researchers need the photons to be as close as possible.

The team circumvented this problem by organizing the loops so that light could still flow freely on the edge of the chip, even in the case of manufacturing defects. This design not only protects the light from disturbances, but also limits the formation of single photons in these edge channels. The loop arrangement essentially forces each pair of photons to be almost identical to the next, regardless of the microscopic differences between the rings. The central part of the chip does not contain protected routes, so that all the photons created in these areas are affected by material defects.

The researchers compared their chips to those without any protected route. They collected pairs of photons from the different chips, counting the number emitted and noting their color. They observed that their quantum light source reliably produced high quality monochrome photons over and over again, while the output of the conventional chip was more unpredictable.

"We first thought that we needed to be more careful with the design and that the photons would be more sensitive to the manufacturing process of our chip," says Sunil Mittal, postdoctoral researcher and lead author of the new study. "But, surprisingly, the photons generated in these shielded edge channels are still almost identical, regardless of the quality of the chips."

Mittal adds that this device has an added advantage over other single photon sources. "Our chip works at room temperature, so I do not need to cool it to cryogenic temperatures like other quantum light sources, which makes it a relatively simple installation."

The team believes that this discovery could open a new avenue of research that links quantum light to photonic devices with built-in protection functions. "Physicists have only recently realized that shielded pathways fundamentally change the way photons interact with matter," explains Mittal. "This could have implications in various areas where the interactions between light and matter play a role, including the science of quantum information and optoelectronic technology."

Explore more:
Physicists Demonstrate New Method for Making Unique Photons

More information:
Sunil Mittal et al. A topological source of quantum light, Nature (2018). DOI: 10.1038 / s41586-018-0478-3