Researchers configure silicon rings on a chip to emit high-quality photons for use in quantum information processing. Credit: E. Edwards/JQI.

Protected Pathways for Light Offer a way to Streamline Single Photon Production.

The smallest amount of light you can have is one photon, so dim that it’s pretty much invisible to humans. While imperceptible, these tiny blips of energy are useful for carrying quantum information around. Ideally, every quantum courier would be the same, but there isn’t a straightforward way to produce a stream of identical photons. This is particularly challenging when individual photons come from fabricated chips.

Now, researchers at the Joint Quantum Institute (JQI) have demonstrated a new approach that enables different devices to repeatedly emit nearly identical single photons. The team, led by JQI Fellow Mohammad Hafezi, made a silicon chip that guides light around the device’s edge, where it is inherently protected against disruptions. Previously, Hafezi and colleagues showed that this design can reduce the likelihood of optical signal degradation. In a paper published online on Sept. 10 in Nature , the team explains that the same physics which protects the light along the chip’s edge also ensures reliable photon production.

Single photons, which are an example of quantum light, are more than just really dim light. This distinction has a lot to do with where the light comes from. “Pretty much all of the light we encounter in our everyday lives is packed with photons,” says Elizabeth Goldschmidt, a researcher at the US Army Research Laboratory and co-author on the study.  “But unlike a light bulb, there are some sources that actually emit light, one photon at time, and this can only be described by quantum physics,” adds Goldschmidt.

Many researchers are working on building reliable quantum light emitters so that they can isolate and control the quantum properties of single photons. Goldschmidt explains that such light sources will likely be important for future quantum information devices as well as further understanding the mysteries of quantum physics. “Modern communications relies heavily on non-quantum light,” says Goldschmidt. “Similarly, many of us believe that single photons are going to be required for any kind of quantum communication application out there.”

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

They injected light into a chip containing an array of miniscule silicon loops. Under the microscope, the loops look like linked-up glassy racetracks. The light circulates around each loop thousands of times before moving on to a neighboring loop. Stretched out, the light’s path would be several centimeters long, but the loops make it possible to fit the journey in a space that is about 500 times smaller. The relatively long journey is necessary to get many pairs single photons out of the silicon chip.  

Such loop arrays are routinely used as single photon sources, but small differences between chips will cause the photon colors to vary from one device to the next. Even within a single device, random defects in the material may reduce the average photon quality. This is a problem for quantum information applications where researchers need the photons to be as close to identical as possible.

The team circumvented this issue by arranging the loops in a way that always allows the light to travel undisturbed around the edge of the chip, even if fabrication defects are present. This design not only shields the light from disruptions—it  also restricts how single photons form within those edge channels. The loop layout essentially forces each photon pair to be nearly identical to the next, regardless of microscopic differences among the rings. The central part of the chip does not contain protected routes, and so any photons created in those areas are affected by material defects.

The researchers compared their chips to ones without any protected routes. 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, single-color photons time and again, whereas the conventional chip’s output was more unpredictable.

“We initially thought that we would need to be more careful with the design, and that the photons would be more sensitive to our chip’s fabrication process,” says Sunil Mittal, a JQI postdoctoral researcher and lead author on the new study. “But, astonishingly, photons generated in these shielded edge channels are always nearly identical, regardless of how bad the chips are.”

Mittal adds that this device has one additional advantage over other single photon sources. “Our chip works at room temperature. I don’t have to cool it down to cryogenic temperatures like other quantum light sources, making it a comparatively very simple setup.”

The team says that this finding could open up a new avenue of research, which unites quantum light with photonic devices having built-in protective features. “Physicists have only recently realized that shielded pathways fundamentally alter the way that photons interact with matter,” says Mittal. “This could have implications for a variety of fields where light-matter interactions play a role, including quantum information science and optoelectronic technology.”

Written by D. Genkina and E. Edwards

Author Affiliations 

Sunil Mittal, Joint Quantum Institute, Institute for Research in Electronics and Applied Physics (IREAP)

Elizabeth Goldschmidt, Joint Quantum Institute and US Army Research Laboratory

Mohammad Hafezi, Joint Quantum Institute, UMD Electrical and Computer Engineering, IREAP and Department of Physics

Reference Publication

"A topological source of quantum light," S. Mittal, Elizabeth A. Goldschmidt, Mohammad Hafezi, Nature, , (2018)

Research Contact

S. Mittal:  mittals [AT] umd [DOT] edu

Ten University of Maryland faculty members earned Faculty Early Career Development Program (CAREER) awards from the National Science Foundation in the past fiscal year.

The five-year awards are the NSF’s most prestigious in support of junior faculty members who have the potential to serve as academic role models in research and education and lead advances in the mission of their department or organization.

Hailu B. Gebremariam has accepted an appointment as a full-time lecturer.  He holds a bachelor's and a master's degree in physics from Addis Ababa University, an ICTPHailu Gebremariam diploma in high energy physics from the Abdus Salam International Center for Theoretical Physics and a master's degree in physics from Syracuse University. He received his UMD doctorate in 2005 under Ted Einstein, with the thesis Terrace Width Distribution and First Passage Probabilities for Interacting Steps. Prior to accepting his new position, he was an assistant professor at Montgomery College and a part-time lecturer in the Department of Physics.

Matt SeversonMatt Severson has accepted an appointment as a full-time lecturer. He holds bachelor’s degrees in atmospheric sciences and meteorology and in physics and mathematics from the University of South Alabama, and received his UMD Ph.D. in 2015 under Rabi Mohapatra, with the thesis Neutrino Mass and Proton Lifetime in a Realistic SUSY SO(10) Model.  Prior to accepting his new position, he was a part-time lecturer in the Department of Physics.

Wes Fuhrman, who recently completed his Ph.D. at Johns Hopkins University, has joined the Center for Nanophysics and Advanced Materials (CNAM) to conduct a one-year research program funded by the Schmidt Science Fellowship program. Fuhrman was one of 14 fellows chosen from 220 applicants for the first round of Schmidt funding.

Fuhrman received his bachelor’s degree at the University of California, Irvine, focusing on magnon decay dynamics and quantum game theory. At Hopkins, his interests turned to strongly interacting topological materials. This is also a focus area for CNAM researchers, making UMD an ideal place for Fuhrman to carry out a highly collaborative, multi-disciplinary research program focused on exploring the prospects of new technologies based on topological and correlated electron materials.

About the fellowship:

Schmidt Science Fellows, in partnership with the Rhodes Trust, aims to develop the next generation of science leaders to transcend disciplines, advance discovery, and solve the world’s most pressing problems. Schmidt Science Fellows was launched in 2017 by Eric and Wendy Schmidt and is a program of Schmidt Futures, delivered in partnership with the Rhodes Trust. The program has an initial commitment of at least $25m for the first three years.

The fellowship includes a $100,000 stipend and participation in a global meeting series. According to the program, fellowship recipients choose “leading laboratories at elite institutions that conduct exciting new research."

Candidate event showing the associated production of a Higgs boson and a Z boson, with the subsequent decay of the Higgs boson to a bottom quark and its antiparticle.

Scientists now know the fate of the vast majority of all Higgs bosons produced in the LHC.

Today at CERN, the Large Hadron Collider experiments ATLAS and CMS jointly announced the discovery of the Higgs boson transforming into bottom quarks as it decays. This is predicted to be the most common way for Higgs bosons to decay, yet was a difficult signal to isolate because it closely mimics ordinary background processes. This new discovery is a big step forward in the quest to understand how the Higgs enables fundamental particles to acquire mass.

After several years of refining their techniques and gradually incorporating more data, both experiments finally saw evidence of the Higgs decaying to bottom quarks that exceeds the 5-sigma threshold of statistical significance typically required to claim a discovery. Both teams found their results were consistent with predictions based on the Standard Model. UMD professors Alberto Belloni, Drew Baden, Sarah Eno, Nick Hadley and Andris Skuja are members of the CMS collaboration.

Higgs bosons are only produced in roughly one out of a billion LHC collisions and live only one-septillionth of a second before their energy is converted into a cascade of other particles. Because it is impossible to see Higgs bosons directly, scientists use these secondary particles to study the Higgs’ properties. Since its discovery in 2012, scientists have been able to identify only about thirty percent of all the predicted Higgs boson decays, while its decays to bottom quarks, which should occur sixty percent of the time, had not yet been observed.

“The Higgs boson is an integral component of our universe and theorized to give all fundamental particles their mass,” said Alberto Belloni of the University of Maryland. “But previously we had only directly seen the Higgs couplings to the tau lepton, and the W and Z bosons. Now we have seen the decay of the Higgs to a quark-antiquark pair. This measurement shows for the first time that the Higgs gives mass to a quark.”

The Higgs field is theorized to interact with all massive particles in the Standard Model, the best theory scientists have to explain the behavior of subatomic particles. But many scientists suspect that the Higgs could also interact with massive particles outside the Standard Model, such as dark matter. By finding and mapping the Higgs bosons’ interactions with known particles, scientists can simultaneously probe for new phenomena.

The next step is to increase the precision of these measurements so that scientists can study this decay mode with a much greater resolution and explore what secrets the Higgs boson might be hiding.

Further information:

ATLAS: https://atlas.cern/updates/press-statement/observation-higgs-boson-decay-pair-bottom-quarks

CMS: http://cms.cern/higgs-observed-decaying-b-quarks-submitted