Quantum Sensing with Whispering Galleries - A Christmas Carol

I already wrote 2 articles about my trip to Latvia (A Hitchhiker's Guide to Normalizing Quantum Technologies and How a small country can win the quantum race). During my visit I had the amazing opportunity to visit the Quantum Optics Laboratory of the University of Latvia (Latvijas Universitāte) lead by Janis Alnis . Inga Brice and her colleagues showed us the laboratories in which we had the opportunity to the see cutting edge research in the field of quantum sensing using diamonds with nitrogen vacancies (NV centers), which I will talk in a later newsletter about, and whispering gallery mode quantum sensor, which we will talk in todays Christmas edition of the 'Entangled Threads' newsletter about.

There are several reasons why I decided to take this topic for the Christmas edition. The first and most obvious is that the glas spheres radiated by LASER light, although very small, make amazing Christmas ornaments. But, the most important reason is that I had some time to dig deeper into this amazing technology.

The Name 'whispering gallery modes' comes form the Whispering Gallery in St. Paul’s Cathedral, where whispers can travel clearly across the dome’s curved walls, carrying sound over 100 feet. St. Paul’s itself has a rich history, rising from the ashes of the Great Fire of London in 1666 to become one of the city’s most iconic landmarks, completed in 1710 under the direction of Sir Christopher Wren. Known not just as an architect but also as a physicist and mathematician, Wren designed the cathedral’s majestic dome as a feat of both engineering and beauty. Interestingly, the Whispering Gallery’s unique acoustics were likely an unintended consequence of his design. At the time, the science explaining how sound waves travel along curved surfaces—what we now call whispering gallery modes—was not yet understood. Despite this, the gallery quickly became a source of wonder, blending architectural brilliance with an almost magical effect that continues to captivate visitors to this day.

The acoustic phenomenon of the Whispering Gallery is rooted in the way sound waves interact with curved surfaces. When someone speaks softly near the gallery’s smooth, circular wall, the sound waves travel along the curve, hugging its surface rather than dispersing outward. This effect is due to the reflection of sound waves, which bounce repeatedly along the dome’s inner surface while losing very little energy. The result is that even the faintest whisper can be carried clearly to the opposite side of the gallery, more than 100 feet away.

Similar to the reflection of sound waves in the Whispering Gallery, laser light can also be reflected on curved surfaces that have a high internal reflection index, so for example within micro spheres made from optical fiber. The size of these microspheres is highly frequency selective, as the lights wave length can only be multiples of the path length of the light within the sphere. This means that whispering gallery modes can be for example used to select very precise laser wave length out of a laser with a broader spectrum. But, it also means that if the size, surface property or the difference between the refractive indexes of the sphere and the environment change, also the selected frequencies change.

Exactly this frequency dependency is used in whispering gallery mode quantum sensors to measure and quantify changes of different environmental factors very precise. A change in temperature for example will lead to thermal expansion, which will change the size of the sphere and will shift the selected frequencies. These microspheres can additionally be coated with differences substances like e.g. gold, that change their refractive index when binding to external substances. This can be used to measure smallest amounts of humidity, gases, biological particles or chemicals. A Nature Magazine paper from 2021 showed for example how Whispering Gallery Mode (WGM) microcavities could be used to measure as little as one virus in a sample.

Another interesting direction of research using WGM microcavities is to combine them with NV center diamonds to create a deterministic single photon source. Deterministic single photon sources would have several applications, the most important being that they would make fully secure Quantum Key Distribution (QKD) possible that could secure our digital communication.

As you can see the application of WGM microspheres is quite broad and the research on them is still on-going. One down side is that microspheres made from optical fibre are quite hard to fabricate in mass, but there are already microcavities that can be produced much easier in mass and are embedded in optical chips. This will allows the production of affordable, high precision measurement applications in the very near future. I will keep you updated about any interesting news in this field.

For now I want to wish you a great holiday season, whatever you may celebrate. I wish you a relaxing time with your family and friends and I hope to read you next time!