Talking Lasers and ‘Superradiance’ with Silvia Cardenas Lopez

Earlier this year, several Columbians who study optical and photonic science made their way down to Charlotte, North Carolina, for the annual Conference on Lasers and Electro-Optics (CLEO). Among them was Silvia Cardenas Lopez, a fourth-year PhD Physics student working with theoretical quantum optics expert and Columbia associate physics professor Ana Asenjo-Garcia. Lopez was invited to give attendees a 30-minute presentation titled “Many-body Steady State Behavior in Wave-guide QED.” 

In our new Q&A, Lopez explains the science behind her talk, which is all about the unique ways groups of atoms behave when you shine light on them. She also shares her path into quantum optics, and how it took her from Mexico City to New York. 

Can you explain what your talk was all about?

In the Ansenjo-Garcia group, we study what happens when atoms interact with light—particularly collective decay, which occurs after the atoms are excited by photons of light. When a single atom decays, it will emit an exponentially decaying pulse of light. But ensembles of atoms behave differently than single ones. When groups decay, they can emit a strong, short pulse of light called the superradiance burst. Superradiance is a manifestation of self-organization and the emergence of spontaneous order from the interaction of many particles. 

Researchers realized that if groups of atoms are placed between two mirrors, forming what is known as an “optical cavity,” they could be used to create a continuous light source with a very narrow spectrum. That kind of light is more stable than that of a conventional laser and robust to outside perturbations and imperfections in experimental setups, which makes it very useful for making super precise measurements. But what happens when you take the atoms outside of a cavity? You will still have collective effects, but the environment becomes much more complicated.

It would be awesome to create superradiant lasers in free space someday. It would mean that more experimental setups could generate ultranarrow spectrum light and, since free space is a more complex environment than a cavity, a superradiant laser in free space would have more "tuning knobs" than a superradiant laser in a cavity. For example, one could change the positions of the atoms in space to modify the properties of the light they produce.

But we need to start simpler. So, we started looking at groups of atoms coupled to a waveguide, a structure that allows light to move in only two directions. That’s one more direction than light can move in a cavity, but the situation is not as complex as in free space, where light can propagate in many directions. We found that you can get intense, narrow light if you pump atoms coupled to a waveguide at the correct strength. We still have some work to do before we can conclude this is a superradiant laser, but it’s an exciting step. 

How did the presentation go, and how do you like to prepare for talks?

CLEO was my first “big” talk—I’ve done smaller, 10-minute talks at conferences like the American Physical Society’s Division of Atomic, Molecular, and Optical Physics (DAMOP) conferences, but I’d never been an invited speaker before. It was really nice. 

I think shorter talks can actually be more challenging. Every minute is critical, so you can’t go into everything. When I have to prepare a talk, it helps to rehearse the first couple of minutes, just to make sure that the beginning runs smoothly, even if I’m nervous. I usually try to focus on giving a general idea of the work and intuitive explanations and mention overly technical details only when necessary. It also helps to ask for the opinion of colleagues. Sometimes an explanation that makes a lot of sense to you, who knows every single detail of your work, is not necessarily the best one for a presentation—especially if the audience is not an expert on your topic.

How did you end up at Columbia studying quantum optics?

I am originally from Mexico City. In high school, I had a great math teacher who showed me that math and physics could be more than just numbers and calculations—that they are really about creativity and asking interesting questions. He encouraged me to study physics at the National Autonomous University of Mexico (UNAM), where I had another amazing teacher for Intro to Quantum Mechanics. After teaching us the basic concepts, he would take us through modern experiments—like how scientists can trap atoms inside cavities to get these cool effects. I pursued quantum optics for my master’s at UNAM and was introduced to many-body physics, the research area that explores how groups behave completely differently from single objects. Ana’s group combines quantum optics and many-body physics, so it felt very natural to apply for my PhD. 

And how do you like New York?

New York is a fantastic place to live, and I love to explore. I also like language meetings, where you get paired up with someone trying to learn a new language. I help people with Spanish, but I’ve also been interested in Italian, German, and French. It’s really interesting how your brain is capable of finding patterns. You start with just a few words, and then your brain starts to put things together, and now you can express yourself!

What are some challenges about your science?

Many-body physics can be hard because of the added complexity you have to consider when working with quantum particles. Going from five atoms to 20 isn’t just scaling by a factor of four: it’s exponentially more complex. That means you have to find creative ways to tackle problems because there’s no way to do all of the calculations involved. 

One way we address this is to look for symmetries, which is a very old idea in physics. Instead of trying to pinpoint the crazy thing each individual atom is doing, you can identify groups of atoms that are doing more or less the same thing. That can help simplify how you think about the system. 

Any advice to share with aspiring physicists?

It can be hard for me to follow this sometimes, but ask a lot of questions! It can be scary, but it’s important to be honest when you don’t fully understand something. Start as an undergrad, and it will pay off.