The background science story

A cavity is a fundamental tool for experiments involving lasers. In the most basic sense, cavity is just two opposing mirrors where light can reflect back and forth between them. The laser power can build up by circulating between the mirrors if they are well aligned, typically also accompanied by tuning the cavity length to a multiple of the laser wavelength, a condition called resonance, which is a very important concept in quantum experiments as we will see later.

We use cavities in many situations, like for monitoring and controlling laser frequencies, as well as for laser power buildup to enhance interactions with molecules. Cavities are also very important for LIGO where laser power is crucial. They use what they call power recycling mirrors in a configuration called Fabry-Perot cavity to enhance their laser power from 50 Watts to 750,000 Watts.

Of course, to achieve such high amplification, extreme fine tuning and environmental control are needed. In fact, most resonant cavities require fine tuning, protection, and active feedback control systems. Thus, for our installation, we should use our non-resonant cavity to amplify our laser power - an instrument we recently developed in our laboratory. We recently published a paper about the design, and are applying for a patent. Our cavity can increase laser power by more than 100 times, without needing active feedback. Most importantly, it’s also very robust against perturbation; We have one as a toy in our office, on a desk which people constantly bump into, and yet it runs stably with no issue.

Spectroscopy is a precise way to study interactions between matter and waves by analyzing the frequency of the wave. In the context of atomic/molecular spectroscopy, we study the exact quantum structure of the atoms, molecules or even their constituent sub-particles by using electromagnetic waves of varying frequencies, like lasers and microwaves, to interact with them. In the context of LIGO, we study the properties of merging celestial bodies by studying the frequencies of the gravitational waves they generated. Also, spectroscopy is a general tool we use in almost every step of the way, from studying the properties of novel molecules to manipulating them using lasers. Same goes to LIGO as well, they use spectroscopy to learn about their optics elements, analyze noise signatures and many others.

Specifically, scanning fluorescence spectroscopy is when we use a laser of varying frequencies to interact with particles and monitor the light emitted by the particles. When the laser frequency matches the energy difference between two quantum states of the particle, a resonant condition is met and it excites the particle from the lower state to the higher state. Then, due to interactions with the fluctuating quantum vacuum, the particle will quickly drop back down by spontaneous emission, a process only explainable in quantum field theory. We detect the photon emitted in the process as a signal showing that our laser is on resonance, and we can calculate the energy difference between the two quantum states based on the laser frequency at that moment.

One can argue that frequency is the most fundamental measurement, which is counting the number of occurrences in a time duration. No measurement is more precise than frequency. That’s why modern SI units are mostly defined by frequency, and why it’s often a good strategy to convert quantities from lengths to voltages to frequencies. In the Nobel lecture: passion for precision, Theodor Hänsch quoted his mentor Arthur Schawlow, also a Nobel laureate, “Never measure anything but frequency!”

Once your measurement precision can reach an extreme degree, interestingly, you can start to explore the most extreme high energy phenomena in the universe. In the case of LIGO, you can study the mergers between massive black holes by measuring the laser length deviations smaller than the width of a proton. In the case of our molecule precision measurement, we can learn about beyond standard model new particles and forces that are higher on the energy scale than the highest energy achieved at the Large Hadron Collider at CERN.

There are underlying reasons why you can study extreme high energy by measuring in extreme high precision. These exotic high energy new particles or forces are interacting with the ordinary particles through the same quantum vacuum fluctuations as mentioned before. The higher energy a particle has, the shorter duration they have when they blink into existence in the fluctuating quantum vacuum, as dictated by the Heisenberg uncertainty principle. While they are high on the energy scale, it doesn’t mean that they interact with ordinary matter strongly, in fact the opposite is likely true. Because of that, these short lived fluctuations have very small effects on the ordinary particles and we have to measure very precisely in order to study them.

In the Hutzler lab, we are specifically interested in a type of new particles and forces that can induce asymmetries in the fundamental workings of the universe. Such asymmetries are the reason why we exist and matters exist, instead of all annihilated moments after creation in the big bang. The asymmetries caused the excess creation of matters over anti-matters, and we are the excess matters that remained after most annihilated in pairs. While such asymmetries allow our existence, it is very puzzling and also exciting to learn that the universe has particular preferences. In the void of intergalactic space, it’s natural to assume that there’s no difference between up and down or left and right, yet somehow, for some deep underlying quantum physical process, perturbed by the most extreme exotic particles and forces, the universe does have a preference of one direction over others.