At the time of this project, the world is currently experencing the COVID-19 pandemic and Scott is a rising senior astrophysics major at UCLA. His current research at UCLA is in experimental condensed matter physics with the Ni Lab. He has a strong interest in experimental high-energy astrophysics, and topics such as dark matter and neutrinos. He plans to pursue a PhD along these lines and hopefully one day a faculty position.

You can contact him at:
sm [at] northwestern [dot] edu

Current detectors like LIGO and VIRGO rely on Michelson interferometers consisting of two orthogonal lasers. To record a gravitational wave signal, the detectors measure the phase difference between two beams of light as a result of a difference in the distances to mirrors placed several kilometers away. These devices are limited by their large size and the quantum noise they generate, which makes them difficult to construct and places limits on their sensitivity.

Our new approach would allow for a gravitational wave detector that is significantly smaller (on the order of several meters) while generating less noise. A much greater number of these new detectors could be built and placed around the world to dramatically increase the amount of gravitational wave data collected.

Using superluminal lasers, we can build a device that has a strong relationship between its length and signal output in order to better detect gravitational waves.

Superluminal lasers have a group velocity that is faster than the speed of light. This is the result of anomalous dispersion effects and only happens when a laser has a specific gain profile. Gain is a property of the laser that measures its power amplification.

In order to become superluminal, the laser first passes through an active medium of atomic vapor which causes photons of specific frequencies to be released, leading to the required profile. In our case, we use two rubidium isotopes to accomplish this: ⁸⁵Rb provides the broad gain profile and ⁸⁷Rb is used to get the absorption dip in the center.

Unique to superluminal lasers is a very strong relationship between the laser optical cavity length and the lasing frequency. When a gravitational wave is incident on the optical cavity, the wave distorts the space around it, causing its length to increase or decrease. This very small change in length leads to a very large change in the frequency of the laser.

Two lasers used together would develop a beat frequency between them as a result of the gravitational wave induced frequency changes. This beat frequency could then be translated directly into a gravitational wave signal.

To simulate the behavior of the superluminal laser, I began with a simplified four-level system for the gain medium. Then, using an algorithm developed by the Shahriar group, I made a simulation that iteratively solves the laser equations for the transition frequencies given a set of initial parameters. Once this simplified system is understood, the complexity of the system can be ramped up by adding in other factors to see their effects on the superluminal laser system.

Further analysis will examine the importance of other factors in the behavior of the detector. This includes very small effects such as the strong Zeeman effect, which causes energy levels to split into hyperfine sublevels. For instance, this effect changes the 4-level system of the ⁸⁵Rb gain to a 39-level system. The increase in complexity greatly increases the time it takes to run the iterative algorith in the simulation but it can shed more light on the way these quantum phenomena influence superluminal lasers. It is important to understand as much as we can about the behavior of superluminal lasers in order to perfect the designs of our detectors.

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