UR: Design of a smaller and more sensitive gravitational wave detector

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Scott Mackey

University of California, Los Angeles (UCLA)

Scott C. Mackey is a senior in astrophysics at UCLA with an interest in instrumentation. This work is the continuation of a REU summer project at Northwestern University’s Center for Interdisciplinary Exploration and Astrophysics Research with Professor Selim Shahriar. The project was presented in poster form at the 237th meeting of the American Astronomical Society.

Current observations of gravitational waves are based on Michelson interferometers like those used in LIGO or VIRGO detectors. These are lasers several kilometers long that interfere when space is expanded or contracted by incident gravitational waves. In addition to the high costs and other challenges associated with building detectors at the scale of thousands of meters, another drawback is that their measurements are limited in accuracy by fundamental quantum noise. In order to solve both size and noise issues, we are exploring the use of a gravitational wave detector based on the use of so called superluminal lasers. These lasers get their name from the fact that their group speed is faster than the rated speed of light. As a result, they exhibit negative dispersion as they propagate and therefore have an ultra-sensitive relationship between their frequency and the length of the cavity through which they move. When an incoming gravitational wave causes space to stretch or contract, we can use this ultra-sensitive relationship to detect changes in the length of the laser cavity over distances much smaller than those traveled by lasers from LIGO and VIRGO. Indeed, we estimate that a detector only 10 meters long could achieve the same accuracy as a LIGO over a slightly larger frequency band. Detectors longer than 10 meters would start to experience much less quantum noise than LIGO for dramatically improved accuracy.

To help design this new detector, I performed superluminal behavior simulations to accurately model these lasers and determine which parameters will optimize our use of lasers – such as cavity size, laser power, and the driving frequency – in order to be able to detect a clear gravitational wave signal. Doing this requires a lot of computation intensive on a supercomputer, as the superluminal laser is generated by exploiting 39 Zeeman sub-levels in atomic rubidium vapor. Using a special algorithm developed by the Shahriar group, we solve a 39 × 39 Hamiltonian to determine the time course of the quantum system that generates the laser. This involves considerations for the coupling between the sublevels and the speed dispersion of the atoms. By running these simulations we are closer to understanding superluminal lasers and ultimately building the gravitational wave detector. Someday many small gravitational wave detectors of this design will be able to be placed in the world and in space, which will give us the opportunity to make many more gravitational wave observations.

Schematic diagram of the proposed gravitational wave detector.
Figure 1. This is a basic diagram of the operation of the proposed detector. The detector uses two superluminal ring lasers (of frequency f1 and f2) which interfere to produce a beat frequency proportional to the deformation of the gravitational wave. Lasers are generated by sending standard lasers into atomic rubidium vapor cavities and coupling them to specific Zeeman sublevel transitions in the atoms.

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