Three Questions: Zach Korth, LIGO

Who are you?

My name is Zach Korth. I’m a graduate student here at Caltech, working in Prof. Rana Adhikari’s group on experimental gravitational wave physics. The bulk of my time goes to developing and testing technology that will be installed at the detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) in Livingston, LA, and Hanford, WA. Run jointly by Caltech and MIT, LIGO is poised to make the first ever direct detection of gravitational waves, ripples of space-time itself propagating across the universe at the speed of light, carrying with them information about the most distant and poorly understood astrophysical phenomena thought to exist.

At the LIGO Hanford control room.

I was born and raised in Miami, FL, and had a relatively privileged upbringing, though my parents worked hard to make me aware of and thankful for it. I went to a small private school called Palmer Trinity School in a suburb of Miami from 7th grade through high school. I was always a need-to-know-how-everything-works kind of kid, but my first real introduction to science was in school, and I credit my junior-year physics teacher, Bailey Edwards, with showing me what I wanted to do in life.

I went to the University of Florida for college—majoring in physics, of course—and it’s there that I first became involved with LIGO under Profs. Guido Mueller and Dave Reitze, who is now LIGO’s executive director here at Caltech. There, I worked on several experiments involving LIGO’s “input optics”—those optics that deliver the meticulously prepared laser light to the rest of the interferometer—while trying to demystify optics and electronics for myself without breaking too much equipment.

Flash-forward a few years and here I am. If you told me in high school that in a few years I’d be working at the highest level on the experiments I read about in popular books at the time, I’d probably think you were crazy. I am extremely grateful to so many people for making that dream come true, and I try never to forget how truly magical it is.

What are you up to?

Well, I gave part of that away, didn’t I?

I’m up to a lot. One of the most amazing things about LIGO and the field of gravitational wave science in general is how incredibly multi-disciplinary they are. A LIGO interferometer is an enormously complicated jigsaw puzzle of a machine, requiring state-of-the-art optics, photo- and control electronics, exquisite environmental noise isolation, absurdly stabilized laser systems and the largest ultra-high vacuum envelope on the face of the earth. All that is to say nothing of the exceedingly complex analytical and numerical waveform modeling and signal characterization that busies the day of a LIGO data analyst, without which even the most sensitive interferometer is destined for failure. As someone who works on the detectors, I don’t have to be an expert in all these areas, but I’d better be familiar with most of them.

Thanks to those who are experts in the individual fields above, the sensitivity of LIGO’s newest incarnation, “Advanced LIGO (aLIGO)” will be determined purely by fundamental effects over most of its operational frequency band. Namely, the noise floor of the detector—at an incomprehensible level of roughly 2 x 10-20 m/√Hz—is dominated by noise arising from the quantization of the light used to perform the measurement. This “quantum noise” can be thought of as an enforcement of the Heisenberg uncertainty principle for the LIGO test masses. The consequence of this is that we have to be pretty clever if we want to increase the sensitivity any further.

In the early 1980s, Carl Caves (UNM) showed that quantum noise could be understood as arising from the so-called vacuum fluctuations entering a normally unused port of an interferometer. He also showed that the noise could be suppressed if an exotic state (e.g., a “squeezed” state) were injected instead of the raw vacuum. Some 30 years later, this effect has recently been demonstrated on both LIGO and GEO600 gravitational wave interferometers.

Demonstrated quantum noise reduction with squeezed light on the LIGO H1 detector. The high-frequency noise spectral density is reduced when an exotic squeezed state is injected into the interferometer.

While the standard squeezed states of light can now be generated readily, there is an extra catch: in order to faithfully suppress the quantum noise at all frequencies, they must have a very precise phase spectrum—one that must be applied via filtering after generation. Such narrow-band (tens of Hz) filtering of optical signals is very difficult to do, and the tentative designs for future LIGO enhancements call for long, expensive, and inherently inflexible suspended optical cavities.

One solution that we are beginning to work on is the use of optomechanical filters to achieve the same effect. Using emergent techniques like optomechanically induced transparency (OMIT), the response of these long, expensive cavities can be emulated with an apparatus that fits on a single optical bench. As an added bonus, optomechanical systems typically exhibit much greater ease of tuning.

It is a long (but exciting!) road ahead. Since the system in question will need to coherently transfer a quantum state, it must be strongly isolated from thermal noise and other effects. Even with today’s best materials and cryogenics, the technology just isn’t quite there yet, and so we are working on less straightforward ways to reduce external noise. One idea in particular is to use optical forces to modify the dynamics of a mechanical resonator, and in so doing reduce the influence of thermal noise.

I’m eager to see where this avenue takes us. For one thing, there is the potential direct improvement of gravitational wave detectors’ sensitivity, and the ensuing expansion of our astrophysical reach to new horizons. On the other hand, the technology we’re studying is very interesting on its own merit. Should we succeed in optomechanically filtering a squeezed state of light, we will also have succeeded in preparing a mechanical system in an exotic quantum state, which has yet to be done and which has profound implications within the burgeoning arenas of macroscopic quantum mechanics and quantum information processing.

An exciting road indeed!

What would you do with $1 billion?

As Spiros put it, this should be an easy question for someone like me. I can think of at least two great candidates for such a generous imaginary donation.

I could say, “I would build the Einstein Telescope (ET).” ET is a proposed 3rd-generation terrestrial gravitational wave detector. If built, it will have a sensitivity 10 times better than aLIGO’s and will also be sensitive to gravitational waves of much lower frequency (~1 Hz, vs. ~10 Hz for aLIGO). It will accomplish this by being situated some 150 m underground, which will shield it from the effects of seismic noise and Newtonian gravity gradients, and by operating in a “xylophone” configuration, where multiple collocated interferometers are tuned independently so as to each be optimized for a distinct frequency band. It is a very ambitious project, but, if completed, ET will be able to detect gravitational waves from sources that lie a sizeable fraction of the observable universe away, far beyond the Local Superclusters. It costs $1 billion.

I could also say, “I would contribute the US’s share of the Laser Interferometer Space Antenna (LISA).” LISA is a long-proposed space-based gravitational wave detector. Like LIGO, it would work by measuring minute changes in length between distant test masses. Whereas LIGO’s test masses are 4 km apart, however, LISA’s would be separated by 5 million km. That enormous distance, coupled with the absence of terrestrial noise, means that LISA can be extraordinarily sensitive at much lower frequencies. This opens up the possibility of detecting gravitational waves from altogether different sources, providing for unprecedented tests of general relativity and a much greater potential to study the birth of the universe through cosmic gravitational wave backgrounds. Of course, it needs to go in space (the plan is to have it trail the earth in its orbit around the Sun), and so it costs a pretty penny. After decades of research, NASA decided last year to descope its involvement in LISA, leaving the ESA to figure out how it may or may not be accomplished. The US may still play at least a minor role in LISA, but the project’s future is murkier than ever. It costs around $2.4 billion.

If I absolutely had to choose one, under the assumption that the other would never be built, I would probably choose LISA. Using some incremental enhancement techniques like the ones discussed above, we can expect to eventually push the performance of the LIGO detectors to within a factor of a few of ET’s, with the exception of a small band between 1 – 10 Hz. Without a space-based detector, however, it’s hard to imagine that we’ll ever be sensitive to gravitational waves below ~1 Hz or so. For this reason, LISA is a scientific imperative, and I sincerely hope that this point is recognized by the people with the real money!

5 thoughts on “Three Questions: Zach Korth, LIGO

  1. Pingback: On the importance of choosing a convenient basis | Quantum Frontiers

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