Building Earth’s “ears”: gravitational wave detectors

Astronomy as the North star: from childhood to doctoral studies

I do not remember when I started liking astronomy. However, there were many books at home about planets, galaxies, and the universe. I particularly liked the children’s book “George’s Secret Key to the Universe” written by Lucy and Stephen Hawking. The main character of the book, George, was a kid around the same age as I was. He explored exoplanets, black holes, and other exotic places using a super-computer named Cosmos. I loved the scale of the universe. Even though scientists currently think the universe does have boundaries, it felt like it was infinite. The book stands out as memory because it was very easy to read and more realistic than other science-fiction books. My interests in physics and mathematics in high school strengthened. As astronomy overlaps with both, it was a natural progression. There are only a few astronomy departments in Japan, so I decided to come to the School of Science in Tokyo from Kobe, where I am originally from. My childhood interest in astronomy naturally led me to graduate school.

Gravitational waves: the music of the universe

Gravitational waves are generated in the curvature of space-time when a mass is accelerated in the universe, thus happening everywhere, all the time. Imagine a bowl of water: waves appear when you move the bowl. A similar thing happens to our space-time, but the effect is so tiny that we usually cannot detect it. Our target is gravitational waves emitted when two-massive compact objects collide, such as two black holes or neutron stars. These events are energetic enough to detect with an integration of our technology.

“Listening” to the quietest “music”

By influencing the curvature of space-time, gravitational waves change the distance between two objects, although the effect is tiny. For example, the effect of a passing gravitational wave on the distance between the Sun and the Earth would be on the order of the diameter of a single hydrogen atom. Using a mirror and a laser is the standard way to detect gravitational waves directly. We project a laser beam at the mirror so that the light is reflected back, and we measure the time it takes for the beam to cover the distance. We know how much time it takes without the fluctuation of gravitational waves, so if it takes more or less time than that, it might be because of the fluctuations of a gravitational wave. There are four detectors, two in the US, one in Italy, and one in Japan, called KAGRA. Gravitational detectors have kilometer-scale arms separating two mirrors. We prepare two, kilometers-long vacuum chambers, which cross each other at a 90-degree angle, mirrors at one end of the chambers. The center station with the laser source is at the other end of the chambers. The detector prepared in its best condition is called the observation mode. Then, we wait for gravitational waves to be detected. A detection period is about one to two years. We say we detect waves because they are not something we see with our eyes but more like something we hear with our ears: "listening to" the vibrations of space-time.

KAGRA illustration (c) KAGRA Collaboration / Rey. Hori

Straining our “ears”

Just like how we might struggle to hear a person talking to us through the “noise” of our surroundings, researchers also struggle to detect a gravitational wave signal through various types of noise. Air disturbances confuse gravitational wave detection, so we put everything into a vacuum chamber because sound waves do not propagate in a vacuum. We also need to keep the mirrors stable, so we use tens of meters long suspensions to isolate as much as possible the mirrors from the motion of the ground. For the detector in Japan, one of the main issues is the ground motion caused by the waves in the Sea of Japan. As the waves hit the Japanese islands, they cause vibrations in the ground, which transfer to the mirrors, causing “noise” in our measurements. Such noise is a big problem as we have to be sure that our detection was caused by a wave of the universe and not by the sea.

Quantum noise: the noise that you cannot get rid of

All of the previously mentioned types of noise are within the realm of classical physics. On the other hand, quantum noise is the more fundamental noise of the detector, which means that quantum mechanics determines the limit beyond which we cannot suppress it. By quantum noise, I mean the uncertainty of the photons, quanta, that make up the laser beams. Heisenberg's uncertainty principle is a theorem that connects the "uncertainty" between the particle-like properties of light (photon number) and its wave-like properties (phase). It reveals a trade-off between the two: raising the certainty of one aspect of light's duality reduces the certainty of the other. As mentioned earlier, gravitational wave detectors use laser light, so these uncertainties ultimately limit the sensitivity of the detector. My research aims to effectively control this wave-particle duality, overcome the barrier imposed by the uncertainty principle, and reduce the quantum noise of the detector. In other words, reaching the maximum potential of a detector. There are many possible detector configurations, so we must choose carefully from them.

Filtering out the quantum noise

I have been working on a method to overcome the limit determined by the Heisenberg uncertainty principle, which tells you the uncertainty of your measurement. Heisenberg’s uncertainty principle gives you the “fundamental” limit of the detectors called the standard limit, but there are many ways to overcome it using cutting-edge technologies. I am doing research on one such method, using a "quantum filter" to overcome the standard quantum limit. To create this filter, I propose to apply quantum teleportation to gravitational wave detectors. Quantum teleportation is a technique where a quantum state is teleported to a faraway location. In quantum computing, you manipulate your target state to obtain a desired quantum state. Then, you can proceed to do some operations. When you teleport your quantum state to the other side of the detector, you might be able to manipulate the quantum state similarly to quantum computing. My proposal is to apply this teleportation protocol to build powerful and efficient quantum filters. This technique is expected to be implemented in future generations of gravitational wave telescopes.

Research experiences in Europe and Australia

I got to go to Maastricht University in the Netherlands thanks to a program run by JPSP (Japan Society for the Promotion of Science) and ERC (European Research Council). I got to experience not only research but also a different culture and environment. The work culture in the Netherlands was very different. I got automatically kicked out of the office at 6pm when the whole building was shut down, so I had more leisure time. Surprisingly, my productivity did not suffer. I had a good work life-balance. In my free time, I hiked the second-highest peak in the Netherlands, which was just 300m above sea level. I also enjoyed the country’s many bicycle roads. Moreover, many people spoke English. Since my research group in Japan is very international and I’m used to speaking in English, I had no issues staying there four months. It was also nice that I could easily visit other research laboratories in Europe. I visited several groups in Paris, Padova and Trento on business trips, and had fruitful discussions. This year I also got to go to Australia. Australia was very convenient as everyone spoke English, and Asian culture was widely present. There were many Japanese products in supermarkets and I could easily find good Asian restaurants. One be reasons why I felt comfortable might have been due to the left-side traffic. On the contrary, right-side traffic made Maastricht feel like I was living in a mirror.

A station building in Maastricht, a city in the Netherlands. As the sun did not set until 10 pm in the summer, Nishino could enjoy his downtime after work to its fullest.

If you are thinking of pursuing a STEM field…

If you are a high school student interested in pursuing astronomy, I suggest learning mathematics and physics first. A strong base will not only make your future research better but also teach you how to think logically.　My advice to graduate and undergraduate students is to go abroad if they get the chance. Had I not gone to Maastricht, my research would be completely different now, and I could not have worked on my quantum teleportation proposal. Although I had received great advice here in Japan, I wanted to broaden my horizons in my very specific field of interest, quantum noise in gravitational-wave detection. So, after spending two years in the master’s program, I wanted to be advised and supervised more directly. It took a lot of courage to send the first email to Dr. Stefan Danilishin, an associate professor at Maastricht University whose achievements in my field of research I respected tremendously. But he was kind and responded quickly, and I got to join the gravitational wave research group at the university. His office was close by, so whenever I had a question, I could go and ask him. The paper we worked on was published this August in the journal Physical Review A. So, do not be afraid of exploring opportunities both inside and outside of Japan.

https://doi.org/10.1103/PhysRevA.110.022601

※Year of Interview:2024
Photography:KAIZUKA Junichi
Text:Belta Emese
/ The interview was edited for brevity and clarity.

NISHINO Yohei

2nd-year doctoral student, department of astronomy

Yohei Nishino has been interested in astronomy from childhood. He entered the University of Tokyo, as it was one of the few universities with an astronomy department in Japan. He spent four months doing research at Maastricht University in the Netherlands. He is currently a second-year doctoral student.