Quantum Sensing

Researchers are exploring the potential to detect gravitons using quantum sensing technologies in hope of linking quantum mechanics with Einstein’s theory of relativity.

Advanced quantum sensing tools, such as those used at LIGO, detect gravitational waves by overcoming quantum noise through techniques like “squeezing.” These tools could also support graviton detection by providing a more precise way to measure gravitational disturbances in lab environments.

While technical and philosophical challenges remain, progress in quantum sensing may narrow the gap between quantum mechanics and gravitational theory, and provide new insights into phenomena like black holes and the Big Bang.

While each tasked with the important role of numerically explaining our reality, gravity and quantum mechanics tend to mix like oil and water — they have long presented a profound challenge to unify in the domain of physics. Albert Einstein’s general theory of relativity, established in 1915, describes gravity as the curvature of space-time. In contrast, quantum mechanics suggests that forces are mediated by particles.   READ MORE...

Quantum Squeezing

Lee McCuller, a physics professor and quantum squeezing expert, is developing innovative techniques to enhance the sensitivity of LIGO, the world’s most advanced gravitational wave detector. His future ambition is to broaden the application of these techniques beyond LIGO.



New Caltech professor Lee McCuller is making quantum measurements even more precise.

From a young age, new Assistant Professor of Physics, Lee McCuller, enjoyed the hands-on process of building things. This interest was fostered by his uncle, who created a power supply for him. McCuller used this in conjunction with electronic hobby kits from RadioShack, performing simple tasks such as operating analog circuits to switch lights and motors on and off. 

Today, McCuller’s engineering prowess is applied to an exceptionally advanced device, what some would call the most advanced measurement device in the world: the Laser Interferometer Gravitational-wave Observatory, or LIGO.


McCuller is a recognized expert in a field known as quantum squeezing, a technique utilized at LIGO to achieve extremely precise measurements of gravitational waves. that travel millions and billions of light-years across space to reach us. When black holes and collapsed stars, called neutron stars, collide, they generate ripples in space-time, or gravitational waves. 

LIGO’s detectors—located in Washington and Louisiana—specialize in picking up these waves but are limited by quantum noise, an inherent property of quantum mechanics that results in photons popping in and out of existence in empty space. Quantum squeezing is a complex method for reducing this unwanted noise.  READ MORE...

Breakthrough for Gravitational Waves

Artist’s concept of gravitational waves propagating through space.

New laser breakthrough to help increase understanding of gravitational waves.

Scientists have created a proof-of-concept setup of a new laser eigenmode sensor that offers over 1,000 times the sensitivity. After translating this work to gravitational wave detectors, they will offer the unprecedented precision needed to test the fundamental limits of general relativity and probe the interiors of neutron stars.

Gravitational wave scientists from The University of Western Australia (UWA) have led the development of a new laser mode sensor with unprecedented precision that will be used to probe the interiors of neutron stars and test the fundamental limits of general relativity.

Research Associate from UWA’s Center of Excellence for Gravitational Wave Discovery (OzGrav-UWA) Dr. Aaron Jones, said UWA co-ordinated a global collaboration of gravitational wave, metasurface, and photonics experts to pioneer a new method to measure structures of light called “eigenmodes.”

“Gravitational wave detectors like LIGO, Virgo, and KAGRA store enormous amount of optical power, and several pairs of mirrors are used to increase the amount of laser light stored along the massive arms of the detector,” Dr. Jones said.  READ MORE...

A Pixelated Space...

The search for signatures of quantum gravity forges ahead.
Sand dunes seen from afar seem smooth and unwrinkled, like silk sheets spread across the desert. But a closer inspection reveals much more. As you approach the dunes, you may notice ripples in the sand. Touch the surface and you would find individual grains. The same is true for digital images: zoom far enough into an apparently perfect portrait and you will discover the distinct pixels that make the picture.

The universe itself may be similarly pixelated. Scientists such as Rana Adhikari, professor of physics at Caltech, think the space we live in may not be perfectly smooth but rather made of incredibly small discrete units. “A spacetime pixel is so small that if you were to enlarge things so that it becomes the size of a grain of sand, then atoms would be as large as galaxies,” he says.

Adhikari and scientists around the world are on the hunt for this pixelation because it is a prediction of quantum gravity, one of the deepest physics mysteries of our time. Quantum gravity refers to a set of theories, including string theory, that seeks to unify the macroscopic world of gravity, governed by general relativity, with the microscopic world of quantum physics. At the core of the mystery is the question of whether gravity, and the spacetime it inhabits, can be “quantized,” or broken down into individual components, a hallmark of the quantum world.

“Sometimes there is a misinterpretation in science communication that implies quantum mechanics and gravity are irreconcilable,” says Cliff Cheung, Caltech professor of theoretical physics. “But we know from experiments that we can do quantum mechanics on this planet, which has gravity, so clearly they are consistent. The problems come up when you ask subtle questions about black holes or try to merge the theories at very short distance scales.”

Because of the incredibly small scales in question, some scientists have deemed finding evidence of quantum gravity in the foreseeable future to be an impossible task. Although researchers have come up with ideas for how they might find clues to its existence—around black holes; in the early universe; or even using LIGO, the National Science Foundation-funded observatories that detect gravitational waves—no one has yet turned up any hints of quantum gravity in nature.  READ MORE...

Black Holes Confirmed

Study offers evidence, based on gravitational waves, to show that the total area of a black hole’s event horizon can never decrease.

There are certain rules that even the most extreme objects in the universe must obey. A central law for black holes predicts that the area of their event horizons — the boundary beyond which nothing can ever escape — should never shrink. This law is Hawking’s area theorem, named after physicist Stephen Hawking, who derived the theorem in 1971.

Fifty years later, physicists at MIT and elsewhere have now confirmed Hawking’s area theorem for the first time, using observations of gravitational waves. Their results appear today (July 1, 2021) in Physical Review Letters.

In the study, the researchers take a closer look at GW150914, the first gravitational wave signal detected by the Laser Interferometer Gravitational-wave Observatory (LIGO), in 2015. The signal was a product of two inspiraling black holes that generated a new black hole, along with a huge amount of energy that rippled across space-time as gravitational waves.

If Hawking’s area theorem holds, then the horizon area of the new black hole should not be smaller than the total horizon area of its parent black holes. In the new study, the physicists reanalyzed the signal from GW150914 before and after the cosmic collision and found that indeed, the total event horizon area did not decrease after the merger — a result that they report with 95 percent confidence.

Physicists at MIT and elsewhere have used gravitational waves to observationally confirm Hawking’s black hole area theorem for the first time. This computer simulation shows the collision of two black holes that produced the gravitational wave signal, GW150914. Credit: Simulating eXtreme Spacetimes (SXS) project. Credit: Courtesy of LIGO

Their findings mark the first direct observational confirmation of Hawking’s area theorem, which has been proven mathematically but never observed in nature until now. The team plans to test future gravitational-wave signals to see if they might further confirm Hawking’s theorem or be a sign of new, law-bending physics.

“It is possible that there’s a zoo of different compact objects, and while some of them are the black holes that follow Einstein and Hawking’s laws, others may be slightly different beasts,” says lead author Maximiliano Isi, a NASA Einstein Postdoctoral Fellow in MIT’s Kavli Institute for Astrophysics and Space Research. “So, it’s not like you do this test once and it’s over. You do this once, and it’s the beginning.”  TO READ THE ENTIRE ARTICLE, CLICK HERE...