Pitt Researcher is Pushing the Boundaries of Quantum Physics

Gurudev Dutt, associate professor of physics and astronomy in the Kenneth P. Dietrich School of Arts and Sciences has received a $2M grant from the Templeton Foundation and $470K from the Sloan Foundation to better understand the fundamental nature of gravity.

Dutt and his team, which includes Tom Purdy, and David Pekker, both assistant professors in physics and astronomy, as well as Jacob Taylor from the University of Maryland, will search for gravitons, a proposed particle that, according to quantum field theory, carries the gravitational force similar to the way a photon carries light, an electromagnetic force.

The team will carry out this ambitious project in two parts, both taking place within the unfamiliar realm of quantum physics where things can be in two places at once  – a state known as superposition – and separated particles can become entangled, seemingly to carry current information about each other even though they are not in contact.

From the beginning, the project will push the boundary of what is possible when it comes to quantum physics experimentation. 

The Schroedinger’s cat thought experiment may be the most well-known reference to superposition: a cat is in a box with a vial of poison and a radioactive substance. If a single atom decays, the vial breaks and the cat dies. If there is no decay, the cat lives. The cat represents a particle in superposition; until there is some kind of interaction with the system, the cat is both dead and alive at the same time,

In reality, superposition is not seen in cats, it’s seen at the smallest of scales: a subatomic particle, a few atoms, even molecules. But gravity acts on mass. The more massive something is, the larger the force of gravity and, if quantum field theory is correct, the more gravitons will be exchanged between two gravitationally attracted objects.

Dutt and his team will need to put something much larger into superposition than a few molecules if they want to increase the odds of seeing a graviton exchange. They’re going to need a diamond. Or at least, a piece of one.

The team will use a small defect inside the diamond similarly to the way an electron or atom is used as a qubit in a quantum computer. Playing quantum tricks with these defects to create superposition is Dutt’s expertise, he’s been doing it for decades. But this time he’ll be working with material much larger than usual: a few microns long.

“It sounds tiny,” Dutt said, “but it’s huge by quantum standards.”

The group also wants to see how long they can keep the diamond dust in superposition. It is a fragile state and, like Schroedinger’s cat, contact with anything - another dust particle, for instance - could knock the diamond out of superposition. It would, physicists say, lose coherence.

Once the team has successfully put this “huge” object into the quantum state of superposition, they plan to do it again, but with two specks of diamond. Because of their size, the two will be gravitationally attracted to each other. If quantum field theory is correct, they will be exchanging gravitons.

Dutt can’t see gravitons, but they will leave a clue that they’ve shown up: their exchange will facilitate the sharing of information between the two quantum systems. This should lead to the entanglement of the two particles. And that is something that Dutt can “see,” that is, the entanglement will act as a witness to the exchange of gravitons.

To get to this point will require an immense amount of patience, equipment, time, experimental ingenuity, and stillness. The particles must be kept in ever-emptier vacuums as the tiniest molecule or vibration could disrupt the system, causing what’s known as a “loss of coherence.” .

Dutt and his team are not the first to try to push the size-limits of superposition or detect the quantum nature of gravity, but as more researchers join the search and larger institutions begin to support the work, experiments with the same goal involve new, high-tech equipment and researchers and support staff that, all told, could run in the billions of dollars.

“We are trying to do essentially the same work with this $2.5 million grant,” Dutt said. “But we have just a roomful of equipment and a few talented students from Pitt and the University of Maryland."

- Written by Brandie Jefferson