Quantum entanglement sensors could test quantum gravity

Ask almost any physicist what the most frustrating problem is in modern-day physics, and they will likely say the discrepancy between general relativity and quantum mechanics. That discrepancy has been a thorn in the side of the physics community for decades.

While there has been some progress on potential theories that could rectify the two, there has been scant experimental evidence to support those theories. That is where Selim Shahriar from Northwestern University, Evanston, comes in. He plans to work on a concept called the Space-borne Ultra-Precise Measurement of the Equivalent Principle Signature of Quantum Gravity (SUPREME-GQ), which he hopes will help collect some accurate experimental data on the subject once and for all.

To put it bluntly, the experiment is complicated. At its heart, it uses a space-based platform carrying a quantum-entangled sensor and some precise positioning systems. But understanding why it is useful to test quantum gravity first requires some explanation. Let’s first look at one of the most famous tenets of General Relativity—the Equivalence Principle.

The Equivalence Principle states that gravity and acceleration are the same. It is at the core of General Relativity, which treats gravity as a curve in spacetime rather than a fundamental force. But plenty of quantum gravity theories predict a deviation from this equivalence on minute scales—where quantum mechanics starts to take over.

To describe that deviation, physicists use a term known as the Eötvös parameter. It explains how closely gravitational mass and inertial mass are related. In General Relativity, at least, they should be the same. However, as things get closer to the realm of quantum mechanics, some theories that claim to offer a theory of quantum gravity start to see a divergence between the two, which is represented as a non-zero value for the Eötvös parameter.

So far, this parameter’s value has been tested down to about 10^-15 by the MICROSCOPE experiment, which was specially designed to test this theory. The researchers leading that project published a report back in 2022, and it remains our most accurate measurement of the Eötvös parameter to this day.

However, MICROSCOPE used traditional accelerometers, which—while they provided an estimate of more than 100 times what could be obtained on Earth—were not accurate enough to measure down to the 10^-18 level, where theories such as string theory predict there might be a deviation in the parameter.

Enter Dr. Shahriar and his team. Their goal is to develop a space-based platform that uses atom interferometers to constrain the parameter value down to 10^-20, at which point it could potentially prove or disprove some theories of quantum gravity. But to get there, they need to do a lot of groundwork first.

One step in the process is to understand how quantum entanglement could be utilized in these atom interferometers (AIs). AIs work by using atoms’ dual nature (which has a similar wave/particle duality to light) and splitting a beam of atoms onto separate paths using lasers. In the case of Dr. Shahriar’s experiment, these would be rubidium atoms. After they are split, if they are not observed, they enter into a state equivalent to the famous Schrödinger’s cat experiment of quantum mechanics.

However, creating such a quantum mechanical state has never been done before, which is the next step in Dr. Shahriar’s development work. His team has developed the “generalized echo squeezing protocol,” which theoretically allows them to maintain the quantum entangled state for long periods. Doing so would allow for precise measurements of discrepancies between the two states when they are eventually recombined, and those discrepancies could lead to a highly accurate measure of the Eötvös parameter.

The underlying technology, which the team called a “Schrödinger’s Cat Atom Interferometer” (SCAI) in a recent press release, could also be used for applications back on Earth. If possible, using their theoretical implementation, these sensors would be thousands of times more accurate than typical accelerometers or gyroscopes, which are already used in various applications like guidance and navigation systems and automotive.

There is still much work to be done, even to prove that the theoretical implementation of this technology is possible in the real world. Once people start operating with quantum uncertainties, things tend to get tricky, not just for theoretical cats. But, if Dr. Shahriar and his team do make a functional space-borne SCAI, we will finally be a few steps closer to truly reconciling one of the biggest problems in modern physics.