A New Kind of Entanglement Helps Quantum Sensors Tune Out Noise

In a quest to build the most accurate sensors in the world, scientists are constantly improving their performance, making them more precise, stable and reliable.

But eventually, physical constraints will prevent further improvements.

“By fully embracing the laws of quantum physics, one can expand the performance limits imposed by these constraints,” says Alexey Gorshkov, a senior investigator in the NSF Quantum Leap Challenge Institute for Robust Quantum Simulation (RQS). “And it's very exciting to come up with protocols that come as close as possible to saturating these limits for different sensing tasks.”

Gorshkov, a physicist at the National Institute of Standards and Technology (NIST), also has an appointment in the Joint Quantum Institute and an adjunct appointment in University of Maryland Institute for Advanced Computer Studies (UMIACS).

Even the most precise sensors in the world are not fully isolated and are limited by noise—subtle disturbances from the environment, such as vibrations, electromagnetic fields or temperature changes.

Seeking a workaround, Gorshkov teamed up with Ana Maria Rey and James K. Thompson—both Fellows at JILA in Boulder, Colorado—and colleagues from the Indian Institute of Technology Madras and the Niels Bohr Institute at the University of Copenhagen in Denmark. Together, they asked a crucial question: How can we improve the next generation of sensors despite these limitations?

One promising idea is to use quantum entanglement, connecting atoms so they work together as a system to form a quantum sensor. When atoms are entangled, they share properties even when separated by distance. In principle, this allows for more precise measurements. But entangled atoms are still subject to noise.

“Entangled states are well understood for estimating a single parameter, but our goal was to create an entangled state that is highly sensitive to a parameter difference between two nodes of a sensor network,” says Raphael Kaubruegger, a research associate at JILA and the lead author of a recent paper published in Physical Review X describing the team's work.

The researchers set out to identify a new class of entangled states that could filter out noise affecting both sensors. They then developed two ways to create these states inside an optical cavity—a pair of mirrors about one inch apart that bounce photons back and forth.

The entangled state they identified uses decoherence-free subspaces, which are protected from certain types of disturbances, to suppress noise affecting both sensors.

Lasers are used to create coherent superpositions between two internal states of an atom, but to accomplish that, the laser's frequency must exactly match the atomic transition.

The challenge, as Rey explains, is that even the most precise lasers cannot maintain a stable frequency long enough. These laser-frequency instabilities generate noise that is experienced equally by both sensors and are among the most significant sources of error in state-of-the-art clocks.

“Ideally, one would like to prepare the atoms in a state that is insensitive to this type of noise,” says Rey who is also a Fellow at NIST.

“The state we create is entanglement between these atoms, but in a way that you cannot distinguish which atom is in which ensemble,” Rey says. “They are fully symmetrized.”

“After the fact, we realized this was the same kind of state people were thinking about to describe antiferromagnets, or quantum magnets,” says Thompson, also a NIST Fellow.

In condensed matter physics, the Lieb-Mattis state describes a quantum version of an antiferromagnet, where two groups of atoms behave as though they point in opposite directions, but without the system choosing one fixed direction in space.

One method the team developed to prepare the desired state involves entangling two nodes of a sensor network by engineering a “spin exchange” in which atoms send photons back and forth through an optical cavity. This leads to a state where each atom in one node is perfectly anticorrelated with an atom in the other. If one atom is “up,” the other atom is “down.”

Thompson likens this approach to baseball, where each ensemble is a baseball team. The teams are throwing balls—or, in this case, photons—to each other. Every time a ball is thrown, the other team catches it. Thompson adds that it's important that no one knows which player threw the ball or who caught it.

“That’s what builds these links,” Thompson says. “If a ball is thrown, it is definitely caught.”

The approach produces Heisenberg scaling—the best possible precision scaling—where all the atoms act as one quantum object.

Optical cavities are not perfect. As Rey explains, sometimes a photon is lost. The team's second approach takes this into account.

Inside the optical cavity, photons can bounce back and forth between highly reflective mirrors about 100,000 times before they accidentally slip through to the other side.

“We are losing photons, but the important part is that the photons are lost in a collective way,” Rey says.

Because it's impossible to tell which atom is responsible, this loss can create entanglement, driving the atoms into a state where they can no longer lose photons.

“At some point they get really good at not dropping the ball anymore,” Thompson says.

“They go into a ‘dark state,’ or a state where the phases of the emitted photons completely cancel out, leading to what is known as destructive interference,” Rey adds.

The team was initially trying to understand the detrimental effects of losing those photons. But, as Rey explains, this type of dissipation ultimately led them to the state they wanted.

“The state we initially wanted to prepare was one in which half the atoms are excited, but the system cannot collectively emit a photon,” Kaubruegger adds.

The team's proposed states can be created quickly and, more importantly, even faster as the system gets larger, making them practical for scaling quantum sensors.

“People have thought about this kind of state when you only have two atoms, which is cool, but you'd like to use more,” Thompson says. “It turns out, the more atoms you have, the better!”

By making quantum sensors more precise, these entangled states could one day help guide navigation when GPS is unavailable or reveal hidden underground resources such as minerals, oil and gas.

Close collaborations between theorists and experimentalists have been key to this work. The groups inspire each other—and keep each other in check. Because they work so closely together, Kaubruegger says they have a deeper understanding of the challenges experimentalists face.

And now, the ball, so to speak, is in Thompson's group's hands to demonstrate the state experimentally.

This text has been adapted from a story written by Kirsten Apodaca and originally published by JILA.