Deep in a Boulder laboratory, physicists have cracked a problem that has haunted quantum sensor designers for years: how to make these instruments less sensitive to the noise that inevitably leaks in from the world around them. Researchers at JILA and NIST, led by Ana Maria Rey and James K. Thompson, have developed a new class of quantum entangled state that filters out environmental interference while keeping sensors sharp enough to detect even the subtlest signals they're hunting for.
The challenge is fundamental and unforgiving. Quantum sensors work by packing atoms tightly together to make measurements more precise. But there's a hard limit. "You cannot pack more atoms in a quantum sensor because at some point, they start colliding and disturbing each other, affecting the performance of the sensor," Rey explains. Even the world's most precise instruments are not fully sheltered from vibrations, electromagnetic fields, and temperature swings—the constant whispers of the environment that introduce errors into measurements.
For years, scientists have known that quantum entanglement—where atoms link together and share properties across distance—offers a path forward. When atoms are entangled, they can theoretically work as a unified system to achieve higher precision. But entangled atoms remain vulnerable to noise. The breakthrough came when Rey's team, working with colleagues from the Niels Bohr Institute, the Joint Quantum Institute, and the Indian Institute of Technology Madras, asked a different question: what if you could create an entangled state sensitive to differences between sensors while filtering out noise that affects both equally?
The answer emerged in the form of decoherence-free subspaces. The problem they were solving is particularly acute in state-of-the-art atomic clocks, where laser frequency instabilities introduce noise equally into both sensors. Even the most precise lasers cannot maintain a stable frequency indefinitely. Rey's solution was elegantly counterintuitive: "The state we create is entanglement between these atoms, but in a way that you cannot distinguish which atom is in which ensemble. They are fully symmetrized."
When James Thompson realized what they had created, he made an unexpected connection to condensed matter physics. The entangled state they had developed matched something physicists call the Lieb-Mattis state, which describes quantum antiferromagnets—systems where two groups of atoms act like they point in opposite directions without the system ever settling on one fixed direction. The team published their findings and two distinct methods for creating these states in Physical Review X.
The first method exploits what Thompson calls a "spin exchange," engineered by having atoms send photons back and forth through an optical cavity—a pair of mirrors just 2.5 centimeters apart. The result is a state where each atom in one node is perfectly anticorrelated with an atom in another. If one atom is "up," the other is "down." This produces Heisenberg scaling, the theoretical best possible precision, where all atoms behave as a single quantum object. Thompson illustrates it with baseball: two teams throwing balls—photons—to each other, with the crucial property that you never know which player threw or caught the ball. That mystery is what builds the quantum links.
The second approach acknowledges physical reality: optical cavities are imperfect, and sometimes photons escape. The team engineered a method that accounts for these losses. Together, these advances represent a fundamental step toward a new generation of quantum sensors—instruments that could revolutionize everything from gravitational wave detection to atomic timekeeping, precisely because they have learned to ignore the noise while listening intently for the signal.