A quantum mechanical method known as ‘spin squeezing’ has garnered attention for its potential to significantly enhance the performance of the world’s leading quantum sensors, but achieving it has proven to be quite challenging. Recent research by physicists sheds light on how spin squeezing can be more accessible.
Measurement is fundamental to science, enabling the comprehension of phenomena. With advancements in quantum sensing, researchers can now measure previously inconceivable aspects, such as atomic vibrations, individual photon properties, and gravitational wave fluctuations.
Spin squeezing, a quantum mechanical effect, is seen as a key technique to boost the functionality of the most accurate quantum sensors available. However, practical realization of spin squeezing has been elusive. In a recent study, scientists from Harvard have revealed ways to bring this concept closer to reality.
Spin squeezing involves a specific form of quantum entanglement that limits how much an ensemble of particles can fluctuate. This allows for more precise measurements of certain observable signals, albeit at the cost of accuracy in measuring other complementary signals—similar to how squeezing a balloon increases its height at the expense of its width.
“Quantum mechanics can improve our ability to measure very small signals,” explained Norman Yao, a physics professor and one of the authors of the new study published in Nature Physics. “We have demonstrated that quantum-enhanced measurement techniques can be applied to a wider range of systems than previously thought.”
Using the balloon analogy, Maxwell Block, a co-author of the paper and former graduate student, clarified that a circle symbolizes the inherent uncertainty in any quantum measurement. “By squeezing this uncertainty, making the balloon more like an ellipse, we can change the sensitivity of measurements,” Block noted. “This allows for certain measurements to achieve levels of precision unattainable without quantum mechanics.”
A variation of spin squeezing has already been implemented to enhance the sensitivity of the gravitational wave detectors in the LIGO experiment, which received a Nobel Prize for its success.
The Harvard team’s findings build upon a significant 1993 paper that first introduced the idea of an entangled state that could be spin squeezed through “all-to-all” interactions among atoms. These interactions are similar to a large Zoom meeting where all participants interact simultaneously. In atomic interactions, this kind of connectivity facilitates the necessary quantum mechanical correlations to create a spin-squeezed state. However, in real-world conditions, atoms usually interact more like a game of telephone, communicating with only a few neighbors at a time.
According to Bingtian Ye, co-lead author of the study and also a former graduate student, “For many years, it was believed that true quantum-enhanced spin squeezing could only occur through all-to-all interactions. But we’ve shown that achieving it is considerably simpler.”
The researchers propose a novel approach to generating spin-squeezed entanglement. They discovered, alongside their collaborators in France, that the conditions for spin squeezing can be found in a common form of magnetism present in nature—ferromagnetism, which is the reason refrigerator magnets stick. They suggest that all-to-all interactions are not a requirement for achieving spin squeezing. Instead, having spins sufficiently connected to synchronize into a magnetic state allows them to dynamically generate spin squeezing.
The researchers remain hopeful that by lowering the hurdles to achieving spin squeezing, their findings will encourage quantum scientists and engineers to develop more portable sensors, beneficial for applications in biomedical imaging, atomic timekeeping, and beyond.
To advance this goal, Yao is currently leading experiments aimed at generating spin squeezing in quantum sensors based on nitrogen-vacancy centers—defects in diamond’s crystal structure that are recognized for their suitability as quantum sensors.
This research was funded by various federal agencies, including the Army Research Office, the Office of Naval Research, the Department of Energy, the Department of Defense, and the National Science Foundation.
