For over a century, physicists have struggled to bridge the gap between quantum mechanics, which governs the behavior of the smallest particles in nature, and general relativity, which describes the universe on the largest scales. These two fundamental theories are incredibly successful within their respective domains, yet they remain incompatible with each other. Understanding how they fit together is one of the greatest challenges in modern physics.
One of the most promising tools in this pursuit is the optical lattice clock, an ultra-precise atomic clock that allows scientists to explore the subtle interplay between quantum mechanics and relativity. Optical lattice clocks operate by trapping atoms in a grid-like structure made of laser beams, holding them in place while precisely controlling their quantum states. These clocks rely on the oscillations of atoms to measure time with unprecedented accuracy, making them powerful instruments for testing fundamental physics.
According to Einstein’s theory of general relativity, time flows differently depending on gravitational strength—a phenomenon known as gravitational time dilation or gravitational redshift. This effect means that an atomic clock placed at a higher altitude, where gravity is slightly weaker, will tick faster than an identical clock at a lower altitude, where gravity is stronger. In an optical lattice clock, this gravitational effect causes slight shifts in the oscillation frequency of trapped atoms, altering the passage of time at the quantum level.
While the influence of relativity on individual atoms is well understood, its impact on many-body quantum systems, where atoms interact and become entangled, remains largely unexplored. Many-body quantum systems involve complex interactions between particles, leading to emergent phenomena that do not appear in isolated atoms. The challenge lies in studying these interactions within the framework of both quantum mechanics and relativity.
A team of researchers led by JILA and NIST Fellows Jun Ye and Ana Maria Rey, in collaboration with scientists from Leibniz University Hannover, the Austrian Academy of Sciences, and the University of Innsbruck, has made significant progress in this direction. They developed innovative experimental protocols to study how gravitational redshift affects quantum entanglement and interactions in an optical atomic clock. Their work, published in Physical Review Letters, reveals surprising ways in which gravity and quantum interactions intertwine.
One of their key discoveries is that interactions between atoms can cause them to synchronize, behaving as a unified system rather than ticking independently. This synchronization is remarkable because it counteracts the natural desynchronization caused by gravitational redshift. In essence, even though gravity tries to pull the timing of individual atoms apart, their quantum interactions help lock them together, preserving their collective coherence.
Dr. Anjun Chu, a former JILA graduate student and the study’s first author, explains, “One of our key findings is that interactions between atoms can help to lock them together so that now they behave as a unified system instead of ticking independently due to the gravitational redshift.” This result highlights a deep connection between quantum entanglement and relativistic effects, bringing us closer to understanding how these two domains of physics interact.
The challenge of detecting gravitational effects in an optical lattice clock lies in distinguishing them from other noise sources that can also cause slight shifts in atomic oscillations. To address this, the researchers devised a clever technique called the dressing protocol, which fine-tunes the gravitational redshift by manipulating atomic energy levels using laser light.
The dressing protocol leverages Einstein’s famous mass-energy equivalence equation, E=mc², which states that energy and mass are interchangeable. When an atom transitions between different energy states, its effective mass changes slightly. By carefully controlling these energy states, researchers can modify how an atom experiences gravitational redshift, effectively tuning the strength of the relativistic effect.
JILA graduate student Maya Miklos explains, “By changing the superpositions of internal levels of the particles you’re addressing, you can change how large the gravitational effects appear. This is a really clever way to probe mass-energy equivalence at the quantum level.” This technique provides an unprecedented level of control over gravitational redshift, enabling researchers to isolate its effects from other perturbations.
Beyond distinguishing gravitational effects, the study also explored quantum many-body dynamics in the presence of relativity. The researchers placed atoms inside an optical cavity, where they could interact by exchanging photons—particles of light. When one atom in an excited state emits a photon, another atom can absorb it, effectively exchanging energy between them. This process, known as photon-mediated interaction, allows atoms to communicate and influence each other’s behavior, even when they are not in direct contact.
Normally, atoms at different heights within a gravitational field tick at slightly different rates due to gravitational redshift. Without interactions, these tiny timing differences would accumulate, causing the atoms to gradually fall out of sync. However, the introduction of photon-mediated interactions led to an unexpected phenomenon: synchronization.
“It’s fascinating,” says Dr. Chu. “You can think of each particle as its own little clock. But when they interact, they start to tick in unison, even though gravity is trying to pull their timing apart.” This synchronization demonstrates that quantum interactions can counteract relativistic effects, revealing a novel interplay between gravity and quantum mechanics.
Even more astonishingly, this synchronization process also leads to the creation of quantum entanglement, a phenomenon in which particles become correlated in such a way that the state of one instantly influences the state of another, regardless of distance. Entanglement is a cornerstone of quantum mechanics and plays a crucial role in technologies such as quantum computing and secure communication.
JILA postdoctoral researcher Dr. Kyungtae Kim notes, “Synchronization is the first phenomenon we can see that reveals this competition between gravitational redshift and quantum interactions. It’s a window into how these two forces balance each other.” The study suggests that measuring how quickly synchronization occurs could serve as an indirect way to detect and quantify entanglement, offering a novel approach to studying the quantum-gravity interface.
The implications of this research extend beyond optical clocks. Understanding how relativity and quantum mechanics interact in many-body systems could have profound consequences for fundamental physics, quantum computing, and even astrophysics. The techniques developed in this study could pave the way for more precise experiments that test the limits of mass-energy equivalence, gravitational time dilation, and quantum coherence.
The researchers believe that future experiments could explore even more intricate scenarios, such as how quantum interactions amplify gravitational effects or how different forms of entanglement emerge in relativistic settings. These studies could bring us closer to unifying the two great pillars of modern physics—general relativity and quantum mechanics.
Dr. Ana Maria Rey emphasizes the significance of this work, stating, “Detecting this GR-facilitated entanglement would be a groundbreaking achievement, and our theoretical calculations suggest that it is within reach of current or near-term experiments.” This research represents a critical step toward unraveling the mysteries of quantum gravity, an endeavor that could reshape our understanding of the universe.
As optical lattice clocks continue to improve in precision, they will provide an ever more powerful platform for probing the fundamental nature of reality. The ability to experimentally study the intersection of quantum mechanics and general relativity will open new frontiers in physics, challenging our deepest assumptions about time, space, and the fabric of the cosmos.
In the coming years, these breakthroughs could revolutionize not only fundamental physics but also practical applications in metrology, timekeeping, and quantum technologies. The quest to reconcile quantum mechanics with general relativity remains one of the greatest scientific challenges of our time, and optical lattice clocks may be one of the keys to unlocking this long-sought unification.
More information: Anjun Chu et al, Exploring the Dynamical Interplay between Mass-Energy Equivalence, Interactions, and Entanglement in an Optical Lattice Clock, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.093201. On arXiv: DOI: 10.48550/arxiv.2406.03804