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For more than a hundred years, scientists have faced an intriguing dilemma: how to reconcile the principles of quantum mechanics, which apply to the tiniest particles, with the laws of general relativity that describe the cosmos on a grand scale.
The optical lattice clock, known for its exceptional precision in timekeeping, is emerging as a significant instrument in addressing this monumental question. Inside an optical lattice clock, atoms are confined within a lattice created by laser beams, and their quantum coherence and interactions are meticulously managed according to the rules of quantum mechanics. Concurrently, according to Einstein’s general relativity, time is affected by gravity, leading to the phenomenon known as gravitational redshift. This results in subtle changes in the internal energy levels of atoms based on their positions in gravitational fields, ultimately influencing their tick rates—the oscillations that mark time in these advanced clocks.
Researchers can investigate the minor shifts in oscillation frequencies of these highly accurate clocks to better understand how Einstein’s theory of relativity impacts quantum systems. While the relativistic effects on individual atoms are well understood, their implications in many-body quantum systems, where atoms interact and entangle, remain largely uncharted territory.
Making progress in this complex area, a research team led by JILA and NIST fellows, along with physicists from the University of Colorado Boulder, has proposed innovative approaches to examine the influence of relativity, such as gravitational redshift, on quantum entanglement and interactions within an optical atomic clock. Their findings uncovered that the relationship between gravitational phenomena and quantum interactions can lead to surprising results, including atomic synchronization and the entanglement of particles. These insights were published in Physical Review Letters.
Dr. Anjun Chu, a postdoctoral researcher at the University of Chicago and the lead author of the study, remarked, “One of our critical insights is how atom interactions can synchronize their behavior so that they function as a unified entity instead of ticking independently due to gravitational redshift. This connection between quantum interactions and gravitational effects is truly exciting.”
Rey further noted, “The relationship between general relativity and quantum entanglement has intrigued physicists for decades. The challenge has been that GR corrections are often minuscule in typical experiments, making them hard to observe. With atomic clocks achieving unprecedented levels of precision, these subtle effects are now within reach of measurement. By analyzing multiple atoms simultaneously, we have a unique opportunity to delve into the intersection of general relativity and many-body quantum physics. Our research focused on a system of atoms interacting through photon exchange within an optical cavity. Fascinatingly, we discovered that while individual interactions may not affect the clock’s ticking, their collective influence on redshift can drastically change dynamics and foster entanglement among atoms.”
Distinguishing Gravitational Effects
To tackle the challenge of identifying gravitational redshift’s impact on quantum behavior, the team devised innovative methods to separate these effects from other sources of noise that might induce slight frequency shifts. They employed a technique termed a dressing protocol, which involves adjusting the internal states of atoms using laser manipulation. While such protocols are familiar in quantum optics, their application here marks one of the initial uses aimed at fine-tuning gravitational effects.
The approach hinges on mass-energy equivalence, as articulated by Einstein’s iconic equation E=mc². This principle dictates that an atom in an excited state displays a slightly greater mass than when it is in its ground state. The variance in mass due to gravitational potential energy corresponds to gravitational redshift. The dressing protocol enables researchers to effectively modify this mass difference, and thereby the corresponding gravitational redshift, by controlling how the atoms remain in a combination of two internal energy states. This allows particles to exist simultaneously in both states, thus providing unprecedented control to fine-tune gravitational effects.
Through this method, the researchers were able to differentiate true gravitational redshift from other influences, such as variations in magnetic fields, affecting the system.
“By adjusting the superpositions of internal levels of the particles you are examining, you can influence the apparent magnitude of gravitational effects,” explained JILA graduate student Maya Miklos. “This represents a clever avenue for probing mass-energy equivalence at the quantum scale.”
Seeing Synchronization and Entanglement
Once the team established a method to pinpoint genuine gravitational influences, they investigated how these effects manifest in quantum many-body dynamics, utilizing the photon-mediated interactions that arise from placing atoms in an optical cavity.
When one atom transitions from an excited state to the ground state, it emits a photon within the cavity. This photon may be absorbed by another atom in the ground state, thereby exciting it as well. This type of energy exchange allows particles to influence each other without direct contact.
These quantum interactions can compete with individual gravitational effects acting on atoms in the cavity. Typically, particles at varying heights in a gravitational field experience slight discrepancies in their tick rates due to gravitational redshift. If the atoms were to interact, these subtle variations would usually lead to a gradual loss of synchronization over time.
However, when photon-mediated interactions were introduced, an extraordinary outcome occurred: the atoms synchronized their ticking, effectively aligning their rates despite the gravitationally induced frequency differences.
Chu described it as “fascinating. You can visualize each particle as an independent clock. But through interaction, they begin to synchronize, even as gravity seeks to disrupt their timing.”
This synchronization highlighted an intriguing relationship between gravitational influences and quantum interactions, demonstrating that the latter can counteract the natural decoupling driven by gravitational redshift.
Moreover, this synchronization also led to the generation of quantum entanglement, wherein particles become intertwined, permitting the state of one to instantaneously affect another. Remarkably, the speed of synchronization also provided a new metric for assessing entanglement, shining a light on the complex interplay between these phenomena. “Synchronization is the first observable outcome that illustrates the competition between gravitational redshift and quantum interactions,” noted JILA postdoctoral researcher Dr. Kyungtae Kim. “It presents a lens through which we can see these two forces at play.”
Advancing Physics Research
This study not only illuminated initial connections between gravitational effects and quantum physics but also laid a foundation for refining experimental approaches, enhancing precision with applications extending from quantum computing to fundamental physics inquiries.
“The potential to detect entanglement facilitated by gravitational effects would be revolutionary, and our theoretical projections suggest that it is achievable with current or upcoming experimental setups,” Rey stated.
Future investigations may focus on how particles react under varying conditions or how interactions could amplify gravitational influences, further advancing the quest to unify the two major pillars of modern physics.
This research received funding from the Sloan Foundation, the Simons Foundation, and the Heising-Simons Foundation, alongside support from the JILA PFC.
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