The Pursuit of Perfect Timekeeping

The challenge of creating the world’s most precise clock is that that even the slightest deviations limit the precision. Atomic clocks, which rely on the coherent evolution of atomic states, are the most accurate timekeeping devices known to humanity. However, achieving this level of precision requires a deep understanding of the interactions between atoms, especially as many atoms are packed in a dense ensemble to increase the signal strength. 

In a recent study published in Science, by JILA and NIST Fellows and University of Colorado Boulder physics professors Jun Ye and Ana Maria Rey, interactions between atoms are explored in depth, focusing on superexchange processes that occur in a three-dimensional optical lattice.

The Role of Superexchange Interactions

Superexchange interactions are second-order tunneling processes between nearest neighbor atomic spins. These interactions are central to understanding magnetic phenomena such as antiferromagnetism and superconductivity. In the context of atomic clocks, superexchange interactions can influence the coherence time and precision of the clock.

To grasp the concept of superexchange, consider a relay race where the baton is passed through several intermediaries before reaching the final runner. Similarly, in superexchange interactions, atoms exchange spins through virtual tunneling processes, leading to coherent spin dynamics. This interaction is ordinarily quite weak, and would have been ignored had the researchers not been pursuing the best possible clock precision. 

In the study, researchers used a degenerate Fermi gas of nuclear spin-polarized ⁸⁷Sr atoms loaded into a three-dimensional optical lattice. By tuning the lattice confinement and applying imaging spectroscopy, they mapped out favorable atomic coherence regimes. The clock laser prepared each atom in a coherent superposition of the two electronic states, which can be considered as a pseudo-half spin. The propagation effect of the clock laser introduced a spin-orbit coupling phase, transforming the Heisenberg spin model into one with XXZ-type spin anisotropy. 

William Milner, first author on the paper, explains, "You want to use as many atoms as possible and get the best precision. As you pack them into this 3D lattice, they can start to interact. These atoms can talk to each other, so you can no longer think of them as isolated atoms."

The experimental setup involved a highly filled Sr 3D lattice, where atoms were confined in the ground band of the lattice. The researchers employed Ramsey spectroscopy to measure atomic coherence and observe superexchange interactions. This technique allowed them to directly probe the coherent nature of superexchange interactions over timescales of multiple seconds.

Balancing Interactions

One of the key findings was the identification of regimes where atomic coherence is maximized. By varying the lattice confinement, researchers observed how both s- and p-wave interactions contribute to decoherence and atom loss. These interactions can be balanced to achieve optimal coherence times, which are crucial for the precision of optical lattice clocks. Imagine balancing a seesaw with two children of different weights. To achieve equilibrium, you need to adjust their positions carefully. Similarly, in the optical lattice, researchers balanced s- and p-wave interactions to minimize decoherence. 

However, at deep transverse confinement, coherent superexchange interactions were directly observed, tunable via on-site interaction and site-to-site energy shift. Milner elaborates, "In this regime is where you get these superexchange interactions. These higher order interactions occur because the atoms can't move around, but they can virtually jump onto a site and then jump back, with spin exchanged."

The study provided direct observations of superexchange dynamics, which were manifested in oscillations of the Ramsey fringe contrast persisting over a timescale of several seconds. These observations were well captured by an anisotropic lattice spin model, breaking the Heisenberg SU(2) symmetry due to the spin-orbital coupling phase. Additionally, the experiments showed the direct tunability of the interactions via lattice strength and potential gradients.

Enhancing Clock Performance

Optical lattice clocks are advancing the fields of fundamental physics, metrology, and quantum simulation. By controlling superexchange interactions, researchers can enhance the performance of these clocks, leading to more precise timekeeping and new insights into quantum magnetism and spin entanglement.

Just as a finely tuned orchestra produces a flawless performance, a well-controlled optical lattice clock can achieve unprecedented precision. The experiment demonstrated that by tuning the lattice confinement and controlling superexchange interactions, researchers can optimize the coherence time of the clock. This has the potential to further advance timekeeping and enable new applications in quantum technologies.

Milner notes, "By changing the confinement, you can make it so these superexchange interactions are very small and pretty much negligible. On the other hand, there's promise that you can use these interactions to create entangled states, which should give you even better precision." 

Stefan Lannig, a postdoc in the Ye group, adds, "We want to trap the atoms in the 3D lattice to get the highest atom number for the best precision, but in a sample as small as possible. This helps us get rid of background effects and achieve optimal performance."

Looking ahead, the research opens new avenues for exploring quantum magnetism and spin entanglement using optical lattice clocks. By leveraging the coherent nature of superexchange interactions, scientists can probe deeper into the quantum dynamics of many-body systems. This could lead to breakthroughs in understanding fundamental physics and developing advanced quantum technologies.

This study by the Ye group represents a significant step forward in the field of atomic clocks and quantum metrology. By unraveling the complexities of superexchange interactions, researchers have laid the groundwork for enhancing the precision and performance of optical lattice clocks. Researchers at JILA are orchestrating the interactions of atoms to unlock the secrets of time itself, pushing the boundaries of what is possible in the quest for perfect timekeeping.

This research is supported by U.S. Department of Energy Center of Quantum System Accelerator, National Science Foundation QLCI, JILA Physics Frontier Center, V. Bush Fellowship, and NIST.

_{Written by Steven Burrows, JILA Science Communications Manager}