# Dipole-dipole interactions: Observing a new clock systematic shift
In a fresh examination disclosed in Science today, JILA and NIST (National Institute of Standards and Technology) Fellow Jun Ye and his group of researchers have taken a remarkable step in comprehending the intricate and combined light-atom interactions within atomic clocks, which are recognized as the most precise timekeepers in the cosmos.
Using a cubic lattice, the specialists gauged particular energy shifts within the group of strontium-87 atoms because of dipole-dipole interactions. With a high density of atoms, these mHz-level frequency shifts—known as cooperative Lamb shifts—were studied spectroscopically. These shifts were researched spatially and contrasted with computed values using imaging spectroscopy methods developed in this experiment.
These cooperative Lamb shifts, titled because the existence of numerous indistinguishable atoms in a tightly restricting space modifies the electromagnetic mode structure around them, are a critical factor as the numbers of atoms in clocks continue to grow.
Atomic clocks, long seen as the pinnacle of precision, run on the concept of measuring the frequency of light absorbed or emitted by atoms. Each tick of these clocks is governed by the oscillations of the quantum superposition of electrons within these atoms, activated by the corresponding energy from a probing laser. The laser excites the atoms into a quantum state known as the clock state.
While more traditional optical lattice clocks use a one-dimensional lattice, suppressing the atoms’ movements only along one strongly constricting direction, the strontium quantum gas clock used in this study confined the atoms in all directions by situating them in a cubic arrangement. While utilizing a 3D lattice is an appealing clock geometry, it also requires initiating an ultracold quantum gas of atoms and cautiously loading them into the lattice.
Utilizing the cubic lattice, Ross Hutson (a recent JILA Ph.D.graduate), Milner, and the other researchers in the Ye lab, were able to facilitate and measure the dipole-dipole interactions between the strontium atoms. These shifts, generally so minute they are overlooked, originate from collective interference between the atoms behaving as dipoles when they are prepared in a superposition of the two clock states.
With more robust dipole-dipole interactions happening within the lattice, the researchers discovered that these interactions produced local energy shifts across the clock system.
“These [shifts were] initially proposed back in 2004 as a futuristic thing to worry about [for clock accuracy],” adds Milner. “Now, they’re suddenly more relevant [as you add more atoms to the lattice].”
As if measuring these shifts wasn’t intriguing enough, even more interesting was that the researchers saw that the cooperative Lamb shifts weren’t uniform across the lattice, but varied depending on each atom’s specific location.
From their measurements, the team realized there was a close connection between the cooperative Lamb shifts and the propagation direction of the clock probe laser within the lattice. This relationship allowed them to find a specific angle where a “zero crossing” was observed and the sign of the frequency shift transitioned from positive to negative.
Beyond controlling and minimizing these dipole-dipole interactions in the cubic lattice, the JILA researchers hope to use these interactions to explore many-body physics in their clock system.
