Twisting and binding of matter waves with photons in a cavity – .

Accurately measuring the energy states of individual atoms is a historic challenge for physicists due to atomic recoil. When an atom interacts with a photon, the atom “recoils” in the opposite direction, making it difficult to precisely measure the atom’s position and momentum. This step back can have big implications for quantum sensing, which detects minute changes in parameters, for example using changes in gravitational waves to determine the shape of the Earth or even detect dark matter.

In a new article published in ScienceJILA and NIST Fellows Ana Maria Rey and James Thompson, JILA Fellow Murray Holland, and their teams proposed a way to overcome this atomic recoil by demonstrating a new type of atomic interaction called momentum exchange interaction, where the atoms exchanged their momentum by exchanging corresponding interactions. photons.

Using a cavity – an enclosed space made of mirrors – the researchers observed that atomic recoil was attenuated by atoms exchanging energy states in the confined space. This process created a collective absorption of energy and dispersed the recoil among the entire particle population.

With these results, other researchers can design cavities to dampen recoil and other external effects in a wide range of experiments, which can help physicists better understand complex systems or discover new aspects of quantum physics. Improved cavity design could also enable more precise simulations of superconductivity, such as in the case of the Bose-Einstein-Condensate-Bardeen-Cooper-Schrift (BEC-BCS) crossover or high-energy physical systems.

For the first time, the momentum exchange interaction was observed to induce on-axis torsional dynamics (OAT), an aspect of quantum entanglement, between atomic momentum states. OAT acts like a quantum braid to entangle different molecules, with each quantum state twisted and connected to another particle.

Previously, OAT was only observed in the internal states of atoms, but now, with these new results, it is believed that OAT induced by momentum exchange could help reduce quantum noise from multiple atoms. Being able to entangle momentum states could also lead to improvements in some physical measurements made by quantum sensors, such as gravitational waves.

Leveraging a Density Network

In the new study, inspired by previous research by Thompson and his team, researchers examined the effects of quantum superposition, which allows particles such as photons or electrons to exist in multiple quantum states simultaneously. .

“In this [new] project, the atoms all share the same spin label; the only difference is that each atom is in a superposition between two momentum states,” explained Chengyi Luo, graduate student and first author.

The researchers found that they could better control atomic recoil by forcing atoms to exchange photons and their associated energies. Similar to a game of dodgeball, an atom can “throw” a “dodgeball” (a photon) and fly back in the opposite direction. This “dodgeball” can be caught by a second atom, which can cause the same recoil for that second atom. This cancels out the two recoils experienced by the two atoms and averages them out for the entire cavity system.

When two atoms exchange their different photon energies, the resulting wave packet (an atom’s wave distribution) in superposition forms a momentum graph known as a density lattice, which resembles a fine-tooth comb.

Luo added: “The formation of the density network indicates two momentum states. [within the atom] are “consistent” with each other, so they could interfere [with each other]” The researchers found that the exchange of photons between the atoms caused the wave packets from the two atoms to link together, so that they were no longer separate measurements.

Researchers could induce momentum exchange by exploring the interaction between the density network and the optical cavity. Because the atoms exchanged energy, any recoil due to the absorption of a photon was dispersed among the entire community of atoms rather than among individual particles.

Dampen Doppler Shift

Using this new control method, the researchers found that they could also use this recoil damping system to help alleviate a separate measurement problem: Doppler shift.

Doppler shift, a phenomenon of classical physics, explains why the sound of a siren or train horn changes pitch as it passes a listener or why some stars appear red or blue in images of the night sky . It is the change in frequency of the wave as the source and the observer move closer (or further away) from each other. In quantum physics, the Doppler shift describes the change in energy of a particle due to relative motion.

For researchers like Luo, Doppler shift can be a challenge to get an accurate measurement. “When absorbing photons, atomic recoil will cause a Doppler shift in the frequency of the photon, which is a big problem when talking about precision spectroscopy,” he explained. By simulating their new method, the researchers found that it could overcome measurement biases due to Doppler shift.

Tangled Momentum Exchange

The researchers also discovered that the exchange of momentum between these atoms could be used as a kind of quantum entanglement. As John Wilson, a graduate student in Holland’s group, said: “When an atom falls, its movement causes the frequency of the cavity to vary. This, in turn, encourages other atoms to collectively sense this feedback mechanism and causes them to correlate their movement through the Holland group. shared oscillations.

To further test this “entanglement,” the researchers created greater separation between the momentum states of the atoms and then induced the momentum exchange. The researchers found that the atoms continued to behave as if they were connected. “This indicates that the two momentum states actually oscillate relative to each other as if connected by a spring,” Luo added.

Looking ahead, the researchers plan to delve deeper into this new form of quantum entanglement, hoping to better understand how it can be used to improve various types of quantum devices.