The atoms inside the optical cavity exchange their state of momentum by "playing catch" with photons. When an atom absorbs a photon from the applied laser, the entire atomic cloud recoil instead of a single atom. Photo credit: Steven Burrows/Rey, Thompson and Holland Groups
Accurately measuring the energy state of individual atoms has been a historic challenge for physicists due to atomic recoil. When an atom interacts with a photon, the atom "kicks back" in the opposite direction, making it difficult to accurately measure the position and momentum of the atom. This recoil can have a significant impact on quantum sensing, which can detect small 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 paper published in the journal Science, JILA and NIST researchers Ana Maria Rey and James Thompson, JILA researcher Murray Holland and their team propose a way to overcome this atomic backlash by demonstrating a new type of atomic interaction called momentum exchange interactions, in which atoms exchange momentum by exchanging corresponding photons.
Using a cavity (an enclosed space made up of mirrors), the researchers observed that atomic recoil was inhibited by atoms exchanging energy states within a confined space. This process creates a collective absorption of energy and disperses the recoil throughout the particle population.
With these results, other researchers can design cavities to suppress recoil and other externalities in a wide range of experiments, which could help physicists better understand complex systems or discover new aspects of quantum physics. Improved cavity design can also enable more accurate superconducting simulations, such as Bose-Einstein-Condensed-Bardeen-Cooper-Schrift (BEC-BCS) crossover or high-energy physics systems.
For the first time, momentum-exchange interactions were observed to induce uniaxial distortion (OAT) dynamics between atomic momentum states, which is an aspect of quantum entanglement. OAT is like a quantum braid that is used to entangle different molecules, as each quantum state twists and connects to another particle.
Previously, OATs only appeared in the internal state of atoms, but now, with these new results, it is thought that momentum exchange-induced OATs can help reduce quantum noise from multiple atoms. Being able to entangle momentum states can also improve some of the physical measurements of quantum sensors, such as gravitational waves.
Utilize density gratings
In the new study, inspired by previous research by Thompson and his team, the researchers investigated the effects of quantum superposition, which allows particles such as photons or electrons to exist in multiple quantum states at the same time.
"In this [new] project, atoms all share the same spin tag; The only difference is that each atom is in a superposition state between the two states of momentum," explains Chengyi Luo, graduate student and first author.
Researchers have found that they can better control atomic recoil by forcing atoms to exchange photons and their associated energy. Similar to a game of dodgeball, an atom may "throw" a "dodgeball" (photon) and recoil in the opposite direction. This "dodgeball" may be caught by the second atom, which causes the second atom to produce the same amount of recoil. This counteracts the two recoils experienced by the two atoms and averages the entire cavity system.
When two atoms exchange their different photon energies, the superposition of the resulting wave packets (the wave distribution of atoms) forms a momentum map called a density grating, which looks like a fine-toothed comb.
"The formation of density gratings shows that the two states of momentum [within the atom] are 'coherent' with each other, so they can interfere with each other," Rowe added. The researchers found that the exchange of photons between atoms causes the wave packets of the two atoms to bind, so they are no longer separate measurements.
Researchers can induce momentum exchange by exploring the interaction between density gratings and cavities. As atoms exchange energy, any recoil of absorbed photons is dispersed throughout the atomic community rather than in individual particles.
Suppresses Doppler shifts
Using this new control method, the researchers found that they could also use this recoil damping system to help alleviate a separate measurement problem: the Doppler shift.
The Doppler shift is a phenomenon in classical physics that explains why the sound of a siren or train horn changes pitch as it passes by the listener, or why certain stars appear red or blue in images of the night sky – this is the change in the frequency of waves when the sound source and observer are close (or farther away) from each other. In quantum physics, the Doppler shift describes the change in energy that occurs in a particle due to its relative motion.
For researchers like Luo, Doppler shifts can be a challenge to get accurate measurements. "When absorbing photons, atomic recoil will result in a Doppler shift in the frequency of the photons, which is a big problem when talking about precision spectroscopy," he explained. By simulating their new method, the researchers found that it could overcome the measurement skew due to the Doppler shift.
Entangled momentum exchange
The researchers also found that the exchange of momentum between these atoms can be used as a type of quantum entanglement. As John Wilson, a graduate student in Holland's group, said, "When an atom falls, its motion swings the cavity frequency." This, in turn, encourages other atoms to collectively feel this feedback mechanism and prompts them to associate their movements through a common oscillation.
To further test this "entanglement", the researchers established a greater separation between the momentum states of the atoms, which then induced momentum exchange. The researchers found that the atoms continued to behave as if they were connected. "This suggests that these two states of momentum are indeed oscillating with each other, as if they were connected by springs," Rowe added.
Going forward, the researchers plan to further explore this new form of quantum entanglement, hoping to better understand how it can be used to improve various types of quantum devices.
更多信息:Chengyi Luo 等人,布拉格原子干涉仪中的动量交换相互作用抑制多普勒去相,《科学》(2024 年)。 DOI: 10.1126/science.adi1393.www.science.org/doi/10.1126/science.adi1393
期刊信息： Science