Researchers reveal microscopic quantum correlations of ultracold molecules

Physicists are increasingly using cryogenic molecules to study the quantum states of matter. Many researchers argue that molecules are superior to other alternatives such as trapped ions, atoms, or photons. These advantages suggest that molecular systems will play an important role in new quantum technologies. For some time, however, the study of molecular systems has progressed only so far due to the long-standing challenges of preparing, controlling and observing molecules in the quantum regime.

Now, as documented in a study published this week, Nature, Princeton researchers have made a major breakthrough by studying molecular gases microscopically at levels never achieved in previous studies. A team at Princeton University, led by associate professor of physics Wasim Bakr, cooled the molecules to very low temperatures, loaded them into artificial crystals of light known as optical lattices, and measured their collective quantum behavior in high space. I was able to study with resolution. We were able to observe individual molecules.

“We prepared molecules in a gas into well-defined internal and kinetic quantum states. strong interaction It provokes subtle quantum correlations between molecules, which we were able to detect for the first time,” Bakr said.

This experiment has great implications for research in fundamental physics, such as many-body physics studies investigating the emergent behavior of ensembles of interacting quantum particles. This research could also accelerate the development of large-scale quantum computer systems.

To build large-scale quantum systems for both quantum computing and more general scientific applications, researchers are using everything from trapped ions and atoms to electrons confined in “quantum dots.” I have used alternatives.

The goal is to transform these different choices into what we call qubits, the building blocks of quantum computer systems. Quantum computers have far greater computing power and capacity (exponentially larger) than classical computer systems, allowing them to solve problems that are difficult for classical computers to solve.

So far, no single type of qubit has emerged as a front-runner, but Bakr and his team believe that: molecular systemwhich has not been explored as much as other platforms, but is particularly promising.

One of the key advantages of using molecules in experimental environments, especially as potential qubits, is the fact that molecules can store quantum information in a wealth of new ways not available in single atoms.

For example, even for a simple molecule consisting of just two atoms, which can be visualized as a small dumbbell, quantum information can be stored in the rotational motion of the dumbbell or the swinging of its constituent atoms relative to each other. Another advantage of molecules is that they often have long-range interactions. They can interact with other molecules far from many sites in the optical lattice, but atoms, for example, can only interact if they occupy the same sites.

These advantages are expected to enable researchers to explore fascinating new quantum phases of matter in these synthetic systems when using molecules to study many-body physics. However, the main problem that Bakr and his team were able to overcome in this experiment is the microscopic characterization of these quantum states.

“The ability to interrogate gases at the level of individual molecules is a new aspect of our research,” said Bakr. “If we can look at individual molecules, we can extract more information about many-body systems.”

What Bakr means by extracting more information is observing and documenting the subtle correlations that characterize molecules in quantum states (e.g., positional correlations in the lattice and rotational state correlations). Ability.

Jason Rosenberg, a graduate student in the Department of Physics at Princeton University and co-first author of the paper, said: paper. “By looking at individual molecules, we can actually characterize and investigate the different quantum phases that we expect to emerge.”

Researchers have used atomic quantum gases to study many-body physics for more than two decades, but molecular quantum gases have been much more difficult to tame. Unlike atoms, molecules can store energy by vibrating and rotating in different ways. These different excitations are known as ‘degrees of freedom’. Their abundance is a feature that makes the molecule difficult to control and manipulate experimentally.

“To study molecules quantum regimewe need to control all those degrees of freedom and put them into a well-defined quantum-mechanical state,” Bakr said.

The researchers first achieved this precise level by cooling two atomic gases, sodium and rubidium, to incredibly low temperatures measured in nanokelvins, or billionths of a kelvin. achieved control of At these extremely low temperatures, each of the two gases transitions into a state of matter known as Bose-Einstein condensation. In this ultracold environment, researchers guide atoms to pair into sodium-rubidium molecules in well-defined internal quantum states. A laser is then used to transfer the molecule to its absolute ground state, where all rotations and vibrations of the molecule are frozen.

To preserve the quantum behavior of the molecules, they are isolated in a vacuum chamber and held within an optical lattice composed of standing waves of light.

Rosenberg said:

In experiments, researchers captured about 100 molecules in this ‘egg box’ lattice. The researchers then pushed the system out of equilibrium and tracked what happened in the strongly interacting system.

“We made a sudden ‘tweak’ to the system,” said graduate student Lysander Christakis, co-lead author of the paper. “We allowed molecules to interact and build quantum entanglementThis entanglement is reflected in subtle correlations, and the ability to investigate systems at this microscopic level allows us to uncover these correlations and learn about them. ”

Entanglement is one of the most fascinating and complex properties of many-body quantum states. It describes the properties of the subatomic world, where quantum elements such as molecules, electrons, and photons are intimately connected to each other regardless of distance. Entanglement is particularly important in quantum computing as it acts as a kind of computational multiplier. This is a key factor underlying the exponential speedup in solving problems on quantum computers.

The unparalleled control researchers have achieved over the preparation and detection of molecules quantum computingUltimately, however, the researchers stress that the experiment isn’t necessarily about creating state-of-the-art qubits. Rather, and most importantly, this is a big step forward. basic physics research.

“This work opens up many possibilities for studying very interesting problems in many-body physics,” said Christakis. “What we’ve demonstrated here is a complete platform to use cryogenic molecules As a system for studying complex quantum phenomena. ”

Rosenberg agreed. “In this experiment, the molecules were frozen to individual sites on the lattice and quantum information Only the rotational state of the molecule was preserved. Going forward, it will be exciting to explore a whole other realm of interesting phenomena that emerge when molecules are allowed to “hop” from site to site. has opened the door for investigating ever more exotic states of the world, and we are now able to characterize them remarkably well.