Two years ago, researchers at the University of Chicago Pritzker School of Molecular Engineering (PME) created a hybrid array of neutral atoms of two different species for the first time. They used optical tweezers, or highly focused laser beams, to hold the atoms in place. By demonstrating independent control of the two species in the same array, the researchers ushered in a new breadth of opportunities for quantum processing
Now, in a recent work published in Nature Physics, this team from the Bernien Lab, led by UChicago PME Assistant Professor Hannes Bernien, has taken the next critical step of demonstrating quantum entanglement between the two different elements using Rydberg interactions. Additionally, by leveraging this entanglement, they showed and benchmarked quantum non-demolition measurements, a key subroutine required for scaling quantum systems.
“Our lab was built to see these interactions between two different species in a controlled manner," said co-first author Shraddha Anand, a PME graduate student. “The exciting thing now is everything this opens up.”
Limiting crosstalk between atoms
The process of using optical tweezers to trap a single atom is still nascent; the technique won the Nobel Prize in Physics in 2018. Entangling two different types of atoms in optical tweezers is a complicated process, Anand said. One of the major challenges is finding two elements that will not crosstalk — or transfer energy — with one another.
The Bernien group identified rubidium and cesium atoms as the two types that would sufficiently work together.
“Encoding quantum information is very sensitive,” Anand said. “We specifically picked these atoms because the resonance frequencies at which we do cooling, imaging, and trapping are really far from each other, so there's minimal crosstalk between the two.”
The limited crosstalk allows the Bernien group to gain more fine-grained and independent control of the individual atoms. With greater control, the researchers can create greater possibilities.
A stepping stone to quantum error correction
Controlling individual atoms is one way to create the qubits — or quantum bits — that are the foundation of quantum computing. This novel approach has become one of the frontrunners in the quest to build a quantum computer and there are now several start-ups that are founded based on this principle.
The outstanding challenge in building a large and stable quantum computer is that quantum systems are inherently noisy. The theory of quantum error correction provides a pathway to deal with the noise, but most protocols work by measuring a subset of qubits to detect the error and apply the required correction. Crucially, this measurement and processing must not induce more errors.
This is where the dual-species nature of their array can help us. “We can now entangle the two species, and measure one of them for error detection without impacting the atoms of the other,” explained Anand.
Quantum simulation
People may hear quantum information and only think of quantum computers, but Anand said it is more than that.
“Quantum information processing is a very broad term,” she said. “It includes using quantum systems to solve computational problems more efficiently … but there's also a simulation aspect.” To explain its significance, Anand referenced physicist Richard Feynman.
“Richard Feynman said we shouldn't be using classical systems to study quantum phenomena because, by the very nature of quantum physics, the encoding is inefficient,” she said. “We should be building quantum simulators to study quantum phenomena.”
Large-scale controlled quantum systems create unique opportunities to explore nature through an atom-by-atom approach. When physicists study materials, for example, they can either study an individual atom in the material at a basic level, or they can study the material in aggregate.
The space in-between those two ends of the spectrum is difficult to probe, Anand said. The group's work with the dual-species Rydberg array makes it a little less difficult.
“Because we can build these systems atom by atom, we can start studying materials at a scale that hasn't been done before,” she said. “And since we're using two different types of species, we get access to a completely new interaction regime.”
The group is now working on exploring the physics of these dual-species interaction regimes and incorporating it into novel quantum protocols.
Citation: “A dual-species Rydberg array,” Anand, S., Bradley, C.E., White, R. et al., Nature Physics. September 20, 2024. DOI: 10.1038/s41567-024-02638-2