Quantum technologies, including advanced sensors and future quantum computers, depend on entanglement - that spooky connection where particles influence each other in ways that would give any classical physicist a headache. Creating these fancy entangled states has traditionally required sophisticated equipment and carefully designed experimental systems, because nothing worthwhile in physics comes easy.
Researchers at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) have now proposed a much simpler approach. Their new theoretical method can generate and control a wide range of entangled quantum states using tools that are already common in many quantum physics laboratories. The work, published in Physical Review X, could help advance ultra-precise quantum sensing and open new opportunities for exploring fundamental physics.
“We wanted to take simple ingredients that you find in a lot of physical platforms and put these together in a minimal way to get something interesting, complex and powerful,” said Aashish Clerk, professor of molecular engineering at UChicago PME and senior author of the new study. The research was supported by Q-NEXT, a U.S. Department of Energy (DOE) National Quantum Information Science Research Center led by DOE's Argonne National Laboratory.
The team's approach is based on cavity quantum electrodynamics (cavity QED), where atoms are placed inside an optical cavity - two mirrors that trap light between them. The particles then interact with the confined light. The problem? In many cavity QED systems, all atoms interact with light in exactly the same way, making them effectively indistinguishable and restricting the range of quantum states that can be produced.
“The challenge has always been that these systems have too much symmetry,” Clerk said. “All the atoms are talking to light in the same way. That really restricts what kind of entangled states you get.”
The researchers found a straightforward fix: while all atoms continue to be driven by the same laser, additional lasers or magnetic fields shift the excited state energies of different groups of atoms. Each atom is paired with another that has an equal but opposite energy offset. This simple modification breaks the symmetry while keeping the system controllable and predictable. By adjusting which atoms receive particular energy shifts, scientists can tune the system to produce a variety of entangled states without changing the hardware.
“You turn these lasers on and wait, and at some point the system stabilizes into an interesting, highly entangled quantum state,” said Anjun Chu, a postdoctoral researcher in the Clerk group and first author of the new work. “By simply adjusting the lasers, we can access kinds of entangled states that no one had thought about before.”
One promising application is quantum sensing. Entangled quantum states can detect tiny differences in magnetic or gravitational fields between separate locations. However, developing states that are both highly sensitive and resistant to noise has been a major challenge. The researchers demonstrated that a version of their system with two groups of atoms could measure field gradients - when the two atomic ensembles are placed in different locations, the resulting quantum state reflects the difference between local magnetic or gravitational fields while rejecting background noise that affects both locations equally.
“You're able to do two things that are normally not compatible with one another: Use entanglement to build an exquisitely sensitive sensor but also have robustness to arbitrarily large amounts of noise,” Clerk said. “Normally, entanglement is very fragile. This approach has some amazing resilience.”
Another advantage: the information stored in these quantum states can be extracted using standard Ramsey measurement techniques, eliminating the need for specialized methods. The researchers also showed that the same platform can generate unusual quantum states that have long attracted interest from physicists, like the AKLT state - a many-body entangled state first introduced in the 1980s to describe unusual magnetic materials. The AKLT state may also have applications in quantum computing.
The work remains theoretical for now, but the researchers are already discussing possible experimental tests with other groups. They are also investigating more sophisticated ways to arrange atoms and exploring the full range of quantum states their method may produce.
“The fact that such simple ingredients can generate such complex and useful quantum states gives us hope that even before we reach the dream of a general all-purpose quantum computer, we can already generate quantum states that let us do things we couldn't do in a purely classical world,” Clerk said.
This material is based upon work supported by the U.S. Department of Energy Office of Science National Quantum Information Science Research Centers as part of the Q-NEXT center. Materials provided by University of Chicago. Original written by Sarah C.P. Williams.