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.

Rethinking Cavity QED Systems

The team's approach is based on cavity quantum electrodynamics, commonly known as cavity QED. In these experiments, atoms or other particles are placed inside an optical cavity, which consists of two mirrors that trap light between them. The particles then interact with the confined light inside the cavity.

A limitation of many cavity QED systems is that all of the atoms interact with the light in exactly the same way. Because the atoms are effectively indistinguishable, the range of quantum states that can be produced is restricted.

"The challenge has always been that these systems have too much symmetry. All the atoms are talking to light in the same way," Clerk said. "That really restricts what kind of entangled states you get."

In a typical cavity QED setup, each atom has a ground state and an excited state separated by a specific energy difference.

The researchers found a straightforward way to reduce the system's symmetry. While all atoms continue to be driven by the same laser, additional lasers or magnetic fields are used to shift the excited state energies of different groups of atoms. The atoms are arranged so that each one is paired with another atom that has an equal but opposite energy offset.

This simple modification allows atoms to behave differently from one another while preserving enough structure for the system to remain controllable and predictable. By changing which atoms receive particular energy shifts, scientists can tune the system to produce a variety of entangled states without altering the physical 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."

Building Better Quantum Sensors

One of the most promising uses for the new approach is quantum sensing.

In theory, entangled quantum states can detect extremely small differences in magnetic fields or gravitational fields between separate locations. However, developing states that are both highly sensitive and resistant to noise has remained a major challenge.

The researchers demonstrated that a version of their proposed system containing two groups of atoms could be used to measure field gradients. When the two atomic ensembles are placed in different locations, the resulting quantum state reflects the difference between the local magnetic or gravitational fields. At the same time, it naturally rejects 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 is that the information stored in these quantum states can be extracted using standard Ramsey measurement techniques, eliminating the need for specialized or exotic measurement methods.

Applications Beyond Sensing

The researchers also showed that the same platform can generate unusual quantum states that have long attracted interest from physicists.

One example is the AKLT state, a well known many body entangled state first introduced in the 1980s to describe unusual magnetic materials. The team found that their relatively simple setup can stabilize this state. In addition to helping scientists study complex magnetic systems, the AKLT state may also have applications in quantum computing.

Next Steps for the Research

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 within the system and exploring the full range of quantum states that their method may be capable of producing.

"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.