Many of the most promising quantum technologies, including advanced sensors and future quantum computers, rely on a phenomenon known as particle entanglement. In this phenomenon, particles become deeply coupled and influence each other in ways that cannot be explained by classical physics. Creating the complex entangled states required for these techniques has traditionally required sophisticated equipment and carefully designed experimental systems.
Researchers at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) have proposed a simpler approach. Their new theoretical method can generate and control a wide range of entangled quantum states using tools already common in many quantum physics labs.
The works published in Physical Review Xcould help advance ultra-high precision quantum sensing and open new opportunities to explore fundamental physics.
“We wanted to take simple ingredients found in many physical platforms and combine them in a minimal way to create something interesting, complex, and powerful,” said Aashish Clark, professor of molecular engineering at the University of Chicago PME and senior author of this new study.
This research was supported by Q-NEXT, the U.S. Department of Energy’s (DOE) National Quantum Information Science Research Center led by Argonne National Laboratory.
Rethinking cavity QED systems
The research 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. The optical cavity consists of two mirrors that trap light between them. The particles then interact with the light trapped inside the cavity.
A limitation of many resonator QED systems is that all atoms interact with light in exactly the same way. Because atoms are virtually indistinguishable, the range of quantum states that can be created is limited.
“The challenge has always been that these systems are so symmetrical; all the atoms interact with light in the same way,” Clark says. “This really limits what kinds of entangled states you can get.”
In a typical cavity QED setup, each atom has a ground state and an excited state separated by a certain energy difference.
Researchers have discovered a simple way to reduce the symmetry of the system. All atoms are still driven by the same laser, but an additional laser or magnetic field is used to shift the excited state energies of different atomic groups. Atoms are placed in pairs with another atom, each with an equal but opposite energy offset.
This simple change allows the atoms to behave differently from each other while maintaining enough structure that the system remains controllable and predictable. By changing which atoms undergo specific energy shifts, scientists can tune the system to create different entangled states without changing the physical hardware.
“If you turn on these lasers and wait, at some point the system stabilizes and enters an interesting, highly entangled quantum state,” said Anjun Chu, a postdoctoral fellow in the Clark group and lead author of the new study. “By simply tuning the laser, we can access types of entangled states that no one has thought of before.”
Building better quantum sensors
One of the most promising applications of the new approach is quantum sensing.
In theory, entangled states can detect extremely small differences in magnetic or gravitational fields between separate locations. However, developing sensitive and noise-resistant states remains a major challenge.
The researchers demonstrated that a version of the proposed system involving two groups of atoms can be used to measure field gradients. When two ensembles of atoms are placed in different locations, the resulting quantum state reflects differences in the local magnetic or gravitational fields. At the same time, it naturally rejects background noise that affects both locations equally.
“We can do two things that are normally not mutually exclusive: use entanglement to build extremely sensitive sensors, but also be robust to arbitrarily large amounts of noise,” Clark said. “Tangles are usually very fragile. This approach has 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 specialized measurement methods.
Applications beyond sensing
The researchers also showed that the same platform can generate unusual quantum states that have long been of interest to physicists.
An example is the AKLT condition. This is a well-known many-body entanglement state that was first introduced in the 1980s to describe unusual magnetic materials. The team found that this condition could be stabilized with a relatively simple setup. In addition to helping scientists study complex magnetic systems, AKLT states may also have applications in quantum computing.
Next steps in research
For now, the study remains theoretical, but the researchers are already discussing the possibility of experimental testing with other groups.
They are also investigating more sophisticated ways of arranging atoms within the system, exploring the full range of quantum states that could be created that way.
“The fact that we can generate such complex and useful quantum states with such simple materials gives us hope that even before we reach the dream of a general-purpose quantum computer, we can already generate quantum states that allow us to do things that were not possible in the purely classical world,” Clark said.
This material is based on research supported by the U.S. Department of Energy’s Office of Science National Quantum Information Research Center as part of the Q-NEXT Center.
