Sometimes the smallest change does the heaviest lifting.
Researchers at the University of Chicago say they have found a simple way to create powerful entangled quantum states that are usually hard to produce. The team proposed making small changes to the energy levels of atoms inside an optical cavity, using equipment already common in many quantum physics laboratories.
The theoretical work, published in Physical Review X, could help advance ultra-precise quantum sensing and create new ways to study 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 the University of Chicago Pritzker School of Molecular Engineering and senior author of the study.
The approach uses cavity quantum electrodynamics, or cavity QED, where atoms or other particles sit inside an optical cavity made of two mirrors that trap light between them. The particles interact with the confined light inside the cavity.
The researchers said many cavity QED systems are limited because all atoms interact with light in the same way. That makes the atoms effectively indistinguishable and restricts the quantum states the system can produce.
“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 team said it found a simple way to reduce that symmetry.
All atoms are still driven by the same laser, but extra lasers or magnetic fields shift the excited-state energies of different groups of atoms. Each atom is paired with another atom that has an equal but opposite energy offset.
The researchers said that lets atoms behave differently from one another while keeping the system controllable and predictable. By changing which atoms get particular energy shifts, scientists can tune the system to produce different 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 study. “By simply adjusting the lasers, we can access kinds of entangled states that no one had thought about before.”
The team said one of the clearest uses for the method is quantum sensing. In theory, entangled states can detect very small differences in magnetic or gravitational fields between separate locations, but researchers have struggled to make states that are highly sensitive and resistant to noise.
The study showed that a version of the 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.”
The researchers also said the information stored in these states can be read out using standard Ramsey measurement techniques, without specialized measurement methods.
The same platform could also generate other quantum states that physicists have long studied, including the AKLT state, a many-body entangled state first introduced in the 1980s to describe unusual magnetic materials. The team said its setup can stabilize that state, which may also have applications in quantum computing.
The work is theoretical for now. The researchers said they are discussing possible experimental tests with other groups and are also studying more advanced ways to arrange atoms in the system and the full range of states the method can produce.
The research was supported by Q-NEXT, a US Department of Energy National Quantum Information Science Research Center led by Argonne National Laboratory.
“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.
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