A Simple Recipe of Lasers and Mirrors Builds Quantum States No One Had Imagined

Trap a few thousand atoms between two mirrors, bathe them in laser light, and something frustrating happens. The atoms all start behaving identically. Each one talks to the trapped light in exactly the same way as its neighbor, and that sameness, that perfect democratic symmetry, turns out to be a cage. It limits the kinds of entangled states the atoms can form, no matter how cleverly you tune the experiment.

For decades this has been the quiet ceiling on a whole class of quantum machines. The setup, known as cavity quantum electrodynamics, or cavity QED, is the workhorse behind some of the most precise sensors ever built. But its symmetry has always held it back.

Now a team at the University of Chicago’s Pritzker School of Molecular Engineering has found a way to break the symmetry without breaking the apparatus. Their trick, described on 1 June in Physical Review X, uses tools already sitting in quantum labs around the world. No exotic new hardware. Just an extra magnetic field, or a second set of lasers, used to give different groups of atoms slightly different identities. The result is a recipe for highly entangled quantum states, some of which physicists had never thought to look for.

“The challenge has always been that these systems have too much symmetry. All the atoms are talking to light in the same way,” says Aashish Clerk, the molecular engineering professor who led the work. The team’s fix sounds almost too modest to matter.

Here is the idea. Each atom has a low-energy ground state and a higher-energy excited state, separated by a fixed gap. While every atom is driven by one common laser, the researchers nudge the excited-state energy of different groups up or down, pairing each group with a partner shifted by an equal and opposite amount. That small asymmetry gives the atoms distinct personalities while keeping enough structure for the maths to stay solvable. Change which atoms get which nudge, and the whole system settles into a different entangled state, all without touching a single physical component.

“You turn these lasers on and wait, and at some point the system stabilizes into an interesting, highly entangled quantum state,” says Anjun Chu, the postdoctoral researcher who is first author on the paper. “By simply adjusting the lasers, we can access kinds of entangled states that no one had thought about before.”

Turning leakage into a tool

What makes the approach genuinely clever is what it does with loss. In a cavity, light leaks out; photons escape, and that escape normally drains away the delicate quantum behavior you are trying to preserve. Most schemes for building entangled states fight this dissipation, or demand a long list of carefully engineered, independent loss channels to tame it. Clerk’s group did something closer to judo. They lean on a single, naturally occurring leakage process, the same collective decay that already happens in every cavity QED experiment, and use it, together with those paired energy shifts, to actively push the atoms toward the state they want. Turn on the drives, let the system bleed, and it relaxes not into mush but into a unique, pure, deeply entangled configuration. The escaping light, oddly, does the assembling.

The states that emerge have a peculiar internal bookkeeping. Atoms with equal and opposite energy shifts end up paired, their fates intertwined, and by reshuffling which atoms pair with which, the team can dial in entanglement of varying complexity. There is even a tidy mathematical analogy hiding in here: arranging the atoms into the right order turns out to mirror “bubble sort,” the schoolbook algorithm for sorting a list by swapping neighbors one pair at a time.

Sensing the difference between two places

The most immediate payoff is in quantum sensing. Entangled atoms can, in principle, detect impossibly small differences in a magnetic or gravitational field between two spots. The trouble is that entanglement is fragile, and the very states most sensitive to a signal tend to be the ones most easily wrecked by background noise. Engineers have long wanted a sensor that is both exquisitely sensitive and stubbornly robust, and the two demands usually pull in opposite directions. Clerk’s team showed that a version of their setup, using two clouds of atoms placed in two locations, can measure the gradient between the local fields while shrugging off noise that rattles both clouds 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,” says Clerk. “Normally, entanglement is very fragile. This approach has some amazing resilience.”

And you do not need fancy equipment to read the answer out. The states can be measured with standard Ramsey measurements, the bread-and-butter technique already used in atomic clocks and interferometers, which matters rather a lot if any of this is to leave the chalkboard. The same logic extends to four clouds of atoms, which the team showed could sense not just a gradient but the curvature of a field, the way it bends across space.

The reach goes beyond sensing, too. The same humble setup can be tuned to stabilize the AKLT state, a famous knot of many-body entanglement first dreamed up in the 1980s to describe exotic magnetic materials, and now prized as a possible resource for quantum computing. That such an elaborate state should fall out of lasers, mirrors and a bit of leakage is the sort of thing that makes theorists sit up.

For now it is all theory. The researchers are talking with experimental groups about putting the scheme to the test, and mapping out the wider zoo of states the method might reach. “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,” says Clerk. The mirrors, it seems, have not finished surprising us.

Source: Chu et al., Physical Review X, 1 June 2026. DOI: 10.1103/qdh9-2pc7

Frequently Asked Questions

Why does breaking the symmetry of these atoms matter so much?

When every atom in a cavity responds to light identically, the system can only ever produce a narrow menu of entangled states, which caps what the technology can do. By giving groups of atoms slightly different energies, researchers unlock a far broader family of states from the same hardware. It is the difference between a piano with three keys and one with the full keyboard.

How can the same leaky cavity that ruins quantum states also build them?

Normally, light escaping a cavity drains the fragile quantum behavior physicists want to keep. The clever move here is to treat that single, natural leakage process not as an enemy but as a steering force that, combined with carefully tuned lasers, pushes the atoms into a precise entangled configuration and holds them there. The system settles into the target state rather than decaying away from it.

Could this actually improve real-world sensors?

That is the hope, and the design was built with practicality in mind. The states can be read out with Ramsey measurements, a standard technique already used in atomic clocks and interferometers, and they stay sensitive to a signal while ignoring noise that affects two locations equally. The work is still theoretical, so the next step is for an experimental group to put it to the test.

What is the AKLT state and why would anyone want one?

The AKLT state is a particular tangle of many-body entanglement first proposed in the 1980s to understand exotic magnetic materials, and it has since become a sought-after ingredient for certain approaches to quantum computing. Producing it usually takes considerable effort, so the fact that a simple arrangement of lasers and mirrors can stabilize it is what caught physicists’ attention here.