Trap several thousand atoms between a pair of mirrors, illuminate them with laser light, and a perplexing phenomenon occurs. The atoms begin to behave in unison. Each atom interacts with the confined light exactly like its neighboring atom, and that uniformity, that flawless democratic symmetry, transforms into a confinement. It restricts the types of entangled states the atoms can create, irrespective of how cleverly you adjust the experiment.
For many years, this has been a subtle limit on an entire category of quantum devices. The configuration, called cavity quantum electrodynamics, or cavity QED, serves as the backbone for some of the most precise sensors ever designed. However, its symmetry has consistently hindered progress.
A group at the University of Chicago’s Pritzker School of Molecular Engineering has now discovered a method to disrupt the symmetry without damaging the apparatus. Their technique, detailed on 1 June in Physical Review X, employs tools readily available in quantum laboratories worldwide. No need for exotic new equipment. Just an additional magnetic field, or a supplementary set of lasers, is used to assign slightly different identities to distinct groups of atoms. The outcome is a blueprint for highly entangled quantum states, some of which physicists had never considered before.
“The main issue has always been that these systems exhibit excessive symmetry. All the atoms are interacting with light uniformly,” remarks Aashish Clerk, the molecular engineering professor leading the study. The team’s solution appears almost too simple to be significant.
Here’s the concept. Each atom possesses a low-energy ground state and a higher-energy excited state, separated by a defined gap. While every atom is influenced by a shared laser, the researchers slightly adjust the excited-state energy of different groups upward or downward, matching each group with a counterpart altered by an equal and opposite measure. This minor asymmetry grants the atoms unique characteristics while maintaining enough structure for the mathematics to remain manageable. Modify which atoms receive which adjustment, and the entire system transitions into a different entangled state, all without altering a single physical element.
“You activate these lasers and wait, and eventually the system stabilizes into an intriguing, highly entangled quantum state,” states Anjun Chu, the postdoctoral researcher who is the main author of the paper. “By merely fine-tuning the lasers, we can access kinds of entangled states that previously no one had considered.”
Transforming leakage into a strategy
What renders the approach genuinely ingenious is its treatment of loss. Within a cavity, light escapes; photons break free, and this escape typically drains the delicate quantum behavior you aim to preserve. Most methods for creating entangled states combat this dissipation or require an extensive list of specifically engineered, independent loss channels to manage it. Clerk’s team took a different route, akin to judo. They utilize a single, naturally occurring leakage process, the same collective decay that occurs in every cavity QED experiment, along with those paired energy adjustments, to actively drive the atoms towards their desired state. Activate the drives, allow the system to release energy, and it relaxes not into chaos but into a distinctive, pure, deeply entangled configuration. Oddly, the escaping light facilitates the assembly.
The states that result possess a distinct internal bookkeeping. Atoms with equivalent and opposite energy adjustments end up paired, their destinies intertwined, and by rearranging which atoms pair with others, the team can fine-tune the entanglement to varying complexities. There is even a neat mathematical analogy lurking here: organizing the atoms in the correct sequence resembles “bubble sort,” the conventional algorithm for sorting a list by exchanging neighboring elements one pair at a time.
Sensing the variance between two locations
The most immediate benefit is in quantum sensing. Entangled atoms can, in theory, detect incredibly minute differences in a magnetic or gravitational field between two locations. The challenge is that entanglement is fragile, and the states that are most sensitive to a signal are often the ones most easily disrupted by background noise. Engineers have long sought a sensor that is both exquisitely sensitive and tenaciously robust, but these two requirements typically conflict. Clerk’s team demonstrated that a variation of their setup, employing two clouds of atoms positioned in two different locations, can measure the gradient between the local fields while remaining unaffected by noise that rattles both clouds uniformly.
“You’re capable of achieving two objectives that are usually incompatible: utilizing entanglement to construct an exquisitely sensitive sensor while also ensuring resilience to arbitrarily large amounts of noise,” notes Clerk. “Typically, entanglement is quite delicate. This method exhibits remarkable resilience.”
And there’s no need for advanced equipment to obtain the results. The states can be assessed using standard Ramsey measurements, the primary technique already applied in atomic clocks and interferometers, which is quite important if any of this is to move beyond theoretical concepts. The same principle applies to four clouds.