Scientists develop theory for an entirely new quantum system based on ‘giant superatoms’

An “echo” that arrives before you finish speaking sounds like a glitch. In quantum hardware, that kind of self-interference can be a feature.

That odd timing sits at the heart of a new theoretical design from Chalmers University of Technology, where physicists propose a hybrid object they call a “giant superatom.” The idea is meant to tackle a familiar obstacle in quantum computing: qubits lose their quantum information when the environment nudges them, even slightly.

“Quantum systems are extraordinarily powerful but also extremely fragile. The key to making them useful is learning how to control their interaction with the surrounding environment,” said Lei Du, a postdoctoral researcher in applied quantum technology at Chalmers.

Du is lead author on the team’s paper, which lays out the model. Their proposal tries to suppress decoherence while also letting multiple qubits act together, which is essential for scaling.

The “giant” part of the concept comes from giant atoms, a term Chalmers researchers introduced a little over a decade ago. In practice, a giant atom is often built as a qubit with multiple connection points to a waveguide that carries light or sound. Because those points are separated in space, the qubit can interact with the same field at more than one location.

That geometry lets the system interfere with itself. Waves emitted at one point can travel through the environment and affect the qubit at another point, after a short delay.

“Waves that leave one connection point can travel through the environment and return to affect the atom at another point, similar to hearing an echo of your own voice before you’ve finished speaking,” said Anton Frisk Kockum, an associate professor of applied quantum physics at Chalmers and a co-author.

In the model, that self-interaction can reduce decoherence and give the system a kind of memory of earlier interactions. The paper also describes how, under certain conditions, a giant atom can suppress its own decay into the waveguide through interference.

Giant atoms, by themselves, do not solve everything. Quantum computers also rely on entanglement, the shared quantum link that lets separate qubits behave like parts of a single state. Building that link reliably, and routing it through a device, is one of the hardest parts of making bigger machines.

The “superatom” idea comes from a different direction. A superatom is made from several natural atoms that share one quantum state and respond to light as a collective unit.

Du and colleagues combine those two constructs into one: the giant superatom. In their description, multiple atoms are entangled internally, while the overall unit couples to a waveguide through a giant-atom-style connection.

“A giant superatom may be envisaged as multiple giant atoms working together as a single entity, exhibiting a non-local interaction between light and matter,” Du said. He added that the design could let quantum information from multiple qubits be stored and controlled in one unit, without piling on more external circuitry.

“Giant superatoms open the door to entirely new capabilities, giving us a powerful new toolbox,” said Janine Splettstoesser, a professor of applied quantum physics at Chalmers and a co-author. “They allow us to control quantum information and create entanglement in ways that were previously extremely difficult, or even impossible.”

The paper digs into how a giant superatom’s interaction with light depends on its internal quantum state. That state-dependence matters because it lets the same hardware behave differently depending on how the atoms inside are entangled.

One configuration keeps multiple giant superatoms tightly coupled in a specific arrangement. In that layout, the authors describe transfers of quantum states between units that are “decoherence-free,” meaning the information does not leak away through the waveguide in the idealized case.

A second configuration puts the units farther apart but matches the connections so the waves stay in phase. That alignment enables directional signaling, which is a route to distributing entanglement across long distances.

The team also highlights “braided” arrangements, where coupling points from one unit sit between coupling points of another. In the braided case, the model supports protected transfers and swaps between internal entangled states. The authors note extremely weak decoherence can still appear due to non-Markovian retardation effects, meaning the environment’s time delays matter.

For separate units, the authors show a different lever: chiral emission, where excitations preferentially travel one way along the waveguide. By setting a phase difference between coupling points, different internal entangled states can emit in opposite directions. With carefully modulated couplings, the model reaches transfer fidelities above 99% in one scenario, and it can generate multipartite “W-class” entangled states between remote nodes in another.

This work is theory. The authors model the waveguide as a one-dimensional chain, though they state their results also hold for continuous waveguides. Several of their strongest effects also depend on tuned parameters, engineered phases, and time-dependent control of coupling strengths that prevents unwanted reflections.

Distance creates tradeoffs. Braided structures require units to sit close together, while long-range protocols rely on directional emission and careful timing.

None of that makes the concept less interesting, but it does frame the next step: turning a mathematical design into something you can fabricate and test.

Research findings are available online in the journal Physical Review Letters.

The original story "Scientists develop theory for an entirely new quantum system based on ‘giant superatoms’" is published in The Brighter Side of News.

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