“Giant superatoms” could finally solve quantum computing’s biggest problem

Quantum computers are expected to transform fields like drug discovery and encryption by solving problems that are far beyond the reach of conventional machines. However, progress has been limited by a major challenge known as decoherence. This occurs when quantum bits, or qubits, lose their information due to interactions with their surroundings. Even small amounts of electromagnetic noise can disrupt the fragile quantum states needed for computation.

"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," says Lei Du, postdoctoral researcher in applied quantum technology at Chalmers.

Lei Du is the lead author of a study that outlines this new type of quantum system. The design is built around giant superatoms, which combine several important features. These systems reduce decoherence, remain stable, and consist of multiple interconnected "atoms" that function together as a single unit.

What Are Giant Superatoms

Giant superatoms bring together two previously separate ideas in quantum physics: giant atoms and superatoms. While each has been studied on its own, this is the first time they have been merged into a single system. These structures behave like atoms but are not found in nature. Instead, they are engineered by scientists (see fact box below).

Giant Atoms and Their "Quantum Echo"

The idea of giant atoms was first introduced by researchers at Chalmers over a decade ago and is now widely used in the field. A giant atom is typically designed as a qubit (which is the smallest unit of quantum information). Unlike ordinary atoms, it connects to light or sound waves at multiple, physically separated points. This allows it to interact with its environment in several places at once, helping it preserve quantum information.

"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. This self-interaction leads to highly beneficial quantum effects, reduces decoherence and gives the system a form of memory of past interactions," explains Anton Frisk Kockum, Associate Professor of Applied Quantum Physics at Chalmers and co-author of the study.

Extending Entanglement Across Distances

Although giant atoms have improved understanding of quantum behavior, they have had limitations when it comes to entanglement. Entanglement allows multiple qubits to share a single quantum state and act as one coordinated system, which is essential for powerful quantum computers.

To overcome this limitation, the research team combined giant atoms with the concept of superatoms. A superatom consists of several natural atoms that share the same quantum state and behave collectively as one larger atom.

This combination is expected to make it easier to create complex quantum states needed for quantum communication, networks, and highly sensitive measurement systems.

"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. This enables quantum information from multiple qubits to be stored and controlled within one unit, without the need for increasingly complex surrounding circuitry," explains Lei Du.

"Giant superatoms open the door to entirely new capabilities, giving us a powerful new toolbox. They allow us to control quantum information and create entanglement in ways that were previously extremely difficult, or even impossible," says Janine Splettstoesser, Professor of Applied Quantum Physics at Chalmers and co-author of the study.

Toward Scalable and Practical Quantum Systems

This work creates new possibilities for building quantum systems that are both scalable and reliable. The researchers plan to move from theory toward actually constructing these systems. Their design could also be integrated with other quantum technologies, serving as a building block for connecting different types of quantum platforms.

"There is currently strong interest in hybrid approaches, in which different quantum systems work together, because each has its own strengths," says Anton Frisk Kockum. "Our research shows that smart design can reduce the need for increasingly complex hardware and giant superatoms are bringing us one step closer to practically applicable quantum technology."

Controlling Quantum Information Flow

More on: Methods for protecting, controlling and distributing quantum information

The study shows that the way giant superatoms interact with light depends on their internal quantum states. This discovery gives researchers greater control over how quantum information moves through a system. They describe two different ways of connecting these structures to achieve useful outcomes.

In one setup, several giant superatoms are closely linked in a specific arrangement. This allows them to pass quantum states between each other without decoherence, meaning no information is lost.

In another setup, the atoms are spaced farther apart but connected in a carefully tuned way so that waves remain synchronized. This makes it possible to direct quantum signals and distribute entanglement over long distances.

Understanding Giant Atoms and Superatoms

Superatoms and giant atoms are engineered systems that behave like atoms rather than naturally occurring ones.

A superatom is a quantum system made up of multiple natural atoms that share a single quantum state and respond to light as one entity.

A giant atom, on the other hand, connects to light or sound waves at several separate points in space. It is called "giant" because it is larger than the wavelength of the light it interacts with.

Giant atoms have defined energy levels and follow the rules of quantum mechanics, yet they can reach sizes of up to millimeters, making them visible to the naked eye. Through electromagnetic or acoustic waves, they can interact with their surroundings at multiple locations at the same time. One way to picture this is as a single atom linked to a wave at several distant points. This unusual setup allows the atom to be influenced by the waves it produces.