In the pursuit of powerful and stable quantum computers, researchers at Chalmers University of Technology, Sweden, have developed the theory for an entirely new quantum system – based on the novel concept of 'giant superatoms'. This breakthrough enables quantum information to be protected, controlled, and distributed in new ways and could be a key step towards building quantum computers at scale.
It is anticipated that quantum computers will revolutionise technologies in areas such as drug development and encryption by tackling problems far beyond the capabilities of today's computers. However, the practical realisation of quantum computers has been slowed by a fundamental challenge known as decoherence – the tendency of quantum bits, or qubits, to lose information when interacting with their environment. Even tiny disturbances from electromagnetic noise can destroy the delicate quantum effects required for reliable 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 scientific paper presenting the theoretical model of an entirely new quantum system developed by a Chalmers research team. Their system is based on the novel concept of giant superatoms and brings together several key properties. It suppresses decoherence and is stable, whilst simultaneously comprising multiple, tightly interconnected "atoms" that act collectively.
Giant superatoms combine two different quantum-mechanical constructs: giant atoms and superatoms. These have been explored separately in recent years but have not previously been combined. They behave like atoms but are not natural atoms. Rather, they are artificial structures that physicists have learned to engineer (see fact box below).
Giant atoms with a quantum echo
The concept of giant atoms was coined by researchers at Chalmers just over a decade ago and has since become a standard term in the field. A giant atom is most often designed as a qubit (which is the smallest unit of quantum information). The atom has multiple, spatially separated coupling points to a light or sound wave, allowing it to interact with its surroundings at several locations simultaneously. This enables the giant atom to protect 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.
Enabling entanglement to be distributed over long distances
While giant atoms have already advanced our understanding of quantum physics, their ability to exploit another key quantum phenomenon – entanglement – has so far been limited. Entanglement allows multiple qubits to share a single quantum state and operate as a single, unified system. This is a prerequisite for building powerful, large-scale quantum computers.
The researchers have addressed this problem by combining giant atoms with the superatom concept. A superatom is a structure comprising several natural atoms that share a common quantum state and behave collectively as a single, larger atom.
It is anticipated that this combination will now make it easier to create the advanced quantum states that are crucial for future quantum communication, quantum networks and highly sensitive sensors.
"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.
A key step toward scalable quantum computers
The results open up new opportunities to build scalable and reliable quantum systems, with the researchers now planning to move from theory to fabrication of the quantum system. Their concept could also be combined with other types of quantum systems; as a building block for connecting multiple systems, for example.
"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."
More on: Methods for protecting, controlling and distributing quantum information
The Chalmers study demonstrates how the interaction between giant superatoms and light depends on their internal quantum states. This insight has given the researchers greater control over how quantum information propagates through a system and they describe two distinct ways of coupling the atoms to produce highly useful effects.
In one configuration, multiple giant superatoms are tightly coupled in a specific arrangement, allowing them to transfer their quantum states to one another without decoherence. In other words, there is no loss of information.
In the second configuration, the atoms are placed farther apart but connected in a carefully matched way so that the light or sound waves remain in phase. This enables quantum signals to be sent in specific directions, allowing entanglement to be distributed over large distances.
More on: Superatoms and giant atoms in quantum physics
Superatoms and giant atoms are not natural atoms; they are artificial structures which behave like atoms.
The term superatom refers to a quantum-mechanical construct made up of multiple natural atoms that together behave like a single, larger atom. They share a common quantum state and respond to light as if they were a single quantum entity.
The term giant atom refers to a structure with multiple, spatially separated coupling points to a light or sound wave. The atom is called "giant" because, unlike natural atoms, it is larger than the wavelength of the light with which it interacts.
Giant atoms have well-defined energy levels and are governed by the principles of quantum mechanics, yet they can be up to millimetres in size and thus visible to the naked eye. Through electromagnetic or acoustic waves, they can interact with their surroundings at several spatially separated locations simultaneously. One might imagine a single atom "connected" to a light or sound wave at multiple, widely separated points. This unusual form of coupling allows the atom to be influenced by the very waves it emits.
