A quantum superhighway for ultrafast NOON states

A NOON state is a superposed quantum state where N particles are in one state “at the same time” and in another “at the same time.” Here, the particles are trapped in two wells, within a trap formed by lasers. The superimposed state therefore consists of a state in which all the particles are in the left-hand well, and a state in which they are trapped in the right-hand well. The particles interact with each other and stick together when they are in the same site, preventing an isolated particle from leaving the trap. Credit: University of Liège / S. Dengis

Until now, creating quantum superpositions of ultra-cold atoms has been a real headache, too slow to be realistic in the laboratory. Researchers at the University of Liège have now developed an innovative new approach combining geometry and “quantum control,” which drastically speeds up the process, paving the way for practical applications in quantum technologies.

The paper is published in the journal Physical Review A.

Imagine being in a supermarket with a cart filled to the brim. The challenge: get to the checkout before the others, without dropping your products on the corners. The solution? Choose a route with as few corners as possible to go faster without slowing down. That’s exactly what Simon Dengis, a doctoral student at the University of Liège, has managed to do, but in the world of quantum physics.

With his colleagues in the PQS (Quantum Statistical Physics) group, Dengis has developed a protocol for rapidly generating what are known as NOON states.

“These states, which look like miniature versions of Schrödinger’s famous cat, are quantum superpositions,” explains the physicist. “They are of major interest for technologies such as ultra-precise quantum sensors or quantum computers.”

The obstacle of time

The main challenge? Manufacturing these states normally takes far too long. We’re talking tens of minutes or more, which often exceeds the lifetime of the experiment. The cause? An energy bottleneck, a “sharp bend” in the system’s evolution that forces it to slow down.

This is where the ULiège team breaks new ground. By combining two powerful concepts—counterdiabatic driving and the optimal geodesic path—they have succeeded in “smoothing the road” for atoms. The result: the system can evolve faster without losing the trajectory of the desired state, just like a driver who anticipates a bend by tilting his tray.

“This strategy saves a considerable amount of time: in some cases, the process is accelerated by a factor of 10,000, while maintaining 99% fidelity, i.e. near-perfection of the result,” says Peter Schlagheck, director of the laboratory. Where previously it would have taken around ten minutes to create such a state, the researchers have succeeded in considerably reducing this waiting time to 0.1 seconds.

A quantum superhighway for ultrafast NOON states
Counter-diabatic control compensates for the inertia of a system by modifying it in a certain way. In this case, to compensate for the movement of the water caused by the movement of the waiter, the latter can tilt his tray to compensate for the inertia of the glass and prevent it from tipping over. Credit: University of Liège / S.Dengis

Towards practical applications

With this breakthrough, it is finally possible to produce NOON states with ultra-cold atoms. This opens up prospects in quantum metrology (ultra-sensitive measurements of time, rotation or gravity) and quantum information technologies. Ultimately, these tools could improve instruments such as quantum gyroscopes or miniature gravity detectors.

This research shows how theory and experimentation can come together to make concrete advances in quantum physics. By combining mathematical concepts, fundamental physics and experimental feasibility, ULiège researchers have made a breakthrough that could well transform ideas that were once theoretical into tomorrow’s technologies.

A quantum superhighway for ultrafast NOON states
The proposed protocol (blue, GCD) makes it possible to widen the energy bottleneck (compared with the usual protocol in red, G) and therefore to have to brake less when approaching it. The image can be understood in the context of a motorbike race: the red motorbike will have to brake much more than the blue motorbike because the turn is less “smooth.” The blue bike will therefore arrive at its destination before its opponent. Here, the change in the system’s energies (and therefore its states) is less abrupt, allowing the process to be accelerated drastically. Credit: University of Liège / S.Dengis

Quantum superposition and the NOON state

Quantum superposition is the idea that a quantum system (such as an atom, an electron or a photon) can exist in several states at the same time, as long as it is not observed. The example most often used to explain this concept is Schrödinger’s cat: A cat is locked in a box. According to quantum mechanics, until the box is opened, the cat is both alive and dead.

This simultaneous combination of two states is called superposition. It is only by opening the box to observe it that we force nature to choose a state: alive or dead. The NOON states are an example of quantum superposition: all the atoms are in both the left-hand well and the right-hand well. It is only at the moment of measurement that they are found in one or the other.

More information:
Simon Dengis et al, Accelerated creation of NOON states with ultracold atoms via counterdiabatic driving, Physical Review A (2025). DOI: 10.1103/PhysRevA.111.L031301. On arXiv: DOI: 10.48550/arxiv.2406.17545

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A quantum superhighway for ultrafast NOON states (2025, March 31)
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