On June 7, 2024, the United Nations proclaimed 2025 as the International Year of Quantum Science and Technology (IYQ). The occasion is the first centenary of the invention of Quantum Mechanics, a giant leap forward in our understanding of the laws of physics at the microscopic/atomic level.

It is worth mentioning that another major advance in theoretical physics was made almost simultaneously at the beginning of the twentieth century: relativity, culminating in general relativity. The relations between these two branches of physics lead to very deep questions, the most famous being the Einstein-Podolski-Rosen paradox which triggered debates between two of the founding fathers Bohr and Einstein, and led much later to experimental evidence of the “quantum entanglement” (see the 2022 Nobel prize in Physics to A. Aspect, J. Clauser and A. Zeilinger). The question - how to reconcile general relativity and quantum mechanics - is still pending. Such a reconciliation would be achieved by the elaboration of a viable theory of quantum gravity.

Quantum theory is an essential ingredient of quantum field theory, which is at the core of elementary particle physics. To foster research in this domain Europe created the Centre Européen de Recherche Nucléaire (CERN). CERN has succeeded wonderfully, by federating national efforts for the last 70 years and it is still active with several projects for the years to come.

Beyond the theoretical questions, technological advances have enabled significant progress in the creation of quantum technologies. These rely on the ability to control the quantum nature of atoms, electrons and photons, leading to advances across a range of application areas: quantum sensors, quantum computing, quantum communications.

Quantum sensors

Quantum properties may be used for measurements in domains as diverse as atomic clocks, superconducting quantum interference devices, or NMR spectroscopy. The devices essentially exploit the ability to play with atoms (possibly ions). For example, quantum gravimeters measure the fall of individual atoms in a vacuum chamber at very low temperature. The precision of these devices is excellent and is expected to exceed that of classical gravimeters. The key word for both present and future applications is “extreme sensitivity”.

Quantum computing

The basic idea of quantum computing is to replace bits (i.e. something taking one of two values 0 and 1) by something taking as value a linear combination of two states. The experimental “qubit” is just a two-state quantum mechanical system. Devices in use at present include Rydberg atoms, Josephson junctions, trapped ions, spin qubits in a silicon transistor, etc. The phase space of N qubits is then much larger than the phase space of N classical bits, as it is a vector in the tensor product of N copies of such two-dimensional spaces. The programming is obtained by switching gates between the qubits and can be used for some extremely specific operations. The outcome will be the result of a measure, and it is probabilistic.

There are huge private investments to develop quantum computers. There are huge private investments to develop quantum computers. Many platforms are bulky, require cryogenic temperatures and have high energy-demands, making it hard to bring them out-of-the-lab and into the real world. Quantum computers are particularly susceptible to errors in the gate operations between their qubits, practically requiring a significantly larger number of qubits to perform a reliable calculation than it is currently possible. Quantum states are notoriously fragile, and lose their coherence through interaction with the environment, a process known as decoherence, which is fundamentally different from classical noise. So, it’s a long way to efficient quantum computing…

Quantum communication

The exchange of quantum information between distant parties is a new way for us to communicate across distances. Technology enables the coherent transfer of information around the globe via satellite or via fiber-optic cable, and quantum theory promises that transmitted messages will be secure from prying eyes. The same technology allows the synchronisation of atomic time-keeping across distances, with applications in navigation and high-frequency online tasks.

To conclude, we must remember that Europe was at the forefront of research at the beginning of the 20^th century. Nowadays there is a fierce competition internationally for all the above topics. We have to maintain the effort at the level of the European community both on the theoretical and experimental levels, keeping in mind the innumerable expected technical and industrial applications of any progress made.

© European Physical Society, EDP Sciences, 2025