Support l'X The second quantum revolution in Ecole Polytechnique's laboratories
L'intérieur d'un cryostat permettant d'étudier les circuits quantiques hybrides à l'École polytechnique
The year 2025 marks the centenary of the framing of quantum mechanics (or quantum . Cphysics) as one of the fundamental theories of physics.
From its very beginnings, quantum mechanics has provided a satisfactory description of the corpuscular and wave-like behaviour of elementary particles of matter and light (“photons”).
In addition to new equations to represent the behavior of systems, quantum physics introduced revolutionary notions compared to classical physics.First of all, quantum superposition.When we observe the state of a classical system, such as the position of a telephone, we find a very specific result: the telephone is in one place and not in another.“But the situation changes radically in quantum physics,” explains Laurent Sanchez-Palencia, CNRS Research Director at the CPHT.
Indeed, contrary to what we're used to in classical physics, the state of a system must be dissociated from what can be measured of it: an object can be in a superposition of two (or more) states, as if it were in two different places, for example. But if we measure its position, we find only one of these possibilities, perfectly randomly. What's most remarkable is that we know this isn't a misunderstanding of the “true position”; this randomness is a fundamental property of quantum physics.”
From the first to the second quantum revolution
Few people know it, but this new physics has not only led to advances in fundamental knowledge, but also to applications that we use frequently. All electronic systems (smartphones, computers, etc.) are based on semiconductor materials in which electrons exhibit quantum behavior. Other examples include lasers, MRI medical imaging and photovoltaic solar cells (which work by the photoelectric effect, the interpretation of which earned Albert Einstein the Nobel Prize). This was the first quantum revolution.
Another quantum concept plays a crucial role in current research: entanglement, whose mysteries researchers are still working to unravel. Experiments by Alain Aspect, professor at École Polytechnique and winner of the 2022 Nobel Prize in Physics, have contributed to the experimental confirmation of this counter-intuitive phenomenon. Imagine having two entangled quantum “balls”, each of which can be red or blue. In this imaginary example, entanglement allows the colors of the two balls to be necessarily distinct, even though the color of each is perfectly indeterminate. If you observe a ball, it may be red or blue at random (a superposed state, as described above). Entanglement implies that if one appears blue when observed, then the other no longer has a choice: it necessarily appears red and vice versa. With entanglement, the determination of the color of the second ball due to the observation of the first is instantaneous, regardless of the distance separating them. “These distance correlations are highly non-classical, and can be used to perform new tasks, forming the basis of what is now being called the second quantum revolution,” emphasizes Laurent Sanchez-Palencia.
Over the last few decades, researchers have learned to control individual quantum systems (atoms, photons, superconducting circuits, etc.) more and more effectively, by maintaining for longer the properties of superposition and entanglement, which tend to disappear very quickly, and all the more rapidly the greater the number of objects involved. This effect is known as quantum decoherence, which determines the transition from quantum to classical behavior. These advances have led to the development of research in several directions:
- Communications: quantum properties can guarantee the security of communications, by detecting spying attempts, and could pave the way for a “quantum internet”
- Metrology: i.e. high-precision measurements. The sensitivity of quantum superpositions and entanglement to perturbations is used to make high-performance sensors.
- Quantum computation: using the quantum properties of systems to create algorithms with no classical equivalent. The challenge is twofold: to find the right physical systems to build a high-performance quantum computer, and to discover algorithms that efficiently solve useful problems.
- Quantum simulation: using highly controlled quantum objects to recreate the behavior of complex materials or systems that are difficult or impossible to simulate classically. Already a powerful tool in fundamental research, this approach could pave the way to solving many industrial optimization problems.
- Quantum materials: this involves studying and learning to control the properties of certain materials whose understanding still eludes us. This is the case, for example, with so-called “high-temperature superconductors”, capable of conducting electricity without loss below a certain temperature, or hybrid superconducting circuits.
The research ecosystem at École polytechnique
While the present and future prospects of quantum technologies are eminently promising, fundamental research is crucial if they are to be fully realized. With this in mind, École polytechnique is actively involved in the French quantum plan launched in early 2021. Several teams from École Polytechnique's laboratories are actively working on these subjects, at the interface between physics and computer science, at the Irradiated Solids Laboratory (LSI), the Laboratory of Condensed Matter for Physics (PMC), the Applied Optics Laboratory (LOA), the Center for Theoretical Physics and the École Polytechnique Computer Science Laboratory (LIX).
Together with Institut Polytechnique de Paris and Université Paris-Saclay, this research is part of Quantum Saclay, the center that federates quantum research, training and innovation activities on the Saclay plateau. The annual Quantum Saclay scientific day was held at École Polytechnique on January 20, 2025, bringing together around a hundred participants, including researchers, students and industrial players, to discuss the latest developments and future challenges.
Downstream from this fundamental research, a young generation of scientists and entrepreneurs is emerging from this ecosystem. A number of quantum companies have been co-founded or are co-directed by X graduates, including Quandela, Pasqal, Alice&Bob, C12 Quantum Electronics and Qobly.