Quantum physics describes the behavior of matter and radiation at the scale of atoms and subatomic particles. At this scale, the laws of classical mechanics no longer apply: particles obey counterintuitive rules, formalized in the early twentieth century by Planck, Einstein, Heisenberg, Schrödinger, and Dirac. These foundations remain the bedrock of contemporary research, including in the field of quantum computing.
Why classical mechanics fails at the atomic scale
Newtonian mechanics accurately predicts the trajectory of a ball or the orbit of a planet. Apply the same equations to an electron, and the predictions collapse.
The fundamental problem lies in the very nature of observation. At our scale, measuring an object does not significantly alter it. At the nanometer scale (the size of an atom), any measurement disturbs the observed system. The energy of the photon used to “see” an electron is enough to change its trajectory.
This experimental observation has led physicists to abandon the classical framework and construct an entirely new mathematical formalism. To learn everything about quantum physics, one must first accept that this theory does not describe what a particle “does,” but what can be predicted from measurement results on that particle.
This distinction is not philosophical: it has direct practical consequences on how current quantum experiments and technologies are conceived.
Superposition and the uncertainty principle: two pillars of quantum physics
Two concepts structure the entire theory. Understanding them is enough to grasp the logic of most quantum phenomena.
Quantum superposition
A particle can exist in multiple states simultaneously as long as no measurement is made. This is not a metaphor: the mathematical formalism (Schrödinger’s wave function) literally describes a combination of several possible states.
When measured, the particle “chooses” one state among the possibilities. This transition from superposition to a unique state is what physicists call wave function collapse. The available data do not allow for a definitive conclusion about the exact mechanism of this collapse, which remains one of the open debates in fundamental physics.
Heisenberg’s uncertainty principle
It is impossible to know both the position and the velocity of a particle simultaneously with arbitrary precision. The more precisely the position is known, the more indeterminate the velocity becomes, and vice versa.
This limit is not a flaw in our instruments. It is embedded in the mathematical structure of the theory. The uncertainty principle sets a fundamental bound on what can be known about a quantum system, regardless of the technology used to observe it.
Wave-particle duality and quantum entanglement
Both wave and particle
Light behaves both as a wave (interference, diffraction) and as a stream of particles (the photoelectric effect, explained by Einstein). Electrons exhibit the same dual behavior.
Wave-particle duality applies to all matter, not just light. Interference experiments have been conducted with increasingly heavy molecules, confirming that this property is not reserved for subatomic objects, even though it becomes imperceptible at larger scales.
Quantum entanglement
Two entangled particles share a common quantum state. Measuring one instantly fixes the state of the other, regardless of the distance separating them. Einstein referred to this phenomenon as “spooky action at a distance.”
Experiments conducted since the 1980s (notably the work of Alain Aspect, Nobel Prize in Physics 2022) have confirmed that this correlation is indeed real and cannot be explained by classical “hidden variables.” Entanglement is now used as a resource in quantum communication protocols and in certain architectures of quantum computers.
Tunneling effect and decoherence: the limits of the quantum world
The tunneling effect
A particle can cross an energy barrier that it should not be able to surpass according to classical laws. This phenomenon is a direct consequence of the wave nature of matter: the wave function does not abruptly drop to zero at the barrier, it gradually decreases. If the barrier is thin enough, a measurable fraction of particles can cross it.
The tunneling effect is not a laboratory curiosity. It plays a role in the functioning of electronic components, in certain chemical reactions, and in nuclear processes at the heart of stars.
Quantum decoherence
Quantum properties (superposition, entanglement) are extremely fragile. As soon as a quantum system interacts with its environment, it gradually loses its quantum behavior and behaves classically. This is decoherence.
Decoherence explains why we do not observe superposition at our scale. A cat cannot be both dead and alive because the billions of particles that compose it are constantly interacting with air, light, and ambient temperature.
This phenomenon also constitutes the main technical obstacle in quantum computing. Recent research focuses on quantum error correction: logical qubits, protected by several physical qubits, help reduce the error rate associated with decoherence. Google announced significant advances with its Willow processor in December 2024, in “sub-threshold” operation, where adding qubits effectively reduces errors instead of amplifying them.
- Superconducting architectures remain the most publicized, but neutral atoms and trapped ions are also being explored as qubit carriers.
- Next-generation error-correcting codes (like qLDPC) aim to reduce the number of physical qubits needed to protect a logical qubit.
- Industrial roadmaps published by major players (IBM, Quantinuum, IonQ) remain just that—roadmaps, not capabilities available today.
Quantum physics provides a theoretical framework validated by over a century of experiments, but several fundamental questions remain open: the exact nature of measurement, the interpretation of the wave function, the boundary between the quantum world and the classical world. Error correction has become the main indicator of maturity in quantum computing, surpassing the simple number of qubits. The theoretical foundations described here, from superposition to decoherence, are precisely those on which these technological advances rest.
