Quantum physics describes the behavior of matter and light at the scale of atoms and particles. At this scale, energy does not flow continuously: it is exchanged in discrete packets called quanta. This property, unlike anything in our everyday experience, underpins a theory that has transformed our understanding of the microscopic world since the early 20th century.
Energy quantization: why quantum physics exists
In the visible world, energy seems to vary without restriction. A ball gradually speeds up, and a lamp turns on in seemingly imperceptible increments. At the scale of an atom, reality is different: an electron can occupy only certain precise energy levels, never the values in between.
This idea arose from a concrete problem. Classical physics could not explain the radiation emitted by a heated object (the famous blackbody radiation). Max Planck proposed that energy is emitted in fixed amounts, proportional to the frequency of light. This postulate paved the way for a complete overhaul of the laws of physics at the smallest scales.
To understand quantum physics, we must accept that familiar rules stop working below a certain scale. Electrons, photons, and other particles obey their own laws, in which the notion of a defined trajectory disappears.
Wave-particle duality: a behavior with no visible equivalent
Light behaves sometimes like a wave and sometimes like a stream of particles (photons). The photoelectric effect, in which light knocks electrons out of a metal, can only be explained by treating light as a collection of particles carrying quantized energy.
This wave-particle duality applies not only to light. Electrons, and even heavier molecules, produce interference patterns when sent through a double slit, just as a wave would. A particle has no single position before it is measured: it is described by a mathematical function (the wave function) that gives the probability of finding it in one place or another.
This idea is puzzling because it has no perfect analogy in everyday life. A tennis ball passes through one slit or the other. An electron, in a sense, passes through both, and it is the act of observation itself that determines the result.
Quantum superposition and entanglement: two related concepts
Superposition follows directly from the wave function. Until a measurement is made, a particle can exist in several states simultaneously. This is not a metaphor: experiments confirm that measurement results are random, distributed according to calculable probabilities, and not predetermined.
Quantum entanglement takes this logic further. Two particles can be prepared in a shared state, so that measuring one instantly determines the state of the other, no matter how far apart they are. Albert Einstein called this phenomenon “spooky action at a distance,” but repeated experiments since the 1980s have confirmed that it is real.
A few clarifications to avoid common misunderstandings:
- Entanglement cannot be used to transmit a message faster than light, because the result of each measurement remains random when considered on its own.
- Superposition collapses when a measurement is made: a single, definite state is then obtained, a phenomenon sometimes called wave function collapse.
- Schrödinger’s cat is a thought experiment designed to show the apparent absurdity of applying superposition to a macroscopic object, not a literal description of reality.
Practical applications of quantum physics in today’s technologies
Quantum mechanics is not confined to research laboratories. Semiconductors, lasers, and LEDs rely on quantum effects. Without an understanding of electron energy levels in solids, none of these components would exist.
The tunnel effect, in which a particle passes through an energy barrier that classical physics says it should not be able to cross, is used in flash memory and in some very high-resolution microscopes. It is a purely quantum phenomenon with no macroscopic equivalent.
In emerging technologies, quantum computers use qubits that exploit superposition to process certain tasks differently from classical processors. In February 2025, a Google team published a result in Nature showing that adding error-correction codes could reduce errors in a logical qubit as the code grew. This is a step toward more reliable computations, but a large-scale, universal quantum computer remains a work in progress.
In Europe, the EuroQCI initiative plans a quantum communication network combining terrestrial fiber links with a space segment. The European Space Agency is working on satellite-based quantum key distribution to secure data exchanges over long distances. Quantum computers are also subject to export controls in the European Union, a sign that these machines are now considered strategic assets.
Probability in quantum physics: accepting fundamental randomness
Classical physics is deterministic: if all the initial conditions are known, the result can be predicted. Quantum mechanics breaks with this principle. Randomness is not a gap in our knowledge but a property of the quantum world.
The wave function provides probabilities, not certainties. We can calculate the probability that an electron will be detected at a particular location, but we can never guarantee the outcome of an individual measurement. This is what distinguishes quantum theory from all earlier physical theories.
This does not make quantum physics imprecise. Its statistical predictions are among the most thoroughly verified in all of science. The theory works with remarkable accuracy, even though it requires us to give up the idea of a universe that is entirely predictable at the particle scale.
Quantum physics describes a world in which energy is quantized, particles have no defined trajectory before they are observed, and randomness is a law of nature. These principles already power everyday technologies and are shaping the next wave of advances, from quantum computing to satellite-secured communication networks.
