Quantum physics describes the behavior of matter and energy at the scale of atoms and subatomic particles. At this scale, the laws of Newtonian mechanics cease to function: a particle can behave like a wave, two objects separated by a great distance can remain correlated, and a physical system can exist in multiple states at once as long as no measurement is made.
Why classical mechanics fails at the atomic scale
Classical physics relies on continuous quantities: position, velocity, energy vary smoothly. In the early twentieth century, experiments showed that the energy emitted by a hot body did not follow this logic. Planck proposed that energy is exchanged in discrete packets, the quanta of energy.
Einstein extended this idea by showing that light itself behaves like a stream of particles (photons) in the photoelectric effect. This result contradicted the classical wave model of light, which had been validated for decades.
To delve deeper into the definition of quantum physics, it is essential to keep this starting point in mind: physical quantities are not continuous at a small scale; they are quantized. All the formalism developed later by Heisenberg, Schrödinger, and Dirac stems from this break.
Wave-particle duality and quantum superposition: two foundational principles
Wave-particle duality
An electron sent towards two slits produces an interference pattern on a screen, just like a wave would. But if a detector identifies which slit it passes through, the pattern disappears and the electron behaves like a particle. Observation alters the outcome of the experiment.
This phenomenon is not a metaphor. It has been reproduced thousands of times in the laboratory, including with molecules composed of dozens of atoms. Wave-particle duality is an experimental fact, not a philosophical interpretation.
Superposition of states
A quantum system can exist simultaneously in multiple states. An atom can be both excited and unexcited, a photon can be polarized vertically and horizontally. Measurement forces the system to “choose” one state among the possibilities.
This principle was pushed to the absurd by Schrödinger with his thought experiment of the cat, both alive and dead. The image became famous, but it aimed to highlight the strangeness of the formalism, not to validate it.
Quantum entanglement: correlation without signal
Two particles can be prepared in an entangled 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” because it seemed to violate relativity.
Entanglement does not allow information to be transmitted faster than light. The measurement results remain random when taken in isolation. It is only by comparing the measurements from both sides that one observes the correlation, and this comparison requires a classical channel.
Alain Aspect, Nobel Prize in Physics 2022, conducted experiments that confirmed the reality of entanglement and ruled out explanations based on “local hidden variables.” His work laid the groundwork for what is called the second quantum revolution, focused on the technological exploitation of these phenomena.
Concrete applications of quantum technologies
The principles of superposition and entanglement are no longer confined to research laboratories. Several technological fields are adopting them, with varying degrees of maturity.
- Quantum sensors: devices exploiting the extreme sensitivity of quantum states to external disturbances are being developed to operate outside the laboratory. Sandia National Laboratories is working on sensors that can be used when GPS is unavailable, although the solution has not yet been integrated on a chip or demonstrated as a complete navigation system.
- Quantum computing: quantum computers use qubits in superposition to simultaneously explore a large number of solutions. Reducing calculation errors remains the main challenge. In June 2025, a team from the University of Oxford reported an operation on an ionic qubit with particularly low error, but two-qubit gates remain more difficult to master.
- Quantum communications: quantum key distribution (QKD) exploits entanglement to detect any attempt at interception. NATO now treats quantum technologies as an operational preparedness issue, and its roadmap includes experimentation with these systems.
The gap between announcements and operational reality remains significant. The precision of an isolated experiment is not enough to demonstrate a fault-tolerant quantum computer. Each advancement solves one technical problem and reveals another.
Post-quantum cryptography: protecting data before the threat
A sufficiently powerful quantum computer could break the asymmetric encryption algorithms used to secure online exchanges. This prospect, even if distant, is prompting security agencies to prepare for the transition to resistant algorithms.
The risk is not only about the future. Data encrypted today could be stored by an attacker and then decrypted later, a strategy known as “harvest now, decrypt later.” The migration to post-quantum cryptography began before the threat became operational, precisely for this reason.
Quantum physics has transformed the understanding of the microscopic world in less than a century. Its technological applications progress in stages, each limited by concrete engineering obstacles. Sensors, secure communications, and quantum computing advance on distinct timelines, with maturity levels that media announcements tend to smooth out.
