A quantum system is an object or an ensemble of objects which follows the laws of quantum mechanics, a fundamental theory which explains natural phenomena. More precisely, at small scales (atomic or subatomic), or low temperatures, classical physics fails to describe physical effects, such as matter cohesion or the propagation of electrons in crystals. While classical physics, which is suited at large scales, can be recovered from quantum mechanics under appropriate conditions, quantum mechanics is fundamental in the sense that the properties of macroscopic systems (density, elasticity or specific heat for example) can only be explained from quantum mechanics.
Key features of quantum systems
The following features are typical of quantum systems, in the sense that they (usually) become irrelevant in macroscopic systems or high temperature systems:
- Wave-particle duality: until the end of the 19th century, electricity and light were understood as waves. With the development of quantum mechanics, we understood that light and electricity were in fact carried by particles (photons and electrons). These two pictures, however, do not contradict each other: light and electricity are carried by particles, which “behave like waves”, in the sense that they can interfere in the same way that waves do. To formally describe this feature, we associate to each particle a wave-function. The squared amplitude of that wave-function gives the probability to observe the particle at a given moment and point in space. The wave-function evolves according to the Schrodinger equation.
- Superposition: Quantum systems can be in a superposition of states. For example, an atom can be in a state such that there is a 50% chance of being in the ground state and 50% chance of being in an excited state.
- Probabilistic outcomes of measurements: this is a consequence of the previous statement. Since superposition is possible, it implies that several measurements of identical quantum systems can yield different results. Using the example above, measuring an atom which is a superposition of ground state and excited state yields the result “ground state” half of the time and “excited state” the rest of the time.
- Entanglement: Quantum systems can be correlated in a way that cannot be explained by classical arguments. This feature is central to quantum information and quantum technologies.
- Quantized energy levels: quantum systems have discrete (quantized) energy levels, as opposed to macroscopic systems whose energy can vary continuously. For example, an atom can either be in its ground state or in an excited state with well-defined energies, but other energies are inaccessible. A large number of atoms (e.g. a battery) can instead be powered up continuously.
Frequently asked questions about quantum systems
- Can quantum phenomena be observed directly? Usually not, since they typically occur at atomic or subatomic scales and/or at low temperature (close to the absolute zero, i.e. -273 degrees Celsius). However, phenomena such as levitation of a magnet above a superconductor can be observed with bare eyes using a high temperature superconductor in liquid nitrogen.
- Are quantum systems useful in every day life? Quantum systems are used in modern technologies, such as lasers, MRI machines, to improve GPS precision, or in emerging technologies such as quantum computers.