Physical phenomena that are described by quantum mechanics, often involving discrete energy states.Quantum phenomena are physical effects that arise from quantum mechanics, the theory describing nature at atomic and subatomic scales. Unlike classical physics, where quantities like energy and momentum vary continuously, quantum systems often exhibit discrete values — for example, electrons in an atom occupy specific energy levels, not a continuous range. Other hallmark phenomena include superposition, where a particle exists in multiple states at once, and entanglement, where particles remain correlated across distances.
The foundations were laid in the early 20th century by Max Planck (1900), Albert Einstein (1905), and Niels Bohr (1913), with the full mathematical framework developed by Werner Heisenberg, Erwin Schrödinger, and others in the mid-1920s.
Quantum mechanics replaces deterministic trajectories with probability waves. A particle's state is described by a wavefunction, and observable quantities correspond to operators whose eigenvalues are the possible measurement outcomes. When a measurement is made, the wavefunction collapses to one of these discrete outcomes. The discreteness of energy levels arises from boundary conditions on the wavefunction, such as those confining an electron in an atom.
Quantum phenomena underpin modern electronics, lasers, MRI scanners, and are the basis for emerging quantum computers and quantum cryptography.
Quantum entanglement was called 'spooky action at a distance' by Einstein, yet it has been experimentally confirmed and is now used in secure communication.