Wave-Particle Duality
Wave-particle duality is the concept that every elementary particle or quantic entity may be partly described in terms not only of particles, but also of waves. It expresses the inability of the classical concepts "particle" or "wave" to fully describe the behavior of quantum-scale objects.
Einstein's explanation of the photoelectric effect demonstrated the particle nature of light, while diffraction experiments proved its wave nature. De Broglie extended this to all matter, positing that electrons and other particles also possess wave-like properties.
Superposition
Quantum superposition is a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two (or more) quantum states can be added together ("superposed") and the result will be another valid quantum state.
Conversely, every quantum state can be represented as a sum of two or more other distinct states. Mathematically, it refers to a property of solutions to the Schrödinger equation; since the Schrödinger equation is linear, any linear combination of solutions will also be a solution.
The most famous thought experiment illustrating this is Schrödinger's Cat, where a cat in a sealed box is simultaneously alive and dead until the box is opened and the state is observed.
Quantum Tunneling
Quantum tunneling refers to the quantum mechanical phenomenon where a wavefunction can propagate through a potential barrier. This behavior has no counterpart in classical physics, where a particle cannot pass through a barrier if its kinetic energy is lower than the barrier's potential energy.
Because of wave-particle duality, an electron's position is defined by a probability wave. Even if a barrier is present, this wave function does not abruptly drop to zero, meaning there is a non-zero probability that the electron can be found on the other side of the barrier.
Tunneling is vital to many modern technologies, including scanning tunneling microscopes and flash memory drives, and is the reason nuclear fusion occurs in the sun.