Introduction To Quantum — Mechanics

The term "quantum" refers to a discrete unit. In the macroscopic world, energy seems continuous. However, in the quantum world, energy is exchanged in specific, individual "packets." An electron in an atom can only exist in certain energy levels, never in between them. The Uncertainty Principle

A particle can exist in multiple states at once until it is observed. This is famously illustrated by Schrödinger’s Cat. INTRODUCTION TO QUANTUM MECHANICS

Two particles can become linked so that the state of one instantly influences the state of the other, regardless of the distance between them. Einstein called this "spooky action at a distance." Why It Matters The term "quantum" refers to a discrete unit