Quantum entanglement and supersymetry conceptual illustration. Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles are generated or interact in ways such that the quantum state of each particle cannot be described independently. Supersymmetry (SUSY) is a particle physics theory that proposes a type of spacetime symmetry relating two basic classes of elementary particles: bosons, which have an integer-valued 'spin', and fermions, which have a half-integer spin. If supersymmetry is a true symmetry of nature, it would explain many mysterious features of particle physics and would help solve paradoxes such as the cosmological constant problem. There is only indirect evidence for the existence of supersymmetry, primarily in the form of evidence for gauge coupling unification.