The notable physicist known for his groundbreaking work in quantum mechanics is Max Planck. He was pivotal in introducing the concept of quantum theory, which revolutionized physics in the early 20th century. Planck's famous equation, known as Planck's constant, introduced the idea that energy exists in discrete units called quanta. His work laid the foundation for future developments in physics, influencing scientists like Albert Einstein and Niels Bohr. Planck's contributions earned him the Nobel Prize in Physics in 1918.