Physics

• Physicists, of course, apply quantum mechanics to understand how matter behaves at the microscopic level. Physicists may use quantum mechanics to think about how light and matter interact, for example, or explain the properties of superconductors. Particle physicists working with colliders like the Large Hadron Collider (LHC) make use of quantum mechanics all the time, since the behavior of the subatomic particles involved in these collisions is governed by quantum mechanics. Many inventions we consider fundamental to modern life -- the laser and the transistor, to name just a couple -- were made possible by quantum physics.
  
  

Chemistry

• The periodic table exhibits many trends that help to predict the properties of elements; only with the aid of quantum mechanics, however, could chemists understand why the periodic table predicted those trends and explain them. Chemists think about bonding between atoms and molecular structure in terms of quantum mechanics and use it for calculations. You may have seen pictures of electric potential or elpot diagrams that show how electron density is distributed over a molecule; the calculations that produce these images are done using molecular orbital theory, which is an extension of quantum mechanics applied to polyatomic ions and molecules.
  
  

Biology

• Human beings and all other organisms are systems of biochemical reactions and interactions. Understanding how these systems function often entails seeking answers at the molecular level -- figuring out which enzymes catalyze which reactions, for example, or how specific biochemical pathways operate. Consequently, biologists also need to understand quantum mechanics as it applies to chemistry. Often, quantum physics also furnishes biology with useful tools. The electron microscope, for instance, was made possible by the realization that electrons behave not only like particles but also like waves, a fundamental insight from quantum mechanics.
  
  

Cosmology

• Cosmology is the branch of science that investigates the beginnings of the universe or tries to understand the universe as a whole. It might seem counterintuitive that quantum mechanics would play any role here, because quantum mechanics is typically used to describe how matter behaves at the microscopic level, while relativity and classical physics describe how matter behaves at the macro level. Nonetheless, some scientists have attempted to apply quantum mechanics to understand more about the early instants of the universe and its formation, although these theories are still very much a work in progress. This approach is called quantum cosmology.