Well, the first point to make is that it would be unusual to find someone with a quantum scientist title or some academic degree in “quantum science.”  Since quantum science has such wide applicability, it’s used by lots of different fields of science, such as chemistry, physics, and many types of engineering.  People usually get academic degrees in these types of fields, but are learning some quantum science while doing so.  If a person gets a degree in chemistry and ends up using a lot of quantum mechanics in their work, they might be identified as a “quantum chemist.”

So, are there also “quantum physicists” and “quantum engineers”?

And are there different subfields of chemistry that use quantum mechanics besides quantum chemistry?

Absolutely.  Almost all chemists are concerned with the making and breaking of chemical bonds between atoms, and, among many other things, quantum mechanics underlies the rules of how bonds are made and broken.  Some chemists will want to spend a lot of time understanding the quantum nature of bonds as part of their work, while others may not feel the need to.  Someone who does biochemistry is less likely to spend time thinking about the things they work on in terms of quantum mechanics, while someone who does physical chemistry is more likely to.

Yes, people in other physical sciences, like earth science and materials science, can require a quantum understanding of some things.  As quantum mechanics can be viewed as a general theory of information, there’s increasing interest in it in computer science as well.  This interest is also related to the development of new technologies like quantum computers, which is also an area where you’re starting to see people who identify as quantum engineers.  There are also an increasing number of examples of people looking into whether quantum concepts are useful for biological systems.

Yes!  This is probably not that surprising since quantum mechanics is such a wide-ranging theory and is understood to be the ground rules for the physical world.  You would expect that a theory that powerful would be useful for many different types of science.  One useful thing about learning more quantum science is that it is knowledge that you can take with you if you go from one field to another.

Quantum mechanics is a very general set of rules governing the physical world that was developed starting in 1925.  The year 2025 was chosen as the International Year of Quantum Science and Technology because it marks 100 years of quantum mechanics.  We’ve talked elsewhere about what quantum means; the mechanics part refers to a systematic set of rules that can be widely applied to describe how things move and change.

Do “quantum mechanics” and “quantum theory” mean the same thing?

Then what changed to go from quantum theory to quantum mechanics?

In the 1900-1925 period, there was no consistency in how and when to apply these quantum hypotheses to explain experiments and make predictions. Sometimes they seemed to work spectacularly well, which gave many people confidence that there must be something to the idea.  But many other times, scientists tried to use these hypotheses to model or predict things, and the model didn’t make any sense, or the predictions were wrong.  The point is that there was no systematic way of applying quantum theory ideas to different physical systems.  A systematic method would be a “mechanics.”

The groundwork for it, yes.  The basic framework and some general sets of principles to follow took a few years to sort out in order to be able to apply them systematically to a wide range of problems.  People are even now still working to revise and extend this framework, but many of the core pieces of quantum mechanics were put in place in 1925.  The term “quantum mechanics” started to be widely used in the 1920s to describe these systematic rules.  It was also a phrase that distinguished this new mechanics from what’s now called “classical mechanics.”

Classical mechanics, or sometimes just “mechanics,” is the framework for describing the motion of massive objects that was initially developed in the 17^th century.  This framework is a set of general rules that can be used to describe how planets orbit the sun or the rate at which a dropped object falls to the ground.

Yes, exactly.  The rules of classical mechanics are still very useful and often easier to use than those of quantum mechanics, but quantum mechanics is an even broader theory that, in many scientists’ assessments, supersedes the rules of classical mechanics.  One way to put it is that by the end of the 19^th century, scientists thought they had a good, systematic theory for how matter moved around – that’s classical mechanics – and a good, systematic theory for how light worked – this is the electromagnetic wave description of light.  However, there were a number of puzzles in trying to understand how light and matter interacted with each other.  In the period from 1900-1925, some of these puzzles seemed to be solved using quantum ideas, but there was no systematic understanding of how light and matter interacted in all cases.

Not only did quantum mechanics provide a full description of how light and matter interact, but in doing so it dramatically revised our understanding of light and matter and the rules governing each of them.  The earlier “classical” rules governing matter and light were revealed to be only approximations of a richer, quantum description of matter, light, and their interactions.