How far can Physics help me in Computer Science, and in which fields?
In all fields and as far as you want it to.
At the least direct, the practical problem solving experience of physics training is invaluable.
More directly, the mathematics-as-a-tool/language POV will help everywhere too.
Most directly - anything involving the interrelation of data and the real world.
It's wide open because physics is so general.
It can help you in the very tangible ways Simon listed, but outside of core math/problem solving skills, it won't offer much direct help in most industries. Unless you get into work with modeling and simulation or other similar research and need to have a grasp on how the universe works, physics is sort of outside the realm of day-to-day activities.
I speak as a software developer who got a minor in physics for fun. If you love physics, definitely do it. It's worth it even if you don't directly use it every day. I work in the banking/finance industry, and, not surprisingly, quantum physics doesn't come up too often.
What about quantum-computing?
Quantum computing is way more commonly done in physics departments rather than comp sci departments and it is not fully developed so any of the comp sci you need to learn can be learned in the physics department.
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