I'm in my last year of my BSc, and I have a couple of things that I am really interested in, and I am hoping to do a MSc and then a PhD. My adviser explained that a PhD is very focused, and it's likely that my future academic career will be mainly in that general topic. The thing is I like two very different topics. One is high level (machine learning), and the other is low level/theoretical CS (systems, parallelism, quantum) or at least that's kind of how these fields are grouped together at my university. I wouldn't mind doing a PhD in either of these fields, but I would like to have opportunities to work with the other field at some point in the future. And the way I see it, these are very different spheres of CS.
So my question is how broad or narrow is your research area after your PhD. Do you get opportunities to work in different areas? I know there isn't a physical barrier stopping me from exploring different fields, but how difficult is it to get such positions when your PhD is in a different topic? How far can you deviate from your initial research?
At present I have better research experience with low level computing, as my dissertation is on parallelism and quantum programming. And I expect it would be easier for me to get admitted to a PhD with a proposal in this field, since I can demonstrate experience. But I like to think that at some point after the PhD, I can also do ML research (like in a postdoc or as an RA).
For context, I intend to continue my education in the UK.