I plan to pursue a graduate school "career" in theoretical condensed matter physics in the US; however, getting into a grad program as a theory student is quite difficult due to limited spacing, little funding, and high competition. Should I instead apply as an experimentalist and then switch over to theory in grad school? Or should I not say "experimentalist' or "theorist" in my statement of purpose and instead focus on the topic(s) in condensed matter physics that interest me and include names of professors in theory and experiment? In the former case, is the action ethical? In the latter case, will it seem that I am not focused? Please advise, since I do not want to narrow my options.
Should I instead apply as an experimentalist and then switch over to theory in grad school? is the action ethical?
It's not unethical to do experimental work while hoping to switch to theory later. If you're hoping to join an experimental group but only work on theory, that may not be possible. Your advisor may not be prepared to advise you on theory and/or may be funding you from experimental projects which require you to do experimental work. There's no guarantee of being able to switch advisors; the lack of space and funding that you mentioned restricts current as well as prospective students.
Or should I not say "experimentalist' or "theorist" in my statement of purpose and instead focus on the topic(s) in condensed matter physics that interest me and include names of professors in theory and experiment? In the latter case, will it seem that I am not focused?
You might seem less focused than someone who explicitly states they want to work in theory or experiments, but that is not necessarily a bad thing. If you have strong preparation, less focus means you appear to be more flexible. Someone who is more focused (like exclusively on theory) might be rejected despite a strong preparation if funding is only available for experimental students. On the other hand, if you appear to be very focused on theory, readers might think you are more motivated by the topic and more knowledgeable about your interests, which might make you more attractive as a theory student.
This is the general problem of how specializing locks some doors and unlocks others. Without knowing the admissions criteria of the specific school, it would be hard to determine which has higher chances of admission.
At this stage it doesn't matter, so you can choose whatever you like (or whatever you think is more promising etc) and switch (or not switch) later.It is quite common for phd students in cond-mat to do both (some) theory and (some) experiments during their phd. I'd also not be worried about being "ethical" here because people do change their interests, and it is totally unreasonable to expect a student to choose a line of research for their entire life.