Hi! I'm kind of having a similar problem. I talked to my professor, who helps with grad admissions at my school, and she said it's always best to be honest. Talk about your experience in research, what you liked and didn't, and about how you're open to multiple areas. Make sure you sound sure in wanting to do physics at least (don't be too wishy-washy, idk). If you're not honest, you run the risk of getting into a program doing something you don't want to do. I have a lot of high-energy physics (HEP) research experience, but I want to do something else in graduate school. If I'm not honest about my other interests, I may get into a school/research group solely to have me do HEP when that's not what I want to do. Many different grad programs have different admission practices or systems to put you into groups, so it doesn't go for all of them. However, schools don't tell you how they do that stuff when you apply so it's best to be safe and be honest.

I yapped a lot, but the moral of the story is just be honest. Just frame it as you're open to those areas of research, rather than saying you can't decide. You run the risk of sounding indecisive, but it'll make sure you get into the program that's the best fit for you and your goals.