How do I tell my advisor that I lost interest in the subject?
Assuming your advisor is a great person and someone who listens, could you simply say: 


  I'd like to work in a new direction for my PhD.


Alternatively, if you feel it necessary to give some reason:


  I'd like to work in a new direction to differentiate my masters work from my PhD thesis.


Preferably in either case you have at least some idea of where you want to proceed so you can continue the discussion in that direction. A desire for some breadth is a fairly normal and healthy part of the academic experience.

If your advisor questions the change of direction, you can simply say that after working on topic ______ for ____ years you are more interested in pursuing another direction.

It's certainly possible your advisor will be somewhat upset if he expected you to work in your previous subject (and especially if he took you on as a student because of that expectation), but it won't serve either of you to be coy about it or to plod through, not enjoying what you are doing, until you decide to quit. Ideally, however, this should have been a discussion you had before you agreed to work on a PhD with this advisor.

Keep in mind that your new direction should still be a fit with your advisor's work; if your previous project was in ideal spheres and your new work is going to be in falling apples, your advisor may not be qualified to advise you. It sounds like in your case that is not an issue, but if you are receiving any grant funding through your advisor, those funds may be particular to the project you are undertaking - hopefully you have some awareness of whether this applies to your situation.

Note that in my suggestions I was careful to phrase things positively and prospectively: you don't need to say you are sick of your previous topic, only that you want to move into something else. By approaching it this way, you come across as forward-thinking and not at all negative about physics in general, and not negative of your past experience as a masters student.