What type of Grad School if I'm interested in Quantum Computing? I'm currently an undergraduate at a prestigious private university in the U.S. I'm majoring in Physics, Math, and Computer Science and currently am a junior. I've taken 5 graduate level courses so far (including Real Analysis and Algorithms) and have undergraduate experience working in a lab in a field unrelated to quantum computing (for which I'm in the process of writing a paper about). I've also taken a course in quantum computing, 2 semester of undergraduate quantum mechanics, 2 semesters of discrete math, and a course on complexity theory. I plan to take 2 semesters of grad quantum in my senior year as well as 2 semesters in mathematical logic. 

I'm interested in going to graduate school for theoretical work in quantum computing and quantum information theory. I'm wondering if I should be aiming to apply to Computer Science, Math (Applied I would guess), or Physics departments if I want to do research in those fields.
You should aim for whichever department has people working on the topic.

Quantum Computing is somewhat multidisciplinary, and not present in every university. So, before applying, check where are the quantum computing groups and follow them. Your background seems to be a good fit for either side.

By the way, I wouldn't be surprised if there was somewhere a department dedicated to quantum computing. Recently I saw a "department of glycoscience".