Allow me to link to you a page of comic:
http://abstrusegoose.com/307
As a physicist, in *today's* industry, you'll likely work on the lowest level of the computer, that is the bottom picture in the comic. You'll either be designing new materials/processes to build individual gates for use in the 2nd picture or you'll be optimizing manufacturing process to mass produce silicon wafers.
To work on the intermediate levels your best bet would be electrical/computer engineering. Towards the top will be computer science/software engineering. The topmost is, well, game design ;)
Now if you think of developing new microprocessors as improving instruction sets or architectures, then I don't think any physicist is able to do that without serious re-training. Computer engineering will be most suitable. If you think of it as developing new processes to make better individual elements or to increase the yield in the factories (called fabs) then physics is involved, and so are almost all physical science/engineering. I tell you this because only Intel and IBM routinely ask for graduates from our department, and they hire PhD people from all physical science/engineering as process engineers in their fabs. I also come from the country of the largest manufacturer, so I have lots of friends in fabs.
Personally, I hate the thought of working in the fab where the only thing you can *own* is a process/machine in manufacturing, which is what an Intel PhD hiring rep told me. I'd much rather *own* a piece of code being a game programmer. But that's my opinion.
I must emphasize that this is *today's* industry. There does not exist a quantum computer industry today, and it's very difficult to predict when you'll have one.