As someone that has spent lots of time interviewing for many tech companies lately, I would highly recommend not disclosing that you're interviewing for anything. It's typically viewed as unprofessional. When/if you are offered a job, then go through the steps of resignation or promotion. This will limit any drama and this is the correct way to advance your career.
In an interview, it's alright to indicate that you are actively interviewing and that they are not the only interview you have had. However, it is considered unprofessional for them to ask who you are interviewing with or vice versa. I walked out of an interview in which the interviewer wanted to know exactly who else I had interviewed with...