Do a Google search on
 "electronic dog door"

A really smart cat might learn to defeat the collar key lock by going
out with/under the dog, but those things are your best bet unless the
cat can be trained to stay in.

What worked for some of my cats is obviously not going to work for
you: the presence of the (cat chasing) dogs in the back yard kept some
of the cats from attempting to go out.  That was *not* desirable, and
I had to give the dogs away to get the cats to settle down and get
back to keeping the property free of mice - *before* they came inside.

--
T.E.D. (tdavis@gearbox.maem.umr.edu)

I'm sure I wouldn't be happy if this was my cat doing this, but the
vision of some cat stalking the neighborhood through the doggie doors
is pretty hilarious...I've been chuckling for five minutes at the
image.