This is a good idea – a electronic catflap that is controlled by the microchip most cats already have these days
The Microchip Catflap No Collar Needed Keeps out unwanted cats by using your cat’s existing microchip number without the need for a collar. Automatically detects light levels and can keep your cat in at night. Designed by a vet. [From Pet Porte Microchip Cat Flap ]
This may be what we need for our cats, as my wife likes to keep them in when it’s dark as well!