There’s lots of different door locks out there that use manual keypads, RFID card, biometric device, WiFi, Bluetooth, etc. Mirko Pavleski shows off an arduino powered door lock that works by correctly timing a light pulse for an exact number of milliseconds.

I wouldn’t call it the most secure kind of lock since it only requires a single timed light pulse. A more secure solution would be a pattern of on/off lights. While clever, it’s likely vulnerable to ‘shoulder surfing’ by others watching from a distance with a detector (even if it was UV). This would be true at night when the light would be quite visible. But it’s a fun idea.