It is possible to define a mapping between the bits in a digital key and the shape of a physical key. Further, it is possible to physically etch a digital key onto a physical one below the resolution at which said data could be easily copied. Consequently, it is possible to design an intelligent lock which possesses a public key and which generates a cryptographic challenge, which is then decrypted using the private (physical) key. A key which could physically open the lock but which does not possess the appropriate private key could trigger an alarm.

Certainly digital keys exist, but this key would require no power source. The lock, on the other hand, would require both power and enough logic to implement the PKI approach of choice. Nevertheless, this strikes me as a practical approach for car or office locks.