Which of the following statements are correct

The QKD can be in principle implemented using a variety of physical systems and not only photons.

The Schmidt number (rank) of an entangled state is greater than one, which is equivalent to the reduced density matrix of one of the subsystems of that state being a projection operator.

Security proofs for QKD systems are based on the assumption that the devices used in the system are perfect.

The security of a QKD system can be increased by using a larger key size.