  Quantum key distribution (QKD) allows two spatially separated parties to
securely generate a cryptographic key. The first QKD protocol, published by C.
H. Bennett and G. Brassard in 1984 (BB84), describes how this is achieved by
transmitting individual qubits and exchanging classical authenticated
information. Any attempt to eavesdrop on the protocol introduces errors
detectable by the legitimate parties.
  This Licentiate Thesis studies the recently introduced EEQKD protocol which
builds on BB84. In EEQKD, the qubits sent individually in BB84 are entangled
and thus not directly available to an eavesdropper who is, in this protocol,
provided only one-by-one access to the transmission. The maximal information an
eavesdropper can gain using a straightforward intercept-resend (IR) attack, is
obtained for a given error rate. The secure key generation rate of EEQKD is
estimated in practical scenarios including qubit loss and quantum channel
noise. In addition, an exquisite vulnerability is exposed: For a particular
setting of qubit entanglement, paradoxically proving most useful in the face of
an IR attack, EEQKD reduces to BB84.
  Part of this research has been published in O. Ahonen et al., Physical Review
A 78, 032314 (2008).
