  We present a new fiber based quantum key distribution (QKD) scheme which can
be regarded as a modification of an idea proposed by Inoue, Waks and Yamamoto
(IWY) [1]. The scheme described here uses a single phase modulator and two
differential delay elements in series at the transmitter that form an
interferometer when combined with a third differential delay element at the
receiver. The protocol is characterized by a high efficiency, reduced exposure
to an attack by an eavesdropper, and higher sensitivity to such an attack when
compared to other QKD schemes. For example, the efficiency with which
transmitted data contribute to the private key is 3/4 compared with 1/4 for
BB84 [2]. Moreover, an eavesdropper can aquire a maximum of 1/3 of the key
which leads to an error probability in the private key of 1/3. This can be
compared to 1/2 and 1/4 for these same parameters in both BB84 and IWY. The
combination of these considerations should lead to increased range and key
distribution rate over present fiber-based QKD schemes.
