  We propose orthogonal frequency division multiplexing (OFDM), as a spectrally
efficient multiplexing technique, for quantum key distribution (QKD) at the
core of trustednode quantum networks. Two main schemes are proposed and
analyzed in detail, considering system imperfections, specifically, time
misalignment issues. It turns out that while multiple service providers can
share the network infrastructure using the proposed multiplexing techniques, no
gain in the total secret key generation rate is obtained if one uses
conventional all-optical passive OFDM decoders. To achieve a linear increase in
the key rate with the number of channels, an alternative active setup for OFDM
decoding is proposed, which employs an optical switch in addition to
conventional passive circuits. We show that by using our proposed decoder the
bandwidth utilization is considerably improved as compared to conventional
wavelength division multiplexing techniques.
