  We describe a quantum key distribution protocol based on pairs of entangled
qubits that generates a secure key between two partners in an environment of
unknown and slowly varying reference frame. A direction of particle delivery is
required, but the phases between the computational basis states need not be
known or fixed. The protocol can simplify the operation of existing setups and
has immediate applications to emerging scenarios such as earth-to-satellite
links and the use of integrated photonic waveguides. We compute the asymptotic
secret key rate for a two-qubit source, which coincides with the rate of the
six-state protocol for white noise. We give the generalization of the protocol
to higher-dimensional systems and detail a scheme for physical implementation
in the three dimensional qutrit case.
