  Employing the fundamental laws of quantum physics, Quantum Key Distribution
(QKD) promises the unconditionally secure distribution of cryptographic keys.
However, in practical realisations, a QKD protocol is only secure, when the
quantum bit error rate introduced by an eavesdropper unavoidably exceeds the
system error rate. This condition guarantees that an eavesdropper cannot
disguise his presence by simply replacing the original transmission line with a
less faulty one. Unfortunately, this condition also limits the possible
distance between the communicating parties, Alice and Bob, to a few hundred
kilometers. To overcome this problem, we design a QKD protocol which allows
Alice and Bob to distinguish system errors from eavesdropping errors. If they
are able to identify the origin of their errors, they can detect eavesdropping
even when the system error rate exceeds the eavesdropping error rate. To
achieve this, the proposed protocol employs an alternative encoding of
information in two-dimensional photon states. Errors manifest themselves as
quantum bit and as index transmission errors with a distinct correlation
between them in case of intercept-resend eavesdropping. As a result, Alice and
Bob can tolerate lower eavesdropping and higher system error rates without
compromising their privacy.
