  In search of a quantum key distribution scheme that could stand up for more
drastic eavesdropping attack, I discover a prepare-and-measure scheme using
$N$-dimensional quantum particles as information carriers where $N$ is a prime
power. Using the Shor-Preskill-type argument, I prove that this scheme is
unconditional secure against all attacks allowed by the laws of quantum
physics. Incidentally, for $N = 2^n > 2$, each information carrier can be
replaced by $n$ entangled qubits. And in this case, I discover an eavesdropping
attack on which no unentangled-qubit-based prepare-and-measure quantum key
distribution scheme known to date can generate a provably secure key. In
contrast, this entangled-qubit-based scheme produces a provably secure key
under the same eavesdropping attack whenever $N \geq 16$. This demonstrates the
advantage of using entangled particles as information carriers to combat
certain eavesdropping strategies.
