  We report an experimental demonstration of a complied version of Shor's
algorithm using four photonic qubits. We choose the simplest instance of this
algorithm, that is, factorization of N=15 in the case that the period $r=2$ and
exploit a simplified linear optical network to coherently implement the quantum
circuits of the modular exponential execution and semi-classical quantum
Fourier transformation. During this computation, genuine multiparticle
entanglement is observed which well supports its quantum nature. This
experiment represents a step toward full realization of Shor's algorithm and
scalable linear optics quantum computation.
