  Quantum entanglement between qudits - the d-dimensional version of qubits -
is relevant for advanced quantum information processing and provides deeper
insights in the nature of quantum correlations. Encoding qudits in the
frequency modes of photon pairs produced by continuous parametric
down-conversion enables access to high-dimensional states. By shaping the
energy spectrum of entangled photons, we demonstrate the creation,
characterization and manipulation of entangled qudits with dimension up to 4.
Their respective density matrices are reconstructed by quantum state
tomography. For qubits and qutrits we additionally measured the dependency of a
d-dimensional Bell parameter for various degrees of entanglement. Our
experiment demonstrates the ability to investigate the physics of
high-dimensional frequency entangled qu$d$it states which are of great
importance for quantum information science.
