  Interferometry using discrete energy levels in nuclear, atomic or molecular
systems is the foundation for a wide range of physical phenomena and enables
powerful techniques such as nuclear magnetic resonance, electron spin
resonance, Ramsey-based spectroscopy and laser/maser technology. It also plays
a unique role in quantum information processing as qubits are realized as
energy superposition states of single quantum systems. Here, we demonstrate
quantum interference of different energy states of single quanta of light in
full analogy to energy levels of atoms or nuclear spins and implement a Ramsey
interferometer with single photons. We experimentally generate energy
superposition states of a single photon and manipulate them with unitary
transformations to realize arbitrary projective measurements, which allows for
the realization a high-visibility single-photon Ramsey interferometer. Our
approach opens the path for frequency-encoded photonic qubits in quantum
information processing and quantum communication.
