Single particles of light can be prepared in a quantum superposition of two different colors, an achievement that could prove useful for quantum information processing.
Interferometry using discrete energy levels of 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 or maser technology.It also plays a unique role in quantum information processing as qubits may be implemented as energy superposition states of simple quantum systems.Here, we demonstrate quantum interference involving energy states of single quanta of light.In full analogy to the energy levels of atoms or nuclear spins, we 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.Our approach opens the path for frequency-encoded photonic qubits in quantum information processing and quantum communication.
Multi-color photons are prominent candidates for carrying quantum information, as their unlimited dimensionality allows for novel qudit-based schemes. The generation and manipulation of such photons takes place in nonlinear optical media, and the coupling between the different frequency bins can be engineered to obtain the desired quantum state. Here, we propose the design of a frequency-domain Stern–Gerlach effect for photons, where quantum entanglement between the spatial and spectral degrees of freedom is manifested. In this scheme, orthogonal frequency-superposition states can be spatially separated, resulting in a direct projection of an input state onto the frequency-superposition basis. We analyze this phenomenon for two-color qubits and three-color qutrits, and present a generalized wavelength-domain analog of the Hong–Ou–Mandel interference with distinguishable photons. Our results pave the way toward realization of single-element, all-optically controlled spectral-to-spatial beam splitters and tritters that can benefit quantum information processing in the frequency domain.