Ramsey Interference with Single Photons

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.

Figure 1

(a) Principle of a Ramsey interference based on discrete energy levels of photons depicted as rotations on the Bloch sphere. (b) Up- or down-conversion for a particular set of frequencies $\{{\nu }_1,{\nu }_2,{\nu }_i,{\nu }_s\}$ via Bragg scattering four-wave mixing corresponds to polar rotation on the Bloch sphere. $\delta \nu ={\nu }_1-{\nu }_2={\nu }_i-{\nu }_s$ and $\mathrm{\Delta }\nu ={\nu }_s-{\nu }_1$.

Figure 2

(a)–(d) Experimental implementation of Ramsey interferometry with photons.

Figure 3

Photon depletion and conversion efficiency to the idler frequency via BS FWM as a function of the pump power measured for decoy states with the setup depicted in Fig. 2. $P$ is the power corresponding to a polar $\pi /2$ rotation on the Bloch sphere.

Figure 4

Ramsey interference fringes observed while varying the phase between two BS-FWM frequency converters.