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. Main Text:The two-state model represents the most fundamental quantum system and can be applied to a wide variety of physical systems. Ramsey interferometry, magnetic resonance imaging, and electron-spin resonance spectroscopy are governed by similar 2-level system dynamics, which involves molecular-atomic levels, nuclear spin, and electronic spin, respectively. The coupling between energy levels is achieved using electromagnetic fields, which can be tailored at will and allows for many advanced techniques such as adiabatic elimination and stimulated Raman adiabatic passage in higher dimensional atomic system, or spin locking in NMR. Quantum interference involving systems in superposition of different energies is at the heart of fundamental and applied physics. For example, quantum coherence has been highly useful in increasing the accuracy of time measurement from the first idea of using NMR suggested by Rabi in 1945 (1) to the first atomic clock relying on the Ramsey interferometry (2,3), which has been recently extended by using trapped single ions (4). In addition, Ramsey interferometry on single Rydberg atoms has allowed the nondestructive measurement of the number of photon in a cavity (5) and single spin manipulation using the same techniques constitutes one of the most promising routes towards quantum processing (6-8). Matter-wave interferometers using collective energy levels of atoms in a BEC have also been demonstrated (9) and used to measure gravity down to record breaking precision (10). Nevertheless, a fundamental quantum system that has not been extensively studied in the context of discrete 2-level energy systems (i.e. frequency) is the single photon. Translating those studies to photonics system can be implemented by controlling light with light using nonlinear optics. For classical light the analogy between atomic/molecular optics and nonlinear optics is well known (11) and there are various cases where the complex dynamics of light propagation in a nonlinear medium can be si...

Parametric single-photon sources are well suited for large-scale quantum networks due to their potential for photonic integration. Active multiplexing of photons can overcome the intrinsically probabilistic nature of these sources, resulting in near-deterministic operation. However, previous implementations using spatial and temporal multiplexing scale unfavorably due to rapidly increasing switching losses. Here, we break this limitation via frequency multiplexing in which switching losses remain fixed irrespective of the number of multiplexed modes. We use low-noise optical frequency conversion for efficient frequency switching and demonstrate multiplexing of three modes. We achieve a generation rate of 4.6 × 104 photons per second with an ultra-low g(2)(0) = 0.07 indicating high single-photon purity. Our scalable, all-fiber multiplexing system has a total loss of just 1.3 dB, such that the 4.8 dB multiplexing enhancement markedly overcomes switching loss. Our approach offers a promising path to creating a deterministic photon source on an integrated chip-based platform.

We demonstrate that nondegenerate four-wave mixing in a Si 3 N 4 microring resonator can result in a nonlinear coupling rate between two optical fields exceeding their energy dissipation rate in the resonator, corresponding to strong nonlinear coupling. We demonstrate that this leads to a Rabi-like splitting, for which we provide a theoretical description in agreement with our experimental results. This yields new insight into the dynamics of nonlinear optical interactions in microresonators and access to novel phenomena.

We review the motivation, goals, and achievements of the Photonic Integrated Networked Energy efficient datacenter (PINE) project, which is part of the Advanced Research Projects Agency-Energy (ARPA-E) ENergyefficient Light-wave Integrated Technology Enabling Networks that Enhance Dataprocessing (ENLITENED) program. The PINE program leverages the unique features of photonic technologies to enable alternative megadatacenters and high-performance computing (HPC) system architectures that deliver more substantial energy efficiency improvements than can be achieved through link energy efficiency alone. In phase 1 of the program, the PINE system architecture demonstrated an average factor of 2.2× improvement in transactions/joule across a diverse set of HPC and datacenter applications. In phase 2, PINE will demonstrate an aggressive 1.0 pJ/bit total link budget with high-bandwidth-density dense wavelength-division multiplexing (DWDM) links to enable additional 2.5× or more efficiency gains through deep resource disaggregation.

We describe a novel technique for performing a single-shot optical cross-correlation in nanowaveguides. Our scheme is based on four-wave mixing between two orthogonally polarized input signals propagating with different velocities due to polarization mode dispersion. The cross-correlation is determined by measuring the spectrum of the idler wave generated by the four-wave mixing process.