Quantum-optics researchers have been trying to achieve strong interactions between individual photons for decades.1 These interactions constitute a fundamental tool toward the ultimate control of light fields "quantum by quantum."
The realization of strong nonlinear interactions between individual light quanta (photons) is a long-standing goal in optical science and engineering, being of both fundamental and technological significance. We describe and demonstrate a medium that is nonlinear at the level of individual quanta, exhibiting strong absorption and phase shift for photon pairs while remaining transparent to single photons. The quantum nonlinearity is obtained by coherently coupling slowly propagating photons to strongly interacting atomic Rydberg states in a cold, dense atomic gas. Finally, we will discuss our progress towards realization of integrated quantum photonic systems, based on deterministic coupling of individual atoms and atom-like systems to photonic crystal cavities.
Nonlinear interactions between photons are fundamentally weak as the photons do not interact directly with each other, and any interaction is mediated by matter. This has motivated researchers over many decades to search for strongly nonlinear materials (by controlling electronic properties) and optical resonators with strong spatial and temporal confinement of light. An extreme form of nonlinear optics is quantum nonlinear optics, where we can realize nonlinear interaction between single photons. Such quantum nonlinear optics is at the heart of any photonic quantum information system including analog quantum simulation and fault-tolerant quantum computing. While engineering light-matter interactions can effectively create photon-photon interactions, the required photon number to observe any nonlinearity are normally very high, where any quantum-mechanical signature disappears. However, with emerging low-dimensional materials and engineered photonic resonators, the photon number can be reduced to reach the quantum nonlinear optical regime. In this review paper, we discuss different mechanisms exploited in solid-state platforms to attain quantum nonlinear optics. We review emerging materials and optical resonator architectures with different dimensionalities. We also present future research directions and open problems in this field.
Enhancing optical nonlinearities so that they become appreciable on the single photon level and lead to nonclassical light fields has been a central objective in quantum optics for many years. After this has been achieved in individual micro-cavities representing an effectively zero-dimensional volume, this line of research has shifted its focus towards engineering devices where such strong optical nonlinearities simultaneously occur in extended volumes of multiple nodes of a network. Recent technological progress in several experimental platforms now opens the possibility to employ the systems of strongly interacting photons, these give rise to as quantum simulators. Here we review the recent development and current status of this research direction for theory and experiment. Addressing both, optical photons interacting with atoms and microwave photons in networks of superconducting circuits, we focus on analogue quantum simulations in scenarios where effective photon-photon interactions exceed dissipative processes in the considered platforms.
Quantum optics is the branch of physics that investigates the interaction of light with matter at the quantum level, where the discrete nature of photons and quantum states of atoms, molecules, and solid-state systems become dominant. Quantum optics systems provide the experimental and theoretical foundation for understanding nonclassical states of light, quantum coherence, entanglement, and measurement at the quantum limit. Over the past few decades, advances in laser technology, nonlinear optics, and precision measurement have transformed quantum optics from a fundamental discipline into a key enabler of quantum technologies, including quantum communication, quantum computing, quantum sensing, and quantum metrology. This review presents a comprehensive overview of the physical principles, system architectures, and experimental platforms of quantum optics. Emphasis is placed on light sources, atom–photon interaction systems, detection techniques, and emerging applications, along with current challenges and future research directions.
Most phenomena around us can be explained by classical optics since photons interact weakly with atoms. When the interaction is strong enough where one photon affects the properties of other photons, we go to the regime of quantum photon-photon nonlinear optics. The work in this thesis explores this regime of physics. This paves the path to exploring strongly interacting many-body physics with photons which is still a largely unexplored domain. The ability to control strongly interacting photons is also of central importance in quantum science and engineering. We use the approach of Electromagnetically Induced Transparency (EIT) in atomic ensembles to create strong interactions and demonstrate coherent control between single photons. Using EIT, we coherently map single photons onto the atoms which travel as coupled excitations of light and matter called dark-state polaritons. When the medium is optically dense, the polariton propagates as a massive particle with a velocity that is much smaller than the speed of light. These polaritons interact with each other via their atomic component. In this thesis, we show robust symmetry-protected collisions between photons that imprint a large phase of π/2 with little loss and extend the nature of photon-photon interactions to the repulsive regime, which are essential for quantum communication and metrology. We use the technique of photon blockade to make a fast optical detector of Rydberg states, directly demonstrating the use of quantum nonlinear optics for quantum computation. We also study the extensions of the attractive and repulsive photons for three particles shedding light into the formation of bound states and crystals of photons and paving the way to studying many-body physics of photons.
Rydberg atoms have emerged as a versatile platform for quantum optics due to their exaggerated properties, particularly their strong long-range interactions, which enable a new regime of light-matter interaction. By mapping the interactions between Rydberg atoms onto photons, effective photon-photon interactions can be realized at the single-photon level, thereby overcoming the intrinsic weakness of conventional optical nonlinearities. In this review, we first introduce the fundamental physical principles of Rydberg-mediated quantum optics, and then discuss some key developments, including single-photon engineering, photonic quantum gates, contactless nonlinear optics, and quantum entanglement, providing a comprehensive overview of the current state and prospects of this rapidly developing field.