Quantum nonlinear optics — photon by photon

Abstract

other. This linearity in light propagation, in combination with the high frequency and hence large bandwidth provided by waves at optical frequencies, has made optical signals the preferred method for communicating information over long distances. In contrast, the processing of information requires some form of interaction between signals. In the case of light, such interactions can be enabled by nonlinear optical processes. These processes, which are now found ubiquitously throughout science and technology, include optical modulation and switching, nonlinear spectroscopy and frequency conversion 1 , and have applications across both the physical 2 and biological 3,4 sciences.A long-standing goal in optical science has been the implementation of nonlinear effects at progressively lower light powers or pulse energies. The ultimate limit may be termed 'quantum nonlinear optics' (Box 1) -the regime where individual photons interact so strongly with one another that the propagation of light pulses containing one, two or more photons varies substantially with photon number. Although this domain is difficult to reach owing to the small nonlinear coefficients of bulk optical materials, the potential payoff is significant. The realization of quantum nonlinear optics could improve the performance of classical nonlinear devices, enabling, for example, fast energy-efficient optical transistors that avoid Ohmic heating 5 . Furthermore, nonlinear switches activated by single photons could enable optical quantum information processing and communication 6 , as well as other applications that rely on the generation and manipulation of non-classical light fields 7,8 . The challenge of making photons interactAt low optical powers, most optical materials exhibit only linear optical phenomena, such as refraction and absorption, which can be described by a complex index of refraction. However, a sufficiently intense light beam can modify a material's index of refraction, such that the light propagation becomes power-dependent. This is the essence of classical nonlinear optics (Box 1). Large optical fields are required to alter the index of refraction of conventional bulk materials because a strong nonlinear response can only be induced if the electric field of the light beam acting on the electrons is comparable to the field of the nucleus. As a result, early experimental observations of nonlinear optical phenomena, such as frequencydoubling or sum-frequency generation 9 , were achievable only after the development of powerful lasers. Advances in nonlinear optics over the past four decades have resulted in progressively more efficient nonlinear processes 10 , thus enabling the observation of nonlinear processes at lower and lower light levels. It is natural to inquire if and how these nonlinear interactions can be made so strong that they become important even at the level of individual quanta of radiation. Although this question was addressed in early theoretical studies [11][12][13] , it has become more pressing with the advent of...

Subcycle measurement of intensity correlations in the terahertz frequency range

“…We believe that this limit can be reached with few technical improvements in the detection scheme and the probing beam power. Recent success in making single photons interact in nonlinear optical media strengthen our ambition and have already changed the general perception that nonlinear effects can only be observed at high photon intensities and are vanishingly small for single photons [38]. As demonstrated, our averaged sensitivity limit corresponds to an equivalent electric field of 90 V/m detected along an effective length of 0.5 mm.…”

Large Bragg Reflection from One-Dimensional Chains of Trapped Atoms Near a Nanoscale Waveguide

“…Beyond the remarkable ability to couple a single emitter to a guided mode [3], the 1D reservoir would also enable the exploration and eventual engineering of photon-mediated long-range interactions between multiple qubits, a challenging prospect in free-space geometries. This emerging field of waveguide quantum electrodynamics promises unique applications to quantum networks, quantum nonlinear optics, and quantum simulation [4][5][6].…”

Collective phases of strongly interacting cavity photons

“…With notable progress towards generating strong optical nonlinearities at the few-photon level, for example with atoms coupled to small-mode-volume optical devices [17][18][19][20][21][22], Rydberg polaritons [23,24], and circuit-QED devices [25][26][27][28], this situation is rapidly changing. The production of strongly interacting, driven and dissipative gases of photons appears to be feasible [29,30], and affords exciting opportunities to explore the properties of open quantum systems in unique contexts, while studying the applicability of theoretical treatments designed with more weakly interacting systems in mind. For example, it is not fully understood how the steady states of these systems relate to the equilibrium states of their "closed" counterparts, or how conventional optical phenomena, such as bistability, manifests in the presence of strong optical nonlinearities and spatial degrees of freedom.…”