Nonlinear π phase shift for single fibre-guided photons interacting with a single resonator-enhanced atom

Abstract

Realizing a strong interaction between individual photons is an important objective of research in quantum science and technology. It requires an optical medium in which light experiences a phase shift that depends nonlinearly on the photon number. Once the additional two-photon phase shift reaches π, such an ultra-strong nonlinearity could enable the implementation of high-fidelity quantum logic operations. However, the nonlinear response of standard optical media is orders of magnitude too weak. Here, we demonstrate a fibre-based nonlinearity that realizes an additional two-photon phase shift close to the ideal value of π. We employ a whispering-gallery-mode resonator, interfaced by an optical nanofibre, where the presence of a single rubidium atom in the resonator mode results in a strongly nonlinear response. We show that this results in entanglement of initially uncorrelated incident photons. This demonstration of a fibreintegrated, ultra-strong nonlinearity is a decisive step towards photon-based scalable quantum logics.O ptical photons are a key ingredient for investigations and applications in modern quantum information science. They are well decoupled from their environment but can, nevertheless, be conveniently manipulated with high precision. In conjunction with the ease of transmitting them over long distances using optical fibres, this makes photons prime candidates for the distribution 1 and processing of quantum information in scalable quantum networks 2 as well as for metrology beyond the standard quantum limit 3 . These and many other applications require the photons to interact with each other in order to prepare and probe entanglement or to perform quantum logic operations. However, a direct photon-photon interaction does not exist in free space. It has been shown that an effective photon-photon interaction can be implemented probabilistically using linear optical elements in combination with projective measurements 4 . Yet, for now, this technique cannot be considered scalable in a practical sense because it requires high-fidelity single-photon sources and detectors that are still beyond present capabilities 5 . Alternatively, a deterministic interaction can be realized by means of an optical medium that exhibits a nonlinearity down to the level of individual photons. Such strong optical nonlinearities have been demonstrated, for example, in atomic ensembles, where the nonlinearity either stems from direct interaction between the excited atoms 6,7 or from the generation of an intrinsic Kerr nonlinearity via electromagnetically induced transparency 8,9 . An alternative approach is based on enhancing the nonlinearity of individual quantum emitters by coupling them to optical resonators. With these systems, nonlinear phase shifts up to a few tens of degrees, single-photon-controlled on-and off-switching of light, as well as photon number-dependent redirection of light have been demonstrated [10][11][12][13][14][15] . In this Article, we demonstrate the realization of an optical fibre-based nonlin...

Dynamical observations of self-stabilizing stationary light

“…(12) 2 Approximations and simplified equations Solutions to equations (8)(9)(10)(11)(12) can be found more easily by eliminating the time derivatives in eqs. (11,12).…”

“…In particular, implementations of quantum repeaters will likely rely on mapping of photonic states onto atomic systems to enable storage of quantum information [3,4]. Deterministic quantum logic gates in optical systems may also rely on atomic state mapping to enable nonlinear photon-photon interactions [5,6,7,8,9]. A fundamental issue facing any attempt to implement nonlinear cross phase modulation (XPM) is that the interaction is inherently very weak.…”

“…Various methods have been used to achieve one or both of these objectives, including photonic crystal fibres with [6] and without [12] injected atoms, tapered optical fibres in atomic vapour [13], optical cavities [9,14,15] and slow light propagation [16,17,18].…”

Polarization-entangled mid-infrared photon generation inp-doped semiconductor quantum wells

“…Quantum correlated photon pairs, also known as entangled photon pairs, are the main ingredient in quantum communications [1], quantum computing [1][2][3], quantum key distribution (QKD) [4], quantum teleportation [5], superdense coding [6] and many other applications of quantum information theory. These photon pairs can be generated in spontaneous parametric down conversion (SPDC), or in spontaneous four wave mixing (SFWM) [7].…”

“…These photon pairs can be generated in spontaneous parametric down conversion (SPDC), or in spontaneous four wave mixing (SFWM) [7]. SPDC, which is also known as parametric fluorescence [8], is based on second order nonlinearity (χ (2) ), while SFWM is based on third order nonlinearity (χ (3) ). Raman scattering noise, which is hard to suppress in SFWM [9], has made the SPDC scheme more attractive.…”