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arXiv:2012.03890 · Physical Review A

Atomic Faraday beam splitter for light generated from pump-degenerate four-wave mixing in a hollow-core photonic crystal fiber

Ioannis Caltzidis, Harald Kübler, Tilman Pfau, Robert Löw, Mark A. Zentile

Published Apr 1, 2021• 19 citations

Abstract

We demonstrate an atomic Faraday dichroic beam splitter suitable for spatially separating signal and idler fields from pump degenerate four-wave mixing in an atomic source. By rotating the plane of polarization of one mode ${90}^{\ensuremath{\circ}}$ with respect to the other, a subsequent polarizing beam splitter separates the two frequencies, which differ by only 13.6 GHz, and achieves a suppression of $(\ensuremath{-}26.3\ifmmode\pm\else\textpm\fi{}0.1)$ and $(\ensuremath{-}21.2\ifmmode\pm\else\textpm\fi{}0.1)$ dB in the two outputs, with a corresponding transmission of 97% and 99%, respectively. This technique avoids the necessity of using spatial separation of four-wave mixing modes and thus opens the door for enhancing the process efficiency in waveguide experiments. As a proof of principle, we generate light via four-wave mixing in $^{87}\mathrm{Rb}$ loaded into a hollow-core photonic crystal fiber and interface it with the atomic Faraday dichroic beam splitter.

DOI: 10.1103/physreva.103.043501

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We demonstrate an atomic Faraday dichroic beam splitter suitable for spatially separating signal and idler fields from pump degenerate four-wave mixing in an atomic source. By rotating the plane of polarization of one mode

{90}^{\ensuremath{\circ}}

with respect to the other, a subsequent polarizing beam splitter separates the two frequencies, which differ by only 13.6 GHz, and achieves a suppression of

(\ensuremath{-}26.3\ifmmode\pm\else\textpm\fi{}0.1)

and

(\ensuremath{-}21.2\ifmmode\pm\else\textpm\fi{}0.1)

dB in the two outputs, with a corresponding transmission of 97% and 99%, respectively. This technique avoids the necessity of using spatial separation of four-wave mixing modes and thus opens the door for enhancing the process efficiency in waveguide experiments. As a proof of principle, we generate light via four-wave mixing in

^{87}\mathrm{Rb}

loaded into a hollow-core photonic crystal fiber and interface it with the atomic Faraday dichroic beam splitter.

We present an application of the Faraday effect to produce a narrowband dichroic beam splitter using an alkali metal vapor. Two Raman beams detuned in frequency by the ground-state hyperfine splitting in (87)Rb are produced using an electro-optic modulator and then separated using the Faraday effect in an isotopically pure (85)Rb thermal vapor. An experimental transmission spectra for the beam splitter is presented along with a theoretical calculation. The performance of the beam splitter is then demonstrated and characterized using a Fabry-Perot etalon. For a temperature of 70 degrees C and a longitudinal magnetic field of 80 G, a suppression of one frequency of 18 dB is achieved, limited by the quality of the polarizers.

We experimentally investigate near-infrared to telecommunications frequency conversion via a diamond four-wave mixing scheme in rubidium vapor contained within a hollow-core photonic-crystal fiber. The strong light-atom interaction in the fiber results in lower pump power requirements and higher conversion efficiency than can be achieved under equivalent conditions in a rubidium vapor cell. We also observe non-intuitive pump and signal frequency dependence of the four-wave mixing efficiency in the fiber due to the large nonlinearities present in the system. These results indicate the potential for hollow-core fibers to provide a scalable solution for quantum information network infrastructure, with additional modelling required for a full understanding of the extreme atom-light interaction effects present.

High fidelity transmission of quantum polarization states is essential for realizing quantum networks. The degenerate ${\ensuremath{\Psi}}^{\ensuremath{-}}$ state is an eigenstate of a transmission fiber. In this paper we propose a method for preparing the degenerate ${\ensuremath{\Psi}}^{\ensuremath{-}}$ state using spontaneous four wave mixing (SFWM) in a fiber Sagnac loop excited by two orthogonally polarized pump waves. This configuration includes a 45\ifmmode^\circ\else\textdegree\fi{} Faraday rotator, which creates a relative phase of $\ensuremath{\pi}$ between the two counterpropagating entangled photon pairs with indistinguishability both in frequencies and generation paths. We confirm that only the amplitude is degraded in the output wave form of nonlinear sum frequency generation in the Bell state analyzer after propagation through the fiber due to the polarization mode dispersion.

In this work, we propose and theoretically analyze a new scheme for generation of counterpropagating photon pairs in photonic crystal waveguides through the process of spontaneous four-wave mixing. Using the fundamental properties of periodic Bloch modes in a standard photonic crystal waveguide, we demonstrate how modal phase-matching can be reached between forward-propagating pump modes and counterpropagating signal and idler modes, for generation of degenerate and non-degenerate photon pairs. We then show how this scheme can be used for generation of photon pairs that are nearly uncorrelated in the spectral degree of freedom. Such a source will be highly interesting as a heralded source of single photons, especially as the spectrally separable signal and idler photons are also spatially separated directly at the source. We conduct our investigation based on a design in silicon, yet our design concept is general and can in principle be applied to any nanostructured material platform.

We demonstrate extremely efficient four-wave mixing with gains greater than 100 at microwatt pump powers and signal-to-idler conversion of 50% in Rb vapor confined to a hollow-core photonic band-gap fiber. We present a theoretical model that demonstrates such efficiency is consistent with the dimensions of the fiber and the optical depths attained. This is, to our knowledge, the largest four-wave mixing gain observed at such low total pump powers and the first demonstrated example of four-wave mixing in an alkali-metal vapor system with a large (approximately 30 MHz) ground state decoherence rate.