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arXiv:1212.0396 · New Journal of Physics

Efficient optical pumping and high optical depth in a hollow-core photonic-crystal fibre for a broadband quantum memory

Michael Sprague, Duncan G. England, A. Abdolvand, Joshua Nunn, Xian‐Min Jin, W. Steven Kolthammer, Marco Barbieri, B. Rigal, Patrick Michelberger, Tessa F. M. Champion, Philip St.J. Russell, Ian A. Walmsley

Published May 20, 2013• 35 citations
arXivHTMLPDF

Abstract

The generation of large multiphoton quantum states—for applications in computing, metrology and simulation—requires a network of high-efficiency quantum memories capable of storing broadband pulses. Integrating these memories into a fibre offers a number of advantages towards realizing this goal: strong light–matter coupling at low powers, simplified alignment and compatibility with existing photonic architectures. Here, we introduce a large-core kagome-structured hollow-core fibre as a suitable platform for an integrated fibre-based quantum memory with a warm atomic vapour. We demonstrate, for the first time, efficient optical pumping in such a system, where 90 ± 1% of atoms are prepared in the ground state. We measure high optical depths (3 × 10 4 ) and narrow homogeneous linewidths (6 ± 2 MHz) that do not exhibit significant transit-time broadening, showing that we can prepare a Λ-level system in a pure state. Our results establish that kagome fibres are suitable for implementing a broadband, room-temperature quantum memory, as well as a range of nonlinear optical effects.

DOI
10.1088/1367-2630/15/5/055013

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The generation of large multiphoton quantum states—for applications in computing, metrology and simulation—requires a network of high-efficiency quantum memories capable of storing broadband pulses. Integrating these memories into a fibre offers a number of advantages towards realizing this goal: strong light–matter coupling at low powers, simplified alignment and compatibility with existing photonic architectures. Here, we introduce a large-core kagome-structured hollow-core fibre as a suitable platform for an integrated fibre-based quantum memory with a warm atomic vapour. We demonstrate, for the first time, efficient optical pumping in such a system, where 90 ± 1% of atoms are prepared in the ground state. We measure high optical depths (3 × 10 4 ) and narrow homogeneous linewidths (6 ± 2 MHz) that do not exhibit significant transit-time broadening, showing that we can prepare a Λ-level system in a pure state. Our results establish that kagome fibres are suitable for implementing a broadband, room-temperature quantum memory, as well as a range of nonlinear optical effects.
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Quantum information networks will facilitate the use of a broad range of quantum technologies including quantum computers, quantum sensors, and quantum communication systems, among others. An important component of these networks are devices that enable light-atom interactions, as they interface photons from communication channels with the matter-based nodes, allowing for processing of photonic quantum information. These devices will need to be scalable and easily integrated to expand the size of networks. One platform of interest is vapour-filled hollow-core fibres (HCF). By overlapping light and atomic vapour within the same volume, strong interactions can occur, including important nonlinear effects such as cross-phase modulation and wave mixing processes. HCFs can be directly integrated into fibre systems, making them a convenient technology for the expansion of quantum networks. This thesis reports on three projects themed around scalable technologies for quantum networks. The first project involved the injection-locking of a diode laser. Nanosecond long pulses had their 20 GHz frequency chirps reduced to near the Fourier-transform limit, enabling them to interact efficiently with rubidium vapour. This project demonstrated that the mature, cost effective, and scalable technology of diode lasers can be compatible with essential atomic ensemble quantum networking devices such as quantum memories and quantum frequency converters. In the second project, a high-bandwidth quantum memory protocol called off-resonant cascaded absorption (ORCA) was investigated. The ORCA memory can be used to synchronise high speed photonic operations that occur in optical quantum information processing. This project used a Kagom´e HCF to enhance the light-atom interactions, resulting in two orders of magnitude reduction in optical power requirements. The third project again used the HCF platform, this time to facilitate a quantum frequency conversion scheme that converts rubidium-compatible, near-IR wavelengths to the telecommunications C-band, which is ideal for long distance fibre transmission. This project demonstrated the FWM process has reduced optical pump powers and higher efficiency in the HCF when compared to a rubidium-filled cell. The research described in this thesis presents a strong argument for the use of HCFs as a scalable technology that could be implemented in quantum networks.
We present an experimental realization of a noise-free and high-bandwidth quantum memory scheme using a rubidium vapor that is confined within the hollow core of a photonic crystal fiber. We achieve the same internal efficiencies as similar free-space experiments (30%) for 4.5-ns-long optical pulses but with a 100-fold reduction in the control-field power required. Modeling indicates that this efficiency could be improved to 88% with higher control powers and the implementation of techniques such as light-induced atomic desorption to increase the optical depth. The compactness, robustness, and low drive power of this approach lends itself to direct integration into large-scale fiber-based quantum processors.
The absorption of broadband photons in atomic ensembles requires either an effective broadening of the atomic transition linewidth, or an off-resonance Raman interaction. Here, we propose a scheme for a quantum memory capable of storing and retrieving ultrafast photons in an ensemble of two-level atoms using a propagation medium with a time–dependent refractive index generated from aligning an ensemble of gas-phase diatomic molecules. The refractive index dynamics generates an effective longitudinal inhomogeneous broadening of the two-level transition. We numerically demonstrate this scheme for storage and retrieval of a weak pulse as short as 50 fs, with a storage time of up to 20 ps. With additional optical control of the molecular alignment dynamics, the storage time can be extended about one nanosecond leading to time–bandwidth products of order 104. This scheme could in principle be achieved using either a hollow-core fibre or a high-pressure gas cell, in a gaseous host medium comprised of diatomic molecules and a two-level atomic vapour at room temperature.
We investigate the formation and propagation of ultraslow weak-light solitons and their memory in the atomic gas filled in a kagome-structured hollow-core photonic crystal fiber (HC-PCF) via electromagnetically induced transparency (EIT). We show that, due to the strong light-atom coupling contributed by the transverse confinement of the HC-PCF, the EIT and hence the optical Kerr nonlinearity of the system can be largely enhanced, and hence optical solitons with very short formation distance, ultraslow propagation velocity, and extremely low generation power can be realized. We also show that the optical solitons obtained can not only be robust during propagation, but also be stored and retrieved with high efficiency through the switching off and on of a control laser field. The results reported herein are promising for practical applications of all-optical information processing and transmission via the ultraslow weak-light solitons and the kagome-structured HC-PCF.