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arXiv:1802.07947 · Physical Review Applied

Two-Way Photonic Interface for Linking the Sr+ Transition at 422 nm to the Telecommunication C Band

Thomas A. Wright, Robert J. A. Francis-Jones, Corin B. E. Gawith, Jonas N. Becker, Patrick M. Ledingham, Peter G. R. Smith, Joshua Nunn, Peter J. Mosley, Benjamin Brecht, Ian A. Walmsley

Published Oct 4, 2018• 18 citations
arXivHTMLPDF

Abstract

Trapped ions have achieved the highest fidelity in local processing of quantum information, but their short emission wavelengths are incompatible with long-distance distribution of information at telecom wavelengths, which prevents their use in a large-scale quantum network. We present an interface capable of two-way frequency conversion of single photons between a Sr${}^{+}$ node-compatible wavelength and the telecom $C$ band, forming part of a critical pathway for future hybrid light-matter quantum networks. This scheme offers bidirectional translation of widely separated frequencies in a single stage, with noise levels low enough for high-fidelity interconnects.

DOI
10.1103/physrevapplied.10.044012

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Trapped ions have achieved the highest fidelity in local processing of quantum information, but their short emission wavelengths are incompatible with long-distance distribution of information at telecom wavelengths, which prevents their use in a large-scale quantum network. We present an interface capable of two-way frequency conversion of single photons between a Sr+{}^{+}+ node-compatible wavelength and the telecom CCC band, forming part of a critical pathway for future hybrid light-matter quantum networks. This scheme offers bidirectional translation of widely separated frequencies in a single stage, with noise levels low enough for high-fidelity interconnects.
We report a single-stage bi-directional interface capable of linking Sr+ trapped ion qubits in a long-distance quantum network. Our interface converts photons between the Sr+ emission wavelength at 422 nm and the telecoms C-band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single photon level with efficiencies of 9.4 % and 1.1 % respectively. Furthermore, we demonstrate that the noise introduced during the conversion process is sufficiently low to implement high-fidelity interconnects suitable for quantum networking.
Fiber-based quantum networks require photons at telecommunications wavelengths to interconnect qubits separated by long distances. Trapped ions are leading candidates for quantum networking with high-fidelity two-qubit gates, long coherence times, and the ability to readily emit photons entangled with the ion's internal qubit states. However, trapped ions typically emit photons at wavelengths incompatible with telecommunications fiber. Here, we demonstrate frequency conversion of visible photons, emitted from the S–P dipole transition of a trapped Ba+ ion into the telecommunications C-band. These results are an important step toward enabling a long-distance trapped ion quantum internet.
Trapped ions are one of the leading candidates for scalable and long-distance quantum networks because of their long qubit coherence time, high-fidelity single- and two-qubit gates, and their ability to generate photons entangled with the qubit state of the ion. One method for creating ion-photon entanglement is to exploit optical transitions from the 6 P 1/2 to 6 S 1/2 levels, which naturally emit spin-photon entangled states. But these optical transitions typically lie in the ultraviolet and visible wavelength regions of the spectrum. These wavelengths exhibit significant fiber-optic propagation loss, thereby limiting the transfer of quantum information to tens of meters. Quantum frequency conversion is essential to convert these photons to telecom wavelengths so that they can propagate over long distances in fiber-based networks, as well as for compatibility with the vast number of telecom-based optoelectronic components. Here, we generate O-band telecom photons via a low-noise quantum frequency conversion scheme from photons emitted from the 6 P 1/2 to 6 S 1/2 dipole transition of a trapped barium ion. We use a two-stage quantum frequency conversion scheme to achieve a frequency shift of 375.4 THz between the input visible photon and the output telecom photon, achieving a conversion efficiency of 11%. We attain a signal-to-background ratio of over 100 for the converted O-band telecom photon with background noise of less than 15 counts/s. These results are an important step toward achieving trapped ion quantum networks over long distances for distributed quantum computing and quantum communication.
We report a resource-efficient scheme in which a single pump laser was used to achieve frequency conversion by Bragg-scattering four-wave mixing in a photonic crystal fiber. We demonstrate bidirectional conversion of coherent light between Sr+ P1/2 - D3/2 emission wavelength at 1092 nm and the telecommunication C band with conversion efficiencies of 4.2 % and 37 % for up- and down-conversion, respectively. We discuss how the scheme may be viably scaled to meet the temporal, spectral and polarisation stability requirements of a hybrid light-matter quantum network.
The realization of a scalable quantum internet requires efficient light-matter interfaces that map stationary qubits onto photonic carriers for long-distance transmission. A central challenge is the generation of entangled photons simultaneously compatible with single-emitter transitions and low-loss telecom fiber infrastructure. Spontaneous parametric down-conversion in integrated photonic platforms offers a promising route toward this goal. Among available material systems, AlGaAs is particularly attractive due to its large second-order nonlinearity and strong potential for monolithic integration. However, engineering the spectral and spatial properties of the generated quantum states requires the simultaneous optimization of numerous geometric and material parameters, a task remaining computationally demanding for conventional numerical approaches. To address this challenge and enable rapid and high-fidelity modeling of complex nonlinear photonic devices, we develop an inverse-design framework based on neural network surrogate models. Using this readily extendable method, we design a transversely pumped AlGaAs waveguide microcavity that produces polarization-entangled photon pairs in distinct spatial modes and frequency channels, one at 1092 nm, resonant with a 88Sr+^{88}\text{Sr}^{+}88Sr+ transition, and the other at 1550 nm in the telecom C-band. This device establishes a direct photonic interface between trapped-ion qubits and long-haul fiber networks, providing a scalable pathway toward hybrid quantum network architectures.