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arXiv:2409.06571 · Advanced Quantum Technologies

Characterization of the Spin and Crystal Field Hamiltonian of Erbium Dopants in Silicon

Adrian Holzäpfel, Stephan Rinner, Kilian Sandholzer, Andreas Gritsch, T. Chanelière, Andreas Reiserer

Published Dec 19, 2024• 9 citations

Abstract

Abstract The integration of coherent emitters into low‐loss photonic circuits is a key technology for quantum networking. In this context, nanophotonic silicon devices implanted with erbium are a promising hardware platform that combines advanced wafer‐scale nanofabrication technology with coherent emission in the minimal‐loss band of optical fibers. Recent studies have reported two distinct sites in the silicon lattice in which erbium can be reproducibly integrated with particularly promising properties. Here, for an in‐depth analysis of these sites, resonant fluorescence spectroscopy is performed on a nanophotonic waveguide in magnetic fields applied along different orientations. In this way, the site symmetry is determined, the spin Hamiltonian is reconstructed and a partial fit of the crystal field Hamiltonian is performed. The obtained quantitative description of the magnetic interaction allows the optimization of Zeeman splittings, optical branching ratios or microwave driving to the needs of future experiments. Beyond that, the derived site symmetry constrains the location of the erbium dopant in the silicon unit cell. This is a key step toward a detailed microscopic understanding of the erbium sites, which may help to improve the integration yield, thus paving the way to efficient nanophotonic quantum memories based on the Er:Si platform.

DOI: 10.1002/qute.202400342

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Abstract The integration of coherent emitters into low‐loss photonic circuits is a key technology for quantum networking. In this context, nanophotonic silicon devices implanted with erbium are a promising hardware platform that combines advanced wafer‐scale nanofabrication technology with coherent emission in the minimal‐loss band of optical fibers. Recent studies have reported two distinct sites in the silicon lattice in which erbium can be reproducibly integrated with particularly promising properties. Here, for an in‐depth analysis of these sites, resonant fluorescence spectroscopy is performed on a nanophotonic waveguide in magnetic fields applied along different orientations. In this way, the site symmetry is determined, the spin Hamiltonian is reconstructed and a partial fit of the crystal field Hamiltonian is performed. The obtained quantitative description of the magnetic interaction allows the optimization of Zeeman splittings, optical branching ratios or microwave driving to the needs of future experiments. Beyond that, the derived site symmetry constrains the location of the erbium dopant in the silicon unit cell. This is a key step toward a detailed microscopic understanding of the erbium sites, which may help to improve the integration yield, thus paving the way to efficient nanophotonic quantum memories based on the Er:Si platform.

Quantum memories integrated into nanophotonic silicon devices are a promising platform for large quantum networks and scalable photonic quantum computers. In this context, erbium dopants are particularly attractive, as they combine optical transitions in the telecommunications frequency band with the potential for second-long coherence time. Here, we show that these emitters can be reliably integrated into commercially fabricated low-loss waveguides. We investigate several integration procedures and obtain ensembles of many emitters with an inhomogeneous broadening of <2 GHz and a homogeneous linewidth of <30 kHz. We further observe the splitting of the electronic spin states in a magnetic field up to 9 T that freezes paramagnetic impurities. Our findings are an important step toward long-lived quantum memories that can be fabricated on a wafer-scale using CMOS technology.

The realization of a scalable architecture for quantum information processing is a major challenge for quantum science.A promising approach is based on emitters in nanostructures that are coupled by light.Here, we show that erbium dopants can be reproducibly integrated at well-defined lattice sites by implantation into pure silicon.We thus achieve a narrow inhomogeneous broadening, less than 1 GHz, strong optical transitions, and an outstanding optical coherence even at temperatures of 8 K, with an upper bound to the homogeneous linewidth of around 10 kHz.Our study thus introduces a promising materials platform for the implementation of on-chip quantum memories, microwave-to-optical conversion, and distributed quantum information processing.

Integrated photonic circuits offer great promise for quantum technologies. However, due to the rapid propagation of light, many envisioned applications require efficient on-chip quantum memories with a programmable delay, compact footprint, and high fidelity. Implementing this based on standard semiconductor processing technology is an outstanding challenge. Here, we realize such memories using erbium-doped silicon waveguides, fabricated as part of a multi-wafer project by a nanophotonic foundry. We demonstrate light storage with a

44.2(9)\ \text{MHz}

bandwidth and a programmable delay exceeding

1\ μ\text{s}

in a device with a footprint of only

1.5\times 10^{-2}\ \text{mm}^2

, outperforming on-chip delay lines by many orders of magnitude. The phase of the read-out light field is preserved with a visibility of

91.3(30)\ \%

. The efficiency of

1.89(28)\times 10^{-8}

can be improved in future devices through resonator enhancement and higher dopant concentrations. With this, the demonstrated approach will pave the way towards applications in photonic quantum computing based on scalable silicon processing technology.

Single photons enable the distribution of quantum information over large distances and thus play a major role in quantum technologies such as communication and computing. Solid-state emitters are practical and efficient sources of single photons that can be manufactured in large numbers. When combined with a spin, the resulting spin-photon interfaces can store quantum states for extended periods and serve as the basis for quantum networks and repeaters. Among the many host materials explored over the past few decades, silicon stands out for its advanced nanofabrication, the maturity of its integrated photonics and microelectronics, and its high isotopic purity, which leads to exceptionally long spin coherence. These properties position silicon single-photon emitters and spin-photon interfaces among the most promising hardware platforms for implementing quantum networks and distributed quantum information processors. This review summarizes the current state of the art and open challenges towards coherent single-photon sources and scalable spin-photon interfaces based on color centers and erbium dopants in nanophotonic silicon structures.