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arXiv:2311.16875 · Advanced Optical Materials

Spectral Multiplexing of Rare‐Earth Emitters in a Co‐Doped Crystalline Membrane

Alexander Ulanowski, Johannes Früh, Fabian Salamon, Adrian Holzäpfel, Andreas Reiserer

Published Mar 6, 2024• 13 citations

Abstract

Abstract The spectral addressing of many individual rare‐earth dopants in optical resonators offers great potential for realizing distributed quantum information processors. To this end, it is required to understand and control the spectral properties of the emitters in micron‐scale devices. Here, erbium emitters are investigated in a Fabry–Perot resonator that contains a 10 µm thin membrane of crystalline yttrium orthosilicate that is co‐doped with europium. The co‐doping allows for tailoring the inhomogeneous distribution of the emitter frequency. With this approach, more than 360 spectrally resolved emitters are observed with Purcell factors exceeding 35, each of which constitutes an individually addressable qubit within the micron‐scale resonator. In addition to this spectral multiplexing, the optical coherence is preserved up to 0.62(3) ms under dynamical decoupling. Without decoupling, the coherence still reaches the lifetime limit for the emitters with the strongest Purcell enhancement that leads up to a 110‐fold lifetime reduction, down to 0.104(9) ms. Future work may combine this with long‐lived nuclear spin memories, which makes the investigated co‐doped membranes a promising platform for quantum repeaters and distributed quantum computers.

DOI: 10.1002/adom.202302897

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Abstract The spectral addressing of many individual rare‐earth dopants in optical resonators offers great potential for realizing distributed quantum information processors. To this end, it is required to understand and control the spectral properties of the emitters in micron‐scale devices. Here, erbium emitters are investigated in a Fabry–Perot resonator that contains a 10 µm thin membrane of crystalline yttrium orthosilicate that is co‐doped with europium. The co‐doping allows for tailoring the inhomogeneous distribution of the emitter frequency. With this approach, more than 360 spectrally resolved emitters are observed with Purcell factors exceeding 35, each of which constitutes an individually addressable qubit within the micron‐scale resonator. In addition to this spectral multiplexing, the optical coherence is preserved up to 0.62(3) ms under dynamical decoupling. Without decoupling, the coherence still reaches the lifetime limit for the emitters with the strongest Purcell enhancement that leads up to a 110‐fold lifetime reduction, down to 0.104(9) ms. Future work may combine this with long‐lived nuclear spin memories, which makes the investigated co‐doped membranes a promising platform for quantum repeaters and distributed quantum computers.

Spectral Multiplexing of Rare-Earth Emitters Individual erbium dopant atoms (yellow) in crystals made from other elements can be detected by measuring fluorescence light after resonant excitation with a laser. When the dopants are located at a site that is neighboring another dopant atom (blue), the resonance frequency is changed because of the mutual interaction. This allows one to tailor the spectral distribution of the emitters even at very low concentrations, such that correspondingly co-doped crystals can be better used for quantum information applications. For further details, see article number 2302897 by Andreas Reiserer and co-workers. [Cover image by C. Hohmann, MCQST.]

In a quantum network, coherent emitters can be entangled over large distances using photonic channels. In solid-state devices, the required efficient light-emitter interface can be implemented by confining the light in nanophotonic structures. However, fluctuating charges and magnetic moments at the nearby interface then lead to spectral instability of the emitters. Here, we avoid this limitation when enhancing the photon emission up to 70(12)-fold using a Fabry-Perot resonator with an embedded 19-micrometer-thin crystalline membrane, in which we observe around 100 individual erbium emitters. In long-term measurements, they exhibit an exceptional spectral stability of <0.2 megahertz that is limited by the coupling to surrounding nuclear spins. We further implement spectrally multiplexed coherent control and find an optical coherence time of 0.11(1) milliseconds, approaching the lifetime limit of 0.3 milliseconds for the strongest-coupled emitters. Our results constitute an important step toward frequency-multiplexed quantum-network nodes operating directly at a telecommunication wavelength.

Abstract Europium‐doped nanocrystals constitute a promising material for a scalable future quantum computing platform. Long‐lived nuclear spin states could serve as qubits addressed via coherent optical transitions. In order to realize an efficient spin‐photon interface, we couple the emission from a single nanoparticle to a fiber‐based microcavity under cryogenic conditions. The spatial and spectral tunability of the cavity permits us to place individual nanoparticles in the cavity, to measure the inhomogeneous linewidth of the ions, and to show a multi‐modal Purcell‐enhancement of two transition in Eu 3+ . A halving of the free‐space lifetime to 1.0 ms is observed, corresponding to a 140‐fold enhancement of the respective transition. Furthermore, we observe a narrow optical linewidth of 3.3 MHz for a few‐ion ensemble in the center of the inhomogeneous line. The results represent an important step towards the efficient readout of single Eu 3+ ions, a key requirement for the realization of single‐ion‐level quantum processing nodes in the solid state.

Atomic-like emitters in the solid state serve as important resources in the advancement of future quantum networks. In particular, intra-4f optical transitions of rare earth ions exhibit excellent coherence properties thanks to the shielding effect of outer electrons. Still, the presence of various dephasing channels in solid state hosts introduces additional decoherence beyond the radiative decay, causing the coherence time of most rare earth doped materials to be over an order of magnitude lower than the radiative limit. Such obstacle prevents the emission of indistinguishable photons from rare earth ions, which is an essential requirement for various quantum applications. In this work, we perform optical coherence study on erbium ions doped in thin-film lithium niobate by patterning photonic crystal resonators with quality factor around 100 k and sub-λ3 mode volume. Leveraging the combination of long coherence and strong Purcell enhancement in the cavity, we show that the coherence time measured with photon echo approaches the radiative limit (80%), representing a 50-fold improvement compared to the waveguide case. Our results present promising prospects toward utilizing rare earth doped materials as quantum repeaters and sources of indistinguishable photons.

Erbium incorporated in a ceria (CeO2) host offers strong potential as a spin qubit platform, providing a pathway toward telecom-compatible quantum memory. But erbium is a dim emitter due to its long excited state lifetime. Methods to enhance its emission rate and efficiency through nanophotonic cavity integration are therefore essential. In this work, we demonstrate cavity-enhanced emission from erbium doped CeO2. We couple colloidally synthesized erbium doped CeO2 nanocrystals to a silicon nanobeam cavity and show a 30-fold improvement in emitter brightness combined with a 2-fold lifetime enhancement. We estimate a lower bound of 12 for the Purcell factor of cavity-coupled emitters after accounting for non-radiative decay. These results pave the way towards utilizing colloidally synthesized optically addressable spin qubits emitting at telecom wavelength for quantum networking and distributed quantum computing.

Individual spins that are coupled to telecommunication photons offer unique promise for distributed quantum information processing once a coherent and efficient spin–photon interface can be fabricated at scale. We implement such an interface by integrating erbium dopants into a nanophotonic silicon resonator. We achieve spin-resolved excitation of individual emitters with <0.1GHz spectral diffusion linewidth. Upon resonant driving, we observe optical Rabi oscillations and single-photon emission with a 78-fold Purcell enhancement. Our results establish a promising platform for quantum networks.