Researchers at the Karlsruhe Institute of Technology (KIT) and collaborating institutions have achieved almost total light extinction, 96% contrast, when coupling tin-vacancy (SnV) centers within a diamond-like microcavity. This precise control over light transmission is a key step toward realizing efficient spin-photon interfaces for quantum networks. The work demonstrates cavity-enhanced light-matter coupling in a tunable Fabry-Pérot microcavity operating at temperatures down to 1 K, maximizing Purcell enhancement and delivering a 4.1(1) factor compared to 1.85(5) in air-like modes. The multi-institutional effort includes researchers from the University of Stuttgart, Ulm University and Fraunhofer Institute.
Tin-Vacancy Centers as Spin-Photon Interfaces for Quantum Networks
Resonant probing of tin-vacancy centers embedded within a diamond microcavity revealed a 96% extinction contrast, nearly eliminating light transmission and demonstrating precise control over light-matter interaction. This level of light blockage is unusual for solid-state quantum systems and signifies a substantial advancement in manipulating these qubits for potential quantum technologies. The experiment used an open Fabry-Pérot microcavity designed to maximize the coupling between the tin-vacancy centers and photons, enhancing the efficiency of this interaction.
Researchers achieved this by integrating low-roughness diamond membranes, concentrating the electromagnetic field within the diamond itself. The engineered microcavity operates as a “diamond-like” structure, differing from conventional designs and maximizing Purcell enhancement, a measure of the increased spontaneous emission rate of the tin-vacancy center. Measurements showed a Purcell factor of 4. 1(1) within the diamond-like mode, a more than two-fold increase compared to the 1. 85(5) observed in an “air-like” configuration.
This improvement simultaneously reduces the mechanical stability requirements for maintaining the delicate alignment of the cavity and emitter. The team, comprised of members from the Karlsruhe Institute of Technology and multiple other German institutions, demonstrated coherent cavity-emitter coupling with a coherent cooperativity of C = 4. 0(14), indicating a strong interaction between the light and the spin of the tin-vacancy center. Further manipulation with a magnetic field enabled spin-resolved cavity extinction, observing spin-selective optical transitions with a contrast of = 0. 91.
This means that the researchers could selectively block light transmission based on the spin state of the tin-vacancy center, allowing for better control and readout of quantum information. “These results establish SnV centers in diamond coupled to open Fabry-Pérot microcavities as a promising platform for efficient spin-photon interfaces,” the authors wrote in their published findings.
The work builds on previous demonstrations of quantum networks based on nitrogen-vacancy centers, aiming to overcome limitations in photon emission rates and improve heralded entanglement rates. The research highlights the potential of tin-vacancy centers as an alternative to nanophotonic approaches for building quantum networks, offering a clear path toward achieving high-fidelity spin-photon entanglement. The team’s success in creating a high-cooperativity coupling and spin-resolved extinction demonstrates a significant step toward realizing practical quantum network nodes capable of distributing entanglement between remote locations for applications in quantum communication and computing.
High-Cooperativity Coupling in Fabry-Pérot Microcavities
Achieving a Purcell enhancement of 4. 1(1) represents a leap in maximizing light-matter interaction with tin-vacancy centers within diamond, exceeding the 1. This improvement stems from a specifically engineered “diamond-like regime” within the Fabry-Pérot microcavity, concentrating the electromagnetic field inside the diamond membrane itself. The integration of low-roughness diamond membranes, with surface irregularities below 0. This approach simultaneously eases the stringent mechanical stability demands typically associated with maintaining delicate cavity alignment.
The demonstrated high cooperativity, quantified as C = 4. This value indicates that the rate of light-matter interaction is four times greater than the rate at which energy is lost from either the cavity or the emitter.
Diamond-Like Modes Enhance Purcell Factor to 4.1
Integrating low-roughness diamond membranes was essential to this process, enabling a maximized Purcell enhancement while simultaneously easing the stringent mechanical stability requirements often associated with such systems. Previous attempts at enhancing Purcell factors largely focused on “air-like” cavity configurations, where the light field resides in the air gap. While less sensitive to surface imperfections, these configurations inherently limit the achievable enhancement. This precise control over the interface is key, as diamond-like modes are particularly susceptible to scattering losses from surface imperfections.
The resulting improvement in the effective Purcell factor, reaching 4. 1 compared to 1. 85 in air-like configurations, demonstrates the efficacy of this approach. This near-total light extinction, the ability to almost completely block light transmission, is particularly noteworthy in solid-state quantum systems. The researchers state, establishing SnV centers coupled to open microcavities as a promising route toward scalable spin-photon interfaces.
The team’s success hinges on a detailed understanding of how the cavity mode interacts with the diamond membrane. By referencing the mode volume to the emitter position, the Purcell factor is influenced by the cavity finesse, beam waist and an intensity ratio quantifying the mode character. Diamond-like modes deliver a more than two-fold increase in the effective Purcell factor over air-like modes, reaching = 4.1(1) compared to = 1.85(5) in the air-like case, while simultaneously relaxing mechanical stability requirements.
This demanding requirement is achieved through the exceptionally low surface roughness of the fabricated membranes, minimizing scattering losses and maximizing the concentration of the field within the diamond. On-resonance lifetime measurements confirmed the Purcell-shortened lifetime at a diamond-like cavity position, further validating the approach.
96% Extinction Contrast Achieved with SnV Centers
This level of control surpasses previous demonstrations and establishes a promising pathway for building efficient spin-photon interfaces important for quantum networks. The high extinction rate, quantified by a coherent cooperativity of C = 4. Fabrication of the microcavity relied on diamond membranes, where the field is concentrated inside the diamond and Purcell enhancement is maximized, while simultaneously relaxing mechanical stability requirements. This improvement is not merely incremental. It relaxes the stringent mechanical stability requirements typically associated with high-cooperativity systems, simplifying the engineering challenges of scaling up these devices.
Further manipulation of the SnV center’s spin state through an applied magnetic field allowed researchers to observe spin-resolved cavity extinction. This capability is essential for creating quantum network nodes capable of transmitting and processing quantum information. The team also investigated the limits of this system, noting that the observed linewidth of the SnV center was twice the theoretically achievable minimum due to fast charge-noise-induced spectral diffusion. Extrapolating to ideal conditions, they predict a potential cooperativity of = 8. 0(5) and a contrast of = 0. 99(1).
Despite this limitation, the current performance, coupled with photon detection efficiencies reaching 10%, suggests that high-fidelity single-shot spin readout is within reach, even with off-axis magnetic fields. The ability to operate effectively with varied magnetic field orientations is a significant advantage.
Spin-Resolved Cavity Extinction via Magnetic Field Control
This high-contrast extinction arises from coherent coupling between the SnV center and the surrounding cavity, demonstrating a coherent cooperativity of C = 4. 0(14). 1, more than double the 1. This enhancement maximizes the rate at which the SnV center emits light, improving the efficiency of photon collection and reducing decoherence.
Consecutive cavity transmission scans, recorded with and without the applied magnetic field, clearly demonstrate the splitting of the extinction feature into two distinct branches corresponding to the two spin states. The superconducting coil used to generate the magnetic field was limited to a current of 9 amps, preventing superconductor quenching.
This controlled magnetic field, oriented fixed relative to the emitters, splits the single extinction feature into two spin-selective transitions, enabling optical spin readout and spin-dependent transmission or reflection of single photons from the cavity. The observed splitting and high contrast are critical for realizing efficient spin-photon interfaces and building scalable quantum network nodes.
Low-Roughness Diamond Membranes Enable Enhanced Coupling
Fabricating diamond membranes with sub-nanometer surface roughness, specifically, a root mean square roughness of 0. 2 nanometers, proved critical to achieving enhanced light-matter coupling, enabling operation within the diamond-like regime of the fabricated Fabry-Pérot microcavity. This level of surface finish minimized scattering losses and maximized Purcell enhancement, a phenomenon where the rate of spontaneous emission from an emitter is increased by placing it in a resonant optical cavity.
The team’s approach directly addressed a key limitation in previous work: maintaining picometer-scale stability of tunable microcavities at the low temperatures required for long-lived electron spin coherence. 91, demonstrating a high degree of control over the interaction between light and the SnV center’s spin.
The observed coherent cooperativity reached 4. The ability to discriminate between spin states with such high contrast is particularly significant for applications in quantum information processing, as it provides a pathway for efficient optical spin readout and the creation of spin-photon entanglement. The combination of low-roughness membranes, tunable microcavities and in-situ magnetic field control represents an advance in the field of solid-state quantum optics.
👉 More information
🗞 High-cooperativity coupling and spin-resolved extinction of tin-vacancy centers in a diamond-like microcavity
✍️ Kerim Köster et al.
🧠 DOI: http://link.aps.org/doi/10.1103/jhhs-kls1
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