Software Framework for Optically Accessible Quantum Memory Using Group-IV Color Centers in Diamond

TL;DR

The paper presents a standardized software framework for modeling optically accessible quantum memories based on group-IV color centers in diamond coupled to nanophotonic Sawfish cavities. It develops a detailed spin–photon interaction model, derives Kraus operators for read-in and read-out, and provides an open-source module (code_strocka_qm) that can optimize cavity parameters, control pulses, and environmental settings to benchmark memory performance for quantum networks. By comparing optical and microwave spin control across SiV$^{-}$ and SnV$^{-}$ centers and performing extensive numerical sweeps, the work demonstrates how memory fidelity and photon loss depend on bandwidth, temperature, and cooperativity, offering actionable insights for device design. The framework enables benchmarking, design optimization, and integration of quantum memories into repeater, QRAM, and secure-communication architectures, with ample room for extension to additional color centers and memory modalities.

Abstract

In the rapidly evolving field of quantum technology, the precise and detailed description of quantum components is not just a necessity but the foundation for advancing research, development, and applications. Optically accessible quantum memories are key building blocks for devices such as quantum repeaters and two-factor authentication. The memory we describe here is based on a tin-vacancy color center coupled to a highly efficient cavity. It leverages state-dependent reflection from the cavity and implements high-fidelity fractional single qubit gates via a train of optical $π/8$ pulses. We also describe its operation under microwave control, further extending our analysis. Our primary contribution in this work is the integration of this device model into a standardized software framework for quantum memory architectures.

Figures (3)

• Figure 1: a) A time-bin qubit $|\psi\rangle$ is stored in a Sawfish bopp_sawfish_2024pregnolato_fabrication_2024 spin-photon interface that hosts a G4V. Storage is achieved via the electronic spin. Fast switches (S) are deployed throughout the circuit to enable efficient measurement and routing of the photonic state. The $X$-basis measurement is performed using a fast switch integrated with an imbalanced Mach-Zehnder interferometer. The storage procedure is completed once the photonic qubit emitted from the single-photon source (SPS 1) is measured in the $X$-basis. For retrieval, the spin state is read out by entangling it with a photon from an additional single-photon source (SPS 2) after a time-bin preparation stage (TBPS) Lee2018Bouchard2022Yu2025. A subsequent $Z$-basis measurement on the spin is performed. b) Using the SiV$^-/$SnV$^{-}$ center as a G4V, spin-photon entanglement is mediated through a Sawfish cavity-to-fiber interface bopp_sawfish_2024pregnolato_fabrication_2024. The SiV$^-$/SnV$^{-}$ is modeled as a four-level system characterized by its spontaneous emission rates $\gamma_{1A},\gamma_{2B}$, spectral contrast $\Delta\omega_s=\omega_{2B}-\omega_{1A}$ spin splitting $\omega_s$, and atomic transition frequencies $\omega_{1A},\omega_{2B}$, while interacting with a cavity mode $a_c$ at frequency $\omega_c$ for the spin-dependent reflection steps. c) Entanglement is generated employing a reflection scheme containing spin-dependent reflection steps and a $\pi/2$ rotation where the spin is initialized in the state $|1\rangle$. The $\pi/2$ rotation is applied to the spin before the final reflection event. This rotation can be implemented either using a sequence of four $\pi/8$ optical Raman pulses or utilizing microwave control. Figures b) and c) are adopted from strocka_token_2025.
• Figure 2: Visualization of the infidelity $1-\langle F\rangle$ and trace $\langle {\rm tr}(\rho_{\rm ph})\rangle$ of the photonic qubit after read-out (see Appendix, Sec. 5 for details) as a function of the system's nanophotonic temperature $T$ assuming the bandwidth of the photonic qubit for the read-in and read-out process is $\gamma_{\rm in,out}=0.1$ GHz, the AC magnetic field for microwave control of the SiV$^{-}$ (SnV$^{-}$) is $B_{\rm ac}=3.7 (1.0)$ mT, the axial strain is $E_x=4\cdot 10^{-5} (0)$ and the shear strain is $\epsilon_{xy}=0 (0)$ for the SiV$^{-}$ (SnV$^{-}$), the fidelity of the auxiliary photon source for read-out is $F_{\rm aux}=1.0$ for all the considered cases and we limit the cooperativity to $C_{\rm max}=12.5$.
• Figure 3: a) Visualization of the infidelity as a function of the bandwidth for the SiV$^{-}$ and SnV$^{-}$ for the read-in and b) read-out process. For the SnV$^{-}$ the trace is ${\rm tr}(\rho_{\rm ph})\approx 0.94$ and for the SiV$^{-}$ it is $\langle {\rm tr}(\rho_{\rm ph})\rangle= 0.80$ on the considered range. c) Visualization of the infidelity as a function of the maximal allowed cooperativity assuming all microwave control for the SnV$^{-}$ with $B_{\rm dc}=0.3$ T, $B_{\rm ac}=1$ mT in a low strain environment at $T=0.1$ K for $\gamma_{\rm in,out}=1$ GHz. The average trace reduction on that range is $\langle {\rm tr}(\rho_{\rm ph})\rangle\approx 0.9580$. Due to a rather noisy dependence of the trace reduction on the cooperativity we omit that graph here. For the SiV$^{-}$ the optimal cooperativity is below $12.5$ leading to no improvement on the considered range which is why we omit the SiV$^{-}$ representation here. d) Visualization of the infidelity as a function of the infidelity of the auxiliary photon source for read-out considering the SiV$^{-}$ as defect center. The trace is $\langle {\rm tr}(\rho_{\rm ph})\rangle=0.7812$ on the considered range.