Surveying optically addressable spin qubits for quantum information and sensing technology

TL;DR

This review surveys optically addressable spin qubits in FOAMs across 3D defects (diamond, SiC), 2D van der Waals hosts (e.g., hBN), and molecular systems (p-, d-, f-block). It analyzes key spin parameters—$T_1$, $T_2$, and $T_2^*$—and how synthesis, defect concentration, and environment govern performance, emphasizing ODMR as a central readout modality and the role of dynamic decoupling and clock transitions in extending coherence. The authors benchmark representative platforms (NV and group-IV centers in diamond and SiC, hBN defects, Pc:PTP and Cr$^{4+}$ complexes, and lanthanide spins) and distill trends that room-temperature operation favors light-element hosts with strong optical cycling, while heavy-atom systems exhibit longer lifetimes at cryogenic temperatures. They argue that cross-platform knowledge transfer and data-driven materials discovery will accelerate device-ready qubits for sensing and quantum optics, with implications for scalable quantum information processing. Overall, the paper provides a structured map of FOAM candidates, their performance envelopes, and actionable strategies to push toward practical quantum technologies.

Abstract

Quantum technologies offer ways to solve certain tasks more quickly, efficiently, and with greater precision than their classical counterparts. Yet substantial challenges remain in the construction of sufficiently error-free and scalable quantum platforms needed to unlock any real benefits to society. Acknowledging that this hardware can take vastly different forms, our review here focuses on so-called spintronic (\textit{i.e.}~spin-electronic) materials that use electronic or nuclear spins to embody qubits. Towards helping the reader to spot trends and pick winners, we have surveyed the various families of optically addressable spin qubits and attempted to benchmark and identify the most promising ones in each. We go on to reveal further trends that demonstrate how qubit lifetimes depend on the material's synthesis, the concentration/distribution of its embedded qubits, and the experimental conditions.

Figures (10)

• Figure 1: Quantum spin parameters important for quantum technology. (a) Bloch-sphere representation of a spin vector showing a quantum state composed of $|\psi\rangle = (|0\rangle + |1\rangle)/\sqrt{2}$. (b) Spin-lattice relaxation $T_1$ transforms an ensemble of polarised spins to a Boltzmann (thermal) spin distribution, approximated by the equation shown, where N$_{|0\rangle/|1\rangle}$ is the population of each state, $\Delta$E$_{|0\rangle-|1\rangle}$ is the energy difference between each state, $k_\textrm{B}$ is the Boltzmann constant, and $T$ is temperature. (c) Bloch sphere representing an incoherent collective superposition for an ensemble of spins after time $T_2$ has elapsed. (d) Homogeneous spin-resonance linewidth, demonstrating the impact of lowering $T_2$, and (e) the inhomogeneous linewidth broadened by neighbouring ($\alpha$ and $\beta$) magnetic spins; $\nu$ and $B_0$ represent the microwave frequency and magnetic field, respectively, as commonly used independent variables in quantum sensing.
• Figure 2: Prominent approaches for addressing quantum spins for quantum applications (inner circle) and their applications in quantum technologies (outer circle). Optically-detected magnetic resonance (ODMR) spin measurements utilise spin-dependent luminescence with light/microwave-based spin manipulation (green third), whilst Electron Paramagnetic Resonance (EPR) spin measurements can employ light to initialise a spin system into a spin-polarised state (pink third) or one can employ microwaves to manipulate a thermally polarised system with spin-dependent microwave readout (blue third).
• Figure 3: A collection of selected quantum spin parameters measured using pulsed-ODMR techniques, including (a) spin-lattice relaxation, (b) spin coherence, (c) spin dephasing times, and (d) optical contrast. Note: not every parameter has been reported for each system and conditions, hence not every material will be visible on all four plots. In each case, we have opted to include the highest measured parameter using standard inversion/saturation recovery, 3-pulse echo, Ramsey technique, regardless of appplied magnetic field. Data adapted from: 0.1$\%$ Pc:PTP Singh2025Mena2024, M$_2$TTM-3FIr-M$_2$TTM Chowdhury2024, CH$_3$-m(TTM)$_2$Kopp2025, 2,2$^{\prime}$-dinaphthylcarbene Roggors2025, 1,2,4,5-tetrachlorobenzene Breiland1975, Cr$^{4+}$ molecular systems Bayliss2020baylissEnhancingSpinCoherence2022; Rh$^{3+}$ molecular systems Glasbeek2001, NV-diamond liuCoherentQuantumControl2019takahashiQuenchingSpinDecoherence2008, isotopically enriched NV-diamond Balasubramanian2009, SiV$^-$diamond Green2017Sukachev2017 SnV$^-$diamond rosenthalMicrowaveSpinControl2023, GeV$^-$diamond Senkalla2024; V$_\textrm{B}^-$ in hBN gottschollRoomTemperatureCoherent2021, carbon defects in hBN Scholten2024sternQuantumCoherentSpinChejanovsky2021, monovacancies in 4H-SiC Nagy2019Babin2022, isotopically purified SiC Lekavicius2022, divacancies in 4H-SiC Christle2015Crook2020Li2022Yan2020Seo2016linTemperatureDependenceDivacancy2021, N$_\textrm{C}$V$_\textrm{Si}$ (hh) in 4H-SiC Wang2020b, N$_\textrm{C}$V$_\textrm{Si}$ (kk) in 4H-SiC Jiang2023, Cr$^{4+}$ in 4H-SiC koehlResonantOpticalSpectroscopy2017, Mo$^{5+}$ in 6H-SiC Gilardoni2020, V$^{4+}$ in 4/6H-SiC Wolfowicz2020, Cr$^{4+}$ in GaN Koehl2017, Eu$^{3+}$ in Y$_{2}$O$_{3}$Zhong2019, Er$^{3+}$ in Y$_{2}$O$_{3}$bottgerOpticalDecoherenceSpectroscopy2024, Y$_{2}$SiO$_{5}$Rancic2018, KTP Bottger2016, and LiNbO$_4$Thiel2010bottgerOpticalDecoherenceSpectroscopy2024; Pr$^{3+}$ in Y$_{2}$SiO$_{5}$equallHomogeneousBroadeningHyperfine1995 and La$_2$(WO$_4$)$_3$Lovric2011; Yb$^{3+}$ in Y$_{2}$SiO$_{5}$Lim2018 and YAG Bottger2016b; Ce$^{3+}$:YAG Azamat2017Belykh2021. Rb in solid Ne dargyteOpticalSpincoherenceProperties2021, T-centre in $^{28}$Si Bergeron2020, EYFP protein Feder2024, F-centre in CaO Glasbeek1981.
• Figure 4: Selected spin defects in diamond. (a) Jablonski diagram for NV-diamond as a prototypical example of an optically addressable material; (b) low temperature fluorescence spectrum for SiV$^-$, SnV$^-$ and GeV$^-$ singlet spin centres showing electronic transitions between electronic and spin-orbit energy levels used for optically addressing spins; (c) example defect structure and energy level diagram for SiV$^-$ and (d) GeV$^-$ spin centres. Fluorescence data adapted from Karapatzakis2024Trusheim2020Muller2014
• Figure 5: Optically addressable point defects in SiC. (a) Structures of the most commonly studied SiC polytypes, 3C-SiH, 4H-SiC and 6H-SiC showing the origin of divancancy nomenclature depending on the lattice substitution site; (b) 2D-depiction of selected SiC ODMR-active defects. (c) Photoluminescence spectrum from a SiC ensemble showing multiple individually addressable defect types (PL1-PL6). (d) Orbital distribution for the ground and excited states of the V$_B^-$-defect with the corresponding quartet spin-level energy diagram with and without an applied magnetic field. Red and blue arrows indicate optical transitions that connect m$_\textrm{s}$=$|1/2\rangle$ and m$_\textrm{s}$=$|3/2\rangle$ states, respectively. (e) ODMR spectrum of 4H-SiC samples at 20 K and (f) 300 K (inset optical initialisation scheme), demonstrating the robust spin-optical properties of the PL8-defect (the highest contrast peak), (g) the corresponding room-temperature Hahn-echo decay trace for PL8. Figures 1a, 1b, 1c, 1d, 1e-g were reproduced from references Davidsson2018Bathen2021Wolfowicz2017Nagy2019Yan2020, respectively, with permission from MDPI and the American Chemical Society under the open access Creative Commons licence.