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arXiv:2610.39713

I$_4$ complex acting as near-infrared emitter with a quantum memory in silicon

Miaomiao Han, Song Li, Gergő Thiering, Thomas Frauenheim, Péter Deák, Bing Huang, Ádám Gali

Published Oct 29, 2026• 0 citations
arXivHTMLPDF

Abstract

Silicon color centers emitting in the near infrared offer a route to spin--photon interfaces compatible with mature complementary metal--oxide--semiconductor fabrication. A key unresolved candidate is the X-line at 1039.7 meV in the photoluminescence (PL) spectrum, long attributed to a complex of four self-interstitials (I$_4$), yet whose microscopic structure and spin properties have remained controversial and uncharacterized at a predictive level. Using hybrid-functional (HSE06) density functional theory, we unambiguously establish the bare I$_4$ complex as the origin of the X-line: our calculations quantitatively reproduce the zero-phonon line, the complete phonon sideband structure---identifying the prominent secondary peaks as one- and two-phonon replicas of strongly coupled quasi-local modes---and the EPR hyperfine constants. The charge transition level is found at VBM$+$0.08~eV, conclusively ruling out the previously proposed association with the B3 EPR center and its correlated deep donor level. Most significantly, we identify a metastable spin-1 triplet excited state separated from the singlet by $\sim$1.6~meV, consistent with the NL51 EPR signal. The $D_{2d}$ symmetry of the defect suppresses intersystem crossing and phosphorescence by first-order spin--orbit coupling, implying an exceptionally long spin--lattice relaxation time. Combined with a spectrally resolvable zero-field splitting and near-infrared emission, the triplet state of the I$_4$ complex constitutes a viable spin quantum memory intrinsic to silicon.

DOI
10.48550/arxiv.2610.39713

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I4_44​ complex acting as near-infrared emitter with a quantum memory in silicon
Silicon color centers emitting in the near infrared offer a route to spin--photon interfaces compatible with mature complementary metal--oxide--semiconductor fabrication. A key unresolved candidate is the X-line at 1039.7 meV in the photoluminescence (PL) spectrum, long attributed to a complex of four self-interstitials (I4_44​), yet whose microscopic structure and spin properties have remained controversial and uncharacterized at a predictive level. Using hybrid-functional (HSE06) density functional theory, we unambiguously establish the bare I4_44​ complex as the origin of the X-line: our calculations quantitatively reproduce the zero-phonon line, the complete phonon sideband structure---identifying the prominent secondary peaks as one- and two-phonon replicas of strongly coupled quasi-local modes---and the EPR hyperfine constants. The charge transition level is found at VBM+++0.08eV, conclusively ruling out the previously proposed association with the B3 EPR center and its correlated deep donor level. Most significantly, we identify a metastable spin-1 triplet excited state separated from the singlet by ∼\sim∼1.6meV, consistent with the NL51 EPR signal. The D2dD_{2d}D2d​ symmetry of the defect suppresses intersystem crossing and phosphorescence by first-order spin--orbit coupling, implying an exceptionally long spin--lattice relaxation time. Combined with a spectrally resolvable zero-field splitting and near-infrared emission, the triplet state of the I4_44​ complex constitutes a viable spin quantum memory intrinsic to silicon.
Nuclear spin qubits in silicon offer exceptionally coherent quantum memory, and optically-interfaced spins are a promising platform for both quantum networking and distributed quantum computing. It has been proposed that emitters with diamagnetic ground states may permit an optical interface to nuclear spin memories via metastable, hyperfine-coupled excited states while suppressing key sources of decoherence. Until now, direct optical observation of suitable transitions in silicon colour centres has remained elusive. Here we characterize the singly-ionized interstitial aluminum donor (Ali+_\mathrm{i}^+i+​) in isotopically purified 28^{28}28Si and find several novel features of this little-studied defect. We measure bright emission and strong optical transitions and, in contrast to previous studies of this centre, attribute its emission to an exchange-split spin triplet and singlet level of the lowest-energy 1s:T2_22​ excited state. We measure the excited-state lifetimes and, as a consequence of its narrow emission linewidth, observe the fine and hyperfine structure of the long-lived triplet state. This constitutes the first measurement of an optically-resolved hyperfine structure in the excited state of a telecommunications-band silicon colour centre.
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We have investigated the optical properties of the ${(\mathrm{NV})}^{\ensuremath{-}}$ center in 3C-SiC to determine the photoluminscence zero phonon line (ZPL) associated with the $^{3}\mathrm{E}\ensuremath{\rightarrow}^{3}\mathrm{A}_{2}$ intracenter transition. Combining electron paramagnetic resonance and photoluminescence spectroscopy, we show that the ${\mathrm{NV}}^{\ensuremath{-}}\phantom{\rule{0.28em}{0ex}}\mathrm{center}$ in 3C-SiC has a ZPL line at 1.468 $\ensuremath{\mu}\mathrm{m}$ in excellent agreement with theoretical predictions. The ZPL line can be observed up to T=100T=100T=100 K. The negatively charged NV center in 3C-SiC is the structural isomorphe of the NV center in diamond and has equally a spin S=1S=1S=1 ground state and a spin S=1S=1S=1 excited state, long spin lattice relaxation times and presents optically induced groudstate spin polarization. These properties make it already a strong competitor to the NV center in diamond, but as its optical domain is shifted in the near infrared at $1.5\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$, the NV center in 3C-SiC is compatible with quantum photonic networks and silicon based microelectronics.
Realization of quantum memory with a photonic interface in the telecommunication bands in a wafer-scalable platform is a central requirement for long-distance quantum networks. Silicon carbide (SiC) provides a technologically mature host for integrated quantum photonics, yet only a limited number of defects combine spin functionality with telecom emission. Here we report on chlorine-based defects in 4H-SiC as a platform for telecom-band quantum memory. The emission of these defects spans the entire telecommunication range with zero-phonon lines in the O- and C-bands and a Debye-Waller factor of up to 39 %39 \, \%39%. Time-resolved photoluminescence measurements reveal a short excited-state lifetime in the sub-nanosecond range. We demonstrate that these defects are spin-active even at room temperature, exhibiting optically detected magnetic resonances (ODMR) in the sub-GHz frequency range. Using ODMR spectroscopy and Ramsey interferometry, we resolve the hyperfine structure arising from the interaction with 35Cl^{35}\mathrm{Cl}35Cl nuclear spins. The ODMR spectra exhibit complex behaviour in an external magnetic field due to mixing of electron-nuclear spin states, which is well reproduced by our simulations. The spin relaxation and coherence times are in the sub-microsecond range, limited by rapid quenching of the ODMR contrast and attributed to charge-state metastability. The combination of telecom-band emission, coherent spin control and compatibility with wafer-scale fabrication positions Cl-related defects in SiC as a promising platform for chip-scale quantum memories with spin-photon interfaces operating in the fiber-optic telecommunication windows.
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