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npj Computational Materials

An L-band emitter with quantum memory in silicon

Péter Udvarhelyi, Anton Pershin, Péter Deák, Ádám Gali

Published Dec 29, 2022• 22 citations
Publisher

Abstract

Abstract Fluorescent centres in silicon have recently attracted great interest, owing to their remarkable properties for quantum technology. Here, we demonstrate that the C-centre in silicon can realise an optically readable quantum register in the L-band wavelength region where the transmission losses in commercial optical fibres are minimal. Our in-depth theoretical characterisation confirms the assignment of the C-centre to the carbon-oxygen interstitial pair defect. We further explore its magneto-optical properties, such as hyperfine and spin-orbit coupling constants from first principles calculations, which are crucial for tight control of the quantum states of the triplet electron spin. Based on this data, we set up quantum optics protocols to initialise and read out the quantum states of the electron spin, and realise a quantum memory by transferring quantum information from the electron spin to proximate 29Si nuclear spins. Our findings establish an optically readable long-living quantum memory in silicon where the scalability of qubits may be achieved by CMOS-compatible technology.

DOI
10.1038/s41524-022-00957-7

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Abstract Fluorescent centres in silicon have recently attracted great interest, owing to their remarkable properties for quantum technology. Here, we demonstrate that the C-centre in silicon can realise an optically readable quantum register in the L-band wavelength region where the transmission losses in commercial optical fibres are minimal. Our in-depth theoretical characterisation confirms the assignment of the C-centre to the carbon-oxygen interstitial pair defect. We further explore its magneto-optical properties, such as hyperfine and spin-orbit coupling constants from first principles calculations, which are crucial for tight control of the quantum states of the triplet electron spin. Based on this data, we set up quantum optics protocols to initialise and read out the quantum states of the electron spin, and realise a quantum memory by transferring quantum information from the electron spin to proximate 29Si nuclear spins. Our findings establish an optically readable long-living quantum memory in silicon where the scalability of qubits may be achieved by CMOS-compatible technology.
Abstract Optically active spin systems in silicon are attracting growing interest for quantum technologies, driven by their potential for large-scale photonic integration, long spin coherence times, and compatibility with CMOS fabrication. Efficient spin-photon interfaces, which allow coupling between spin degrees of freedom and photons are crucial for quantum networks, enabling entanglement distribution and information transfer over long distances. While a variety of optically active quantum emitters in silicon have been investigated, no spin-active defect with optical transitions in the telecom L-band (1565–1625 nm)—a key wavelength range for low-loss fiber-based communication—has been experimentally demonstrated. Here, we demonstrate the optical detection of spin states in the C center, a carbon-oxygen defect in silicon that exhibits a zero-phonon line at 1571 nm. By combining optical excitation with microwave driving, we achieve optically detected magnetic resonance (ODMR), enabling spin-state readout via telecom-band optical transitions. These findings provide experimental validation of recent theoretical predictions and mark a significant step toward integrating spin-based quantum functionalities into silicon photonic platforms, paving the way for scalable quantum communication and memory applications in the telecom L-band.
Silicon-based quantum technologies have gained increasing attention due to their potential for large-scale photonic integration, long spin coherence times, and compatibility with CMOS fabrication. Efficient spin-photon interfaces are crucial for quantum networks, enabling entanglement distribution and information transfer over long distances. While several optically active quantum emitters in silicon have been investigated, no spin-active defect with optical transitions in the telecom L-band-a key wavelength range for low-loss fiber-based communication-has been experimentally demonstrated. Here, we demonstrate the optical detection of spin states in the C center, a carbon-oxygen defect in silicon that exhibits a zero-phonon line at 1571 nm. By combining optical excitation with microwave driving, we achieve optically detected magnetic resonance, enabling spin-state readout via telecom-band optical transitions. These findings provide experimental validation of recent theoretical predictions and mark a significant step toward integrating spin-based quantum functionalities into silicon photonic platforms, paving the way for scalable quantum communication and memory applications in the telecom L-band.
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.
Color centers in silicon are emerging as promising platforms for quantum technologies. Among them, the G center has attracted considerable interest owing to its bright telecom O-band single-photon emission and its optically addressable metastable electron-spin triplet state. Here we investigate the spin properties of ensembles of G centers under above-band-gap excitation. We elucidate the spin photo-dynamics giving rise to the optical detected magnetic resonance (ODMR) response of G centers. The optimal pulsed sequence for measuring the ODMR spectrum of the G defects is identified, along with the temperature and optical-power regimes maximizing the spin readout contrast. Through magneto-optical measurements, we detect a level-anticrossing of the G center electron spin states. At last, we demonstrate coherent spin control of the defects, and characterize their spin-coherence properties. Unveiling the spin degree of freedom of the G center opens new avenues for the realization of quantum memories and quantum registers based on silicon color centers.
Combining the long-coherence of spin qubits and the capability to transmit information and entanglement through photons, spin-photon interfaces (SPIs) are a promising platform for networked quantum computation and long-distance quantum communication. SPIs that possess local `memory' qubits in addition to the optically coupled `communication' qubit can improve remote entanglement fidelities through brokered entanglement schemes and entanglement purification. In these schemes, it is critical to protect the memory qubit from decoherence during entanglement operations on the communications qubit. Silicon, a platform with mature microelectronic and nanophotonic fabrication, is host to the T centre, an SPI with emission in the telecommunications O-band that directly integrates with silicon nanophotonics. Cavity-coupled T centres are a platform for brokered entanglement distribution in silicon photonic circuits and over long-distance optical fibre links. The T centre's electron and nuclear spin qubits are an intrinsic register of communication and memory qubits respectively, with anisotropic hyperfine coupling. In this work we determine the T centre's hydrogen hyperfine coupling tensor. We also introduce schemes to protect against dephasing or eliminate relaxation of the T centre's hydrogen memory qubit during optical excitation. These results address a key challenge for practical T centre quantum networks.
The performance of modular, networked quantum technologies will be strongly dependent upon the quality of their quantum light-matter interconnects. Solid-state colour centres, and in particular T centres in silicon, offer competitive technological and commercial advantages as the basis for quantum networking technologies and distributed quantum computing. These newly rediscovered silicon defects offer direct telecommunications-band photonic emission, long-lived electron and nuclear spin qubits, and proven native integration into industry-standard, CMOS-compatible, silicon-on-insulator (SOI) photonic chips at scale. Here we demonstrate further levels of integration by characterizing T centre spin ensembles in single-mode waveguides in SOI. In addition to measuring long spin T 1 times, we report on the integrated centres’ optical properties. We find that the narrow homogeneous linewidth of these waveguide-integrated emitters is already sufficiently low to predict the future success of remote spin-entangling protocols with only modest cavity Purcell enhancements. We show that further improvements may still be possible by measuring nearly lifetime-limited homogeneous linewidths in isotopically pure bulk crystals. In each case the measured linewidths are more than an order of magnitude lower than previously reported and further support the view that high-performance, large-scale distributed quantum technologies based upon T centres in silicon may be attainable in the near term.