Their exceptional coherence makes nuclear spins in solids a prime candidate for quantum memories in quantum networks and repeaters. Still, the direct all-optical initialization, coherent control, and readout of individual nuclear spin qubits have been an outstanding challenge. Here, this is achieved by embedding 167-Er dopants in yttrium orthosilicate in a cryogenic Fabry-Perot cavity, whose linewidth of 65 MHz is much smaller than the 0.9 GHz separation of neighboring hyperfine levels. Frequency-selective emission enhancement thus enables a single-shot readout fidelity of 91(2)%. Furthermore, a large magnetic field freezes paramagnetic impurities, leading to coherence times exceeding 0.2 s. The combination of nuclear-spin qubits with frequency-multiplexed addressing and lifetime-limited photon emission in the minimal-loss telecommunications C-band establishes 167-Er as a leading platform for long-range, fiber-based quantum networks.
Their exceptional coherence makes nuclear spins in solids a prime candidate for quantum memories in quantum networks and repeaters. Still, the direct all-optical initialization, coherent control, and readout of individual nuclear spin qubits have been an outstanding challenge. Here, this is achieved by embedding 167-Er dopants in yttrium orthosilicate in a cryogenic Fabry-Perot cavity, whose linewidth of 65 MHz is much smaller than the 0.9 GHz separation of neighboring hyperfine levels. Frequency-selective emission enhancement thus enables a single-shot readout fidelity of 91(2)%. Furthermore, a large magnetic field freezes paramagnetic impurities, leading to coherence times exceeding 0.2 s. The combination of nuclear-spin qubits with frequency-multiplexed addressing and lifetime-limited photon emission in the minimal-loss telecommunications C-band establishes 167-Er as a leading platform for long-range, fiber-based quantum networks.
The ability to coherently control and read out qubits with long coherence times in a scalable system is a crucial requirement for any quantum processor. Nuclear spins in the solid state have shown great promise as long-lived qubits. Control and readout of individual nuclear spin qubit registers has made major progress in the recent years using individual electron spin ancilla addressed either electrically or optically. Here, we present a new platform for quantum information processing, consisting of 183W nuclear spin qubits adjacent to an Er3+ impurity in a CaWO4 crystal, interfaced via a superconducting resonator and detected using a microwave photon counter at 10mK. We study two nuclear spin qubits with T2∗ of 0.8(2)s and 1.2(3)s, T2 of 3.4(4)s and 4.4(6) s, respectively. We demonstrate single-shot quantum non-demolition readout of each nuclear spin qubit using the Er3+ spin as an ancilla. We introduce a new scheme for all-microwave single- and two-qubit gates, based on stimulated Raman driving of the coupled electron-nuclear spin system. We realize single- and two-qubit gates on a timescale of a few milliseconds, and prepare a decoherence-protected Bell state with 88% fidelity and T2∗ of 1.7(2)s. Our results are a proof-of-principle demonstrating the potential of solid-state nuclear spin qubits as a promising platform for quantum information processing. With the potential to scale to tens or hundreds of qubits, this platform has prospects for the development of scalable quantum processors with long-lived qubits.
Optically active solid-state spin qubits thrive as an appealing technology for quantum interconnects and quantum networks, thanks to their atomic size, scalable synthesis, long-lived coherence, and ability to coherently interface with flying qubits. Trivalent erbium dopants, in particular, emerge as an attractive candidate due to their emission in the telecom C band and shielded 4f intra-shell spin and optical transitions. Nevertheless, prevailing top-down architectures for rare-earth qubits and devices have not yet achieved simultaneous long optical and spin coherence, which is necessary for efficient long-distance quantum networks. Here, we demonstrate dual Er3+ telecom spin-photon interfaces in two distinct lattice symmetry sites within an epitaxial thin-film platform. By leveraging high matrix crystallinity, controlled proximity of dopants to surfaces, and exploiting host lattice symmetry, we simultaneously achieve kilohertz-level optical linewidth in a strongly symmetry-protected site, and erbium qubit spin coherence times exceeding 10 milliseconds. Additionally, we realize single-shot readout and microwave coherent control of erbium qubits in a fiber-integrated package, enabling rapid deployment and scalability. These advancements highlight the significant potential of high-quality rare-earth qubits and quantum memories assembled using a bottom-up method, paving the way for scalable development of quantum light-matter interfaces tailored for telecommunication quantum networks. Erbium quantum emitters operating in the telecom C band are promising for spin-photon interfaces but achieving both optical and spin coherence has been challenging. Gupta et al. report two types of erbium dopants in epitaxial Y2O3 thin films occupying distinct lattice sites and exhibiting long spin and optical coherence times
Quantum networking is a crucial technology towards constructing large-scale distributed quantum systems. The structure of photonic link and distributed network nodes allows high-speed and high-fidelity interconnect, enabling various applications such as distributed quantum computing and sensing, as well as secure quantum communications. Optically interfaced solid-sate spins are promising candidates for building a quantum network due to their active spin-photon interfaces, compatibility, and scalability for device integration. Especially for defect spins in silicon, the platform benefits from mature silicon photonic industry, and can access to the integration techniques developed over the past decades. T center in silicon particularly holds great expectations because of the telecom-O band optical interface, ground state doublet spin manifold, and long-lived nuclear spin coherence. Their telecom spin-photon interface provides a direct link between stationary quantum memory and flying qubits that are compatible with the existing telecommunication fiber network infrastructure. This work focuses on investigating cavity quantum electrodynamics (cQED) for cavity-coupled single T centers, its applications in quantum optical control of T center optical properties, and in realizing single-shot spin state readout. The cavity enhanced light-matter interaction mitigates the caveats of T center, such as long radiative lifetime, low Debye-Waller factor, and lack of polarization selection rule for the optical transitions. In this work, by integrating single T centers with a low-loss, small mode volume silicon photonic cavity, we have achieved the Purcell enhancement for boosting the T center fluorescence rate with a factor of 6.89. We build up the theoretical framework based on the Jaynes-Cummings model and Lindblad master equation to explain experimental observations and reveal cQED parameters. The further study on the electronic spin state readout is carried out as the fast, high-fidelity single-shot readout is critical for quantum information processing. Two methods are proposed based on realistic cQED parameters: resonant fluorescence readout in the low-cooperativity regime and spin-dependent cavity-reflection readout for intermediate- and high-cooperativity regimes. Both approaches demonstrate feasibility of high-fidelity (F > 0.95) readout, particularly the reflection readout allows F > 0.99 within a short readout time of 50 μs. This work demonstrates a promising telecom spin-photon interface with high-fidelity readout based on cavity-coupled single T centers in silicon, which are promising candidates for building future large-scale quantum networks.
Echo-based spectroscopy of the superhyperfine interaction of an electronic spin with nuclear spins in its surroundings enables detailed insights into the microscopic magnetic environment of spins in solids. Still, it is an outstanding challenge to resolve individual nuclear spins in a dense bath, in which many of them exhibit a comparable coupling strength. This simultaneously requires a high spectral resolution and a large signal-to-noise ratio. However, when probing spin ensembles, dipolar interactions between the dopants can lead to a concentration-dependent trade-off between resolution and signal. Here, we fully eliminate this limitation of previous optical-echo-envelope-modulation spectroscopy experiments by integrating the emitters into a high-finesse resonator, which allows for strong optical echoes even at very low concentrations. To demonstrate its potential, the technique is applied to erbium dopants in yttrium orthosilicate ( Er : YSO ). Achieving an unprecedented spectral resolution enables precise measurements of the superhyperfine interaction with four of the Y nuclear spins densely surrounding each emitter. The achieved boost of the signal, enabled by the resonator, allows for extending the approach to the lowest concentration possible—to the level of single dopants, thereby providing a tool for detecting and studying individual nuclear spins. Thus, our technique paves the way for an improved understanding of dense nuclear-spin baths in solids.
Quantum networks will enable the transmission of quantum information over long distances, allowing us to link quantum computers for applications such as quantum cryptography, astronomical interferometry, and blind quantum computing. The silicon-vacancy center in diamond has emerged as a promising scalable solid-state platform capable of efficiently interfacing its electron spin with photons for long-range communication. This thesis investigates two long-lived nuclear spin qubits as quantum memories to significantly expand the quantum state storage and processing capabilities of this networking node. First, we demonstrate that selective isotope implantation gives a deterministic 29Si high-fidelity nuclear memory with coherence times over two seconds, enabling its integration into a two-node quantum network through 35 km of optical fiber deployed through the Boston metropolitan area. Next, we explore the control of a nearby 13C nuclear spin robust against repeated readouts and resets of the electron networking qubit, and utilize it to show a quadratic rate enhancement for entanglement swapping between two incoming photonic qubits. Together, these results advance our three-qubit node toward realistic operation as a quantum repeater to boost entanglement rates in a long-range network.
Individually addressed Er 3+ ions in solid-state hosts are promising resources for quantum repeaters, because of their direct emission in the telecom band and their compatibility with silicon photonic devices.While the Er 3+ electron spin provides a spin-photon interface, ancilla nuclear spins could enable multiqubit registers with longer storage times.In this work, we demonstrate coherent coupling between the electron spin of a single Er 3+ ion and a single I = 1/2 nuclear spin in the solid-state host crystal, which is a fortuitously located proton ( 1 H).We control the nuclear spin using dynamical-decoupling sequences applied to the electron spin, implementing one-and two-qubit gate operations.Crucially, the nuclear spin coherence time exceeds the electron coherence time by several orders of magnitude, because of its smaller magnetic moment.These results provide a path toward combining long-lived nuclear spin quantum registers with telecom-wavelength emitters for long-distance quantum repeaters.