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arXiv:2401.04465 · Physical Review Letters

High-Fidelity Optical Readout of a Nuclear-Spin Qubit in Silicon Carbide

Erik Hesselmeier, Pierre Kuna, Wolfgang Knolle, Florian Kaiser, Nguyên Tiên Són, Misagh Ghezellou, Jawad Ul‐Hassan, Vadim Vorobyov, Jörg Wrachtrup

Published May 2, 2024• 26 citations
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Abstract

Quantum state readout is a key requirement for a successful qubit platform. In this work, we demonstrate a high-fidelity quantum state readout of a V2 center nuclear spin based on a repetitive readout technique. We demonstrate up to 99.5% readout fidelity and 99% for state preparation. Using this efficient readout, we initialize the nuclear spin by measurement and demonstrate its Rabi and Ramsey nutation. Finally, we use the nuclear spin as a long-lived memory for quantum sensing application of a weakly coupled diatomic nuclear-spin bath.

DOI
10.1103/physrevlett.132.180804

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Quantum state readout is a key requirement for a successful qubit platform. In this work, we demonstrate a high-fidelity quantum state readout of a V2 center nuclear spin based on a repetitive readout technique. We demonstrate up to 99.5% readout fidelity and 99% for state preparation. Using this efficient readout, we initialize the nuclear spin by measurement and demonstrate its Rabi and Ramsey nutation. Finally, we use the nuclear spin as a long-lived memory for quantum sensing application of a weakly coupled diatomic nuclear-spin bath.
Robust entanglement at room temperature is a central challenge for solid-state quantum information processing and quantum-enhanced sensing. Here we demonstrate room-temperature storage of entanglement in a silicon carbide (SiC) quantum node by coherently transferring an electron-nuclear entangled state onto long-lived nuclear-spin memory qubits. Using a shallow single color center in 4H-SiC, conventionally denoted PL6, we realize a fully addressable three-qubit register composed of one electron-spin processor and two strongly coupled 29^{29}29Si nuclear-spin memory qubits. This platform enables the deterministic generation of high-fidelity entangled states, including a nuclear-spin Bell state with a fidelity of 94±2%94 \pm 2\%94±2% and a three-qubit Greenberger-Horne-Zeilinger (GHZ)-type state with a fidelity of 89±4%89 \pm 4\%89±4%. By implementing a SWAP-gate protocol in the strong hyperfine-coupling regime, the electron-nuclear entanglement is transferred to the nuclear-spin memory with a fidelity of 92.5±2.5%92.5 \pm 2.5\%92.5±2.5%, extending the entanglement lifetime by a factor of 240. We further confirm the generality of this approach in an additional heterogeneous 29^{29}29Si-13^{13}13C nuclear-spin register and, through a statistical survey of 200 single PL6 centers, show that multi-nuclear-spin registers occur naturally with probabilities above 10%. These results position shallow SiC color centers as a powerful platform for entanglement-assisted quantum sensing and scalable room-temperature quantum technologies.
Optically addressable spin defects are excellent candidate platform for quantum sensing and quantum network. Nuclear spins coupled to color centers naturally enable long lived quantum memories and local qubits registers. To fully leverage this potential precise characterization of the surrounding nuclear-spin environment augmented with refined DFT models is required. In this work, we report angstrom-level 3D localization of 25 nuclear spins around a single V2 center in 4H Silicon Carbide. Utilizing specially placed robust nuclear memory as a highly efficient readout ancilla for readout, we apply correlation based spectroscopy and by selecting multi-spin chains up to length four, we access and characterize extended nuclear spin cluster. Using the coupling map we reconstruct their couplings to the central electron spin and neighboring nuclei. This work paves the way towards advanced quantum register applications on Silicon Carbide platform.
We demonstrate high-fidelity repetitive measurements of nuclear spin qubits in an array of neutral ytterbium-171 ( 171 Yb) atoms.We show that the qubit state can be measured with a spin-flip probability of 0.004(4) for a single tweezer and 0.012(3) averaged over the array.This is accomplished by high cyclicity of one of the nuclear spin qubit states with an optically excited state under a magnetic field of B = 58 G, resulting in a spin-flip probability of approximately 10 -5 per scattered photon during fluorescence readout.The performance improves further as ∼ 1/B 2 .The state discrimination fidelity is 0.993(4) with a state-averaged readout survival of 0.994(3), limited by off-resonant scattering to dark states.We combine our measurement technique with high-contrast rotations of the nuclear spin qubit via an ac magnetic field to explore two paradigmatic scenarios, including the noncommutativity of measurements in orthogonal bases, and the quantum Zeno mechanism in which measurements "freeze" coherent evolution.Finally, we employ real-time feedforward to repetitively and deterministically prepare the qubit in the +z or -z direction after initializing it in a different basis and performing a measurement in the Z basis.These capabilities constitute an important step towards adaptive quantum circuits with atom arrays.
Abstract Quantum state readout is a key component of quantum technologies, including applications in sensing, computation, and secure communication. Readout fidelity can be enhanced by repeating readouts. However, the number of repeated readouts is limited by measurement backaction, which changes the quantum state that is measured. This detrimental effect can be overcome by storing the quantum state in an ancilla qubit, chosen to be robust against measurement backaction and to allow error correction. Here, we protect the electronic-spin state of a diamond nitrogen-vacancy center from measurement backaction using a robust multilevel 14N nuclear-spin memory and perform repetitive readout, as demonstrated in previous work on bulk diamond devices. We achieve additional protection using error correction based on the quantum logic of coherent feedback to reverse measurement backaction. The repetitive spin readout scheme provides a 13-fold enhancement of readout fidelity over conventional readout and the error correction a 2-fold improvement in the signal. These experiments demonstrate full quantum control of a nitrogen-vacancy center electronic-spin coupled to its host 14N nuclear spin inside a ~25 nm nanodiamond, creating a sensitive and biologically compatible platform for nanoscale quantum sensing. Our error-corrected repetitive readout scheme is particularly useful for quadrupolar nuclear magnetic resonance imaging in the low magnetic field regime where conventional repetitive readout suffers from strong measurement backaction. More broadly, methods for correcting longitudinal (bit-flip) errors described here could be used to improve quantum algorithms that require non-volatile local memory, such as correlation spectroscopy measurements for high resolution sensing.
Single-shot readout of qubits is required for scalable quantum computing. Nuclear spins are superb quantum memories due to their long coherence time, but are difficult to be read out in a single shot due to their weak interaction with probes. Here we demonstrate single-shot readout of a weakly coupled ^{13}C nuclear spin at room temperature, which is unresolvable in traditional protocols. States of the weakly coupled nuclear spin are trapped and read out projectively by sequential weak measurements, which are implemented by dynamical decoupling pulses. A nuclear spin coupled to the nitrogen-vacancy (NV) center with strength 330 kHz is read out in 200 ms with a fidelity of 95.5%. This work provides a general protocol for single-shot readout of weakly coupled qubits at room temperature and therefore largely extends the range of physical systems for scalable quantum computing.
Abstract High-fidelity projective readout of a qubit’s state in a single experimental repetition is a prerequisite for various quantum protocols of sensing and computing. Achieving single-shot readout is challenging for solid-state qubits. For Nitrogen-Vacancy (NV) centers in diamond, it has been realized using nuclear memories or resonant excitation at cryogenic temperature. All of these existing approaches have stringent experimental demands. In particular, they require a high efficiency of photon collection, such as immersion optics or all-diamond micro-optics. For some of the most relevant applications, such as shallow implanted NV centers in a cryogenic environment, these tools are unavailable. Here we demonstrate an all-optical spin readout scheme that achieves single-shot fidelity even if photon collection is poor (delivering less than 10 3 clicks/second). The scheme is based on spin-dependent resonant excitation at cryogenic temperature combined with spin-to-charge conversion, mapping the fragile electron spin states to the stable charge states. We prove this technique to work on shallow implanted NV centers, as they are required for sensing and scalable NV-based quantum registers.