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Nature Physics

A many-body quantum register for a spin qubit

Martin Hayhurst Appel, Alexander Ghorbal, Noah Shofer, Leon Zaporski, Santanu Manna, Saimon Filipe Covre da Silva, Urs Haeusler, Claire Le Gall, Armando Rastelli, Dorian A. Gangloff, Mete Atatüre

Published Jan 24, 2025• 38 citations
Publisher

Abstract

Abstract Quantum networks require quantum nodes with coherent optical interfaces and several stationary qubits. In terms of optical properties, semiconductor quantum dots are highly compelling, but their adoption as quantum nodes has been impaired by the lack of auxiliary qubits. Here we demonstrate that the dense, always-present, nuclear spin ensemble surrounding a gallium arsenide quantum dot can be used as a functional quantum register. We prepared 13,000 host nuclear spins in a single many-body dark state that acts as a logical state of the register. A second logical state is defined as a single nuclear-magnon excitation, enabling controlled quantum-state transfer between an electron spin qubit in the quantum dot and the nuclear magnonic register. Using SWAP gates, we implemented a full write–store–retrieve-read-out protocol with 68.6(4)% raw overall fidelity and a storage time of 130(16) μs, which could be extended to 20 ms or beyond using dynamical decoupling techniques. Our work establishes how many-body physics can add functionality to quantum devices, in this case transforming quantum dots into multi-qubit quantum nodes with deterministic registers.

DOI
10.1038/s41567-024-02746-z

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•Jan 1, 2006•0 citations
Abstract Quantum networks require quantum nodes with coherent optical interfaces and several stationary qubits. In terms of optical properties, semiconductor quantum dots are highly compelling, but their adoption as quantum nodes has been impaired by the lack of auxiliary qubits. Here we demonstrate that the dense, always-present, nuclear spin ensemble surrounding a gallium arsenide quantum dot can be used as a functional quantum register. We prepared 13,000 host nuclear spins in a single many-body dark state that acts as a logical state of the register. A second logical state is defined as a single nuclear-magnon excitation, enabling controlled quantum-state transfer between an electron spin qubit in the quantum dot and the nuclear magnonic register. Using SWAP gates, we implemented a full write–store–retrieve-read-out protocol with 68.6(4)% raw overall fidelity and a storage time of 130(16) μs, which could be extended to 20 ms or beyond using dynamical decoupling techniques. Our work establishes how many-body physics can add functionality to quantum devices, in this case transforming quantum dots into multi-qubit quantum nodes with deterministic registers.
Quantum networks require quantum nodes with coherent optical interfaces and multiple stationary qubits. In terms of optical properties, semiconductor quantum dots are highly compelling, but their adoption as quantum nodes has been impaired by the lack of auxiliary qubits. Here, we demonstrate a functional quantum register in a semiconductor quantum dot leveraging the dense, always-present nuclear spin ensemble. We prepare 13,000 host nuclear spins into a single many-body dark state to operate as the register logic state ∣0⟩|0\rangle∣0⟩. The logic state ∣1⟩|1\rangle∣1⟩ is defined as a single nuclear magnon excitation, enabling controlled quantum-state transfer between the electron spin qubit and the nuclear magnonic register. Using 130-ns SWAP gates, we implement a full write-store-retrieve-readout protocol with 68.6(4)% raw overall fidelity and a storage time of 130(16) μμμs in the absence of dynamical decoupling. Our work establishes how many-body physics can add step-change functionality to quantum devices, in this case transforming quantum dots into multi-qubit quantum nodes with deterministic registers.
Coherent excitation of an ensemble of quantum objects offers the opportunity to realise robust entanglement generation and information storage in a quantum memory [1]. Thus far, interfacing with such a collective excitation deterministically has remained elusive owing to the difficulty of controlling a probe spin in the midst of a complex many-body system. In the strained atomic lattice of a semiconductor quantum dot, nuclear quadrupole effects generate an electron-nuclear interaction that can be engineered by driving the electron spin (Fig. 1a). By implementing an all-optical approach to access the individual quantised electronic-nuclear spin transitions, we have experimentally demonstrated coherent optical rotations of a single collective nuclear spin excitation corresponding to a spin wave called a nuclear magnon [2] (Fig. 1b).
States with long coherence are a crucial requirement for qubits and quantum memories. Nuclear spins in epitaxial GaAs/AlGaAs quantum dots are a great candidate, offering excellent isolation from external environments and on-demand coupling to optical flying qubits. However, coherence times are limited to ≲ 1 ms by the dipole-dipole interactions between the nuclei and by the nuclear quadrupolar coupling to inhomogeneous crystal strain. Here, we combine strain engineering of the nuclear spin ensemble and tailored dynamical decoupling sequences to achieve nuclear spin coherence times exceeding 100 ms. Recently, a reversible transfer of quantum information into nuclear spin ensembles has been demonstrated in quantum dots: our results provide a path to develop this concept into a functioning solid-state quantum memory suitable for quantum repeaters in optical quantum communication networks.
Recently, an ensemble of nuclear spins in a quantum dot have been proposed as a long-lived quantum memory. A quantum state of an electron spin in the dot can be faithfully transfered into nuclear spins through controlled hyperfine coupling. Here we study the decoherence of this memory due to nuclear spin dipolar coupling and inhomogeneous hyperfine interaction during the storage period. We calculated the maximum fidelity of writing, storing, and reading operations. Our results show that nuclear spin dynamics can severely limit the performance of the proposed device for quantum information processing and storage based on nuclear spins.
Inspired by recent developments in the control and manipulation of quantum dot nuclear spins, which allow for the transfer of an electron spin state to the surrounding nuclear-spin ensemble for storage, we propose a quantum repeater scheme that combines individual quantum dot electron spins and nuclear-spin ensembles, which serve as spin-photon interfaces and quantum memories respectively. We consider the use of low-strain quantum dots embedded in high-cooperativity optical microcavities. Quantum dot nuclear-spin ensembles allow for the long-term storage of entangled states, and heralded entanglement swapping is performed using cavity-assisted gates. We highlight the advances in quantum dot technologies required to realize our quantum repeater scheme which promises the establishment of high-fidelity entanglement over long distances with a distribution rate exceeding that of the direct transmission of photons.
Single electron spin in a semiconductor quantum dot (QD) and single photon wavepacket propagating in an optical waveguide are investigated as carriers of quantum bit (qubit) for information processing. Cavity quantum electrodynamics of the coupled system composed of charged QD, microcavity and waveguide provides a quantum interface for the interplay of stationary spin qubits and flying photon qubits via cavity assisted optical control. This interface forms the basis for a wide range of essential functions of a quantum network, including transferring, swapping, and entangling qubits at distributed quantum nodes as well as a deterministic source and an efficient detector of a single photon wavepacket with arbitrarily specified shape. The cavity assisted optical process also made possible ultrafast initialization and QND readout of the spin qubit in QD. In addition, the strong optical nonlinearity of dot-cavity-waveguide coupled system enables phase gate and entanglement operation for flying single photon qubits in waveguides. The coherence of the electron spin is the wellspring of these quantum applications being investigated. At low temperature and strong magnetic field, the dominant cause of electron spin decoherence is the coupling with the interacting lattice nuclear spins. We present a quantum solution to the coupled dynamics of the electron with the nuclear spin bath. The decoherence is treated in terms of quantum entanglement of the electron with the nuclear pair-flip excitations driven by the various nuclear interactions. A novel nuclear interaction, mediated by virtue spin-flips of the single electron, plays an important role in single spin free-induction decay (FID). The spin echo not only refocuses the dephasing by inhomogeneous broadening in ensemble dynamics but also eliminates the decoherence by electron-mediated nuclear interaction. Thus, the decoherence times for single spin FID and ensemble spin echo are significantly different. The quantum theory of decoherence also leads to a method of coherence recovery of the electron by disentanglement, realized through maneuvering the nuclear bath evolution by control of the electron spin-flip. The studies form the basis to outline the construction of a solid-state quantum network for scalable and distributed processing of quantum information