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arXiv:2108.04790 · Nature Communications

Assembly and coherent control of a register of nuclear spin qubits

Katrina Barnes, P. Battaglino, Benjamin Bloom, Kayleigh Cassella, Robin Coxe, Nicole Crisosto, Jonathan P. King, Stanimir Kondov, Krish Kotru, Stuart C. Larsen, J. Lauigan, Brian Lester, Mickey McDonald, E. Megidish, S. Narayanaswami, C. Nishiguchi, Remy Notermans, Lucas Peng, Albert Ryou, Tsung-Yao Wu, Michael Yarwood

Published May 19, 2022• 117 citations
arXivHTMLPDF

Abstract

Abstract The generation of a register of highly coherent, but independent, qubits is a prerequisite to performing universal quantum computation. Here we introduce a qubit encoded in two nuclear spin states of a single87Sr atom and demonstrate coherence approaching the minute-scale within an assembled register of individually-controlled qubits. While other systems have shown impressive coherence times through some combination of shielding, careful trapping, global operations, and dynamical decoupling, we achieve comparable coherence times while individually driving multiple qubits in parallel. We highlight that even with simultaneous manipulation of multiple qubits within the register, we observe coherence in excess of 105times the current length of the operations, with $${T}_{2}^{{{{{\mathrm{echo}}}}}}=\left(40\pm 7\right)$$ T2echo=40±7 seconds. We anticipate that nuclear spin qubits will combine readily with the technical advances that have led to larger arrays of individually trapped neutral atoms and high-fidelity entangling operations, thus accelerating the realization of intermediate-scale quantum information processors.

DOI
10.1038/s41467-022-29977-z

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jun 3, 2010•0 citations
Abstract The generation of a register of highly coherent, but independent, qubits is a prerequisite to performing universal quantum computation. Here we introduce a qubit encoded in two nuclear spin states of a single87Sr atom and demonstrate coherence approaching the minute-scale within an assembled register of individually-controlled qubits. While other systems have shown impressive coherence times through some combination of shielding, careful trapping, global operations, and dynamical decoupling, we achieve comparable coherence times while individually driving multiple qubits in parallel. We highlight that even with simultaneous manipulation of multiple qubits within the register, we observe coherence in excess of 105times the current length of the operations, with T2echo=(40±7){T}_{2}^{{{{{\mathrm{echo}}}}}}=\left(40\pm 7\right)T2echo​=(40±7) T2echo=40±7 seconds. We anticipate that nuclear spin qubits will combine readily with the technical advances that have led to larger arrays of individually trapped neutral atoms and high-fidelity entangling operations, thus accelerating the realization of intermediate-scale quantum information processors.
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 183^{183}183W nuclear spin qubits adjacent to an Er3+^{3+}3+ impurity in a CaWO4_44​ crystal, interfaced via a superconducting resonator and detected using a microwave photon counter at 10mK. We study two nuclear spin qubits with T2∗T_2^*T2∗​ of 0.8(2) 0.8(2)~0.8(2) s and 1.2(3) 1.2(3)~1.2(3) s, T2T_2T2​ of 3.4(4) 3.4(4)~3.4(4) s and 4.4(6) 4.4(6)~4.4(6)  s, respectively. We demonstrate single-shot quantum non-demolition readout of each nuclear spin qubit using the Er3+^{3+}3+ 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∗T_2^*T2∗​ of 1.7(2) 1.7(2)~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.
Abstract Phosphorus atoms in silicon are an outstanding platform for quantum computing as their nuclear spins exhibit coherence time over seconds [1, 2]. By placing multiple phosphorus atoms within a radius of a few nanometers, they couple via the hyperfine interaction to a single, shared electron. Such a nuclear spin register enables multi-qubit control above the fault-tolerant threshold [3] and the execution of small-scale quantum algorithms [4]. To achieve quantum error correction, fast and efficient interconnects have to be implemented between spin registers while maintaining high fidelity across all qubit metrics. Here, we demonstrate such integration with a fully controlled 11-qubit atom processor composed of two multi-nuclear spin registers which are linked via electron exchange interaction. Through the development of scalable calibration and control protocols, we achieve coherent coupling between nuclear spins using a combination of single- and multi-qubit gates with all fidelities ranging from 99.5% to 99.99%. We verify the efficient all-to-all connectivity by preparing both local and non-local Bell states with a record state fidelity beyond 99% and extend entanglement through the generation of Greenberger-Horne-Zeilinger (GHZ) states over all data qubits. By establishing high-fidelity operation across interconnected nuclear-spin registers, we ealise a key milestone towards fault-tolerant quantum computation with atom processors.
Spins associated with single defects in solids provide promising qubits for quantum-information processing and quantum networks.Recent experiments have demonstrated long coherence times, highfidelity operations, and long-range entanglement.However, control has so far been limited to a few qubits, with entangled states of three spins demonstrated.Realizing larger multiqubit registers is challenging due to the need for quantum gates that avoid cross talk and protect the coherence of the complete register.In this paper, we present novel decoherence-protected gates that combine dynamical decoupling of an electron spin with selective phase-controlled driving of nuclear spins.We use these gates to realize a ten-qubit quantum register consisting of the electron spin of a nitrogen-vacancy center and nine nuclear spins in diamond.We show that the register is fully connected by generating entanglement between all 45 possible qubit pairs and realize genuine multipartite entangled states with up to seven qubits.Finally, we investigate the register as a multiqubit memory.We demonstrate the protection of an arbitrary single-qubit state for over 75 s-the longest reported for a single solid-state qubit-and show that two-qubit entanglement can be preserved for over 10 s.Our results enable the control of large quantum registers with long coherence times and therefore open the door to advanced quantum algorithms and quantum networks with solid-state spin qubits.
Arrays of optically trapped neutral atoms are a promising architecture for the realization of quantum computers. In order to run increasingly complex algorithms, it is advantageous to demonstrate high-fidelity and flexible gates between long-lived and highly coherent qubit states. In this work, we demonstrate a universal high-fidelity gate set with individually controlled and parallel application of single-qubit gates and two-qubit gates operating on the ground-state nuclear-spin qubit in arrays of tweezer-trapped 171 Yb atoms. We utilize the long lifetime, flexible control, and high gate fidelity of our system to characterize native gates using single- and two-qubit Clifford and symmetric subspace randomized-benchmarking circuits with more than 200 controlled- Z () gates applied to one or two pairs of atoms. We measure our two-qubit entangling gate fidelity to be 99.72(3)% (99.40(3)%) with (without) postselection. In addition, we introduce a simple and optimized method for calibration of multiparameter quantum gates. These results represent important milestones toward executing complex and general quantum computation with neutral atoms.
Abstract Single electron spins bound to multi-phosphorus nuclear spin registers in silicon have demonstrated fast (0.8 ns) two-qubit SWAP\sqrt{\mathrm{SWAP}}SWAP​ SWAP gates and long spin relaxation times (~30 s). In these spin registers, when the donors are ionized, the nuclear spins remain weakly coupled to their environment, allowing exceptionally long coherence times. When the electron is present, the hyperfine interaction allows coupling of the spin and charge degrees of freedom for fast qubit operation and control. Here we demonstrate the use of the hyperfine interaction to enact electric dipole spin resonance to realize high-fidelity ( F=100−6+0F=10{0}_{-6}^{+0}F=100−6+0​ F = 10 0 − 6 + 0 %) initialization of all the nuclear spins within a four-qubit nuclear spin register. By controllably initializing the nuclear spins to ∣⇓⇓⇓⟩\left\vert \Downarrow \Downarrow \Downarrow \right\rangle∣⇓⇓⇓⟩ ⇓ ⇓ ⇓ , we achieve single-electron qubit gate fidelities of F = 99.78 ± 0.07% (Clifford gate fidelities of 99.58 ± 0.14%), above the fault-tolerant threshold for the surface code with a coherence time of $${T}_{2}^{\,* }=12\,\upmu {{{\rm{s}}}}$$ T 2 * = 12 μ s .
Building scalable quantum information systems is a central challenge facing modern science. Single spins in diamond are a promising platform for distributed quantum information networks and precision measurements. I will discuss recent progress in this field demonstrating coherent operations with coupled electron-nuclear spin quantum registers and nanoscale precision magnetometry. Our experiments demonstrate addressing, preparation, and coherent control of individual nuclear spin qubits in the diamond lattice at room temperature. We have measured spin coherence times exceeding milliseconds, and observed coherent coupling to nearby electronic and nuclear spins. Robust initialization of a two-qubit register and transfer of arbitrary quantum states between electron and nuclear spin qubits has been achieved. Our results show that coherent operations are possible with individual solid-state qubits whose coherence properties approach those for isolated atoms and ions. The resulting electron-nuclear few-qubit registers can potentially serve as small processor nodes in a quantum network where the electron spins are coupled by optical photons.