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arXiv:1905.02094 · Physical Review X

A Ten-Qubit Solid-State Spin Register with Quantum Memory up to One Minute

C. E. Bradley, J. Randall, M. H. Abobeih, Remon Berrevoets, Maarten J. Degen, Michiel A. Bakker, M. Markham, D. J. Twitchen, T. H. Taminiau

Published Sep 11, 2019• 545 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevx.9.031045

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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.
The electron spin of a nitrogen-vacancy center in diamond lends itself to the control of proximal 13C^{13}\mathrm{C}13C nuclear spins via dynamical decoupling methods, possibly combined with radio-frequency driving. Long-lived single-qubit states and high-fidelity electron-nuclear gates required for the realization of a multiqubit register have already been demonstrated. Towards the goal of a scalable architecture, linking multiple such registers in a photonic network represents an important step. Multiple pairs of remotely entangled qubits can enable advanced algorithms or error correction protocols. We investigate how a photonic architecture can be extended from the intrinsic nitrogen spin to multiple 13C^{13}\mathrm{C}13C spins per node. Applying decoherence-protected gates sequentially, we simulate the fidelity of creating multiple pairs of remotely entangled qubits. Even though the currently achieved degree of control of 13C^{13}\mathrm{C}13C spins might not be sufficient for large-scale devices, the two schemes are compatible in principle. One requirement is the correction of unconditional phases acquired by unaddressed nuclear spins during a decoupling sequence.
Abstract Solid-state quantum registers are exceptional for storing quantum information at room temperature with long coherence time. Nevertheless, practical applications toward quantum supremacy require even longer coherence time to allow for more complex algorithms. In this work we propose a quantum register that lies in a decoherence-protected subspace to be implemented with nuclear spins nearby a nitrogen-vacancy center in diamond. The quantum information is encoded in two logical states composed of two carbon-13 nuclear spins, while an electron spin is used as ancilla for initialization and control. Moreover, by tuning an off-axis magnetic field we enable non-nuclear-spin-preserving transitions that we use for preparing and manipulating the register through stimulating Raman adiabatic passage. Furthermore, we consider more elaborated sequences to improve simultaneous control over the system yielding decreased gate time.
Quantum memories are critical for solid-state quantum computing devices and a good quantum memory requires both long storage time and fast read/write operations. A promising system is the nitrogen-vacancy (NV) center in diamond, where the NV electronic spin serves as the computing qubit and a nearby nuclear spin as the memory qubit. Previous works used remote, weakly coupled 13 C nuclear spins, trading read/write speed for long storage time. Here we focus instead on the intrinsic strongly coupled 14 N nuclear spin. We first quantitatively understand its decoherence mechanism, identifying as its source the electronic spin that acts as a quantum fluctuator. We then propose a scheme to protect the quantum memory from the fluctuating noise by applying dynamical decoupling on the environment itself. We demonstrate a factor of 3 enhancement of the storage time in a proof-of-principle experiment, showing the potential for a quantum memory that combines fast operation with long coherence time.