Robust quantum-network memory based on spin qubits in isotopically engineered diamond

Abstract

Quantum networks can enable long-range quantum communication and modular quantum computation. A powerful approach is to use multi-qubit network nodes which provide the quantum memory and computational power to perform entanglement distillation, quantum error correction, and information processing. Nuclear spins associated with optically-active defects in diamond are promising qubits for this role. However, their dephasing during entanglement distribution across the optical network hinders scaling to larger systems. In this work, we show that a single 13 C spin in isotopically engineered diamond offers a long-lived quantum memory that is robust to the optical link operation of an NV centre. The memory lifetime is improved by two orders-of-magnitude upon the state-of-the-art, and exceeds the best reported times for remote entanglement generation. We identify ionisation of the NV centre as a newly limiting decoherence mechanism. As a first step towards overcoming this limitation, we demonstrate that the nuclear spin state can be retrieved with high fidelity after a complete cycle of ionisation and recapture. Finally, we use numerical simulations to show that the combination of this improved memory lifetime with previously demonstrated entanglement links and gate operations can enable key primitives for quantum networks, such as deterministic non-local two-qubit logic operations and GHZ state creation across four network nodes. Our results pave the way for test-bed quantum networks capable of investigating complex algorithms and error correction.

Entanglement of Spin-Pair Qubits with Intrinsic Dephasing Times Exceeding a Minute

“…These spin pairs provide inherently long-lived quantum states due to a combination of three physical phenomena: a decoherence-free subspace, a clock transition, and a variant of motional narrowing. This inherent coherence protection makes spin pairs promising systems for a variety of applications, such as robust memories for optically connected quantum networks [18][19][20]43] and memory-enhanced sensing [44][45][46][47][48][49].…”

“…For quantum networks, the long coherence time and small effective coupling to the NV electron spin (a few hertz) might enable faithful storage of quantum states during the probabilistic generation of NV-NV entanglement through optical channels. Such a robust memory is a key requirement for progressing toward larger-scale networks based on defect spins [19,43,50]. For sensing, a hybrid system consisting of a sensitive quantum sensor (e.g., the NV electron spin) in conjunction with a robust quantum memory can increase sensitivity and enhance sensor properties [44][45][46][47][48][49].…”

Performance of quantum registers in diamond in the presence of spin impurities

“…In fact, the 13 C's which are strongly coupled to the NV − can be initialized to the desired state and are addressable via either radio-frequency drives or via the dipole interaction with the NV − [11][12][13][14]. The long coherence properties of the nuclear spins also lead to the proposal to use them as quantum memory or quantum repeaters for quantum information networks [15][16][17][18][19].…”

“…Based on the idea of using several 13 C's as being part of the register, quantum gates and small quantum algorithms have successfully been implemented experimentally [17,[20][21][22]. These algorithms can be initiated by local control of the full register: MW-driving the NV − and additional radiofrequency drives for the 13 C. Here, only the multi-qubit processes rely on the dipole interaction between the 13 C and the NV − .…”