Diamond Nuclear Spin Register Enables Millisecond-Scale Electron Spin Lifetime and Control

Future quantum networks require robust and stable quantum bits, and researchers are increasingly turning to solid-state systems to provide them. Marco Klotz, Andreas Tangemann, and David Opferkuch, along with colleagues at Ulm University, now demonstrate a significant step forward by creating and controlling entanglement within a three-qubit nuclear spin register embedded in diamond. The team achieved this by cleverly linking the nuclear spins to a single electron spin, effectively using the electron as a mediator for quantum connections. This approach overcomes limitations found in other systems, offering a pathway to an optically accessible and highly stable quantum register that could prove crucial for building scalable quantum technologies, and opens new possibilities for sensing extremely weak interactions between atomic nuclei.

This work demonstrates control of, and entanglement in, a fully connected three-qubit 13C nuclear spin register in diamond. The register is coupled to the quasi-free electron spin-1/2 of a silicon-vacancy centre (SiV). High strain decouples the SiV’s electron spin from disruptive interactions, reducing its susceptibility to environmental noise and extending its lifetime to hundreds of milliseconds. This extended coherence enables sensing of nuclear-nuclear couplings down to a few Hertz. To detect and control the register, researchers leverage continuous decoupling using shaped, low-power microwave and direct radio frequency driving, and they further implement a nuclear spin conditional phase-gate.

Nanodiamond Integration for SiV Qubit Control

This research details the creation of a quantum register using silicon-vacancy (SiV) centers embedded within nanodiamonds, integrated into a nanophotonic platform for strong light-matter interaction and efficient qubit control. Researchers control and manipulate the nuclear spins associated with the SiV center, using them to enhance the coherence of the electron spin qubit. The system employs both optical and microwave control to manipulate qubit states, utilizing truncated sinc-shaped pulses for precise control and a digital step attenuator to control pulse amplitude. A post-selection scheme, involving single-shot readout (SSR) on each nuclear spin, improves the fidelity of register initialization. Dynamical decoupling techniques suppress noise and extend qubit coherence times, while single-shot readout (SSR) measures qubit states without destroying them, essential for quantum operations and verification. Researchers use numerical simulations, utilizing the QuTiP software package, to model the system’s behavior and optimize experimental parameters.

Millisecond Coherence Enables Nuclear Spin Control

Researchers have achieved a significant advance in quantum information science by creating and controlling a three-qubit register using the nuclear spins of atoms within a diamond sample. This register is uniquely linked to an electron spin within a silicon-vacancy defect, allowing for precise control and measurement of the nuclear spins. A key breakthrough lies in decoupling the electron spin from environmental disturbances, extending its coherence to hundreds of milliseconds. This improvement enables the detection of subtle interactions between the nuclear spins at frequencies as low as a few Hertz.

The team employed precisely shaped microwave and radio frequency pulses to prepare the nuclear spin states and measure their interactions, demonstrating the ability to entangle the nuclear spins with the electron spin. This approach differs from previous methods, as it is not limited by the properties of the electron spin itself, opening new possibilities for optically accessible quantum registers within a solid-state material. The results reveal exceptionally long coherence times for the electron spin, exceeding previous measurements by three orders of magnitude. This extended coherence allows for more complex quantum operations to be performed before quantum information is lost. Furthermore, researchers detected and controlled the interactions between individual nuclear spins with remarkable precision, resolving couplings as small as 160 kHz. By dynamically decoupling the nuclear spins from their environment, they extended their coherence and demonstrated the ability to rotate them conditionally based on the state of the electron spin, paving the way for more complex quantum algorithms and exploring new avenues in quantum sensing and computation.

Millisecond Coherence Enables Nuclear Spin Entanglement

This research demonstrates the creation and control of entanglement within a three-qubit nuclear spin register in diamond, coupled to an electron spin associated with a silicon-vacancy center. The team achieved bipartite entanglement with a fidelity of 0. 689 and implemented control over individual nuclear spins with high readout fidelity exceeding 96%. High strain decouples the electron spin from disruptive interactions, extending its coherence time and enabling sensitive detection of nuclear-nuclear couplings. The results present an alternative method for achieving nuclear spin entanglement, circumventing limitations found in dynamically decoupled systems and offering a pathway towards optically-accessible, solid-state quantum registers.

While the current entanglement fidelity is limited by gate and readout imperfections, the authors suggest improvements through optimal control techniques and increased magnetic field strength. Future work focuses on scaling the register size towards a five-qubit fault-tolerant memory and exploring spin-photon entanglement for potential applications in quantum networks and long-range quantum communication. The demonstrated sensitivity and coherence times represent a significant step towards realizing practical, solid-state quantum information processing.