Entanglement of a nuclear spin qubit register in silicon photonics

This paper explores the integration of T centres in silicon photonics to demonstrate a multi-qubit register for quantum information processing. Specifically, it focuses on the use of the T centre defect in silicon, which includes an electron spin and two coupled nuclear spins (hydrogen and silicon), forming a coherent three-qubit quantum register. The work highlights the potential for using T centres as a spin-photon interface for scalable quantum communication and quantum computing.

Key Contributions:

1. T Centre as a Spin-Photon Interface:

• The T centre in silicon consists of a point defect with two carbon atoms, one hydrogen atom, and an unpaired electron at a silicon substitutional site.

• The system exhibits telecom O-band emission around 1,325 nm, which makes it compatible with telecommunication systems, while also providing coherent electron spin properties essential for quantum memory and quantum communication.

2. Multi-Qubit Register in Silicon Photonics:

• The electron spin of the T centre,

• A hydrogen nuclear spin (I = 1/2),

• A silicon nuclear spin (I = 1/2).

• The study successfully demonstrates a three-qubit register that includes:

• The qubits are coherently controlled and manipulated using microwave (MW) and radio-frequency (RF) pulses, while the electron spin serves as an intermediary for controlling nuclear spins.

3. Spin Coherence Times:

• Electron coherence: The electron spin shows a spin echo coherence time of 411(15) μs after dynamical decoupling, with further extension to 1.68 ms using XY8 dynamical decoupling sequences.

• Nuclear coherence: The hydrogen nuclear spin exhibits coherence times of 112(10) ms and the silicon nuclear spin has a coherence time of 67(7) ms, both extended using Hahn-echo sequences.

• These long coherence times are crucial for quantum memory and are much longer than those observed in many other quantum systems.

4. Entanglement of Nuclear Spins:

• Using controlled-NOT (CNOT) gates, the hydrogen and silicon nuclear spins were entangled, demonstrating a Bell state fidelity of 77(3)%.

• This marks a significant achievement in nuclear spin entanglement, which is essential for quantum computation and quantum communication networks.

5. Device Integration and Characterization:

• The T centre was embedded into single-mode photonic waveguides on a silicon-on-insulator (SOI) platform, facilitating efficient coupling and integration with other photonic and electronic components.

• Optical spectroscopy and microwave spectroscopy were used to manipulate and read out the spin states of the T centre, allowing for high-fidelity quantum state initialization, control, and readout.

6. Challenges and Future Prospects:

• Despite the impressive performance, the experiment highlights challenges, such as detuning and the need for high Rabi frequencies to address weakly coupled nuclear spins.

• Future work will focus on pulse optimization, improving gate fidelities, and extending the register size to include more qubits by better controlling spin interactions and improving coherence times further.

Conclusion:

This work demonstrates the potential of T centres in silicon as a scalable, photonic-compatible platform for quantum information processing. The demonstrated three-qubit register is a significant step towards multi-qubit quantum memory and quantum communication in telecom-compatible silicon photonics. The results indicate that T centres could play a key role in building large-scale quantum networks and facilitating photonic quantum computing. Future work will explore extending the qubit register, improving control fidelity, and addressing noise sources to scale the system for more complex quantum tasks.