Diamond Spin Qubit Interfaces with Photons for Quantum Networking.

The development of robust quantum networks necessitates efficient connections between stationary quantum bits, or qubits, and flying qubits, typically photons. Achieving strong coupling between these systems is a significant challenge, demanding both effective light collection and precise control over the qubit’s quantum state. Researchers at QuTech and the Kavli Institute of Nanoscience, Delft University of Technology, including Julius Fischer, Yanik Herrmann, Cornelis F. J. Wolfs, Ronald Hanson, Stijn Scheijen, and Maximilian Ruf, detail a system achieving this through the integration of a diamond nitrogen-vacancy (NV) centre with an open microcavity, as reported in their article, “Spin-Photon Correlations from a Purcell-enhanced Diamond Nitrogen-Vacancy Center Coupled to an Open Microcavity”. Their work demonstrates enhanced light collection via the Purcell effect, alongside coherent control of the NV centre’s electron spin, ultimately enabling the generation and measurement of correlated spin-photon states —a crucial step toward scalable quantum communication.

Recent advances in quantum networking demonstrate a successful coupling of a diamond nitrogen-vacancy (NV) centre with an open microcavity, establishing a critical interface for developing practical quantum technologies. This integration facilitates efficient collection of resonant photons and precise control over the spin qubit, a fundamental requirement for constructing scalable quantum networks and enabling future quantum communication protocols. A spin qubit, essentially a quantum bit of information encoded in the spin of an electron, requires precise manipulation to function effectively.

Measurements reveal a Purcell factor of 7.3 ± 1.6, indicating a substantial enhancement of the spontaneous emission rate due to the microcavity. The Purcell effect describes the increase in spontaneous emission rate of a dipole emitter when placed within a resonant cavity, directly improving the efficiency of light-matter interaction and boosting signal strength. This enhancement is crucial for overcoming losses inherent in quantum systems.

Researchers employ on-chip microwave lines to control the spin qubit at a Rabi frequency of 10 MHz, enabling rapid manipulation of its quantum state. The Rabi frequency represents the rate at which a qubit oscillates between its quantum states under the influence of an external field, and a higher frequency allows for faster processing and the creation of more complex quantum systems. Through careful tuning of the microcavity to the NV centre’s transition frequency, they achieve a coherent photon detection probability of 0.5%, a significant improvement over previous solid immersion lens devices and enhancing the reliability of quantum information transfer.

The study details the generation and measurement of multi-qubit spin-photon states, specifically two- and three-qubit systems, demonstrating the potential for creating complex entangled networks. Entanglement, a key feature of quantum mechanics, links the fates of two or more particles, regardless of the distance separating them. By utilising resonant pulses for both initialization and readout of the electron spin, the team measures heralded Z-basis correlations between photonic time-bin qubits and the spin qubit, confirming the entanglement between the spin and photonic qubits. These correlations validate the functionality of the integrated system as a platform for quantum information processing and open doors for advanced quantum algorithms.

Researchers extend their investigations beyond NV centres, exploring V2 centres in 4H-SiC with fiber-based Fabry-Perot microcavities and investigating spectral multiplexing of rare-earth emitters in co-doped crystalline membranes. This broader investigation highlights the potential of microcavity-based systems for diverse quantum technologies and demonstrates the versatility of the approach. The development of a Python 3 framework, QMI, for controlling laboratory equipment further underscores the commitment to creating a versatile and accessible platform for quantum research and facilitating collaboration within the scientific community.