Good Vibrations: A Microcavity-based Diamond Spin-Photon Interface for Quantum Networking

Abstract

Quantum networks promise exciting applications that are not possible with their classical counterparts, such as distributed quantum computation or fundamentally secure quantum communication. Optically active spins in solid states are among the prime candidates for realizing quantum network nodes, while photons are used to distribute entanglement between nodes. The nitrogen-vacancy (NV) center in diamond is a pioneering system with the demonstrations of teleportation of quantum states within a three-node network and metropolitan-scale heralded entanglement. However, scaling to more nodes or longer distances is limited by the low extraction of resonant photons, creating a high interest in optical resonators (cavities) to improve the optical interface. Moreover, the diamond tin-vacancy (SnV) center with better optical properties emerged as a promising alternative candidate for quantum network nodes.
This dissertation presents the building blocks and the experimental realization of an open, fiber-based, cryogenic Fabry-Pérot microcavity enabling the Purcell enhancement of diamond NV and SnV centers, incorporated into the microcavity via a diamond membrane. The background on color centers in the context of quantum networking (Chapter 2) and optical cavities for diamond color centers is summarized (Chapter 3). A novel laser-cutting patterning method is introduced, which can be used to fabricate micrometer-thin diamond devices with arbitrary lateral shapes in the range of tens to hundreds of micrometers (Chapter 4). Microdevices fabricated by this method are characterized by scanning cavity microscopy, revealing a high cavity finesse. Furthermore, SnV and NV centers in the microdevices maintain bulk-like optical properties, which are required for quantum networking. Next to the sample fabrication, the detailed design, construction, and operation of a cryogenic microcavity system is presented, reaching a reproducible cavity length stability level of around 25 picometer with a sample temperature of about 8 kelvin on the cavity mirror (Chapter 5), a prerequisite for the following cavity experiments. Two cavity quantum optics experiments are conducted, exploring the regimes of coherent cavity coupling and efficient photon extraction with the cavity. In the first experiment, a single SnV center is coupled to the cavity, achieving a coherent cooperativity of 0.7. This enables the observation of nonlinear quantum effects, such as the modulation of the cavity resonance by an individual SnV center and the altered photon statistics of light transmitted through the cavity (Chapter 6). In the second experiment, a single NV center is coupled to the cavity, and the Purcell enhancement combined with a high cavity outcoupling leads to a resonant photon extraction (end-to-end) efficiency of 0.5 %. The NV center’s electron spin qubit initialization, manipulation with an on-chip microwave stripline, and readout are utilized to generate spin-photon correlated states, a precondition for remote entanglement with a second color center (Chapter 7).
The presented platform combines an efficient optical interface with microwave control of the spin state and can be used for the exploration of optically active defects in solid states, as a bright source of single photons, and for cavity-enhanced quantum networking.