Diamond Quantum Network Nodes with Open Microcavities

Abstract

The technology of building quantum networks by interconnecting multiple quantum nodes has the potential to revolutionize the world, much like its classical equivalent - the internet - did. The possibility of processing quantum information on a shared network promises exciting applications that are not possible with today’s technologies. Quantum systems with stationary qubits and qubit-photon interfaces, like diamond color centers, are prime candidates for these quantum network nodes. Here, the qubit-photon interface efficiency is crucial for their connectivity, which propels the development of cavity systems that enhance this interface via the Purcell effect. This dissertation presents advances in coupling nitrogen-vacancy (NV) and tin-vacancy (SnV) centers in diamond to open microcavities and explores their capabilities for quantum networks.
The basis for experiments with cavity-coupled color centers is laid by the construction of a cryogenic fiber-based open microcavity system. This system uses a flat sample mirror opposite to a laser-ablated spherical fiber mirror to confine optical cavity modes. It is designed to achieve a low passive cavity length fluctuation level, microwave integration as well as full optical access through the fiber-mirror and free-space via the sample mirror. A closed-cycle optical cryostat hosts this system and enables continuous operation in a controlled high-vacuum environment.
Besides the microcavity developments, a novel patterning method for the fabrication of micrometer-thin diamond membranes is presented. The method involves laser-cutting to pattern diamonds with micrometer-scale feature sizes and subsequent bonding to a sample mirror. Comparing the laser-cutting method to established electron-beam lithography and a two-step transfer pattern process with a silicon nitride hard mask validates the fabrication of high-quality diamond devices for microcavity applications.
By integrating a diamond device hosting SnV centers into the cryogenic microcavity system, single cavity-coupled SnV centers are investigated. The coherent coupling regime is reached as a result of the achieved Purcell enhancement and the coherence of the optical transition. The coupled system of SnV center and cavity exhibits quantum nonlinear behavior, as evidenced by dips in the cavity transmission spectrum and changes in the photon statistics of the transmitted light. These effects can be exploited in remote entanglement protocols, underlining the potential of these systems to serve as quantum network nodes with Purcell-enhanced photonic interfaces.
Moreover, the cryogenic microcavity system is employed to equip NV centers with efficient qubit-photon interfaces. The cavity-coupling is used in combination with a crosspolarized resonant excitation and detection scheme to initialize and read out the NV center electron spin qubit. In addition, the electron spin is coherently controlled with on-chip delivered microwave pulses, and pulsed resonant excitation enables the generation of spin-photon correlated states. The quantum networking capabilities of the system are demonstrated by measuring heralded Z-basis correlations between photonic time-bin qubits and the spin qubit. In these experiments, a tenfold improvement in resonant photon detection probability is achieved over state-of-the-art NV center quantum network nodes, paving the way for cavity-enhanced quantum networking with NV centers.