Quantum Networks with Diamond Color Centers
Local Control and Multi-Node Entanglement
Hans K.C. Beukers (TU Delft - QID/Hanson Lab)
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Abstract
The ability to send quantum information over long distances can enable fundamentally new applications, such as intrinsically secure communication, enhanced metrology, and distributed quantumcomputation. Entangled links serve as powerful resources for sending quantum information between nodes in a quantum network. However, generating entanglement in sufficient quantity and quality across a network, such that they can be used for applications, remains an open challenge.
In this thesis, we explore the use of color centers in diamond as network nodes. Their electron spin serves as a matter qubit with an optical interface, enabling the entanglement of two distant color centers, mediated by photons. The surrounding nuclear spins are used as memory qubits for local computation and entanglement storage. In this thesis, we investigate both the well-established nitrogen-vacancy (NV) center in diamond and the recently discovered tin-vacancy (SnV) center in diamond. The physics and control methods for both types of color centers are discussed in Chapter 2.
Remote entanglement between matter qubits can be achieved with many different entanglement protocols. In Chapter 3, we present a framework that explains and categorizes different protocols and quantum network hardware components. This framework is then used to compare the performance of various protocols while using similar hardware.
There have been many realisations of rudimentary network links between two network nodes using various quantum hardware. In Chapters 4 and 5, we realize the first entanglement-based three-node quantum network employing NV centers in diamond. In this network, we demonstrate fundamental network capabilities such as the creation of a remote three-party Greenberger–Horne–Zeilinger (GHZ) state and entanglement swapping to connect non-neighboring network nodes, as detailed in Chapter 4. These advancements are facilitated by storing an entangled state in a network node while generating a second entangled link. In Chapter 5 we demonstrate quantum teleportation between two non-neighboring network nodes by adding a fifth qubit to the network and utilizing the entangled link generated through the entanglement swapping.
The three-node network experimentswere enabled by the nuclear spin memory, emphasizing the importance of nuclear spin control for quantum networks based on color centers. In Chapter 6, we explore nuclear spin control with the SnV center in diamond. This recently discovered color center promises enhanced entanglement rates compared to the NV center due to its superior optical interface. We control single nuclear spins and show entanglement between the electron and nuclear spin. These experiments provide insights into the challenges and opportunities of controlling nuclear spins using an electron spin-1/2.