Entanglement between nodes of a quantum network

Thesis

Quantum devices are rapidly gaining momentum as a technology that will induce a paradigm shift in computing, communication and cryptography. Trapped ion qubits are one of the leading candidates for implementing a quantum computer, having previously demonstrated all of the required criteria. Local gate fidelities between ions exceed those for all other platforms, but the total number of ions in a trap is limited by unavoidable issues – one possibility for scaling the ion trap quantum processor is to create entanglement between ions in separate traps via single photons. The work in this thesis demonstrates the generation of remote entanglement between ions at high fidelity and rate, paving the way towards protocols using multiple entangled pairs for computations.

We describe the construction from scratch of a twin-trap apparatus designed to entangle separated ions. We demonstrate the entanglement of strontium ions in traps separated by ∼ 2 m by swapping entanglement from single photons emitted by the ⁸⁸Sr⁺ ions. A novel photon collection geometry is used, maximising the entanglement between the ion and photon without impeding optical access for standard ion trap laser beam geometries. The Bell state fidelity of the ion-photon state, which is itself a valuable entanglement resource for blind computation, is at least 97.70(12) % in either trap, with entangled pairs detected at a rate of at least 3.98 × 10³ s⁻¹. The remote ion-ion Bell state fidelity is 94.0(5) %, and is generated at a rate of 182 s⁻¹, representing the highest fidelity remote entanglement reported in ions by a large margin, at a rate more than an order of magnitude faster than previous experiments.

The two identical trap systems are designed with capabilities beyond those demonstrated here. We can co-trap ⁸⁸Sr⁺ and ⁴³Ca⁺ in a microfabricated trap suitable for local ion transport operations, opening up the possibility of performing distillation of remote entanglement in the same apparatus. Entanglement distillation can be used to make remote entanglement with similar fidelity to local operations, and would represent a significant step towards a fully scalable ion trap quantum computer architecture.

Authors: Stephenson, L

• DOI: 10.5287/ora-7rmjw4yak
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Quantum networking; Trapped ions
• Subjects: Atomic, Molecular and Optical Physics; Quantum theory