Trapped ion quantum information processing using multiple qubit encodings

Thesis

Trapped atomic ions are one of the most promising platforms for developing a quantum computer. On a small scale, they have shown the longest qubit coherence times [1] and the highest single- and two-qubit gate fidelities [2–5]. The challenge is finding ways to scale up the system while maintaining this level of performance to build usefully powerful quantum computers. In this thesis, we describe experimentally validated techniques to overcome some of the difficulties in scaling trapped-ion quantum computers.

We use the vast state space available within a single ion to encode qubits both in the ground level and the metastable level [6]. This architecture enables measurement, cooling, and reset of the ground-level qubits while protecting the state of the metastable level ones. We demonstrate the building blocks of this architecture using trapped 137Ba+ ions. We implement a metastable level clock qubit in 137Ba+ and measure a coherence time of 31(5)s. We show that the coherence time of this clock qubit in the presence of the laser light for cooling and measurement is 21.9(21)s. We further demonstrate a two-qubit entangling gate between two metastable level qubits necessary for building a universal gate set. Moreover, we perform an entangling gate between a ground-level qubit and a metastable level qubit, the equivalent of the mixed-species gate [7]. This gate facilitates the implementation of quantum non-demolition measurements (QNDs) in our architecture directly without having to redefine the qubit encoding.

Additionally, we present a new state preparation and measurement (SPAM) protocol that uses the ability to perform mid-circuit measurements to detect errors that occur during SPAM effectively. These errors can then be removed using post-selection or converted into erasures, one of the most straightforward errors to correct. We demonstrate the use of the protocol for three different qubit encodings in 137Ba+ : an optical qubit (O), a metastable level qubit (M), and a ground level qubit (G). For all three qubit types, we achieved the lowest reported SPAM infidelities of any qubit of O: 7(4) × 10−6, M: 5(4) × 10−6, and G: 8(4) × 10−6. The combination of these techniques can be used in building large-scale quantum computers based on trapped ions in the future.

Authors: Sotirova, AS

• DOI: 10.5287/ora-xrj88azx5
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: barium; trapped ions; quantum computing; ion traps
• Subjects: Trapped ions; Physics; Quantum computing; Ion traps