Quantum information processing with mixed-species ion crystals

Trapped ions are among the leading platforms for the realization of scalable quantum information processing (QIP). A hybrid system composed of different species of ions extends the control capabilities of the system by providing new capabilities and extra degrees of freedom. A significant advantage of using two species of ion is the ability to individually manipulate and read out the state of one ion species without disturbing the quantum information stored in the internal states of the other. In this thesis I demonstrate different elements required for the control of mixedspecies ion chains composed of calcium and beryllium ions. One of the main results is the first realization of repeated measurements of quantum correlations of two beryllium ions that preserve their quantum entanglement. This is possible by reading out a cotrapped calcium ion that has previously interacted with the beryllium ions. Real-time feedback of the classical control system allows us to prepare and retain entangled states for up to fifty measurement rounds. The measurements consist of a series of quantum gates between the three ions, which involve mixed-species gates between beryllium hyperfine states and calcium electronic states. Independent experiments, also described in this thesis, show mixed-species entangled state fidelities of 98.4(9)% for two ions, and 93.8(5)% for three ions. The low-crosstalk mixed-species operations demonstrated in this work can be useful in quantum error correction as well as quantum metrology. The abilities of the classical control system and the precise control of mixed-species chains are further exploited to deterministically realize an arbitrary unitary operation on a single ion two-level system (qubit) using measurement-base quantum computing (MBQC). Within this framework, sequences of adaptive measurements on entangled states in a larger multi-qubit Hilbert space are used to realize quantum operations on the two-dimensional space of a single target qubit. As opposed to standard MBQC, entanglement is created on demand when needed in this thesis to minimize the additional resources required to control the target qubit. The work presented in this thesis provides the first not post-selected realization of single-qubit unitaries in the MBQC paradigm. Future algorithms will involve the manipulation of multiple ion chains of different types whose structure is continuously reconfigured through transport and splitting/recombination. This thesis also discusses the characterization of transport and separation of single and mixed-species ion chains. It shows the reliable splitting of single-species chains composed of two ions with a residual motional excitation below 1.6 quanta for calcium and below 0.7 quanta for beryllium. Mixed-species transport and splitting are also investigated. However, more characterization is required to make these processes reliable. Finally, this thesis also describes technical advancements in the software interface used for controlling and calibrating experiments. The interface consists of a low-level structure written in C++ that controls all the physical details of the apparatus, and provides a set of command structures that are independent of details of the physical setup and allow the users to write control scripts at a high level of abstraction. These tools are complemented by an improved graphical user interface that provides users with a significant degree of automation and sophisticated scheduling of calibration experiments through a Python-based scripting interface. This is the second edition of the thesis, released on the 22th of May 2020. The first edition was released on the 25th of March 2020.