Doctoral thesis
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Marina Kudra, Quantum Technology

Titel: Building a bosonic microwave qubit


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Opponent: Professor Steven Girvin, Yale University, New Haven, CT, USA
Main supervisor: Professor Per Delsing
Examiner: Professor August Yurgens

Abstract: Superconducting circuits is a promising platform for quantum computing. Quantum information is usually stored in discrete two-level qubits e.g. in transmon qubits. These qubits are interconnected and placed in grids to form logical qubits, and many logical qubits together form a quantum computer.

In this thesis, we consider encoding quantum information in a resonator instead of the two-level qubit. Resonators can host bosonic modes that have, in principle, an infinite number of quantum levels in which we redundantly can encode a discrete qubit. This makes bosonic qubits hardware efficient, since we can perform error correction directly on a single hardware component, namely the resonator. However, we will still need to use an ancilla two-level qubit to universally control the bosonic qubit. This thesis can be interpreted as an instruction guide on creating a bosonic microwave qubit and it contains the following chapters.

We first introduce the cryogenic setup and the state-of-the-art room-temperature hardware that generates the microwave pulses we need to perform all the experiments in this thesis. We discuss the latest generation of the room-temperature measurement- and control-system we used for both bosonic and discrete variable qubit systems.

We then introduce the hardware components that are needed to form a bosonic qubit, namely a superconducting transmon qubit and a 3D superconducting cavity. We explore the fluctuations of their coherence properties, and we try to understand the sources of noise that limit those properties.

Next, we create arbitrary bosonic states and gates by using interleaved sequences of displacements and optimized selective number-dependent arbitrary phase gates. We characterize a bosonic gate, the X-gate on the binomially encoded qubit, by coherent state process tomography.

We then characterize the selective photon addition gate. We implement this gate by a comb of off-resonant drives that simultaneously excite the qubit and add a photon to the cavity depending on its state. Supplemented by an unconditional qubit reset, this gate is suitable for single photon error correction.