The field of quantum computing is growing at a rapid rate with the promise of dramatic improvements in the ability to solve critical computational problems. A leading approach for implementing fault-tolerant quantum computers is based on superconducting circuits containing Josephson junctions to form qubits. These qubits have many attractive qualities, but predicting the device performance requires detailed knowledge of the junction properties. Thus, precise characterization is crucial for realizing different qubit designs. The coherence of superconducting qubits is limited by several sources in the circuit en- vironment. Building a quantum computer requires effort to reduce gate errors caused by decoherence. This is typically achieved with quantum error correction. An alternative ap- proach involves designing qubits that are intrinsically protected from noise sources. One such design is the charge-parity qubit, which combines Josephson junctions in a novel regime. The capacitance of the junctions is a critical parameter for this qubit design that directly affects the engineered protection against decoherence. Beyond the conventional geometric capaci- tance, there is an excess contribution from virtual quasiparticle tunneling; we refer to this component as the electronic capacitance. This thesis focuses on the characterization of the capacitance of Josephson junctions, with the goal of isolating and measuring the electronic capacitance contribution, particularly for use in the charge-parity qubit. The thesis first reviews the initial implementation of the charge-parity qubit and the challenges that were encountered. Next, the theoretical origin is described and a framework for measuring the junction capacitance using superconducting resonant circuits is described. This technique is then applied to arrays of junctions fabricated with different parameters and the scaling of the electronic capacitance is presented. Finally, based on this knowledge of the junction capacitance, second-generation charge-parity qubits are implemented and the experimental results are presented.
We outline the physical principles underlying the design of finite size superconducting arrays that may serve as protected qubits. These systems are built from fully frustrated rhombi consisting of four Josephson junctions; their low energy spectrum is composed of localized Z2 charge excitations and π phase vortices. Protection from both the phase and flip errors requires a balance between charging and Josephson energies; optimization of the array size results in the hierarchical construction of these arrays. We have developed computational methods, based on a combination of perturbation theory and numerical diagonalization, which allow us to compute the global phase stiffness and the energy of localized charge excitations in various geometries. We present the most promising designs, which are currently under experimental investigation.
Impacts from high-energy particles cause correlated errors in superconducting qubits by increasing the quasiparticle density in the vicinity of the Josephson junctions (JJs). Such errors are particularly harmful as they cannot be easily remedied via conventional error correcting codes. Recent experiments reduced correlated errors by making the difference in superconducting gap energy across the JJ larger than the qubit transition energy. In this work, we assess gap engineering near the JJ (
δΔJJ) and the capacitor/ground-plane (
δΔM1) by exposing arrays of transmon qubits to two sources of radiation. For
α-particles from an
241Am source, we observe
T1 errors correlated in space and time, supporting a hypothesis that hadronic cosmic rays are a major contributor to the
10−10 error floor observed in Ref. 1. For electrons from a pulsed linear accelerator, we observe temporally correlated
T1 and
T2 errors, this measurement is insensitive to spatial correlations. We observe that the severity of correlated
T1 errors is reduced for qubit arrays with a greater degree of gap engineering at the JJ. For both
T1 and
T2 errors, the recovery time is hastened by an increased
δΔM1, which we attribute to the trapping of quasiparticles into the capacitor/ground-plane. We construct a model of quasiparticle dynamics that qualitatively agrees with our observations. This work reinforces the multifaceted influence of radiation on superconducting qubits and provides strategies for improving radiation resilience.