Simone Fasciati

Abstract:

Using quantum systems with more than two levels, or qudits, can scale the computational space of quantum processors more efficiently than using qubits, which may offer an easier physical implementation for larger Hilbert spaces. However, individual qudits may exhibit larger noise, and algorithms designed for qubits require to be recompiled to qudit algorithms for execution. In this work, we implemented a two-qubit emulator using a 4-level superconducting transmon qudit for variational quantum algorithm applications and analyzed its noise model. The major source of error for the variational algorithm was readout misclassification error and amplitude damping. To improve the accuracy of the results, we applied error-mitigation techniques to reduce the effects of the misclassification and qudit decay event. The final predicted energy value is within the range of chemical accuracy.

Abstract:

Superconducting circuits are a leading platform for quantum information processing, partly due to the great freedom of tailoring circuit parameters which enables the implementation of a wide variety of Hamiltonians. Many simple superconducting circuits commonly employed as qubits contain a variation of superconducting quantum interference device (SQUID) that adds in-situ magnetic flux tunability to the system and further increases its flexibility. At the same time, this control parameter requires additional dedicated circuitry and can introduce flux noise which is detrimental to qubit performance. This increases hardware complexity and hinders the scaling to large qubit numbers. In this thesis, we develop and test a simple 3D-integrated architecture for individual flux control of tileable, coaxial, gradiometric superconducting qubits, achieving highly selective flux bias (low crosstalk) and incorporating both charge and flux control into a single off-chip element. The addition of flux tunability fully retains the simplicity of the fabrication and packaging process of the original, fixed-frequency coaxial architecture. We then use this experimental platform to study the inductively shunted transmon (IST), an interesting qubit species based on a radio-frequency (RF) SQUID loop with a small linear shunting inductance. Its qualitative behavior is similar to a transmon qubit but with a positive instead of negative anharmonicity. We design, simulate, fabricate and experimentally characterize gradiometric IST qubits, showing that they can be easily integrated into an existing transmon-based quantum processor architecture. Further, by directly coupling an IST to a transmon via a mutual capacitance, we demonstrate how the opposite signs of anharmonicity can be exploited to effectively reduce the undesired static longitudinal coupling (ZZ interaction) to zero. We also investigate microwave sideband transitions in this two-qubit system and benchmark a controlled-Z (CZ) entangling gate. This work paves the way towards hardware-efficient, crosstalk-suppressed superconducting quantum processors based on multi-species qubit lattices.