Measurement-Induced Transitions in Dispersive Qubit Readout Schemes

To make quantum computers practically useful, the underlying building blocks, called qubits, must be protected from the environment so that no errors occur during the computations. The current superconducting qubit technology is limited by the time it takes to check and fix these errors. A major roadblock to improving the performance of superconducting quantum processors comes from the inefficiencies in determining the quantum states of individual qubits because of the tendency of the qubits to leave the intended states during the measurement process. This project focuses on understanding the underlying physical mechanisms that govern the qubit transitions during the measurement process and aims to develop and experimentally verify methods for predicting these transitions in the state-of-the-art superconducting qubits to improve the performance of modern quantum processors and to expand the capabilities of superconducting quantum technologies. The societal benefits of the project include advancing the fundamental understanding of measurements in quantum mechanics and fostering novel design paradigms in the development of quantum devices and sensors. Graduate students working on the project will acquire theoretical and experimental skills essential for long-term economic growth driven by advanced technologies. The error rates required for performing practically useful quantum computations must be significantly below the rates achievable with the currently available physical qubits. Quantum error correction offers a way to effectively reduce error rates by encoding logical qubits within an array of physical qubits. The present-day implementations of the error correction codes with superconducting qubits are largely limited by the time that it takes to readout and reset the qubits used to detect the errors in the remaining part of the circuit. Further progress in shortening the readout time and increasing the measurement fidelity of the widely used dispersive qubit measurement scheme is hindered by the measurement-induced qubit transitions into the out-of-computational space. This project aims to provide a universal framework for systematically identifying the measurement-induced transitions and analyzing the limitations imposed on the dispersive readout by these transitions. The theoretical framework for estimating the maximum number of photons will be experimentally verified in model qubit systems such as transmons and fluxonia. Once verified, it will be applied to optimize the dispersive readout of fluxonium qubits. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.