Fluxon readout for superconducting qubits

Long B. Nguyen, Yen‐Hsiang Lin, Nicholas Grabon +2 more

Hayato Nakano, Hideaki Takayanagi

Kirill G. Fedorov, Anastasia V. Shcherbakova, Michael Johannes Wolf +2 more

An experiment demonstrating a link between classical single-flux quantum digital logic and a superconducting quantum circuit is reported. We implement coupling between a moving Josephson vortex (fluxon) and a flux qubit by reading out of a state of the flux qubit through a frequency shift of the fluxon oscillations in an annular Josephson junction. The energy spectrum of the flux qubit is measured using this technique. The implemented hybrid scheme opens an opportunity to readout quantum states of superconducting qubits with the classical fluxon logic circuits.

Konstantin Nesterov, Long B. Nguyen, Aaron Somoroff +7 more

Shunsuke Kamimura, Aree Taguchi, Masamitsu Tanaka +1 more

We theoretically investigate a readout scheme of the quantum state of a superconducting qubit based on time delay of a single flux quantum (SFQ), also known as a fluxon, propagating in a Josephson transmission line (JTL). We concretely study the time-delay readout based on capacitive coupling between a transmon qubit and a JTL, and we evaluate the time delay depending on the qubit state. We also reveal a feature of the absence of fluxon pinning and exponential suppression of nonadiabatic transitions caused by the propagating fluxon, which is advantageous for the time-delay readout. We extend the analysis to a multi-level transmon as well. Owing to the spectral gap in the JTL, the radiative decay of the qubit mediated by the JTL is exponentially suppressed, and thus the transmission line itself also serves as a filter protecting the qubit. The readout scheme requires neither complicated wiring to low-temperature stages nor bulky microwave components, which are bottlenecks for integration of a large-scale superconducting quantum computer.

Kirill G. Fedorov

This thesis presents experimental studies on developing of a novel fluxon readout for superconducting flux qubits. It is based on a scheme for detecting the microwave radiation of an oscillating fluxon in an annular Josephson junction (AJJ). The readout was implemented for a superconducting flux qubit coupled as a current dipole to the AJJ. An energy spectrum of the flux qubit was measured via detecting a frequency shift of the fluxon oscillations versus a flux bias through the qubit.