Superconducting Flux Qubits

Tokunoshin Uchida, Shingo Nozaki, Daiki Kurihara +5 more

We report a demonstration of a self-shunted superconducting flux qubit (SSFQ) as an ultracompact, strongly anharmonic, and long-lifetime qubit by using the full epitaxially grown NbN/AlN/NbN Josephson junctions. The NbN-junction-based SSFQ achieved a high anharmonicity of 797 MHz as well as drastically reduced footprint of the qubit compared to capacitively shunted (C-shunt) flux qubits with a large shunt capacitor. The lifetime of the SSFQ reached 13 - 25 $μ$s, which is comparable to or relatively higher than that of NbN-based C-shunt flux qubits and Al-based flux qubits with more than 10 times smaller junctions. This indicates that the large epitaxial NbN-based junction can be adopted for SSFQ as well as merged-element transmon with the relatively large junctions. The results presented in this work will offer a new choice of the scalable qubits as well as the material platform toward the realization of large-scale superconducting quantum computers.

Eddy Collin, Grégoire Ithier, P. Joyez +2 more

Evgeni V. Il’ichev

Tokunoshin Uchida, Hiroki Kutsuma, Taro Yamashita

We proposed and theoretically investigated the self-shunted superconducting flux qubits (SSFQs) with relatively large Josephson junctions with the diameter over$\text{1}\; \mu \text{m}$. Because of the large self-capacitance of several 10 fF in the junctions, the large shunt capacitor with dimensions of several 100$ \mu \text{m}$of the conventional capacitively shunted flux quantum bits (qubits) can be eliminated, resulting in significant reduction of the footprint. We systematically calculated the SSFQ performances, such as the anharmonicity, and investigated their dependences on the circuit parameters. As a result, it was shown that the SSFQs have the large anharmonicity of 400–2000 MHz for the reasonable parameters, such as the large junction area of around 1$ \mu \text{m}^{2}$and the small critical current density of around 10$\text{A/cm}^{2}$. Furthermore, we derived and estimated the relative relaxation time of the SSFQs due to the charge and flux noises. As a result, we found that the high anharmonicity over 800 MHz could be achieved without degrading the relaxation time from that of the high-coherence flux qubits. The SSFQs offer the new choice of the highly scalable qubits, which achieve both the small footprint and high anharmonicity.

Gianluigi Catelani

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.