The Null Flux Qubit

Hideaki Takayanagi

S. Vallés-Sanclemente, T. H. F. Vroomans, Timo R. van Abswoude +8 more

We experimentally optimize the frequency of flux-tunable couplers in a superconducting quantum processor to minimize the impact of spectator transmons during quantum operations (single-qubit gates, two-qubit gates, and readout) on other transmons. We adapt a popular transmonlike tunable-coupling element, achieving high-fidelity, low-leakage controlled-Z gates with unipolar, fast-adiabatic pulsing only on the coupler. We demonstrate the ability of the tunable coupler to null residual-ZZ coupling as well as exchange couplings in the one- and two-excitation manifolds. However, the nulling of these coherent interactions is not simultaneous, prompting the exploration of trade-offs. We present experiments pinpointing spectator effects on specific quantum operations. We also study the combined effect on the three types of operations using repeated quantum parity measurements.

David K. Ferry

A. Yu. Dmitriev, O. V. Astafiev

Flux qubits are among the first qubits that were ever demonstrated. They have some advantages when compared to capacitively shunted charge qubits, which are now commonly used for building prototypes of quantum processors. Specifically, flux qubits are intrinsically nonlinear systems and they remain so even with low charging energies, which is important for the suppression of large charge noise in solids. In spite of the clear advantages of flux qubits, their applications in multi-qubit devices—prototypes of quantum computers and simulators—are still limited. Flux qubits are also a very powerful tool for fundamental research. In this paper, we discuss the basic properties of flux qubits using the radio frequency superconducting quantum interference device geometry—the most fundamental realization of flux qubits. We also compare and analyze experimental realizations of flux qubits and propose further directions for research.

David K. Ferry

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.