Quantum computing based on superconducting qubits requires the understanding and control of the materials, device architecture, and operation. However, the materials for the central circuit element, the Josephson junction, have mostly been focused on using the AlO x tunnel barrier. Here, we demonstrate Josephson junctions and superconducting qubits employing two-dimensional materials as the tunnel barrier. We batch-fabricate and design the critical Josephson current of these devices via layer-by-layer stacking N layers of MoS 2 on the large scale. Based on such junctions, MoS 2 transmon qubits are engineered and characterized in a bulk superconducting microwave resonator for the first time. Our work allows Josephson junctions to access the diverse material properties of two-dimensional materials that include a wide range of electrical and magnetic properties, which can be used to study the effects of different material properties in superconducting qubits and to engineer novel quantum circuit elements in the future.
Abstract Decoherence in quantum bits (qubits) is a major challenge for realizing scalable quantum computing. One of the primary causes of decoherence in qubits and quantum circuits based on superconducting Josephson junctions is the critical current fluctuation. Many efforts have been devoted to suppressing the critical current fluctuation in Josephson junctions. Nonetheless, the efforts have been hindered by the defect-induced trapping states in oxide-based tunnel barriers and the interfaces with superconductors in the traditional Josephson junctions. Motivated by this, along with the recent demonstration of 2D insulator h -BN with exceptional crystallinity and low defect density, we fabricated a vertical NbSe 2 / h -BN/Nb Josephson junction consisting of a bottom NbSe 2 superconductor thin layer and a top Nb superconductor spaced by an atomically thin h -BN layer. We further characterized the superconducting current and voltage ( I – V ) relationships and Fraunhofer pattern of the NbSe 2 / h -BN/Nb junction. Notably, we demonstrated the critical current noise (1/ f noise power) in the h -BN-based Josephson device is at least a factor of four lower than that of the previously studied aluminum oxide-based Josephson junctions. Our work offers a strong promise of h -BN as a novel tunnel barrier for high-quality Josephson junctions and qubit applications.
Nb/Al-AlOx/Nb tunnel junctions with controllable critical current density JC are fabricated using the standard selective Nb etching process. Tunnel barriers are formed in different oxygen exposure conditions (oxygen pressure P and oxidation time t), giving rise to JC ranging from 100 A/cm2 to above 2000 A/cm2. JC shows a familiar linear dependence on P × t in logarithmic scales. We calculate the energy levels of the phase- and flux-type qubits using the achievable junction parameters and show that the fabricated Nb/Al-AlOx/Nb tunnel junctions can be used conveniently for quantum computation applications in the future.
The Josephson energy
EJ is a key parameter governing the properties of transmon superconducting qubits. In Al/AlO
x/Al junctions,
EJ is set by electron tunneling through an ultrathin oxide barrier and therefore depends exponentially on the atomic structure of the barrier. We compute
EJ by first-principles device modeling based on the NEGF-DFT quantum-transport method, comparing a junction with a crystalline Al
2O
3 barrier against ten junctions with melt--quenched amorphous Al
2O
3 barriers of the same thickness. From the Fermi-level transmission and the Ambegaokar--Baratoff relation, we obtain a mean
EJ/h of
2.78 GHz for the amorphous ensemble, with a standard deviation of
4.67 GHz, compared with
0.73 GHz for the crystalline reference; individual amorphous values span nearly two orders of magnitude. Scattering-state analysis shows that transport is quantum tunneling and that the variability originates from stoichiometric inhomogeneity of the amorphous oxide: Al-rich, low-barrier regions can connect into percolation-like tunneling pathways that strongly enhance the conductance. A realistic
200×200 nm
2 junction self-averages over more than
2×104 such microscopic regions. These results establish a quantitative atomistic route from oxide microstructure to the superconducting-circuit energy scale
EJ.
In superconducting quantum circuits, Josephson junctions are a key component to provide nonlinearity and enable superconducting qubits. Fabricating junctions with precise critical currents is crucial to defining qubit frequency. Here we suppress the critical current variation of Josephson junctions by varying the critical current density Jc and junction areas. The relative standard deviation of the critical current Ic is controlled to be below 0.8%. This uniform fabrication is promising for realizing scalable superconducting quantum computation.
The superconductive Josephson junction is the key device for superconducting quantum computation. We have fabricated Al/Al2O3/Al tunnel junctions using a double angle evaporation method based on a suspended shadow mask. The Al2O3 junction barrier has been formed by introducing pure oxygen into the chamber during the fabrication process. We have adjusted exposure conditions by changing either the oxygen pressure or the oxidizing time during the formation of tunnel barriers to control the critical current density J c and the junction specific resistance R c . Measurements of the leakage in Al/Al2O3/Al tunnel junctions show that the devices are suitable for qubit applications.