Enhanced coherence of all-nitride superconducting qubits epitaxially grown on silicon substrate

Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation. However, coherence times of quantum circuits made from conventional aluminum-based Josephson junctions are limited by the presence of microscopic two-level systems in the amorphous aluminum oxide tunnel barriers. Here, we have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on silicon substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation, resulting in fewer two-level fluctuators, feasibility for epitaxial tunnel barriers that reduce energy relaxation and dephasing, and a larger superconducting gap of ~5.2 meV for NbN, compared to ~0.3 meV for aluminum, which suppresses the excitation of quasiparticles. By replacing conventional MgO by a silicon substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time $${T}_{1}=16.3\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 1 = 16.3 μ s and a spin-echo dephasing time $${T}_{2}=21.5\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 2 = 21.5 μ s . These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to the previously reported $${T}_{1}\approx {T}_{2}\approx 0.5\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 1 ≈ T 2 ≈ 0.5 μ s of NbN-based qubits on MgO substrates. These results are an important step towards constructing a new platform for superconducting quantum hardware.

A Novel Approach for Design and Investigation of Dual Material Stack Gate Oxide TFET using Oxide Strip Layer Mechanism

In this paper, for the first time, we use a distinctive approach based on oxide strip layer in dual material stack gate oxide-tunnel field-effect transistor (DMSGO-OSL-TFET) to improve the DC, analog/RF, and linearity performance. For this, a stack gate oxide with workfunction is considered to enhance the ONstate current (ION ) and reduce the ambipolar current (Iamb). For this case, the gate electrode is tri-segmented, named as tunnel gate (M1), control gate (M2) and auxiliary gate (M3) with different gate lengths (L1, L2, L3) and work functions (φ1, φ2, φ3), respectively. To maintain dual-work functionality, the possible combinations of these work functions are considered. Technology computer-aided design (TCAD) simulations are performed and noted that the workfunction combination (φ1 = φ3 < φ2) outperforms compared to other structures. Where φ1 on the source side is used to enhance the ION , while φ3 (equal to φ1) is used on the drain side to minimize the Iamb. To further enhance the device performance, a high-K oxide strip layer is considered on the drain side to suppress the (Iamb) whereas, a low-K oxide strip layer is used at the source junction to maximize the ION . Moreover, length of gate segments, oxide strip layer height, and thickness are optimized to achieve a better ION , switching ratio, subthreshold swing (SS) and reduce the (Iamb) which helps in the gain of device and design of analog/RF circuits. The proposed device as compared to dual material control gate-oxide strip layer-TFET (DMCG-OSL-TFET) shows improvement in ION /IOF F (∼ 4.23 times), 84 % increase in transconductance (gm), 136 % increase in cut-off frequency (fT ), 126 % increase in gain bandwidth product (GBP), point subthreshold swing (15.8 mV/decade) and other significant improvements in linearity performance parameters such as gm3, VIP3, IIP3, IMD3 making the proposed device useful for low power switching, analog/RF and linearity applications.

Simulating the fabrication of aluminium oxide tunnel junctions

Aluminium oxide (AlOx) tunnel junctions are important components in a range of nanoelectric devices including superconducting qubits where they can be used as Josephson junctions. While many improvements in the reproducibility and reliability of qubits have been made possible through new circuit designs, there are still knowledge gaps in the relevant materials science. A better understanding of how fabrication conditions affect the density, uniformity, and elemental composition of the oxide barrier may lead to the development of lower noise and more reliable nanoelectronics and quantum computers. In this paper, we use molecular dynamics to develop models of Al–AlOx–Al junctions by iteratively growing the structures with sequential calculations. With this approach, we can see how the surface oxide grows and changes during the oxidation simulation. Dynamic processes such as the evolution of a charge gradient across the oxide, the formation of holes in the oxide layer, and changes between amorphous and semi-crystalline phases are observed. Our results are widely in agreement with previous work including reported oxide densities, self-limiting of the oxidation, and increased crystallinity as the simulation temperature is raised. The encapsulation of the oxide with metal evaporation is also studied atom by atom. Low density regions at the metal–oxide interfaces are a common feature in the final junction structures which persists for different oxidation parameters, empirical potentials, and crystal orientations of the aluminium substrate.