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Acta Physica Sinica

Preparation of niobium based superconducting qubits and auxiliary devices

中国科学院大学,, 中科院物理所固态量子信息与计算实验室,, 中国科学院物理研究所,

Published Jan 1, 2021• 0 citations
Publisher

Abstract

近年来超导量子计算的研究方兴未艾，随着谷歌宣布首次实现“量子优势”和对时间晶体的观察，超导量子计算的研究受到了人们进一步的广泛关注。超导量子比特是具有量子化能级、量子态叠加和量子态纠缠等典型量子特性的宏观器件，通过电磁脉冲信号控制磁通量、电荷或具有非线性电感和无能量耗散的约瑟夫森结上的位相差，可对量子态进行精确调控，从而实现量子计算和量子信息处理。超导量子比特有着诸多方面的优势，很有希望成为普适量子计算的核心组成部分。以铌或其它硬金属（如钽等）为首层大面积材料制备的超导量子比特及辅助器件（本文简称铌基器件）拥有其独特的优点以及进一步发展的空间，目前已引起人们越来越多的兴趣。本文将介绍常见的多种超导量子比特的基本构成和工作原理，进而按照器件加工的一般顺序，从基片选择和预处理、薄膜生长、图形转移、刻蚀和约瑟夫森结的制备等方面详细介绍铌基超导量子比特及其辅助器件的多种制备工艺，为超导量子比特的制备提供一个可借鉴的清晰的工艺过程。最后，我们将介绍若干制备铌基超导量子比特与辅助器件的具体例子，并对器件制备的工艺与方法的优化做一展望。

DOI
10.7498/aps.70.20211865

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张量网络方法是研究量子多体物理的重要理论框架，其核心思想是利用量子态内部的纠缠结构，将具有指数复杂度的量子态拆解为由低维张量构成的网络，从而实现对多体系统的高效表达和模拟。近年来，张量网络的研究已由封闭系统拓展至非厄米及开放量子系统，在理论发展和实际应用等方面均取得显著进展。本文系统回顾了张量网络方法的发展历程，重点介绍其在量子多体物理中的代表性应用，包括强关联电子系统、量子计算与量子信息、拓扑物态等方向。研究表明，张量网络能够高效精确地模拟强关联量子多体系统，刻画各种有序态之间的竞争。混态张量网络可描述含噪声的量子态，为理解本征拓扑序和对称性保护拓扑序提供统一的理论框架。总之，张量网络已成为连接量子多体计算、量子计算、拓扑物态的通用语言，并将在量子物理的研究中发挥更关键的作用。
Over the past two decades significant advances have been made in the research of superconducting quantum computing and quantum simulation, in particular of the device design and fabrication that leads to ever-increasing superconducting qubit coherence times and scales. With Google’s announcement of the realization of “quantum supremacy”, superconducting quantum computing has attracted even more attention. Superconducting qubits are macroscopic objects with quantum properties such as quantized energy levels and quantum-state superposition and entanglement. Their quantum states can be precisely manipulated by tuning the magnetic flux, charge, and phase difference of the Josephson junctions with nonlinear inductance through electromagnetic pulse signals, thereby implementing the quantum information processing. They have advantages in many aspects and are expected to become the central part of universal quantum computing. Superconducting qubits and auxiliary devices prepared with niobium or other hard metals like tantalum as bottom layers of large-area components have unique properties and potentials for further development. In this paper the research work in this area is briefly reviewed, starting from the design and working principle of a variety of superconducting qubits, to the detailed procedures of substrate selection and pretreatment, film growth, pattern transfer, etching, and Josephson junction fabrication, and finally the practical superconducting qubit and their auxiliary device fabrications with niobium base layers are also presented. We aim to provide a clear overview for the fabrication process of these superconducting devices as well as an outlook for further device improvement and optimization in order to help establish a perspective for future progress.
光谱测量与成像技术的进步，已成为生命科学与材料科学研究中的关键表征手段。然而，在针对光学响应微弱的样品（如低维材料、活体细胞等）进行实际观测时，高功率激发光往往引入显著经典噪声，并造成不可忽视的光致损伤，从而限制了光学测量的信噪比与应用范围。在此背景下，具备特殊量子统计特性的非经典光源（如纠缠光与压缩光）为实现超越经典极限的信噪比提升提供了极具潜力的解决方案。本文聚焦于量子增强光谱与成像领域，系统梳理了基于纠缠光与压缩光两类重要量子光源的最新研究进展。纠缠光凭借光子间的量子关联特性，在关联成像、非探测光子成像及超快干涉测量等应用中展现出卓越的抗噪声能力；而压缩光则通过压缩光场噪声，在位移传感、等离激元探测及非线性显微成像等方面显著提升了探测灵敏度与信噪比。本文进一步系统阐述了量子光源在提高信噪比、降低光致损伤、提升时间分辨率以及增强非线性转换效率等方面的独特优势，并分析了当前制约量子成像技术实用化的关键挑战，包括光源亮度低，系统损耗大等问题。最后，对该领域未来的发展方向进行了展望。
Niobium films are a key component in modern two-dimensional superconducting qubits, yet their contribution to the total qubit decay rate is not fully understood. The presence of different layers of materials and interfaces makes it difficult to identify the dominant loss channels in present two-dimensional qubit designs. In this paper, we present the study that directly correlates measurements of RF losses in such films to material parameters by investigating a high-power impulse magnetron sputtered (HiPIMS) film atop a three-dimensional niobium superconducting radio frequency (SRF) resonator. By using a 3D SRF structure, we are able to isolate the niobium film loss from other contributions. Our findings indicate that microwave dissipation in the HiPIMS-prepared niobium films, within the quantum regime, resembles that of record-high intrinsic quality factor of bulk niobium SRF cavities, with lifetimes extending into seconds. Microstructure and impurity level of the niobium film do not significantly affect the losses. These results set the scale of microwave losses in niobium films and show that niobium losses do not dominate the observed coherence times in present two-dimensional superconducting qubit designs, instead highlighting the dominant role of the dielectric oxide in limiting the performance. We can also set a bound for when niobium film losses will become a limitation for qubit lifetimes.
Abstract We present a transmon qubit fabrication technique that yields systematic improvements in T 1 relaxation times. We encapsulate the surface of niobium and prevent the formation of its lossy surface oxide. By maintaining the same superconducting metal and only varying the surface, this comparative investigation examining different capping materials, such as tantalum, aluminum, titanium nitride, and gold, as well as substrates across different qubit foundries demonstrates the detrimental impact that niobium oxides have on coherence times of superconducting qubits, compared to native oxides of tantalum, aluminum or titanium nitride. Our surface-encapsulated niobium qubit devices exhibit T 1 relaxation times 2–5 times longer than baseline qubit devices with native niobium oxides. When capping niobium with tantalum, we obtain median qubit lifetimes above 300 μs, with maximum values up to 600 μs. Our comparative structural and chemical analysis provides insight into why amorphous niobium oxides may induce higher losses compared to other amorphous oxides.