Reducing Quasiparticle Decoherence in Superconducting Quantum Circuits

量子信息科学-使用量子力学来执行新的计算，模拟，通信和传感-准备彻底改变计算，生物化学，凝聚态物理学，密码学和天文学，以及许多其他领域。最有前途的技术平台之一是基于由超导材料制成并在低温下运行的电路。基于这些量子电路的量子计算机已经被创建并用于简单的应用。然而，这些电路受到退相干的限制，退相干是由于环境噪声导致的量子信息的丢失或扰乱。这种退相干的一个限制性来源是超导材料中过量准粒子激发的存在。这些准粒子在量子电路中无处不在，但它们的起源和行为仍然知之甚少。在这个项目中，PI正在研究准粒子产生和破坏的机制，描述捕获准粒子远离敏感电路元件的不同技术，并利用所获得的知识来减轻它们对量子电路的有害影响。该项目还培养了一名研究生和一名博士后学者在快速发展的量子信息技术领域，为学术界和工业界增加了劳动力。技术摘要：即使在非常低的温度下，超导量子电路中也存在准粒子的非平衡种群，限制了这些电路的相干性，并导致量子处理器的错误。这些准粒子的来源，它们在量子电路中的行为，以及减轻其影响的最佳方法都知之甚少。本项目的目标是描述这些源的特征，降低准粒子产生率，更好地理解准粒子湮灭和捕获的机制，提高湮灭和捕获率，并普遍降低准粒子减轻退相干。所涉及的实验是基于对相干超导电路的高保真测量，该电路被设计成对准粒子敏感。这些包括谐振器结合相位偏置的纳米桥约瑟夫森结，其内部的Andreev状态作为准粒子陷阱，作为敏感的非饱和准粒子探测器。这些Andreev设备，准粒子之间的时间和空间的相关性进行测量和准粒子的弛豫和激发机制的特点。具有可调频率的量子比特被用来测量准粒子的能量分布;具有准粒子敏感光谱的transmons被用来测量准粒子的密度和输运特性。最后，利用线性波导谐振腔的品质因数测试，测量了准粒子的密度和输运。所有这些设备都用于测试减轻准粒子的方法，包括辐射屏蔽，增加准粒子陷阱，改变电路材料，改变电路设计和新的操作模式。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Nontechnical abstract:Quantum information science -the use of quantum mechanics to perform novel computing, simulation, communication, and sensing -is poised to revolutionize computing, biochemistry, condensed matter physics, cryptography, and astronomy, as well as a host of other fields. One of the most promising technology platforms is based on electrical circuits made of superconducting materials and operated at cryogenic temperatures. Quantum computers based on these quantum circuits have already been created and used for simple applications. However, these circuits are limited by decoherence, the loss or scrambling of quantum information due to environmental noise. A limiting source of this decoherence is the presence of excess quasiparticle excitations in the superconducting material. These quasiparticles are ubiquitous in quantum circuits, but their origins and behavior remain poorly understood. In this project, the PI is studying the mechanisms by which quasiparticles are created and destroyed, characterizing different techniques for trapping quasiparticles away from sensitive circuit elements, and using the knowledge gained in order to mitigate their harmful effects on quantum circuits. The project also trains one graduate student and one postdoctoral scholar in the rapidly expanding field of quantum information technology, growing the workforce for both academia and industry.Technical abstract:Nonequilibrium populations of quasiparticles exist in superconducting quantum circuits even at very low temperatures, limiting the coherence of these circuits and causing errors in quantum processors. The sources of these quasiparticles, their behavior in quantum circuits, and the best methods for mitigating their effects are all poorly understood. The goal of this project is to characterize these sources and reduce the quasiparticle generation rate, to better understand the mechanisms of quasiparticle annihilation and trapping and to increase the annihilation and trapping rates, and to generally reduce quasiparticle-mitigated decoherence. The experiments involved are based on high-fidelity measurements of coherent superconducting circuits engineered to be sensitive to quasiparticles. These include resonators incorporating phase-biased nanobridge Josephson junctions, whose internal Andreev states serve as quasiparticle traps, as sensitive non-saturating quasiparticle detectors. With these Andreev devices, temporal and spatial correlations between quasiparticles are measured and mechanisms of quasiparticle relaxation and excitation are characterized. Transmon qubits with tunable frequency are used to measure quasiparticle energy distributions; transmons engineered with quasiparticle-sensitive spectra are used to measure quasiparticle density and transport characteristics. Finally, tests of linear waveguide resonators' quality factors are used to measure quasiparticle density and transport. All these devices are used to test ways of mitigating quasiparticles, including radiation shielding, adding quasiparticle traps, changing circuit materials, altering circuit design, and novel modes of operation. The use of these circuits as quasiparticle-based detectors is also explored.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.