[论文解读] High-fidelity parametric beamsplitting with a parity-protected converter

Yao Lu, Aniket Maiti+7 more|arXiv (Cornell University)|Mar 2, 2023

Quantum Information and Cryptography参考文献 69被引用 7

一句话总结

该论文展示了一种快速高保真参数浴分器，在两个高Q微波腔之间通过差分驱动的对称性保护DC-SQUID 转换器实现，耦合抑制后达到超过99.98%的浴分保真度，并抑制耦合器引起的去相干。

ABSTRACT

Fast, high-fidelity operations between microwave resonators are an important tool for bosonic quantum computation and simulation with superconducting circuits. An attractive approach for implementing these operations is to couple these resonators via a nonlinear converter and actuate parametric processes with RF drives. It can be challenging to make these processes simultaneously fast and high fidelity, since this requires introducing strong drives without activating parasitic processes or introducing additional decoherence channels. We show that in addition to a careful management of drive frequencies and the spectrum of environmental noise, leveraging the inbuilt symmetries of the converter Hamiltonian can suppress unwanted nonlinear interactions, preventing converter-induced decoherence. We demonstrate these principles using a differentially-driven DC-SQUID as our converter, coupled to two high-Q microwave cavities. Using this architecture, we engineer a highly-coherent beamsplitter and fast ($\sim$ 100 ns) swaps between the cavities, limited primarily by their intrinsic single-photon loss. We characterize this beamsplitter in the cavities' joint single-photon subspace, and show that we can detect and post-select photon loss events to achieve a beamsplitter gate fidelity exceeding 99.98$\%$, which to our knowledge far surpasses the current state of the art.

研究动机与目标

• 推动微波谐振腔之间用于玻色子量子计算与仿真的快速高保真两模相互作用。
• 解决限制先前浴分器的寄生过程和驱动引起的去相干。
• 利用对称性（奇偶保护）和差分驱动抑制不希望的非线性相互作用。
• 演示两个高Q腔之间的浴分器，其相干性主要受固有光子损耗限制。

提出的方法

• 使用一个对称DC-SQUID（奇偶保护）作为非线性转换器耦合两个高Q腔。
• 通过缓冲器的差分驱动（λ/4 模式）实现腔之间的共振、可调浴分器。
• 推导浴分哈密顿量，其中 g_BS 与 J1(|φ_d1|)J1(|φ_d2|) 成正比，Δ_BS = Δ_ab − Δ_d + Δ_Z,ab。
• 设计差分驱动以使耦合器保持在基态，抑制驱动引起的激发。
• 在联合单光子子空间（双轨量子比特）中表征性能，并进行带泄露检测的随机基准测试。
• 从 κ_BS 和 g_BS 估计由去相干限制的保真度，以评估门控性能。

Figure 1: The differentially driven SQUID as a parity protected converter. a , The symmetric DC-SQUID contains two orthogonal modes SQUIDModesLecocq2011 ; KamalSQUIDModes , the common mode (coupler) and the differential mode (actuator). We selectively couple the former to two bosonic modes and the l

实验结果

研究问题

• RQ1 对称性保护的差分驱动转换器是否能够在两个高Q腔之间实现快速高保真的浴分？
• RQ2 在参数浴分过程中，主导的去相干通道是什么？对称性抑制是否能减轻耦合器引起的误差？
• RQ3 在现实驱动强度和损耗下，单光子（双轨）子空间中浴分器的性能如何？
• RQ4 光子丢失事件的后选（post-selection）在多大程度上提高了有效门保真度？
• RQ5 此方法是否可扩展到其他玻色模或超越示范的多模操作？

主要发现

• 浴分器在工作点达到 g_BS/2π > 5 MHz。
• 在一系列驱动强度下，去相干限制的保真度超过99.9%。
• 在工作点的驱动振幅下，耦合器激发仍然可忽略，未随驱动单调增加。
• 耦合器的加热抑制至约4×10^−5 次换的激发量，与无驱动的背景水平相符。
• 原始随机基准测试给出有效未筛选浴分器保真度为99.941(1)%，带泄漏检测的保真度为99.985(1)%。
• 保真度最终受腔光子损耗限制，保留了对玻色编码有利的噪声偏置。

Figure 2: Beamsplitting with the differentially-driven SQUID. a , Beamsplitting implements an effective driven Rabi evolution in the Bloch sphere of the dual-rail qubit formed by the single photon subspace Alice and Bob, where decay can be detected by monitoring the vacuum state. b , Resonant evolut