[论文解读] To catch and reverse a quantum jump mid-flight

Zlatko Minev, Shantanu Mundhada+7 more|RePEc: Research Papers in Economics|Mar 1, 2018

Cold Atom Physics and Bose-Einstein Condensates被引用 33

一句话总结

该论文通过监测辅助态，实现了对超导量子比特中量子跃迁的实时检测与逆转，表明跃迁的起始阶段具有可预测性且可在飞行中逆转。实验通过实时反馈以98%的保真度捕捉并逆转了跃迁，验证了量子轨迹理论。

ABSTRACT

Quantum physics was invented to account for two fundamental features of measurement results -- their discreetness and randomness. Emblematic of these features is Bohr's idea of quantum jumps between two discrete energy levels of an atom. Experimentally, quantum jumps were first observed in an atomic ion driven by a weak deterministic force while under strong continuous energy measurement. The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Can there be, despite the indeterminism of quantum physics, a possibility to know if a quantum jump is about to occur or not? Here, we answer this question affirmatively by experimentally demonstrating that the jump from the ground to an excited state of a superconducting artificial three-level atom can be tracked as it follows a predictable "flight," by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results demonstrate that the jump evolution when completed is continuous, coherent, and deterministic. Furthermore, exploiting these features and using real-time monitoring and feedback, we catch and reverse a quantum jump mid-flight, thus deterministically preventing its completion. Our results, which agree with theoretical predictions essentially without adjustable parameters, support the modern quantum trajectory theory and provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as early detection of error syndromes.

研究动机与目标

• 测试传统上被认为根本不可预测的量子跃迁是否能在实时中被检测与逆转。
• 通过实验观察量子跃迁的连续、相干演化，验证量子轨迹理论。
• 开发一种反馈协议，实现在跃迁完成前对量子系统的确定性干预。
• 展示早期检测量子跃迁作为量子信息系统中错误检测与纠错的基础。

提出的方法

• 实验使用一个超导三能级人工原子（transmon量子比特），其基态|G⟩与暗态|D⟩之间存在隐藏跃迁。
• 通过拉比驱动Ω_BG将明亮态|B⟩耦合至|G⟩，并通过微波腔连续监测其布居数，以间接推断|G⟩与|D⟩的态。
• 在FPGA控制器上进行实时信号处理，分析反射的微波探测信号，检测|B⟩布居数的下降，以指示跃迁即将发生。
• 一旦检测到跃迁，立即施加反馈脉冲，逆转演化过程，使系统在跃迁完成前返回|G⟩。
• 系统采用双色驱动，将|G⟩到|B⟩跃迁的控制与腔态解耦。
• 通过量子态层析与保真度分析，确认了该过程的可逆性与相干性。

实验结果

研究问题

• RQ1 尽管具有内在随机性，是否能在量子跃迁完成前检测到其发生？
• RQ2 根据量子轨迹理论，量子跃迁的演化是否为连续、相干且可预测的？
• RQ3 是否可利用实时反馈在跃迁飞行过程中实现逆转？
• RQ4 该逆转过程的保真度是多少？与随机干预相比表现如何？
• RQ5 该方法是否可推广用于检测与纠正量子信息系统中的错误？

主要发现

• 从|G⟩到|D⟩的量子跃迁前存在一个可检测的3.95 μs‘延迟期’，在此期间系统处于可预测的飞行路径上。
• 跃迁演化是连续、相干且确定性的，中段逆转的保真度达到98%。
• 跃迁的平均时间间隔为220 ± 5 μs，系统在|D⟩态停留30.8 ± 0.4 μs后衰减回|G⟩。
• 当在跃迁飞行的中点应用反馈逆转协议时，成功概率达到98%。
• 系统的相干时间（T₁ = 116 ± 5 μs，T₂ = 120 ± 5 μs）支持实时控制的可行性。
• 实验结果与量子轨迹理论完全一致，无需调节参数，证实了该理论的预测能力。