关于超强耦合基态虚激发子的非破坏测量研究

The ultra-strong coupling Quantum electrodynamics (QED) system, which is quite different from the weak coupling case, is one of the most hot topic in the field of quantum control. Exploring the quantum properties of the analytical eigenstates and dynamic evolution of an ultra-strong coupling QED system will help us exploit its potential applications in both quantum optics and quantum information processing. Lots of evidence indicate that, in the ground state of an ultra-strong coupling system, there are virtual photons that can not spontaneously emit outside in the cavity, and the atom (qubit) also has a certain probability in its excited state. Observing the quantum effects of these virtual excitations will give us a better understanding of the QED theories. Recent studies find that, by employing parametrically modulated couplings, one can achieve better quantum nondemolition (QND) measurements, and the signal-to-noise ratio can be significantly enhanced. In our present research plan, we intend to realize the QND measurements of the virtual excitations of an superconducting circuit-QED system with ultra-strong coupling. Our purposes include proposing novel superconducting circuit layouts with parametrically modulated longitudinal and optomechanical-like coupling, and finding the method for performing the QND measurement based on the virtual excitations. With theoretical analyzing the dynamics of the measurement, we want to find the ways to improve the signal-to-noise ratio and precision of the QND measurements. The realization of these targets can not only help us have a better understanding of the virtual excitations in the ultra-strong coupling QED systems, but also can provide us more probability to extend the field of quantum control to the ultra-strong coupling regime.

超强耦合的量子电动力学系统（QED）是近几年量子调控领域的研究热点，它和弱耦合QED系统有着显著的区别。研究超强耦合系统本征态性质和动力学特性，对量子光学和量子信息处理等应用领域研究内容的扩展有重要的意义。有研究发现，超强耦合系统的基态中包含无法自发辐射出的虚光子和比特虚激发态。实现这些虚激发子的观测，是超强耦合QED研究中的一个基本问题。最新的研究证据表明，参数调制耦合可以实现较好的非破坏测量，在提高测量精度上有重大应用潜力。在这个研究计划中，基于超导量子电路系统。我们尝试利用参数调制耦合来实现超强耦合QED系统中基态虚激发子的测量。我们的目标是在理论上构造对应的调制耦合测量系统。通过分析，找到提高虚光子测量信噪比的方法。这些研究结果的实现，不仅能加深我们对超强耦合QED系统的性质和动力学演化的理解，还可以扩展为一般的光子和比特测量方法,为量子操控提供更多的可能性。

超强耦合的量子电动力学系统（QED）是和弱耦合QED系统有着显著的区别。超强耦合下的基态不再是腔的真空态和比特基态的直积形式，其中叠加有虚光子态，比特也有一定概率处于激发态。本项目重点关注超导量子电路系统中的强耦合和超强耦合现象，并找出超强耦合虚激发子探测的非破坏测量手段。.基于这些目的，一方面我们要关注如何实现用于非破坏测量的新型耦合电路系统，另外一方面，我们还要找出快速制备超强耦合高保真基态的方法。针对以上问题，我们在超导量子电路系统中的超强耦合、量子非破坏测量和和量子调控等问题展开了一系列的研究。这些包括1) 提出用于超强耦合虚激发子测量的非微扰的色散耦合，该方案可以避免常见的色散耦合Purcell效应带来的测量误差。借助这个方案，我们就可以很好地进行超强耦合的比特虚激发态的测量。2）利用绝热捷径制备超强耦合系统的基态。该方案解决了调制过程中制备时间过慢和非绝热引起的基态向高能态跃迁的错误概率，通过构造绝热捷径哈密顿量，不仅可以有效降低非绝热效应，还能缩短制备超强耦合基态的时间。3）构造了用于光子测量的类光机械系统。该系统可以用于超强耦合系统中虚光子的非破坏测量。.除了以上工作，在该项目的支持下，我们还在超导量子电路系统中关注了声子混合系统和手性量子光学中的强耦合现象，以上连续性的工作较好地完成了项目的既定目标，为探索和利用超强耦合系统的特性提供了新的思路和方案。

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