Measurement and Control in Open Quantum Systems

根据量子力学，粒子没有明确的属性，比如位置和动量，而是用“复值波函数”来描述，用概率的数学方式描述它们。量子粒子的波动特性意味着这些粒子可以以看似不同状态的叠加态存在，例如一个量子粒子可以同时出现在两个地方，或者朝着两个不同的方向前进，或者占据两个不同能级的叠加态。这个波函数的演化遵循1925年制定的Schrödinger方程。自从Schrödinger方程形成以来，它就被应用于理解原子和分子的性质以及化学和材料的基础。然而，Schrödinger方程只适用于孤立的量子系统。如果要以适当的精度测量量子粒子的性质，即使粒子处于叠加态，也可能得到一个明确的答案。因此，测量行为将波函数从初始叠加态坍缩为确定态。这个坍缩过程不能用Schrödinger方程来描述，并且自量子理论起源以来，调和测量的“开放”量子系统的演化与理论一直是激烈辩论和研究的主题。该项目的目标是加深我们对量子测量的理解，并利用测量相互作用来控制量子粒子的演化。该方法将使用微观超导电路作为人造原子，并将这些原子与微波光相互作用以创建开放量子系统。该团队将进行一系列实验，探索测量过程以及如何使用测量来控制量子进化，量子进化是新兴量子技术的重要组成部分。该项目将利用人造量子系统（超导人造原子）和空腔量子电动力学物理来创建对量子环境具有前所未有控制的系统。该团队将进行一系列实验，探索量子测量、量子控制和基本对称性的物理学。第一个实验检验了量子信息和经典信息之间的边界。将少量光子信号与人造原子的能态纠缠，然后用超导参数放大器可控地放大（或压缩），扩大少量光子指针态的希尔伯特空间。然后使用第二个放大器来探测放大后的指针态和原子之间的纠缠。第二个实验考察了辐射衰变的过程，以及如何检测自发发射的光子来控制原子状态的演化。一个参数放大器将被用来对量子发射器发射的辐射进行同差测量。该团队将研究如何利用差测角的选择来引导发射器状态的演变。第三个项目将创建一个由两个具有时空反转对称性的人造原子组成的系统，以研究奇偶时对称性破缺相变。通过量子储层工程，将产生一个原子损失而另一个原子获得的宇称时间对称系统。系统的稳定状态将通过光谱和量子态层析成像作为两个原子之间耦合的函数来探测。

According to quantum mechanics, particles do not have definite properties such as position and momentum, but are instead described by a "complex valued wavefunction" which describes them mathematically in terms of probabilities. The wave nature of quantum particles means that these particles can exist in superpositions of seemingly disparate states, for example a quantum particle could be in two places at once, or heading in two different directions, or occupy a superposition of two different energy levels. The evolution of this wavefunction obeys the Schrödinger equation which was formulated in 1925. Since its formulation, the Schrödinger equation has been applied to understand the properties of atoms and molecules and the basis for chemistry and materials. Yet, the Schrödinger equation only applies to isolated quantum systems. If one is to measure the properties of a quantum particle with suitable precision, a definite answer may result even if the particle is in a superposition of states. Thus the act of measurement collapses the wavefunction from an initial superposition to a definite state. This collapse process cannot be described by the Schrödinger equation and reconciling the evolution of measured "open" quantum systems with the theory has been a topic of intense debate and research since the origins of quantum theory. The goal of this project is to deepen our understanding of quantum measurement and to harness the measurement interaction to control the evolution of quantum particles. The approach will use microscopic superconducting circuits as artificial atoms and the interaction of these atoms with microwave light to create open quantum systems. The team will conduct a series of experiments that explore the measurement process and how measurement can be used to control quantum evolution, an important component of emerging quantum-based technologies.This project will utilize fabricated quantum systems (superconducting artificial atoms) and the physics of cavity quantum electrodynamics to create systems with unprecedented control of the quantum environment. The team will undertake a series of experiments that explore the physics of quantum measurement, quantum control, and fundamental symmetries. The first experiment examines the boundary between quantum and classical information. A few photon signal will be entangled with the energy states of an artificial atom and then controllably amplified (or squeezed) with a superconducting parametric amplifier, enlarging the Hilbert space of the few photon pointer state. A second amplifier will then be used to probe the resulting entanglement between the amplified pointer state and atom. The second experiment examines the process of radiative decay and how detection of spontaneously emitted photons can be used to control the evolution of the atomic states. A parametric amplifier will be used to perform homodyne measurement of radiation emitted from a quantum emitter. The team will study how the choice of homodyne measurement angle can be used to steer the evolution of the emitter's state. The third project will create a system of two artificial atoms that exhibits space-time inversion symmetry to study the parity-time symmetry breaking phase transition. The parity-time symmetric system will be created through quantum reservoir engineering, inducing loss for one atom and gain for the other atom. The steady states of the system will be probed through spectroscopy and quantum state tomography as a function of the coupling between the two atoms.