创新背景

1935年，奥地利理论物理学家埃尔温·薛定谔设想了一个著名的思想实验，以阐述量子力学中的悖论。设定实验中的原子因为量子状态可以相互叠加，可能同时存在两种相反的状态。如果在原子和宏观物体建立起一定的联系，让宏观物体也发生“纠缠”现象。假如把一只猫和一瓶毒药放进一个封闭空间里，量子叠加会导致原子处在衰变或未衰变的状态，触发某种机械装置打翻毒药，猫会在毒药是否打翻的情况下生和死共存。

创新过程

量子力学的发展延伸了设想：如果房间中不止一只猫，按照量子理论的自然逻辑，这些猫不仅处于同时活着和死去的状态，并且“同生共死”。因为它们不仅处在多体的量子叠加态，并且互相存在超越经典关联的量子纠缠。

量子技术中宏观物体和经典态的量子纠缠被重要应用，但制备多体“薛定谔猫”在技术上挑战性极大。因为用来模拟“猫”生死的经典态一般处在高维度的希尔伯特空间(Hilbert Space)中，存在严重的环境退相干，导致其中的量子效应很难被观测。

模拟猫的生和死往往采用具有不同相位相干态的微波光子。相干态是因为其最接近经典的一种态，因此被称为半经典态，这种状态的光子来模拟“薛定谔的猫”实验是最直接的选择。耶鲁大学在2016年最早实现了两体的半经典态之间的量子纠缠，其成果发表在《科学》(Science)上。但试验系统的扩展对象被极度限制，每增加多体量子态就需要很多超导微波谐振腔。

清华大学的研究团队采用了另外一种思路，利用在波导中传播的相位相反的相干态微波脉冲光子定义“薛定谔的猫”，并设法将他们纠缠起来，实现了“飞”起来的多体“薛定谔猫”。这种方法显然很容易制备多体的“猫”态，该团队也在实验中实现了四体的“薛定谔猫”态。2022年3月11日，相关论文《多方纠缠态中飞行的薛定谔猫》发表Science Advances 上。

该研究以清华大学交叉信息研究院段路明教授与其合作者提出的 Duan-Kimble 可扩展光量子计算方案为理论基础，借助腔电动力学体系实现了飞行光量子和原子之间的量子纠缠。在此基础上，从多体飞行微波光子态的密度矩阵出发，利用可局域量子纠缠的方法验证了直到四体“猫”态中的量子纠缠，首次在实验中成功制备超过两体的半经典态之间量子纠缠。

之后，通过重构超导量子比特和多体“猫”态这个混合量子系统的密度矩阵，团队确认了这两种量子纠缠本质上的量子态完全不同，提供了一种高度可扩展的多体“薛定谔猫”态制备方案。

研究院表示，实验的主要装置类似一台超导量子计算机，同时包括一台给超导量子芯片降温的稀释制冷机给，一些微波信号源和波形生成装置提供实验需要的微波信号。实验装置的核心是超导量子电路，包含一个超导微波谐振腔和一个超导量子比特，‘薛定谔猫’态的制备依赖于研究者对超导量子比特量子态的精确控制。

此实验研究希望借助飞行微波光子，实现超导量子比特之间的远程量子纠缠。

创新关键点

借助飞行光量子和微波光子促进量子纠缠发展，推动量子计算和量子网络发展。

创新价值

助力实现超导量子电路的量子网络和模块化量子计算。

Microwave pulsed photons using phase-opposite coherent states define the quantum four-body "Schrödinger's cat" state

The research team from Tsinghua University adopted another idea, using microwave pulse photons of coherent states with opposite phases propagating in the waveguide to define "Schrödinger's cat", and managed to entangle them to realize the "flying" many-body" Schrödinger's cat". This method is obviously easy to prepare a many-body "cat" state, and the team also achieved a four-body "Schrödinger's cat" state in the experiment. On March 11, 2022, the related paper "A flying Schrödinger's cat in multipartite entangled states" was published in Science Advances.
The research is based on the Duan-Kimble scalable optical quantum computing scheme proposed by Professor Duan Luming from the Institute of Interdisciplinary Information Research, Tsinghua University and his collaborators, and realizes the quantum entanglement between flying photons and atoms with the help of cavity electrodynamics system. On this basis, starting from the density matrix of the many-body flying microwave photon state, the quantum entanglement up to the four-body "cat" state was verified by the method of localized quantum entanglement. Quantum entanglement between classical states.
Then, by reconstructing the density matrix of the hybrid quantum system of superconducting qubits and many-body "cat" states, the team confirmed that the two quantum states of quantum entanglement are fundamentally different, providing a highly scalable many-body "Schrödinger's cat" state preparation scheme.
The research institute said that the main device of the experiment is similar to a superconducting quantum computer, and it also includes a dilution refrigerator to cool the superconducting quantum chip. Some microwave signal sources and waveform generation devices provide the microwave signals required for the experiment. The core of the experimental setup is a superconducting quantum circuit, which contains a superconducting microwave resonator cavity and a superconducting qubit. The preparation of the 'Schrödinger's cat' state relies on the researchers' precise control of the quantum state of the superconducting qubit.
This experimental study hopes to realize long-range quantum entanglement between superconducting qubits with the help of flying microwave photons, and promote the realization of quantum networks and modular quantum computing of superconducting quantum circuits.