Tsinghua University successfully fabricated many-body "Schrödinger's cat" states of flying microwave photons with the help of superconducting quantum circuits
Recently, the research group of Duan Luming, Institute of Interdisciplinary Information, Tsinghua University, has made important progress in the field of microwave quantum information processing. For the first time, with the help of superconducting quantum circuits, the many-body "Schrödinger's cat" state of coherent flying microwave photons has been successfully prepared and verified. Quantum entanglement between different "cat" states and between many-body "cat" states and superconducting qubits, the paper "A flying Schrödinger's cat in multipartite entangled states" recently Published in the international academic journal Science Advances.
In 1935, physicist Erwin Schrödinger proposed a famous thought experiment to illustrate the paradox in quantum mechanics. According to Schrödinger's idea, atoms may exist in two different states at the same time. This is called quantum superposition. If there is an interaction between atoms and macroscopic objects, they "entangle" and the macroscopic objects may be in a strange state. superposition state. Schrödinger used a cat to illustrate this situation. Imagine a closed room with a cat and a bottle of poison. If the decay of atoms can trigger a mechanism to break the bottle and release the poison, the cat will die. But given that atoms may be in a superposition state that decays or does not decay, this means that there may be a "dead and alive" cat in the room, the famous "Schrödinger's cat".
What if there is more than one cat in the room? According to the natural logic of quantum theory, these cats are not only "dead and alive", but also "dead together", as shown in Figure 1. That is, these cats are not only in the quantum superposition state of many bodies, but there is quantum entanglement between them that goes beyond the classical correlation. Such quantum entanglement between macroscopic objects or classical states is not only an interesting scientific problem, but also has important applications in many quantum technologies. The preparation of many-body "Schrödinger's cat" is technically very challenging, because the classical states used to simulate the life and death of "cats" are generally in high-dimensional Hilbert spaces, and there is often severe decoherence. As a result, quantum effects are difficult to observe.
The theoretical basis of this work is the Duan-Kimble scalable optical quantum computing scheme proposed by Professor Duan Luming and his collaborators, which realizes quantum entanglement between flying photons and atoms by means of cavity electrodynamics system. In this experiment, the researchers used coherent state flying microwave photons with opposite phases to simulate the "birth" and "death" of cats, and realized superconducting qubits and Quantum entanglement of coherent microwave photons, namely the preparation of "Schrödinger's cat" state; the preparation of multipartite "Schrödinger's cat" state is realized by continuously reflecting multiple coherent microwave photon pulses. As shown in Figure 2, the researchers used quantum state tomography to reconstruct the quantum state of flying microwave photons in high-dimensional Hilbert space, confirming the successful preparation of the four-body "cat" state. Starting from the density matrix of many-body flying microwave photon states, the researchers used the method of localized quantum entanglement to verify quantum entanglement up to the four-body "cat" state. Quantum entanglement between them. Furthermore, by reconstructing the density matrix of the hybrid quantum system of superconducting qubits and many-body "cat" states, the researchers confirmed quantum entanglement between these two intrinsically distinct quantum states.
This work presents a highly scalable scheme for the preparation of many-body "Schrödinger's cat" states. Many-body "cat" states based on flying microwave photons have important applications in many quantum technologies, such as fault-tolerant superconducting qubit remote entanglement based on many-body "cat" states, making microwave photon-based quantum networks and modular Quantum computing becomes possible. In addition, the use of quantum entanglement in many-body "cat" states may also improve the detection accuracy of radar and realize "quantum radar" with higher noise immunity.
The co-first authors of the paper are doctoral students Wang Zhiling and Bao Zenghui from the Institute of Interdisciplinary Information, Tsinghua University, the corresponding authors are Associate Researcher Zhang Hongyi and Professor Duan Luming, and other authors include Li Yan, Cai Weizhou, Wang Weiting, Ma Yuwei, Cai Tianqi, doctoral students of the Interdisciplinary Information Institute , Han Xiyue, Wang Jiahui, Assistant Professor Wu Yukai, Researcher Song Yipu and Associate Professor Sun Luyan. This project has been supported by the National Natural Science Foundation of China (Project Nos. 11874235, 11925404), the National Key R&D Program (Project No. 2017YFA0304303, 2020YFA0309500), the Quantum Information Frontier Science Center of the Ministry of Education, the Tsinghua University Scientific Research Start-up Project, and the Guangdong Provincial Key Field R&D Program (Project No. 2017YFA0304303, 2020YFA0309500). No. 2020B0303030001) and Guoqiang Research Institute of Tsinghua University (Project No. 2019GQG1024).
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