Source: The Paper
Recently, the Tsinghua University team has made progress in the field of microwave quantum information processing, and for the first time, with the help of superconducting quantum circuits, successfully prepared a multibody "Schrödinger cat" state of coherent flying microwave photons, making it possible to make quantum networks and modular quantum computing based on microwave photons possible. It is reported that there are many time-honored thought experiments in the field of quantum mechanics. Most of them are used to point out possible flaws in quantum mechanics.
Schematic diagram of the multibody "Schrödinger's Cat", picture from Tsinghua University
Famous thought experiment: "Schrödinger's Cat"
In 1935, theoretical physicist Erwin Schr dinger envisioned a famous thought experiment to illustrate paradoxes in quantum mechanics.
If a cat is kept in a closed room containing radioactive sources and toxic gases, the radioactive sources have a certain chance of decaying per unit of time. When the decay of the radioactive source is detected, toxic gases are released and the cat dies. If the radioactive source had not decayed, the cat would have survived.
According to the Copenhagen interpretation in quantum mechanics, the properties of a physical system are not determined, and their properties can only be measured using the probabilistic terminology of quantum mechanics, and the measurement behavior will have an impact on the system, causing the probability set to shrink to one of many possible values, which is called wave-function collapse.
Therefore, observation plays an important role in quantum mechanics. Going back to the "Schrödinger Cat" thought experiment, this means that after some time, the cat is in a state of living and dying at the same time. But when you look into the room, in this instant you see that the cat is alive or dead in a certain state.
According to Schrödinger, atoms may exist in two different states at the same time, namely quantum superposition, and if there is an interaction between atoms and macroscopic objects, "entangled" them, then macroscopic objects may be in a strange superposition state.
Limited to the two-body "Schrödinger cat"
As scientists further investigate the theory of quantum mechanics, a new question arises: What if there is more than one cat in the room? According to the natural logic of quantum theory, these cats are not only in a state of living and dying at the same time, but also "co-living and co-dying", that is, they are not only in a multi-body quantum superposition state, but also have quantum entanglements with each other that transcend classical associations.
Quantum entanglement between aforementioned macroscopic objects or classical states is an interesting scientific problem and has important applications in many quantum technologies. But preparing a multibody "Schrödinger cat" is technically challenging. Because the classical state used to simulate the life and death of a "cat" is generally in the high-dimensional Hilbert Space, there is a serious environmental decoherence, making the quantum effects difficult to observe.
Previously, only the preparation of the two-body "Schrödinger cat" had been realized internationally. "The main reason is the lack of a suitable experimental protocol." Zhang Hongyi, an associate researcher at Tsinghua University's Institute for Interdisciplinary Information Technology, told the www.thepaper.cn that the Yale research team first realized quantum entanglement between the semi-classical states of the two bodies in 2016, and its results were published in Science. "They used two superconducting microwave chambers each carrying a quantum superposition of the two-body microwave coherent state, but the scalability of the system was poor: on the one hand, many superconducting microwave resonators were required to achieve a multibody quantum state; on the other hand, it was very difficult to couple the same qubit with these resonators."
Multi-body "Schrödinger cat" who can "fly"
In order to realize the preparation of multibody "Schrödinger cat", Duan Luming, chair professor of the Institute for Interdisciplinary Information Sciences of Tsinghua University, Zhang Hongyi, associate researcher, and other research groups adopted another research idea to simulate the "birth" and "death" of cats by using phase-opposite coherent flight microwave photons.
This time, with the help of flying microwave photons, the team realized the quantum entanglement of superconducting qubits and coherent microwave photons in the reflection process of the resonator port containing superconducting qubits, and finally realized the multibody "Schrödinger cat" that "flew" by continuously reflecting multiple coherent state microwave photon pulses. The paper, "A flying Schr dinger's cat in multipartite entangled states," was published in Science Advances.
Image from Science Advances
"We use flying microwave photon pulses at different moments to define multibody quantum states, and use microwave resonators and superconducting qubit systems to achieve quantum entanglement between microwave photon pulses at different moments." Zhang Hongyi told the surging news (www.thepaper.cn) reporter, "Our scheme has been significantly improved in terms of scalability compared with previous experiments, ensuring that we have successfully observed quantum entanglement between multibody semiclassical states." ”
He said that flying microwave photons have better scalability than previous experiments. "Second, compared with experimental systems in the optical band (wavelengths in the 100-nanometer range), superconducting quantum circuits in the microwave band have good regulation, which helps us achieve higher quality implementation systems and more accurate system parameters."
The "four-body" breakthrough from theory to practice
The theoretical basis of the Tsinghua University team is Duan Luming and Jeff Kimble, a professor at California Institute of Technology, who collaborated in 2004 to propose the Duan-Kimble scalable optical quantum computing solution.
The scheme proposes that a high-quality resonator can be used to assist in the operation of controlled quantum gates between flying optical qubits, thereby enabling scalable optical quantum computing. Its core content refers to the process of reflecting flying photons from a resonator containing qubits, which can realize the controlled quantum gate operation and quantum entanglement of flying photons and qubits.
"This scheme is concise and technically feasible, and is currently one of the mainstream methods to achieve entanglement between flying light qubits and local qubits." Zhang Hongyi said, "This also provides a theoretical basis for many important subsequent experiments, such as the realization of non-destructive single-photon detectors, single-photon diodes and other quantum devices." ”
Based on the above scheme, Duan Luming and his collaborators proposed in 2005 that quantum superposition of coherent flying photons, the so-called "Schrödinger cat" state, can be achieved. "This provides the most direct theoretical basis for our research."
On the basis of this theory, the Duan Luming Research Group of Tsinghua University started from the density matrix of the multibody flying microwave photon state and verified the quantum entanglement in the four-body "cat" state by using the method of local quantum entanglement, which is also the first time that the quantum entanglement between the semi-classical states of more than two bodies has been successfully prepared in the experiment.
The density matrix of the multibody "cat" state reconstructed in the experiment is from Tsinghua University
In addition, by reconstructing the density matrix of the hybrid quantum system of superconducting qubits and the multibody "cat" state, the team confirmed the quantum entanglement between these two essentially distinct quantum states. This work proposes a highly scalable multibody "Schrödinger" state preparation protocol.
Reviewers commented that for the first time, the Tsinghua University team achieved the preparation of up to four-body "Schrödinger cat" states, that is, the entanglement between multiple coherent microwave photons and between them and superconducting qubits.
Pushing the "Schrödinger cat" into the field of application
"The main device of this experiment is similar to a superconducting quantum computer, including a dilution chiller to cool the superconducting quantum chip, and some microwave signal sources and waveform generation devices provide the microwave signals needed for the experiment." Zhang Hongyi said, "The core of the experimental device is a superconducting quantum circuit, containing a superconducting microwave resonator and a superconducting qubit, and the preparation of the 'Schrödinger cat' state depends on our precise control of the superconducting qubit quantum state." ”
He said that one of the main motivations for the team to design this experiment is to use flying microwave photons to achieve remote quantum entanglement between superconducting qubits, and then to achieve quantum networks and modular quantum computing of superconducting quantum circuits. "In this study, we have achieved quantum entanglement between the 'Schrödinger cat' state and the superconducting qubits of flying microwave photons, and later we hope to achieve remote quantum entanglement of superconducting qubits with the help of the 'Schrödinger cat' state, and study the advantages of the 'cat' state in resisting quantum state distortion caused by microwave line loss."
Therefore, multibody "cat" states based on flying microwave photons have important applications in many quantum technologies. In addition to making microwave photonic-based quantum networks and modular quantum computing possible, quantum entanglement in the multibody "cat" state may also improve the detection accuracy of radar and achieve "quantum radar" with higher noise resistance. "On the other hand, we also plan to explore the application of the 'Schrödinger cat' state in microwave quantum optics and the realization of new microwave light quantum devices."