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Experimental Quantum Superradiant Phase Transition Beyond No-Go Theorem Achieved for the First Time

The research team led by Prof. PENG Xinhua from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences, and the collaborators, achieved for the first time the experimental quantum superradiant phase transition (SPT) beyond no-go theorem by introducing anti-squeezing. The study was published in Nature Comunications.

Since the theoretical prediction of SPT in thermal equilibrium in the 1970s, it has become an important subject for statistical physics and electrodynamics. However, it has never been observed in real cavity quantum electrodynamics (QED) systems due to the stern conditions for implementing equilibrium SPT. More importantly, as predicted by the no-go theorem, the squared term of electromagnetic vector potential (namely A2 term) in the cavity QED forbids the occurrence of equilibrium SPT.

In this work, based on a nuclear magnetic resonance (NMR) quantum simulator, the research team simulated the occurrence of SPT in the Rabi model without A2 term, as well as the mechanism of no-go theorem. Then, the research team observed SPT when including A2 term by introducing auxiliary anti-squeezing effect. The anti-squeezing effect exponentially enhanced the zero point fluctuation of the system, thus breaking the limit of no-go theorem.

To be more Specific, the team prepared the quantum simulator into the corresponding ground state of cavity QED Hamiltonian by adopting adiabatic quantum control. They realized the anti-squeezing effect on the basis of high-precision quantum control technology, and observed the re-appearance of SPT by measuring the order parameter (the average photon number).

Besides, the experiment also demonstrated that the system was prepared into highly entangled and squeezed Schrödinger cat states. The rich quantum sources obtained in the superradiant phase were of great significance for quantum metrology and quantum computation.

In conclusion, anti-squeezing effects could effectively recover the equilibrium superradiant phase even with the existence of A2 term. The result is not in contrast with no-go theorem. In fact, the experiment relaxes the limit of A2 term on SPT via anti-squeezing.

This study not only broke the potential barriers that the no-go theorem posed on the development of quantum physics, but also shed light on exploring novel quantum optical effect with the platform of NMR or other spin systems.

"This is an important experiment for the whole field of quantum simulation and well-suited for a publication in Nature Communications." commented by the reviewer.

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