China University of science and technology provides a new way for quantum precision measurement
Peng Xinhua research group, Key Laboratory of micro magnetic resonance, Chinese Academy of Sciences, University of science and technology of China, and Professor Lv Xinyou of Huazhong University of science and technology have made important progress in the experimental research of quantum simulation of superradiative phase transition. By introducing the anti compression operation and with the help of high-precision quantum control technology, the cooperative research group successfully realized the equilibrium hyperradiative phase transition beyond the no go theorem on the nuclear magnetic resonance quantum simulator for the first time, promoted the development of quantum phase transition theory and quantum simulation, and provided a new way for quantum precision measurement. Relevant research results were published online in the international academic journal Nature communication on November 1 under the title of "experimental quantum simulation of superadian phase transition beyond no go theory via antisqueezing" [NAT. Common. 12, 6281 (2021)].
Since the superradiative phase transition of equilibrium state was predicted by theory in the early 1970s, it has been an important research topic in statistical physics and electrodynamics, and provides key quantum resources for quantum information science. However, the superradiative phase transition in equilibrium has not been observed in the real cavity QED system. On the one hand, the current cavity QED technology is usually difficult to meet the critical parameters required for equilibrium superradiative phase transition and the preparation of ultra-low temperature ground state; More importantly, the natural square term of vector potential in cavity QED system makes the phase transition point fall in the parameter region that cannot be reached physically, that is, the so-called no go theorem. Therefore, the experimental study of equilibrium hyperradiative phase transition is very challenging.
Figure. 1 (a) corresponding scheme from spin system to Rabi model. (b) Phase transition diagram of Rabi model including vector potential square term and inverse compression term.
Based on the nuclear magnetic resonance quantum simulator, the research group simulated and verified the hyperradiative phase transition of Rabi model without vector potential term and the mechanism of no go theorem; Furthermore, the additional anti compression operation is skillfully introduced to exponentially enhance the zero fluctuation of the system. The superradiative phase transition is successfully observed in the presence of the square term of vector potential, which breaks through the limitation of no go theorem. Specifically, the research group experimentally prepared the quantum simulation system into the ground state corresponding to the Hamiltonian of the cavity QED system by using the adiabatic quantum control method, and realized the key anti compression operation based on the precise quantum control technology. Then, the recovery of the hyperradiative phase transition was observed by measuring the change of the order parameter (average photon number). In addition, the experiment also shows that the system is prepared into a highly entangled squeezed Schrodinger cat state with entering the super radiation phase by quantum state chromatography.
Figure. 2: experimental results (a) order parameter behavior of the system without anti compression term. (b) The behavior of order parameters with different parameters after introducing anti compression term. (c) Wigner function of quantum state of superradiative phase and normal phase system.
The results show that the compression / anti compression operation can effectively regulate the quantum phase transition dot, and can restore the equilibrium hyperradiative phase transition even in the presence of the square term of vector potential. This not only breaks the potential obstacles caused by no go theorem to the further development of related fields, but also inspires subsequent researchers to apply more advanced quantum control technology to the experimental research of complex systems such as light matter interaction and condensed matter; The highly entangled states (squeezed states) prepared in the experiment are also expected to provide key quantum resources for the field of quantum measurement and fault-tolerant quantum computing. The reviewer spoke highly of this work: "this is an important experiment for the whole field of quantum simulation and well proposed for a publication in nature communications.".
Professor Peng Xinhua's research group has been committed to simulating various valuable complex quantum systems on NMR systems and has made a series of progress, including the experimental observation of Li Yang zero (Phys. Rev. Lett. 114, 010601), the measurement of non temporal correlation (otoc) (Phys. Rev. X 7, 031011), and the experimental detection of quantum topological order (NAT. Phys. 14, 160 – 165, 2018), It has accumulated rich experience in quantum simulation and developed international cutting-edge quantum control technology.
Chen Xi (graduated) and Wu Ze, doctoral students of the Key Laboratory of micro magnetic resonance, Chinese Academy of Sciences, are the co first authors of the article, and Peng Xinhua and LV Xinyou are the co corresponding authors of the article. The research was supported by the Ministry of science and technology, the National Natural Science Foundation of China and Anhui Province.
Link：https://www.nature.com/articles/s41467-021-26573-5
