Breakthrough in quantum simulation of many-body system

In 1952, Lee and Yang proved that the partition functions of the ferromagnetic Ising systems can vanish at certain points, termed Lee-Yang Zeros, in the complex plane of magnetic field. These Lee-Yang Zeros fully determine the partition functions or free energy of spin systems, and in turn the thermodynamic properties of the systems. Experimental observation of Lee-Yang Zeros, however, have been regarded as impossible since they occur only at an imaginary magnetic field, which are unphysical.

Prof. Jiangfeng Du and Xinhua Peng from University of Science and Technology of China, collaborated with Prof. Renbao Liu from the Chinese University of HongKong, have observed Lee-Yang Zeros experimentally for the first time by quantum simulator. The results are published in Physical Review Letters as Editors’ Suggestion [Phys. Rev. Lett. 114, 010601 (2015)], and selected for a ViewPoint in APS’s Physics entitled "Imaginary Magnetic Fields in the Real World ".

Recently, a theoretical discovery was made by Prof. Liu that the coherence of a probe spin coupled to a many-body system vanishes at times in one-to-one correspondence to the Lee-Yang Zeros of the partition function of the many-body system, which makes it experimentally feasible to observe the Lee-Yang Zeros. Following this theoretical proposal, Prof. Peng et.al succeeded in simulating the 9-spin ferromagnetic Ising system in the liquid-state nuclear magnetic resonance (NMR), and first direct observation of Lee-Yang Zeros in experiments. From the measured Lee-Yang Zeros, the researchers also reconstructed the free energy of the thermal system and determined its phase transition temperature. This experiment demonstrate feasibility of using probe spin coherence to determine the thermodynamic properties of the probed system and, more generally, to access thermodynamics on the complex plane of physical parameters.

 

This work was financially supported by National Natural Science Foundation, Ministry of Science and Technology, Chinese Academy of Sciences, and Synergetic Innovation Center of Quantum Information & Quantum Physics.

 

 

(ZHOU Hui,School of Physical Sciences)