The Chinese research team realized the loophole-free test of quantum intertextuality for the first time

Recently, Wang Pengfei, Zhang Jingning, assistant researchers of Beijing quantum Institute, and concurrently employed Professor Jin Qihuan, in conjunction with researchers from Tsinghua University, the University of Seville in Spain, the Southern University of science and technology and other researchers, have realized the leak free test of quantum intertextuality in experiments for the first time. The study entitled "significant loophole free test of kochen specker context using two specialties of atomic ions" was published in science advances on February 9, 2022.

Quantum intertextuality is the essential difference between quantum system and classical system. In the classical world we are familiar with, when we measure the same observation of the same system in different order or at different time points, the output results are always the same. Therefore, we naturally believe that these measured observations exist long ago and still exist after measurement. However, this assumption does not hold in quantum mechanics. A pair of measurements in a quantum system can also produce measurement results that do not interfere with each other and produce the same results when repeated measurements, but even in this case, the correlation of their measurement results cannot be explained by the assumption of pre-existing intrinsic values. This phenomenon was proposed by kochen, specker and bell in 1960, resulting in the bell kochen specker theorem of quantum intertextuality in quantum mechanics. This property has now proved to be the internal reason why quantum computers outperform classical computers.

However, how to test quantum intertextuality experimentally without closing all "loopholes" has always been a very challenging problem. The quantum intertextuality test is very similar to the bell test, but it does not need the condition of class space separation. Recently, some bell test experiments have been proved without major vulnerabilities. However, many quantum systems, including quantum computers, are difficult to be large enough to accommodate subassemblies in space like relationships. In this case, how do we know that the system operates in a quantum state and is controlled by the principles of quantum mechanics? Quantum intertextuality testing can provide a solution. The violation of quantum intertextuality inequality has been observed in the systems of photons, neutrons, ions, molecular nuclear spin and superconducting systems, but they can not close all the loopholes.

picture 1 Schematic diagram of mixed ion trap experimental system

Professor Jin Qihuan's research team proposed an experimental method for vulnerability free quantum intertextuality testing using composite systems. In the experiment, two different ions are used to ensure that they do not interfere with each other, and the ideal observation conditions are ensured by repeated measurement of the two ions, and no measurement results are missed in the experiment. Finally, the experimental results realize the violation of 15 standard deviations of quantum intertextuality inequality. In detail, the experimental verification of quantum intertextuality mainly involves two vulnerabilities, exploratory vulnerability and ideal observation vulnerability. The ideal observation vulnerability can be divided into sharpness vulnerability and compatibility vulnerability. The hybrid ion system used in this study uses fluorescence detection technology to detect the state of qubits, and achieves 100% detection efficiency and 98% detection fidelity. Thus, the detection vulnerability and sharp vulnerability are closed. The mixed ion system consists of a BA ion and a Yb ion trapped in a Paul trap. Each pair of observations of quantum intertextuality inequality corresponds to a different pair of ions. Moreover, the two ions have completely different operating lasers, detection lasers and detection devices, ensuring the closure of compatibility vulnerabilities. The initial entangled state required to test quantum intertextuality is prepared by M ø lmer-s ø rensen (M-S) quantum gate. The experimental results clearly show the violation of quantum intertextuality inequality.

picture 2 data diagram of M-S quantum operation gate used in the experiment

In addition to being of great significance for basic research, the research results can also be widely used in other fields of quantum information. For example, many quantum systems cannot be bell tested because their size cannot achieve class space separation. The scheme designed in this study has no requirement of class space separation, and can be used to verify the authenticity of quantum systems. For example, it is used to detect whether the "quantum computer" is a real quantum computer. In addition, it can also be used as a self verifying quantum random number generator, blind quantum computing and so on.

The corresponding authors of the paper are Jin Qihuan, a concurrent professor of our college, and ad á n Cabello, a professor at the University of Seville, Spain. The co first authors are Wang Pengfei, an assistant researcher of our college, and Zhang Junhua, an assistant researcher of Southern University of science and technology. Zhang Jingning, an assistant researcher of the superconducting quantum computing team of our college, provided theoretical support for this work. This work is supported by the national key research and development plan, the major research plan of the National Natural Science Foundation of China "construction and manipulation of the second generation quantum system" and the National Natural Science Foundation of China.

Paper link:https://www.science.org/doi/full/10.1126/sciadv.abk1660?af=R