News Source: School of Physics

Editor: News Agency of BIT

Translator: Lin Zixin, News Agency of BIT

Recently, professor Zhang Xiangdong and associate researcher Shang Jiangwei from Beijing Institute of Technology(BIT), School of Physics cooperated with doctor Han Rui from Center of Quantum Technology, National University of Singapore, and made significant progress in the field of quantum verification, putting forward a new quantum verification protocol based on non-demolition measurements, which not only achieves an optimal verification efficiency, but can also fulfill other tasks such as fidelity estimation and state preparation. With an arbitrary measuring order, the protocol is easy to realize in an experiment. Related research findings are published in the top international journal Physical Review Letters.

Quantum verification is an eminent quantum system characterization protocol, which is capable of efficiently determining whether the fidelity of quantum states is within the threshold value as required and etc. Compared with classic characterization methods such as quantum state tomography and direct fidelity estimation, the resource consumption of quantum verification based on precision is reduced from quadratic to first power, so it is superior in high precision quantum system characterization tasks. In the past two years, together with Dr. Han, the team has made a series of considerable progress. A feasible numerical optimization algorithm of the verification protocol was first presented in the study of the verification of bipartite entangled states in arbitrary dimensions, with an accordingly presented optimal bipartite entangled state verification protocol. This study also demonstrated the advantages of adaptive measurement for improving the efficiency of quantum verification [npj Quantum Inf. 5, 112 (2019)]. Based on this point, with adaptive measurement, for the first time we gave a valid verification of Dicke states, the complex multipartite entangled states, and accordingly presented the non-adaptive measurement protocol, which proved the general relationship between the adaptive measurement protocol and the non-adaptive one [Phys. Rev. Applied 12, 044020 (2019)]. Meanwhile, we extended quantum state verification to process verification, and validated the effective verification of quantum gate and quantum measurement [Phys. Rev. A 101, 042315 (2020)].

On the foundation of previous work, we found that there are several problems in quantum verification based on the initial protocol framework. First, it is generally difficult to present verification protocols of optimal efficiency for arbitrary quantum state. Second, the initial quantum verification protocol is a probabilistic measurement of multiple groups of measurement settings, which is difficult to directly implement. In experiments, each measurement setting is measured multiple times in a fixed order. This technical compromise leaves loopholes for potential deception. Third, the unknown state to be verified has been destructed after verification, and therefore cannot be used in subsequent tasks. To solve this problem, we introduced non-destructive measurement technology into quantum verification, in which we replaced probabilistic measurement with continuous measurement, which conducted multiple groups of different measurement settings for quantum states. We proved that such continuous non-destructive measurement verification protocol is always optimal in terms of measurement efficiency, so that we need to focus on measurement settings only. In this process, we also found that such continuous measurement with optimal efficiency is independent of the measurement order of each setting, which makes it easy to construct experiments. Furthermore, our measurement protocol not only protects the verified target state from being damaged, but also projects the unknown non-target state directly to the target state with certain probability, which is equal to the fidelity between the unknown state and the target state. Therefore, the protocol can also be used for fidelity estimation and state preparation tasks. With the development of quantum technology, non-destructive measurement has been realized on various platforms, and is gradually becoming a standard technical means. It is expected that the corresponding experiments can effectively demonstrate the characteristics and strengths of non-destructive quantum verification protocol.

Liu Yechao, a PhD student of School of Physics, Class of 2016, is the first author of this essay. Professor Zhang Xiangdong and associate researcher Shang Jiangwei are the corresponding authors. This research is supported by the National Natural Science Foundation of China and the National Key R&D Program, as well as the Beijing Institute of Technology Research Fund Program for Young Scholars.

Links of related articles:

https://www.nature.com/articles/s41534-019-0226-z

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.12.044020

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.101.042315

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.090504