The CAS key lab of quantum information makes a significant progress in the field of quantum metrology. Under the lead of Prof. GUO Guangcan, LI Chuanfeng, XIANG Guoyong and collaborators introduced a general recipe for performing deterministic collective measurements on two identically prepared quantum systems based on quantum walks and achieved the highest efficiency in quantum state tomography. These results were published online by Nature Communications Quantum measurement is inevitable to extract information from quantum systems. Exploring the power and limitation of quantum measurement has profound implications not only to foundational studies, such as uncertainty relations and nonlocality, but also to many practical applications, ranging from quantum metrology, quantum imaging, characterization of fault-tolerant devices, and detection of gravitational waves. In most of these tasks, many identically prepared quantum systems are measured to acquire sufficient information. Although there is no entanglement and even no classical correlation among these identically prepared systems, surprisingly, collective measurements on them may extract more information than local measurements on individual systems, thereby leading to higher efficiency and precision in these tasks. Although the significance of collective measurements has been recognized for two decades, it is still very challenging to demonstrate their advantage in experiments. This is because the generalized entangling measurements on many identically prepared quantum systems are very difficult to realize deterministically in experiments. Actually, the most current measurement methods are restricted local measurements on a single copy. Any progress on collective measurement would be important to quantum information.
LI Chuanfeng, XIANG Guoyong, and collaborators introduced a general recipe for performing deterministic collective measurements on two identically prepared quantum states based on quantum walks. Using photonic quantum walks, they implement an optimized collective measurement deterministically for the first time in the world. Then they use this new method in quantum state tomography and achieved the highest tomographic efficiency. Experimental results of two-copy collective measurements demonstrated at least 50% improvement of tomographic efficiency and the efficiency would be increased with the level of purity and the number of copies of quantum states. For example, when the number of copies of general pure states is 2048, the efficiency of two-copy collective measurements is about three times higher than that of optimal adaptive local measurements.
Their work demonstrated a truly nonclassical phenomenon because of nonlocality in quantum measurement. It offers a completely new measurement method for multi-parameter quantum metrology (including quantum state tomography) to beat the precision limit of local measurements. Furthermore, it opens an avenue for exploring quantum information and the fundamental problem of quantum mechanics via collective measurements
The first author of this work is Dr. HOU Zhibo. This work was supported by the National Key Research and Development Program of China，the National Natural Science Foundation of China，Key Research Program of Frontier Sciences, CAS and the Fundamental Research Funds for the Central Universities.
Individual and collective measurements; parts of experimental results.
URL: https://www.nature.com/articles/s41467-018-03849-x
