The team Li Chengfeng, Chen Geng and others from CAS Key Lab of Quantum Informaiton collaborated with Nanjing University optimizes the measurement method of quantum weak measurement, and improves the measurement accuracy of single photon Kerr effect by an order of magnitude. The experimental results first approaches the optimal Heisenberg limit. The research results were published on August 8th in Physics Review Letters.

Using limited resources to achieve higher measurement accuracy is an important requirement for scientific development. An important goal of quantum precision measurement is to make the measurement accuracy inversely proportional to the number N of photons or atoms used in a single measurement, i.e., to achieve Heisenberg limit accuracy. The accuracy of the classical measurement method can only be inversely proportional to the square root of n, the so-called standard quantum limit. Obviously, when N is large enough, the accuracy of quantum precision measurement will be much better than that of classical methods. Quantum precision measurement methods generally require quantum resources such as entangled states or squeezed states. Limited by the current technology, these methods are not yet practical. In the previous work, Li Chuanfeng's group combined the mixed-state probe with the imaginary weak measurement technology to achieve Heisenberg-scaling accuracy of the single-photon Kerr effect with the probe photon utilization rate 16% [Nature Communications 9, 93 (2018)]. In this work, the research group further optimizes the measurement method to perform single-photon projective measurement so as to extract more information, thereby increasing the probe photon utilization rate to 83% (that is, the measurement accuracy is about 1.2/N). Approaching the optimal Heisenberg limit (1/N). The single-photon Kerr effect measured in the experiment is about 6E-8 radians, and the measurement accuracy is nearly one order of magnitude compared with the previous experiment, reaching 9.5E-11 radians. The measuring device has also become simpler, and the experiment can be completed with a normal laser pulse.

This achievement demonstrates the superiority of quantum precision measurement in practical measurement tasks, and provides new ideas for the development of quantum metrology and quantum weak measurement. The specific task of the single photon Kerr effect is considered in this experiment. How to extend this efficient experimental method to various important practical application scenarios will be further explored by the research group.

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(School of Physical Science)