In a study published in Physics Review Letters, researchers from University of Science and Technology of China of Chinese Academy of Sciences and Nanjing University optimized the method of quantum weak measurement, which improves the measurement accuracy of single photon Kerr effect by an order of magnitude, and first approached the optimal Heisenberg limit. 

To achieve higher measurement accuracy, an important goal is to make the accuracy inversely proportional to the number N of photons or atoms used in a single measurement, i.e., to achieve Heisenberg limit accuracy (1/N). Yet the accuracy of the classical measurement method can only be inversely proportional to the square root of n, the so-called standard quantum limit. 

Based on the month-before scheme of this team, researchers optimized the method and performed single-photon projective measurement to extract more information, thereby increasing the probe photon utilization rate to 83% (i.e., the measurement accuracy is about 1.2/N), which approached the optimal Heisenberg limit (1/N).

Besides, they measured the single-photon Kerr effect and the result was about 6E-8 radians, which is nearly one order of magnitude compared with the previous experiment, reaching 9.5E-11 radians. 

 In this study, the measuring device was simpler, and the experiment was completed with a normal laser pulse.

The study demonstrated the superiority of quantum precision measurement in practical measurement tasks and provided new insights into the development of quantum metrology and quantum weak measurement.

How to extend this efficient experimental method to practical application will be further explored.

Experimental facility to measure single photon Kerr effect (Image by LI Chuanfeng)