Measurement precision in the Heisenberg limit can be much higher than classical measurement methods. Because it is experimentally difficult to prepare entangled states with a photon number greater than 10, this method can demonstrate in principle the possibility of exceeding the standard quantum limit, but does not yet have practical measurement capabilities.
Quantum precision measurements are an important new development in quantum information science, aiming to utilize quantum resources and effects to achieve measurement accuracy beyond classical methods. A previous important discovery in this field was that optical phase measurements with Heisenberg limit precision can be realized by using multiphoton entangled states as probes. In principle, the measurement accuracy in the Heisenberg limit can be much higher than that of classical measurement methods. Since it is experimentally difficult to prepare entangled states with photon numbers greater than 10, this method can demonstrate in principle the possibility of exceeding the standard quantum limit, but does not yet have practical measurement capabilities. Thus, designing a quantum precision measurement technique that can be practically applied and reach the Heisenberg limit has been a long-standing academic endeavor.
Chuanfeng Li's research group abandoned conventional thinking and combined the preparation of mixed-state probes with the technique of measuring the weak value of the imaginary part to experimentally achieve the Heisenberg limit of precision and use it to measure the Kerr effect induced by a single photon in a commercial photonic crystal fiber. This method does not need to utilize quantum resources such as entanglement, and the probes used originate from conventional laser pulses, thus escaping the photon number limit. The group experimentally utilized a laser pulse containing about 100,000 photons, which is two orders of magnitude higher than the highest precision previously measured by classical methods.