We study the projective nature of quantum measurements to achieve quantum-enhanced magnetometry using a spin chain probe for remotely detecting a local magnetic field [1, 2]. Our protocol involves performing a sequence of local measurements followed by free evolution on a spin chain probe initialized in a separable state. The local magnetic field to be estimated acts on one end of the probe, inducing an evolution through the whole system. On the other hand, the readout spin, measured sequentially at regular intervals on a fixed basis, is located at the other end, enabling remote sensing of the local field. The system is not reset until the entire sequence of measurements is completed. Increasing the sequence length demonstrates that sensing precision can be enhanced beyond the standard limit. The sequential protocol has both fundamental and practical implications. From a fundamental point of view, it introduces a new methodology for achieving quantum-enhanced sensitivity by exploiting the quantum nature of measurements and their subsequent wave-function collapse. From a practical perspective, our protocol provides remote quantum sensing while avoiding the need for complex entangled states and challenging adaptive measurements.
[1] V Montenegro, GS Jones, S Bose, A Bayat - Physical Review Letters 129, 120503, 2022
[2] Y Yang, V Montenegro, A Bayat - Physical Review Research 5, 043273, 2023