QUROPE Quantum Information Processing and Communication in Europe

R. Demkowicz-Dobrzański, L. Maccone
Phys. Rev. Lett. 113, 250801 (2014)

Quantum metrology provides super-classical scaling in measurement precision by exploiting quantum effects. A crucial question in the field is to understand when entangled states lead to super-classical scaling.

In their work, Demkowicz-Dobrzański and Maccone analyze the role of entanglement among probes and with external ancillas in quantum metrology. In the absence of noise, it is known that unentangled sequential strategies can achieve the same Heisenberg scaling of entangled strategies and that external ancillas are useless. This changes in the presence of noise; the work proves that entangled strategies can have higher precision than unentangled ones and that the addition of passive external ancillas can also increase the precision. They also analyze some specific noise models and use the results to conjecture a general hierarchy for quantum metrology strategies in the presence of noise.