Experimental quantum probing measurements with no knowledge of the system-probe interaction

Abstract

In any natural science, measurements are the essential link between theory and observable reality. Is it possible to obtain accurate and relevant information via measurement whose action on the probed system is unknown? In other words, can one be convinced to know something about the nature without knowing in detail how the information was obtained? In this paper, we show that the answer is, surprisingly, yes. We construct and experimentally implement a quantum optical probing measurement where measurements on the probes, i.e., the photons' polarization states, are used to extract information on the systems, i.e., the frequency spectra of the same photons. Unlike the preexisting probing protocols, our measurement does not require any knowledge of the interaction between the probe and the system.

Experimental Snapshot Verification of non-Markovianity with Unknown System-Probe Coupling

“…Interestingly, this result does not depend on knowing anything about how the system and probe interact, as it is based on the same approach as studied in [29,30]. This means that the protocol is not sensitive to imperfections in the implementation of the system-probe coupling U , and the same strategy can be used for multiple different couplings to obtain even tighter bounds.…”

“…Here ∆n = n H − n V is the birefringence of the medium, where n H and n V are the refractive indices in the horizontal (H) and vertical (V) directions, respectively. This polarization-frequency model has been recently popular in the studies of quantum information in open quantum systems [17,19,30,[39][40][41][42][43][44][45][46]. If the fast axes of the quartz plates in the combination are not aligned, the dynamics becomes significantly more complicated.…”

“…Motivated by the analysis of pure dephasing channels in [29,30], we choose the initial probe states of the polarization system as ρ 1 = ρ 2 = ρ 3 = |+ +|. After the polarization has interacted with the frequency for an unknown time, we perform tomographic measurements on the evolved states Φ 1 (ρ 1 ), Φ 2 (ρ 2 ), and Φ 3 (ρ 3 ) and calculate the α-fidelities F α Φ 1 (ρ 1 ), Φ 2 (ρ 2 ) and F α Φ 1 (ρ 1 ), Φ 3 (ρ 3 ) .…”

“…Recently, a method for probing the lower bound for ∆η was experimentally implemented in the cases of unknown coupling between the system (frequency) and probe (polarization) [30]. For our notation, the probed bound is given by…”

“…Later, such protocol was experimentally implemented in a single photon experiment [30]. In that paper, the upper bound for the width of a Gaussian frequency spectrum was probed by measurements on a polarization probe qubit that had interacted with frequency in a combination of quartz plates rotated in randomly chosen angles, corresponding to an unknown system-probe coupling.…”