Interaction-free measurement of the degree of polarization of an atomic ensemble

Full text: In the last years, several proposals for quantum information and quantum communication schemes require the use of atomic media together with single photons. Among the possible interactions, much interest has been generated by non destructive techniques, like the quantum non-demolition measurement. We present here instead a characterization of the polarization state of an atomic sample via the non-interaction of our sample with the probe, thus reducing, in principle, the damage to the sample for successful event to zero. The term 'interaction-free' applies to a measurement process where the probe carries information about a system without interacting with it, possible thanks to the quantum nature of the interference process. After the proposal of Elitzur and Vaidman, an experimental demonstration was provided by Kwiat et al.. We have realized an interaction free measurement of the spectrum and polarization state of a hot ensemble of 87Rb by inserting it in a polarization interferometer, where a different optical density for one of the polarization modes is revealed by detection of light at the output dark in the balanced case The probe is a narrowband coherent light, strongly attenuated to the single photon level, with a central frequency that can be scanned by some GHz in the region of the Rb D1 line. The atoms are optically pumped by an intense beam into the mF =1,2 Zeeman substates of the F=2 hyperfine level. Observing the output of the dark port while scanning the probe frequency, the obtained trace presents a peak in correspondence with the transitions involving the hyperfine ground level F=2. The profile of the trace corresponds to the profile of the transition, power broadened by the intense pumping necessary for polarization. The demonstrated scheme suffers from limited statistical efficiency, equal to 1/4 in theory and further reduced because of experimental imperfections. It has been demonstrated that combining the interaction free measurement approach and an implementation of the quantum Zeno effect it is possible to reach a theoretical efficiency close to unity. Moreover, this scheme could be used to measure the degree of polarization of a sample of atoms reducing the damage compared with standard absorption techniques. (author)