Figure 1: Interaction-free measurements.
From: Interaction-free measurements by quantum Zeno stabilization of ultracold atoms
(a) Classic proposal for an IFM2: A single photon entering the Mach–Zehnder interferometer never reaches the dark port D due to destructive interference. Only if an absorbing object (depicted as a bomb) is placed in one arm of the interferometer, the photon can trigger detector D, thereby witnessing the existence of the object. (b) Our implementation relies on a Bose–Einstein condensate in the Zeeman level (F, mF)=(1, 0), which can decay to the levels (1, ±1) by spin changing collisions. The analogue of an absorbing object is realized by a resonant laser beam, providing an effective loss of atoms in the level (1, −1). Owing to the quantum Zeno effect, this object will prevent the generation of atomic pairs by spin dynamics. An IFM of the object can thus be realized by counting the number of atoms in the level (1, 1): if the result is zero, the object exists and has not interacted with atoms. (c) Without the absorbing object, the number of atoms in level (1, 1) (open circles) grows exponentially according to 〈N+1〉=sinh2(Ωt) (solid line), which can be used as a calibration of the spin dynamics rate Ω. The error bar presents the standard error of the mean number of transferred atoms.
