Interaction-Free Measurement of a Quantum Object: On the Breeding of “Schrödinger Cats”

Abstract

The possibility of an "interaction-free" determination of the presence of an object was first discussed by Renninger and by Dicke,l who examined the effect on a quantum system due to the non-observation of a particular result (e.g., the non-scattering of a photon). Elitzur and Vaidman extended these ideas, so that the presence of an object modified the interference of a photon, even though the photon and object need not have interacted. 2 In the best case, their method works only 50% of the time. We have recently reported a different technique,3 based on the quantum Zeno effect,4 which allows the fraction of interaction-free measurements (IFMs) to be arbitrarily close to 1. As a result, one even has the possibility to employ multi-photon pulses for the interrogation. When the object being observed is in a quantum superposition state, one can prepare superpositions and entanglements of these macroscopic states of light. There are many ways to perform interaction-free measurements. 3 For example, consider the scheme shown in Fig. 1. A vertically-polarized photon is directed into the system at timeT = 0, and removed after N cycles (by a fast switch, not shown). Its polarization is rotated each cycle by !:!.(} = 7r /2N, e.g., with an optically active material, or a waveplate.