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Physical Review Letters

Better Schemes for Quantum Interrogation in Lossy Experiments

Terry Rudolph

Published Oct 2, 2000• 15 citations
Publisher

Abstract

We show that several schemes exist for improving the efficiency of quantum interrogations (or "interaction-free" measurements), which in principle are limited by lossy experimental components. We show how one new scheme (the "Helstrom" scheme) can efficiently detect the presence of a bomb which has been designed to foil standard quantum interrogations by arming itself when there is high probability of a photon being in the region. This particular new scheme can probe a region of space with a greater probability of success than a person in that region determining they are being probed.

DOI
10.1103/physrevlett.85.2925

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We show that several schemes exist for improving the efficiency of quantum interrogations (or "interaction-free" measurements), which in principle are limited by lossy experimental components. We show how one new scheme (the "Helstrom" scheme) can efficiently detect the presence of a bomb which has been designed to foil standard quantum interrogations by arming itself when there is high probability of a photon being in the region. This particular new scheme can probe a region of space with a greater probability of success than a person in that region determining they are being probed.
The phenomenon of quantum interrogation allows one to optically detect the presence of an absorbing object, without the measuring light interacting with it. In an application of the quantum Zeno effect, the object inhibits the otherwise coherent evolution of the light, such that the probability that an interrogating photon is absorbed can in principle be arbitrarily small. We have implemented this technique, achieving efficiencies of up to 7373%73, and consequently exceeding the 5050%50 theoretical maximum of the original ``interaction-free'' measurement proposal. We have also predicted and experimentally verified a previously unsuspected dependence on loss.
We propose a scheme for nondistortion quantum interrogation defined as an interaction-free measurement that preserves the internal state of the object being detected. In our scheme, two Einstein-Podolsky-Rosen entangled photons are used as the probe and polarization-sensitive measurements are performed at the four ports of the Mach-Zehnder interferometer. In comparison with the previous single-photon scheme, it is shown that the two-photon approach has a higher probability of initial state preserving interrogation of an atom prepared in a quantum superposition. In the case that the presence of the atom is not successfully detected, the experiment can be repeated since the initial state of the atom is unperturbed.
In ``interaction-free'' measurements, one typically wants to detect the presence of an object without touching it with even a single photon. One often imagines a bomb whose trigger is an extremely sensitive measuring device whose presence we would like to detect without triggering it. We point out that all such measuring devices have a maximum sensitivity set by the uncertainty principle, and thus can only determine whether a measurement is ``interaction-free'' to within a finite minimum resolution. We further discuss exactly what can be achieved with the proposed ``interaction-free'' measurement schemes.
We present some applications of high-efficiency quantum interrogation (``interaction-free measurement'') for the creation of entangled states of separate atoms and of separate photons. The quantum interrogation of a quantum object in a superposition of object-in and object-out leaves the object and probe in an entangled state. The probe can then be further entangled with other objects in subsequent quantum interrogations. By then projecting out those cases in which the probe is left in a particular final state, the quantum objects can themselves be left in various entangled states. In this way, we show how to generate two-, three-, and higher-qubit entanglement between atoms and between photons. The effect of finite efficiency for the quantum interrogation is delineated for the various schemes.
We consider the nondistortion quantum interrogation of an atom prepared in a quantum superposition. By manipulating the polarization of the probe photon and making connections to interaction-free measurements of opaque objects, we show that nondistortion interrogation of an atom in a quantum superposition can be done with efficiency approaching unity. However, if any component of the atom's superposition is completely transparent to the probe wave function, a nondistortion interrogation of the atom is impossible.
We consider the nondistortion quantum interrogation (NQI) of an atom prepared in a quantum superposition. By manipulating the polarization of the probe photon and making connections to interaction free measurements of opaque objects, we show that nondistortion interrogation of an atom in a quantum superposition can be done with efficiency approaching unity. However, if any component of the atom's superposition is completely transparent to the probe wave function, a nondistortion interrogation of the atom is impossible.