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Physical Review A

“Interaction-free” interaction: Entangling evolution coming from the possibility of detection

Paweł Horodecki

Published Jan 17, 2001• 18 citations
Publisher

Abstract

The dynamical version of the so-called ``interaction-free'' measurement [Phys. Rev. Lett. 74 (1995) 4763] is presented, in which the object called ``bomb'' is replaced with the atom evolving coherently. It is pointed out that, due to the quantum Zeno effect, the ``explosion'' symbolized by the absorption of a photon by the atom will never take place. It is shown, however, that this is accompanied by a strong modification of the involved evolution which now has a highly entangling character. Properties of the new evolution are discussed. It is also proved that beyond the quantum Zeno effect regime there is a natural possibility of conditional (i.e., followed by the sequence of some, but not necessarily frequent, probes) freezing of the evolving system in a maximally entangled state. An explanation of the effects as well as some general conclusions are provided.

DOI
10.1103/physreva.63.022108

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Summary form only given.Using the complementary wave-like and particle-like natures of photons, one can make interaction-free measurements (IFM)-the presence of an object can be determined with no photons being absorbed. By using an application of the quantum Zeno effect (weakly and repeatedly testing for the object's presence so that the otherwise coherent evolution of the interrogating photon is inhibited), we have demonstrated the feasibility of IFMs with an efficiency of >85%, higher than that possible in the original IFM scheme, where, at most, half of the measurements could be interaction free. We also investigate various interaction-free imaging systems, demonstrate imaging with a resolution <5 /spl mu/m, and define the limits to these systems. We also discuss in which sense interaction-free measurements really are not.

Conference on Lasers and Electro-Optics•Jan 1, 1998•10 citations
The dynamical version of the so-called interaction-free'' measurement [Phys. Rev. Lett. 74 (1995) 4763] is presented, in which the object called bomb'' is replaced with the atom evolving coherently. It is pointed out that, due to the quantum Zeno effect, the ``explosion'' symbolized by the absorption of a photon by the atom will never take place. It is shown, however, that this is accompanied by a strong modification of the involved evolution which now has a highly entangling character. Properties of the new evolution are discussed. It is also proved that beyond the quantum Zeno effect regime there is a natural possibility of conditional (i.e., followed by the sequence of some, but not necessarily frequent, probes) freezing of the evolving system in a maximally entangled state. An explanation of the effects as well as some general conclusions are provided.
Abstract The quantum Zeno effect is one in which a quantum system never jumps to a new state if an experimenter keeps observing it. This effect is illustrated in several examples. The chapter describes the measurement of the state of a two-level atom under radiation that would normally cause the atom to change to the other state. Polarized photons can pass through optically active material multiple times, with polarization measurement at each stage, so that the polarization does not change. One can use superposition and interference to do interaction-free measurements. A hypothetical bomb-detection device (the EV effect) uses this method to tell that a bomb is not a dud without exploding it. An actual experimental used interference to detect the position of a mirror, which is never touched by any photon. Experimenters have also formed an image using photons that never touch the object imaged.
The quantum Zeno effect is the inhibition of the free evolution of a system when that system is subject to a measurement process. The authors of one recent experiment study, W. M. Itano et al., [Phys. Rev. A 41, 2295 (1990)] claimed a demonstration of the quantum Zeno effect. This was done by performing discontinuous atomic-level measurements on a coherently evolving two-level atomic stystem. The measurement was implemented using a pulsed laser strong enough to saturate the two-level atomic system. In this paper, we examine the simpler case where the measurement laser is continuously on and does not saturate the two-level atomic system. In contrast to previous treatments, we show certain parameter regimes wherein an effective state reduction does occur. We ascertain the experimental parameter regimes under which a measurement can be said to have been performed. We show that, in some regimes, fully efficient photon detection does not give good information about the atomic-level populations and hence no measurement has been performed. Throughout, we emphasize that the design of an atomic level-measurement device requires more than the analysis of a mechanism for photon detection or of coherence loss. We gain insight into the operation of a particular measurement process by following the selective evolution of various systems using the quantum-trajectories approach.
In 1977, Mishra and Sudarshan [J. Math. Phys. 18, 756 (1977)] showed that an unstable particle would never be found decayed while it was continuously observed. They called this effect the quantum Zeno effect (or paradox). Later it was realized that the frequent measurements could also accelerate the decay (quantum anti-Zeno effect). In this paper, we investigate the quantum Zeno effect using the definite model of the measurement. We take into account the finite duration and the finite accuracy of the measurement. A general equation for the jump probability during the measurement is derived. We find that the measurements can cause inhibition (quantum Zeno effect) or acceleration (quantum anti-Zeno effect) of the evolution, depending on the strength of the interaction with the measuring device and on the properties of the system. However, the evolution cannot be fully stopped.
Abstract. If frequent measurements ascertain whether a quantum system is still in its initial state, transitions to other states are hindered and the quantum Zeno effect takes place. However, in its broader formulation, the quantum Zeno effect does not necessarily freeze everything. On the contrary, for frequent projections onto a multidimensional subspace, the system can evolve away from its initial state, although it remains in the subspace defined by the measurement. The continuing time evolution within the projected “quantum Zeno subspace ” is called “quantum Zeno dynamics: ” for instance, if the measurements ascertain whether a quantum particle is in a given spatial region, the evolution is unitary and the generator of the Zeno dynamics is the Hamiltonian with hard-wall (Dirichlet) boundary conditions. We discuss the physical and mathematical aspects of this evolution, highlighting the open mathematical problems. We then analyze some alternative strategies to obtain a Zeno dynamics and show that they are physically equivalent.
Summary form only given.Using the complementary wave-like and particle-like natures of photons, one can make interaction-free measurements (IFM)-the presence of an object can be determined with no photons being absorbed. By using an application of the quantum Zeno effect (weakly and repeatedly testing for the object's presence so that the otherwise coherent evolution of the interrogating photon is inhibited), we have demonstrated the feasibility of IFMs with an efficiency of >85%, higher than that possible in the original IFM scheme, where, at most, half of the measurements could be interaction free. We also investigate various interaction-free imaging systems, demonstrate imaging with a resolution <5 /spl mu/m, and define the limits to these systems. We also discuss in which sense interaction-free measurements really are not.