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arXiv:0710.1086

Comment on "Nonexponential Decay Via Tunneling in Tight-Binding Lattices and the Optical Zeno Effect"

Elena Rufeil Fiori, Horacio M. Pastawski

Published Oct 4, 2007• 1 citation

Abstract

S. Longhi [1] studied the survival probability P(t) of an unstable state coupled to a tight-binding lattice finding an exact analytical solution that describes the nonexponential decay. When the first coupling is smaller than the others, he shows that P(t) has a natural decomposition into two terms; one is the exponential decay, consistent with the Gamow's approach, and the other is the correction to this decay. The first purpose of this Comment is to show that the condition imposed on the system; the site energy of the first site is set in the center of the band, limits the generality of the results. A decomposition based on the spectral properties of the system removes this restriction [2]. Other point of this Comment is that, for the weak coupling limit, the author stated that a Zeno effect occurs for t less than t*, where t* is the smallest root of the equation g_eff(t*)=g_0, and for the strong coupling limit an anti-Zeno effect occurs for t close to t**, where t** is the first peak of g_eff. By using a spectral formulation we show that, the anti-Zeno effect not only requires a strong coupling limit, also is necessary to choose the resonance energy and width such that a destructive interference can occur. We present this interference and give an analytical expression and physically interpretation for t* and t**.

DOI: 10.48550/arxiv.0710.1086

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S. Longhi [1] studied the survival probability P(t) of an unstable state coupled to a tight-binding lattice finding an exact analytical solution that describes the nonexponential decay. When the first coupling is smaller than the others, he shows that P(t) has a natural decomposition into two terms; one is the exponential decay, consistent with the Gamow's approach, and the other is the correction to this decay. The first purpose of this Comment is to show that the condition imposed on the system; the site energy of the first site is set in the center of the band, limits the generality of the results. A decomposition based on the spectral properties of the system removes this restriction [2]. Other point of this Comment is that, for the weak coupling limit, the author stated that a Zeno effect occurs for t less than t*, where t* is the smallest root of the equation g_eff(t*)=g_0, and for the strong coupling limit an anti-Zeno effect occurs for t close to t**, where t** is the first peak of g_eff. By using a spectral formulation we show that, the anti-Zeno effect not only requires a strong coupling limit, also is necessary to choose the resonance energy and width such that a destructive interference can occur. We present this interference and give an analytical expression and physically interpretation for t* and t**.

We report the first observation of the quantum Zeno and anti-Zeno effects in an unstable system. Cold sodium atoms are trapped in a far-detuned standing wave of light that is accelerated for a controlled duration. For a large acceleration the atoms can escape the trapping potential via tunneling. Initially the number of trapped atoms shows strong nonexponential decay features, evolving into the characteristic exponential decay behavior. We repeatedly measure the number of atoms remaining trapped during the initial period of nonexponential decay. Depending on the frequency of measurements we observe a decay that is suppressed or enhanced as compared to the unperturbed system.

Controlling the decay dynamics of an unstable state coupled to a continuum is a longstanding problem of major interest in different areas of quantum and classical physics. Suppression of quan- tum mechanical decay and slow-down of the escape dynamics in macroscopic quantum and optical tunneling via dynamical decoupling methods have been proposed and experimentally demonstrated in different physical systems over the last two decades. So far most of dynamical decoupling schemes have been focused to Hermitian dynamics. In this work we consider the decay of an unstable state coupled to a continuum via a time-dependent non-Hermitian coupling with finite (albeit arbitrarily long) duration, and show rather generally that for a wide class of coupling temporal shapes, in which the real and imaginary parts of the coupling are related each other by a Hilbert transform, the metastable state returns to its initial condition after the interaction with the continuum. Re- markably, the absence of decay holds even beyond the weak-coupling (Markovian) limit and for arbitrarily slow (adiabatic) interactions. The method is illustrated by considering a paradigmatic example of decay dynamics of a metastable state coupled to a tight-binding continuum described by a Fano-Anderson model and realized by a semi-infinite tight-binding lattice. Non-Hermitian coupling can be implemented by fast modulation of the complex site energy of the edge site in the lattice.

The quantum Zeno effect--suppression of decay by frequent measurements--was believed to occur only when the response of the detector is so quick that the initial tiny deviation from the exponential decay law is detectable. However, we show that it can occur even for exactly exponentially decaying systems, for which this condition is never satisfied, by considering a realistic case where the detector has a finite energy band of detection. The conventional theories correspond to the limit of an infinite bandwidth. This implies that the Zeno effect occurs more widely than expected thus far.