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Physical Review A

Population dynamics and phase effects in periodic level crossings

B. M. Garraway, Nikolay V. Vitanov

Published Jun 1, 1997• 67 citations
Publisher

Abstract

We present an analytic study of the population dynamics of a two-state system interacting with an external field and subjected to periodic level crossings. We apply an evolution matrix approach to calculate the excited-state population at the crossings (the nodes) and at the antinodes. The results are expressed in terms of only two parameters: the transition probability p for a quarter period from a crossing to an antinode, and the transition probability P for a half period between two successive crossings. We find that the values of the excited-state population at the antinodes can form global (gross) structures. We show that these structures and the population dynamics as a whole are very sensitive to the initial phase \ensuremath{\varphi} of the frequency-modulated field, particularly in the limits \ensuremath{\varphi}=0 (cosine modulation) and \ensuremath{\varphi}=\ensuremath{\pi}/2 (sine modulation). We calculate the parameters p and P by using two analytic approaches: one based on the original Landau-Zener model, and the other based on the finite Landau-Zener model. Both approaches unexpectedly lead to the same results. The notion of the global structures and the relevant parametrization in terms of p and P allow us to find various distinctive cases of population dynamics, such as population swapping, completely periodic evolution, superpositional trapping, and stepwise evolution.

DOI
10.1103/physreva.55.4418

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We present an analytic study of the population dynamics of a two-state system interacting with an external field and subjected to periodic level crossings. We apply an evolution matrix approach to calculate the excited-state population at the crossings (the nodes) and at the antinodes. The results are expressed in terms of only two parameters: the transition probability p for a quarter period from a crossing to an antinode, and the transition probability P for a half period between two successive crossings. We find that the values of the excited-state population at the antinodes can form global (gross) structures. We show that these structures and the population dynamics as a whole are very sensitive to the initial phase \ensuremath{\varphi} of the frequency-modulated field, particularly in the limits \ensuremath{\varphi}=0 (cosine modulation) and \ensuremath{\varphi}=\ensuremath{\pi}/2 (sine modulation). We calculate the parameters p and P by using two analytic approaches: one based on the original Landau-Zener model, and the other based on the finite Landau-Zener model. Both approaches unexpectedly lead to the same results. The notion of the global structures and the relevant parametrization in terms of p and P allow us to find various distinctive cases of population dynamics, such as population swapping, completely periodic evolution, superpositional trapping, and stepwise evolution.
We clarify some aspects of population dynamics of a level-crossing system, in which a periodic modulation of the relative energy is superimposed on the energy change with a constant velocity. For moderate values of the off-diagonal coupling, the temporal evolution of the system is well described by a transfer-matrix formalism. It is shown that the existence probability on one level shows a series of steplike changes both in the high-frequecy limit and the low-frequency limit of the periodic modulation, reflecting the quasiquantization of the oscillating field in the former case, while reflecting the multiple real crossings in the latter case.
A model of the time evolution of two interacting Rydberg manifolds of energy levels subject to a linearly ramped electric field is solved exactly in the Landau-Zener (LZ) approximation. Each manifold's levels are treated as linear in time, parallel, equally spaced, and infinite in number. Their pairwise interactions produce a regular two-dimensional grid of isolated anticrossings. The time development of an initially populated state is then governed by two-level LZ transitions at avoided crossings and adiabatic evolution between them, parametrized by the LZ transition probability DDD and a dynamical phase unit \ensuremath{\varphi}. The resulting probability distributions of levels are given analytically in the form of recursion relations, generating functions, integral representations involving DDD and \ensuremath{\varphi}, and in certain limits by Bessel or Whittaker functions. Level populations are mapped out versus location on the grid for a range of cases. Interference effects lead to two principal types of probability distributions: a braiding adiabatic pattern with revivals for small DDD and a diabatic pattern for $D\ensuremath{\rightarrow}1$ in which only certain levels parallel to the initial one are appreciably populated. The sensitivity of the coherent evolution to \ensuremath{\varphi} is discussed, along with the relation of this model to others and to selective-field ionization.
The excitation amplitude after multiple crossings is not a mere product of Landau-Zener transition probabilities at each crossing, due to coherent evolution of the system in between crossings. In the three-level ladder system, the trapping of population by frequency modulated fields ensures coherent evolution, and inclusion of phase effects for population redistribution after multiple crossings becomes necessary. The relative phase accumulated by various adiabatic states as they evolve along different paths is tailored to show the existence of quantum interference effects. We present a method of inverting the population in a three-level system, without affecting the population in the intermediate state. We also present an all optical implementation of the three-level ladder system, where these effects can be realized.
The time evolution groups of parallel energy levels (m, m{prime} = 0, 1, 2,...) is considered as a model of dynamic state mixing of interacting Rydberg manifolds by ramped electric fields used in selective field ionization (SFI). Each intersection [m, m{prime}] of the triangular grid of level crossings is treated as an isolated 2-level Landau-Zener anticrossing with amplitudes d and a for diabatic and adiabatic transitions, respectively ({vert_bar}d{vert_bar}{sup 2} + {vert_bar}a{vert_bar}{sup 2} = 1). Beginning on the uppermost upward-going level (m = 0) and following successive time steps {tau}{sup (N)} (N = 0,1,2,...), a path analysis leads to complex interference patterns among the many paths arriving at an intersection [m, m{prime}] after passing through N = m + m{prime} previous ones. The relative phase between any two paths is a multiple of the unit of action {var_phi} defined by the two pairs of adiabatic levels connecting four neighboring anticrossings. Compared to results from an analysis neglecting interference effects, there occurs here a general {open_quotes}emptying out{close_quotes} of population at the center of the interaction region (where the majority of paths lead). Most striking are resonances at high ramp rates (d {yields} 1), which with high probability enhance upward-going levels whose indexmore » m is an integral multiple of 27{pi}/{var_phi}. Though {var_phi} is sensitive to ramp rate, it is nevertheless expected that observed diabatic SFI signals should exhibit sequences of secondary peaks traceable to such resonances, which should not be assigned to extraneous initial states.« less
We have explored a genuinely time-dependent problem consisting of periodic crossings of energy levels. The model can be realized in several physical systems, but we refer it explicitly to a two-level ion that swings in a harmonic trap and concurrently interacts with a traveling light wave. We have solved the Schr\"odinger equation for this model by numerical integration as well as by the use of matrix continued fractions for the steady state. We investigate both the time and the frequency behavior of the model. We have also compared the exact behavior with an intuitive model based on a Landau-Zener description of each crossing followed by a simple relaxation behavior. In addition, the coherent interaction with the field is found to lead to a resonancelike behavior of the population transfer, which we can attribute to the accumulated phase of the Bloch vector. The various time scales of the problem are identified and their physical significance is elucidated.
A specific constant-amplitude periodic level-crossing model of the semi-classical quantum time-dependent two-state problem that belongs to a general Heun class of field configurations is presented. The exact analytic solution for the probability amplitude, generally written for this class in terms of the general Heun functions, in this specific case admits series expansion in terms of the incomplete Beta functions. Terminating this series results in an infinite hierarchy of finite-sum closedform solutions each standing for a particular two-state model, which generally is only conditionally integrable in the sense that for these field configurations the amplitude and phase modulation functions are not varied independently. However, there exists at least one exception when the model is unconditionally integrable, that is the Rabi frequency and the detuning of the driving optical field are controlled independently. This is a constant-amplitude periodic level-crossing model, for which the detuning in a limit becomes a Dirac delta-comb configuration with variable frequency of the levelcrossings. The exact solution for this model is derived, the Floquet exponents are determined and study of the population dynamics in the system for various regions of the input parameters is done.