Short-Time Expansion of Amplitudes for Time-Dependent Potentials
In this Demonstration we show that short-time quantum-mechanical transition amplitudes can be very accurately calculated if their expansion in the time of propagation is known to high orders. Here we consider imaginary-time amplitudes for the one-dimensional forced harmonic oscillator, with the time-dependent potential . For a quantum system in a time-dependent potential, the probability of a transition from an initial state to a final state in time is equal to , where is the transition amplitude. The evolution operator has to take into account explicit time-dependence of the potential. In a recently developed effective action approach, the amplitude is expressed in terms of an effective potential, and a set of recursive relations allows the systematic analytic derivation of the terms in the expansion of the effective potential in time t. The effective action thus obtained is characterized by a chosen level corresponding to the maximal order in its expansion. If level effective action is used, errors in the calculation of the transition amplitudes are proportional to .
The effective action approach was originally developed in a series of papers:
 A. Bogojevic, A. Balaz, and A. Belic, "Systematically Accelerated Convergence of Path Integrals," Phys. Rev. Lett., 94(18), 2005.
 A. Bogojevic, A. Balaz, and A. Belic, "Systematic Speedup of Path Integrals of a Generic N-Fold Discretized Theory," Phys. Rev. B, 72(6), 2005.
 A. Bogojevic, A. Balaz, and A. Belic, "Generalization of Euler's Summation Formula to Path Integrals," Phys. Lett. A, 344(2–4), 2005 pp. 84–90.
The extension of the approach to many-body systems is presented in the following papers:
 A. Bogojevic, I. Vidanovic, A. Balaz, and A. Belic, "Fast Convergence of Path Integrals for Many-Body Systems," Phys. Lett. A, 372(19), 2008 pp. 3341–3349.
 A. Balaz, A. Bogojevic, I. Vidanovic, and A. Pelster, "Recursive Schrödinger Equation Approach to Faster Converging Path Integrals," Phys. Rev. E, 79(3), 2009.
This method has been successfully applied to numerical studies of properties of various quantum systems:
 I. Vidanovic, A. Bogojevic, and A. Belic, "Properties of Quantum Systems via Diagonalization of Transition Amplitudes I: Discretization Effects," Phys. Rev. E, 80(6), 2009.
 I. Vidanovic, A. Bogojevic, A. Balaz, and A. Belic, "Properties of Quantum Systems via Diagonalization of Transition Amplitudes II: Systematic Improvements of Short-Time Propagation," Phys. Rev. E, 80(6), 2009.
Recently, this approach has been extended to systems in time-dependent potentials:
 A. Balaz, I. Vidanovic, A. Bogojevic, and A. Pelster, "Fast Converging Path Integrals for Time-Dependent Potentials," arXiv, 2009.
Mathematica programs developed for symbolic derivation of higher-order effective actions, as well as the C programs developed and used in numerical simulations in the above papers, can be found at http://www.scl.rs/speedup.