Time-dependent quantum dynamics

Yu Hua Cao, Shi Jin, Nana Liu

Recently, there has been growing interest in simulating time-dependent Hamiltonians using quantum algorithms, driven by diverse applications, such as quantum adiabatic computing. While techniques for simulating time-independent Hamiltonian dynamics are well established, time-dependent Hamiltonian dynamics is less explored and it is unclear how to systematically organize existing methods and to find new methods. Sambe-Howland's continuous clock elegantly transforms time-dependent Hamiltonian dynamics into time-independent Hamiltonian dynamics, which means that by taking different discretizations, existing methods for time-independent Hamiltonian dynamics can be exploited for time-dependent dynamics. In this work, we systemically investigate how Sambe-Howland's clock can serve as a unifying framework for simulating time-dependent Hamiltonian dynamics. First, we demonstrate the versatility of this approach by showcasing its compatibility with analog quantum computing and digital quantum computing. Second, for digital quantum computers, we illustrate how this framework, combined with time-independent methods (e.g., product formulas, multiproduct formulas, qDrift, and linear combination of unitaries-Taylor), can facilitate the development of efficient algorithms for simulating time-dependent dynamics. This framework allows us to (1) resolve the problem of finding minimum-gate time-dependent product formulas; (2) establish a unified picture of both Suzuki's and Huyghebaert and De Raedt's approaches; (3) generalize Huyghebaert and De Raedt's first- and second-order formula to arbitrary orders; (4) answer an unsolved question in establishing time-dependent multiproduct formulas; (5) and recover continuous qDrift on the same footing as time-independent qDrift. Third, we demonstrate the efficacy of our higher-order Huyghebaert and De Raedt's algorithm through digital adiabatic simulation.

Abraham Nitzan

This chapter focuses on the time-dependent Schrödinger equation and its solutions for several prototype systems. It provides the basis for discussing and understanding quantum dynamics in condensed phases, however, a full picture can be obtained only by including also dynamical processes that destroy the quantum mechanical phase. Such a full description of quantum dynamics cannot be handled by the Schrödinger equation alone; a more general approach based on the quantum Liouville equation is needed. This important part of the theory of quantum dynamics is discussed in Chapter 10. Given a system characterized by a Hamiltonian Ĥ , the time-dependent Schrödinger equation is For a closed, isolated system Ĥ is time independent; time dependence in the Hamiltonian enters via effect of time-dependent external forces. Here we focus on the earlier case. Equation (1) is a first-order linear differential equation that can be solved as an initial value problem.

Zoltán Papp

Abraham Nitzan

Abstract This chapter the reviews essential concepts and methods of time-dependent quantum mechanics. (1) Formal solutions of the time-dependent Schrödinger equation. (2) The two-level system. (3) Interacting systems: The time-dependent self-consistent field approximation and the Ehrenfest approximation. (4) The Born–Oppenheimer approximation and nuclear potential surfaces. (5) Time-dependent Hamiltonians: A two-level system in a time-dependent field. (6) Non-adiabatic dynamics: The surface-hopping method. (7) Expressing the time evolution in terms of the Green operator. (8) Representations (the Schrödinger and Heisenberg representations, the interaction representation). (9) Time-dependent perturbation theory. (10) Quantum dynamics of the free particles. (11) Quantum dynamics of the harmonic oscillator. (12) The raising/lowering operators formalism. (13) Tunneling.