Adiabatic time evolution in spin systems (11 pages)

J. Chee

An extension of the adiabatic factorization of the time evolution operator is studied for spin in a general time-varying magnetic field B ( t ). When B ( t ) changes adiabatically, such a factorization reduces to the product of the geometric operator which embodies the Berry phase phenomenon and a usual dynamical operator. For a general time variation of B ( t ), there should be a non-adiabatic operator in the factorization that is responsible for non-adiabatic transitions. A simple and explicit expression for the instantaneous angular velocity of the non-adiabatic operator is derived. This is done in a way that is independent of any specific representation of spin. Two classes of simple conditions are given under which the non-adiabatic operator itself can be made explicit. These conditions are used to find new expressions for B ( t ) that allow the explicit construction of the time evolution operator.

Stephan Appelt, Gerhard Wäckerle, M. Mehring

K. Napi�rkowski, W. Pusz

Bela Bauer, Lei Wang, Iztok Pižorn +1 more

We explore the role of entanglement in adiabatic quantum optimization by performing approximate simulations of the real-time evolution of a quantum system while limiting the amount of entanglement. To classically simulate the time evolution of the system with a limited amount of entanglement, we represent the quantum state using matrix-product states and projected entangled-pair states. We show that the probability of finding the ground state of an Ising spin glass on either a planar or non-planar two-dimensional graph increases rapidly as the amount of entanglement in the state is increased. Furthermore, we propose evolution in complex time as a way to improve simulated adiabatic evolution and mimic the effects of thermal cooling of the quantum annealer.

Takuya Hatomura, Kazutaka Takahashi

Adiabatic gauge potential is the origin of nonadiabatic transitions. In counterdiabatic driving, which is a method of shortcuts to adiabaticity, adiabatic gauge potential can be used to realize identical dynamics to adiabatic time evolution without requiring slow change of parameters. We introduce an algebraic expression of adiabatic gauge potential. Then, we find that the explicit form of adiabatic gauge potential can be easily determined by some algebraic calculations. We demonstrate this method by using a single-spin system, a two-spin system, and the transverse Ising chain. Moreover, we derive a lower bound for fidelity to adiabatic time evolution based on the quantum speed limit. This bound enables us to know the worst case performance of approximate adiabatic gauge potential. We can also use this bound to find dominant terms in adiabatic gauge potential to suppress nonadiabatic transitions. We apply this bound to magnetization reversal of the two-spin system and to quantum annealing of the transverse Ising chain. Adiabatic gauge potential reflects structure of energy eigenstates, and thus we also discuss detection of quantum phase transitions by using adiabatic gauge potential. We find a signature of a quantum phase transition in the transverse Ising chain.

Konstantin Yu Bliokh