In this two-hour lecture we will make a review of the theory and the (numerical) experiments on the behavior of quantum fidelity of classically chaotic and regular hamiltonian systems.
We will derive a simple and general relation between the fidelity of
quantum motion, characterizing the stability of quantum dynamics with
respect to arbitrary static perturbation of the unitary evolution
propagator, and the integrated time auto-correlation function of the
generator of perturbation. Quite surprisingly, this relation predicts
the slower decay of fidelity the faster decay of correlations. In
particular, for non-ergodic and non-mixing dynamics, where asymptotic
decay of correlations is absent, a qualitatively different and faster
decay of fidelity is predicted on a time scale
as
opposed to mixing dynamics where the fidelity is found to decay
exponentially on a time-scale
, where
is
proportional to the strength of perturbation. A detailed discussion
of a semi-classical regime of small effective values of Planck
constant
is given where classical correlation functions can be
used to predict quantum fidelity decay. Note that the correct and
intuitively expected classical stability behavior is recovered in the
classical limit
, as the two limits
and
do not commute. In addition we also discuss a non-trivial
dependence on the number of degrees of freedom and the role of the
thermodynamic limit.
The theoretical predictions will be demonstrated mainly in two families of models: (i) a quantized kicked top and a quantized pair of coupled kicked tops where the semiclassical regime is emphasized, and (ii) kicked Ising spin 1/2 chain where the thermodynamic regime is emphasized. We also need to stress that these results have important implications for the stability of quantum computation, and may be used in order to optimize the accuracy of quantum algorithms.
References
T. Prosen, ``General relation between quantum ergodicity and fidelity of quantum dynamics'',
Phys. Rev. E 65, 036208 (2002).
T. Prosen and M. Znidaric, ``Can quantum chaos enhance the stability of quantum computation?'',
J. Phys. A: Math. Gen. 34, L681 (2001).
T. Prosen and M. Znidaric, ``Stability of quantum motion and correlation decay'',
J. Phys. A: Math. Gen. 35, 1455 (2002).
T. Prosen and T. H. Seligman, ``Decoherence of spin echoes'',
J. Phys. A: Math. Gen. 35, 4707 (2002); nlin.CD/0201038.
T. Prosen, T. H. Seligman and M. Znidaric, ``Stability of quantum coherence and
correlation decay'', preprint, quant-ph/0204043
The questions about the mechanisms and the conditions for the relaxation to equilibrium in the thermodynamic limit of a generic isolated hamiltonian system constitute important open problems in statistical mechanics. In this talk we consider an established technique in describing relaxation of strongly chaotic single-particle classical systems, namely the concept of Perron-Frobenius-Ruelle resonance spectrum, and use it for the dynamical description of non-integrable quantum many-body systems in thermodynamic limit.
We define a quantum Perron-Frobenius master operator over a suitable
normed space of translationally invariant states adjoint to the
quasi-local algebra of quantum lattice gasses (e.g. spin
chains), whose spectrum determines the exponents of decay of time
correlation functions. The gap between the leading eigenvalue and the
unit circle signals the exponential mixing (universal asymptotic
exponential decay of arbitrary time correlation functions), whereas
closing the gap typically corresponds to a transition to non-ergodic
dynamics, which may as a consequence, lead to important anomalous
transport properties. In particular, the conservation laws of
completely integrable quantum lattices represent degenerate eigenvalue
1 eigenvectors of the Perron-Frobenius operator.
Theoretical ideas are applied and validated in a generic example of
kicked Ising spin chains, namely a one dimensional spin
lattice with nearest neighbor Ising interaction kicked with periodic
pulses of a tilted homogeneous magnetic field. We show that the
'chaotic eigenmodes' corresponding to leading Perron-Frobenius-Ruelle
eigenvalue resonances have fractal structure in the basis of local
operators.
References
T. Prosen, ``Chaotic resonances in quantum many-body dynamics'', preprint.