Abstract
We report on the anomalous behavior of control pulses for spins subject to
classical noise with a singular autocorrelation function. This behavior is not
detected for noise with analytic autocorrelation functions. The effect is
manifest in the different scaling behavior of the deviation of a real pulse to
the ideal, instantaneous one. While a standard pulse displays scaling $\propto
\tau_p^1$, a first-order refocusing pulse normally shows scaling
$\tau_p^2$. But in presence of cusps in the noise
autocorrelation the scaling $\tau_p^3/2$ occurs. Cusps in
the autocorrelation are characteristic for the omnipresent Ornstein-Uhlenbeck
process. We prove that the anomalous exponent cannot be avoided; it represents
a fundamental limit. On the one hand, this redefines the strategies one has to
adopt to design refocusing pulses. On the other hand, the anomalous exponent,
if found in experiment, provides important information on the noise properties.
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