  Quantum phase estimation is an important component in diverse quantum
algorithms. However, it suffers from spectral leakage, when the reciprocal of
the record length is not an integer multiple of the unknown phase, which incurs
an accuracy degradation. For the existing single-sample estimation scheme,
window-based methods have been proposed for spectral leakage mitigation. As a
further advance, we propose a dual-frequency estimator, which asymptotically
approaches the Cramer-Rao bound, when multiple samples are available. Numerical
results show that the proposed estimator outperforms the existing window-based
methods, when the number of samples is sufficiently high.
