  We develop a reliable parameter-free analytic continuation method for quantum
many-body calculations. Our method is based on a kernel grid, a causal spline,
a regularization using the second-derivative roughness penalty, and the L-curve
criterion. We also develop the L-curve averaged deviation to estimate the
precision of our analytic continuation. To deal with statistically obtained
data more efficiently, we further develop a bootstrap-averaged analytic
continuation method. In the test using the exact imaginary-frequency Green's
function with added statistical error, our method produces the spectral
function that converges systematically to the exact one as the statistical
error decreases. As an application, we simulate the two-orbital Hubbard model
for various electron numbers with the dynamical-mean field theory in the
imaginary time and obtain the real-frequency self-energy with our analytic
continuation method, clearly identifying a non-Fermi liquid behavior as the
electron number approaches the half filling from the quarter filling. Our
analytic continuation can be used widely and it will facilitate drawing clear
conclusions from imaginary-time quantum many-body calculations.
