  Along with the scaling of dimensions in quantum systems, transitions between
the system's energy levels would become close in frequency, which are
conventionally resolved by weak and lengthy pulses. Here, we extend and
experimentally demonstrate analytically based swift quantum control techniques
on a four-level trapped ion system, where we perform individual or simultaneous
control over two pairs of spectrally nearby transitions with tailored
time-varied drive, achieving operational fidelities ranging from 99.2(3)\% to
99.6(3)\%. Remarkably, we achieve approximately an order of magnitude speed up
comparing with the case of weak square pulse for a general control. Therefore,
our demonstration may be beneficial to a broad range of quantum systems with
crowded spectrum, for spectroscopy, quantum information processing and quantum
simulation.
