  Avoiding the loss of coherence of quantum mechanical states is an important
prerequisite for quantum information processing. Dynamical decoupling (DD) is
one of the most effective experimental methods for maintaining coherence,
especially when one can access only the qubit-system and not its environment
(bath). It involves the application of pulses to the system whose net effect is
a reversal of the system-environment interaction. In any real system, however,
the environment is not static, and therefore the reversal of the
system-environment interaction becomes imperfect if the spacing between
refocusing pulses becomes comparable to or longer than the correlation time of
the environment. The efficiency of the refocusing improves therefore if the
spacing between the pulses is reduced. Here, we quantify the efficiency of
different DD sequences in preserving different quantum states. We use 13C
nuclear spins as qubits and an environment of 1H nuclear spins as the
environment, which couples to the qubit via magnetic dipole-dipole couplings.
Strong dipole-dipole couplings between the proton spins result in a rapidly
fluctuating environment with a correlation time of the order of 100 us. Our
experimental results show that short delays between the pulses yield better
performance if they are compared with the bath correlation time. However, as
the pulse spacing becomes shorter than the bath correlation time, an optimum is
reached. For even shorter delays, the pulse imperfections dominate over the
decoherence losses and cause the quantum state to decay.
