Quantum memory is a core component of a quantum computer where quantum
mechanics, instead classical mechanics, becomes dominant if the device size
approaches nanometer scale. Because of it potential scalability and its long
relaxation time T_1, an electron spin in a semiconductor quantum dot is one
of the most promising candidate for a solid-state-based quantum computer
architecture. Decoherence of the electron spin in quantum dots, through
hyperfine coupling to its surrounding nuclear spins, is the major obstacle
for the realization of a quantum memory. Typical decoherence time T_2^* is
10 ns for GaAs quantum dots at low temperature (~100 mK) and in sub-Tesla
magnetic field [1]. I address the decoherence mechanism and the decoherence
process of an electron spin in quantum dots, as well as quantum control of
the decoherence to extend the coherence time 1000 times longer via dynamical
decoupling method by subjecting the electron spin to a sequence of control
pulses. Experimental related considerations, such as external magnetic
field, initial bath polarization, and control pulse imperfection, are also
discussed [2].

[1] J. R. Petta et al., Science 309, 2180 (2005).
[2] W. Zhang et al., Phys. Rev. B 74, 205313 (2006); ibid, 75, 201302(R) (2007).