The initialization and the readout of the spin system state are
as important as its manipulation. Different methods are used for
initializing the spin system. In diamond, the preparation of the
state j0↑〉 is technically controlled by means of an optical excitation
[12,36]. Nevertheless, at low magnetic field, the state of the spin
system is always a superposition of the basis states jmSms〉 (see
Ref. [22]). The purification requires to bring the spin system close
to the level anti-crossings (LACs) [35,22]. For example, the input
state j + ↓〉 can be obtained as follows. With increasing the static
magnetic field strength, the spin-21 flips beyond the LAC which
occurs at about 100 G while the spin-1 flips beyond the LAC which
occurs above 0.1 T (see Ref. [22]). At high magnetic field, the spin
system is in the desired state. The second input state j + ↑〉 can be
deduced from the first one by means of a selective RF pulse
[27,28]. An iterative method for purifying the spin system is given
in Ref. [21]. The readout of the spin system is realized by means of
photoluminescence which is responsive to the electron spin
polarization [36].