A physics potential adding positron polarization has recently been demonstrated in a precise measurement of on Z-pole.  With % and %, the systematic error( )can be 5-times smaller than that of single polarization(%), that is 0.00005. This accuracy would significantly constrain the mass of heavy Higgs boson if the first-phase collider can not find a Higgs boson. In addition, a new physics signal can be ambiguously measured in the initial states of and for hardly interacting in the standard model. For instance, the process of can be observed with no SM background. 
T.Hirose described the R&D program and the present status of polarized positrons.  The two methods have been considered as follows. The first is to use the decay of an unstable nucleus, that is , where is made in the process of by protons bombarding an aluminum plate. The present setup is based on a commercial proton cyclotron, CYPRIS, of Sumitomo Heavy Industries Ltd., which accelerates protons up to 18MeV with a beam intensity of 1A. The positrons from the decay must be monochromized energetically in a moderator having a 25m tungsten plate, and are then accelerated. The present intensity of positrons is expected to be /sec for a polarization of more than 80% with a 10 efficiency() of the moderator. The first R&D milestone is to measure the actual intensity and polarization of positrons. In order to fulfill the requirement of 10/sec for the JLC-I, 40MeV proton beams of 100mA are needed in addition to a dramatic improvement in the efficiency of the moderator ().
The second method is to use pair creations by polarized photons which are back-scattered from Compton scattering events between circularly polarized laser light and high-energy electrons. A test experiment is being conducted using 1.5 GeV beams of the ATF-linac at KEK. A beam of 210/bunch collides with a 2.33eV YAG-laser beam of 0.55J/pulse to produce polarized photons(<80MeV) of 4.410/bunch. Polarized positrons (>80%) are expected to be created with an intensity of 1.410/bunch from a 3mm tungsten target hit by the photons.  For JLC-I, a high-intensity 5-GeV linac of more than 10/bunch is needed together with a multi-converter system comprizing 100(number of bunches) CO lasers of 5J/500ps which are operational at 150Hz and are driven by a Nd:YAG laser so as to precisely control their timings. Different methods have recently been proposed to produce high-energy polarized photons emitted in undulators at 100200 GeV linac and at a relativistic ion-storage ring for a more efficient source of polarized positrons. 
Although many R&D and breakthroughs are necessary to realize a polarized positron beam, it is worth pursuing.