Test for nonreciprocal circular birefringence in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

A fiber-optic Sagnac interferometer has been used to search for broken time-reversal symmetry in single crystals of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. The measurements reveal no nonreciprocal circular birefringence to a sensitivity of 10 μrad. Data were taken in transmission at 1060 and 670 nm, from 20 to 300 K, in an applied field of up to 200 Oe.