Test for nonreciprocal circular birefringence in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin films as evidence for broken time-reversal symmetry

We have measured the amount of nonreciprocal circular birefringence of 50 to 800 Å ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ films in transmission with a 15-μm beam diameter. A novel instrument with a sensitivity of 2 μrad for nonreciprocal phase shifts was developed by modifying a fiber-optic gyroscope. It is insensitive to reciprocal phase shifts. We observed no nonreciprocal phase shifts in any samples.