The mass and resonance frequencies of the tuning forks used are listed in Table I. To reflect the laser beam efficiently, a mirror has to be attached to the end of the tuning forks. The mirror must be light because otherwise it could modify the resonance frequency of the tuning forks. We chose a 10 mm10 mm0.5 mm piece of a Si111 wafer. 6 Because a wafer of 75 mm in diameter has a mass of 3 g, the mass of each small mirror can be estimated to be 0.0660.004 g.