Vacuum tunneling probe: a nonreciprocal, reduced-back-action transducer

The vacuum tunneling probe used in the scanning tunneling microscope is shown to represent a new class of nonreciprocal reduced-back-action transducers. It is demonstrated that nonreciprocity allows the amplifier limit obeyed by conventional receivers to be greatly exceeded. The present analysis indicates that the quantum limit is enforced by the momentum shot noise associated with the tunneling current. It is suggested that if the difficulty of mechanically impedance matching a tunneling-probe transducer to a massive gravitational-wave antenna could be overcome, the transducer may offer a means of reaching the quantum limit for gravitational-wave detectors. (R.R.)