  We propose a quantum Wheatstone bridge as a fully quantum analogue to the
classical version. The bridge is a few-body boundary-driven spin chain
exploiting quantum effects to gain an enhanced sensitivity to an unknown
coupling. The sensitivity is explained by a drop in population of an entangled
Bell state due to destructive interference as the controllable coupling
approaches the unknown coupling. A simple criteria for the destructive
interference is found, and an approximate expression for the width of the drop
is derived. The sensitivity to the unknown coupling is quantified using the
quantum Fisher information, and we show that the state of the bridge can be
measured indirectly through the spin current. Our results are robust towards
calibration errors and generic in the sense that several of the current
state-of-the-art quantum platforms could be used as a means of realization. The
quantum Wheatstone bridge may thus find use in fields such as sensing and
metrology using near-term quantum devices.
