Quantum Wheatstone bridge

The quantum mechanical effects of entanglement and interference are the drivers of modern quantum technologies. Combining these with the unidirectional dynamics of cold thermal baths yield additional possibilities in areas such as error correction, heat control, and metrology. One example of such a device is a quantum version of the classical Wheatstone bridge. The quantum Wheatstone bridge is comprised of four spins coupled to thermal baths at the boundaries and exploits quantum effects to gain an enhanced sensitivity to an unknown coupling strength. The four spins are arranged in a double slit formation allowing an entangled bell state to develop on the interface. A controllable coupling is varied until the bell state is destroyed due to destructive interference which is called the balance point. At the balance point the unknown coupling is the same as the controllable coupling which is robust towards calibration errors and decoherence. Alternatively, the balance point can be found by maximizing the spin transport between the two baths. This makes for a device for measuring an unknown coupling working through quantum effects and driven by the non-unitarity of the thermal baths.