Heisenberg limited measurements with superconducting circuits
  We describe an assembly of N superconducting qubits contained in a single-mode cavity. In the dispersive regime, the correlation between the cavity field and each qubit results in an effective interaction between qubits that can be used to dynamically generate maximally entangled states. With only collective manipulations, we show how to create maximally entangled quantum states and how to use these states to reach the Heisenberg limit in the determination of the qubit bias control parameter (gate charge for charge qubits, external magnetic flux for rf-SQUIDs). 
Alexandre Guillaume (1) and Jonathan P. Dowling (2 and 3) ((1) Jet   Propulsion Laboratory, California Institute of Technolo...