Quadratic Quantum Measurements
  We develop a theory of quadratic quantum measurements by a mesoscopic detector. It is shown that quadratic measurements should have non-trivial quantum information properties, providing, for instance, a simple way of entangling two non-interacting qubits. We also calculate output spectrum of a quantum detector with both linear and quadratic response continuously monitoring coherent oscillations in two qubits. 
Wenjin Mao, Dmitri V. Averin, Rusko Ruskov and Alexander N. Korotkov