Experimental Comparison of Efficient Tomography Schemes for a Six-Qubit   State
  Quantum state tomography suffers from the measurement effort increasing exponentially with the number of qubits. Here, we demonstrate permutationally invariant tomography for which, contrary to conventional tomography, all resources scale polynomially with the number of qubits both in terms of the measurement effort as well as the computational power needed to process and store the recorded data. We demonstrate the benefits of combining permutationally invariant tomography with compressed sensing by studying the influence of the pump power on the noise present in a six-qubit symmetric Dicke state, a case where full tomography is possible only for very high pump powers. 
Christian Schwemmer, Geza Toth, Alexander Niggebaum, Tobias Moroder,   David Gross, Otfried G\"uhne, Harald Weinfurter
quant-ph