Ancilla-assisted quantum state tomography in multiqubit registers

摘要

The standard method of quantum state tomography (QST) relies on the measurement of a set of noncommutingnobservables, realized in a series of independent experiments. Ancilla-assisted QST (AAQST) proposed bynNieuwenhuizen and co-workers [Phys. Rev. Lett. 92, 120402 (2004)] greatly reduces the number of independentnmeasurements by exploiting an ancilla register in a known initial state. In suitable conditions AAQST allowsnmapping out density matrix of an input register in a single experiment. Here we describe methods for explicitnconstruction of AAQST experiments in multiqubit registers. We also report nuclear magnetic resonance studiesnon AAQST of (i) a two-qubit input register using a one-qubit ancilla in an isotropic liquid-state system andn(ii) a three-qubit input register using a two-qubit ancilla register in a partially oriented system. The experimentalnresults confirm the effectiveness of AAQST in such multiqubit registers.