State tomography via weak measurements

TL;DR

This paper introduces efficient quantum state tomography methods using weak measurements that can directly access all elements of a quantum state's density matrix, applicable to both pure and mixed states.

Contribution

The authors develop new tomography schemes utilizing weak measurements that do not discard data and can directly measure any matrix element of a quantum state.

Findings

Schemes work for pure and mixed states

All density matrix elements can be directly measured

No data is discarded in the process

Abstract

Recent work has revealed that the wave function of a pure state can be measured directly and that complementary knowledge of a quantum system can be obtained simultaneously by weak measurements. However, the original scheme applies only to pure states, and it is not efficient because most of the data are discarded by post-selection. Here, we propose tomography schemes for pure states and for mixed states via weak measurements, and our schemes are more efficient because we do not discard any data. Furthermore, we demonstrate that any matrix element of a general state can be directly read from an appropriate weak measurement. The density matrix (with all of its elements) represents all that is directly accessible from a general measurement.