Beijing Institute of Mathematical Sciences and Applications

Yu Wang

Tianfeng Feng

Thursday, September 7, 2023 8:00 PM - 9:30 PM

A3-2-301

A direct measurement protocol allows reconstructing specific elements of the density matrix of a quantum state without using quantum state tomography. However, the direct measurement protocols to date are primarily based on weak or strong measurements with an ancillary pointer, which interacts with the investigated system to extract information about the specified elements. In this talk, I will review previous protocol of direct measurement and introduce a direct measurement scheme based on phase-shifting operations which do not need ancillary pointers. In this method, estimates of at most six expectation values of projective observables suffice to determine any specific element of an unknown quantum density matrix. A concrete quantum circuit to implement this direct measurement protocol for multiqubit states is provided, which is composed of just single-qubit gates and two multiqubit controlled-phase gates. This scheme is also extended for the direct measurement of the density matrix of continuous-variable quantum states. It is shown that the method can be used in quantum information applications where only partial information about the quantum state needs to be extracted, for example, problems such as entanglement witnessing, fidelity estimation of quantum systems, and quantum coherence estimation.

Tianfeng Feng is a postdoctoral fellow at the University of Hong Kong. He focuses on theories and experiments related to quantum information processing and quantum foundations, and has published more than 10 papers in high-impact journals, including Phys. Rev. D, npj Quantum Information, Photonics Research, Phys. Rev. A, etc.