Direct characterization of arbitrary qubit systems without auxiliary states

Zhiyuan Wang, Zijing Zhang

Zhiyuan Wang, Zijing Zhang

Abstract Quantum process tomography is an effective method for measuring the quantum channel parameters in the field of quantum information. However, standard quantum process tomography (SQPT) cannot obtain arbitrarily specific process–matrix elements because it requires a global reconstruction algorithm. Direct quantum tomography can be used to obtain specific matrix elements of interest without the need for a global reconstruction algorithm. Therefore, direct process quantum tomography (DQPT) has attracted widespread attention. DQPT based on weak measurements has been recently proposed. However, additional auxiliary states and post‐selection processes are necessary in these schemes, which increases the complexity of practical experiments and reduces the utilization of measurement resources. In this study, a DQPT protocol without the assistance of pointer states is proposed, which reduces the complexity of practical experiments. Subsequently, the scheme is generalized to arbitrary qubit systems and experimentally perform the protocol on a nuclear magnetic resonance (NMR) system provided by a quantum cloud platform. Finally, considering that when the qubit resources are insufficient, a superimposed coherent state is used as the pointer state to complete the direct characterization of a quantum process. The measurement protocol is easily scalable and integrated, laying the foundation for direct tomography on a chip.

Hong‐Fu Wang, Zhang Shou

This paper proposes a method to measure directly the concurrence of an arbitrary two-qubit pure state based on a generalized Grover quantum iteration algorithm and a phase estimation algorithm. The concurrence can be calculated by applying quantum algorithms to two available copies of the bipartite system, and a final measurement on the auxiliary working qubits gives a better estimation of the concurrence. This method opens new prospects of entanglement measure by the application of quantum algorithms. The implementation of the protocol would be an important step toward quantum information processing and more complex entanglement measure of the finite-dimensional quantum system with an arbitrary number of qubits.

Masahito Hayashi, Satoshi Ishizaka, Akinori Kawachi +2 more

K. Anjali, Akshata Shenoy H., H. S. Karthik +3 more

Zhaobing Fan, Zewen Shan, Haitao Ma

Coherence-assisted transformation under incoherent operations is discussed. For transformation from the pure state to the mixed state, we show that the coherence loss can be partially recovered by adding auxiliary coherent states. First, we discuss the coherence-assisted transformation for qubit states and give the sufficient and necessary condition for the partial recovery of coherence loss, and the maximum of the recovery of coherence loss is also studied in this case. Second, the maximally coherent state can be obtained in the above recovery scheme, so we give the full characterization of obtaining the maximally coherent state in a qubit system. Finally, we show that the coherence-assisted transformation for arbitrary finite-dimensional main coherent states and low-dimensional auxiliary coherent states is always possible, and the coherence loss also can be partially recovered in these cases.