Average fidelity in n-qubit systems

Meng Qin, Xu Sheng-Long, Xiao-Yue Zhai

In this article, we derive the explicit entanglement and fidelity expressions for a larger class of two-qubit states, namely, a seven-parameter family of so called X-states. The analytical expressions of the entanglement, the output entanglement and the average fidelity are obtained for this general model by using the concept of concurrence and average fidelity. We study the relations between quantum entanglement, the output entanglement, and the average fidelity for standard teleportation of one-qubit and partial teleportation of two-qubit state. We discover that the average fidelity and entanglement is not a simple dependence relation. The higher entangled system is helpful for teleportation.

M. D. Bowdrey, Daniel K. L. Oi, Anthony J. Short +2 more

Rosario Fazio, G. Massimo Palma, Emilio Sciacca +1 more

Yangsen Ye, Sirui Cao, Yulin Wu +20 more

High-fidelity two-qubit gates are essential for the realization of large-scale quantum computation and simulation. Tunable coupler design is used to reduce the problem of parasitic coupling and frequency crowding in many-qubit systems and thus thought to be advantageous. Here we design an extensible 5-qubit system in which center transmon qubit can couple to every four near-neighboring qubits via a capacitive tunable coupler and experimentally demonstrate high-fidelity controlled-phase (CZ) gate by manipulating central qubit and one near-neighboring qubit. Speckle purity benchmarking and cross entropy benchmarking are used to assess the purity fidelity and the fidelity of the CZ gate. The average purity fidelity of the CZ gate is 99.69±0.04% and the average fidelity of the CZ gate is 99.65±0.04%, which means that the control error is about 0.04%. Our work is helpful for resolving many challenges in implementation of large-scale quantum systems.

Shivang Pandey, Debabrata Goswami

Ngwa Engelbert Afuoti, Lukong Fai, Nguenang Pierre +3 more

We revisit the teleportation of two qubits entanglement using a single channel made of a single Majorana wire connecting two pairs of qubits each at the sending and receiving ends of the wire. The scheme is different from that use in our early work (Phys. Scr. 95 035803 (2020)) where the two qubit teleportation required two copies of the Majorana wires setup as required to implement the Lee and Kim teleportation scheme. The two pairs of qubit interact via the exchange coupling. It is in this setting that we investigate the role of the exchange coupling, the channel parameters, the spin chain parameters on the average fidelity of entanglement teleportation. We equally consider the fact that the geographically located two pairs of qubits interact differently with their respective spin chain environment. The dynamics of the average fidelity is shown to be enhanced when using a set-up with symmetric coupling between qubits and Majorana wire at both ends as compared to when using asymmetric coupling. Small values of the coupling between the qubits and spin environments should however be used. Large values of the exchange coupling between qubits in the two sub-systems should be used to have high fidelity of the teleported state. The dynamics of the average fidelity of entanglement teleportation is seen to depend on the size of the spin environment as compared to the teleportation protocol used in our earlier work where the dynamics is mildly sensitive to the size of the spin environment. Increasing the strength of the transverse magnetic field and large values of the anisotropy degree enhances the average fidelity of entanglement teleportation. The same observation is seen when considering the same values of the coupling between the two qubits sub-systems at the opposite ends of the Majorana wire and their respective spin environment. The dynamics of the system is however seen to observe almost a similar trend as that of the above cited work for the single Majorana wire setup.