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Quantum Process Tomography of a Controlled-Phase Gate for Time-Bin Qubits
摘要： Time-bin qubits, where information is encoded in a single photon at different times, have been widely used in optical-fiber- and waveguide-based quantum communications. With the recent developments in distributed quantum computation, it is logical to ask whether time-bin encoded qubits may be useful in that context. We have recently realized a time-bin qubit controlled-phase (C-phase) gate using a 2 × 2 optical switch based on a lithium-niobate waveguide, with which we demonstrated the generation of an entangled state. However, the experiment was performed with only a pair of input states and thus the functionality of the C-phase gate was not fully verified. In this research, we use quantum process tomography to establish a process fidelity of 97.1%. Furthermore, we demonstrate the controlled-NOT gate operation with a process fidelity greater than 94%. This study confirms that typical two-qubit logic gates used in quantum computational circuits can be implemented with time-bin qubits and thus it is a significant step forward for the realization of distributed quantum computation based on time-bin qubits.
关键词： quantum computation,time-bin qubits,controlled-phase gate,controlled-NOT gate,quantum process tomography
更新于2025-09-16 10:30:52
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Selection of spectrum model in estimation of Brillouin frequency shift for distributed optical fiber sensor
摘要： To improve real-time performance in distributed optical ?ber sensor, the in?uence of Brillouin spectrum models on performance in estimation of Brillouin frequency shift (BFS) is systematically investigated. First, the related Brillouin spectrum models, objective functions, optimization algorithms and convergence criterion in estimation of BFS are introduced. The BFS estimation algorithms based on the Lorentzian, Gaussian, pseudo-Voigt and Voigt models are written in MATLAB. The algorithms are used to estimate BFS for numerically generated Brillouin spectra with di?erent values of SNR (Signal to Noise Ratio) and measured Brillouin spectra with di?erent values of pulse width and average time. The results reveal that regardless of the simpli?ed Lorentzian and Gaussian models or more accurate pseudo-Voigt and Voigt model-based algorithms used, the estimated BFS between di?erent algorithms is quite similar. However, the ?tting error of the simpli?ed model-based algorithms is larger than that of the more accurate model-based algorithms especially for the cases with high value of SNR. The computational burden of the former is less than that of the latter. The Voigt model-based algorithm requires the most computational e?ort. If the nonlinear least-square ?t method is used, the selection of spectrum model has no big in?uence on the accuracy in the estimated BFS. Therefore, the algorithm based on simpli?ed Lorentzian and Gaussian models should be selected in BFS estimation. Thus, not only high accuracy in BFS estimation is ensured, but also the computation time can, at best, decrease to one-tenth of the maximum value.
关键词： Spectrum model,Accuracy,Computation time,Brillouin frequency shift,Distributed optical ?ber sensing
更新于2025-09-12 10:27:22