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arXiv:0912.0881

Play building blocks on population distribution of multilevel superconducting flux qubit with quantum interference

Xueda Wen, Yiwen Wang, Yu, Yang

Published Dec 4, 2009• 1 citation
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Abstract

Recent experiments on Landau-Zener interference in multilevel superconducting flux qubits revealed various interesting characteristics, which have been studied theoretically in our recent work by simply using rate equation method [PRB 79, 094529, (2009)]. In this note we extend this method to the same system but with larger driving amplitude and higher driving frequency. The results show various anomalous characteristics, some of which have been observed in a recent work.

DOI
10.48550/arxiv.0912.0881

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Recent experiments on Landau-Zener interference in multilevel superconducting flux qubits revealed various interesting characteristics, which have been studied theoretically in our recent work by simply using rate equation method [PRB 79, 094529, (2009)]. In this note we extend this method to the same system but with larger driving amplitude and higher driving frequency. The results show various anomalous characteristics, some of which have been observed in a recent work.
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We proposed an analytical model to analyze the Landau-Zener interference in a multilevel superconducting flux qubit driven by large-amplitude external fields. Our analytical results agree remarkably well with those of Berns et al. [Nature (London) 455, 51 (2008)]. Moreover, we studied the effect of driving frequency and dephasing rate on the interference. The dephasing generally destroys the interference, while increasing frequency rebuilds the interference at large dephasing rate. At a certain driving frequency and dephasing rate, the interference shows some anomalous features, as observed in recent experiments.
We irradiated an rf superconducting quantum interference device qubit with large-amplitude and high-frequency electromagnetic field. Population transitions between macroscopic distinct quantum states due to multiple Landau-Zener transitions at energy-level avoided crossings were observed. The qubit population on the excited states as a function of flux detuning and microwave power exhibits interference patterns. Some features are found in the interference and a model based on rate equations can well address these features.
Landau–Zener–Stückelberg (LZS) interference has drawn renewed attention to quantum information processing research because it is not only an effective tool for characterizing two-level quantum systems but also a powerful approach to manipulate quantum states. Superconducting quantum circuits, due to their versatile tunability and degrees of control, are ideal platforms for studying LZS interference phenomena. We use a superconducting Xmon qubit to study LZS interference by parametrically modulating the qubit transition frequency nonlinearly. For dc flux biasing of the qubit slightly far away from the optimal flux point, the qubit excited state population shows an interference pattern that is very similar to the standard LZS interference in linear regime, except that all bands shift towards lower frequencies when increasing the rf modulation amplitude. For dc flux biasing close to the optimal flux point, the negative sidebands and the positive sidebands behave differently, resulting in an asymmetric interference pattern. The experimental results are also in good agreement with our analytical and numerical simulations.
Microwave resonances between discrete macroscopically distinct quantum states with single photon and multiphoton absorption are observed in a strongly driven radio frequency superconducting quantum interference device flux qubit. The amplitude of the resonant peaks and dips are modulated by the power of the applied microwave irradiation and a population inversion is generated at low flux bias. These results, which can be addressed with Landau–Zener transition, are useful to develop an alternative means to initialize and manipulate the flux qubit, as well as to do a controllable population inversion used in a micromaser.
We distinguish different mechanisms for population inversion in flux qubits driven by dc+ac magnetic fields. We show that for driving amplitudes such that there are Landau-Zener-Stückelberg interferences, it is possible to have population inversion solely mediated by the environmental bath. Furthermore, we find that the degree of population inversion can be controlled by tailoring a resonant frequency Ω(p) in the environmental bath. To observe these effects experiments should be performed for long driving times after full relaxation.