Within the unified framework of exploiting the relative entropy as a distance measure of quantum correlations, we make explicit the hierarchical structure of quantum coherence, quantum discord and quantum entanglement in multipartite systems. On this basis, we introduce a new measure of quantum coherence, the basis-free quantum coherence and prove that this quantity is exactly equivalent to quantum discord. Furthermore, since the original relative entropy of coherence is a basis-dependent quantity, we investigate the local and nonlocal unitary creation of quantum coherence, focusing on the two-qubit unitary gates. Intriguingly, our results demonstrate that nonlocal unitary gates do not necessarily outperform the local unitary gates. Finally, the additivity relationship of quantum coherence in tripartite systems is discussed in detail, where the strong subadditivity of von Neumann entropy plays an essential role.

We investigate the performance of quantum fisher information under the Unruh-Hawking effect, where one of the observers (eg, Rob) is uniformly accelerated with respect to other partners. In the context of relativistic quantum information theory, we demonstrate that quantum fisher information, as an important measure of the information content of quantum states, has a rich and subtle physical structure comparing with entanglement or Bell nonlocality. In this work, we mainly focus on the parameterized (and arbitrary) pure two-qubit states, where the weight parameter θ and phase parameter φ are naturally introduced. Intriguingly, we prove that F θ keeps unchanged for both scalar and Dirac fields. Meanwhile, we observe that F φ decreases with the increase of acceleration r but remains finite in the limit of infinite acceleration. More importantly, our results show that the symmetry of F φ (with respect to θ = π/4) has been broken by the influence of Unruh effect for both cases. PACS numbers: 03.67.-a,06.20.-f,04.62.+v I. INTRODUCTIONQuite recently, quantum fisher information (QFI) [1,2] has attracted considerable attention, not only due to its own significance in quantum estimation theory and quantum information theory, but also associated with recent rapid progress in quantum-enhanced metrology [3][4][5]. In fact, as an important measure of information content of quantum states, QFI has already played a critical role in quantum statistical inference through its inextricable relationship with Cramér-Rao inequality [6,7]. Moreover, QFI also has various applications in other quantum information tasks such as characterization of non-Markovianity [8], investigation of uncertainy relations [9-13] and entanglement detection [14], just to name a few. Besides, since every realistic system will inevitably suffer from the decoherence induced by the coupling to its surroundings, it is natural to explore the dynamics of QFI in all sorts of quantum noise channels [15,16], as we do when we analyse quantum entanglement, quantum discord and Bell nonlocality. A great deal of research has been devoted to this perspective of QFI, which is well motivated within the framework of noisy quantum metrology [17,18].On the other hand, the combination of quantum information science and relativity theory leads us to a deeper interpretation of quantum mechanics [19,20] and opens up a new way to understand the information paradox when black holes are involved [21][22][23][24]. In particular, what the community care about is how the Unruh-Hawking effect affects the "information contents" (or more especially, correlation measures) in quantum states. Therefore, diverse efforts have been made to investigate * Electronic address: xgwang@zimp.zju.edu.cn † Electronic address: cpsun@csrc.ac.cn

A high-speed steady-state visual evoked potentials (SSVEP)-based brain-computer interface (BCI) system using dry EEG electrodes was demonstrated in this study. The dry electrode was fabricated in our laboratory. It was designed as claw-like structure with a diameter of 14 mm, featuring 8 small fingers of 6 mm length and 2 mm diameter. The structure and elasticity can help the fingers pass through the hair and contact the scalp when the electrode is placed on head. The electrode was capable of recording spontaneous EEG and evoked brain activities such as SSVEP with high signal-to-noise ratio. This study implemented a twelve-class SSVEP-based BCI system with eight electrodes embedded in a headband. Subjects also completed a comfort level questionnaire with the dry electrodes. Using a preprocessing algorithm of filter bank analysis (FBA) and a classification algorithm based on task-related component analysis (TRCA), the average classification accuracy of eleven participants was 93.2% using 1-second-long SSVEPs, leading to an average information transfer rate (ITR) of 92.35 bits/min. All subjects did not report obvious discomfort with the dry electrodes. This result represented the highest communication speed in the dry-electrode based BCI systems. The proposed system could provide a comfortable user experience and a stable control method for developing practical BCIs.