EPJ Quantum Technology最新文献

The nanoscale fabrication of μm-spaced single-photon emitter arrays is crucial for the development of integrated photonic chips. We report on the fabrication and systematic characterization of germanium-vacancy (GeV) color centers arrays in diamond obtained upon ion implantation at the nanoscale. Ge²⁺ ion implantations at 35 keV and 70 keV energies were carried out using a focused ion beam (FIB) equipped with a liquid metal alloy ion source. The arrays of emitters are subsequently aligned to ø300 nm nanopillar waveguiding structures, fabricated using a combination of electron-beam lithography and plasma etching. The photon collection efficiency and photoluminescence (PL) signal-to-background ratio increased by a factor 8 with respect to the unstructured sample. The photophysical properties of the GeV emitters fabricated by this approach were unaltered with respect to those found in unprocessed diamond. The efficiency of the overall manufacturing process to fabricate individual GeV centers was assessed. Up to 33% of the fabricated nanopillars, depending on ion implantation parameters, were found to contain single emitters.

High relative velocities and large distances in space-based quantum communication with satellites in lower earth orbits can lead to significant Doppler shifts and delays of the signal impairing the achievable performance if uncorrected. We analyze the influence of systematic and stochastic Doppler shift and delay in the specific case of a continuous variable quantum key distribution (CV-QKD) protocol and identify the generalized correlation function, the ambiguity function, as a decisive measure of performance loss. Investigating the generalized correlations as well as private capacity bounds for specific choices of spectral amplitude shape (Gaussian, single- and double-sided Lorentzian), we find that this choice has a significant impact on the robustness of the quantum communication protocol to spectral and temporal synchronization errors. We conclude that optimizing the pulse shape can be a building block in the resilient design of quantum network infrastructure.

As the demand for computing power continues to rise, it is difficult for a single type of computing device or architecture to satisfy the current situation. Diversity and heterogeneity are becoming more and more popular. Seamlessly integrating the realms of high performance computing and quantum computing, and harnessing their collective potential, has emerged as a consensus approach to effectively address the pressing need for increased computing power. In the emerging computing scenario, various different types of computing devices have super-heterogeneous characteristics, and there are significant differences in computational principles, programming models, parallelism, etc. Effectively harnessing these disparate resources and achieving a unified programming paradigm have become urgent imperatives. To address the above problems, this paper introduces QCCP, a taskflow programming model that enables efficient collaborative computing between classical computers and quantum computers. QCCP establishes a unified programming abstraction, shields the super-heterogeneous characteristics of the underlying network and hardware, and supports flexible scheduling for different computational backends. The experimental results indicate that QCCP can support the processing of hybrid classical-quantum applications with diverse program structures. In particular, QCCP reveals its immense potential and superiority in tackling real-world challenges, specifically in the realm of quantum circuit cutting and reconstruction.

The field of Quantum Information Science and Technology (QIST) education presents unique challenges for both students and educators, such as the necessity of understanding abstract properties of quantum systems. To provide a more intuitive understanding of quantum systems, a multitude of qubit representations have been developed in recent years. Given the diversity of the field, a specific representation may be more suitable in one content area of than in another. Consequently, the choice of representation may vary considerably depending on the course orientation. However, no exhaustive analysis has been conducted into the differences between the representation of single- and multi-qubit systems in higher education QIST courses. Furthermore, the factors which influence the selection of a suitable representation remain open. To close this gap, we conducted an online survey with 25 educators at different German and Austrian universities on their use of representations in QIST-related courses. The results confirm the pivotal role of mathematical formalism in QIST education regardless of the specific course characteristics but also reveal an untapped potential for enhancing student learning through the intentional and comprehensive use of multiple external representations (MERs), especially in the case of multi-qubit systems. The findings are discussed within the context of the field of QIST and current insights into learning with MERs.

The violation of a Bell inequality implies the existence of nonlocality, making device-independent randomness certification possible. This paper derives a tight upper bound for the maximal quantum violation of Gisin’s elegant Bell inequality (EBI) for arbitrary two-qubit states, along with the constraints required to achieve this bound. This method provides the necessary and sufficient conditions for violating the EBI for several quantum states, including pure two-qubit states and the Werner states. The lower bound of certifiable global randomness is analyzed based on the tight upper bound of the EBI for pure two-qubit states, with a comparison to the Clauser-Horne-Shimony-Holt (CHSH) inequality. The relationship between the noise level and the lower bound of certifiable global randomness with respect to the Werner states is also explored, and the comparisons with both the CHSH inequality and the chained inequality are given. The results indicate that when the state approaches a maximally entangled state within specific quantified ranges, the EBI demonstrates advantages over both the CHSH inequality and the chained inequality, providing theoretical guidance for experimental device-independent quantum random number generation.

Characterization and exploitation of multiple channels between the transmitter and the receiver in multiple-input multiple-output (MIMO) communications brought a paradigm shift in classical communication systems. The techniques developed around MIMO communication systems not only brought unprecedented advancements in communication rates but also substantially improved the reliability of communication, measured by low error rates. We develop a framework for MIMO quantum communications with discrete-variable quantum systems. We propose a general model of MIMO quantum channels incorporating noise, losses, and crosstalk among multiple channels. We leverage the approximate quantum cloning to transmit imperfect clones of the input state over this channel setup. We demonstrate that transmitting multiple imperfect clones achieves better communication fidelity as compared to transmitting a single perfect copy of the state due to the diversity of the MIMO setup. We also demonstrate a practical tradeoff between fidelity and rate of communication and call it quantum diversity multiplexing tradeoff (DMT) due to its similarity with the well-known DMT in classical MIMO setups.

In quantum cryptography, secret communications are delivered through a quantum channel. One of the most important breakthroughs in quantum cryptography has been the quantum key distribution (QKD). This process enables two distant parties to share secure communications based on physical laws. However, eavesdroppers can still interrupt the communication. To overcome this, we propose a different way to detect the presence of Eve through the polynomial interpolation technique. This technique also allows us for key verification. This approach prevents the receiver as well as the intruder from discovering the sender’s fundamental basis. To fully utilize IBM quantum computers’ quantum computing capabilities, this paper attempts to show % error against alpha (strength of eavesdropping) and the impact of noise on the success probability of the desired key bits. Furthermore, the success probability under depolarizing noise is explained for different qubit counts. In the enhanced QKD protocol, using polynomial interpolation for reconciliation shows a 50% probability of successful key generation. This is even when the noise is increased to the maximum capacity.

Routing design is an important aspect in aiding the completion of the Quantum Processing Unit (QPU) layout design for large-scale superconducting quantum processors. One of the research focuses is how to generate reliable routing schemes within a short time. In this study, we propose a superconducting quantum processor auto-routing method for supporting scalable architecture, which is mainly implemented through the bidirectional A star algorithm, the backtracking algorithm, and the greedy strategy. By using this method, the number of crossovers and corners can be reduced while efficiently completing the processor routing. To verify the effectiveness of our method, we selected 5 types of qubit numbers for processor routing experiments. The experimental results show that compared to the improved A star algorithm of Qiskit Metal, our method reduces the average execution time by at least 43.61% and 41.68% in serial and parallel, respectively. Compared with four other routing algorithms, our method has a minimum average reduction of 10.63% and 1.21% in the number of crossovers and corners, respectively. In addition, our method supports the processor routing design of planar and flip-chip architectures, and can automatically process both airbridge and insulation types of crossovers. Therefore, our method can provide efficient and reliable automated routing design to assist the development of large-scale superconducting quantum processors.

In previous research, it has been argued that many of the student (mis-)conceptions of quantum concepts described in the literature as widespread among learners can be traced back to poorly developed (quantum) model perceptions that hinder the learning of quantum physics. In particular, it has been shown that the degrees of two cognitive dimensions, namely Functional Fidelity and Fidelity of Gestalt, in students’ thinking account for a substantial amount of the variance in students’ model perceptions in quantum physics and may therefore be useful for describing and understanding the (development of) students’ conceptions of quantum physics topics. So far, however, the cognitive dimensions Functional Fidelity and Fidelity of Gestalt have only been investigated in exploratory studies. In this article, we report the results of a confirmatory factor analysis of data collected from N = 179 secondary school students using an instrument adapted from the literature to assess learners’ perceptions of the photon model. The results of our study provide empirical evidence that the two-factor model of learners’ model perceptions in the quantum context is indeed a good fit to the data. Together with literature from science education research on students’ conceptual development, and taking into account earlier findings on Fidelity of Function and Gestalt Fidelity we derive a plausible description of students’ conceptual development in the context of quantum physics – leading to what we call the Fidelities-Model of Conceptual Development. We discuss this framework in the light of previous research and argue for its potential generalisability beyond the teaching and learning of quantum physics topics. The implications of our findings for both science education research and practice are presented.