Browsing by Author "Zeng, Junkai"

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    Collection Management Tweets Project Fall 2017
    Khaghani, Farnaz; Zeng, Junkai; Bhuiyan, Momen; Tabassum, Anika; Bandyopadhyay, Payel (Virginia Tech, 2018-01-17)
    The report included in this submission documents the work by the Collection Management Tweets (CMT) team, which is a part of the bigger effort in CS5604 on building a state-of-the-art information retrieval and analysis system for the IDEAL (Integrated Digital Event Archiving and Library) and GETAR (Global Event and Trend Archive Research) projects. The mission of the CMT team had two parts: 1) Cleaning 6.2 million tweets from two 2017 event collections named "Solar Eclipse" and "Las Vegas Shooting", and loading them into HBase, an open source, non-relational, distributed database that runs on the Hadoop distributed file system, in support of further use; and 2) Building and storing a social network for the tweet data using a triple-store. For the first part, our work included: A) Making use of the work done by the previous year's class group, where incremental update was done, to introduce a faster development process of data collection and storing; B) Improving the performance of work done by the group from last year. Previously, the cleaning part, e.g., removing profanity words, plus extracting hashtags and mentions, utilized Python. This becomes very slow when the dataset scales up. We introduced parallelization in our tweet cleaning process with the help of Scala and the Hadoop cluster, and made use of different Natural Language Processing libraries for stop word and profanity removal; C) Along with tweet cleaning we also identified and stored Named-Entity-Recognition (NER) entries and Part-of-speech (POS) tags, with the tweets which was not done by the previous team. The cleaned data in HBase from this task is provided to the Classification team for spam detection and to the Clustering and Topic Analysis team for topic analysis. Collection Management Webpage team uses the extracted URLs from the tweets for further processing. Finally, after the data is indexed by the SOLR team, the Front-End team visualizes the tweets to users, and provides access for searching and browsing. In addition to the aforementioned tasks, our responsibilities also included building a network of tweets. This entailed doing research into the types of database that are appropriate for this graph. For storing the network, we used a triple-store database to record different types of edges and relationships in the graph. We also researched methods ascribing importance to nodes and edges in our social networks once they were constructed, and analyzed our networks using these techniques.
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    Designing arbitrary single-axis rotations robust against perpendicular time-dependent noise
    Li, Bikun; Calderon-Vargas, Fernando A.; Zeng, Junkai; Barnes, Edwin Fleming (2021-09)
    Low-frequency time-dependent noise is one of the main obstacles on the road toward a fully scalable quantum computer. The majority of solid-state qubit platforms, from superconducting circuits to spins in semiconductors, are greatly affected by 1/f noise. Among the different control techniques used to counteract noise effects on the system, dynamical decoupling sequences are one of the most effective. However, most dynamical decoupling sequences require unbounded and instantaneous pulses, which are unphysical and can only implement identity operations. Among methods that do restrict to bounded control fields, there remains a need for protocols that implement arbitrary gates with lab-ready control fields. In this work, we introduce a protocol to design bounded and continuous control fields that implement arbitrary single-axis rotations while shielding the system from low-frequency time-dependent noise perpendicular to the control axis. We show the versatility of our method by presenting a set of non-negative-only control pulses that are immediately applicable to quantum systems with constrained control, such as singlet-triplet spin qubits. Finally, we demonstrate the robustness of our control pulses against classical 1/f noise and noise modeled with a random quantum bath, showing that our pulses can even outperform ideal dynamical decoupling sequences.
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    Dynamically Corrected Quantum Control: A Geometrical Framework
    Zeng, Junkai (Virginia Tech, 2019-10-22)
    Implementing high-fidelity quantum control and suppressing the unwanted environmental noise has been one of the essential challenges in developing quantum information technologies. In the past, driving pulse sequences based on Dirac delta functions or square wave functions, such as Hahn spin echo or CPMG, have been developed to dynamically correcting the noise effects. However, implementing these ideal pulses with high fidelity is a challenging task in real experiments. In this thesis, we provide a new and simple method to explore the entire solution space of driving pulse shapes that suppress environmental noise in the evolution of the system. In this method, any single-qubit phase gate that is first-order robust against quasi-static transversal noise corresponds to a closed curve on a two-dimensional plane, and more general first-order robust single-qubit gates correspond to closed three-dimensional space curves. Second-order robust gates correspond to closed curves having the property that their projection onto any two-dimensional planes shall enclose a zero net area. The driving pulse shapes that implement the gates can be determined by the curvature, torsion, and the length of the curve. By utilizing the framework it is possible to obtain globally optimal solutions in pulse shaping in respect of experimental constraints by mapping them into geometrical optimization problems. One such problem we solved is to prove that the fastest possible single-qubit phase gates that are second-order noise-resistant shall be implemented using sign-flipping square functions. Since square waves are not experimentally feasible, we provide a method to smooth these pulses with minimal loss in gate speed while maintaining the robustness, based on the geometrical framework. This framework can also be useful in diagnosing the noise-cancellation properties of pulse shapes generated from numerical methods such as GRAPE. We show that this method for pulse shaping can significantly improve the fidelity of single-qubit gates through numerical simulation.
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    General solution to inhomogeneous dephasing and smooth pulse dynamical decoupling
    Zeng, Junkai; Deng, Xiu-Hao; Russo, Antonio; Barnes, Edwin Fleming (Institute of Physics, 2018-03-26)
    In order to achieve the high-fidelity quantum control needed for a broad range of quantum information technologies, reducing the effects of noise and system inhomogeneities is an essential task. It is well known that a system can be decoupled from noise or made insensitive to inhomogeneous dephasing dynamically by using carefully designed pulse sequences based on square or delta-function waveforms such as Hahn spin echo or CPMG. However, such ideal pulses are often challenging to implement experimentally with high fidelity. Here, we uncover a new geometrical framework for visualizing all possible driving fields, which enables one to generate an unlimited number of smooth, experimentally feasible pulses that perform dynamical decoupling or dynamically corrected gates to arbitrarily high order.Wedemonstrate that this scheme can significantly enhance the fidelity of singlequbit operations in the presence of noise and when realistic limitations on pulse rise times and amplitudes are taken into account.
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    Noise-resistant Landau-Zener sweeps from geometrical curves
    Zhuang, Fei; Zeng, Junkai; Economou, Sophia E.; Barnes, Edwin Fleming (2022-02-02)
    Landau-Zener physics is often exploited to generate quantum logic gates and to perform state initialization and readout. The quality of these operations can be degraded by noise fluctuations in the energy gap at the avoided crossing. We leverage a recently discovered correspondence between qubit evolution and space curves in three dimensions to design noise-robust Landau-Zener sweeps through an avoided crossing. In the case where the avoided crossing is purely noise-induced, we prove that operations based on monotonic sweeps cannot be robust to noise. Hence, we design families of phase gates based on non-monotonic drives that are error-robust up to second order. In the general case where there is an avoided crossing even in the absence of noise, we present a general technique for designing robust driving protocols that takes advantage of a relationship between the Landau-Zener problem and space curves of constant torsion.