Browsing by Author "Truger, Felix"

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Open Access
Configurable readout error mitigation in quantum workflows
(2022) Beisel, Martin; Barzen, Johanna; Leymann, Frank; Truger, Felix; Weder, Benjamin; Yussupov, Vladimir
Current quantum computers are still error-prone, with measurement errors being one of the factors limiting the scalability of quantum devices. To reduce their impact, a variety of readout error mitigation methods, mostly relying on classical post-processing, have been developed. However, the application of these methods is complicated by their heterogeneity and a lack of information regarding their functionality, configuration, and integration. To facilitate their use, we provide an overview of existing methods, and evaluate general and method-specific configuration options. Quantum applications comprise many classical pre- and post-processing tasks, including readout error mitigation. Automation can facilitate the execution of these often complex tasks, as their manual execution is time-consuming and error-prone. Workflow technology is a promising candidate for the orchestration of heterogeneous tasks, offering advantages such as reliability, robustness, and monitoring capabilities. In this paper, we present an approach to abstractly model quantum workflows comprising configurable readout error mitigation tasks. Based on the method configuration, these workflows can then be automatically refined into executable workflow models. To validate the feasibility of our approach, we provide a prototypical implementation and demonstrate it in a case study from the quantum humanities domain.
Open Access
Secure distributed paillier key generation with application to the Ordinos e-voting system
(2020) Truger, Felix
Ordinos is a novel verifiable tally-hiding e-voting system. At its heart, a homomorphic encryption scheme and secure multi-party computation (MPC) are used to tally votes and securely determine the voting result, without necessarily revealing the full tally (e.g., the number of votes per candidate)The proof of concept implementation of Ordinos is based on a threshold variant of the Paillier encryption scheme and two MPC protocols for the comparison of encrypted numbers (greater-than and equality). Due to the threshold construction, the decryption key is shared among a set of trustees. The MPC protocols for comparison require precomputed encrypted randomness of certain shape. Formerly, a trusted party was employed to generate the key shares and randomness and distribute them to the trustees. In this thesis, the trusted party was replaced by MPC protocols that allow to generate the key shares and randomness among the trustees. The protocols provide security against malicious parties in the honest-majority setting. The key generation follows a proposal by Nishide and Sakurai (2010) that is based on verifiable secret sharings and zero-knowledge proofs for committed values. We introduce a few adaptations to reduce its runtime using mostly standard techniques. The generation of randomness is based on the Paillier encryption scheme as an arithmetic black box and standard zero-knowledge proofs for Paillier encrypted values. The protocols were implemented and their performance was evaluated in a local network. Most notablythe implemented key generation protocol for threshold Paillier showed an expected average runtime around 95 minutes for generating 2048-bit keys among 3 trustees with a threshold of 2. Since existing implementations provide security only in the semi-honest setting, this is the first time that an approach with security against malicious parties was implemented and evaluated. Overall, the distributed generation of both key shares and randomness takes considerably more time compared to the use of a trusted party, but avoids security risks and trust problems that occur with trusted parties.
Open Access
Selection and optimization of hyperparameters in warm-started quantum optimization for the MaxCut problem
(2022) Truger, Felix; Beisel, Martin; Barzen, Johanna; Leymann, Frank; Yussupov, Vladimir
Today’s quantum computers are limited in their capabilities, e.g., the size of executable quantum circuits. The Quantum Approximate Optimization Algorithm (QAOA) addresses these limitations and is, therefore, a promising candidate for achieving a near-term quantum advantage. Warm-starting can further improve QAOA by utilizing classically pre-computed approximations to achieve better solutions at a small circuit depth. However, warm-starting requirements often depend on the quantum algorithm and problem at hand. Warm-started QAOA (WS-QAOA) requires developers to understand how to select approach-specific hyperparameter values that tune the embedding of classically pre-computed approximations. In this paper, we address the problem of hyperparameter selection in WS-QAOA for the maximum cut problem using the classical Goemans-Williamson algorithm for pre-computations. The contributions of this work are as follows: We implement and run a set of experiments to determine how different hyperparameter settings influence the solution quality. In particular, we (i) analyze how the regularization parameter that tunes the bias of the warm-started quantum algorithm towards the pre-computed solution can be selected and optimized, (ii) compare three distinct optimization strategies, and (iii) evaluate five objective functions for the classical optimization, two of which we introduce specifically for our scenario. The experimental results provide insights on efficient selection of the regularization parameter, optimization strategy, and objective function and, thus, support developers in setting up one of the central algorithms of contemporary and near-term quantum computing.