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arXiv 2025年

作者： Henze, Fabian Tran, Viet Ostermann, Birte Kueng, Richard de Wolff, Timo Gross, David Institute for Theoretical Physics University of Cologne Germany Institute for Integrated Circuits and Quantum Computing JKU Linz Austria Institute for Analysis and Algebra TU Braunschweig Germany

quantum computers can solve semidefinite programs (SDPs) using resources that scale better than state-of-the-art classical methods as a function of the problem dimension. At the same time, the known quantum algorithms scale very unfavorably in the precision, which makes it non-trivial to find applications for which the quantum methods are well-suited. Arguably, precision is less crucial for SDP relaxations of combinatorial optimization problems (such as the Goemans-Williamson algorithm), because these include a final rounding step that maps SDP solutions to binary variables. With this in mind, Brandão, França, and Kueng have proposed to use quantum SDP solvers in order to achieve an end-to-end speed-up for obtaining approximate solutions to combinatorial optimization problems. They did indeed succeed in identifying an algorithm that realizes a polynomial quantum advantage in terms of its asymptotic running time. However, asymptotic results say little about the problem sizes for which advantages manifest. Here, we present an analysis of the non-asymptotic resource requirements of this algorithm. The work consists of two parts. First, we optimize the original algorithm with a particular emphasis on performance for realistic problem instances. In particular, we formulate a version with adaptive step-sizes, an improved detection criterion for infeasible instances, and a more efficient rounding procedure. In a second step, we benchmark both the classical and the quantum version of the algorithm. The benchmarks did not identify a regime where even the optimized quantum algorithm would beat standard classical approaches for input sizes that can be realistically solved at all. In the absence of further significant improvements, these algorithms therefore fall into a category sometimes called galactic: Unbeaten in their asymptotic scaling behavior, but not practical for realistic problems. © 2025, CC BY.

关键词： quantum computers

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Variational quantum thermalizers based on weakly-symmetric nonunitary multi-qubit operations

arXiv

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arXiv 2025年

作者： Zapusek, Elias Kirova, Kristina Hahn, Walter Marthaler, Michael Reiter, Florentin Institute for Quantum Electronics ETH Zürich Zürich8093 Switzerland Institute for Integrated Circuits and Quantum Computing Johannes Kepler University Linz Linz4040 Austria Fraunhofer Institute for Applied Solid State Physics IAF Tullastr. 72 Freiburg79108 Germany HQS Quantum Simulations GmbH Rintheimer Str. 23 Karlsruhe76131 Germany

We propose incorporating multi-qubit nonunitary operations in Variational quantum Thermalizers (VQTs). VQTs are hybrid quantum-classical algorithms that generate the thermal (Gibbs) state of a given Hamiltonian, with applications in quantum algorithms and simulations. However, current algorithms struggle at intermediate temperatures, where the target state is nonpure but exhibits entanglement. We devise multi-qubit nonunitary operations that harness weak symmetries and thereby improve the performance of the algorithm. Utilizing dissipation engineering, we create these nonunitary multi-qubit operations without the need for measurements or additional qubits. To train the ansatz, we develop and benchmark novel methods for entropy estimation of quantum states, expanding the toolbox for quantum state characterization. We demonstrate that our approach can prepare thermal states of paradigmatic spin models at all temperatures. Our work thus creates new opportunities for simulating open quantum many-body systems. Copyright © 2025, The Authors. All rights reserved.

关键词： Qubits

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Fast and robust quantum state tomography from few basis measurements 16

Fast and robust quantum state tomography from few basis meas...

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16th Conference on the Theory of quantum Computation, Communication and Cryptography, TQC 2021

作者： França, Daniel Stilck Brandão, Fernando G.S.L. Kueng, Richard QMATH Department of Mathematical Sciences University of Copenhagen Denmark AWS Center for Quantum Computing PasadenaCA United States Institute for Quantum Information and Matter California Institute of Technology PasadenaCA United States Institute for Integrated Circuits Johannes Kepler University Linz Austria

ISBN: (纸本)9783959771986

quantum state tomography is a powerful but resource-intensive, general solution for numerous quantum information processing tasks. This motivates the design of robust tomography procedures that use relevant resources as sparingly as possible. Important cost factors include the number of state copies and measurement settings, as well as classical postprocessing time and memory. In this work, we present and analyze an online tomography algorithm designed to optimize all the aforementioned resources at the cost of a worse dependence on accuracy. The protocol is the first to give provably optimal performance in terms of rank and dimension for state copies, measurement settings and memory. Classical runtime is also reduced substantially and numerical experiments demonstrate a favorable comparison with other state-of-the-art techniques. Further improvements are possible by executing the algorithm on a quantum computer, giving a quantum speedup for quantum state tomography. © Daniel Stilck França, Fernando G.S L. Brandão, and Richard Kueng;licensed under Creative Commons License CC-BY 4.0

关键词： Tomography

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Electric-field control of photon indistinguishability in cascaded decays in quantum dots

arXiv

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arXiv 2025年

作者： Undeutsch, Gabriel Aigner, Maximilian Garcia, Ailton J. Reindl, Johannes Peter, Melina Mader, Simon Weidinger, Christian Covre da Silva, Saimon F. Manna, Santanu Schöll, Eva Rastelli, Armando Institute of Semiconductor and Solid State Physics Johannes Kepler University Linz Austria Instituto de Física Gleb Wataghin Universidade Estadual de Campinas Campinas Brazil Department of Electrical Engineering Indian Institute of Technology Delhi India Institute for Integrated Circuits and Quantum Computing Johannes Kepler University Linz Austria

Photon indistinguishability, entanglement, and antibunching are key ingredients in quantum optics and photonics. Decay cascades in quantum emitters offer a simple method to create entangled-photon-pairs with negligible multi-pair generation probability. However, the degree of indistinguishability of the photons emitted in a cascade is intrinsically limited by the lifetime ratio of the involved transitions. Here we show that, for the biexciton-exciton cascade in a quantum dot, this ratio can be widely tuned by an applied electric field. Hong-Ou-Mandel interference measurements of two subsequently emitted biexciton photons show that their indistinguishability increases with increasing field, following the theoretically predicted behavior. At the same time, the emission linewidth stays close to the transform-limit, favoring applications relying on the interference among photons emitted by different sources. Copyright © 2025, The Authors. All rights reserved.

关键词： Semiconductor quantum dots

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Blind calibration of a quantum computer

arXiv

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arXiv 2025年

作者： Jeanette, Liam M. Wilkens, Jadwiga Roth, Ingo Than, Anton Green, Alaina M. Hangleiter, Dominik Linke, Norbert M. Joint Quantum Institute University of Maryland College ParkMD20742 United States Duke Quantum Center Duke University DurhamNC27701 United States Institute for Integrated Circuits Johannes Kepler University Linz Linz4040 Austria Quantum Research Center Technology Innovation Institute Abu Dhabi United Arab Emirates Simons Institute for the Theory of Computing University of California at Berkeley BerkeleyCA94720 United States Joint Center for Quantum Information and Computer Science NIST University of Maryland College ParkMD20742 United States

quantum system calibration is limited by the ability to characterize a quantum state under unknown device errors. We develop an accurate calibration protocol that is blind to the precise preparation of a specific quantum state. It extracts device errors from simple tomographic data only, and does not require bespoke experiments for a priori specified error mechanisms. Using a trapped-ion quantum computer, we experimentally demonstrate the accuracy of the method by recovering intentional miscalibrations. We then use blind calibration to estimate the native calibration parameters of the experimental system. The recovered calibrations are close to directly measured values and perform similarly in predicting state properties. © 2025, CC BY.

关键词： quantum computers

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High Mobility SiGe/Ge 2DHG Heterostructure quantum Wells for Semiconductor Hole Spin Qubits

arXiv

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arXiv 2024年

作者： Kong, Zhenzhen Li, Zonghu Zhou, Yuchen Cao, Gang Li, Hai-Ou Su, Jiale Zhang, Yiwen Liu, Jinbiao Guo, Guo-Ping Li, Junfeng Luo, Jun Zhao, Chao Ye, Tianchun Wang, Guilei Hefei National Laboratory Hefei230088 China Integrated Circuit Advanced Process R&D Center Institute of Microelectronics Chinese Academy of Sciences Beijing100029 China School of Integrated Circuits University of Chinese Academy of Sciences Beijing100049 China CAS Key Laboratory of Quantum Information University of Science and Technology of China Hefei230026 China Origin Quantum Computing Company Limited Hefei230026 China

Strong spin-orbit coupling and relatively weak hyperfine interactions make germanium hole spin qubits a promising candidate for semiconductor quantum processors. The two-dimensional hole gas structure of strained Ge quantum wells serves as the primary material platform for spin hole qubits. A low disorder material environment is essential for this process. In this work, we fabricated a Ge/SiGe heterojunction with a 60 nm buried quantum well layer on a Si substrate using reduced pressure chemical vapor deposition technology. At a temperature of 16 mK, when the carrier density is 1.87×1011/cm2, we obtained a mobility as high as 308.64×104cm2/Vs. Concurrently, double quantum dot and planar germanium coupling with microwave cavities were also successfully achieved. This fully demonstrates that this structure can be used for the preparation of higher-performance hole spin qubits. Copyright © 2024, The Authors. All rights reserved.

关键词： Nanocrystals

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Short-time simulation of quantum dynamics by Pauli measurements

arXiv

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arXiv 2024年

作者： Faehrmann, Paul K. Eisert, Jens Kieferová, Mária Kueng, Richard Dahlem Center for Complex Quantum Systems Freie Universität Berlin Berlin14195 Germany Institute for Integrated Circuits and Quantum Computing Johannes Kepler University Linz Austria Helmholtz-Zentrum Berlin für Materialien und Energie Hahn-Meitner-Platz 1 Berlin14109 Germany Fraunhofer Heinrich Hertz Institute Berlin10587 Germany Centre for Quantum Software and Information School of Computer Science Faculty of Engineering & Information Technology University of Technology SydneyNSW2007 Australia

Simulating the dynamics of complex quantum systems is a central application of quantum devices. Here, we propose leveraging the power of measurements to simulate short-time quantum dynamics of physically prepared quantum states in classical post-processing using a truncated Taylor series approach. While limited to short simulation times, our hybrid quantum-classical method is equipped with rigorous error bounds. It is extendable to estimate low-order Taylor approximations of smooth, time-dependent functions of tractable linear combinations of measurable operators. These insights can be made use of in the context of Hamiltonian learning and device verification, short-time imaginary time evolution, or the application of intractable operations to sub-universal quantum simulators in classical post-processing. © 2024, CC BY.

关键词： Hamiltonians

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Efficient Estimation of Pauli Observables by Derandomization

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Physical Review Letters 2021年第3期127卷 030503-030503页

作者： Hsin-Yuan Huang Richard Kueng John Preskill Institute for Quantum Information and Matter Caltech Pasadena California 91125 USA Department of Computing and Mathematical Sciences Caltech Pasadena California 91125 USA Institute for Integrated Circuits Johannes Kepler University Linz A-4040 Austria Walter Burke Institute for Theoretical Physics Caltech Pasadena California 91125 USA AWS Center for Quantum Computing Pasadena California 91125 USA

We consider the problem of jointly estimating expectation values of many Pauli observables, a crucial subroutine in variational quantum algorithms. Starting with randomized measurements, we propose an efficient derandomization procedure that iteratively replaces random single-qubit measurements by fixed Pauli measurements; the resulting deterministic measurement procedure is guaranteed to perform at least as well as the randomized one. In particular, for estimating any L low-weight Pauli observables, a deterministic measurement on only of order log(L) copies of a quantum state suffices. In some cases, for example, when some of the Pauli observables have high weight, the derandomized procedure is substantially better than the randomized one. Specifically, numerical experiments highlight the advantages of our derandomized protocol over various previous methods for estimating the ground-state energies of small molecules.

关键词： quantum computation quantum protocols quantum tomography

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Models of quantum Complexity Growth

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PRX quantum 2021年第3期2卷 030316-030316页

作者： Fernando G.S.L. Brandão Wissam Chemissany Nicholas Hunter-Jones Richard Kueng John Preskill Institute for Quantum Information and Matter Caltech Pasadena California USA AWS Center for Quantum Computing Pasadena California USA Department of Computing and Mathematical Sciences Caltech Pasadena California USA Walter Burke Institute for Theoretical Physics Caltech Pasadena California USA Perimeter Institute for Theoretical Physics Waterloo Ontario Canada Institute for Integrated Circuits Johannes Kepler University Linz Austria

The concept of quantum complexity has far-reaching implications spanning theoretical computer science, quantum many-body physics, and high-energy physics. The quantum complexity of a unitary transformation or quantum state is defined as the size of the shortest quantum computation that executes the unitary or prepares the state. It is reasonable to expect that the complexity of a quantum state governed by a chaotic many-body Hamiltonian grows linearly with time for a time that is exponential in the system size; however, because it is hard to rule out a shortcut that improves the efficiency of a computation, it is notoriously difficult to derive lower bounds on quantum complexity for particular unitaries or states without making additional assumptions. To go further, one may study more generic models of complexity growth. We provide a rigorous connection between complexity growth and unitary k-designs, ensembles that capture the randomness of the unitary group. This connection allows us to leverage existing results about design growth to draw conclusions about the growth of complexity. We prove that local random quantum circuits generate unitary transformations whose complexity grows linearly for a long time, mirroring the behavior one expects in chaotic quantum systems and verifying conjectures by Brown and Susskind. Moreover, our results apply under a strong definition of quantum complexity based on optimal distinguishing measurements.

关键词： Computational complexity quantum correlations in quantum information quantum entanglement quantum measurements quantum chaotic systems Strongly correlated systems Many-body techniques Random matrix theory

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Non-Hermitian Waveguide Cavity QED with Tunable Atomic Mirrors

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Physical Review Letters 2023年第10期131卷 103602-103602页

作者： Wei Nie Tao Shi Yu-xi Liu Franco Nori Center for Joint Quantum Studies and Department of Physics School of Science Tianjin University Tianjin 300350 China Institute of Theoretical Physics Chinese Academy of Sciences P.O. Box 2735 Beijing 100190 China CAS Center for Excellence in Topological Quantum Computation University of Chinese Academy of Sciences Beijing 100049 China School of Integrated Circuits Tsinghua University Beijing 100084 China Theoretical Quantum Physics Laboratory Cluster for Pioneering Research RIKEN Wakoshi Saitama 351-0198 Japan Center for Quantum Computing RIKEN Wakoshi Saitama 351-0198 Japan Physics Department The University of Michigan Ann Arbor Michigan 48109-1040 USA

Optical mirrors determine cavity properties by means of light reflection. Imperfect reflection gives rise to open cavities with photon loss. We study an open cavity made of atom-dimer mirrors with a tunable reflection spectrum. We find that the atomic cavity shows anti-PT symmetry. The anti-PT phase transition controlled by atomic couplings in mirrors indicates the emergence of two degenerate cavity supermodes. Interestingly, a threshold of mirror reflection is identified for realizing strong coherent cavity-atom coupling. This reflection threshold reveals the criterion of atomic mirrors to produce a good cavity. Moreover, cavity quantum electrodynamics with a probe atom shows mirror-tuned properties, including reflection-dependent polaritons formed by the cavity and probe atom. Our Letter presents a non-Hermitian theory of an anti-PT atomic cavity, which may have applications in quantum optics and quantum computation.

关键词： Cavity quantum electrodynamics PT-symmetric quantum mechanics quantum optics with artificial atoms

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