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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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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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Characterize and optimize the four-wave mixing in dual-interferometer coupled silicon microrings

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Chinese Physics B 2019年第10期28卷 198-203页

作者： Chao Wu Yingwen Liu Xiaowen Gu Shichuan Xue Xinxin Yu Yuechan Kong Xiaogang Qiang Junjie Wu Zhihong Zhu Ping Xu Institute for Quantum Information and State Key Laboratory of High Performance Computing College of ComputerCollege of Advanced Interdisciplinary StudiesNational University of Defense TechnologyChangsha 410073China Science and Technology on Monolithic Integrated Circuits and Modules Laboratory Nanjing Electronic Devices InstituteNanjing 210016China National Laboratory of Solid State Microstructures and School of Physics Nanjing UniversityNanjing 210093China

By designing and fabricating a series of dual-interferometer coupled silicon microrings, the coupling condition of the pump, signal, and idler beams can be engineered independently and then we carried out both the continuous-wave and pulse pumped four-wave mixing experiments to verify the dependence of conversion efficiency on the coupling conditions of the four interacting beams, respectively. Under the continuous-wave pump, the four-wave mixing efficiency gets maximized when both the pump and signal/idler beams are closely operated at the critical coupling point, while for the pulse pump case, the efficiency can be enhanced greatly when the pump and converted idler beams are all overcoupled. These experiment results agree well with our theoretical calculations. Our design provides a platform for explicitly characterizing the four-wave mixing under different pumping conditions, and offers a method to optimize the four-wave mixing, which will facilitate the development of on-chip all-optical signal processing with a higher efficiency or reduced pump power.

关键词： silicon resonators four-wave mixing Mach-Zehnder interferometer

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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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Bright photon-pair source based on a silicon dual-Mach-Zehnder microring

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Science China(Physics,Mechanics & Astronomy) 2020年第2期63卷 3-6页

作者： Chao Wu YingWen Liu XiaoWen Gu XinXin Yu YueChan Kong Yang Wang XiaoGang Qiang JunJie Wu ZhiHong Zhu XueJun Yang Ping Xu Institute for Quantum Information and State Key Laboratory of High Performance Computing College of ComputerCollege of Advanced Interdisciplinary StudiesNational University of Defense TechnologyChangsha 410073China Science and Technology on Monolithic Integrated Circuits and Modules Laboratory Nanjing Electronic Devices InstituteNanjing 210016China National Laboratory of Solid State Microstructures and School of Physics Nanjing UniversityNanjing 210093China

Single photons and photon pairs are typically generated by spontaneous parametric down conversion or quantum dots;however,spontaneous four-wave mixing(SFWM)in silicon microring resonators[1]is also an appealing source of entangled photons,offering a strong cavity-enhanced nonlinear interactions while maintaining features,such as compact,simple to fabricate,and allowing for thermal ***,silicon ring-resonators usually suffer from a trade-off between providing a high pair generation rate(PGR)and high extraction *** achieve high PGR,devices are generally operated with the signal and idler photons in the undercoupling regime and pump photons at the critical coupling point,while high extraction rates require the converted photons to be ***,the optimal conditions for achieving maximal output photon pair flux are critical coupling for the pump photons and overcoupling for the converted photons[2,3].

关键词： coupling photon converted

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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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Concentration for Random Product Formulas

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PRX quantum 2021年第4期2卷 040305-040305页

作者： Chi-Fang Chen Hsin-Yuan Huang Richard Kueng Joel A. Tropp Department of Physics Caltech Pasadena California USA Institute for Quantum Information and Matter Caltech Pasadena California USA Department of Computing and Mathematical Sciences Caltech Pasadena California USA Institute for Integrated Circuits Johannes Kepler University Linz Austria

quantum simulation has wide applications in quantum chemistry and physics. Recently, scientists have begun exploring the use of randomized methods for accelerating quantum simulation. Among them, a simple and powerful technique, called qDRIFT, is known to generate random product formulas for which the average quantum channel approximates the ideal evolution. qDRIFT achieves a gate count that does not explicitly depend on the number of terms in the Hamiltonian, which contrasts with Suzuki formulas. This work aims to understand the origin of this speedup by comprehensively analyzing a single realization of the random product formula produced by qDRIFT. The main results prove that a typical realization of the randomized product formula approximates the ideal unitary evolution up to a small diamond-norm error. The gate complexity is already independent of the number of terms in the Hamiltonian, but it depends on the system size and the sum of the interaction strengths in the Hamiltonian. Remarkably, the same random evolution starting from an arbitrary, but fixed, input state yields a much shorter circuit suitable for that input state. In contrast, in deterministic settings, such an improvement usually requires initial state knowledge. The proofs depend on concentration inequalities for vector and matrix martingales, and the framework is applicable to other randomized product formulas. Our bounds are saturated by certain commuting Hamiltonians.

关键词： quantum algorithms quantum simulation

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