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quantum Geometry and Bounds on Dissipation in Slowly Driven quantum Systems

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Physical Review Letters 2025年第14期134卷 146603-146603页

作者： Iliya Esin Étienne Lantagne-Hurtubise Frederik Nathan Gil Refael Department of Physics and Institute for Quantum Information and Matter California Institute of Technology Pasadena California 91125 USA Department of Physics Bar-Ilan University 52900 Ramat Gan Israel Département de Physique and Institut Quantique Université de Sherbrooke Sherbrooke Québec J1K 2R1 Canada Center for Quantum Devices and NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen 2100 Copenhagen Denmark

We show that energy dissipation in slowly driven, Markovian quantum systems at low temperature is linked to the geometry of the driving protocol through the quantum (or Fubini-Study) metric. Utilizing these findings, we establish lower bounds on dissipation rates in two-tone protocols, such as those employed for topological frequency conversion. Notably, in appropriate limits these bounds are only determined by the topology of the protocol and an effective quality factor of the system-bath coupling. Our results bridge topological and geometric phenomena with energy dissipation in open quantum systems, and further provide design principles for optimal driving protocols.

关键词： Topology

Breadth-first graph traversal union-find decoder

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arXiv 2024年

作者： Löbl, Matthias C. Chen, Susan X. Paesani, Stefano Sørensen, Anders S. The Niels Bohr Institute University of Copenhagen Blegdamsvej 17 Copenhagen ØDK-2100 Denmark Quantum Engineering Centre for Doctoral Training University of Bristol Bristol United Kingdom NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Blegdamsvej 17 Copenhagen ØDK-2100 Denmark

Fast decoding algorithms are decisive for real-time quantum error correction and for analyzing properties of error correction codes. Here, we develop variants of the union-find decoder that simplify its implementation and provide potential decoding speed advantages. Furthermore, we show how these methods can be adapted to decode non-topological quantum lowdensity- parity-check (qLDPC) codes. All the developed decoders can directly include both qubit erasures and Pauli errors in the decoding step, thus addressing the dominant noise mechanisms for photonic quantum computing. We investigate the strengths and weaknesses of the different decoder variants, benchmark their speed and threshold error rates on several codes, and provide the used source code. Copyright © 2024, The Authors. All rights reserved.

关键词： quantum computers

Efficient percolation simulations for lossy photonic fusion networks

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arXiv 2023年

作者： Löbl, Matthias C. Paesani, Stefano Sørensen, Anders S. The Niels Bohr Institute University of Copenhagen Blegdamsvej 17 Copenhagen ØDK-2100 Denmark NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Blegdamsvej 17 Copenhagen ØDK-2100 Denmark

The study of percolation phenomena has various applications ranging from social networks or materials science to quantum information. The most common percolation models are bond- or site-percolation for which the Newman-Ziff algorithm enables an efficient simulation. Here, we consider several non-standard percolation models that appear in the context of measurement-based photonic quantum computing with so-called graph states and fusion networks. The associated percolation thresholds determine the tolerance to photon loss in such systems and we develop modifications of the Newman-Ziff algorithm to perform the corresponding percolation simulation efficiently. We demonstrate our algorithms by using them to characterize exemplary fusion networks and graph states. The used source code is provided as an open-source repository. Copyright © 2023, The Authors. All rights reserved.

关键词： Percolation (computer storage)

Superconducting Proximity Effect in Two-Dimensional Hole Gases

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arXiv 2024年

作者： Babkin, Serafim Joecker, Benjamin Flensberg, Karsten Serbyn, Maksym Danon, Jeroen Am Campus 1 Klosterneuburg3400 Austria Center for Quantum Devices Niels Bohr Institute University of Copenhagen CopenhagenDK-2100 Denmark NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Denmark Department of Physics Norwegian University of Science and Technology TrondheimNO-7491 Norway

Technology involving hybrid superconductor–semiconductor materials is a promising avenue for engineering quantum devices for information storage, manipulation, and transmission. Proximity-induced superconducting correlations are an essential part of such devices. While the proximity effect in the conduction band of common semiconductors is well understood, its manifestation in confined hole gases, realized for instance in germanium, is an active area of research. Lower-dimensional hole-based systems, particularly in germanium, are emerging as an attractive platform for a variety of solid-state quantum devices, due to their combination of efficient spin and charge control and long coherence times. The recent experimental realization of the proximity effect in germanium thus calls for a theoretical description that is tailored to hole gases. In this work, we propose a simple model to describe proximity-induced superconductivity in two-dimensional hole gases, incorporating both the heavy-hole (HH) and light-hole (LH) bands. We start from the Luttinger–Kohn model, introduce three parameters that characterize hopping across the superconductor–semiconductor interface, and derive explicit intraband and interband effective pairing terms for the HH and LH bands. Unlike previous approaches, our theory provides a quantitative relationship between induced pairings and interface properties. Restricting our general model to an experimentally relevant case where only the HH band crosses the chemical potential, we predict the coexistence of s-wave and d-wave singlet pairings, along with triplet-type pairings, and modified Zeeman and Rashba spin–orbit couplings. Our model thus presents a starting point for theoretical modeling of quantum devices based on proximitized hole gases, fueling further progress in quantum technology. Copyright © 2024, The Authors. All rights reserved.

关键词： Shear waves

Computational discovery of high-refractive-index van der Waals materials: The case of HfS2

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arXiv 2025年

作者： Zambrana-Puyalto, Xavier Svendsen, Mark Kamper Søndersted, Amalie H. Sarbajna, Avishek Sandberg, Joakim P. Riber, Albert L. Ermolaev, Georgy Boland, Tara Maria Tselikov, Gleb Volkov, Valentyn S. Thygesen, Kristian S. Raza, Søren Department of Physics Technical University of Denmark Fysikvej Kongens Lyngby DK-2800 Denmark NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Denmark Emerging Technologies Research Center XPANCEO Internet City Emmay Tower Dubai United Arab Emirates

New high-refractive-index dielectric materials may enhance many optical technologies by enabling efficient manipulation of light in waveguides, metasurfaces, and nanoscale resonators. Van der Waals materials are particularly promising due to their excitonic response and strong in-plane polarizability. Here we combine ab initio calculations and experiments to discover new high-refractive-index materials. Our screening highlights both known and new promising optical materials, including hafnium disulfide (HfS2), which shows an in-plane refractive index above 3 and large anisotropy in the visible range. We confirm these theoretical predictions through ellipsometry measurements and investigate the photonic potential of HfS2 by fabricating nanodisk resonators, observing optical Mie resonances in the visible spectrum. Over the course of seven days, we observe a structural change in HfS2, which we show can be mitigated by storage in either argon-rich or humidity-reduced environments. This work provides a comparative overview of high-index van der Waals materials and showcases the potential of HfS2 for photonic applications in the visible spectrum. © 2025, CC BY-SA.

关键词： Van der Waals forces

Fault-tolerant quantum simulation of generalized Hubbard models

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arXiv 2025年

作者： Bay-Smidt, Andreas Juul Klausen, Frederik Ravn Sünderhauf, Christoph Izsák, Róbert Solomon, Gemma C. Blunt, Nick S. NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Denmark Nano-Science Center Department of Chemistry University of Copenhagen Denmark Princeton University Department of Mathematics Fine Hall United States Riverlane CambridgeCB2 3BZ United Kingdom

quantum simulations of strongly interacting fermionic systems, such as those described by the Hubbard model, are promising candidates for useful early fault-tolerant quantum computing applications. This paper presents Tile Trotterization, a generalization of plaquette Trotterization (PLAQ), which allows simulation of Hubbard models on arbitrary lattices and provides a framework that enables the simulation of more complex models, including the extended Hubbard model and the PPP model. We consider applications of Tile Trotterization to simulate Hubbard models on hexagonal lattice fragments and periodic hexagonal lattices, analyze gate costs, and provide commutator bounds for evaluating Trotter errors, including new commutator bounds for the extended Hubbard model. We compare the resource requirements of Tile Trotterization for performing quantum phase estimation to a qubitization-based approach, which we optimize for the hexagonal lattice Hubbard model, and demonstrate that Tile Trotterization scales more efficiently with system size. These advancements significantly broaden the potential applications of early fault-tolerant quantum computers to models of practical interest in materials research and organic chemistry. Copyright © 2025, The Authors. All rights reserved.

关键词： Hubbard model

Gatemon Qubit Revisited for Improved Reliability and Stability

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arXiv 2024年

作者： Feldstein-Bofill, David Sun, Zhenhai Wied, Casper Singh, Shikhar Isakov, Brian D. Krøjer, Svend Hastrup, Jacob Gyenis, András Kjaergaard, Morten Center for Quantum Devices Niels Bohr Institute University of Copenhagen Denmark NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Denmark Department of Electrical Computer & Energy Engineering University of Colorado Boulder BoulderCO80309 United States Department of Physics University of Colorado Boulder BoulderCO80309 United States

The development of quantum circuits based on hybrid superconductor-semiconductor Josephson junctions holds promise for exploring their mesoscopic physics and for building novel superconducting devices. The gate-tunable superconducting transmon qubit (gatemon) is the paradigmatic example of such a superconducting circuit. However, gatemons typically suffer from unstable and hysteretic qubit frequencies with respect to the applied gate voltage and reduced coherence times. Here we develop methods for characterizing these challenges in gatemons and deploy these methods to compare the impact of shunt capacitor designs on gatemon performance. Our results indicate a strong frequency- and design-dependent behavior of the qubit stability, hysteresis, and dephasing times. Moreover, we achieve highly reliable tuning of the qubit frequency with 1 MHz precision over a range of several GHz, along with improved stability in grounded gatemons compared to gatemons with a floating capacitor design. © 2024, CC BY.

关键词： Qubits

Many-body effects of two-level systems in superconducting qubits

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arXiv 2025年

作者： Heath, Joshuah T. Tyner, Alexander C. Thann, Thue Christian Michal, Vincent P. Krogstrup, Peter Svendsen, Mark Kamper Balatsky, Alexander V. Nordita Stockholm University KTH Royal Institute of Technology Hannes Alfvéns väg 12 StockholmSE-106 91 Sweden Department of Physics University of Connecticut StorrsCT06269 United States NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Denmark

Superconducting qubits are often adversely affected by two-level systems (TLSs) within the Josephson junction, which contribute to decoherence and subsequently limit the performance of the qubit. By treating the TLS as a soft (i.e., low-frequency) bosonic mode localized in real space, we find that a single TLS in either the amorphous oxide surface or the superconducting bulk may result in a localized "hot spot" of amplified Josephson energy. Such amplification is shown to have a non-negligible effect on the T1 time of certain superconducting qubits, regardless of whether or not the TLS is on resonance with the qubit frequency. With this study, we identify sources of decoherence unique to the superconducting element of superconducting quantum devices. © 2025, CC BY.

关键词： Qubits

High-Threshold quantum computing by Fusing One-Dimensional Cluster States

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Physical Review Letters 2023年第12期131卷 120603-120603页

作者： Stefano Paesani Benjamin J. Brown Center for Hybrid Quantum Networks (Hy-Q) Niels Bohr Institute University of Copenhagen Blegdamsvej 17 Copenhagen 2100 Denmark NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Blegdamsvej 17 Copenhagen 2100 Denmark IBM Quantum T. J. Watson Research Center Yorktown Heights New York 10598 USA IBM Denmark Prøvensvej 1 Brøndby 2605 Denmark

We propose a measurement-based model for fault-tolerant quantum computation that can be realized with one-dimensional cluster states and fusion measurements only; basic resources that are readily available with scalable photonic hardware. Our simulations demonstrate high thresholds compared with other measurement-based models realized with basic entangled resources and 2-qubit fusion measurements. Its high tolerance to noise indicates that our practical construction offers a promising route to scalable quantum computing with quantum emitters and linear-optical elements.

关键词： Measurement-based quantum computing Optical quantum information processing quantum computation quantum error correction quantum information architectures & platforms