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quantum Determinant Estimation

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arXiv 2025年

作者： Agerskov, J. Splittorff, K. NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Denmark Blegdamsvej 17 Copenhagen ØDK-2100 Denmark

A quantum algorithm for computing the determinant of a unitary matrix U ∈ U (N) is given. The algorithm requires no preparation of eigenstates of U and estimates the phase of the determinant to t binary digits accuracy with O(N log2 N + t2) operations and tN controlled applications of U 2m with m = 0, . . ., t − 1. For an orthogonal matrix O ∈ O(N) the algorithm can determine with certainty the sign of the determinant using O(N log2 N) operations and N controlled applications of O. An extension of the algorithm to contractions is discussed. © 2025, CC BY.

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Measurement-Induced Multimode Squeezed Light Interferometers with Scalable Architectures

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arXiv 2025年

作者： Verma, Abhinav Hastrup, Jacob Neergaard-Nielsen, Jonas S. Andersen, Ulrik L. Department of Physics Technical University of Denmark Fysikvej Kongens Lyngby 2800 Denmark Center for Quantum Devices and NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Denmark

In the rapidly expanding realm of photonic quantum information processing, the role of multimode squeezed light interferometers is critical. These devices are pivotal in enabling a wide range of applications, from quantum computing and sensing to advanced communication systems. Although linear interferometers with full programmability have been realized with up to 32 modes [1, 2], scaling to hundreds of modes is hampered by exponential increases in optical losses with circuit depth. Here we present and experimentally demonstrate highly scalable, low-loss multimode squeezed light interferometer schemes, capitalizing on measurement-induced operations and a shallow circuit approach. We validate our approach through a 6-mode interferometer and subsequently demonstrate its scalability by extending the design to 400 modes. This development marks a step toward realizing large-scale Noisy Intermediate-Scale quantum (NISQ) algorithm applications, employing shallow circuits and addressing the prevalent issue of optical losses in photonic quantum information processing. Copyright © 2025, The Authors. All rights reserved.

关键词： Squeezed light

A Digital quantum Algorithm for Non-Markovian Electron Transfer Dynamics Using Repeated Interactions

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arXiv 2025年

作者： Northcote, Lea K. Teynor, Matthew S. Solomon, Gemma C. NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Copenhagen ØDK-2100 Denmark Nano-Science Center Department of Chemistry University of Copenhagen Copenhagen ØDK-2100 Denmark

quantum algorithms have the potential to revolutionize our understanding of open quantum systems in chemistry. In this work, we demonstrate that quantum algorithms leveraging a repeated interaction model can effectively reproduce non-Markovian electron transfer processes under four different donor-acceptor parameter regimes and for a donor-bridge-acceptor system. We systematically explore how the algorithm scales for the regimes. Notably, our approach exhibits favorable scaling in the required repeated interaction length as the electronic coupling, temperature, damping rate, and system size increase. Furthermore, a single Trotter step per repeated interaction leads to an acceptably small error, and high-fidelity initial states can be prepared with a short time evolution. This efficiency highlights the potential of the algorithm for tackling increasingly complex systems. When fault-tolerant quantum hardware becomes available, the method could be extended to incorporate structured baths, additional energy levels, or more intricate coupling schemes, enabling the simulation of real-world open quantum systems that remain beyond the reach of classical computation. Copyright © 2025, The Authors. All rights reserved.

关键词： Markov processes

Strong Intrinsic Longitudinal Coupling in Circuit quantum Electrodynamics

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Physical Review Letters 2025年第15期134卷 153603-153603页

作者： C. A. Potts R. C. Dekker S. Deve E. W. Strijbis G. A. Steele Kavli Institute of Nanoscience Delft University of Technology PO Box 5046 2600 GA Delft The Netherlands Niels Bohr Institute University of Copenhagen Blegdamsvej 17 2100 Copenhagen Denmark NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Denmark

Radiation-pressure interactions between harmonic oscillators have enabled exquisite measurement precision and control, made possible by using strong sideband drives, enhancing the coupling rate while also linearizing the interaction. In this Letter, we demonstrate a strong intrinsic longitudinal coupling, a circuit quantum electrodynamics analog of the radiation-pressure interaction, between a transmon qubit and a linear microwave resonator. A red-detuned sideband drive results in an on-demand Jaynes-Cummings interaction with a high on-off ratio. We measure a longitudinal coupling rate an order of magnitude larger than all decay rates, placing the device in the strong coupling regime. The intrinsic longitudinal interaction demonstrated here will enable the development of high-connectivity quantum information processing hardware and the exploration of the gravitational decoherence of quantum objects.

关键词： Microwave resonators

Andreev spin relaxation time in a shadow-evaporated InAs weak link

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arXiv 2025年

作者： Lu, Haoran Bofill, David F. Sun, Zhenhai Kanne, Thomas Nygård, Jesper Kjaergaard, Morten Fatemi, Valla School of Applied and Engineering Physics Cornell University IthacaNY14853 United States Center for Quantum Devices Niels Bohr Institute University of Copenhagen Copenhagen2100 Denmark NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Copenhagen2100 Denmark

Andreev spin qubits are a new qubit platform that merges superconductivity with semiconductor physics. The mechanisms dominating observed energy relaxation remain unidentified. We report here on three steps taken to address these questions in an InAs nanowire weak link. First, we designed a microwave readout circuit tuned to be directly sensitive to the spin-dependent inductance of the weak link so that higher orbital states are not necessary for readout – this resulted in larger windows in parameter space in which the spin state properties can be probed. Second, we implemented a successful gap-engineering strategy to mitigate quasiparticle poisoning. Third, the weak link was fabricated by in situ shadow evaporation, which has been shown to improve atomic-scale disorder. We show how our design allows characterization of the spin stability and coherence over the full range of magnetic flux and gate voltage of an odd parity bias point. The spin relaxation and dephasing rates are comparable with the best devices previously reported, suggestive that surface atomic-scale disorder and QP poisoning are not linked to spin relaxation in InAs nanowires. Our design strategies are transferrable to novel materials platforms for Andreev qubits such as germanium and carbon. © 2025, CC BY.

关键词： Nanowires

Tailoring Fusion-Based Photonic quantum computing Schemes to quantum Emitters

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PRX quantum 2025年第2期6卷 020304-020304页

作者： Ming Lai Chan Thomas J. Bell Love A. Pettersson Susan X. Chen Patrick Yard Anders S. Sørensen Stefano Paesani Sparrow Quantum Blegdamsvej 104A DK-2100 Copenhagen Ø Denmark Quantum Engineering Technology Labs H. H. Wills Physics Laboratory and Department of Electrical and Electronic Engineering University of Bristol Bristol BS8 1FD United Kingdom Quantum Engineering Centre for Doctoral Training University of Bristol Bristol United Kingdom NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Blegdamsvej 17 DK-2100 Copenhagen Ø Denmark

Fusion-based quantum computation is a promising quantum computing model where small-sized photonic resource states are simultaneously entangled and measured by fusion gates. Such operations can be readily implemented with scalable photonic hardware: resource states can be deterministically generated by quantum emitters and fusions require only shallow linear-optical circuits. Here, we propose fusion-based architectures tailored to the capabilities and noise models in quantum emitters. We show that high tolerance to dominant physical error mechanisms can be achieved, with fault-tolerance thresholds of 8% for photon loss, 4% for photon distinguishability between emitters, and spin noise thresholds well above memory-induced errors for typical spin-photon interfaces. Our construction and analysis provide guidelines for the development of photonic quantum hardware targeting fault-tolerant applications with quantum emitters.

关键词： quantum computers

Identification of soft modes in amorphous Al2O3 via first-principles

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arXiv 2025年

作者： Tyner, Alexander C. Heath, Joshuah T. Thann, Thue Christian Michal, Vincent P. Krogstrup, Peter Svendsen, Mark Kamper Balatsky, Alexander V. Nordita KTH Royal Institute of Technology Stockholm University Stockholm106 91 Sweden Department of Physics University of Connecticut StorrsCT06269 United States NNF Quantum Computing Programme Niels Bohr Institute University of Copenhagen Denmark

Amorphous Al2O3 is a fundamental component of modern superconducting qubits. While amphorphous oxides offer distinct advantages, such as directional isotropy and a consistent bulk electronic gap, in realistic systems these compounds support two-level systems (TLSs) which couple to the qubit, expediting decoherence. In this work, we perform a first-principles study of amorphous Al2O3 and identify low-energy modes in the electronic and phonon spectra as a possible origin for TLSs. Copyright © 2025, The Authors. All rights reserved.

关键词： Qubits

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

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