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Friday 26 December

Quantum Physics

Variational Quantum Eigensolver for Real-World Finance: Scalable Solutions for Dynamic Portfolio Optimization Problems

Irene De León, Danel Arias, Manuel Martín-Cordero et al. (+9 more)

We present a scalable, hardware-aware methodology for extending the Variational Quantum Eigensolver (VQE) to large, realistic Dynamic Portfolio Optimization (DPO) problems. Building on the scaling strategy from our previous work, where we tailored a VQE workflow to both the DPO formulation and the target QPU, we now put forward two significant advances. The first is the implementation of the Ising Sample-based Quantum Configuration Recovery (ISQR) routine, which improves solution quality in Quadratic Unconstrained Binary Optimization problems. The second is the use of the VQE Constrained method to decompose the optimization task, enabling us to handle DPO instances with more variables than the available qubits on current hardware. These advances, which are broadly applicable to other optimization problems, allow us to address a portfolio with a size relevant to the financial industry, consisting of up to 38 assets and covering the full Spanish stock index (IBEX 35). Our results, obtained on a real Quantum Processing Unit (IBM Fez), show that this tailored workflow achieves financial performance on par with classical methods while delivering a broader set of high-quality investment strategies, demonstrating a viable path towards obtaining practical advantage from quantum optimization in real financial applications.
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All authors Irene De León, Danel Arias, Manuel Martín-Cordero, María Esperanza Molina, Pablo Serrano, Senaida Hernández-Santana, Miguel Ángel Jiménez Herrera, Joana Fraxanet, Ginés Carrascal, Escolástico Sánchez, Inmaculada Posadillo, Álvaro Nodar

Quantum Physics · Cross-listed from gr-qc

Entanglement Islands and Thermodynamics of the Black Hole in Asymptotically Safe Quantum Gravity

Sobhan Kazempour, Sichun Sun and Chengye Yu

We study thermodynamic properties and the entanglement island of a black hole in asymptotically safe quantum gravity, analyzing key thermodynamic quantities such as the Hawking temperature, heat capacity, and entropy, as well as the mass-horizon radius relation. Unlike Schwarzschild black holes, the temperature decreases with mass near the evaporation endpoint, signaling a phase transition and possible stable remnant. The entanglement entropy of Hawking radiation is obtained both with and without island contributions. Without islands, the radiation entropy grows linearly indefinitely, leading to the information paradox. By including island contributions and extremizing the generalized entropy functional, we resolve this paradox. At late times, the radiation entropy saturates at the Bekenstein-Hawking entropy, confirming unitary evolution. From this, we derive the Page time and scrambling time by equating early- and late-time entanglement entropies. The result of this study establishes the finiteness of the radiation entropy and consistency with quantum mechanics.
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Quantum Physics

Measure of entanglement and the monogamy relation: a topical review

Yu Guo and Zhixiang Jin

Characterizing entanglement, including quantifying and distribution of entanglement, which lies at heart of the quantum resource theory, have been investigated extensively ever since Bennett \etal proposed three seminal measures of entanglement in 1996. Up to now, there are numerous measures of entanglement that have been proposed from different point of view and plenty of monogamy relations have been explored which make the distribution of entanglement became more and more clear. While this is relatively easy in the case of pure states, it is much more intricate for the case of mixed quantum states especially with higher dimension and more particles in the system. We present here an overview of the theory along this line. We outline most of the results in this field historically and focus on the finite-dimensional systems. In particular we emphasize the point of view that (i) which yardsticks haven been applied in quantifying entanglement and its distribution, (ii) what are the substantive characteristics and interrelations of these measures and their monogamy relations mathematically by comparing, and (iii) which concepts should be improved or revised and how they were developed accordingly.
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Quantum Physics

Spacetime Spins: Statistical mechanics for error correction with stabilizer circuits

Cory T. Aitchison and Benjamin Béri

A powerful method for analyzing quantum error-correcting codes is to map them onto classical statistical mechanics models. Such mappings have thus far mostly focused on static codes, possibly subject to repeated syndrome measurements. Recent progress in quantum error correction, however, has prompted new paradigms where codes emerge from stabilizer circuits in spacetime – a unifying perspective encompassing syndrome extraction circuits of static codes, dynamically generated codes, and logical operations. We show how to construct statistical mechanical models for stabilizer circuits subject to independent Pauli errors, by mapping logical equivalence class probabilities of errors to partition functions using the spacetime subsystem code formalism. We also introduce a modular language of spin diagrams for constructing the spin Hamiltonians, which we describe in detail focusing on independent circuit-level X-Z error channels. With the repetition and toric codes as examples, we use our approach to analytically rank logical error rates and thresholds between code implementations with standard and dynamic syndrome extraction circuits, describe the effect of transversal logical Clifford gates on logical error rates, and perform Monte Carlo simulations to estimate maximum likelihood thresholds. Our framework offers a universal prescription to analyze, simulate, and compare the decoding properties of any stabilizer circuit, while revealing the innate connections between dynamical quantum systems and noise-resilient phases of matter.
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Quantum Physics

Linear Program Witness for Network Nonlocality in Arbitrary Networks

Salome Hayes-Shuptar, Daniel Bhatti, Ana Belen Sainz et al. (+1 more)

Network nonlocality extends Bell nonlocality to settings with multiple independent sources and parties. Certifying it in quantum information processing tasks requires suitable witnesses. However, in contrast to local correlations, the set of network-local correlations is non-convex. This non-convexity makes certifying network nonlocality a highly non-trivial task. Existing approaches involve leveraging network-specific properties, or inflation-based methods whose constraints grow combinatorially in the number of local variables. In this work, we introduce a linear programming witness for network nonlocality built from five classes of linear constraints. These classes are network-agnostic, although the explicit forms of the constraints must be tailored to a specific network's structure. We use the procedure to construct network nonlocality witnesses for a family of ring networks and certify network nonlocality for a concrete example, relying only on observed probabilities and a tunable experimental parameter. Our work advances the search for efficient witnesses to certify network nonlocality across diverse quantum network architectures.
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All authors Salome Hayes-Shuptar, Daniel Bhatti, Ana Belen Sainz, David Elkouss

Quantum Physics

Double-Layered Silica-Engineered Fluorescent Nanodiamonds for Catalytic Generation and Quantum Sensing of Active Radicals

Jia Su, Zenghao Kong, Fei Kong et al. (+9 more)

Fluorescent nanodiamonds (FNDs) hosting nitrogen-vacancy (NV) centers have attracted considerable attention for quantum sensing applications, particularly owing to notable advancements achieved in the field of weak magnetic signal detection in recent years. Here, we report a practical quantum-sensing platform for the controlled production and real-time monitoring of ultra-short-lived reactive free radicals using a double-layered silica modification strategy. An inner dense silica layer preserves the intrinsic properties of NV centers, while an outer porous silica layer facilitates efficient adsorption and stabilization of hydroxyl radicals and their precursor reactants. By doping this mesoporous shell with gadolinium (III) catalysts, we achieve sustained, light-free generation of hydroxyl radicals via catalytic water splitting, eliminating reliance on external precursors. The mechanism underlying this efficient radical generation is discussed in detail. The radical production is monitored in real time and in situ through spin-dependent T1 relaxometry of the NV centers, demonstrating stable and tunable radical fluxes, with concentration tunable across a continuous range from approximately 100 mM to molar levels by adjusting the catalyst condition. This study extends the technical application of nanodiamonds from relaxation sensing to the controlled synthesis of reactive free radicals, thereby providing robust experimental evidence to support the advancement of quantum sensing systems in intelligent manufacturing.
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All authors Jia Su, Zenghao Kong, Fei Kong, Xing Liu, Linyu Zeng, Zhecheng Wang, Zijian Zeng, Jie Liu, Jihu Su, Junhua Yuan, Guosheng Shi, Fazhan Shi

Quantum Physics · Cross-listed from cond-mat.mtrl-sci

Zeeman-like coupling to valley degree of freedom in Si-based spin qubits

S Akbar Jafari, Hendrik J Bluhm and David P Divincenzo

Increasing the valley splitting in Si-based heterostructures is critical for improving the performance of semiconductor qubits. This paper demonstrates that the two low-energy conduction band valleys are not independent parabolic bands. Instead, they originate from the X-point of the Brillouin zone, where they are interconnected by a degeneracy protected by the non-symmorphic symmetry of the diamond lattice. This semi-Dirac-node degeneracy gives rise to the $\Delta_1$ and $\Delta_{2'}$ bands, which constitute the valley degrees of freedom. By explicitly computing the two-component Bloch functions $X_1^\pm$, using the wave vector group at the X-point, we determine the transformation properties of the object $(X_1^+,X_1^-)$. We demonstrate that these properties are fundamentally different from those of a spinor. Consequently, we introduce the term "valleyor" to emphasize this fundamental distinction. The transformation properties of valleyors induce corresponding transformations of the Pauli matrices $\tau_1,\tau_2$ and $\tau_3$ in the valley space. Determining these transformations allows us to classify possible external perturbations that couple to each valley Pauli matrix, thereby identifying candidates for valley-magnetic fields, ${\mathsf B}$. These fields are defined by a Zeeman-like coupling ${\mathsf B}\cdot\vec\tau$ to the valley degree of freedom. In this way, we identify scenarios where an applied magnetic field $\vec B$ can leverage other background fields, such as strain, to generate a valley-magnetic field ${\mathsf B}$. This analysis suggests that beyond the well-known mechanism of potential scattering from Ge impurities, there exist additional channels (mediated by combinations of magnetic and strain-induced vector potentials) to control the valley degree of freedom
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Quantum Physics

Poincaré Duality and Multiplicative Structures on Quantum Codes

Yiming Li, Zimu Li, Zi-Wen Liu et al. (+1 more)

Quantum LDPC codes have attracted intense interest due to their advantageous properties for realizing efficient fault-tolerant quantum computing. In particular, sheaf codes represent a novel framework that encompasses all well-known good qLDPC codes with profound underlying mathematics. In this work, we generalize Poincaré duality from manifolds to both classical and quantum codes defined via sheaf theory on $t$-dimensional cell complexes. Viewing important code properties including the encoding rate, code distance, local testability soundness, and efficient decoders as parameters of the underlying (co)chain complexes, we rigorously prove a duality relationship between the $i$-th chain and the $(t-i)$-th cochain of sheaf codes. We further build multiplicative structures such as cup and cap products on sheaved chain complexes, inspired by the standard notions of multiplicative structures and Poincaré duality on manifolds. This immediately leads to an explicit isomorphism between (co)homology groups of sheaf codes via a cap product. As an application, we obtain transversal disjoint logical $\mathrm{C}Z$ gates with $k_{\mathrm{C}Z}=\Theta(n)$ on families of good qLDPC and almost-good quantum locally testable codes. Moreover, we provide multiple new methods to construct transversal circuits composed of $\mathrm{C}\mathrm{C}Z$ gates as well as for higher order controlled-$Z$ that are provably logical operations on the code space. We conjecture that they generate nontrivial logical actions, pointing towards fault-tolerant non-Clifford gates on nearly optimal qLDPC sheaf codes. Mathematically, our results are built on establishing the equivalence between sheaf cohomology in the derived-functor sense, Čech cohomology, and the cohomology of sheaf codes, thereby introducing new mathematical tools into quantum coding theory.
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All authors Yiming Li, Zimu Li, Zi-Wen Liu, Quynh T. Nguyen

Quantum Physics

Logical Consistency as a Dynamical Invariant: A Quantum Model of Self-Reference and Paradox

Nikolaos Cheimarios and Spyridoula Cheimariou

Logical paradoxes and inconsistent information pose deep challenges in epistemology and the philosophy of logic. Classical systems typically handle contradictions only through external checks or by altering the logical framework, as in Tarski's hierarchy or paraconsistent logics. We propose a novel approach: a quantum circuit architecture that intrinsically enforces logical consistency during its unitary evolution. By encoding self-referential or contradictory propositions into a quantum state, the circuit uses interference effects to suppress inconsistent outcomes while preserving coherent ones. We demonstrate this with the Liar Paradox ("This statement is false"), showing that the quantum model naturally stabilizes truth values that would be paradoxical classically. The framework builds on orthomodular quantum logic treating logical propositions as subspace projectors and connects to belief revision and cognitive modeling by providing a physical mechanism for coherence restoration in epistemic states. This work bridges formal logic and quantum computation, suggesting that consistency can be embedded as a structural property of reasoning systems rather than imposed externally.
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Quantum Physics

Evaluating Supervised Learning Approaches for Quantification of Quantum Entanglement

Shruti Aggarwal, Trasha Gupta, R. K. Agrawal et al. (+1 more)

Quantum entanglement is a key resource in quantum computing and quantum information processing tasks. However, its quantification remains a major challenge since it cannot be directly extracted from physical observables. To address this issue, we study a few machine-learning based models to estimate the amount of entanglement in two-qubit as well as three-qubit systems. We use measurement outcomes as the input features and entanglement measures as the training labels. Our models predict entanglement without requiring the full state information. This demonstrates the potential of machine learning as an effcient and powerful tool for characterizing quantum entanglement
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All authors Shruti Aggarwal, Trasha Gupta, R. K. Agrawal, S. Indu

Quantum Physics

Magic State Distillation using Asymptotically Good Codes on Qudits

Michael J. Cervia, Henry Lamm, Diyi Liu et al. (+2 more)

Qudits offer the potential for low-overhead magic state distillation, although previous results for asymptotically good codes have required qudit dimension $q\gg 100$ or code length $\mathcal{N}\gg 100$. These parameters far exceed experimental demonstrations of qudit platforms, and thus motivate the search for better codes. Using a novel lifting procedure, we construct the first family of good triorthogonal codes on the $\mathbb{F}_{2^{2m}}$ alphabet with $m \geq 3$ that lies above the Tsfasman-Vladut-Zink bound. These codes yield a family of asymptotically good quantum codes with transversal CCZ gates, enabling constant space overhead magic state distillation with qudit dimension as small as $q=64$. Further, we identify a promising code with parameters $[[42,14,6]]_{64}$. Finally, we show that a distilled $|{CCZ}\rangle_{2^{2m}}$ can be reduced to a $|{CCZ}\rangle_{2^n}$ state for arbitrary $n$ with a constant-depth Clifford circuit of at most 9 computational basis measurements, 12 single-qudit and 9 two-qudit Clifford gates.
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All authors Michael J. Cervia, Henry Lamm, Diyi Liu, Edison M. Murairi, Shuchen Zhu

Quantum Physics

Ergotropy in Quantum Batteries

Cheng-Jie Wang and Fu-Quan Dou

Ergotropy–a key figure of merit for quantum battery (QB) performance–plays a crucial role. However, the dynamics and physical mechanisms governing ergotropy evolution remain open challenges. Here, we investigate the ergotropy of a general QB model and find that the charging process is accompanied by the variation and inversion of the energy level populations. In the absence of population inversion, the ergotropy is fully consistent with coherent ergotropy; in local and global population inversion, it is determined by both coherent and incoherent ergotropy. Via random sampling of quantum states and Hamiltonians, we show that coherence and the participation ratio enhance coherent ergotropy, whereas incoherent ergotropy–whether enhanced, unchanged, or suppressed–depends on diagonal entropy, the participation ratio, and energy level population ordering. We demonstrate that the ergotropy lower bound is incoherent ergotropy, the upper bound is the QB stored energy, and enhanced QB purity suppresses locked energy and boosts charging efficiency. Furthermore, we use the Tavis-Cummings (TC) and Jaynes-Cummings (JC) batteries as paradigms to validate our findings. Our work elucidates ergotropy underlying mechanisms in general QBs and establishes a rigorous framework for optimizing ergotropy and charging efficiency, paving the way for high-performance quantum energy-storage devices.
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Quantum Physics

A General Framework for Constructing Local Hidden-state Models to Determine the Steerability

Yanning Jia, Fenzhuo Guo, Mengyan Li et al. (+2 more)

Not all entangled states can exhibit quantum steering, and determining whether a given entangled state is steerable is a crucial problem in quantum information theory. The main challenge lies in verifying the existence of a local hidden-state (LHS) model capable of reproducing all post-measurement assemblages generated by arbitrary measurements. To address this, we propose a machine learning-based framework that employs batch sampling of measurements and gradient-based optimization to construct an optimal LHS model. We validate our method by analyzing the steerability of two-qubit Werner and two-qutrit isotropic states. For Werner states, our approach saturates the analytical visibility bounds under three Pauli measurements, arbitrary projective measurements (PVMs), and arbitrary positive operator-valued measurements (POVMs). For isotropic states, we achieve the known analytical bounds under arbitrary PVMs. We further investigate the steerability of this class of states under arbitrary POVMs, and our results suggest that POVMs can offer an advantage over PVMs in revealing the steerability of such states.
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All authors Yanning Jia, Fenzhuo Guo, Mengyan Li, Haifeng Dong, Fei Gao

Quantum Physics · Cross-listed from cond-mat.mes-hall

A Cohomological Framework for Topological Phases from Momentum-Space Crystallographic Groups

T. R. Liu, Zheng Zhang and Y. X. Zhao

Crystallographic groups are conventionally studied in real space to characterize crystal symmetries. Recent work has recognized that when these symmetries are realized projectively, momentum space inherently accommodates nonsymmorphic symmetries, thereby evoking the concept of momentum-space crystallographic groups (MCGs). Here, we reveal that the cohomology of MCGs encodes fundamental data of crystalline topological band structures. Specifically, the collection of second cohomology groups, $H^2(\Gamma_F,\mathbb{Z})$, for all MCGs $\Gamma_F$, provides an exhaustive classification of Abelian crystalline topological insulators, serving as an effective approximation to the full crystalline topological classification. Meanwhile, the third cohomology groups $H^3(\Gamma_F,\mathbb{Z})$ across all MCGs exhaustively classify all possible twistings of point-group actions on the Brillouin torus, essential data for twisted equivariant K-theory. Furthermore, we establish the isomorphism $H^{n+1}(\Gamma_F,\mathbb{Z})\cong H^n\big(\Gamma_F,\operatorname{\mathcal{F}}(\mathbb{R}^d_F,U(1))\big)$ for $ n\ge 1$, where $\operatorname{\mathcal{F}}(\mathbb{R}^d_F,U(1))$ denotes the space of continuous $U(1)$-valued functions on the $d$D momentum space $\mathbb{R}^d_F$. The case $n=1$ yields a complete set of topological invariants formulated in purely algebraic terms, which differs fundamentally from the conventional formulation in terms of differential forms. The case $n=2$, analogously, provides a fully algebraic description for all such twistings. Thus, the cohomological theory of MCGs serves as a key technical framework for analyzing crystalline topological phases within the general setting of projective symmetry.
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Quantum Physics · Cross-listed from math-ph

The Loring–Schulz-Baldes Spectral Localizer Revisited

Gregory Berkolaiko, Jacob Shapiro and Beyer Chase White

The spectral localizer, introduced by Loring in 2015 and Loring and Schulz-Baldes in 2017, is a method to compute the (infinite volume) topological invariant of a quantum Hamiltonian on $\ZZ^d$, as the signature of the (finite) localizer matrix. We present a direct and elementary spectral-theoretic proof treating the $d=1$ and $d=2$ cases on an almost equal footing. Moreover, we re-interpret the localizer as a higher-dimensional topological insulator via the bulk-edge correspondence.
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Quantum Physics

Delayed Choice Lorentz Transformations on a Qubit

Lucas Burns, Sacha Greenfield and Justin Dressel

A continuously monitored quantum bit (qubit) exhibits competition between unitary Hamiltonian dynamics and non-unitary measurement-collapse dynamics, which for diffusive measurements form an enlarged transformation group equivalent to the Lorentz group of spacetime. We leverage this equivalence to develop a four-dimensional generalization of the three-dimensional Bloch ball to visualize the state of a monitored qubit as the four-momentum of an effective classical charge affected by a stochastic electromagnetic force field. Unitary qubit dynamics generated by Hermitian Hamiltonians correspond to elliptic spatial rotations of this effective charge while non-unitary qubit dynamics generated by non-Hermitian Hamiltonians or stochastic measurement collapse correspond to hyperbolic Lorentz boosts. Notably, to faithfully emulate the stochastic qubit dynamics arising from continuous qubit measurement, the stochastic electromagnetic fields must depend on the velocity of the charge they are acting on. Moreover, continuous qubit measurements admit a dynamical delayed choice effect where a future experimental choice can appear to retroactively determine the type of past measurement backaction, so the corresponding point charge dynamics can also exhibit delayed choice Lorentz transformations in which a future experimental choice determines whether stochastic force fields are electric or magnetic in character long after they interact with the particle.
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Thursday 25 December

Quantum Physics

Quantization of Physical Interaction Strengths via Singular Moduli

Prasoon Saurabh

Since the 2019 redefinition of the SI units, precision metrology has sought to anchor all physical quantities to fundamental constants and integer invariants. While the optical frequency comb revolutionized timekeeping by discretizing the continuum of light into countable teeth, and the Quantum Hall Effect standardized resistance via topological invariants, a comparable standard for interaction strength remains elusive. Currently, measuring the coupling constant ($g$) between quantum systems is an estimation problem, inherently subject to drift, noise, and fabrication variance. Here, we introduce Interaction Metrology, a protocol that transforms the measurement of coupling strengths from an analog estimation into a topological counting problem. By engineering a specific class of algebraic catastrophe – the Unimodal $X_9$ singularity – in a driven-dissipative lattice, we prove that the system's interaction moduli are topologically forced to take discrete, quantized values, forming a "Geometric $k$-Comb." We derive the universality class of this quantization, showing that it arises from the discrepancy between the Milnor ($\mu$) and Tjurina ($\tau$) numbers of the effective potential, a strictly non-Hermitian effect forbidden in standard quantum mechanics. Finally, we provide an ab-initio blueprint for a silicon nitride implementation, demonstrating that this quantization is robust against disorder levels exceeding current foundry tolerances. This discovery establishes a universal standard for force sensing and quantum logic gates, enabling the calibration of interaction strengths with topological certainty.
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Quantum Physics

Decoherence challenges in Nanoscience: A Quantum Phase Space perspective

Ravo Tokiniaina Ranaivoson, Angelo Mamitiana Ralaikoto, Diary Lova Ratsimbazafy et al. (+5 more)

Quantum decoherence is both the fundamental mechanism underlying the quantum-to-classical transition and a major challenge for the development of scalable nanoscale quantum technologies. This work introduces a Quantum Phase Space (QPS) framework that provides a unified geometric description of decoherence. The framework is based on a dual structure consisting of an overcomplete continuous frame of minimum-uncertainty states that defines the QPS geometry, and an orthonormal basis that satisfies the strict mathematical requirements for decoherence within the Spectrum Broadcast Structure (SBS) objectivity criterion for pointer states. Within this framework, the variance-covariance matrix of the QPS ground states serves as a universal indicator of decoherence regimes:it is time-independent for Markovian (memoryless) dynamics, and time-dependent for non-Markovian dynamics with memory and information backflow. To illustrate the formalism, the Hu-Paz-Zhang (HPZ) model is generalized to include simultaneous position and momentum couplings to the environment, leading to a generalized non-Markovian master equation characterized by a spectral-density matrix. The corresponding evolution equations for the first moments$\ (\left\langle p(t) \right\rangle,\left\langle x(t) \right\rangle)$ and for the ground covariance matrix establish a direct connection between microscopic environmental properties, classical-like trajectories and the QPS geometry. The proposed framework provides a unified theoretical foundation for modeling decoherence in nanoscale systems involving simultaneous position and momentum interactions with the environment. The QPS framework may thus bridge fundamental theory and practical quantum engineering, offering a promising coherent pathway to understand, control, and exploit decoherence at the nanoscience frontier.
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All authors Ravo Tokiniaina Ranaivoson, Angelo Mamitiana Ralaikoto, Diary Lova Ratsimbazafy, Rivo Herivola Manjakamanana Ravelonjato, Roland Raboanary, Raoelina Andriambololona, Nomenjanahary Tanjonirina Manampisoa, Fanamby Sahondraniandriana

Quantum Physics

Phase-space description of photon emission

D.V. Karlovets, A.A. Shchepkin, A.D. Chaikovskaia et al. (+4 more)

Interactions between charged particles and light occur in real space and time, yet quantum field theory usually describes them in momentum space. Whereas this approach is well suited for calculating emission probabilities and cross sections, it is insensitive to spatial and temporal phenomena such as, for instance, radiation formation, quantum coherence, and wave packet spreading. These effects are becoming increasingly important for experiments involving electrons, photons, atoms, and ions, particularly with the advent of attosecond spectroscopy and metrology. Here, we propose a general method for describing the emission of photons in phase space via a Wigner function. Several effects for Cherenkov radiation are predicted, absent in classical realm or in quantum theory in momentum space, such as a finite spreading time of the photon, finite duration of the flash and a quantum shift of the photon arrival time. The photon spreading time turns out to be negative near the Cherenkov angle, the flash duration is defined by the electron packet size, and the temporal shift can be both positive and negative. The characteristic time scales of these effects lie in the atto- and femtosecond ranges, thereby illustrating atomic origins of these macroscopic phenomena. The near-field distribution of the photon field resembles the electron packet shape, thus making “snapshots” of the emitter wave function. Our approach can easily be generalized to the other types of radiation and extended to scattering, decay, and annihilation processes, bringing tomographic methods of quantum optics to particle physics.
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All authors D.V. Karlovets, A.A. Shchepkin, A.D. Chaikovskaia, D.V. Grosman, D.A. Kargina, U.G. Rybak, G.K. Sizykh

Quantum Physics

Properties of a Three-Level $\Lambda$-Type Atom Driven by Coherent and Stochastic Fields

Sajad Ahmadi, Mohsen Akbari, Shahpoor Saeidian et al. (+1 more)

We present a theoretical investigation of a three-level $\Lambda$-type atom driven by a strong coherent laser and a weak stochastic field exhibiting amplitude and phase fluctuations. The stochastic field is modeled as a complex Gaussian-Markovian random process with finite bandwidth to describe realistic laser noise. Using the Born-Markov and rotating-wave approximations, we derive a Lindblad-form master equation that incorporates spontaneous emission and noise-induced terms, and we solve for the steady-state regime. We examine level populations in both the bare and dressed bases and compute the incoherent resonance-fluorescence spectrum. Our analysis shows that the stochastic drive is not merely a source of decoherence but a versatile control parameter. By detuning the stochastic-field central frequency relative to the coherent drive (especially for narrow bandwidths), we observe pronounced changes in emission characteristics, including selective enhancement or suppression, and reshaping of the multi-peaked fluorescence spectrum when the detuning matches the generalized Rabi frequency. Numerical results reveal nontrivial steady-state modifications distinct from purely coherent driving, enabling precise control of populations and suggesting applications in quantum control, quantum technologies, spectroscopy, and noise-assisted manipulation of atomic systems.
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All authors Sajad Ahmadi, Mohsen Akbari, Shahpoor Saeidian, Ali Motazedifard

Quantum Physics

A Lyapunov Framework for Quantum Algorithm Design in Combinatorial Optimization with Approximation Ratio Guarantees

Shengminjie Chen, Ziyang Li, Hongyi Zhou et al. (+3 more)

In this work, we develop a framework aiming at designing quantum algorithms for combinatorial optimization problems while providing theoretical guarantees on their approximation ratios. The principal innovative aspect of our work is the construction of a time-dependent Lyapunov function that naturally induces a controlled Schrödinger evolution with a time dependent Hamiltonian for maximizing approximation ratios of algorithms. Because the approximation ratio depends on the optimal solution, which is typically elusive and difficult to ascertain a priori, the second novel component is to construct the upper bound of the optimal solution through the current quantum state. By enforcing the non-decreasing property of this Lyapunov function, we not only derive a class of quantum dynamics that can be simulated by quantum devices but also obtain rigorous bounds on the achievable approximation ratio. As a concrete demonstration, we apply our framework to Max-Cut problem, implementing it as an adaptive variational quantum algorithm based on a Hamiltonian ansatz. This algorithm avoids ansatz and graph structural assumptions and bypasses parameter training through a tunable parameter function integrated with measurement feedback.
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All authors Shengminjie Chen, Ziyang Li, Hongyi Zhou, Jialin Zhang, Wenguo Yang, Xiaoming Sun

Quantum Physics

Investigation of quantum chaos in local and non-local Ising models

Reza Pirmoradian, Elham Sadoogh, Maryam Teymouri et al. (+3 more)

We investigate signatures of quantum chaos within Ising spin chains subjected to transverse and longitudinal fields, incorporating both local (nearest-neighbor) and non-local (long-range) couplings. While local Ising models may exhibit integrable or chaotic dynamics contingent on interaction strengths and field parameters, systems with non-local interactions generally display a stronger propensity toward chaos, even when the non-local couplings are weak. By examining the distribution of energy level spacings through the level spacing ratio, we delineate the transition from integrable to chaotic regimes and characterize the emergence of quantum chaos in these systems. Our analysis demonstrates that non-local couplings facilitate faster operator spreading and more intricate dynamical behavior, enabling these systems to approach maximal chaos more readily than their local counterparts. Additionally, we analyze Krylov complexity as a dynamical probe of chaos, observing a characteristic peak followed by a plateau at late times in chaotic regimes. This behavior provides a quantitative means to distinguish between integrable and chaotic phases, with the growth rate and saturation level of the complexity serving as effective indicators. Our findings underscore the role of non-local interactions in accelerating the onset of chaos and modifying dynamical complexity in quantum spin chains.
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All authors Reza Pirmoradian, Elham Sadoogh, Maryam Teymouri, Negar Abolqasemi-Azad, Mohammad Reza Lahooti, Zahra Mohammad-Ali

Quantum Physics · Cross-listed from physics.comp-ph

GaDE – GPU-acceleration of time-dependent Dirac Equation for exascale

Johanne Elise Vembe, Marcin Krotkiewski, Magnar Bjørgve et al. (+2 more)

Modern heterogeneous high-performance computing (HPC) systems powered by advanced graphics processing unit (GPU) architectures enable accelerating computing with unprecedented performance and scalability. Here, we present a GPU-accelerated solver for the three-dimensional (3D) time-dependent Dirac equation optimized for distributed HPC systems. The solver named GaDE is designed to simulate the electron dynamics in atoms induced by electromagnetic fields in the relativistic regime. It combines MPI with CUDA/HIP to target both NVIDIA and AMD GPU architectures. We discuss our implementation strategies in which most of the computations are carried out on GPUs, taking advantage of the GPU-aware MPI feature to optimize communication performance. We evaluate GaDE on the pre-exascale supercomputer LUMI, powered by AMD MI250X GPUs and HPE's Slingshot interconnect. Single-GPU performance on NVIDIA A100, GH200, and AMD MI250X shows comparable performance between A100 and MI250X in compute and memory bandwidth, with GH200 delivering higher performance. Weak scaling on LUMI demonstrates exceptional scalability, achieving 85% parallel efficiency across 2048 GPUs, while strong scaling delivers a 16x speedup on 32 GPUs - 50% efficiency for a communication-intensive, time-dependent Dirac equation solver. These results demonstrate GaDE's high scalability, making it suitable for exascale systems and enabling predictive simulations for ultra-intense laser experiments probing relativistic quantum effects.
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All authors Johanne Elise Vembe, Marcin Krotkiewski, Magnar Bjørgve, Morten Førre, Hicham Agueny

Quantum Physics · Cross-listed from hep-th

On prethermal time crystals from semi-holography

Toshali Mitra, Sukrut Mondkar, Ayan Mukhopadhyay et al. (+1 more)

We demonstrate the existence of a pair of almost dissipationless oscillating modes at low temperatures in both the shear and sound channels of a hybrid quantum system, comprised of a weakly self-interacting perturbative sector coupled to strongly self-interacting holographic degrees of freedom described by a black hole geometry. We argue that these modes realize prethermal time-crystal behavior in semi-holographic systems without fine-tuning and can be observed by measuring operators that probe either the hard (perturbative) or the soft (holographic) sector. We also find novel {short wavelength} instabilities that lead to the formation of inhomogeneities even at higher temperatures. These results provide evidence that black holes with planar horizons and dynamical boundary conditions can develop both inhomogeneous and metastable time-crystal phases over a wide range of temperatures set by an intermediate scale given by the intersector coupling. Furthermore, they suggest that such phases can be realized without external driving in non-Abelian plasmas of asymptotically free gauge theories in the large-$N$ limit.
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All authors Toshali Mitra, Sukrut Mondkar, Ayan Mukhopadhyay, Alexander Soloviev

Quantum Physics

Asymmetric polaron picture for the quantum Rabi model

Feng Qiao, Qiu-Yi Chen and Zu-Jian Ying

The experimental access to ultra-strong couplings in light-matter interactions has made the quantum phase transition (QPT) in the quantum Rabi model practically relevant, while the physics of the QPT has not yet been fully explored. The polaron picture is a method capable of analyzing in the entire coupling regime and extracting the essential physics behind the QPT. However, the asymmetric deformation of polarons is missing in the current polaron picture. In the present work we propose an improved variational method in asymmetric polaron picture (APP). Our APP not only increases the method accuracy but also reveals more underlying physics concerning the QPT. We find that in the ground state both the polarons and antipolarons are asymmetrically deformed to a large extent, which leads to a richer phase diagram. We also analyze the first excited state in which we unveil an asymmetry direction reversal for the polarons and an attraction/replusion transition differently from the ground state. Finally, we apply the APP in quantum Fisher information analysis and critical coupling extraction, the improvements indicate that the polaron asymmetry makes a considerable contribution to the quantum resource in quantum metrology and plays an unnegligible role in the QPT. Our results and mechanism clarifications expose more subtle energy competitions and abundant physics, and the method potentially might have broader applications in light-matter interactions.
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Quantum Physics

Insufficiency of Pure-State Ensembles in Characterizing Transformations of Entangled States under LOCC

C. L. Liu, Baoqing Sun and D. L. Zhou

The conditions for transforming pure entangled states under local operations and classical communication (LOCC) are well understood. A natural question then arises: Can we determine the transformation conditions for mixed entangled states under LOCC based on the properties of their pure-state ensembles? While much effort has been devoted to this issue, in this paper, we rule out this possibility. Our findings address several open questions, including: (i) The conditions \( E_f^{cr}(\rho) \geq E_f^{cr}(\sigma) \) for all convex roof entanglement measures \(E_f^{cr}\) is insufficient to guarantee the existence of an LOCC transformation \(\Lambda^L(\cdot)\) from \(\rho\) to \(\sigma\); and (ii) The inequalities \(\sum_j p_j E(\varphi_j) \geq \sum_l q_l E(\psi_l)\) for all entanglement monotones \(E\) are not sufficient to ensure the existence of an LOCC transformation from \(\{p_j, \ket{\varphi_j}\}\) to \(\{q_l, \ket{\psi_l}\}\).
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Quantum Physics

Spectroscopic Search for Topological Protection in Open Quantum Hardware: The Dissipative Mixed Hodge Module Approach

Prasoon Saurabh

Standard spectroscopic protocols model the dynamics of open quantum systems as a superposition of isolated, exponentially decaying eigenmodes. This paradigm fails fundamentally at Exceptional Points, where the eigenbasis collapses and the response becomes dominated by non-diagonalizable Jordan blocks. We resolve this ambiguity by introducing a geometric framework based on Dissipative Mixed Hodge Modules (DMHM). By replacing the scalar linewidth with a topological Weight Filtration, we derive “Weight Filtered Spectroscopy” (WFS)–a protocol that spatially separates decay channels based on the nilpotency rank of the Liouvillian. We demonstrate that WFS acts as a dissipative x-ray, quantifying dissipative leakage in molecular polaritons and certifying topological isolation in Non-Hermitian Aharonov-Bohm rings. This establishes that topological protection persists as an algebraic invariant even when the spectral gap is closed.
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Quantum Physics

Trade-off relation between integrated metrological gain and local dissipation in magnetic-field sensing by quantum spin ensemble

Nozomu Takahashi, Le Bin Ho and Hiroaki Matsueda

Quantum metrology plays a central role in precision sensing, where quantum enhancement of detection performance is crucial for both fundamental studies and practical applications. In this work, we derive a tight performance bound for magnetic-field sensing with a spin ensemble in the presence of dissipation. The metrological performance is quantified by the integrated metrological gain (IMG), which explicitly incorporates the time evolution of the measurement apparatus. By combining the Lindblad master equation with the quantum Fisher information, we obtain analytically exact trade-off relations between the IMG and the dissipation rate for local dephasing and local emission processes, showing that the gain scales inversely with the dissipation strength. This trade-off complements the Heisenberg limit, which addresses only the scaling with the number of spins and neglects dissipative dynamics. We analyze various initial state preparations and elucidate the role of quantum entanglement in the presence of dissipation. Notably, while entanglement is essential for achieving Heisenberg scaling at short times, it also accelerates dissipative degradation during time evolution. Consequently, for sufficiently long observation times, comparable metrological performance can be achieved even without entanglement.
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Quantum Physics

Hybrid Quantum Repeater Chains with Atom-based Quantum Processing Units and Quantum Memory Multiplexers

Shin Sun, Daniel Bhatti, Shaobo Gao et al. (+2 more)

Quantum repeaters enable the generation of reliable entanglement across long distances despite the underlying channel noise. Nevertheless, realizing quantum repeaters poses a difficult engineering challenge due to various device constraints and design tradeoffs. Herein we propose and analyze an efficient hybrid quantum repeater design that integrates atom-based quantum processing units, spontaneous parametric down-conversion photon sources, and atomic frequency comb quantum memories. Our design leverages the strong spectrotemporal multiplexing capability of the quantum memory to enable high-rate elementary-link entanglement generation between repeater nodes. Transferring the photonic entanglement into matter-qubit entanglement, together with deterministic quantum operations, further enables reliable long-distance entanglement distribution. We analyze photon-loss channels in the hybrid architecture and propose suitable error-suppression strategies that are natively incorporated into our repeater protocol. Using numerical simulations, we demonstrate the advantages of our hybrid design for end-to-end secret key rates in a linear repeater-chain model. With continued advances in relevant hardware technologies, we envision that the proposed hybrid design is well suited for large-scale quantum networks.
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All authors Shin Sun, Daniel Bhatti, Shaobo Gao, David Elkouss, Hiroki Takahashi

Quantum Physics

Waveguide-array-based multiplexed photonic interface for atom array

Yuya Maeda, Toshiki Kobayashi, Takuma Ueno et al. (+12 more)

The growing demand for high-capacity quantum communication and large-scale quantum computing underscores the importance of networking quantum processing units via multiplexed photonic channels. A neutral atom array with multiplexed atom-photon entanglement is a promising platform for its realization. Here, we demonstrate a key multiplexed photonic interface guiding the photons from an atom array to a single-mode waveguide array fabricated on a glass-based photonic integrated circuit. Remarkable 10 channels out of the 32-channel waveguide array with 25 $\mu$m pitch couple to photons from 10 sites of the atom array with Rydberg gate-enabled separation. Based on the observed correlation between the atomic states and the polarization of the photon with a visibility of 0.87, we anticipate its applicability to a large-scale multiplexed atom-photon entanglement generation for networking quantum processing units.
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All authors Yuya Maeda, Toshiki Kobayashi, Takuma Ueno, Kentaro Shibata, Shinichi Takenaka, Kazuki Ito, Yuma Fujiwara, Shigehito Miki, Hirotaka Terai, Tsuyoshi Kodama, Hideki Shimoi, Rikizo Ikuta, Makoto Yamashita, Shuta Nakajima, Takashi Yamamoto