Quantum Physics · Cross-listed from cond-mat.supr-con
Geometry-enabled magnetic resilience in superconducting nanowire single-photon detectors
Marinus C. van der Maas, Lin Jin, Ilhan Tunç et al. (+7 more)
While
magnetic fields and
superconductors are both central to classical and
quantum technologies, their combined use is often challenging, as
magnetic fields significantly affect superconducting device performance. In superconducting nanowire single-
photon detectors (SNSPDs),
magnetic fields drastically reduce detection efficiencies, hampering their application in magnetically-active classical and
quantum photonics. Here, we systematically characterize the performance of NbTiN SNSPDs under
magnetic fields and show the enhancement of their intrinsic detection efficiency (IDE) at lower bias currents and its suppression at higher currents. This leads to SNSPD performance degradation through reduced or disappearing saturation plateaus. We show that the magnitude of this degradation is highly dependent on nanowire width and demonstrate width-optimized SNSPDs with saturating IDE for a wide range of
photon energies under application-relevant
magnetic fields. Minimizing degradation in superconducting devices under
magnetic fields enables applications like detector-integrated
spin-optic and atomic
quantum processors, high-sensitivity magnetometry, and
quantum transduction.
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All authors Marinus C. van der Maas, Lin Jin, Ilhan Tunç, Raymond Vermeulen, Henri Ervasti, Ravi Gopie, Jan Riegelmeyer, Marco Colangelo, Ryoichi Ishihara, Carlos Errando-Herranz
Quantum Physics
Testing Catability and Coherent Superposition of $2\mathcal{D}$ Graphene Quantum system
Abdelmalek Bouzenada
We develop a theoretical framework for describing superposed coherent states in
graphene quantum systems using the concept of catability as a phase-sensitive metric functional measure. In this case, the formalism quantifies interference stability and coherence structure via phase-dependent contributions of
quantum superposition states. Catability is defined as a functional measure sensitive to relative phase variations within coherent state combinations, serving as a diagnostic tool for
quantum interference effects in
graphene-based systems. Also, the formulation is extended using Lie
algebra techniques, where the underlying
symmetry structure of
graphene quantum states is represented through operator
algebras governing state transformations in
quantum space. In this context, to describe nonlocal propagation and phase-resolved dynamics, a Green function approach is incorporated, enabling systematic treatment of
quantum correlations in a spatially extended structures framework. A unified framework is constructed by combining Lie algebraic
symmetry analysis with Green function propagation theory, yielding a consistent description of phase-sensitive catability in complex
graphene quantum configurations within the framework approach. Results provide a structured route for testing coherence, interference stability, and
quantum state control in low-dimensional
quantum materials systems.
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Quantum Physics
Negative refraction with low absorption using EIT in a four-level left-handed atomic system
Shun-Cai Zhao, Zheng-Dong Liu, Gen Li et al. (+1 more)
We suggest a scheme for obtaining negative refraction with low absorption in a left-handed atomic system.Under the the appropriate conditions,the atomic system displays negative refraction with negative permittivity and permeability(Left-handedness)in a common frequency range,simultaneously.And the imaginary parts of permittivity and permeability show transparently propagate in the same frequency range.Finally,the negative refraction show low absorption due to the EIT effect,and the figure of merit demonstrated this in this resonant atomic system.
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All authors Shun-Cai Zhao, Zheng-Dong Liu, Gen Li, Nian Liu
Quantum Physics
Microwave Power-to-Frequency Transduction via Injection Pulling of a Self-Sustained Oscillator for Rydberg Superheterodyne Sensing
Darmindra Arumugam
A Rydberg superheterodyne sensing architecture is demonstrated in which a self-sustained oscillator (SSO) serves as a dynamically perturbed local oscillator (LO) for microwave detection. The SSO is realized by a phase-controlled
radio-frequency (RF) feedback loop coupled to a transverse electromagnetic (TEM) cavity containing a Rydberg vapor
cell. The system operates near 5.49 GHz using a cesium ladder scheme with an 852 nm probe and 510 nm coupling
laser addressing the 6S to 6P to 49D transition, with microwave coupling to the 50P state. Injection of a microwave signal pulls the SSO frequency via nonlinear dynamics, converting input power into a measurable frequency shift read out optically as a Rydberg probe intermediate-frequency (IF) signal. The response follows Adler-type injection-pulling behavior, with continuous IF tuning with input power. A peak responsivity of 35 kHz/dB is observed, with enhanced sensitivity near synchronization. These results demonstrate power-to-frequency transduction using a dynamically perturbed LO combined with Rydberg atomic readout.
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Quantum Physics
Machine-learned, finite temperature Fermi-operator expansions suitable for GPUs and AI-hardware
Stanislaw Kowalski, Christian F. A. Negre, Anders M. N. Niklasson et al. (+2 more)
We present several finite-temperature recursive
Fermi-operator expansion schemes based on the second-order spectral projection (SP2) method. Our approach builds on a previous observation that the electronic structure problem, as formulated through a recursive SP2 expansion, can be mapped onto the architecture of a deep neural network. Using this perspective, we generalize SP2 to finite electronic temperatures by constructing machine learning models that determine optimized recursive expansion coefficients. The same approach is also applied to the prediction of the electronic
entropy for fractional occupation numbers. The coefficients are trained for a specified chemical potential and electronic temperature and are not available in closed analytical form. However, by employing an appropriate affine rescaling strategy to the
Hamiltonian matrix, we eliminate the need to retrain the model for different temperatures and chemical potentials. Our approach avoids explicit diagonalization and relies solely on highly optimized
matrix-
matrix multiplication kernels. Compared to state-of-the-art diagonalization, we achieve an order-of-magnitude speedup in the single-particle finite-temperature density
matrix calculation for small and moderately sized
matrices on modern GPUs and dense
matrix multiply units.
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All authors Stanislaw Kowalski, Christian F. A. Negre, Anders M. N. Niklasson, Kipton Barros, Joshua Finkelstein
Quantum Physics · Cross-listed from physics.ed-ph
Graduate Training in Quantum Information Science and Engineering: Lessons, Challenges, and a Roadmap from the NSF Research Traineeship Programs
Yohannes Abate, Victor Acosta, Alessandro Alabastri et al. (+31 more)
Since 2019, eighteen NSF Research Traineeship (NRT) awards in
quantum information science and engineering (QISE) and adjacent fields have been funded, constituting the largest NSF-coordinated investment in graduate QISE training in the United States. Synthesizing lessons from our programs, we work through the central tensions that every QISE graduate program must negotiate: between depth in a home discipline and breadth across the field, between structured instruction and open-ended experiential and hands-on learning, and between training individual specialists and cultivating teams that collectively cover all areas of QISE. We describe the structural and pedagogical innovations the NRT programs have developed in response, assess what is working and what remains unresolved, and sketch 12 open problems the community will need to address as QISE graduate education scales beyond the well-resourced research universities where it has up till now been mainly concentrated. Eight concrete recommendations follow: (1) adopt the startup model of team-based training as an organizing philosophy; (2) invest immediately in sensing and communication curriculum development; (3) build student agency into program governance, not just activities; (4) establish structural mechanisms for industrial engagement rather than depending on goodwill; (5) design for sustainability from year one; (6) develop graduate-level textbooks spanning all three QISE pillars: computing, sensing, and communications; (7) establish shared outcome assessment instruments across programs; and (8) develop structured mechanisms for faculty professional development in QISE.
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All authors Yohannes Abate, Victor Acosta, Alessandro Alabastri, Mehmet Aydeniz, Viktoriia E. Babicheva, Lincoln D. Carr, I-Tung Chen, Wandi Ding, Tara Drake, Mattias Fitzpatrick, Kai-Mei C. Fu, Jay Gupta, Kaden R. A. Hazzard, Sophia E. Hayes, Jin Hu, Hilary M. Hurst, Sohrab Ismail-Beigi, Ehsan Khatami, Junichiro Kono, Cheng-Yu Lai, Xiuling Li, Yingmei Liu, Sara Mouradian, Kater Murch, Borja Peropadre, Zoe Phillips, Daniela R. Radu, Akshay Sawhney, James Saslow, James Scoville, Meenakshi Singh, George Siopsis, David Weld, Chee Wei Wong
Quantum Physics · Cross-listed from physics.atom-ph
Design and fabrication of a micro-ion trap with a 3D-printed loading zone for improved hot-ion capture
Sayan Patra, Abhinav Parakh, Xiaoxing Xia et al. (+3 more)
We leverage recent advances in 3D-printing technology to design and fabricate a micro-ion trap with a spatially distinct loading zone for more efficient loading of ions from effusive thermal ovens. The design reduces the Mathieu-$q$ parameter in the loading zone by increasing the ion-electrode separation $r_0$, thereby potentially facilitating more effective
laser cooling of hot ions. This circumvents the temporary thermal instability that arises when the rf potential is reduced during ion loading, a common practice to enable efficient
laser cooling of hot ions. Simulations predict that expanding $r_0$ maintains a high trapped ion fraction from a simulated thermal source across a wide range of Mathieu-$q$ parameters. We demonstrate the manufacturability of this design by 3D-printing the rf rails of a four-rod ion trap and discuss the limitations imposed by state-of-the-art additive manufacturing techniques. We briefly compare hot-ion capture in the three-dimensional design presented here with that in a representative planar trap, illustrating one instance in which the former may be better for loading. The article concludes with an outlook for how this design may be incorporated into a
quantum-CCD architecture to enhance ion loading and reduce associated experimental overheads.
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All authors Sayan Patra, Abhinav Parakh, Xiaoxing Xia, Juergen Biener, Hartmut Häffner, Kristin M. Beck
Quantum Physics
In-Situ Measurement of Beam Divergence in a High Efficiency SNSPD Platform
Daniel W. Sorensen, Martin J. Stevens, Lynden K. Shalm et al. (+1 more)
We implement a time-of-flight imaging technique utilizing a differential-readout SNSPD to spatially resolve detection events in a fiber-coupled detector platform. We measure the spatial detection profiles for ultra-high numerical aperture fiber, standard single-mode fiber, and thermally-expanded core fiber (mode-field diameters 4.1{\mu}m, 10.4{\mu}m, 30{\mu}m respectively) in an active area surrounded by an all-dielectric optical stack designed for near-unity detection efficiency. We see no beam divergence in all but the smallest fiber optic modes. This contradicts previously-held beliefs that beam divergence during the detection process necessitates activate areas much larger than coupled optical modes, opening new paths toward smaller and better-optimized detectors.
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All authors Daniel W. Sorensen, Martin J. Stevens, Lynden K. Shalm, Dileep V. Reddy
Quantum Physics
Spin Chains for Quantum Information Processing
Eduardo K. Soares
Classical computation relies heavily on information manipulation. Each component of a hardware needs to communicate with others, and this is done by encoding information into strings of bits and application of logical operations. When dealing with
quantum technologies, there arises a new set of paradigms and devices, based on manipulations of
qubits, the
quantum analogues of conventional bits. This work investigates the generation and distribution of
quantum entanglement, a uniquely non-classical correlation, across
spin chains, which serve as promising platforms for
quantum information processing. We systematically compare two distinct
entanglement generation protocols: Protocol 1 (P1), based on alternating weak and strong couplings that create a band structure enabling an effective trimer-model approximation, and Protocol 2 (P2), which employs symmetric boundary couplings and virtual excitations to establish a direct effective interaction between the chain ends. Our results demonstrate that a protocol based on virtual excitations and optimized boundary couplings consistently outperforms its counterpart in speed, achieved
entanglement, and robustness against fabrication imperfections and noise. Furthermore, by employing effective model reductions and open
quantum systems techniques we provide a comprehensive framework for understanding the resilience of distributed
entanglement in solid-state
quantum devices. The characteristics of the virtual-coupling protocol highlight its potential for experimental implementation in scalable
quantum technologies.
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Quantum Physics
The extended Wigner's friend, many- and single-worlds and reasoning from observation
Andrew Steane
The concept of an isolated system, and Frauchiger and Renner's extended `Wigner's friend' scenario are discussed. It is argued that: (i) it is questionable whether the approximation of the isolated system is valid when measurement-like processes are involved; (ii) one may infer, from Frauchiger and Renner's thought-experiment, and similar thought-experiments, that any interpretation of
quantum theory involving *subjective collapse* fails; (iii) this does not distinguish single-world from many-world (relative-state) interpretations of
quantum theory; (iv)
reasoning from observations has to take into account the possible
quantum-erasure of those observations if it is to be valid
reasoning; (v) a single-world interpretation is valid if certain kinds of outcome are not
quantum-erased in the future.
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Quantum Physics
Mesoscopic Regimes of Temporal Entanglement in Ergodic Quantum Systems
Sergio Cerezo-Roquebrún, Jan Thorben Schneider, Stefano Carignano et al. (+4 more)
We study temporal correlations in interacting
quantum systems through the influence functional of a half-infinite
quantum Ising chain. Using Rényi
entropies and temporal mutual information, we confirm that integrable dynamics is captured by the quasiparticle picture. In contrast, generic ergodic
Hamiltonian dynamics exhibits pronounced deviations from random-
circuit universality, and its generalization including a
symmetry accounting for energy conservation. Instead, we find a long mesoscopic regime suggestive of a slow spectral reorganization of the influence functional. Our results reveal a rich temporal structure in generic
Hamiltonian dynamics and point to limitations of existing random-
circuit paradigms at experimentally and numerically relevant timescales.
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All authors Sergio Cerezo-Roquebrún, Jan Thorben Schneider, Stefano Carignano, Aleix Bou-Comas, Mari Carmen Bañuls, Esperanza López, Luca Tagliacozzo
Quantum Physics
Higher-order quantum processes respecting closed labs in a spacetime have quantum controlled causal order
Matthias Salzger and V. Vilasini
In
quantum causality and
quantum information, there is a vast landscape of abstract
quantum protocols permitting cyclic or non-acyclic causal structures between operations, including frameworks for indefinite causal order and higher-order
quantum processes such as process
matrices. A longstanding open question is what is the largest class of abstract processes that admit physical realisations without post-selection. In this work, we provide a rigorous answer using a top-down approach grounded in relativistic causality principles. Building on the framework of causal boxes, which characterise the most general
quantum information-processing protocols compatible with fixed background spacetimes, we formalise additional constraints (Acting Once + Local Order) capturing the closed-laboratory assumptions of the process
matrix framework at a fine-grained spacetime level. We prove that any protocol in a classical acyclic spacetime satisfying these conditions is behaviourally equivalent to a
quantum circuit with
quantum control of causal order (QC-QC), providing a top-down derivation of QC-QCs from physical principles. Our results show that QC-QCs constitute precisely the class of higher-order
quantum processes, including those with indefinite order, that can be physically realised within classical spacetime, ruling out more general non-causal processes under the closed-labs assumption. This clarifies the relationship between abstract higher-order process
matrix frameworks and experimentally accessible
quantum protocols, as well as the interplay between coarse-grained cyclic and fine-grained acyclic operational causal structures. We also develop characterisation techniques for process box protocols that lead to new causality-based open questions concerning spacetime
quantum protocols and relativistic
quantum experiments.
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Quantum Physics
Quantum trajectory simulation of two-dimensional non-equilibrium steady states with a trapped ion quantum processor
Anna Dalmasso, Arash Jafarizadeh, Julian Boesl et al. (+8 more)
Digital
quantum computers offer a promising route for studying complex many-body systems that are otherwise inaccessible by their classical counterparts. Capabilities including mid-
circuit measurements and feedback allow for simulating the dynamics of interacting open
quantum systems. Using the Quantinuum System Model H1 trapped-ion
quantum computer, we experimentally realise
quantum trajectories for a two-dimensional system of (interacting) particles-hard-core
bosons or
fermions-undergoing
stochastic driving at a source and drain at opposite corners of a square
lattice. We study the non-
equilibrium steady state with persistent current resulting from the this in/out flow of particles. The particle statistics, presence of interactions, and introduction of a
magnetic field produce measurable effects on the steady state. Our findings highlight the rich physics in this corner driven two-dimensional setup and showcases both the power and current limitations of
quantum computers as a platform to study it.
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All authors Anna Dalmasso, Arash Jafarizadeh, Julian Boesl, Jared Jeyaretnam, Sheng-Hsuan Lin, Andrew G. Green, Frank Pollmann, Michael Knap, Juan P. Garrahan, Henrik Dreyer, Adam Gammon-Smith
Quantum Physics
Quantum metrology via partial quantum error correction
Yinan Chen, Zongyuan Wang and Sisi Zhou
We introduce a
quantum metrology scheme in which partial
quantum error correction suppresses local noise while preserving sensitivity beyond the standard
quantum limit. The probes are superpositions of energetically distinct stabilizer sectors, whose sensing coherence is protected by measuring only a subset of local checks. We propose an adaptive weight strategy to balance two competing effects. Increasing the stabilizer-generator weight suppresses parallel noise to $O(p^{\delta})$, with $\delta$ increasing with the weight, but also increases susceptibility to perpendicular noise. Our main
theorem establishes a sufficient noise condition under which adapting this weight to the system size preserves superlinear
quantum Fisher information under simultaneous independent $X$ and $Z$ noise. We illustrate this strategy using repetition, Bacon-Shor, and toric codes, and analyze noise both before and after phase imprinting. In all examples, the error-correction checks and phase-imprinting operators are local and require no nonlocal connectivity.
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Quantum Physics · Cross-listed from hep-ph
Entanglement Requirements for Coherent Enhancement in Detectors
Zachary Bogorad and Roni Harnik
Coherent enhancement is a powerful mechanism for improving the sensitivity of a wide range of detectors, but its practical use is often limited by the difficulty of preparing the required
quantum states. We show that this difficulty has a fundamental origin: coherent enhancement of a signal interacting with a detector is quantitatively constrained by
entanglement. We prove general bounds on how the strength of coherent effects can scale with system size, as a function of the single-mode
entanglement entropy of the detector. These bounds smoothly interpolate between the incoherent and fully coherent regimes, and apply both to parameter-estimation problems and to scattering processes. We discuss these results from two complementary perspectives: First, they appear as bounds on the
quantum Fisher information of many-body states, which translate directly into limits on parameter sensitivity via the
quantum Cramér-Rao bound. Second, they can be interpreted as limits on a class of scattering cross sections, leading to predictions for how minimum detectable interaction strengths scale with target size. Together, these results provide a unified view of coherent enhancement in metrology and scattering experiments, and motivate the development of new techniques for generating entangled detector states.
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Quantum Physics
Optimal FALQON for Quantum Approximate Optimization via Layer-wise Parameter Tuning
Michael Mancini and Shabnam Sodagari
Feedback-based adaptive
quantum optimization (FALQON) is a promising approach for solving combinatorial problems on noisy intermediate-scale
quantum (NISQ) devices, requiring only single
circuit evaluations per layer. However, standard FALQON relies on fixed hyperparameters that severely limit convergence speed, requiring hundreds to thousands of layers for acceptable solutions. This paper proposes Optimal FALQON, an optimization-based formulation that treats the per-layer time step ($\delta_k$) and scaling factor ($M_k$) as decision variables optimized via classical methods. We present a comprehensive empirical study on all 94 non-isomorphic 3-regular graphs with 12 vertices, comparing Optimal FALQON with standard FALQON and multiple QAOA variants. Results demonstrate statistically significant improvements in success probability, evaluation efficiency, and depth-normalized cost across the evaluated
benchmarks. Furthermore, initializing QAOA with parameters from Optimal FALQON yields superior warm-start performance compared to fixed initialization.
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Quantum Physics · Cross-listed from physics.bio-ph
Chiral-Induced Spin Selectivity Regulates Triplet formation in Heliobacterial Photosynthesis
Parul Raghuvanshi and Vishvendra Singh Poonia
Triplet formation and its regulation have always been of central interest in understanding the photophysical behavior of living systems. In organic systems, excessive triplet formation poses significant challenges, as it can promote photochemical damage and reduce the efficiency of charge separation processes, making its regulation critically important.Here, we present a theoretical investigation of the intrinsic
quantum spin dynamics governing triplet formation in the heliobacterial reaction center, a system that operates without any internal
magnetic field. Using an open
quantum systems approach based on the Lindblad formalism, we simulate the
spin-correlated radical pair dynamics occurring during charge separation in the heliobacterial reaction center. The study systematically examines how triplet formation is regulated by variations in two key parameters, hyperfine coupling strengths and recombination rates, and how this regulation is further influenced by the inclusion of chirality-induced
spin selectivity (CISS) in conjunction with the radical pair mechanism (RPM). Our results demonstrate that the CISS effect significantly suppresses triplet formation across the parameter space relevant to the heliobacterial molecular environment, revealing an intrinsic
quantum protective mechanism operating through
spin control in heliobacterial photosynthesis.
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Quantum Physics
Advances in quantum learning theory with bosonic systems
Francesco Anna Mele
This paper reviews recent advances in
quantum learning theory for continuous-variable (CV) systems.
Quantum learning theory investigates how to extract classical information from
quantum systems as efficiently as possible. CV systems are ubiquitous in nature and in
quantum technologies, as they describe bosonic and
quantum-optical systems. While
quantum learning theory for finite-dimensional systems has been extensively studied, the corresponding theory for CV systems has only recently begun to develop; here we provide a concise review. We focus on the following questions: what is the minimum number of copies (the sample complexity) required to learn a non-Gaussian state, possibly under energy constraints? What is the sample complexity for learning Gaussian states? How does the performance of CV state learning depend on non-Gaussianity? How can one test whether a state is Gaussian or far from the set of Gaussian states? And how can Gaussian processes be learned efficiently? Central to these topics, we also review several bounds on the trace distance between CV states in terms of their covariance
matrices, which may be of independent interest. Overall, this work summarises selected developments in
tomography of CV systems and highlights a selection of open problems.
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Quantum Physics
Andrew Steane and Haru Ishizaka
The structure of
entanglement in the ground state of the harmonic chain is studied. A class of two-mode squeezed states, useful for this purpose, is identified. The
entanglement of the local modes at the ends of the chain, after tracing out the centre, rapidly falls to zero as the length of the chain increases. However, if the central modes are measured, and the result communicated to systems interacting with the outer modes, the latter exhibit greatly enhanced
entanglement, including in conditions where none was otherwise available. These ideas can be demonstrated in experiments in trapped ions, among other systems. The extension to the continuous case yields enhanced
entanglement extracted from the
vacuum state of a bosonic
quantum field.
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Quantum Physics
Covert Signaling for Communication and Sensing over the Bosonic Channels
Tianrui Tan, Evan J.D. Anderson, Michael S. Bullock et al. (+1 more)
Preventing signal detection in communication and active sensing requires careful control of transmission power. In fact, the square-root laws (SRL) for covert classical and
quantum communication and sensing prescribe that the average output energy per channel use scales as $1/\sqrt{n}$ for $n$ channel uses. Diffuse and sparse signaling achieve this. The former transmits signals whose energy decays as $1/\sqrt{n}$ over all $n$ channel uses, which is convenient for mathematical analysis. The latter transmits constant-energy signals only approximately $\propto\sqrt{n}$ times out of $n$ channel uses, remaining silent on the others. This offers significant practical advantages in compatibility with modern digital transmitters. Here, we study sparse signaling over the lossy thermal-noise bosonic channel, which is a
quantum model of many practical channels (including optical, microwave, and
radio-frequency). We characterize the input signal state that minimizes detectability. We find an unintuitive optimal
quantum state structure: a mixture of just two consecutive
photon-number states. In particular, in the low-brightness regime, the optimal signal state is a mixture of
vacuum and a single
photon. Since these states are generally suboptimal for both communication and active sensing, we explore the resulting trade-off and identify input-power thresholds for transitions between optimizing covertness and performance in communication and sensing tasks.
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All authors Tianrui Tan, Evan J.D. Anderson, Michael S. Bullock, Boulat A. Bash
Quantum Physics
Boundary-localized non-Hermitian modes in an absorbing quantum walk
Francisco Riberi
We introduce and solve from first principles a continuous-time
quantum walk with absorption generated by a Lindblad boundary sink of arbitrary strength. Projecting onto the surviving walker sector maps the problem to a non-Hermitian tight-binding
Hamiltonian with a rank-one imaginary defect on the semi-infinite line. We obtain closed-form expressions for the exact propagator, from which the first-passage statistics follow. Weak coupling limits absorption through inefficient transfer into the sink, whereas for strong dissipation, occupation at the boundary is suppressed by Zeno-like reflection, accompanied by the emergence of a localized non-Hermitian mode. Despite the different physical origins of these suppression mechanisms, their respective absorption probabilities exhibit an asymptotic duality. The
evolution is conveniently visualized in phase space, where the localized mode has an exponentially confined Wigner representation near the edge site.
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Quantum Physics · Cross-listed from hep-th
The two-flavor Schwinger model at 50: Solving Coleman's puzzles
Gabriel Cuomo, Ross Dempsey, Andrei Katsevich et al. (+4 more)
In his 1976 paper "More about the massive Schwinger model", Coleman introduced $1+1$-dimensional
Quantum Electrodynamics coupled to two charged massive
fermions. By applying Abelian bosonization, he elucidated much of the physics of this two-
flavor Schwinger model, but he listed three puzzles at the end of his paper. We present new analytical and numerical calculations to solve Coleman's three puzzles and thereby deepen our understanding of this model. These puzzles pertain to the theory with equal
fermion masses at $\theta = 0$ and at $\theta = \pi$, as well as the size of isospin-breaking effects when the
fermion masses are unequal. For the puzzle at $\theta = \pi$, the solution is related to the structure of the zero-temperature phase diagram arXiv:2305.04437: for equal
fermion masses $m$, the model exhibits spontaneous breaking of charge conjugation
symmetry and absence of confinement for any value of the gauge coupling $g$, so that there is a smooth interpolation from weak to strong coupling. Using two-loop Renormalization Group and integrability methods, we show that the mass gap behaves as $\sim m e^{-0.111 g^2/m^2}$ in the strong coupling regime $m\ll g$. Our numerical results using the
lattice Hamiltonian are in good agreement with this behavior. For the puzzle at $\theta = 0$, the solution is related to a level crossing between two isosinglet particles with different discrete
quantum numbers; we demonstrate the necessity of such a crossing by comparing integrability and weak coupling calculations, and we also exhibit the crossing numerically. Finally, we provide a new estimate for the size of isospin-breaking effects caused by different
fermion masses at strong coupling.
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All authors Gabriel Cuomo, Ross Dempsey, Andrei Katsevich, Igor R. Klebanov, Ilia V. Kochergin, Silviu S. Pufu, Benjamin T. Søgaard
Quantum Physics
Analytical Solution to the Kronig-Penney Model with Harmonic Oscillator Wells: Insights to Tight-Binding
Christopher Moore and Frank Marsiglio
The celebrated Kronig-Penney model traditionally has been formulated with square well potentials representing atomic centres. Here, we use a slightly more realistic potential, the truncated harmonic oscillator, in lieu of square well potentials, and solve the model analytically. We derive the energy dispersion and wave functions for this model. This configuration has some important similarities and differences compared to the usual model. In particular, we write the governing equation in a form suggestive of the tight-binding approximation, as can be done for the usual model. In this way, it is straightforward to derive an expression for the tunneling amplitude used in tight-binding in terms of the harmonic oscillator potential parameters.
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Quantum Physics
Generalized master equation for driven quantum oscillators: microscopic origin of nonlinear dissipation and asymmetric resonances
Jakob Wagner, Jeff Maki, Oded Zilberberg et al. (+1 more)
Driven nonlinear
quantum oscillators are a central platform for
quantum technologies, yet their dissipative dynamics are typically described using Lindblad or Caldeira-Leggett master equations derived under assumptions that exclude nonlinearities and driving. Here, we derive a generalized Caldeira-Leggett master equation for driven nonlinear oscillators by retaining the full nonlinear and time-dependent system dynamics in the construction of the dissipator. For position- and momentum-dependent system-bath coupling, the dissipator itself becomes dynamically dressed, generating nonlinear and drive-dependent dissipative channels beyond conventional fixed-dissipator approaches. This produces nonlinear damping together with dissipation-induced corrections to the effective drive. The resulting dissipative dynamics suppress large-amplitude excitations and reduce phase-space fluctuations. For a driven Kerr oscillator, this leads to the suppression of bistability, asymmetric resonance responses, and strongly modified fluctuation distributions. More broadly, our results establish a microscopic framework in which nonlinear dynamics and driving directly reshape the dissipative sector of driven open
quantum systems.
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All authors Jakob Wagner, Jeff Maki, Oded Zilberberg, Kilian Seibold
Quantum Physics
Error Correction of Beamsplitter-Generated Entangled GKP States
Moritz Fontboté-Schmidt, Jeremy Metzner, Florence Berterottière et al. (+8 more)
To be useful,
quantum computers will be required to successfully correct errors occurring at the hardware level. Bosonic codes provide a hardware-efficient option for error correction, but fault-tolerance further requires that the available gate interactions be compatible with the code. A promising bosonic code is the Gottesman-Kitaev-Preskill (GKP) code, for which a linear beamsplitter-like coupling between two bosonic modes is fault-tolerant, making this a key primitive for building larger systems. Here, using two motional modes of a trapped ion, we demonstrate the generation of entangled states of GKP
qubits by interfering two qunaught states, which have a grid structure but carry no logical information, on a beamsplitter. We generate all four Bell states with an average fidelity of 69%, and subsequently demonstrate an extension of the entangled state lifetime through the use of
quantum error correction. These results complete the set of Gaussian operations required for
quantum computing with GKP codes and enable explorations of multi-mode bosonic encodings as well as fundamental tests of information channels.
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All authors Moritz Fontboté-Schmidt, Jeremy Metzner, Florence Berterottière, Ivan Rojkov, Alexander Ferk, Martin Stadler, Bahadir Dönmez, Ralf Berner, Stephan Welte, Daniel Kienzler, Jonathan P. Home
Quantum Physics
Price and Payoff: Non-Determinism in Fault Tolerant Quantum Computation
Aditi Awasthi, Sayam Sethi, Sahil Khan et al. (+2 more)
A promising approach to achieving scalable fault-tolerant
quantum computation is the use of
quantum error correction (QEC) codes augmented with magic states i.e. resource states produced via
distillation, cultivation, or $R_z$ synthesis and teleported into the
circuit as needed. Because magic-state production dominates the space-time volume of fault-tolerant programs, system architects must decide how many production units to allocate. Current approaches rely on deterministic analysis that either provisions for worst-case peak demand (wasting valuable
qubit resources on factories that are never simultaneously utilized) or assumes average demand, which increases execution time. In this work, we build a simulation framework that couples
circuit scheduling with different
stochastic magic state production models, and use it to quantify the impact of non-determinism on
circuit execution. We show that non-determinism has a dual effect that deterministic models cannot capture: it inflates total execution time (the price), while deflating peak per-cycle resource demand (the payoff). For
distillation-based architectures, this demand smoothing shifts the space-time-optimal provisioning point: fewer factories are needed to minimize space-time volume than deterministic analysis predicts. Across
benchmarks,
stochastic-aware provisioning reduces space-time volume by up to 27% compared to the deterministic optimum for
distillation, while requiring up to 30% fewer factories. We characterize these effects across each preparation mechanism, map the resulting design-space tradeoffs, and demonstrate that static resource estimation systematically mis-characterizes the cost of fault-tolerant execution. Our results establish that
stochastic-aware analysis is necessary for right-sizing the factory allocations and should replace deterministic heuristics as the standard methodology for FTQC resource planning.
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All authors Aditi Awasthi, Sayam Sethi, Sahil Khan, Gokul Subramanian Ravi, Jonathan Mark Baker
Quantum Physics
Reducibility of native weighted graphs on Rydberg Arrays
J. Kombe and J. D. Pritchard
We investigate the classical reducibility of random unit-disk graph (UDG) instances of the maximum independent set (MIS) and maximum weighted independent set (MWIS) problems, which can be natively realised in Rydberg atom
quantum processors. Using state-of-the-art kernelisation techniques, we systematically probe how far classical preprocessing can simplify such native optimisation problems of varying size and connectivity. While many small or sparse instances can be fully reduced, dense graphs often retain finite irreducible kernels even after extensive reductions. Introducing vertex weights tends to increase reducibility, whereas extending the interaction range in the underlying UDG connectivity suppresses the reduction efficiency. By exploring where classical reductions cease to be effective, we aim to delineate the regime of problem instances that remain computationally demanding - those most relevant for testing and benchmarking near-term
quantum optimisation hardware. We find that for the remaining finite kernels,
quantum execution would require non-native
embeddings with substantial resource overheads, suggesting that directly running native instances may be more practical than
embedding a reduced kernel.
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