Quantum Physics
Engineering the non-Hermitian Su-Schrieffer-Heeger model with skin effects in Rydberg atom arrays
J. N. Bai, F. Yang, D. Yan et al. (+2 more)
We propose and systematically analyze a practical scheme for implementing a one-dimensional non-Hermitian (NH) Su-Schrieffer-Heeger model using individually addressable Rydberg atom arrays. Our setup consists of an atomic chain with three-atom unit
cells, in which a synthetic gauge field is generated by applying multicolor
laser fields. By engineering fast dissipative channels for one auxiliary atom in each unit
cell, adiabatic elimination effectively gives rise to a NH skin effect. We examine how fluctuations in the experimental parameters influence both the skin effect and the topological invariant in real space and find that both features remain highly robust. This work establishes a versatile, controllable, and programmable open-system
quantum simulator with neutral atoms, providing a clear route for exploring rich NH topological phenomena.
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All authors J. N. Bai, F. Yang, D. Yan, Weibin Li, X. Q. Shao
Quantum Physics
Krypton-sputtered tantalum films for scalable high-performance quantum devices
Maciej W. Olszewski, Lingda Kong, Simon Reinhardt et al. (+9 more)
Superconducting
qubits based on tantalum (Ta) thin films have demonstrated the highest-performing microwave resonators and
qubits. This makes Ta an attractive material for superconducting
quantum computing applications, but, so far, direct deposition has largely relied on high substrate temperatures exceeding \SI{400}{\celsius} to achieve the body-centered cubic phase, BCC (\textalpha-Ta). This leads to compatibility issues for scalable fabrication leveraging standard
semiconductor fabrication lines. Here, we show that changing the sputter gas from argon (Ar) to krypton (Kr) promotes BCC Ta synthesis on silicon (Si) at temperatures as low as \SI{200}{\celsius}, providing a wide process window compatible with back-end-of-the-line fabrication standards. Furthermore, we find these films to have substantially higher electronic conductivity, consistent with clean-limit superconductivity. We validated the microwave performance through coplanar waveguide resonator measurements, finding that films deposited at \SI{250}{\celsius} and \SI{350}{\celsius} exhibit a tight performance distribution at the state of the art. Higher temperature-grown films exhibit higher losses, in correlation with the degree of Ta/Si intermixing revealed by cross-sectional transmission electron microscopy. Finally, with these films, we demonstrate transmon
qubits with a relatively compact, \SI{20}{\micro\meter} capacitor gap, achieving a median quality factor up to 14 million.
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All authors Maciej W. Olszewski, Lingda Kong, Simon Reinhardt, Daniel Tong, Xinyi Du, Gabriele Di Gianluca, Haoran Lu, Saswata Roy, Luojia Zhang, Aleksandra B. Biedron, David A. Muller, Valla Fatemi
Quantum Physics
Spectral Transitions and Singular Continuous Spectrum in A New Family of Quasi-periodic Quantum Walks
Xinyu Yang, Long Li and Qi Zhou
This paper introduces and rigorously analyzes a new class of one-dimensional discrete-time
quantum walks whose dynamics are governed by a parametrized family of extended CMV
matrices. The model generalizes the
unitary almost Mathieu operator (UAMO) and exhibits a richer spectral phase diagram, closely resembling the extended Harper's model. It provides the first example of a solvable quasi-periodic
quantum walk that exhibits a stable region of purely singular continuous spectrum.
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Quantum Physics
Local Distinguishability of Multipartite Orthogonal Quantum States: Generalized and Simplified
Ian George and Mohammad A. Alhejji
In a seminal work [PRL85.4972], Walgate, Short, Hardy, and Vedral prove in finite dimensions that for every pair of pure multipartite orthogonal
quantum states, there exists a one-way local operations and classical communication (LOCC) protocol that perfectly distinguishes the pair. We extend this result to infinite dimensions with a simpler proof. For states on $\mathbb{C}^{d_A \times d_A} \otimes \mathbb{C}^{d_B \times d_B}$, we strengthen this existence result by constructing an $O(d_A^2 d_B^2)$-time
algorithm that specifies such a perfect one-way LOCC protocol. Finally, we establish the equivalence between Walgate et al.'s result and the fact that the one-shot environment-assisted classical capacity of every
quantum channel is at least 1 bit per channel use, thereby clarifying the literature on these notions. At the core of all of these results is the fact that every operator with vanishing trace admits a basis where its diagonal entries are all zero.
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Quantum Physics
Ensemble-Based Quantum Signal Processing for Error Mitigation
Suying Liu, Yulong Dong, Dong An et al. (+1 more)
Despite rapid advances in
quantum hardware, noise remains a central obstacle to deploying
quantum algorithms on near-term devices. In particular, random coherent errors that accumulate during
circuit execution constitute a dominant and fundamentally challenging noise source. We introduce a noise-resilient framework for
Quantum Signal Processing (QSP) that mitigates such coherent errors without increasing
circuit depth or ancillary
qubit requirements. Our approach uses ensembles of noisy QSP
circuits combined with measurement-level averaging to suppress random phase errors in Z rotations. Building on this framework, we develop robust QSP
algorithms for implementing
polynomial functions of Hermitian
matrices and for estimating observables, with applications to
Hamiltonian simulation,
quantum linear systems, and ground-state preparation. We analyze the trade-off between approximation error and hardware noise, which is essential for practical implementation under the stringent depth and coherence constraints of current
quantum hardware. Our results establish a practical pathway for integrating error mitigation seamlessly into algorithmic design, advancing the development of robust
quantum computing, and enabling the discovery of scientific applications with near- and mid-term
quantum devices.
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All authors Suying Liu, Yulong Dong, Dong An, Murphy Yuezhen Niu
Quantum Physics
Comment on "Determining angle of arrival of radio-frequency fields using subwavelength, amplitude-only measurements of standing waves in a Rydberg atom sensor"
M. Chilcott and N. Kjærgaard
We discuss the consequence of excluding allowed RF-transition between substates of a field-dressed Rydberg
manifold when predicting the spectrum that will be observed if the dressed system is probed in an optical EIT scheme.
Quantum Physics · Cross-listed from cond-mat.quant-gas
Superfluidity in the spin-1/2 XY model with power-law interactions
Muhammad Shaeer Moeed, Costanza Pennaforti, Adrian Del Maestro et al. (+1 more)
In trapped-ion
quantum simulators, effective
spin-1/2 XY interactions can be engineered via
laser-induced coupling between internal atomic states and collective
phonon modes. In the simplest one-dimensional ($1d$) traps, these interactions decay as a power-law with distance $1/r^{\alpha}$, with a tunable exponent $\alpha$. For small $\alpha$, the resulting long-range $1d$ XY model exhibits continuous
symmetry breaking, in marked contrast to its nearest neighbor counterpart. In this paper, we examine this model near the phase transition at $\alpha_c$ from the lens of the
spin stiffness, or superfluid density. We develop a
stochastic series expansion (SSE)
quantum Monte Carlo (QMC) simulation and a generalized winding number
estimator to measure the superfluid density in the presence of power-law interactions, which we test against exact diagonalization for small
lattice sizes. Our results show how conventional superfluidity in the $1d$ XY model is enhanced in the long-range interacting regime. This is observed as a diverging superfluid density as $\alpha \rightarrow 0$ in the thermodynamic limit, which we show is consistent with linear
spin-wave theory. Finally, we define a normalized superfluid density
estimator that clearly distinguishes the short, medium, and long-range interacting regimes, providing a novel QMC probe of the critical value $\alpha_c$.
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All authors Muhammad Shaeer Moeed, Costanza Pennaforti, Adrian Del Maestro, Roger G. Melko
Quantum Physics
Quantum Channels on Graphs: a Resonant Tunneling Perspective
Giuseppe Catalano, Farzad Kianvash and Vittorio Giovannetti
Quantum transport on structured networks is strongly influenced by interference effects, which can dramatically modify how information propagates through a system. We develop a
quantum-information-theoretic framework for scattering on graphs in which a full network of connected scattering sites is treated as a
quantum channel linking designated input and output ports. Using the Redheffer
star product to construct global scattering
matrices from local ones, we identify resonant concatenation, a nonlinear composition rule generated by internal back-reflections. In contrast to ordinary channel concatenation, resonant concatenation can suppress noise and even produce super-activation of the
quantum capacity, yielding positive capacity in configurations where each constituent channel individually has zero capacity. We illustrate these effects through models exhibiting resonant-tunneling-enhanced transport. Our approach provides a general methodology for analyzing coherent information flow in
quantum graphs, with relevance for
quantum communication, control, and simulation in structured environments.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Correlated dynamics of three-particle bound states induced by emergent impurities in Bose-Hubbard model
Wenduo Zhao, Boning Huang, Yongguan Ke et al. (+1 more)
Bound states, known as particles tied together and moving as a whole, are profound correlated effects induced by particle-particle interactions. While dimer-monomer bound states are manifested as a single particle attached to a dimer bound pair, it is still unclear about
quantum walks and Bloch oscillations of dimer-monomer bound states. Here, we revisit three-particle bound states in the Bose-Hubbard model and find that interaction-induced
impurities adjacent to bound pair and boundaries cause two kinds of bound states: one is dimer-monomer bound state and the other is bound edge state. In
quantum walks, the spread velocity of dimer-monomer bound state is determined by the maximal group velocity of their energy band, which is much smaller than that in the single-particle case. In Bloch oscillations, the period of dimer-monomer bound states is one third of that in the single-particle case. Emergence of bound edge states also requires that interaction-induced defects are greater than the effective tunneling strength of three-particle bound state. Our work provides new insights to basic mechanics and collective dynamics of three-particle bound states.
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All authors Wenduo Zhao, Boning Huang, Yongguan Ke, Chaohong Lee
Quantum Physics
A Cyclic Layerwise QAOA Training
Enhyeok Jang, Zihan Chen, Dongho Ha et al. (+6 more)
The
quantum approximate optimization
algorithm (QAOA) is a hybrid
quantum-classical
algorithm for solving combinatorial optimization problems. Multi-angle QAOA (MA-QAOA), which assigns independent parameters to each
Hamiltonian operator term, achieves superior approximation performance even with fewer layers than standard QAOA. Unfortunately, this increased expressibility can raise the classical computational cost due to a greater number of parameters. The recently proposed Layerwise MA-QAOA (LMA-QAOA) reduces this overhead by training one layer at a time, but it may suffer from obtaining the precise solution due to the previously fixed parameters. This work addresses two questions for efficient MA-QAOA training: (i) What is the optimal granularity for parameter updates per epoch, and (ii) How can we get precise final cost function results while only partially updating the parameters per epoch? Despite the benefit of reducing the parameters that update per epoch can reduce the classical computation overhead, too fine or coarse a granularity of
Hamiltonian update can degrade the MA-QAOA training efficiency. We find that optimizing one complete layer per epoch is an efficient granularity. Moreover, selectively retraining each layer by tracking gradient variations can achieve a final cost function equivalent to the standard MA-QAOA while lowering the parameter update overhead. Based on these insights, we propose Orbit-QAOA, which cyclically revisits layers and selectively freezes stabilized parameters. Across diverse graph
benchmarks, Orbit-QAOA reduces training steps by up to 81.8%, reduces approximation ratio error by up to 72x compared to the unified stop condition-applied enhanced LMA-QAOA, and achieves equivalent approximation performance compared to the standard MA-QAOA.
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All authors Enhyeok Jang, Zihan Chen, Dongho Ha, Seungwoo Choi, Yongju Lee, Jaewon Kwon, Eddy Z. Zhang, Yipeng Huang, Won Woo Ro
Quantum Physics
Foundry-Enabled Patterning of Diamond Quantum Microchiplets for Scalable Quantum Photonics
Jawaher Almutlaq, Alessandro Buzzi, Anders Khaykin et al. (+6 more)
Quantum technologies promise secure communication networks and powerful new forms of information processing, but building these systems at scale remains a major challenge. Diamond is an especially attractive material for
quantum devices because it can host atomic-scale defects that emit single
photons and store
quantum information with exceptional stability. However, fabricating the optical structures needed to control light in diamond typically relies on slow, bespoke processes that are difficult to scale. In this work, we introduce a manufacturing approach that brings diamond
quantum photonics closer to industrial production. Instead of sequentially defining each device by lithography written directly on diamond, we fabricate high-precision silicon masks using commercial
semiconductor foundries and transfer them onto diamond via microtransfer printing. These masks define large arrays of nanoscale optical structures, shifting the most demanding pattern-definition steps away from the diamond substrate, improving uniformity, yield, and throughput. Using this method, we demonstrate hundreds of diamond "
quantum microchiplets" with improved optical performance and controlled interaction with
quantum emitters. The chiplet format allows defective devices to be replaced and enables integration with existing photonic and electronic
circuits. Our results show that high-quality diamond
quantum devices can be produced using scalable, foundry-compatible techniques. This approach provides a practical pathway toward large-scale
quantum photonic systems and hybrid
quantum-classical technologies built on established
semiconductor manufacturing infrastructure.
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All authors Jawaher Almutlaq, Alessandro Buzzi, Anders Khaykin, Linsen Li, William Yzaguirre, Maxim Sirotin, Gerald Gilbert, Genevieve Clark, Dirk Englund
Quantum Physics
Alternating ZX Circuit Extraction for Hardware-Adaptive Compilation
Ludwig Schmid, Korbinian Staudacher and Robert Wille
We present a novel
quantum circuit extraction scheme that tightly integrates graph-like ZX diagrams with hardware-adaptive routing. The method utilizes the degrees of freedom during the conversion from a ZX diagram to a
quantum circuit (extraction). It alternates between generating multiple extraction options and evaluating them based on hardware constraints, allowing the routing
algorithm to inform and guide the extraction process. This feedback loop extends existing graph-like ZX extraction and supports modular integration of different extraction
algorithms, routing strategies, and target hardware, making it a versatile building block during compilation. To perform numerical evaluations, a reference instance of the scheme is implemented with SWAP-based routing for neutral atom hardware and evaluated using various
benchmark collections on small-to mid-scale
circuits. The reference code is available as open-source, allowing fast integration of other extraction and/or routing tools to stimulate further research and foster improvements of the proposed scheme.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Scattering-state theory of open Floquet lattices: transfer matrices, branch openness, and robust asymmetry
Ren Zhang, Xiao-Yu Ouyang, Xu-Dong Dai et al. (+1 more)
We establish a scattering-state theory for open one-dimensional Floquet
lattices based on a frequency-domain transfer-
matrix formulation. For real quasienergy, the conjugate-
symplectic structure of the transfer
matrix separates bulk Floquet–Bloch modes into propagating and evanescent sectors, enabling a consistent treatment of interface matching and the shrinking-window smoothing required for long-sample transport. By tracking how incoming states populate deep-bulk propagating branches, we define branch-resolved weights \(p_{\mu\alpha}\) and total branch weights \(p_\mu\). We prove that \(p_\mu\) equals the escape probability of a wave packet initialized on the corresponding branch. In the open geometries considered here, true bound trapping of propagating branches is nongeneric, yielding \(p_\mu=1\) for generic parameters. This generic openness implies that long-sample transport is governed by deep-bulk branch populations rather than by boundary-sensitive interference. Consequently, the integrated left–right transmission asymmetry reduces to the net chirality, and hence the winding contribution, of an isolated Floquet band. The robust topological observable is therefore the accumulated asymmetry plateau, not the detailed transmission line shape, which remains strongly reshaped by nonadiabatic boundaries. A spatially adiabatic boundary serves only as a transparent
benchmark for resolving the branch structure, not as the origin of the topological response.
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All authors Ren Zhang, Xiao-Yu Ouyang, Xu-Dong Dai, Xi Dai
Quantum Physics
Optically Addressable Molecular Spins at 2D Surfaces
Xuankai Zhou, Yan-Tung Kong, Cheuk Kit Cheung et al. (+18 more)
Optically addressable
spins at material surfaces have represented a long-standing ambition in
quantum sensing, providing atomic resolution and
quantum-limited sensitivity. However, they are constrained by a finite depth at which the
quantum spins can be stabilized. Here, we demonstrate a hybrid molecular-2D architecture that realizes
quantum spin sensors directly on top of the surface. By anchoring
spin-active molecules onto hexagonal boron nitride (hBN), we eliminate the depth of the
quantum sensor while also exhibiting robust
spin properties from 4~K to room temperature (RT). The Hahn-echo
spin coherence time exceeds \(T_2 = 3.4~\upmu\text{s}\) at 4~K, outperforming values in bulk organic
crystals and overturning the prevailing expectation that
spin inevitably deteriorates upon approaching the surface. By chemically tuning the molecule through deuteration, \(T_2\) improves by more than 10-fold, and under dynamic decoupling, coherence is prolonged to the intrinsic lifetime limit, exceeding 300~\(\upmu\text{s}\). Proximal
proton spins and the
magnetic response of two-dimensional magnets beneath the hBN layer have been detected at RT. These molecular
spins form surface
quantum sensors with long coherence, optical addressability, and interfacial versatility, enabling a scalable, adaptable architecture beyond what conventional solid-state platforms offer.
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All authors Xuankai Zhou, Yan-Tung Kong, Cheuk Kit Cheung, Guodong Bian, Reda Moukaouine, King Cho Wong, Yumeng Sun, Cheng-I Ho, Vladislav Bushmakin, Nils Gross, Chun-Chieh Yen, Tim Priessnitz, Malik Lenger, Sreehari Jayaram, Takashi Taniguchi, Kenji Watanabe, Anton Pershin, Ruoming Peng, Ádám Gali, Jurgen Smet, Jörg Wrachtrup
Quantum Physics · Cross-listed from cond-mat.quant-gas
Stability and Decay of Macrovortices in Rotating Bose Gases Beyond Mean Field
Paolo Molignini, M. A. Caracanhas, V. S. Bagnato et al. (+1 more)
We study the formation, stability, and decay of macrovortices in a rotating Bose gas confined by a Mexican-hat potential with a multiconfigurational ansatz. By systematically including correlations beyond the mean-field level, we map the
equilibrium phase diagram and identify regimes of coexistence between
vortex lattices and multiply charge central
vortices. Quench dynamics reveals that macrovortices are robust under changes in rotation or interaction strength, sustaining clean monopole oscillations with well-separated, vorticity-dependent breathing frequencies. In contrast, trap quenches trigger a universal decay process mediated by
vortex-
phonon coupling, in which rotational energy is progressively transferred to compressible modes until the macrovortex splits into singly quantized
vortices. Our results demonstrate that macrovortex lifetimes and decay pathways can be tuned by trap confinement, providing experimentally accessible signatures of
vortex-
phonon interactions and collective energy transfer in correlated
quantum fluids.
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All authors Paolo Molignini, M. A. Caracanhas, V. S. Bagnato, Barnali Chakrabarti
Quantum Physics
Superradiant Phase is a Finite Size Effect in Two-photon Processes
Fabrizio Ramírez, David Villaseñor, Nahum Vázquez et al. (+1 more)
Two-
photon light-matter interactions exhibit distinctive features such as spectral collapse. The two-
photon Dicke model has been reported to exhibit a superradiant phase which could be useful in
quantum applications. Here we show that this superradiant phase is not a genuine thermodynamic phase but a finite-size effect. Combining analytical and numerical analyses, we demonstrate that the superradiant region shrinks with increasing system size and disappears in the thermodynamic limit, while spectral collapse remains. Our results clarify the nature of superradiant conditions in two-
photon systems and constrain its realization in
quantum platforms.
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All authors Fabrizio Ramírez, David Villaseñor, Nahum Vázquez, Jorge G. Hirsch
Quantum Physics · Cross-listed from hep-th
Timelike Entanglement Signatures of Ergodicity and Spectral Chaos
Rathindra Nath Das, Arnab Kundu and Nemai Chandra Sarkar
We investigate timelike
entanglement measures derived from the spacetime density kernel in the Rosenzweig-Porter model and show that they sharply diagnose both eigenvector ergodicity and spectral chaos. For several
Hilbert-space bipartitions, we compute the second Tsallis
entropy, the
entanglement imagitivity that quantifies non-Hermiticity, and Schatten-norm diagnostics of the kernel. The imagitivity and Frobenius norm exhibit rapid growth and high late-time plateaus in the ergodic regime, are suppressed in the localized regime, and show intermediate behavior in the fractal phase. The real part of the second Tsallis
entropy displays a spectral form factor-like dip-ramp-plateau throughout the chaotic window and a suppressed ramp in the localized regime. We further introduce a kernel negativity, defined as the negative spectral weight of the Hermitian part of the kernel. This negativity equals the trace-norm distance to the set of positive semidefinite operators and the maximal witnessable negative quasiprobability, and its time-averaged value decreases across the ergodic-fractal-localized crossover in close correspondence with the fractal dimension.
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Quantum Physics · Cross-listed from hep-th
Exploring the holographic entropy cone via reinforcement learning
Temple He, Jaeha Lee and Hirosi Ooguri
We develop a reinforcement learning
algorithm to study the
holographic entropy cone. Given a target
entropy vector, our
algorithm searches for a graph realization whose min-cut
entropies match the target vector. If the target vector does not admit such a graph realization, it must lie outside the cone, in which case the
algorithm finds a graph whose corresponding
entropy vector most nearly approximates the target and allows us to probe the location of the facets. For the $\sf N=3$ cone, we confirm that our
algorithm successfully rediscovers monogamy of mutual information beginning with a target vector outside the
holographic entropy cone. We then apply the
algorithm to the $\sf N=6$ cone, analyzing the 6 "mystery" extreme rays of the subadditivity cone from arXiv:2412.15364 that satisfy all known
holographic entropy inequalities yet lacked graph realizations. We found realizations for 3 of them, proving they are genuine extreme rays of the
holographic entropy cone, while providing evidence that the remaining 3 are not realizable, implying unknown
holographic inequalities exist for $\sf N=6$.
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Quantum Physics
A Surface-Scaffolded Molecular Qubit
Tian-Xing Zheng, M. Iqbal Bakti Utama, Xingyu Gao et al. (+24 more)
Fluorescent
spin qubits are central building blocks of
quantum technologies. Placing these
qubits at surfaces maximizes coupling to nearby
spins and fields, enabling nanoscale sensing and facilitating integration with photonic and superconducting devices. However, reducing the dimensions or size of established
qubit systems without sacrificing the
qubit performance or degrading the coherence lifetime remains challenging. Here, we introduce a surface molecular
qubit formed by pentacene molecules scaffolded on a two-dimensional (2D) material, hexagonal boron nitride (hBN). The
qubit exhibits stable fluorescence and optically detected
magnetic resonance (ODMR) from cryogenic to ambient conditions. With fully deuterated pentacene, the Hahn-echo coherence reaches 22 $\mu$s and further extends to 214 $\mu$s under dynamical decoupling, outperforming state-of-the-art shallow NV centers in diamond, despite being positioned directly on the surface. We map the local
spin environment, resolving couplings to nearby
nuclear and electron
spins that can serve as auxiliary
quantum resources. This platform combines true surface integration, long
qubit coherence, and scalable fabrication, opening routes to
quantum sensing,
quantum simulation, and hybrid
quantum devices. It also paves the way for a broader family of 2D material-supported molecular
qubits.
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All authors Tian-Xing Zheng, M. Iqbal Bakti Utama, Xingyu Gao, Moumita Kar, Xiaofei Yu, Sungsu Kang, Hanyan Cai, Tengyang Ruan, David Ovetsky, Uri Zvi, Guanming Lao, Yu-Xin Wang, Omri Raz, Sanskriti Chitransh, Grant T. Smith, Leah R. Weiss, Magdalena H. Czyz, Shengsong Yang, Alex J. Fairhall, Kenji Watanabe, Takashi Taniguchi, David D. Awschalom, A. Paul Alivisatos, Randall H. Goldsmith, George C. Schatz, Mark C. Hersam, Peter C. Maurer
Quantum Physics · Cross-listed from cond-mat.str-el
Real-Time Iteration Scheme for Dynamical Mean-Field Theory: A Framework for Near-Term Quantum Simulation
Chakradhar Rangi, Aadi Singh and Ka-Ming Tam
We present a time-domain iteration scheme for solving the Dynamical Mean-Field Theory (DMFT) self-consistent equations using retarded Green's functions in real time. Unlike conventional DMFT approaches that operate in imaginary time or frequency space, our scheme operates directly with real-time quantities. This makes it particularly suitable for near-term
quantum computing hardware with limited
Hilbert spaces, where real-time propagation can be efficiently implemented via Trotterization or variational
quantum algorithms. We map the effective
impurity problem to a finite one-dimensional chain with a small number of bath sites, solved via exact diagonalization as a proof-of-concept. The hybridization function is iteratively updated through time-domain fitting until self-consistency. We demonstrate stable convergence across a wide range of interaction strengths for the half-filled Hubbard model on a Bethe
lattice, successfully capturing the metal-to-
insulator transition. Despite using limited time resolution and a minimal bath discretization, the spectral functions clearly exhibit the emergence of Hubbard bands and the suppression of spectral weight at the
Fermi level as interaction strength increases. This overcomes major limitations of two-site DMFT approximations by delivering detailed spectral features while preserving efficiency and compatibility with
quantum computing platforms through real-time dynamics.
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Quantum Physics
Distinguishing synthetic unravelings on quantum computers
Eloy Piñol, Piotr Sierant, Dustin Keys et al. (+6 more)
Distinct monitoring or intervention schemes can produce different conditioned
stochastic quantum trajectories while sharing the same unconditional (ensemble-averaged) dynamics. This is the essence of unravelings of a given Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equation: any trajectory-ensemble average of a function that is linear in the conditional state is completely determined by the unconditional density
matrix, whereas applying a nonlinear function before averaging can yield unraveling-dependent results beyond the average
evolution. A paradigmatic example is resonance fluorescence, where direct photodetection (jump/Poisson) and homodyne or heterodyne detection (diffusive/Wiener) define inequivalent unravelings of the same GKSL dynamics. In earlier work, we showed that nonlinear trajectory averages can distinguish such unravelings, but observing the effect in that optical setting requires demanding experimental precision. Here we translate the same idea to a digital setting by introducing synthetic unravelings implemented as
quantum circuits acting on one and two
qubits. We design two unravelings - a projective measurement unraveling and a random-
unitary "kick" unraveling - that share the same ensemble-averaged
evolution while yielding different nonlinear conditional-state statistics. We implement the protocols on superconducting-
qubit hardware provided by IBM
Quantum to access trajectory-level information. We show that the variance across trajectories and the ensemble-averaged von Neumann
entropy distinguish the unravelings in both theory and experiment, while the unconditional state and the ensemble-averaged expectation values that are linear in the state remain identical. Our results provide an accessible demonstration that
quantum trajectories encode information about measurement backaction beyond what is fixed by the unconditional dynamics.
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All authors Eloy Piñol, Piotr Sierant, Dustin Keys, Romain Veyron, Miguel Angel García-March, Tanner Reese, Morgan W. Mitchell, Jan Wehr, Maciej Lewenstein
Quantum Physics
Symmetric and Antisymmetric Quantum States from Graph Structure and Orientation
Matheus R. de Jesus, Eduardo O. C. Hoefel and Renato M. Angelo
Graph states provide a powerful framework for describing multipartite
entanglement in
quantum information science. In their standard formulation, graph states are generated by controlled-$Z$ interactions and naturally encode symmetric exchange properties. Here we establish a precise correspondence between graph
topology and exchange
symmetry by proving that a graph state is fully symmetric under particle permutations if and only if the underlying graph is complete. We then introduce a generalized graph-based construction using a non-commutative two-qudit gate, denoted $GR$, which requires directed edges and an explicit vertex ordering. We show that complete directed graphs generate fully antisymmetric multipartite states when endowed with appropriate orientations. Together, these results provide a unified graph-theoretic description of bosonic and fermionic exchange
symmetry based on graph completeness and edge orientation.
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Quantum Physics
Theory of low-weight quantum codes
Fuchuan Wei, Zhengyi Han, Austin Yubo He et al. (+2 more)
Low check weight is a crucial code property for fault-tolerant
quantum computing, which underlies the strong interest in
quantum low-density parity-check (qLDPC) codes. Here, we initiate the theory of weight-constrained stabilizer codes from various foundational perspectives including the complexity of computing code weight and the explicit boundary of feasible low-weight codes in both theoretical and practical settings. We first prove that computing the optimal generator weight of a stabilizer code is $\mathsf{NP}$-hard, motivating efficiently computable bounds. We derive analytical lower bounds on check weight in terms of code rate and distance, identifying the minimum weights needed for single-
qubit error detection and correction, as well as the sharp distance and rate limits of weight-three error-detecting codes. Matching constructions show that these bounds are tight in several regimes. To establish refined finite-size constraints, we develop a linear programming framework based on
quantum weight enumerators subject to generator-weight constraints, yielding exact optimal weights for all parameter combinations with $n\le9$. Finally, we show that the same framework can incorporate architecture-dependent constraints, using the 127-
qubit IBM Eagle chip as a concrete example. Our study brings the weight as a crucial parameter into coding theory and provides guidance for code design and utility in practical scenarios.
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All authors Fuchuan Wei, Zhengyi Han, Austin Yubo He, Zimu Li, Zi-Wen Liu
Quantum Physics
A Folded Surface Code Architecture for 2D Quantum Hardware
Zhu Sun and Zhenyu Cai
Qubit shuttling has become an indispensable ingredient for scaling leading
quantum computing platforms, including
semiconductor spin, neutral-atom, and trapped-ion
qubits, enabling both crosstalk reduction and tighter integration of control hardware. Cai et al. (2023) proposed a scalable architecture that employs short-range shuttling to realize effective three-dimensional connectivity on a strictly two-dimensional device. Building on recent advances in
quantum error correction, we show that this architecture enables the native implementation of folded surface codes on 2D hardware, reducing the runtime of all single-
qubit logical Clifford gates and logical CNOTs within subsets of
qubits from $\mathcal{O}(d)$ in conventional surface code
lattice surgery to constant time. We present explicit protocols for these operations and demonstrate that access to a transversal $S$ gate reduces the spacetime volume of 8T-to-CCZ magic-state
distillation by more than an order of magnitude compared with standard 2D
lattice surgery approaches. Finally, we introduce a new "virtual-stack" layout that more efficiently exploits the quasi-three-dimensional structure of the architecture, enabling efficient multilayer routing on these two-dimensional devices.
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Quantum Physics
Enhanced quantum state discrimination under general measurements with entanglement and nonorthogonality restrictions
Swati Choudhary, Aparajita Bhattacharyya and Ujjwal Sen
The minimum error probability for distinguishing between two
quantum states is bounded by the Helstrom limit, derived under the assumption that measurement strategies are restricted to positive operator-valued measurements. We explore scenarios in which the error probability for discriminating two
quantum states can be reduced below the Helstrom bound under some constrained access of resources, indicating the use of measurement operations that go beyond the standard positive operator-valued measurements framework. We refer to such measurements as non-positive operator-valued measurements. While existing literature often associates these measurements with initial
entanglement between the system and an auxiliary, followed by joint projective measurement and discarding the auxiliary, we demonstrate that initial
entanglement between system and auxiliary is not necessary for the emergence of such measurements in the context of state discrimination. Interestingly, even initial product states can give rise to effective non-positive measurements on the subsystem, and achieve sub-Helstrom discrimination error when discriminating
quantum states of the subsystem.
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Quantum Physics
Simple broadband signal detection at the fundamental limit
Anthony M. Polloreno and Graeme Smith
Broadband detection of a weak oscillatory field with unknown carrier frequency underlies magnetometry,
axion searches and gravitational-wave sensing. We show that the Grover-like integration-time lower bound for this task is a
geometric corollary of an upper bound on the integrated
quantum Fisher information and present an all-analog protocol with near-optimal scaling.
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Quantum Physics
Covariant formulation of the Berry connection in non-Hermitian systems
Ievgen I. Arkhipov
Non-Hermitian systems exhibit spectral and topological phenomena absent in Hermitian physics; however, their
geometric characterization remains subtle due to the intrinsic ambiguity of biorthogonal eigenspaces. Since left and right eigenvectors are not related by Hermitian conjugation, the associated Berry connection is generally nonunique, leading to complex
geometric phases and ambiguously defined holonomies. Here we formulate a covariant
geometric framework for non-Hermitian
quantum systems based on the metric structure of the underlying
Hilbert space. We show that, in the
quantum regime with continuous state
evolution, the conventional Berry connection and the associated Berry holonomy over closed parameter-space loops can be consistently defined only in the pseudo-Hermitian limit, where the spectrum is real. For generic non-Hermitian
Hamiltonians with complex spectra, the relevant
geometric object is instead the Aharonov–Anandan holonomy associated with cyclic
evolution in projective
Hilbert space. Within the pseudo-Hermitian regime, we construct a unique Hermitian Berry connection that is covariant under arbitrary ${\rm GL}(N,\mathbb C)$ frame transformations and reduces to the standard Berry connection in the Hermitian limit. The resulting formalism separates the intrinsic geometry of the
Hamiltonian eigenspace from contributions arising from the parameter dependence of the
Hilbert-space metric, revealing that the conventional biorthogonal formulation generally mixes these distinct
geometric effects. Consequently,
geometric phases, synthetic gauge fields, and topological characteristics commonly attributed to non-Hermitian eigenspace geometry may, in part, originate from the underlying metric structure. Our framework therefore provides a consistent
geometric foundation for Berry phases, non-Abelian holonomies, and topological invariants in non-Hermitian
quantum systems.
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Quantum Physics
Spectral Codes: A Geometric Formalism for Quantum Error Correction
Satoshi Kanno and Yoshi-aki Shimada
We present a new
geometric perspective on
quantum error correction based on spectral triples in noncommutative geometry. In this approach,
quantum error correcting codes are reformulated as low energy spectral projections of Dirac type operators that separate global logical degrees of freedom from local, correctable errors. Locality, code distance, and the Knill Laflamme condition acquire a unified spectral and
geometric interpretation in terms of the induced metric and spectrum of the Dirac operator. Within this framework, a wide range of known error correcting codes including classical linear codes, stabilizer codes, GKP type codes, and topological codes are recovered from a single construction. This demonstrates that classical and
quantum codes can be organized within a common
geometric language. A central advantage of the spectral triple perspective is that the performance of error correction can be directly related to spectral properties. We show that leakage out of the code space is controlled by the spectral gap of the Dirac operator, and that code preserving internal perturbations can systematically increase this gap without altering the encoded logical subspace. This yields a
geometric mechanism for enhancing error correction thresholds, which we illustrate explicitly for a stabilizer code. We further interpret Berezin Toeplitz quantization as a mixed spectral code and briefly discuss implications for
holographic quantum error correction. Overall, our results suggest that
quantum error correction can be viewed as a universal low energy phenomenon governed by spectral geometry.
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Quantum Physics
Inter-branch message transfer on superconducting quantum processors: a multi-architecture benchmark
Cameron V. Cogburn
We treat inter-branch message transfer in a Wigner's-friend
circuit as a practical
benchmark for near-term superconducting
quantum processors. Implementing Violaris'
unitary message-transfer primitive, we compare performance across IBM Eagle, Nighthawk, and Heron (r2/r3) processors for message sizes up to $n=32$, without error mitigation. We study three message families – sparse (one-hot), half-weight, and dense – and measure conditional string success $p_{\mathrm{all}}=\Pr(P=\mu\mid R=0)$,
memory erasure after uncomputation, and correlation diagnostics (branch contrast and bitwise mutual information). The sparse family compiles to essentially constant two-
qubit depth, yielding a depth-controlled probe of device noise: at $n=32$ we observe $p_{\mathrm{all}}$ spanning $\approx0.07$ to $\approx0.68$ across backends. In contrast, half and dense messages incur rapidly growing routing overhead, and transpiler-seed variability becomes a practical limitation near the coherence frontier. We further report an amplitude sweep (no-amplification test) and a divergence “cousins” sweep that quantifies degradation with branch-conditioned complexity. All data and figure-generation scripts are released.
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Quantum Physics
Quantum Circuit Pre-Synthesis: Learning Local Edits to Reduce $T$-count
Daniele Lizzio Bosco, Lukasz Cincio, Giuseppe Serra et al. (+1 more)
Compiling
quantum circuits into Clifford+$T$ gates is a central task for fault-tolerant
quantum computing using stabilizer codes. In the near term, $T$ gates will dominate the cost of fault tolerant implementations, and any reduction in the number of such expensive gates could mean the difference between being able to run a
circuit or not. While exact synthesis is exponentially hard in the number of
qubits, local synthesis approaches are commonly used to compile large
circuits by decomposing them into substructures. However, composing local methods leads to suboptimal compilations in key metrics such as $T$-count or
circuit depth, and their performance strongly depends on
circuit representation. In this work, we address this challenge by proposing Q-PreSyn, a strategy that, given a set of local edits preserving
circuit equivalence, uses a RL
agent to identify effective sequences of such actions and thereby obtain
circuit representations that yield a reduced $T$-count upon synthesis. Experimental results of our proposed strategy, applied on top of well-known synthesis
algorithms, show up to a $20\%$ reduction in $T$-count on
circuits with up to 25
qubits, without introducing any additional approximation error prior to synthesis.
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All authors Daniele Lizzio Bosco, Lukasz Cincio, Giuseppe Serra, M. Cerezo