Quantum Physics
Nonlinear Diamagnetic Interactions in Ultrastrongly Coupled 2D Electrons
Dasom Kim, Kiran M. Kulkarni, Vaibhav Sharma et al. (+15 more)
The
quantum Hopfield model is widely used to describe ultrastrong light–matter coupling between cavity
photons and collective bosonic excitations in solids, where the diamagnetic interaction is conventionally assumed to be a constant. We experimentally demonstrate that the diamagnetic response of Landau polaritons is reduced under strong terahertz field excitation. We show that this behavior originates from field-driven redistribution of electrons into the nonparabolic regime of the conduction band of GaAs, which reduces the
plasma frequency and consequently the diamagnetic interaction strength. A microscopic hot-electron model reproduces the observed nonlinear response. Motivated by this microscopic picture, we propose a nonlinear extension of the Hopfield model with a Kerr-like interaction. Our results establish a route toward nonlinear cavity
quantum electrodynamics and driven ultrastrong light–matter coupling beyond the conventional linear Hopfield description, which is capable of creating uniquely
quantum optical effects such as squeezed light generation.
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All authors Dasom Kim, Kiran M. Kulkarni, Vaibhav Sharma, Dukhyung Lee, Geon Lee, Sunghwan Kim, Jonas Grumm, Shuang Liang, Hongjing Xu, Fuyang Tay, Andrey Baydin, Motoaki Bamba, Andreas Knorr, Christopher J. Stanton, Michael J. Manfra, Minah Seo, Stephen Hughes, Junichiro Kono
Quantum Physics · Cross-listed from cond-mat.mes-hall
$\mathcal{PT}$ and anti-$\mathcal{PT}$ phase transitions in a trimerized Su–Schrieffer–Heeger chain with nonreciprocal Rashba spin-orbit coupling
Milad Jangjan, Linhu Li, Longwen Zhou et al. (+1 more)
We theoretically investigate a one-dimensional trimerized Su–Schrieffer–Heeger chain with three sublattices per unit
cell subjected to a nonreciprocal Rashba
spin-orbit coupling. Invoking a
spin-flip
symmetry, the non-Hermitian
Hamiltonian decomposes into two independent
spin sectors, enabling a
spin-resolved analysis of non-Hermitian skin effects and system
symmetries. We identify a rich phase diagram consisting of four bulk phases and two edge-state phases. The bulk phases include fully $\mathcal{PT}$-unbroken (real-spectrum) and fully anti-$\mathcal{PT}$-unbroken (imaginary-spectrum) regimes, as well as two mixed phases where one band remains on the real or imaginary axis while the other two form complex-conjugate pairs. The two edge-state phases correspond to topological edge modes with either $\mathcal{PT}$-unbroken (real) or anti-$\mathcal{PT}$-unbroken (imaginary) energies. Using non-Bloch band theory and Cardano's method, we derive closed-form expressions for phase boundaries and establish the bulk-edge correspondence for each
spin sector. Calculations of Berry phase and directional inverse participation ratios confirm our analytical predictions. Our results provide a minimal platform for realizing
spin-resolved non-Hermitian
topology and edge-selective
symmetry preservation, in which the bulk and edge states belong to distinct
symmetry classes.
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All authors Milad Jangjan, Linhu Li, Longwen Zhou, Mir Vahid Hosseini
Quantum Physics
Energetics in daemonic work extraction protocols via non-ideal QND-energy measurement
Daniele Morrone, Francesco Albarelli, Vittorio Giovannetti et al. (+2 more)
We address the problem of extracting work from a
quantum system assisted by a
quantum non-demolition (QND) energy measurement. When a perfect QND measurement can be performed and an auxiliary zero-temperature bath is available, the full energy of the
quantum state can in principle be extracted even without any prior information on the input state. Owing to the presence of a zero-temperature bath, this is achieved at no energetic cost for the measurement process itself. On the contrary, here we consider what happens when the same protocol is implemented in non-ideal scenarios, specifically when the auxiliary bath has a finite temperature. In this case, not only is it impossible to extract the entire energy from the system, but the measurement strategy also acquires a non-zero energetic cost, accounting for both the interaction between system and measurement apparatus, and the corresponding Landauer erasure cost. We quantitatively assess the performance of these work-extraction protocols, both in absolute terms and through the so-called daemonic net gain, which explicitly includes the energetic cost of the measurement. We rigorously prove that, when access to a thermal bath is allowed in the extraction protocol, the daemonic net gain is always non-positive for any temperature of the auxiliary bath. Conversely, when only
unitary operations are considered, the daemonic net gain can attain positive values. We further discuss these different figures of merit by analyzing a paradigmatic example for a single
qubit system.
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All authors Daniele Morrone, Francesco Albarelli, Vittorio Giovannetti, Mauro Paternostro, Marco G. Genoni
Quantum Physics · Cross-listed from math.CO
Perfect state transfer and Cayley presentations
Arnbjörg Soffía Árnadóttir and Krystal Guo
We study perfect state transfer on Cayley graphs from the point of view that state transfer is a property of a graph and not of a group. This paper is a bridge between the classical question about isomorphic Cayley graphs of non-isomorphic groups and
quantum walks on graphs. We show that a Cayley graph of a group with an abelian subgroup of index two is a Cayley graph of an abelian group under any one of three hypotheses, two drawn from the theory of isomorphic Cayley graphs. A statement of the same kind holds for extraspecial groups: every Cayley graph of an extraspecial $p$-group of order $p^{2n+1}$ with a conjugacy-closed connection set is a Cayley graph of $Z_p^{2n+1}$. From these results we deduce that every explicit construction of perfect state transfer in the six papers we survey, on dihedral, dicyclic, generalized dihedral, $V_{8n}$ and extraspecial $2$-groups, is a non-abelian presentation of an abelian Cayley graph. Moreover, we show that a non-abelian group with an abelian subgroup of index two admits a connected Cayley graph with perfect state transfer if and only if its order is divisible by four. Genuinely non-abelian examples do exist. We prove that, for every odd
prime power $q\ge 5$, the $SL(2,q)$ graph of Pantangi and Sin, which they showed to admit perfect state transfer, is a Cayley graph of no abelian group; to our knowledge, this is the first infinite family of Cayley graphs with perfect state transfer provably admitting no abelian Cayley presentation. We also construct an infinite family of Cayley graphs with peak state transfer and determine all regular subgroups of the
automorphism group of every member. An appendix records a census of the connected vertex-transitive graphs with perfect state transfer on at most $30$ vertices.
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Quantum Physics
Collective Quantum Logic Spectroscopy
Raphael Kaubruegger, Matthew Patkowski, Yicheng Zhang et al. (+3 more)
Scaling trapped-ion
quantum sensors from single ions to large ensembles is a key challenge for next-generation precision measurements. At the same time, many ion species of interest for optical clocks and tests of fundamental physics lack closed cycling transitions required for direct
laser cooling and state detection. Collective
quantum logic spectroscopy addresses both limitations by coupling an ensemble of sensor, or spectroscopy, ions to one or more logic ions that provide sympathetic cooling and state readout. Here, we establish the fundamental performance limits and operating regimes of this protocol, identifying how the interaction strength, interrogation time, and logic-ensemble size govern sensitivity, dynamic range, and robustness to experimental imperfections. We show that
quantum-limited sensitivity can be retained even with a single logic ion, while increasing the number of logic ions substantially improves readout efficiency and robustness. Beyond precision metrology, the same collective interface enables many-body measurements relevant to
quantum information processing, including parity measurements and stabilizer-like syndrome extraction. Our results establish collective
quantum logic spectroscopy as a scalable framework for optical clocks,
quantum-enhanced sensing, and trapped-ion
quantum information processing.
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All authors Raphael Kaubruegger, Matthew Patkowski, Yicheng Zhang, Robert J. Lewis-Swan, David B. Hume, Ana Maria Rey
Quantum Physics · Cross-listed from hep-ph
First Search for Ultraheavy Dark Matter Using a Magnetically Levitated Particle
Dennis G. Uitenbroek, Dorian W. P. Amaral, Juehang Qin et al. (+4 more)
We present the first search for ultraheavy dark matter using a magnetically levitated particle. The POLONAISE experiment uses a milligram-scale ferromagnet levitated in a superconducting trap, admitting a force sensitivity of $0.07\,\mathrm{fN\,Hz^{-1/2}}$ and resolving impulses as small as $1\,\mathrm{TeV}/c$. Treating every candidate impulse as a possible dark matter event, we set optimum-interval upper limits on the
neutron coupling $\alpha_n$ for dark matter interacting through a new light mediator. For dark matter masses $10^6\,\mathrm{GeV}/c^2\text{-}10^{15}\,\mathrm{GeV}/c^2$ and mediators lighter than $30\,\mathrm{meV}/c^2$, we exclude couplings as low as $\alpha_n = 3.2\times 10^{-9}$ at $95\%$ confidence level and set leading constraints on the dark matter-
neutron cross section for composite dark matter. Our results extend levitated sensing beyond the mass reach of optical levitation by seven orders of magnitude into the ultraheavy dark matter frontier.
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All authors Dennis G. Uitenbroek, Dorian W. P. Amaral, Juehang Qin, Jurriaan Langendorff, Andrew Gingerich, Tjerk H. Oosterkamp, Christopher D. Tunnell
Quantum Physics
To Scale Up or To Scale Out: Evaluating Space-Time Costs of Compiled Logical Circuits on Modular Superconducting Quantum Processors
Nikiforos Paraskevopoulos, Sebastian de Bone, Mick Christophersen et al. (+4 more)
Modular integration has emerged as the main pathway for scaling superconducting
quantum processing units (QPUs) beyond the constraints of fabrication yield and physical footprint. Currently, two primary strategies lead this effort. Mirroring the "Scaling Up" and "Scaling Out" approaches in GPU architectures and AI infrastructures, these are: chiplet-based scaling, which preserves dense connectivity and high gate fidelity at the expense of engineering complexity, and distributed architectures, which decouple system scaling from monolithic QPU advancements at the expense of sparser connectivity and lower interconnect quality. To evaluate these approaches, we introduce a quantitative stress test measuring the execution cost of a dense workload of random logical entangling operations using a surface code scheme. Using a dedicated compiler, we compute the space-time cost as the number of network nodes increases, analysing this scaling behaviour across various surface code distances, Bell-state fidelities, and Bell-pair generation times. We find that distributed architectures incur an up to exponential space-time performance penalty compared to an effectively monolithic architecture across all simulations. Our results also show that as the network grows, this penalty manifests in two distinct scaling regimes: a noise-dominated regime constrained by insufficient Bell-state fidelity and generation rates, and a connectivity-dominated regime bottlenecked by
lattice-surgery routing congestion.
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All authors Nikiforos Paraskevopoulos, Sebastian de Bone, Mick Christophersen, Simon Storz, A. Mert Bozkurt, Arno Bargerbos, Sebastian Feld
Quantum Physics
Practical Error Suppression and Mitigation for Reliable Quantum Computing
Han-Ze Li, Mengjie Yang, Xianquan Yan et al. (+3 more)
Quantum computing is entering a transitional regime between noisy intermediate-scale
quantum (NISQ) processing and early fault-tolerant
quantum computation (FTQC), in which increasingly capable hardware is beginning to support repeated syndrome measurements, partial error correction, and logical-
qubit operations, while residual physical and logical errors remain non-negligible. In this regime, error suppression, error mitigation, and
quantum error correction are increasingly better viewed as complementary layers of a unified error-reduction strategy rather than as separate approaches, with each acting at a different stage of the
quantum computation to improve simulation reliability. Thus, in this review, we provide a practical and forward-looking overview of the principal hardware error sources and the corresponding error suppression and mitigation methods for reducing their impact across the current NISQ-FTQC transition. We discuss hardware-aware
circuit design, coherent-error suppression, readout mitigation, noise extrapolation, classical inference, and software-supported workflows, with particular emphasis on their implementation on actual
quantum processors. We further examine how error mitigation techniques can be adapted to encoded and logical-
qubit settings so that they can operate alongside
quantum error correction to suppress residual logical errors and improve the accuracy of computation in the early fault-tolerant regime.
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All authors Han-Ze Li, Mengjie Yang, Xianquan Yan, Dax Enshan Koh, Ching Hua Lee, Ruizhe Shen
Quantum Physics
Granthi: Higher-Order Quantum Programming via Unitary Wiring
Samson Abramsky and Radha Jagadeesan
Many mainstream
quantum programming
languages confine higher-order structure to a classical host while restricting the
quantum layer to first-order operations on
qubits. This paper presents Granthi, a purely
unitary higher-order
quantum programming
language built on three design commitments:
quantum programs are first-class values that may be passed, returned, and coherently composed; additive structure is tag-preserving routing rather than observational branching, so control may remain in superposition; and programmer-facing finite label types with staged reversible-operation bindings provide domain-level control spaces without exposing tag management. These bindings are eliminated by elaboration before Source typing. Granthi deterministically normalizes each Source program to a canonical wiring form. Every well-typed Source program, including a term of function type, has a
unitary boundary interpretation. Under backend correctness (BC), the reference compiler produces a
unitary circuit realizing that interpretation. Granthi's currently supported executable fragment is implemented end-to-end: an OCaml DSL elaborates surface programs through a higher-order Core IR to executable
quantum circuits via pytket. The
language directly supports the pure-
unitary quantum switch for explicitly supplied operations; closed instances compile to static
circuits. It also supports interference on control-flow history and structured finite control, all within the purely
unitary fragment
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Quantum Physics
Comment on "Scalable Quantum Machine Learning: Trainability, Expressivity and Efficiency": Polynomial Evaluation of the Triplet-Block Readout
Erfan Amidi
We examine the classical-cost claim for the triplet-block two-body readout in arXiv:2607.24014v1. The Gaussian-state expansion used there gives an $O(2^{2k/3}\mathrm{poly}(n))$ classical
algorithm, but it is not necessary for fixed-body observables. The triplet-block input has an explicitly computable diagonal two-particle reduced density
matrix, which passive fermionic linear optics propagates through $\bigwedge^2 W$. This gives a deterministic $O(n^4)$
algorithm for the complete correlator vector $(\langle n_i n_j\rangle)_{i<j}$, independently of $k$ and the fermionic-linear-optics extent. More generally, every number-conserving fixed-$r$-body expectation is polynomially computable whenever the input $r$-particle reduced density
matrix is classically available; if that
matrix is diagonal, all diagonal correlators are computable in $O(n^{2r})$ time. This invalidates the
algorithm-relative exponential-cost conclusion for the supervised two-body readout, without affecting the gradient-variance, barren-plateau, parameter-shift, or sampling-hardness results.
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Quantum Physics
Group-theoretic treatment of strong light-matter coupling with an arbitrary number of excitations
Antti Peltola, Olli Siltanen, Kimmo Luoma et al. (+1 more)
Strong light-matter interactions in optical microcavities give rise to hybrid light-matter states known as polaritons. While actively used in modern technologies, theoretical descriptions of such systems are often restricted to the single-excitation case, limiting their ability to capture many-excitation physics and hindering further technological advancements. Here, by exploiting the combinatorial structure of
quantum emitters, we investigate the Tavis-Cummings model with arbitrary number of excitations. We derive the structure and properties of its eigensystem and identify allowed radiative transitions in systems of realistic size scales. Our work reveals new behavior inaccessible to the few-excitation regime, while also providing a framework to reduce the computational complexity of similar systems with exponentially growing
Hilbert spaces.
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All authors Antti Peltola, Olli Siltanen, Kimmo Luoma, Konstantinos S. Daskalakis
Quantum Physics · Cross-listed from physics.atom-ph
A Protocol for Shielding-Enhanced Loading of Single Polar Molecules into Optical Tweezers
Reuben R. W. Wang, Christian H. Nunez, Conner Williams et al. (+4 more)
We propose the high-fidelity preparation of single bosonic molecules in optical tweezers starting from small tweezer-trapped molecular ensembles. Our scheme combines a static electric field and a microwave field to generate strong, tunable, anisotropic interactions that shield the molecules against two-body collisional loss. We show that this shielding eliminates all long-range bound states, preventing three-body recombination. This elimination persists for all microwave ellipticities, including the experimentally practical limit of linear polarization. Application of an additional electric field gradient can be used to induce controlled spilling of strongly interacting molecules out of the trap until one remains. With realistic experimental parameters, we estimate that single tweezer-trapped NaCs molecules can be isolated from a pair with fidelities exceeding 99%, and $> 95\%$ per site across an array. These results establish collisional shielding with electric fields as an effective tool for preparing highly-filled tweezer arrays of polar molecules.
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All authors Reuben R. W. Wang, Christian H. Nunez, Conner Williams, Amanda Younes, Li Du, H. R. Sadeghpour, Kang-Kuen Ni
Quantum Physics · Cross-listed from cond-mat.quant-gas
Competing triangular and stripe supersolid orders in a dipolar quantum gas
Karthik Chandrashekara, Christian Gölzhäuser, Lily Platt et al. (+6 more)
Supersolids are exotic
quantum states in which long-range phase coherence coexists, and may interplay, with emergent spatial orders. A particularly rich phase diagram featuring several competing spatial orders is predicted for dipolar supersolids with two-dimensional
crystals, yet the experimental observation of this structural variety has remained limited. Here we experimentally form competing triangular and stripe density-modulated states in a
quantum gas of highly
magnetic atoms confined in a surfboard-shaped trap by tuning contact interaction strength and dipole orientation. We define a structural order parameter and study its statistical behavior. Thereby, we identify both the triangular and stripe phases and the transition between them, the associated critical behavior being marked by enhanced non-Gaussian fluctuations. Furthermore, we observe each spatial structure in both the phase-coherent supersolid regime and the phase-incoherent insulating one, near and far from the unmodulated-to-modulated transition, respectively. Our results establish a versatile platform in which multiple phases of the two-dimensional-supersolid phase diagram, and more generally, intertwined
symmetry-breaking phenomena, can be investigated.
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All authors Karthik Chandrashekara, Christian Gölzhäuser, Lily Platt, Jianshun Gao, Julian Kusch, Lennart Hoenen, Manon Ballu, Wyatt Kirkby, Lauriane Chomaz
Quantum Physics · Cross-listed from cond-mat.quant-gas
Imaging the vacuum fluctuations of a quantum field
Yansheng Zhang, Feiyang Wang, Yi Jiang et al. (+5 more)
Heisenberg uncertainties lead to inevitable fluctuations in the measurement outcomes for
quantum-mechanical observables. For
quantum fields, these uncertainties result in random spatial structures in snapshots of a field, even when the field is in its ground (
vacuum) state. Such `
vacuum fluctuations' are at the heart of a wide range of phenomena, from spontaneous decay processes to the Casimir force and Hawking radiation. Their existence is a key manifestation of the quantumness of the physical world, but usually it is only their consequences that are directly observed. Here, we directly observe spatial
vacuum fluctuations of a bosonic
quantum field. Our experiments are based on a homogeneous planar atomic Bose–Einstein condensate. The condensate comprises two coherently coupled interacting components (
spin states), and the
quantum field describes its
spin degrees of freedom. In the regime where the interactions dominate over the coherent coupling, our system emulates a (massive relativistic) sine-Gordon field. Images of the field reveal simultaneous fluctuations on different length scales, with scale-dependent amplitudes consistent with theoretical predictions for a
vacuum state. Observing such fluctuations in the sine-Gordon limit opens many possibilities for laboratory simulations of relativistic fields in regimes that are presently not theoretically tractable.
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All authors Yansheng Zhang, Feiyang Wang, Yi Jiang, Alexander C. Jenkins, Paul H. C. Wong, Christoph Eigen, Gehrig Carlse, Zoran Hadzibabic
Quantum Physics · Cross-listed from nlin.CD
Coherent states in quantum billiards constructed in the basis of the continued eigenfunctions
I.D. Burkov and S.S. Seidov
In the article a new approach to construction of generalized coherent states in
quantum billiards is proposed. The coherent states are defined as the projections of a Gaussian wave function on the basis of the eigenstates of the
quantum billiard, continued outside. The continuation is built as the solution of the equivalent Balian–Bloch equation, defined on the entire $\mathbb{R}^2$ and the projection operator is built using the resolvent of the Balian–Bloch equation. In the case of the one–dimensional potential well and billiards, belonging to the Coxeter group, the wave functions of the coherent states were expressed analytically via the Jacobi and Riemann theta functions.
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Quantum Physics
Parallel Quantum Advantage with Limited Adaptivity Requires Structure
Qipeng Liu and Saachi Mutreja
Aaronson and Ambainis (Theory of Computing, 2014) conjectured that
quantum query
algorithms admit efficient almost-everywhere classical simulation: for any $T$-query
quantum algorithm, its acceptance probability can be approximated on a $(1-\delta)$ fraction of inputs, up to $\epsilon$ additive error, using $\mathrm{poly}(T, 1/\epsilon, 1/\delta)$ classical queries. At a high level, the
conjecture suggests that exponential
quantum speedups are possible only on sufficiently structured inputs. In this work, we make progress on this
conjecture by proving it for
quantum algorithms that make massively parallel
quantum queries. In contrast, Yamakawa and Zhandry (Journal of the ACM, 2024) showed that
quantum algorithms restricted to parallel queries can still achieve exponential speedups over classical
algorithms for sampling problems. We establish our simulation
theorem by proving the stronger statement that parallel-query
quantum algorithms cannot distinguish the uniform distribution over oracles from oracles drawn from so-called "dense distributions". Our main technical contribution is a coupling
theorem that relates the uniform distribution over oracles to oracles drawn from dense distributions. We further extend this approach beyond the purely parallel setting. We prove simulation
theorems for
quantum algorithms with constant rounds of adaptivity, and for
quantum algorithms with a bounded
quantum-query prefix followed by a massively parallel
quantum-query stage.
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Quantum Physics
Programmable cavity QED with a fiber-integrated atomic array
Stephan Roschinski, Johannes Schabbauer, Franz von Silva-Tarouca et al. (+3 more)
Strong atom-
photon interactions in optical cavities are a key resource for
quantum information processing,
quantum networking, and the exploration of
quantum optical effects. Optical tweezer arrays offer scalable, site-resolved control of neutral atoms, but their integration with high-cooperativity cavity QED systems remains challenging. Here we combine a twelve-site $^{87}$Rb optical tweezer array with a high-cooperativity fiber Fabry-Pérot microcavity. The array is positioned within the cavity mode and individual sites are controlled with subwavelength precision, enabling continuous tuning of the single-atom coupling strength via deterministic displacement through the standing-wave field. For up to five atoms coupled to the cavity, we measure collectively enhanced
vacuum Rabi splitting and implement cavity-based non-destructive readout of the number of coupled atoms. These results establish a scalable architecture for cavity-mediated
entanglement generation and many-body cavity QED with single-atom control, and they lay the foundation for fiber-integrated
quantum network nodes.
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All authors Stephan Roschinski, Johannes Schabbauer, Franz von Silva-Tarouca, Marvin Holten, Damien Bloch, Julian Léonard
Quantum Physics
Quantum-mechanical wave functions in singular potentials: linear and nonlinear states
Hidetsugu Sakaguchi and Boris A. Malomed
It is known that the attractive singular inverse-square potential gives rise to the critical
quantum collapse in the framework of the three-dimensional (3D) linear Schroedinger equation. This article summarizes theoretical results which demonstrate suppression of the collapse, caused by this singular potential, and the creation of the otherwise missing ground state (GS) in a 3D gas of bosonic particles, carrying an electric dipole moment, which are pulled to the central electric charge, with repulsive contact interactions between the particles. In the mean-field approximation, the repulsive interactions are represented by the cubic term in the respective Gross-Pitaevskii (GP) equation. In addition to the GS, excited states with angular momentum are briefly considered too. Another topic considered in the article is 1D and 2D bound states in the linear Schroedinger and GP equations with the repulsive potential, which demonstrates a singularity at r –> infinity. A very recent result is that such a potential, growing faster than the negative harmonic-oscillator potential, produces a full spectrum of counter-intuitive normalizable (localized) bound states. The article puts forward perspectives for further studies of linear and nonlinear bound states existing under the action of the potentials with the singularity at r –> 0 or r –> infinity.
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Quantum Physics
Analytical Solutions of the Generalized Klein-Gordon Oscillator in Som-Raychaudhuri Space-Time via the Extended Nikiforov-Uvarov Method
Hale Karayer, Tolga Celik and Dogan Demirhan
In this study, we present an analytical approach based on the extended Nikiforov-Uvarov method to solve the generalized Klein-Gordon oscillator in the presence of a uniform
magnetic field within the Som-Raychaudhuri space-time. Exact eigenstate solutions are obtained for two distinct potential models, namely the linear potential and the Cornell potential. The corresponding energy
eigenvalues are determined, and the associated eigenfunctions are analyzed and illustrated graphically.
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Quantum Physics · Cross-listed from cond-mat.stat-mech
Learning Potts Models and $Z_3$ Toric Codes: Higher and Ordinary Nishimori Criticality
Rushikesh A. Patil, Malte Pütz, Rohit Mukherjee et al. (+3 more)
Motivated by a previous Ising study, we identify a ${\it higher}$ Nishimori line in the learning phase diagram of the $2D$ $q$-state Potts model $(2 < q\leq 4)$ under bond-energy measurements. This ${\it higher}$ Nishimori line meets the critical temperature line of the Potts model, in a ${\it higher}$ Nishimori critical point – a tricritical point at finite inference strength that separates a paramagnetic, a
ferromagnetic and a '
spin-glass' phase. With analytical tools, we discuss the general structure of the rich phase diagram, which contains two unstable and three stable fixed points, and obtain a number of exact results for universal quantities, including the decay exponent of the Edwards-Anderson correlator, using a Gaussian measurement protocol which allows for exact calculations. Using extensive numerical tools, we confirm these statements for a generic, discrete $q$-state measurement protocol and determine precise numerical estimates for the location of higher and ordinary Nishimori critical points as well as RG flows between the various fixed points. We also discuss the Casimir effective central charges of the critical points in the learning phase diagram, and their monotonic ${\it decrease}$ along measurement-induced RG flows, as established non-perturbatively by the c-effective
theorem and its extensions, and contrast it to the monotonic increase along the corresponding RG flows in the random-bond Potts model. Finally, we discuss a general argument based on ${\it Elitzur's \; theorem}$ that establishes stability of the ordinary Nishimori critical points in their respective learning phase diagrams. Equivalently, our results describe a monitored deformed $\mathbb{Z}_q$ toric code where the tricritical ${\it higher}$ Nishimori point is an 'information' critical point that separates stable
quantum, classical, and no
memory phases.
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All authors Rushikesh A. Patil, Malte Pütz, Rohit Mukherjee, Guo-Yi Zhu, Simon Trebst, Andreas W. W. Ludwig
Quantum Physics
Logarithmic depth compression of Heisenberg Hamiltonian simulation by fan-out parallelization, with built-in error detection
Artemiy Burov and Clément Javerzac
Noisy intermediate-scale
quantum computers are constrained by
circuit depth, while product-formula simulation of
spin systems leads to narrow and deep
circuits. Here we introduce a fan-out-based gadget compiler that trades
circuit depth for width in simulations of Heisenberg-type
nuclear magnetic resonance (NMR)
Hamiltonians. Each logical
spin is encoded into a small repetition-code register sized by its interaction degree, so that all pairwise interactions of a given Pauli type execute in parallel after a logarithmic-depth CNOT fan-out, and the redundant registers provide error detection for post-selection at no additional algorithmic overhead. The central result is a fixed-protocol resource comparison of the two compilations, transpiled to heavy-hex superconducting and all-to-all trapped-ion targets across a set of NMR
spin systems. For interaction graphs with a high-degree hub the volume-optimal schedule halves the two-
qubit depth and reduces the volume 1.7-fold for the 13-
spin demonstration, which on heavy-hex also lowers the two-
qubit gate count, and the depth reduction rises to 2.5-fold on all-to-all for the highest-degree molecule studied. On all-to-all the two-
qubit gate count rises for every system, so the volume reduction is a benefit on depth-limited hardware. The gain grows with the degree inhomogeneity of the interaction graph and vanishes for dense uniform graphs, where the optimum is the sequential
circuit. We simulate the zero-field NMR spectrum of tetramethylsilane, a 13-
spin star system. Under a noise model scaled from a published present-day processor calibration, the shallower gadget
circuits match or surpass the sequential compilation only after post-selection on their built-in error detection, once error rates improve by one to one and a half orders of magnitude. We verify the spectra against an independent classical computation.
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Quantum Physics
Jamming Extensions of Quantum Correlations Lead to Hidden Superluminal Signaling
Ravishankar Ramanathan, Xie Sicheng, Michał Eckstein et al. (+1 more)
Relativistic causality in certain spacetime configurations permits jamming - superluminal causal influences that nevertheless do not enable superluminal signaling. We fully characterize the correlations compatible with relativistic causality (RC) by extending the operator framework of PRL 104, 140404. When the underlying state is required to be
quantum, we prove that any nontrivial jamming - whether state-independent or state-dependent - necessarily leads to hidden superluminal signaling. Thus, the only consistent possibilities are standard
quantum correlations without jamming or the full relativistically causal correlation set, with intermediate jamming extensions of
quantum correlations leading to signaling. As an application of our characterization, we investigate the security of device-independent (DI) cryptographic primitives against adversaries constrained only by relativistic causality. We construct explicit RC attacks that break DI bit commitment and DI secret sharing in jamming geometries, even when these protocols remain secure against no-signaling adversaries. Our results show that security against relativistic adversaries requires behaviors to be specified together with the spacetime locations of their measurement events; input-output statistics alone are insufficient.
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All authors Ravishankar Ramanathan, Xie Sicheng, Michał Eckstein, Paweł Horodecki
Quantum Physics
Nonthermal Dynamics of a One-dimensional Rydberg-atom Chain with Constraint Four-body Interactions
Tianyi Yan and Weibin Li
We investigate dynamics of a linear chain of Rydberg atoms driven by a constrained four-body interaction, where two neighboring atoms are excited simultaneously from the electronic ground state $|0\rangle$ to Rydberg state $|1\rangle$ only when their closest neighbors are in $|0\rangle$ state. By employing an ansatz for the many-body ground state, the low-energy
Hamiltonian is given by a tridiagonal form. The many-body ground state energy, which scales linearly with the chain length $L$, is obtained analytically in the thermodynamic limit and agrees with the one of the exact diagonalization. Our model supports
quantum many-body scar eigenstates that are nearly equally spaced energetically. The scar states overlap strongly with the basis state $|\mathbf{0}\rangle=|0\cdots 0\rangle$. We show that the overlap distribution is tilted by the state-dependent four-body interaction. This leads to non-ergodic dynamics, evidenced by the revival of the initial state. The four-body constrained model can be realized with Rydberg atoms in a Peierls array with alternating bond lengths, where atoms on the shorter and longer bonds experience Rydberg blockade and antiblockade, respectively. Our study provides a pathway to explore constrained non-ergodic dynamics with four-body interactions by combining the Rydberg blockade and antiblockade.
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Quantum Physics
Hybrid Qubit-Rotor Quantum Systems: Clifford Structure, Universal Control, and Applications
Dengyao Luo, Arvin Kushwaha, Mastawal Tirfe et al. (+1 more)
A $U(1)$
quantum rotor pairs a periodic angle with an integer-valued conjugate momentum, and occurs in molecular rotation, superconducting phase-charge
circuits, and compact gauge fields. Coupling such a rotor coherently to
qubits gives a hybrid register whose control structure is not inherited from either the oscillator-
qubit or the qudit case. We develop a Clifford theory, together with a universal-control result, for registers of $n$
qubits and $r$ rotors. We classify all
automorphisms of the hybrid phase space $\mathbb{F}_2^{2n}\times\mathbb{Z}^r\times\mathbb{T}^r$ that preserve the Weyl commutation relations, and give an explicit finite Clifford
circuit for each one. The classification is directional: rotor momentum parity may control
qubit Pauli operations within the Clifford group, while every nonzero
qubit-controlled rotor momentum shift is non-Clifford. It also yields normal forms for the mixed
qubit-rotor couplings and the exact minimum number of elementary mixed gates needed to synthesize them. Adding a rotor cosine potential and one fixed
qubit-rotor conditional phase to the local Clifford operations gives universal control on the full
Hilbert space in the strong operator
topology. We then apply this structure in three settings: an exact controlled-shift realization of gauge-covariant matter hopping, which is necessarily non-Clifford; rotor phase estimation with direct angle readout and probe optimization under momentum-support and energy constraints; and finite
Fourier transforms on rotor momentum codes, where the one-rotor transform for $d=2^s$ compiles into $O(s)$ momentum-selective and controlled-shift instructions and each cross-register
Fourier factor is implemented by one quadratic rotor Clifford gate.
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All authors Dengyao Luo, Arvin Kushwaha, Mastawal Tirfe, Bojko N. Bakalov
Quantum Physics
Rigorous existence and location of quantum phase transitions in lattice Hamiltonian systems
Massimo Ostilli and Carlo Presilla
We extend the analysis of the class of
quantum phase transitions (QPTs) that can be interpreted as condensations in state space, first introduced in [M. Ostilli and C. Presilla, J. Phys. A 54, 055005 (2021)], by generalizing the arguments of [M. Ostilli and C. Presilla, Phys. Rev. Lett. 127, 040601 (2021)] to prove the existence and determine the location (via simple bounds) of QPTs in general one-parameter
lattice Hamiltonians. Unlike our original formulation, this extension also encompasses second-order QPTs, for which we provide the explicit example of the transverse-field Ising model. Our analysis suggests that, under conditions typically satisfied in physical contexts, any QPT taking place in
lattice systems can be interpreted as a condensation in state space.
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Quantum Physics
Characterization of a damping channel as a mixture of amplitude damping and anti-damping channels of different parameters
Vijay Pathak and R. Srikanth
Non-unital noise is a fundamental feature of open
quantum systems, governing energy exchange and inducing state translations on the Bloch sphere. While such translations can be useful for tasks such as state preparation and channel capacity, unital dynamics where no translation occurs are often preferred in
quantum information processing, particularly for error correction. Phase-covariant dynamics provide a general framework encompassing dissipation, excitation, and dephasing processes in
qubit systems; however, commonly used models such as the generalized amplitude damping (GAD) channel offer only limited control over these features. In this work, we present a constructive framework for generating a broader class of phase-covariant dynamics by mixing amplitude-damping and anti-damping channels with unequal decay parameters and time-dependent mixing probabilities. This approach enables independent control over contraction and translation, allows continuous tuning between non-unital and unital regimes, and yields an effective dephasing contribution absent in GAD. To characterize these dynamics, we employ the general
theorem for P-divisibility and show that it can be used constructively by evaluating it in suitably chosen
Hilbert space bases, leading to simplified conditions for both unital and non-unital cases. The framework captures a wide range of behaviors, including CP-divisible, P-divisible but not CP-divisible, and non-P-divisible dynamics. We further show that appropriate mixing can reduce the deviation from the ideal noiseless
evolution and that such improvement persists even after tuning the dynamics to the unital regime. Our results provide a flexible approach to engineering open system dynamics beyond standard thermal models, with potential applications in noise control and
quantum information processing.
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Quantum Physics
Large Scale Entanglement Structure Detection in 100-Qubit Systems via Local Joint Measurements
Rui Li, Yuhang Wang, Chunxiao Du et al. (+5 more)
Identifying the
entanglement structure of a many-body
quantum state, namely how its constituents partition into unentangled blocks, is a central task in
quantum information science, yet conventional
tomography scales exponentially with system size. Here we introduce a scalable framework that recognizes large-scale
entanglement structures directly from local correlation fingerprints. By choosing a representative local Pauli basis that satisfies a boundary-matching condition p_1 = p_R, the entire chain is read out in a single measurement configuration, keeping the measurement effort independent of system size. In noisy simulations, this single-basis protocol classifies GHZ-, W-, and cluster-type structures among 30 candidate partitions with a mean accuracy exceeding 95% for systems of up to 100
qubits. We further validate the protocol on a superconducting
quantum processor, where it reliably classifies block structures for systems of up to 13
qubits before noise- and depth-induced degradation sets in at larger sizes. By mapping these failure modes explicitly, our results delineate the boundary of hardware-level scalability and point to a concrete strategy for characterizing
entanglement structure on near-term
quantum devices.
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All authors Rui Li, Yuhang Wang, Chunxiao Du, Shikun Zhang, Zheng Qin, Wenxiu Li, Hao Zhang, Zhisong Xiao
Quantum Physics · Cross-listed from cond-mat.mtrl-sci
Quantum Dissipative Paraelectricity
A. Cano
Whether a
quantum system with a double-well effective potential undergoes spontaneous
symmetry breaking depends not only on the potential landscape but also on the kinetics and the coupling with additional degrees of freedom. Here we introduce a quasi-exactly solvable model to study this problem in the context of ferroelectrics, with results that apply to a broad class of
quantum phase transitions. Exploiting the analytical solutions, we provide a strict definition of the
quantum paraelectric regime and identify a distinct
quantum ferroelectric regime in which
symmetry breaking can be realized without tunneling features. We then show that explicit
symmetry breaking cannot be inferred from the order-parameter
Hamiltonian alone, but requires additional couplings. This leads us to identify a regime of
quantum dissipative paraelectricity, in which observable
symmetry breaking is suppressed during the
evolution toward the ground state, even when the double-well structure dominates over zero-point
quantum fluctuations.
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Quantum Physics
Architecture and Compilation Co-Design for High-Rate Quantum Product Codes on Neutral Atom Arrays
Adrian Liu, Wan-Hsuan Lin, Daniel Bochen Tan et al. (+2 more)
Achieving fault-tolerant
quantum computing at a practical scale demands
quantum error correction (QEC) codes with high encoding rates.
Quantum low-density parity-check (qLDPC) codes emerge as a promising candidate, especially given the rise of neutral atom arrays that provide dynamic long-range connectivity via atom movements. In general, synthesizing valid and efficient physical execution plans for QEC is a provably hard combinatorial problem, forming a critical compilation bottleneck that worsens as code sizes grow. To overcome this complexity, we focus on an important product family of qLDPC codes with dimension-reduction properties, and propose ONEX. This framework decomposes complex 2D physical execution planning into independent 1D subproblems, each solved to optimal execution depth within practical compilation time. First, we formulate the 1D execution plan with an explicit satisfiability modulo theories (SMT) encoding. This protocol produces provably depth-optimal solutions with substantial duration reduction. Second, we develop a multi-stage compilation pipeline featuring anytime optimization, movement compaction, and iterative feedback. This pipeline maintains practical wall-clock times while providing progressive refinement and on-demand
retrieval of quality solutions. Third, we evaluate ONEX in the application of hypergraph product (HGP) code
memory mapped onto neutral atom arrays, achieving 3.7x to 6.1x and 29.8x to 42.1x higher clock rates than the constructive 1D
algorithm and the general 2D compiler, respectively, while scaling efficiently to codes with 2,500 data
qubits. Finally, we extend ONEX to zoned layouts, revealing architectural insights into the associated trade-offs, and demonstrate its applicability to the broader lifted-product (LP) code family through a representative example.
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All authors Adrian Liu, Wan-Hsuan Lin, Daniel Bochen Tan, Qian Xu, Jason Cong
Quantum Physics
Proper Learning of Shallow All-to-All Quantum Circuits
Steven Kordonowy and Jacob Watkins
This work considers a variation on the problem of learning shallow
quantum circuits. Given query access to the
circuit, as well as knowledge of its gate layout, we consider the task of learning the specific gates used in the
circuit, producing an operationally-equivalent
circuit matching this structure. Building on recent work for learning Haar random brickwork
circuits, we identify a meta-algorithmic framework for learning broad classes of
circuits based on iterative local gate inversions at the front and back of the
circuit. We apply these techniques to study random, all-to-all, two-local
circuits, and provide analytical and numerical evidence that this ensemble undergoes a sharp learnability transition at depth $d^* \sim \log_2 n + \log_2\log_2 n$ in the large size limit, based on an analysis of lightcone growth. These results have implications for recently proposed
quantum cryptographic schemes based on the difficulty of
circuit learning, though there are important distinctions with respect to our setting that suggest avenues for future study.
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