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Thursday 8 January

Quantum Physics

Hardy nonlocality for entangled pairs in a four-particle system

Duc Manh Doan and Hung Q. Nguyen

Nonlocality can be studied through different approaches, such as Bell's inequalities, and it can be found in numerous quantum states, including GHZ states or graph states. Hardy's paradox, or Hardy-type nonlocality, provides a way to investigate nonlocality for entangled states of particles without using inequalities. Previous studies of Hardy's nonlocality have mostly focused on the fully entangled systems, while other entanglement configurations remain less explored. In this work, the system under investigation consists of four particles arranged in a cyclic entanglement configuration, where each particle forms entangled pairs with two neighbors, while non-neighboring particles remain unentangled. We found that this entanglement structure offers a larger set of conditions that lead to the contradiction with the LHV model, compared to the fully entangled systems. This enhancement can be attributed to the presence of multiple excluded states and correlations, in which the measurement result of a particle only influences the result of its paired partners. We implement quantum circuits compatible with the cyclic entanglement structure, and through simulation, the correlation patterns and the states of interest are identified. We further execute the proposed circuits on IBM Brisbane, a practical backend; however, the results show considerable deviations from the simulation counterparts.
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Quantum Physics · Cross-listed from physics.chem-ph

Classical computational simulation of the FeMo-cofactor model to chemical accuracy and its implications

Huanchen Zhai, Chenghan Li, Xing Zhang et al. (+3 more)

We use classical computational methods to estimate the ground-state energy to chemical accuracy in a model of the FeMo-cofactor of nitrogenase which is widely studied as a target of quantum computing. Our result relies on the insight that the ground-state problem can be characterized as one of ranking many competing, but largely simple, states. This allows a combination of systematic high-order coupled cluster and density matrix renormalization group calculations together with an extrapolation protocol to obtain an accurate energy. Within the model we identify several spin isomer candidates for the ground-state that are degenerate to chemical accuracy. Beyond this model, we characterize the impact of additional electronic excitations and the cluster and protein geometric fluctuations on the low-lying electronic landscape. We find that many features of the landscape are retained in more detailed representations of nitrogenase, which points to the complexity of spectroscopic interpretations of the electronic structure of the FeMo-cofactor.
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All authors Huanchen Zhai, Chenghan Li, Xing Zhang, Zhendong Li, Seunghoon Lee, Garnet Kin-Lic Chan

Quantum Physics

Path Integral Lindblad Dynamics in Presence of Time-Dependent Fields

Amartya Bose

The path integral Lindblad dynamics (PILD) method [A. Bose, J. Phys. Chem. Lett. 15(12), 3363-3368 (2024)] had been introduced as a way of incorporating the impact of certain empirical processes like pumps and drains on the dynamics of quantum systems interacting with thermal environments. The method being based on the time-translational invariance of the Nakajima-Zwanzig memory kernel, however, was not able to account for time-dependent external fields. In this communication, we give an alternate, simpler formulation of PILD, that allows us to go beyond this limitation. It does not require the evaluation of the non-Markovian memory kernel directly, and consequently can be applied to Floquet systems as well.
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Quantum Physics

Multimode Phonon-Number Measurement and Single-shot Superparity Measurement using Dispersive Shifts in a Trapped Ion

Wonhyeong Choi, Jiyong Kang, Kyunghye Kim et al. (+3 more)

Dispersive shifts are a widely used tool for bosonic readout and control in circuit quantum electrodynamics, yet they remain relatively unexplored in trapped-ion motional systems. Here we introduce a unified framework for multimode phonon-number measurement and nondestructive single-shot superparity measurement, i.e., phonon-number measurement modulo 2^k, using dispersive shifts in the far-detuned multimode Jaynes-Cummings interaction of a trapped ion system. We implement a Ramsey sequence that realizes a multimode spin-dependent rotation (SDR) together with a selective decoupling scheme that cancels the phase induced by the carrier AC-Stark shift while preserving the phonon-number-dependent phase induced by the dispersive shift. Within this framework, we infer single-mode and two-mode Fock-state distributions from spin-population dynamics, use SDR-based conditional parity operators with postselection to generate cat states and entangled coherent states, and realize nondestructive single-shot measurements of phonon number modulo 2, 4, and 8 in the single-mode setting. These results open a new avenue for the use of multimode parity operators in trapped-ion bosonic systems.
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All authors Wonhyeong Choi, Jiyong Kang, Kyunghye Kim, Jaehun You, Kyungmin Lee, Taehyun Kim

Quantum Physics

Observation of {\Delta}J=0 Rotational Excitation in Dense Hydrogens

Jie Feng, XiaoDi Liu, Haian Xu et al. (+3 more)

Raman measurements performed on dense H2, D2 and H2+D2 in a wide pressure-temperature range reveal the presence of the {\Delta}J=0 rotational excitation. In the gas/fluid state this excitation has zero Raman shift, but in the solid, the crystal field drive s it away from the zero value e.g. 75 cm-1 at around 50 GPa and 10 K for both isotopes and their mixture. In the case of deuterium, the {\Delta}J=0 mode splits upon entering phase II suggesting a very complex molecular environment of the broken symmetry phase (BSP). In the fluid state and phases I and II the frequencies (energies) of the {\Delta}J=0 transition for H2 and D2 do not scale either as rotational (by factor of 2) nor vibrational (by square 2) modes and appear to be completely isotope independent. This independence on mass marks this transition as unique and a fundamentally different type of excitation from the commonly considered harmonic oscillator and quantum rotor.
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All authors Jie Feng, XiaoDi Liu, Haian Xu, Pu Wang, Graeme J. Ackland, Eugene Gregoryanz

Quantum Physics

Increasing the secret key rates and point-to-multipoint extension for experimental coherent-one-way quantum key distribution protocol

Venkat Abhignan, Mohit Mittal, Aditi Das et al. (+1 more)

Using quantum key distribution (QKD) protocols, a secret key is created between two distant users (transmitter and receiver) at a particular key rate. Quantum technology can facilitate secure communication for cryptographic applications, combining QKD with one-time-pad (OTP) encryption. In order to ensure the continuous operation of QKD in real-world networks, efforts have been concentrated on optimizing the use of experimental components and effective QKD protocols to improve secret key rates and increase the transmission between multiple users. Generally, in experimental implementations, the secret key rates are limited by single-photon detectors, which are used at the receivers of QKD and create a bottleneck due to their limited detection rates (detectors with low detection efficiency and high detector dead-time). We experimentally show that secret key rates can be increased by combining the time-bin information of two such detectors on the data line of the receiver for the coherent-one-way (COW) QKD protocol with a minimal increase in quantum bit error rate (QBER, the proportion of erroneous bits). Further, we implement a point-to-multipoint COW QKD protocol, introducing an additional receiver module. The three users (one transmitter and two receivers) share the secret key in post-processing, relying on OTP encryption. Typically, the dual-receiver extension can improve the combined secret key rates of the system; however, one has to optimise the experimental parameters to achieve this within security margins. These methods are general and can be applied to any implementation of the COW protocol.
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All authors Venkat Abhignan, Mohit Mittal, Aditi Das, Megha Shrivastava

Quantum Physics

Maximal Entanglement and Frozen Information: A Unified Framework for Dynamical Quantum Phase Transitions

Kaiyuan Cao, Mingzhi Li, Xiang-Ping Jiang et al. (+2 more)

Dynamical quantum phase transitions (DQPTs) are temporal singularities marked by zeros of the Loschmidt echo, yet their underlying quantum-information structure remains elusive. Here, we introduce a momentum-resolved entanglement entropy as a direct probe of DQPTs in translation-invariant free systems. We analytically establish that every critical momentum mode $k^{*}$ associated with a DQPT saturates its entanglement to the maximal value $\ln{2}$, coinciding with the vanishing of the Loschmidt echo. Crucially, we demonstrate that this maximal entanglement universally suppresses information scrambling: a momentum-resolved out-of-time-ordered correlator (OTOC) vanishes identically for all times at $k^{*}$. These three signatures – Fisher zeros, maximal entanglement, and vanished OTOC – are proved to be equivalent in both the transverse-field Ising and Su-Schrieffer-Heeger models, despite their distinct bipartitions (momentum-pair vs. sublattice). Our results establish a unified, information-theoretic framework for DQPTs, revealing them a points where quantum correlations saturate and information flow halts. This work elevates entanglement and scrambling to central dynamical order parameters, offering a universal perspective on nonequilibrium quantum critically.
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All authors Kaiyuan Cao, Mingzhi Li, Xiang-Ping Jiang, Shu Chen, Jian Wang

Quantum Physics

Holographic codes seen through ZX-calculus

Kwok Ho Wan, H.C.W. Price, Qing Yao et al. (+2 more)

We re-visit the pentagon holographic quantum error correcting code from a ZX-calculus perspective. By expressing the underlying tensors as ZX-diagrams, we study the stabiliser structure of the code via Pauli webs. In addition, we obtain a diagrammatic understanding of its logical operators, encoding isometries, Rényi entropy and toy models of black holes/wormholes. Then, motivated by the pentagon holographic code's ZX-diagram, we introduce a family of codes constructed from ZX-diagrams on its dual hyperbolic tessellations and study their logical error rates using belief propagation decoders. Finally, we show how to construct spacetime ZX-diagrams that realise a fault-tolerant quantum channel in which every internal edge is a protected fault location.
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All authors Kwok Ho Wan, H.C.W. Price, Qing Yao, Zhenghao Zhong, Ainhoa Zapirain

Wednesday 7 January

Quantum Physics

Investigation of Hardware Architecture Effects on Quantum Algorithm Performance: A Comparative Hardware Study

Askar Oralkhan and Temirlan Zhaxalykov

Cloud-accessible quantum processors enable direct execution of quantum algorithms on heterogeneous hardware platforms. Unlike classical systems, however, identical quantum circuits may exhibit substantially different behavior across devices due to architectural variations in qubit connectivity, gate fidelity, and coherence times. In this work, we systematically benchmark five representative quantum algorithms - Bell state preparation, GHZ state generation, Quantum Fourier Transform (QFT), Grover's Search, and the Quantum Approximate Optimization Algorithm (QAOA) - across trapped-ion, superconducting, and simulator backends using Amazon Braket. Performance metrics including fidelity, CHSH violation, success probability, circuit depth, and gate counts are evaluated. Our results demonstrate a strong dependence of algorithmic performance on hardware topology and noise characteristics. For example, 10-qubit GHZ states achieved fidelities above 0.8 on trapped-ion hardware, while superconducting platforms dropped below 0.15 due to routing overhead and accumulated two-qubit gate errors. These findings highlight the importance of hardware-aware algorithm selection and provide practical guidance for benchmarking in the NISQ era.
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Quantum Physics

Nearly Time-Optimal Pure State Tomography with Pauli Measurements

Sabee Grewal, Meghal Gupta, William He et al. (+2 more)

We give an algorithm for pure state tomography with near-optimal copy and time complexity using only single-qubit measurements. Specifically, given $\widetilde{O}(2^n/\epsilon)$ copies of an unknown $n$-qubit pure state $|\psi\rangle$, the algorithm performs only nonadaptive Pauli measurements, runs in time $\widetilde{O}(2^n/\epsilon)$, and outputs $|\widehat{\psi} \rangle$ with fidelity at least $1-\epsilon$ with $|\psi\rangle$ with high probability. This is the first algorithm for pure state tomography that achieves near-optimal running time.
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All authors Sabee Grewal, Meghal Gupta, William He, Aniruddha Sen, Mihir Singhal

Quantum Physics

A Broadband Nanowire Quantum Dot Cavity Design for the Efficient Extraction of Entangled Photons

Sayan Gangopadhyay, Sasan V. Grayli, Sathursan Kokilathasan et al. (+1 more)

A bright source of on-demand entangled photons is needed for quantum networks. A single quantum dot in a site-selected nanowire waveguide is a promising candidate for realizing such sources. However, such sources are associated with poor single-photon indistinguishability, limiting their applicability in quantum networks. A common approach for enhancing the single-photon indistinguishability in quantum dot-based entangled photon sources is to implement a broadband optical cavity. Achieving a high-Purcell cavity while retaining the advantages of the nanowire, such as directional emission, a broad operational bandwidth, and high light extraction efficiency, has been a significant challenge. Here, we propose a nanowire cavity based on quasi-bound states in the continuum formed by the strong coupling of two resonant optical modes. We numerically predict this design to support a cavity mode with 4 nm bandwidth and a Purcell enhancement of $\sim$17. This cavity mode enables a directional far-field emission profile (88% overlap with a Gaussian) with a light extraction efficiency of $\sim$74%. Our solution opens up a route for generating entangled photon pairs with enhanced extraction efficiency and single-photon indistinguishability for the practical realization of quantum networks.
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All authors Sayan Gangopadhyay, Sasan V. Grayli, Sathursan Kokilathasan, Michael E. Reimer

Quantum Physics

Solving nonlinear differential equations on noisy $156$-qubit quantum computers

Karla Baumann, Youcef Modheb, Roman Randrianarisoa et al. (+3 more)

In this paper, we report on the resolution of nonlinear differential equations using IBM's quantum platform. More specifically, we demonstrate that the hybrid classical-quantum algorithm H-DES successfully solves a one-dimensional material deformation problem and the inviscid Burgers' equation on IBM's 156-qubit quantum computers. These results constitute a step toward performing physically relevant simulations on present-day Noisy Intermediate-Scale Quantum (NISQ) devices.
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All authors Karla Baumann, Youcef Modheb, Roman Randrianarisoa, Roland Katz, Aoife Boyle, Frédéric Holweck

Quantum Physics

Implementation of Tensor Network Simulation TN-Sim under NWQ-Sim

Aaron C. Hoyt, Jonathan S. Bersson, Sean Garner et al. (+2 more)

Large-scale tensor network simulations are crucial for developing robust complexity-theoretic bounds on classical quantum simulation, enabling circuit cutting approaches, and optimizing circuit compilation, all of which aid efficient quantum computation on limited quantum resources. Modern exascale high-performance computing platforms offer significant potential for advancing tensor network quantum circuit simulation capabilities. We implement TN-Sim, a tensor network simulator backend within the NWQ-Sim software package that utilizes the Tensor Algebra for Many-body Methods (TAMM) framework to support both distributed HPC-scale computations and local simulations with ITensor. To optimize the scale up in computation across multiple nodes we implement a task based parallelization scheme to demonstrate parallelized gate contraction for wide quantum circuits with many gates per layer. Through the integration of the TAMM framework with Matrix Product State (MPS) tensor network approaches, we deliver a simulation environment that can scale from local systems to HPC clusters. We demonstrate an MPS tensor network simulator running on the state-of-the-art Perlmutter (NVIDIA) supercomputer and discuss the potential portability of this software to HPC clusters such as Frontier (AMD) and Aurora (Intel). We also discuss future improvements including support for different tensor network topologies and enhanced computational efficiency.
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All authors Aaron C. Hoyt, Jonathan S. Bersson, Sean Garner, Chenxu Liu, Ang Li

Quantum Physics · Cross-listed from cs.LG

Distribution-Guided and Constrained Quantum Machine Unlearning

Nausherwan Malik, Zubair Khalid and Muhammad Faryad

Machine unlearning aims to remove the influence of specific training data from a learned model without full retraining. While recent work has begun to explore unlearning in quantum machine learning, existing approaches largely rely on fixed, uniform target distributions and do not explicitly control the trade-off between forgetting and retained model behaviour. In this work, we propose a distribution-guided framework for class-level quantum machine unlearning that treats unlearning as a constrained optimization problem. Our method introduces a tunable target distribution derived from model similarity statistics, decoupling the suppression of forgotten-class confidence from assumptions about redistribution among retained classes. We further incorporate an anchor-based preservation constraint that explicitly maintains predictive behaviour on selected retained data, yielding a controlled optimization trajectory that limits deviation from the original model. We evaluate the approach on variational quantum classifiers trained on the Iris and Covertype datasets. Results demonstrate sharp suppression of forgotten-class confidence, minimal degradation of retained-class performance, and closer alignment with the gold retrained model baselines compared to uniform-target unlearning. These findings highlight the importance of target design and constraint-based formulations for reliable and interpretable quantum machine unlearning.
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Quantum Physics

Exact Multimode Quantization of Superconducting Circuits via Boundary Admittance and Continued Fractions

Mustafa Bakr and Robin Wopalenski

Accurate extraction of linearized quantum circuit models from electromagnetic simulations is essential for the design of superconducting circuits. We present a quantization framework based on the driving-point admittance $Y_{\mathrm{in}}(s)$ seen by a Josephson junction embedded in an arbitrary passive linear environment. By taking the Schur complement of the nodal admittance matrix, we show that the linearized coupled system obeys an eigenvalue-dependent boundary condition, $s Y_{\mathrm{in}}(s) + 1/L_J = 0$, whose roots determine the dressed linear mode frequencies. This boundary condition admits an exact continued fraction representation: any positive-real admittance can be realized as a canonical Cauer ladder, yielding a tridiagonal (Jacobi) structure that enables certified convergence bounds via interlacing theorems.For the full nonlinear Hamiltonian, we treat Josephson junctions in the charge basis, where each cosine potential is exactly tridiagonal, and couple them to cavity modes in the Fock basis; in the general multi-junction case this yields a block-tridiagonal structure solvable by matrix continued fractions, enabling systematic diagonalization across all coupling regimes from dispersive through ultrastrong and deep strong coupling. The resulting quantization procedure is: (i)~compute or measure $Y_{\mathrm{in}}(s)$, (ii)~solve the boundary condition to obtain dressed eigenfrequencies, (iii)~synthesize an equivalent passive network, and (iv)~quantize while retaining the full cosine nonlinearity of the Josephson junction. We prove that junction participation decays as $\mathcal{O}(\omega_n^{-1})$ at high frequencies for any circuit with finite shunt capacitance, ensuring ultraviolet convergence of perturbative corrections without imposed cutoffs.
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Quantum Physics

Thermodynamic significance of QUBO encoding on quantum annealers

Emery Doucet, Zakaria Mzaouali, Reece Robertson et al. (+3 more)

Quadratic unconstrained binary optimization (QUBO) is the standard interface to quantum annealers, yet a single constrained task admits many QUBO encodings whose penalty choices reshape the energy landscape experienced by hardware. We study a Job Shop Scheduling instance using a two-parameter family of encodings controlled by penalty weights $p_{\rm sum}$ (one-hot/sum constraints) and $p_{\rm pair}$ (precedence constraints). Sweeping $(p_{\rm sum},p_{\rm pair})$, we observe sharp transitions in feasibility and solver success across classical annealing-inspired heuristics and on a D-Wave Advantage processor. Going beyond solution probability, we treat the annealer as an open thermodynamic system and perform cyclic reverse-annealing experiments initialized from thermal samples, measuring the stochastic processor energy change. From the first two moments of this energy change we infer lower bounds on entropy production, work, and exchanged heat via thermodynamic uncertainty relations, and corroborate the observed trends with adiabatic master equation simulations. We find that the same encoding transitions that govern computational hardness also reorganize dissipation: weak penalties generate low-energy infeasible manifolds, while overly strong penalties suppress the effective problem energy scale and increase irreversibility, reducing the thermodynamic efficiency. Our results establish QUBO penalties as thermodynamic control knobs and motivate thermodynamics-aware encoding strategies for noisy intermediate-scale quantum annealers.
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All authors Emery Doucet, Zakaria Mzaouali, Reece Robertson, Bartłomiej Gardas, Sebastian Deffner, Krzysztof Domino

Quantum Physics

Solving nonlinear PDEs with Quantum Neural Networks: A variational approach to the Bratu Equation

Nikolaos Cheimarios

We present a variational quantum algorithm (VQA) to solve the nonlinear one-dimensional Bratu equation. By formulating the boundary value problem within a variational framework and encoding the solution in a parameterized quantum neural network (QNN), the problem reduces to an optimization task over quantum circuit parameters. The trial solution incorporates a predictor from the previous continuation step and boundary-enforcing terms, allowing the circuit to focus on minimizing the residual of the differential operator. Using a noiseless quantum simulator, we demonstrate that the method accurately captures both solution branches of the Bratu equation and shows excellent agreement with classical pseudo arc-length continuation results.
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Quantum Physics

Quantum sensing with critical systems: impact of symmetry, imperfections, and decoherence

Yinan Chen, Sara Murciano, Pablo Sala et al. (+1 more)

Entangled many-body states enable high-precision quantum sensing beyond the standard quantum limit. We develop interferometric sensing protocols based on quantum critical wavefunctions and compare their performance with Greenberger-Horne-Zeilinger (GHZ) and spin-squeezed states. Building on the idea of symmetries as a metrological resource, we introduce a symmetry-based algorithm to identify optimal measurement strategies. We illustrate this algorithm both for magnetic systems with internal symmetries and Rydberg-atom arrays with spatial symmetries. We study the robustness of criticality for quantum sensing under non-unitary deformations, symmetry-preserving and symmetry-breaking decoherence, and qubit loss – identifying regimes where critical systems outperform GHZ states and showing that non-unitary deformation can even enhance sensing precision. Combined with recent results on log-depth preparation of critical wavefunctions, interferometric sensing in this setting appears increasingly promising.
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All authors Yinan Chen, Sara Murciano, Pablo Sala, Jason Alicea

Quantum Physics · Cross-listed from cond-mat.quant-gas

Scalable cold-atom quantum simulator of a $3+1$D U$(1)$ lattice gauge theory with dynamical matter

Simone Orlando, Guo-Xian Su, Bing Yang et al. (+1 more)

The stated overarching goal of the highly active field of quantum simulation of high-energy physics (HEP) is to achieve the capability to study ab-initio real-time microscopic dynamics of $3+1$D quantum chromodynamics (QCD). However, existing experimental realizations and theoretical proposals for future ones have remained restricted to one or two spatial dimensions. Here, we take a big step towards this goal by proposing a concrete experimentally feasible scalable cold-atom quantum simulator of a U$(1)$ quantum link model of quantum electrodynamics (QED) in three spatial dimensions, employing linear gauge protection to stabilize gauge invariance. Using tree tensor network simulations, we benchmark the performance of this quantum simulator through near- and far-from-equilibrium observables, showing excellent agreement with the ideal gauge theory. Additionally, we introduce a method for analog quantum error mitigation that accounts for unwanted first-order tunneling processes, vastly improving agreement between quantum-simulator and ideal-gauge-theory results. Our findings pave the way towards realistic quantum simulators of $3+1$D lattice gauge theories that can probe regimes well beyond classical simulability.
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All authors Simone Orlando, Guo-Xian Su, Bing Yang, Jad C. Halimeh

Quantum Physics

Rare-Event Quantum Sensing using Logical Qubits

Robert Ott, Torsten V. Zache, Soonwon Choi et al. (+2 more)

We present a novel protocol to detect rare signals in a noisy environment using quantum error correction (QEC). The key feature of our protocol is the discrimination between signal and noise through distinct higher-order correlations, realized by the non-linear processing that occurs during syndrome extraction in QEC. In this scheme, QEC has two effects: First, it sacrifices part of the signal $\epsilon$ by recording a reduced, stochastic, logical phase $\phi_L = \mathcal{O}(\epsilon^3)$. Second, it corrects the physical noise and extends the (logical) coherence time for signal acquisition. For rare signals occurring at random times in the presence of local Markovian noise, we explicitly demonstrate an improved sensitivity of our approach over more conventional sensing strategies.
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All authors Robert Ott, Torsten V. Zache, Soonwon Choi, Adam M. Kaufman, Hannes Pichler

Quantum Physics

Microscopic Dynamics of False Vacuum Decay in the $2+1$D Quantum Ising Model

Umberto Borla, Achilleas Lazarides, Christian Groß et al. (+1 more)

False vacuum decay, which is understood to happen through bubble nucleation, is a prominent phenomenon relevant to elementary particle physics and early-universe cosmology. Understanding its microscopic dynamics in higher spatial dimensions is currently a major challenge and research thrust. Recent advances in numerical techniques allow for the extraction of related signatures in tractable systems in two spatial dimensions over intermediate timescales. Here, we focus on the $2+1$D quantum Ising model, where a longitudinal field is used to energetically separate the two $\mathbb{Z}_2$ symmetry-broken ferromagnetic ground states, turning them into a “true” and “false” vacuum. Using tree tensor networks, we simulate the microscopic dynamics of a spin-down domain in a spin-up background after a homogeneous quench, with parameters chosen so that the domain corresponds to a bubble of the true vacuum in a false-vacuum background. Our study identifies how the ultimate fate of the bubble – indefinite expansion or collapse – depends on its geometrical features and on the microscopic parameters of the Ising Hamiltonian. We further provide a realistic quantum-simulation scheme, aimed at probing bubble dynamics on atomic Rydberg arrays.
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All authors Umberto Borla, Achilleas Lazarides, Christian Groß, Jad C. Halimeh

Quantum Physics · Cross-listed from hep-th

Chiral Lattice Gauge Theories from Symmetry Disentanglers

Ryan Thorngren, John Preskill and Lukasz Fidkowski

We propose a Hamiltonian framework for constructing chiral gauge theories on the lattice based on symmetry disentanglers: constant-depth circuits of local unitaries that transform not-on-site symmetries into on-site ones. When chiral symmetry can be realized not-on-site and such a disentangler exists, the symmetry can be implemented in a strictly local Hamiltonian and gauged by standard lattice methods. Using lattice rotor models, we realize this idea in 1+1 and 3+1 spacetime dimensions for $U(1)$ symmetries with mixed 't Hooft anomalies, and show that symmetry disentanglers can be constructed when anomalies cancel. As an example, we present an exactly solvable Hamiltonian lattice model of the (1+1)-dimensional "3450" chiral gauge theory, and we argue that a related construction applies to the $U(1)$ hypercharge symmetry of the Standard Model fermions in 3+1 dimensions. Our results open a new route toward fully local, nonperturbative formulations of chiral gauge theories.
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Quantum Physics

Improved Lower Bounds for Learning Quantum Channels in Diamond Distance

Aadil Oufkir and Filippo Girardi

We prove that learning an unknown quantum channel with input dimension $d_A$, output dimension $d_B$, and Choi rank $r$ to diamond distance $\varepsilon$ requires $ \Omega\!\left( \frac{d_A d_B r}{\varepsilon \log(d_B r / \varepsilon)} \right)$ channel queries when $d_A= rd_B$, and $\Omega\!\left( \frac{d_A d_B r}{\varepsilon^2 \log(d_B r / \varepsilon)} \right)$ channel queries when $d_A\le rd_B/2$. These lower bounds improve upon the best previous $\Omega(d_A d_B r)$ bound by introducing explicit, near-optimal $\varepsilon$-dependence. Moreover, when $d_A\le rd_B/2$, the lower bound is optimal up to a logarithmic factor. The proof constructs ensembles of channels that are well separated in diamond norm yet admit Stinespring isometries that are close in operator norm.
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Quantum Physics

Below-shot-noise capacity in phase estimation using nonlinear interferometers

Cristofero Oglialoro, Gerard J. Machado, Felix Farsch et al. (+7 more)

Over the past decade, several schemes for imaging and sensing based on nonlinear interferometers have been proposed and demonstrated experimentally. These interferometers exhibit two main advantages. First, they enable probing a sample at a chosen wavelength while detecting light at a different wavelength with high efficiency (bicolor quantum imaging and sensing with undetected light). Second, they can show quantum-enhanced sensitivities below the shot-noise limit, potentially reaching Heisenberg-limited precision in parameter estimation. Here, we compare three quantum-imaging configurations using only easily accessible intensity-based measurements for phase estimation: a Yurke-type SU(1,1) interferometer, a Mandel-type induced-coherence interferometer, and a hybrid scheme that continuously interpolates between them. While an ideal Yurke interferometer can exhibit Heisenberg scaling, this advantage is known to be fragile under realistic detection constraints and in the presence of loss. We demonstrate that differential intensity detection in the Mandel interferometer provides the highest and most robust phase sensitivity among the considered schemes, reaching but not surpassing the shot-noise limit, even in the presence of loss. Intensity measurements in a Yurke-type configuration can achieve genuine sub-shot-noise sensitivity under balanced losses and moderate gain; however, their performance degrades in realistic high-gain regimes. Consequently, in this regime, the Mandel configuration with differential detection outperforms the Yurke-type setup and constitutes the most robust approach for phase estimation.
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All authors Cristofero Oglialoro, Gerard J. Machado, Felix Farsch, Daniel F. Urrego, Alejandra A. Padilla, Raj B. Patel, Ian A. Walmsley, Markus Gräfe, Juan P. Torres, Enno Giese

2601.04139 · 7 Jan · 16 pages, 6 figures, are data and code required to reproduce the figures presented in this paper are publicly available in an online repository at https://doi.org/10.5281/zenodo.21703011, based on https://github.com/Cristofero1/below-shot-noise-phase-estimation

Abstract PDF $QNTM ×3

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

Bridging the Linear-Quadratic Gap: A Quantum-Classical Hybrid Approach to Robust Supply Chain Design

Rudraksh Sharma, Ravi Katukam and Arjun Nagulapally

The design of supply chain networks in densely populated urban logistics systems faces a timely dilemma: the traditional optimisation approaches are effective to maximise the level of demand perfusion, but they are limited to embracing large expenses in overlapping the facilities and cannibalisation in the market. When tested on a high-fidelity digital twin of the Delhi NCR road network of thirty candidate sites, we establish that Classical Greedy algorithms using the theoretical maximum demand of (473 units) lack any theoretical overlap penalty, but incur a prohibitive overlap penalty (5.08). Here, in comparison, the Quantum-Inspired solution only losses 3.2% of demand (450 compared to 465 units relative to the optimal solution), but the solution preserves 21.8% less operational overlap risk (3.26 compared to 4.17), which can be viewed as a 35.8% improvement compared to the Greedy solution. Geospatial analysis shows that it can be attributed to a shift in strategies: This, in contrast to Classical approaches, which focus on locating facilities in the high-density central areas (North/Central Delhi), the quantum-inspired solver autonomously chooses the diversified topology of the North-south network, penetrating into the underserved periphery growth markets. This is a spatially balanced arrangement which is congruent to the polycentric structure of modern time megacities, and displays better stability to volatility in demand. We have shown that quantum-inspired optimisation methods can close the so-called Linear-Quadratic Gap phenomenon, i.e. the systematic inability of greedy methods to capture the actual quadratic interactions between facilities, and offer a way of computing the pathway to operationally robust and risk-optimised supply chain networks in dense urban conditions.
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Quantum Physics

Scattering phase shift in quantum mechanics on quantum computers

Peng Guo, Paul LeVan, Frank X. Lee et al. (+1 more)

We investigate the feasibility of extracting infinite volume scattering phase shift on quantum computers in a simple one-dimensional quantum mechanical model, using the formalism established in Ref.~\cite{Guo:2023ecc} that relates the integrated correlation functions (ICF) for a trapped system to the infinite volume scattering phase shifts through a weighted integral. The system is first discretized in a finite box with periodic boundary conditions, and the formalism in real time is verified by employing a contact interaction potential with exact solutions. Quantum circuits are then designed and constructed to implement the formalism on current quantum computing architectures. To overcome the fast oscillatory behavior of the integrated correlation functions in real-time simulation, different methods of post-data analysis are proposed and discussed. Test results on IBM hardware show that good agreement can be achieved with two qubits, but complete failure ensues with three qubits due to two-qubit gate operation errors and thermal relaxation errors.
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All authors Peng Guo, Paul LeVan, Frank X. Lee, Yong Zhao

Quantum Physics

Surface Optimization of Superconducting Aluminum Resonators for Robust Quantum Device Fabrication

Simon J. K. Lang, Ignaz Eisele, Alwin Maiwald et al. (+8 more)

Aluminum (Al) remains the central material for superconducting qubits, and considerable effort has been devoted to optimizing its deposition and patterning for quantum devices. However, post-processing strategies focused on oxide removal of niobium (Nb) and tantalum (Ta) -based resonators using buffered oxide etch (BOE), which can not be used for Al. This challenge becomes particularly relevant for industry-scale fabrication with multi-chip bonding, where delays between sample preparation and cooldown require surface treatments that preserve low dielectric loss during extended exposure to ambient conditions. In this work, we investigate surface modification approaches for Al resonators subjected to a 24-hour delay prior to cryogenic measurement. Passivation using self-limiting oxygen and fluorine chemistries was evaluated utilizing different plasma processes. Remote oxygen plasma treatment reduced dielectric losses, in contrast to direct oxygen plasma. A fluorine-based plasma process was developed that passivated the Al surface for subsequent BOE treatment. However, the fluorine content in the surface resulted in higher loss, identifying fluorine as an unsuitable passivation material for Al resonators. Above all, selective oxide removal using HF (hydrogen fluoride) vapor and phosphoric acid yielded median dielectric losses as low as $\tilde{\delta}_\mathrm{LP} = 5.7 \times 10^{-7}$ ($Q_\mathrm{LP} \approx 1.7\,\mathrm{M}$) with $\tilde{\delta}_\mathrm{TLS} = 3.6 \times 10^{-7}$ ($Q_\mathrm{TLS} \approx 2.8\,\mathrm{M}$) in the single photon regime. Selective oxide removal provides a promising pathway for robust Al-based qubit fabrication, as it preserves low dielectric losses for a 24-hour delay before cooldown.
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All authors Simon J. K. Lang, Ignaz Eisele, Alwin Maiwald, Emir Music, Luis Schwarzenbach, Carla Moran-Guizan, Johannes Weber, Daniela Zahn, Thomas Mayer, Rui N. Pereira, Christoph Kutter

Quantum Physics · Cross-listed from q-fin.CP

Quantum computing for multidimensional option pricing: End-to-end pipeline

Julien Hok and Álvaro Leitao

This work introduces an end-to-end framework for multi-asset option pricing that combines market-consistent risk-neutral density recovery with quantum-accelerated numerical integration. We first calibrate arbitrage-free marginal distributions from European option quotes using the Normal Inverse Gaussian (NIG) model, leveraging its analytical tractability and ability to capture skewness and fat tails. Marginals are coupled via a Gaussian copula to construct joint distributions. To address the computational bottleneck of the high-dimensional integration required to solve the option pricing formula, we employ Quantum Accelerated Monte Carlo (QAMC) techniques based on Quantum Amplitude Estimation (QAE), achieving quadratic convergence improvements over classical Monte Carlo (CMC) methods. Theoretical results establish accuracy bounds and query complexity for both marginal density estimation (via cosine-series expansions) and multidimensional pricing. Empirical tests on liquid equity entities (Credit Agricole, AXA, Michelin) confirm high calibration accuracy and demonstrate that QAMC requires 10-100 times fewer queries than classical methods for comparable precision. This study provides a practical route to integrate arbitrage-aware modelling with quantum computing, highlighting implications for scalability and future extensions to complex derivatives.
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Quantum Physics

Phase-Randomized Laser Pulse Generation at 10 GHz for Quantum Photonic Applications

Yuen San Lo, Adam H. Brzosko, Peter R. Smith et al. (+7 more)

Gain-switching of laser diodes is a well-established technique for generating optical pulses with random phases, where the quantum randomness arises naturally from spontaneous emission. However, the maximum switching rate is limited by the time required for phase diffusion: at high repetition rates, residual photons in the cavity seed subsequent pulses, leading to phase correlations, which degrade randomness. We present a method to overcome this limitation by employing an external source of spontaneous emission in conjunction with the laser. Our results show that this approach substantially suppresses interpulse phase correlations, restoring phase randomization at repetition rates as high as 10 GHz. This technique opens new opportunities for high-rate quantum key distribution and quantum random number generation.
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All authors Yuen San Lo, Adam H. Brzosko, Peter R. Smith, Robert I. Woodward, Davide G. Marangon, James F. Dynes, Sergio Juárez, Taofiq K. Paraïso, R. Mark Stevenson, Andrew J. Shields

Quantum Physics

Limitations for adaptive quantum state tomography in the presence of detector noise

Adrian Skasberg Aasen and Martin Gärttner

Assumption-free reconstruction of quantum states from measurements is essential for benchmarking and certifying quantum devices, but it remains difficult due to the extensive measurement statistics and experimental resources it demands. An approach to alleviating these demands is provided by adaptive measurement strategies, which can yield up to a quadratic improvement in reconstruction accuracy for pure states by dynamically optimizing measurement settings during data acquisition. A key open question is whether these asymptotic advantages remain in realistic experiments, where readout is inevitably noisy. In this work, we analyze the impact of readout noise on adaptive quantum state tomography with readout-error mitigation, focusing on the challenging regime of reconstructing pure states using mixed-state estimators. Using analytical arguments based on Fisher information optimization and extensive numerical simulations using Bayesian inference, we show that any nonzero readout noise eliminates the asymptotic quadratic scaling advantage of adaptive strategies. We numerically investigate the behavior for finite measurement statistics for single- and two-qubit systems with exact readout-error mitigation and find a gradual transition from ideal to sub-optimal scaling. We furthermore investigate realistic scenarios where detector tomography is performed with a limited number of state copies for calibration, showing that insufficient detector characterization leads to estimator bias and limited reconstruction accuracy. Although our result imposes an upper bound on the reconstruction accuracy that can be achieved with adaptive strategies, we nevertheless observe numerically a constant-factor gain in reconstruction accuracy, which becomes larger as the readout noise decreases. This indicates potential practical benefits in using adaptive measurement strategies in well-calibrated experiments.
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