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Friday 7 August

Quantum Physics

Ultrafast quantum gate operations in a Kramers-Henneberger atom Qubit

A. Tasnim Aynul, C. Li, L. Cruz Rodriguez et al. (+1 more)

We propose and demonstrate the Kramers-Henneberger KH) atom as a novel qubit platform for ultrafast single-qubit gate operations. In the KH frame, the time-averaged strong laser field engineers a double-well potential whose two lowest eigenstates define the qubit basis, so that the computational structure is created and maintained by the driving field itself. A weak resonant control field drives coherent gate operations: full time-dependent Schrödinger equation simulations with the complete time-dependent KH potential confirm a Z gate and S gate of the order of femtoseconds, six orders of magnitude faster than laser-driven superconducting qubit gates. Decoherence characterisation gives longer decoherence times than those required for the gate operations. The complete six-gate single-qubit set is demonstrated in the time-averaged two-level limit, with strong agreement between the full and time-averaged descriptions confirming that fidelity is limited by structured leakage rather than stochastic decoherence. In principle, this is an error channel suppressible through pulse engineering. These results constitute the first demonstration of coherent single-qubit gates in a strong-field setting, with a clear pathway toward attosecond-scale operations.
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All authors A. Tasnim Aynul, C. Li, L. Cruz Rodriguez, C. Figueira de Morisson Faria

Quantum Physics

Exploring the Relaxation Landscape of a 2D Quantum Magnet on a 256-Qubit Processor

Tiago Mendes-Santos, Joseph Vovrosh, Sergi Julià-Farré et al. (+33 more)

How quantum matter relaxes far from equilibrium is a central open problem in many-body physics, and one for which analog quantum simulators are well positioned to move from confirming theory to discovering new physics. Here, we use a two-dimensional Rydberg atom array of 256 qubits to map the relaxation landscape of the two-dimensional transverse-field Ising model across its phase diagram. Beyond the expected rapid thermalization, we identify two further regimes. The first is a prethermal regime whose dynamics are governed by an effective XY model. The second, and most unexpected, is a crossover regime characterized by a slowdown in relaxation. This slowdown occurs precisely where state-of-the-art classical tensor-network methods lose control at late times, whereas the quantum simulation remains consistent across system sizes. These results establish Rydberg atom arrays as a platform for scientific discovery in nonequilibrium quantum many-body dynamics.
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All authors Tiago Mendes-Santos, Joseph Vovrosh, Sergi Julià-Farré, Dorian Claveau, Guillaume Villaret, Lucas Béguin, Lucas Leclerc, Laurin Brunner, Wladislaw Krinitsin, Matthias Hecker, Fergus Hayes, Boris Albrecht, Lilian Bourachot, Clémence Briosne-Frejaville, Antoine Cornillot, Julius de Hond, Djibril Diallo, Clément Dupays, Robin Dupont, Thomas Eritzpokhoff, Loïc Henriet, Lucas Lassablière, Arvid Lindberg, Yohann Machu, Hadriel Mamann, Thomas Pansiot, Julien Ripoll, Bruno Ximenez, Henrique Silvério, Joseph Tindall, Markus Schmitt, Markus Heyl, Adrien Signoles, Constantin Dalyac, Antoine Browaeys, Alexandre Dauphin

Quantum Physics

Rate-Fidelity Control for Wide-Area Quantum Links

Connor Clayton, Cory Nunn, Quinn Carmack et al. (+4 more)

Quantum network links must distribute entanglement at high rates while satisfying application-specified fidelity demands. However, wide-area deployed fiber links suffer from polarization drift which destabilizes end-to-end fidelity and forces periodic compensation. Current deployments often use active stabilization with fixed control policies, and improvements generally stem from advances in quantum hardware. Meanwhile, software control remains relatively underexplored. Here, we formulate quantum link operation as a joint control problem over tunable rate-fidelity tradeoffs and uncontrollable link drift. From this framework, we construct a link control protocol that dynamically adapts source pump power and polarization compensation to maximize entanglement distribution rate subject to a minimum fidelity constraint. We evaluate the protocol through trace-driven simulations driven by data from a 64 km deployed optical fiber. Compared with optimized static policies, our adaptive controller improves mean entanglement distribution rate by 14% over a 24 hour trace, without requiring any offline policy optimization. Our results show that software-based physical layer control can provide a practical mechanism for improving near-term quantum link performance without requiring additional quantum hardware.
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All authors Connor Clayton, Cory Nunn, Quinn Carmack, Wayne McKenzie, Anne Marie Richards, Xiaodi Wu, Bobby Bhattacharjee

Quantum Physics

Many-Body Localization Induced by Correlated Disorder in Interacting Superconducting Qubits

Thiago R. Girão Souza, Andreia Saguia, Alan C. Santos et al. (+1 more)

The failure of quantum thermalization due to Many-Body Localization (MBL) has evolved from a theoretical concept in spin chains to an experimental reality in synthetic quantum platforms, most notably superconducting circuits based on transmon qubits. Despite its significance, the MBL transition has been studied primarily under purely random disorder and local couplings, leaving more complex and realistic configurations largely unexplored. Here, we investigate the quantum dynamics of transmon networks subject to the unavoidable competing effects of correlated disorder and network-mediated non-local interactions. First, we show how to engineer the physical parameters of the quantum hardware to systematically control the correlated disorder patterns emerging in the system. Then, we demonstrate that the MBL phase transition is robust against such correlations, which is essential for tuning localization properties in realistic transmon devices. This robustness is established through the analysis of the block entanglement entropy variance across disorder realizations, which precisely locates the MBL critical point. Independently, we introduce a local memory parameter, whose dynamics at long evolution times reveals memory retention in the localized phase and yields a critical point consistent with the entropy analysis. These results provide a framework for understanding localization in complex quantum network topologies, with potential implications for multi-qubit processor design.
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All authors Thiago R. Girão Souza, Andreia Saguia, Alan C. Santos, Marcelo S. Sarandy

Quantum Physics

From triangles to prisms: towards a geometric extension of concurrence fill for three-qubit mixed states

Divyanshu K. Verma and Girish Kulkarni

The quantification of multipartite entanglement remains a long-standing open challenge in quantum information theory with major implications for quantum foundations and quantum technologies. In the context of three-qubit pure states, the concurrence triangle underlies geometric interpretations for a variety of multipartite entanglement measures. In particular, the concurrence fill, which is proportional to the square root of the area of the concurrence triangle, has been shown to be a valid measure of genuine tripartite entanglement. As a result, there is now significant interest in extending the concurrence triangle and concurrence fill to three-qubit mixed states. In this work, we show that any rank-2 three-qubit mixed state can be visualized in terms of concurrence prisms associated with their pure state decompositions. Thus, the concurrence fill of the mixed state obtained through convex-roof extension is proportional to the total prism fill area, defined as the square root of the base area times the height, associated with the prisms minimized over all pure state decompositions. We then consider three-qubit mixed states that are incoherent mixtures of Greenberger-Horne-Zeilinger (GHZ) and W states and analytically perform their convex-roof extension to obtain a closed-form expression for the concurrence fill of these rank-2 mixtures. Finally, we apply the geometric picture of concurrence prisms to these mixtures and their pure state decomposition corresponding to minimum concurrence fill as a tool for visualization.
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Quantum Physics

A unifying framework for quantum algorithms for time-dependent non-unitary dynamics

Xiaojing Dong, Chuwen Ma, Yizhe Peng et al. (+1 more)

We present an improved autonomization method for quantum simulation of time-dependent homogeneous dissipative linear systems, combining a clock-variable reformulation with Schrödingerization to obtain a time-independent Hamiltonian system. To control both discretization error and recovery probability, we construct the clock profile from a compactly supported window function and a normalized Dirichlet kernel: the former imposes endpoint regularity, while the latter concentrates the sampled mass and keeps the discrete normalization bounded. For the subsequent Schrödingerization step, we use an exactly periodic version of an error-function initial profile, whose explicit Fourier coefficients allow direct error estimates and simplify the analysis of smooth initialization. By combining Fourier projection with weighted recovery, we retain a single logarithmic precision factor in the full algorithm. Under finite-order derivative bounds, sampled matrix access, and exact state-preparation access, we prepare the normalized solution at time $T$ to accuracy $\varepsilon$ with success probability $\Theta(1)$ using $\mathcal O\!\bigl(g\alpha_AT\log(g\alpha_AT/\varepsilon)\bigr)$ matrix queries and $\mathcal O(g)$ queries to each state-preparation oracle, where $g=\|\boldsymbol{x}_0\|/\|\boldsymbol{x}(T)\|$ and $\alpha_A$ is the matrix-oracle normalization. We illustrate the construction and its recovery probabilities through a numerical experiment on a time-dependent two-cavity system.
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All authors Xiaojing Dong, Chuwen Ma, Yizhe Peng, Yue Yu

Quantum Physics

Niobium Titanium Nitride as a High Tensile Stress Material for Nanomechanics

Korbinian Rubenbauer, Burak E. Bülbüloglu, Thomas Luschmann et al. (+6 more)

Over the past decades, high-coherence mechanical resonators have been continuously pushed to new limits, using techniques such as dissipation dilution and clamp-tapering to enhance their quality factor beyond intrinsic material limitations. Today, these mechanical resonators are often fabricated from silicon-nitride, silicon-carbide or aluminum. Recently, however, interest in novel material platforms has grown, especially those allowing for the integration within superconducting circuits. Among these, superconducting nitrides stand out as particularly promising due to their high transition temperatures compared to elementary superconductors. Here, we introduce them as nanomechanical resonators and report on the fabrication and characterization of highly stressed, doubly clamped niobium titanium nitride (NbTiN) nanostring resonators. Using optical interferometry, we determine the elastic properties and mechanical quality factor from room temperature to 13 K. We observe high tensile stress up to 0.81 GPa along with a Young's modulus of 181 GPa and an intrinsic mechanical quality factor up to 850. With these favorable mechanical properties, NbTiN constitutes a promising material platform for future applications in cavity electro- and nanomechanics.
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All authors Korbinian Rubenbauer, Burak E. Bülbüloglu, Thomas Luschmann, Manuel Müller, Stephan Geprägs, Witlef Wieczorek, Eva M. Weig, Matthias Althammer, Hans Huebl

Quantum Physics · Cross-listed from physics.optics

Non-monotonic dependence of OAM Schmidt spectrum on crystal thickness

Harshal Jain, Suman Karan, Radhika Prasad et al. (+1 more)

The orbital angular momentum (OAM) of photons provides a high-dimensional resource for quantum information protocols. The dimensionality of OAM-entangled states generated via spontaneous parametric down-conversion (SPDC) is quantified by the angular Schmidt spectrum. Here, we experimentally investigate the dependence of the angular Schmidt spectrum on the thickness of the nonlinear crystal. Contrary to previous studies reporting a monotonic decrease in the Schmidt number with increasing crystal thickness, we report the first experimental observation of a nonmonotonic behavior, as we demonstrate an increase in the Schmidt number beyond a certain crystal thickness. We attribute this to the spatial walk-off effect in the anisotropic nonlinear crystal and explain it using a theoretical model that is devoid of standard phase-matching approximations. These findings can have important implications for high-dimensional entangled state generation.
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All authors Harshal Jain, Suman Karan, Radhika Prasad, Anand K. Jha

Quantum Physics · Cross-listed from cond-mat.dis-nn

Many-Body Mobility Edge and Non-Hermitian Skin Effect in an Interacting Quasi-Periodic Spin Chain

Lavoisier Wah, Ayan Banerjee and Flore K. Kunst

Non-Hermitian many-body physics reveals a rich interplay between topology, localization, and boundary effects, yet their collective behavior in interacting disordered systems remains largely unexplored. In this work, we study an interacting non-Hermitian spin chain subject to a quasi-periodic longitudinal field, providing a unified and controlled setting, where non-Hermitian dynamics, interactions, and localization mechanisms intertwine. Remarkably, we discover a "D-shaped" many-body mobility edge that separates extended and localized eigenstates, while simultaneously delineating regimes of many-body localization and the many-body skin effect (where many-body eigenstates acquire an anomalous drift towards a boundary under open boundaries) emerging from the combined action of interactions, non-Hermiticity, and driving amplitude. We demonstrate that the skin effect induces multifractal scaling in the non-Hermitian eigenstates, providing a clear signature of the many-body skin effect. Employing diagnostics such as the fractal dimension, complex eigenvalue fractions, and many-body inverse participation ratios, we map out a unified phase diagram in which all measures consistently identify the "D-shaped" mobility edge. Finally, we probe this interplay using both complex level-spacing statistics and dynamical observables such as density imbalance, entanglement growth, and wave-packet evolution, culminating in a rich many-body mobility phase diagram that captures both the many-body skin effect and localization transitions. Our results identify a clear, defining signature of the "D-shaped" many-body mobility edge, and underscore its pivotal role in shaping the physics of open quantum many-body systems.
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Quantum Physics

Multimode phonon-mediated enhancement of entanglement and competing synchronization in cavity magnomechanics

Z.Imara, Jia-Xin Peng, E.K. Berinyuy et al. (+2 more)

The generation of quantum correlations in hybrid quantum systems remains a central challenge due to the intrinsic limitations of linear interactions. In cavity magnomechanical platforms, the cavity-magnon coupling gives rise to hybridized cavity-magnon polaritons (CMPs). However, as a beam-splitter-type interaction, it does not by itself generate entanglement between the polariton modes in the absence of additional nonlinear or parametric processes. Here, we propose a mechanism based on multimode phonon mediation, in which multiple vibrational modes act as parallel scattering channels that couple the polaritons through Stokes and anti-Stokes processes. We show that , in the parameter regime explored here, the presence of multiple phonon modes leads to a monotonic enhancement of steady-state entanglement, thereby going beyond the limitations of conventional single-mode schemes. Furthermore, we demonstrate that quantum synchronization between the polariton modes originates from the same underlying scattering processes responsible for entanglement generation, yet exhibits an opposite scaling behavior with increasing phonon number for the phase quadrature, while the amplitude synchronization reveals collective squeezing that grows with the number of phonon channels. Our results provide new insights into the role of multimode interactions in shaping quantum correlations and establish a viable pathway for controlling entanglement and collective dynamics in hybrid magnomechanical platforms.
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All authors Z.Imara, Jia-Xin Peng, E.K. Berinyuy, S.K. Singh, A. El Allati

Quantum Physics

Fugacity-Resolved Stabilizer Entropy in Critical Quantum Chains: Discrete Selberg Sums and Exactly Solvable Rényi Indices

Reyhaneh Khasseh and M. A. Rajabpour

Stabilizer Rényi entropy quantifies the nonstabilizerness of a quantum state through Rényi moments of its Pauli expectation-value distribution. Its standard form sums over Pauli-string degrees and retains only the total Rényi weight. We introduce a fugacity-resolved partition function for the critical transverse-field Ising chain that resolves this degree in the balanced Majorana representation and generates its full counting statistics. This Ising problem extends beyond a single model: exact decimation identities and the correspondence with the half-filled \(XX\) chain establish it as a common finite-size building block for stabilizer and computational-basis Shannon–Rényi entropies. We map the fugacity-resolved all-minors sum, for every positive Rényi index, to a checkerboard-weighted discrete Selberg ensemble on a half-filled doubled root lattice. For positive integer indices, it reduces to a finite aliased Dyson constant term. At \(\alpha=\tfrac12,1,2\), determinant and Pfaffian compressions yield product formulas. At \(\alpha=4\), the generic-fugacity problem admits an exact inverse Jack–Kostka representation, while at unit fugacity a complementary middle-minor identity relates it to the square of the \(\alpha=2\) result. These generating functions determine the balanced-degree statistics. Complement symmetry makes the distribution symmetric about \(k=L/2\), with an index-dependent width. It is exactly binomial at \(\alpha=1\), has variance proportional to \(L\) at \(\alpha=\tfrac12\), and develops an \(L\log L\) enhancement at \(\alpha=2\). After variance rescaling, the centered distributions converge to Gaussian limits at all three indices. Beyond these solvable cases, finite-size numerics reveal an evolution from a central peak to a symmetric bimodal profile and eventually to endpoint dominance as the Rényi index increases.
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Quantum Physics · Cross-listed from physics.atom-ph

An ionic clock qubit inside a circular Rydberg atom

Fabian Thielemann, Aaron Götzelmann, Marius Thomas et al. (+4 more)

Neutral atoms trapped in optical tweezers and excited to Rydberg states, together with trapped ions, are among the most advanced platforms for quantum simulation and quantum computing. Current experiments often rely on additional atoms in neighboring traps to encode ancilla qubits for local manipulation and readout. Here, we demonstrate a dual ion-Rydberg system comprising two qubits encoded in two individually controlled electrons of the same alkaline-earth atom. The first, a microwave qubit, is encoded in a pair of circular Rydberg states, while the second, an optical qubit, is encoded on a narrow quadrupole transition of the Rydberg atom's ionic core. We demonstrate coherent control of the optical qubit and achieve coherence times of several hundred microseconds under dynamical decoupling. Furthermore, we realize coherent coupling between the two electrons via electrostatic quadrupole interactions over the large separation between the Rydberg electron and the ionic core, and map out its angular tunability. Finally, we demonstrate a two-qubit operation, reminiscent of a Mølmer-Sørensen gate, that evolves through an entangled state of the two qubits driven by the quadrupole coupling. Our work opens a pathway to exploit a pair of individually controlled electronic qubits with tunable coupling for quantum simulation and quantum metrology.
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All authors Fabian Thielemann, Aaron Götzelmann, Marius Thomas, Einius Pultinevicius, Armin Humić, Christian Hölzl, Florian Meinert

Quantum Physics

Single-qubit detection by collective phase imprinting

Kritsana Srakaew, Pascal Weckesser, Daniel Adler et al. (+4 more)

The amplification of quantum information carried by a single quantum excitation is a recurring challenge across diverse quantum platforms. The coupling between a single qubit and a mesoscopic ensemble of spins, for example, can be leveraged to realize non-destructive detection of the qubit state. However, realizing robust couplings between such systems is experimentally challenging and typically requires programmable quantum gates or native long-range interactions. Here, we introduce a platform that couples a single qubit, encoded in the ground-to-Rydberg transition of a control atom, to a Rydberg-dressed target ensemble of ground-state atoms trapped in an optical lattice. We show that the state of the control qubit can be coherently mapped onto the ensemble via a qubit-controlled collective phase shift. By Rydberg-dressing the ensemble, the controlled phase shift per target atom becomes independent of the number of target atoms, making the protocol intrinsically insensitive to atom-number fluctuations and atom loss, which are the dominant experimental imperfections in our system. Exploiting the collective response of up to eight target spins, we demonstrate the efficacy of the scheme by realizing non-destructive detection of a single Rydberg excitation with a state-assignment fidelity of $\mathcal{F} = 99.81^{+0.17}_{-1.47}\,\%$. Our approach demonstrates the key ingredients for high-fidelity transfer of quantum information from a single qubit to a mesoscopic ensemble, opening a route to non-destructive mid-circuit readout of Rydberg states and to efficient interfaces between single qubits and photonic modes.
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All authors Kritsana Srakaew, Pascal Weckesser, Daniel Adler, Suchita Agrawal, David Gröters, Immanuel Bloch, Johannes Zeiher

Quantum Physics

Architecture-Aware Reinforcement Learning for Communication-Efficient Distributed Quantum Circuit Compilation

Chien-Tung Kuo, Felix Burt, Samuel Yen-Chi Chen et al. (+2 more)

Distributed quantum computing provides a scalable route for executing quantum circuits beyond the capacity limits of a single quantum processing unit (QPU), but it introduces a communication-aware compilation problem involving strict hardware constraints and circuit dependencies. This paper presents an architecture-aware reinforcement-learning framework that formulates distributed quantum compilation as a constrained Markov Decision Process (MDP). The compiler-level communication actions dynamically update logical-qubit placement and enable subsequent gate execution. A heterogeneous graph model represents interactions among hardware, logical qubits, and circuit operations, while a policy trained via Proximal Policy Optimization optimizes EPR-pair consumption and communication makespan. Evaluation across benchmark circuits shows that our policy matches state-of-the-art heuristics on structured workloads, with lookahead reward shaping yielding modest improvements on unstructured circuits. These results demonstrate that reinforcement learning is a flexible alternative to manual heuristics, though scalability remains a key bottleneck for practical use.
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All authors Chien-Tung Kuo, Felix Burt, Samuel Yen-Chi Chen, Kin K. Leung, Kuan-Cheng Chen

Quantum Physics

Correlation Geometry of Quantum Sensor Networks: Local-Global Information Flow and Local Privacy

Gong-Chu Li, Lei Chen, Xu-Song Hong et al. (+7 more)

Quantum sensor networks (QSN) typically encode N unknown parameters while targeting a single linear combination, rendering the N-1 remaining parameters as nuisance directions. To rigorously quantify estimation precision under such nuisances, we use the effective quantum Fisher information (EQFI) and establish a “barrel-effect” bottleneck: the global EQFI cannot exceed the weakest weighted local sensing capacity. To elucidate the information allocation mechanism underlying this bottleneck, we derive an exact local–global phase map that delineates how the trade-off between local and global EQFI depends dynamically on quantum correlations, and accordingly we identify concrete conditions for saturating the bottleneck bound. Notably, this geometric map uncovers a counterintuitive “overcorrelated” regime where excessive correlations actively degrade both local and global performance. Finally, we apply the phase map to intrinsic local privacy and identify the condition under which every local parameter is inaccessible while the desired global combination remains estimable. Overall, our work provides a principled methodology for engineering optimal network states in quantum sensing architectures.
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All authors Gong-Chu Li, Lei Chen, Xu-Song Hong, Hua-Qing Xu, Yuancheng Liu, Si-Qi Zhang, Jia-Hao Zhao, Geng Chen, Chuan-Feng Li, Guang-Can Guo

Quantum Physics

QCORE: A Quantum-Control-Oriented Real-Time Execution Architecture with Extensible Closed-Loop Services and Shared AI Acceleration

Heyue Li, Yanshu Guo, Qichun Liu et al. (+3 more)

Scalable quantum processors require control, readout, feedback, calibration, and error correction to coexist under bounded latency and shared-resource constraints, whereas existing platforms typically optimize only a subset of these capabilities. This article presents QCORE (Quantum-Control-Oriented Real-Time Execution), a QPU-side digital control reference architecture positioned between the Host and a platform-specific analog/mixed-signal front end. QCORE separates task management, shared resources, hard-real-time execution, and long-timescale services into four hardware partitions. A fast-result sideband closes same-round feedback, a Measurement Packet provides a traceable measurement and service interface, and a common service-control skeleton, Tile-local QEC, and versioned safe-point commit organize calibration, error correction, and long-term state updates. Transaction-level, event-driven, and quantum-behavioral models are used for evaluation. At a background load of 0.8, the $P_{99}$ latency of the shared Measurement Packet/Event feedback path is $(1.984\pm0.004)L_{\max}$. Closed-loop operation reduces the mean frequency error by $83.2\%\pm0.8\%$ and lowers the state-assignment error at maximum readout drift from $10.39\%\pm0.54\%$ to $5.37\%\pm0.29\%$. No unsafe acceptance or mixed-version observation is observed in 100,000 configuration transactions, and Tile-local QEC reduces modeled global-boundary demand and yields a $2.08\times$ capacity-normalized scaling estimate.
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All authors Heyue Li, Yanshu Guo, Qichun Liu, Tiefu Li, Zhihua Wang, Hanjun Jiang

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

Spin Qubits in Photon-Coupled Microwave Cavities

Samuel Johnson and Nancy Sandler

Electron spin qubits in microwave cavities provide a promising platform for scalable quantum computing hardware, leveraging long coherence times, charge-noise robustness and cavity mediated qubit-qubit interactions. While the strong spin-photon coupling regime is accessible via on-chip micromagnets, scaling conventional architectures by placing multiple qubits within a single shared resonator degrades transmission amplitudes, hence limiting large-scale efficiency. To overcome this limitation, we analyze a modular architecture where individual cavities containing a limited number of qubits are coupled via single-photon-exchange waveguides. Using input/output theory, we compute the transmission amplitudes for networks of two and three coupled cavities in various configurations. We map out the distinct physical regimes accessible by tuning key system parameters, offering a viable pathway for scalable cavity-based quantum spin qubit networks.
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Quantum Physics

Investigating Quantum-Embedded Transformers on Classical Datasets for Cross-Modality Classification

Hao-Yuan Chen

We test whether a parameterized quantum circuit (PQC) improves a hybrid quantum-classical model's performance on classical datasets, using an interface-matched classical map as the control while holding all other components fixed. Our architecture, Quantum-Embedded Attention (QEA), uses a learnable projector to compress backbone features into an $n_q$-dimensional angle vector, a shallow PQC to map those angles to one- and two-qubit Pauli expectations, and a classical attention decoder to produce class logits. We hypothesized the PQC would improve accuracy or seed-to-seed stability over a classical map with matched input/output dimensions. We test this with an interface-matched $2\times2$ factorial on Breast Cancer Wisconsin at $n_q\in\{4,8\}$, independently swapping the PQC for a classical map and the attention decoder for a linear head, across five paired seeds per cell. Three of four paired quantum-minus-classical $95\%$ confidence intervals include zero; the fourth, a $+1.63$ percentage-point contrast for the attention decoder at $n_q=4$, reverses sign at $n_q=8$ and does not survive correction across the four contrasts. The experiment thus shows no consistent PQC contribution and cannot establish equivalence. A five-dataset cross-modality grid shows comparable accuracy on AG~News, Breast Cancer Wisconsin, and BirdCLEF but a large deficit on CIFAR-10; these cells are not interface-matched and are interpreted descriptively. We report all planned canonical runs, distinguish current Pauli-readout results from legacy probability-readout experiments, and analyze bottleneck, simulation, finite-shot, and noise limitations. The results do not establish a quantum advantage; they demonstrate why controlled component attribution is necessary before crediting a hybrid model's performance to its quantum layer.
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Quantum Physics

Entanglement-enhanced optical magnetometry beyond the standard quantum limit

Jun Jia, Túlio Brito Brasil, Maimouna Bocoum et al. (+6 more)

Optical atomic magnetometry is a powerful tool for continuous sensing applications, yet, in the absence of quantum correlations, its sensitivity is limited by the standard quantum limit (SQL) stemming from a trade-off between optical probe imprecision and quantum measurement backaction. Beyond-SQL sensitivity requires quantum correlations that modify these measurement noise sources. Here we demonstrate such sensitivity by using entangled state of the probe light and by engineering correlations between measurement imprecision and backaction. Having first explored SQL in a broad range of frequencies, we demonstrate overcoming the limit by combining variational readout with coupling the magnetometer to one mode of a bipartite entangled light state and conditioning the results on the other entangled mode. Tuning the detected light quadratures and combining the signals from the two measurement channels, we achieve sensitivity beyond the SQL in a broad range of acoustic frequencies which has so far remained inaccessible to quantum-noise-limited optical magnetometry.
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All authors Jun Jia, Túlio Brito Brasil, Maimouna Bocoum, Andrea Grimaldi, Laurits Møberg, Mikhail Balabas, Jörg Helge Müller, Emil Zeuthen, Eugene Simon Polzik

Quantum Physics

Quantum-State Projectors on Grassmannian: Geometry, Holonomy, and Topology

Shin-Ming Huang

An isolated group of $k$ bands in an $N$-level quantum system defines a rank-$k$ spectral projector and hence a map into the complex Grassmannian $\mathrm{Gr}(k,N)$. For a smooth gapped Hamiltonian, this projector is globally smooth and periodic even when band topology obstructs a globally smooth periodic, or symmetry-compatible, Bloch frame. We take the globally defined differential $\dd P$ as the central object: it is the tangent field of the Grassmannian map, removes unphysical rotations within the selected subspace, and retains the physical interband transition. The interband block of this tangent data simultaneously determines the quantum metric and Berry curvature; the associated horizontal generator produces finite Grassmannian motion, while wedge products of $\dd P$ enter topological forms. We construct a shortest path between two projectors in the ambient Grassmannian and show that the singular values of its horizontal generator block are the principal angles between the endpoint subspaces. This construction provides a piecewise-geodesic interpretation of discrete geometric phases. In a separate development, we derive a basis-independent expression for the determinant of a multiband Wilson loop from traces of powers of an ordered projector product, without decomposing a degenerate band multiplet into individual bands. Finally, the same tangent-vector calculus organizes Chern characters, chiral winding numbers, and the time-reversal $\mathbb Z_2$ index. The resulting framework unifies local quantum geometry, finite subspace distance, holonomy, and topology while avoiding global gauge fixing.
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Quantum Physics

Robust device-independent characterization of sharpness and incompatibility of unsharp instruments

Qian Zhang, Kai-Yu Yuan, Yan-Xin Rong et al. (+3 more)

Unsharp measurements are key resources for tasks that balance information gain and disturbance, but certifying them without device assumptions remains a challenge. We propose a fully device-independent protocol for characterizing unsharp instruments, based on an entanglement-assisted sequential quantum random access code, where the first decoder is allowed to communicate her measurement setting to the second. This communication-enhanced scheme creates a decoding regime in which both decoders surpass classical bounds, enabling tight quantification of sharpness and direct quantification of measurement incompatibility beyond noncommunicating protocols. Experimentally, we implement tunable unsharp measurements using a Mach-Zehnder interferometer, observing the predicted sequential enhancement in decoding probability. Additionally, we achieve significantly narrower sharpness intervals and incompatibility quantification across multiple target sharpness values. Our results show that communication is a powerful operational resource for certifying precisely unsharp instruments and advancing device-independent quantum information protocols.
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All authors Qian Zhang, Kai-Yu Yuan, Yan-Xin Rong, Zhen Shang, Yong-Jian Gu, Ya Xiao

Quantum Physics

Sub-Vacuum Jamming for Secure Communication

Sh. Barzanjeh, A. Poostindouz and B. C. Sanders

Artificial-noise jamming improves physical-layer security by degrading an eavesdropper's channel, but the injected noise also interferes with the legitimate receiver. We introduce a correlated-jamming protocol based on classical and quantum correlations that suppresses this self-interference while preserving the jamming penalty experienced by the eavesdropper. Bob broadcasts one mode of a correlated two-mode source and retains the second as a local reference. Eve, who has no access to the retained mode, receives the full thermal jamming field, whereas Bob uses an optimized joint measurement to cancel its correlated fluctuations. The residual self-jamming noise is governed by the conditional variance of the transmitted jamming quadrature given the retained reference. Any classically correlated source is bounded by the vacuum-noise floor and therefore restores Bob, at best, to his unjammed receiver noise. An entangled two-mode squeezed source surpasses this limit and produces sub-vacuum residual noise, with the maximum quantum advantage set by the transmissivity of the jamming path back to Bob. In the bright-source regime, this advantage becomes a fixed reduction in Bob's noise floor, independent of the signal and jamming powers. Within a bounded-collection wiretap model, correlated jamming preserves positive secrecy even when Eve has a stronger direct channel than Bob and remains effective against collective measurements on Eve's Gaussian output states. The protocol supports bright classical message transmission and requires no end-to-end quantum channel. Possible applications include shot-noise-limited optical communication, cryogenic microwave networks, and covert or power-constrained secure links.
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Quantum Physics

Conditions for Quantum Advantage in AC Power Flow

Parikshit Pareek, Abhijith Jayakumar, Carleton Coffrin et al. (+1 more)

This paper aims to contextualize the requirements for Quantum Computing (QC) algorithms to achieve a quantum advantage in solving the alternating current power flow (ACPF) problem, with a focus on runtime complexity. First, we establish a benchmark for a QC iterative solver to demonstrate an advantage over the classical Newton-Raphson Load Flow (NRLF) algorithm. Next, we derive a baseline expression for the end-to-end runtime complexity of any Gate-based QC algorithm as $\Omega(N \kappa/\varepsilon),$ reflecting dependence on system size $N$, condition number $\kappa$, and error tolerance $\varepsilon$. Finally, we highlight key areas where QC algorithms may offer potential benefits over NRLF in addressing the standard ACPF problem.
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All authors Parikshit Pareek, Abhijith Jayakumar, Carleton Coffrin, Sidhant Misra

Quantum Physics

Shor's algorithm requires Fanout

Lucas Gretta and Malvika Raj Joshi

Shor's algorithm is a canonical quantum supremacy target whose core operation relies on the Quantum Fourier Transform (QFT). In this note, we resolve an open question of Fang, Fenner, Green, Homer and Zhang from 2006 by showing that approximating QFT in constant depth, for any $n$-qubit modulus, necessarily requires the $n$-qubit Fanout operation. Formally, let $\mathsf{QFT}_q$ be the gate acting on $n = \lceil \log q \rceil$ qubits that computes the QFT under modulus $q$. It is known that any $n$-qubit $\mathsf{QFT}_q$ can be implemented in constant depth using $\mathsf{FANOUT}_n$, i.e. $\mathsf{QFT}_q \in \mathsf{QAC}^0_f$. We prove the converse by using a $\mathsf{QFT}_q$ gate to construct a state of "non-negligible felinity". Consequently, $\mathsf{QFT}_q \in \mathsf{QAC}^0 \iff \mathsf{FANOUT}_n \in \mathsf{QAC^0}$. In the case of $q = 2^n$, such as in Shor's, we approximate $\mathsf{FANOUT}_n$ using a single $\mathsf{QFT}_{2^n}$ gate and $O(1)$ two-qubit local gates, thus tying the feasibility of realizing Shor's algorithm with NISQ circuits to that of Fanout.
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Quantum Physics

High-Performance Quantum Transduction with Correlated Noise

Yu-Bo Hou, Xiaoan Ai, Pengbo Li et al. (+1 more)

Quantum transduction, which coherently converts quantum states between microwave and optical frequency domains, is a key technology for hybrid quantum architectures. Its performance, however, is fundamentally limited by thermal noise. Direct quantum transduction is particularly susceptible to noise and often fails to achieve positive quantum capacity. Entanglement-based quantum transduction, which realizes state conversion through quantum teleportation assisted by microwave-optical entanglement, is intrinsically more robust against thermal noise. However, generating sufficiently strong entanglement in a realistic thermal environment remains a major challenge. In this paper, we exploit correlated noise as a resource for quantum transduction. For direct quantum transduction, it is shown that the noise correlations give rise to controllable interference terms that substantially suppress the effective channel noise. For entanglement-based quantum transduction, the same correlations enhance the generation of microwave-optical entanglement, thereby improving the fidelity of teleportation-based conversion. As a result, both transduction protocols exhibit broad regions of positive quantum capacity over experimentally relevant ranges of cooperativity. We further discuss a possible physical mechanism for engineering the required noise correlations, providing theoretical guidance for experimental implementations. These results suggest that correlated noise can substantially relax the stringent cryogenic requirements for microwave-optical quantum transduction and facilitate the realization of practical hybrid quantum networks.
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All authors Yu-Bo Hou, Xiaoan Ai, Pengbo Li, Changchun Zhong

Thursday 6 August

Quantum Physics

Optimal Temporal Hiding in Correlated Quantum Reference-Frame Processes

Maxim V. Churilov

A finite group-valued temporal reference frame defines a correlated random-unitary process rather than a list of independent channels. We identify a regime in which its complete trajectory law is an exact operational coordinate. If the carrier contains every irreducible representation of the finite group $G$, one ancilla-assisted input resolves all group elements, and for arbitrary laws $\mu,\nu$ on $G^n$ the strategy half-distance equals $d_{\mathrm{TV}}(\mu,\nu)$. Optimal output-side causal post-processing likewise reduces to classical convolution on $G^n$. This converts temporal hiding into a $q$-ary coding problem. Uniform coset laws of an $[n,k]_q$ code are identical on every set of fewer than $d(C^\perp)$ slots and perfectly distinguishable globally. For $M$ sectors with leakage at most $\eta$ from $t$ selected slots and global decoding error $\epsilon$, we obtain $(1-\epsilon)\log M \leq (n-t)\log q+\eta+h_2(\epsilon)$. For nested codes $C\subset D$, the payload $R=\dim D-\dim C$, invisible depth $t=d_{\mathrm{rel}}(C^\perp,D^\perp)-1$, and sector distance $d_{\mathrm{rel}}(D,C)$ obey $R+t+d_{\mathrm{rel}}(D,C)-1\leq n$ and $R+t+2e+f\leq n$, where $e$ and $f$ are adversarial errors and known erasures. Nested generalized Reed-Solomon codes attain the bounds in their existence range. We also give the exact projection-rank leakage profile and show that hiding arbitrary coherent superpositions from $t$ slots is precisely quantum erasure correction, yielding $\log_q K+2t\leq n$. The results separate classical trajectory privacy from coherent temporal privacy and quantify the robust payload hidden from reduced process tomography.
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Quantum Physics · Cross-listed from physics.ed-ph

Improving student understanding via interactive learning tutorial on quantum key distribution using entanglement

Liam Doyle and Chandralekha Singh

We describe the development, validation and implementation of a Quantum Interactive Learning Tutorial (QuILT) on quantum key distribution (QKD) using entanglement, a context which involves a practical application of quantum concepts relevant for the second quantum revolution. The QuILT helps students learn quantum concepts relevant for quantum cryptography using a simple two-particle system. The protocol uses two entangled particles and two Stern-Gerlach Apparati to generate a random shared key over a public channel for encrypting and decrypting information. It actively engages students in the learning process and helps them build links between the concepts learned in class and their real world applications. The QuILT was implemented as a homework in a traditional quantum mechanics course and a quantum computing and quantum information course. The evaluation suggests that the QuILT is helpful in improving students' understanding of the concepts related to QKD in both courses. Also, the evaluation of the QuILT in both types of courses with or without lecture-based instruction in relevant QKD concepts suggests that the QuILT can be given as a homework after lecture-based instruction on entanglement without in-class discussion on QKD. Therefore, entanglement can be covered without taking up much in-class time, while also providing students with an understanding of the QKD method and how it protects from eavesdroppers.
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Quantum Physics

Approaching the Fundamental Limit of Single-Shot Qubit Frequency Tracking with an Adiabatic Tangentially-Modulated Pulse

Itamar Oren, Luke I. Dyer, Calum Holker et al. (+2 more)

Understanding and mitigating noise in two-level quantum systems is essential for achieving high-fidelity qubit control. Conventional frequency tracking techniques, such as Ramsey interferometry, are fundamentally limited by trade-offs between sensitivity, bandwidth, and dynamic range. Here we introduce the adiabatic tangentially-modulated (ATM) pulse, a pulse derived from quantum adiabatic theory that maps qubit detuning onto a sigmoidal, near-binary response. Using numerical simulations supported by analytical modeling, we show that pulse sensitivity and detuning range can be independently engineered through simple design parameters. We derive scaling relations governing these quantities and demonstrate their agreement with simulation. A single-shot ATM measurement achieves sensitivity comparable to that obtained from multi-shot Ramsey averaging, enabling tracking of substantially higher frequency noise components with a lower closed-loop white-noise floor. In addition, this pulse exhibits strong robustness to amplitude fluctuations compared with binary response pulses derived from the Shinnar-Le Roux formalism. Together, these properties establish the ATM pulse as a promising approach for robust qubit frequency tracking.
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All authors Itamar Oren, Luke I. Dyer, Calum Holker, Gerardo A. Paz-Silva, Chih Hwan Yang

Quantum Physics

Matrix Product State Theory of Few-Photon Squeezed Pulses Interacting with a Two-Level Emitter in a Waveguide

Sofia Arranz Regidor, Matthew Kozma and Stephen Hughes

Squeezed light states have a special place in quantum optics with potentially profound applications in emerging quantum technologies. We present a numerically-exact matrix product states (MPS) approach to model quantum pulses of squeezed light, at the few-photon level, interacting with a two-level system in a waveguide environment. We represent the squeezed state as a coherent superposition of Fock states and explore the nonlinear population dynamics as well as multi-photon correlation functions that emerge. We show how the squeezed pulse can create quantum correlations that are unique to squeezed pulses, including $\langle b(t) b(t+t')\rangle$ for transmitted fields as well as $\langle b^\dagger(t) b^\dagger(t+t') b(t+t') b(t) \rangle$. We also demonstrate how $\langle b(t) b(t+t') \rangle$, a first-order correlation function, shows nonlinear photon correlations that are similar to those known and measured for two-photon scattering states. Finally, we also study the squeezed spectra of the pulse before and after interacting with the two-level system, and highlight the role of the spectral bandwidth of the incident pulse. The MPS theory allows the modeling of arbitrary bandwidth squeezing without making any Markov and Born approximations for the light-matter interaction processes, and can easily be extended to waveguide systems with multiple emitters and time-delayed feedback.
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Quantum Physics

Exact quantum circuits for a lattice Boltzmann realization of the Dirac equation

Nilesh Sawant, Ethan Young, Kevin Patrick Griffin et al. (+1 more)

The quantum lattice Boltzmann scheme of Succi and Dellar advances a four-component Dirac spinor on a lattice by a fixed sequence of local, exactly norm-preserving operations: a basis rotation, a collision, a streaming shift, and the inverse rotation. While this correspondence has been noted qualitatively, it has remained unclear whether it can be realized exactly and unitarily as a gate-model quantum circuit. This unitarity is a structural property of the scheme, which should, in principle, enable an operator-by-operator port onto quantum hardware. We give a gate-level construction of every operation of the three-dimensional Dirac QLB scheme: the fixed rotation gates, the collision gate, the streaming shift as a controlled increment on a position register, the position-dependent potential as a phase oracle, and periodic and reflecting boundary conditions as unitary circuits. Together, these constructions provide the first complete demonstration that the scheme is exactly representable in the gate model without any approximation beyond the classical method itself. We then compose them into single-axis, two- and three-dimensional time steps. On a state-vector emulator the resulting circuits reproduce the classical QLB solver to machine precision, thus verifying our quantum implementation. Our aim is representability and correctness rather than computational speedup; questions of state preparation, asymptotic cost, and hardware noise are identified as open avenues for future work. Appendices restore physical units, derive the scheme's Zitterbewegung in closed form and compare it with a trapped-ion simulation, and execute a reduced two-site instance on a Rigetti superconducting processor, where choosing the qubits from the live calibration lowers the state-preparation error. All operators, circuits, and figures are reproducible from the open-source quantumKineticMethods library.
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All authors Nilesh Sawant, Ethan Young, Kevin Patrick Griffin, Michael Martin