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Friday 7 November

Quantum Physics

Semi-device-independent randomness certification on discretized continuous-variable platforms

Moisés Alves, Vitor L. Sena, Santiago Zamora et al. (+4 more)

Randomness is fundamental for secure communication and information processing. While continuous-variable optical systems offer an attractive platform for this task, certifying genuine quantum randomness in such setups remains challenging. We present a semi-device-independent scheme for randomness certification tailored to continuous-variable implementations, where the dimension assumption is operationally implemented by restricting state preparations to the two-level Fock subspace. Using standard homodyne and displacement-based measurements, we show that simple optical setups can achieve dimension-witness violations that certify positive min-entropy, even in the presence of realistic losses and misaligned reference frames. These results demonstrate that practical and scalable quantum randomness generation is achievable with minimal experimental complexity on continuous-variable platforms.
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All authors Moisés Alves, Vitor L. Sena, Santiago Zamora, Tailan S. Sarubi, A. de Oliveira Junior, Alexandre B. Tacla, Rafael Chaves

Quantum Physics

Memory effects in repeated uses of quantum channels

Hayden Zammit, Roberto Salazar, Gianluca Valentino et al. (+2 more)

Quantum Information Processing (QIP) tasks can be efficiently formulated in terms of quantum dynamical maps, whose formalism is able to provide the appropriate mathematical representation of the evolution of open quantum systems. A key QIP task is quantum state transfer (QST) aimed at sharing quantum information between distant nodes of a quantum network, enabling, e.g. quantum key distribution and distributed quantum computing. QST has primarily been addressed insofar by resetting the quantum channel after each use, thus giving rise to memoryless channels. Here we consider the case where the quantum channel is continuously used, without implementing time- and resource- consuming resetting operations. We derive a general, analytical expression for the $n^{\mathrm{th}}$-use average QST fidelity for $U(1)$-symmetric channels and apply our formalism to a perfect QST channel in the presence of imperfect readout timing. We show that even relatively small readout timing errors give rise to memory effects which have a highly detrimental impact on subsequent QST tasks.
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All authors Hayden Zammit, Roberto Salazar, Gianluca Valentino, Johann A. Briffa, Tony J. G. Apollaro

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

Controlled generation of 3D vortices in driven atomic Josephson junctions

Vijay Pal Singh, Ludwig Mathey, Herwig Ott et al. (+1 more)

We propose an ac-driven atomic Josephson junction as a clean and tunable source of three dimensional (3D) solitary waves in quantum fluids. Depending on the height of the junction barrier, the emitted excitations appear as vortex rings at low velocity or vorticity-free rarefaction pulses near the sound velocity, thus spanning the complete Jones-Roberts family of solitons. The Shapiro-step phenomenon renders the emission deterministic: on the first, second, third Shapiro steps, the junction ejects one, two, and three solitary excitations per drive cycle. This enables controlled generation of single- and multi-excitation configurations, allowing detailed studies of the full crossover between vortex rings and rarefaction pulses and their interaction dynamics. In particular, deterministic multi-ring emission provides insights into leapfrogging dynamics of two and three coaxial rings and their decay via boundary-assisted, sound-mediated processes. This ac-driven protocol establishes a compact and reproducible platform for generating, classifying, and controlling 3D solitonic excitations, paving the way for precision studies of nonlinear vortex dynamics, dissipation, and quantum turbulence in trapped superfluids.
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All authors Vijay Pal Singh, Ludwig Mathey, Herwig Ott, Luigi Amico

Quantum Physics

Quantum Data Representation via Circuit Partitioning and Reintegration

Ziqing Guo, Jan Balewski, Kewen Xiao et al. (+1 more)

Quantum data encoding (QDE) enables faster com-putations than classical algorithms through superposition and en-tanglement. Circuit cutting and knitting are effective techniques for ameliorating current noisy quantum processing unit (QPUs) errors via a divide-and-conquer approach that splits quantum circuits into subcircuits and recombines them using classical postprocessing. Unfortunately, the existing QDE frameworks fail to consider quantum hardware limitations, such as the topology of the chip. Designing a computation model that supports the algorithm level of quantum computation and optimizes non-all-to-all connected quantum circuit simulations remains underde-veloped. In this study, we introduce shardQ, a method that leverages the SparseCut algorithm with matrix product state (MPS) compilation and a global knitting technique to mitigate the quantum error rates. This method elucidates the optimal trade-off between the computational time and error rate for quantum encoding with a theoretical proof, evidenced by an ablation analysis using an IBM Heron-type QPUs with 15% error reduction. This study also presents the results of quantum image encoding readiness. The proposed model advances the current quantum computation towards the fault-tolerant regime as QDE is the input of grand unified quantum algorithms.
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All authors Ziqing Guo, Jan Balewski, Kewen Xiao, Ziwen Pan

Quantum Physics · Cross-listed from cond-mat.stat-mech

Exact strong zero modes in quantum circuits and spin chains with non-diagonal boundary conditions

Sascha Gehrmann and Fabian H. L. Essler

We construct exact strong zero mode operators (ESZM) in integrable quantum circuits and the spin-1/2 XXZ chain for general open boundary conditions, which break the bulk U(1) symmetry of the time evolution operators. We show that the ESZM is localized around one of the boundaries and induces infinite boundary coherence times. Finally, we prove that the ESZM becomes spatially non-local under the map that relates the spin-1/2 XXZ chain to the asymmetric simple exclusion process, which suggests that it does not play a significant role in the dynamics of the latter.
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Quantum Physics

Further improvements to stabilizer simulation theory: classical rewriting of CSS-preserving stabilizer circuits, quadratic form expansions of stabilizer operations, and framed hidden variable models

Vsevolod I. Yashin, Vladimir V. Yatsulevich, Aleksey K. Fedorov et al. (+1 more)

Simulation of stabilizer circuits is a well-studied problem in quantum information processing, with a number of highly optimized algorithms available. Yet, we argue that further improvements can arise from the theoretical structure of stabilizer operations themselves. We focus on the subclass of stabilizer circuits composed of Calderbank-Shor-Steane (CSS)-preserving stabilizer operations, which naturally appear in fault-tolerant computations over CSS stabilizer codes. Using elementary circuit transformation techniques, we show that such circuits can be exactly rewritten as classical probabilistic circuits that reproduce measurement statistics. This rewriting introduces no computational overhead, in contrast to the general case of stabilizer circuits. To clarify the origin of this simplification, we introduce the standard quadratic form representation of general stabilizer operations (Clifford channels). It provides an efficient way to describe compositions of stabilizer operations and thus to simulate stabilizer circuits. CSS-preserving operations correspond to purely linear forms, which under a Walsh-Hadamard-Fourier transform yield a noncontextual hidden variable model, providing an alternative proof of the introduced rewriting. Finally, we develop a theory of reference frames for multiqubit systems, where frames are encoded by quadratic forms. This allows us to express stabilizer operations as probabilistic maps for proper reference frames. Non-CSS-preserving stabilizer circuits require dynamical modifications of reference frames, embodying a contextuality resource that leads to the computational overhead. This framework provides a new perspective on simulating stabilizer and near-stabilizer circuits within dynamically evolving quasiprobability models.
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All authors Vsevolod I. Yashin, Vladimir V. Yatsulevich, Aleksey K. Fedorov, Evgeniy O. Kiktenko

Quantum Physics

Helios: A 98-qubit trapped-ion quantum computer

Anthony Ransford, M.S. Allman, Jake Arkinstall et al. (+183 more)

We report on Quantinuum Helios, a 98-qubit trapped-ion quantum processor based on the quantum charge-coupled device (QCCD) architecture. Helios features $^{137}$Ba$^{+}$ hyperfine qubits, all-to-all connectivity enabled by a rotatable ion storage ring connecting two quantum operation regions by a junction, speed improvements from parallelized operations, and a new software stack with real-time compilation of dynamic programs. Averaged over all operational zones in the system, we achieve average infidelities of $2.5(1)\times10^{-5}$ for single-qubit gates, $7.9(2)\times10^{-4}$ for two-qubit gates, and $4.8(6)\times10^{-4}$ for state preparation and measurement, none of which are fundamentally limited and likely able to be improved. These component infidelities are predictive of system-level performance in both random Clifford circuits and random circuit sampling, the latter demonstrating that Helios operates well beyond the reach of classical simulation and establishes a new frontier of fidelity and complexity for quantum computers.
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All authors Anthony Ransford, M.S. Allman, Jake Arkinstall, J.P. Campora III, Samuel F. Cooper, Robert D. Delaney, Joan M. Dreiling, Brian Estey, Caroline Figgatt, Alex Hall, Ali A. Husain, Akhil Isanaka, Colin J. Kennedy, Nikhil Kotibhaskar, Ivaylo S. Madjarov, Karl Mayer, Alistair R. Milne, Annie J. Park, Adam P. Reed, Riley Ancona, Molly P. Andersen, Pablo Andres-Martinez, Will Angenent, Liz Argueta, Benjamin Arkin, Leonardo Ascarrunz, William Baker, Corey Barnes, John Bartolotta, Jordan Berg, Ryan Besand, Bryce Bjork, Matt Blain, Paul Blanchard, Robin Blume-Kohout, Matt Bohn, Agustin Borgna, Daniel Y. Botamanenko, Robert Boutelle, Natalie Brown, Grant T. Buckingham, Nathaniel Q. Burdick, William Cody Burton, Varis Carey, Christopher J. Carron, Joe Chambers, John Children, Victor E. Colussi, Steven Crepinsek, Andrew Cureton, Joe Davies, Daniel Davis, Matthew DeCross, David Deen, Conor Delaney, Davide DelVento, B.J. DeSalvo, Jason Dominy, Ross Duncan, Vanya Eccles, Alec Edgington, Neal Erickson, Stephen Erickson, Christopher T. Ertsgaard, Bruce Evans, Tyler Evans, Maya I. Fabrikant, Andrew Fischer, Cameron Foltz, Michael Foss-Feig, David Francois, Brad Freyberg, Charles Gao, Robert Garay, Jane Garvin, David M. Gaudiosi, Christopher N. Gilbreth, Josh Giles, Erin Glynn, Jeff Graves, Azure Hansen, David Hayes, Lukas Heidemann, Bob Higashi, Tyler Hilbun, Jordan Hines, Ariana Hlavaty, Kyle Hoffman, Ian M. Hoffman, Craig Holliman, Isobel Hooper, Bob Horning, James Hostetter, Daniel Hothem, Jack Houlton, Jared Hout, Ross Hutson, Ryan T. Jacobs, Trent Jacobs, Melf Johannsen, Jacob Johansen, Loren Jones, Sydney Julian, Ryan Jung, Aidan Keay, Todd Klein, Mark Koch, Ryo Kondo, Chang Kong, Asa Kosto, Alan Lawrence, David Liefer, Michelle Lollie, Dominic Lucchetti, Nathan K. Lysne, Christian Lytle, Callum MacPherson, Andrew Malm, Spencer Mather, Brian Mathewson, Daniel Maxwell, Lauren McCaffrey, Hannah McDougall, Robin Mendoza, Michael Mills, Richard Morrison, Louis Narmour, Nhung Nguyen, Lora Nugent, Scott Olson, Daniel Ouellette, Jeremy Parks, Zach Peters, Jessie Petricka, Juan M. Pino, Frank Polito, Matthias Preidl, Gabriel Price, Timothy Proctor, McKinley Pugh, Noah Ratcliff, Daisy Raymondson, Peter Rhodes, Conrad Roman, Craig Roy, Ciaran Ryan-Anderson, Fernando Betanzo Sanchez, George Sangiolo, Tatiana Sawadski, Andrew Schaffer, Peter Schow, Jon Sedlacek, Henry Semenenko, Peter Shevchuk, Susan Shore, Peter Siegfried, Kartik Singhal, Seyon Sivarajah, Thomas Skripka, Lucas Sletten, Ben Spaun, R. Tucker Sprenkle, Paul Stoufer, Mariel Tader, Stephen F. Taylor, Travis H. Thompson, Raanan Tobey, Anh Tran, Tam Tran, Grahame Vittorini, Curtis Volin, Jim Walker, Sam White, Douglas Wilson, Quinn Wolf, Chester Wringe, Kevin Young, Jian Zheng, Kristen Zuraski, Charles H. Baldwin, Alex Chernoguzov, John P. Gaebler, Steven J. Sanders, Brian Neyenhuis, Russell Stutz, Justin G. Bohnet

Quantum Physics

Trade-off between complexity and energy in quantum phase estimation

Yukuan Tao, Madalin Guta and Gerardo Adesso

Quantifying the energetic cost of implementing quantum operations is essential for assessing the efficiency and scalability of quantum sensing and information-processing technologies. Here, we introduce a framework for analysing the interplay between complexity and energy cost of quantum processes. In particular, we apply our framework to a sequential quantum phase estimation protocol, where a phase of physical significance is encoded in a quantum channel. The channel is applied to a probe state repeatedly until the probe is measured and the outcome leads to an estimate on the phase. We establish a trade-off relation between the total energy cost of the protocol and the number of times the channel is applied (complexity), while reaching a desired estimation precision. A sweet spot is located where the two quantities are co-optimised. The principles of our analysis can be adapted to benchmark the energetic requirements in other quantum protocols and devices.
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Quantum Physics

Realization of Thread Level Parallelism on Quantum Devices

Keren Li, Zidong Lin, Zheng An et al. (+4 more)

Scaling up quantum devices is a central challenge for realizing practical quantum computation. Modular quantum architectures promise scalability, yet experiments to date have relied on either $\sim\!10^{3}$-qubit monolithic chips or fragile interconnects with high loss. Here, we introduce a classical linkage scheme that merges multiple independent quantum processing units (QPUs) into a single logical device, enabling thread-level parallelism (TLP). Theoretically, we show that quantum routines with product-state inputs and low-rank entangling layers can be re-expressed in an efficient parallelizable form. Experimentally, we validate this architecture on clusters comprising up to sixteen benchtop nuclear magnetic resonance (NMR) quantum nodes. A four-qubit Greenberger-Horne-Zeilinger (GHZ) state is partitioned into parallel two-qubit subcircuits, achieving a fidelity of $93.8\,\%$ with respect to the ideal state. A non-Hermitian evolution, implemented via a truncated Cauchy integral on Hermitian Hamiltonians, reproduces exact observables with high accuracy. Our results demonstrate that classical links suffice to scale up the logical size of quantum computations and realize general, non-unitary channels on today's hardware, opening an experimentally accessible route toward software-defined, clustered quantum accelerators.
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All authors Keren Li, Zidong Lin, Zheng An, Guanru Feng, Zipeng Wu, Shiyao Hou, Jingen Xiang

Quantum Physics

Anticoncentrated $n$-bit distribution from $\log(n)$ qubits

Bingzhi Zhang and Quntao Zhuang

Random circuit sampling (RCS) is a leading approach to demonstrate quantum advantage, with its believed classical hardness rooted in anticoncentration of output distributions and average-case hardness of probability estimation. Here we show that this association is not fundamental. We introduce holographic random circuit sampling (HRCS), a spatiotemporal protocol that interleaves random unitary evolution with mid-circuit measurements. We prove that $n$ classical bits exhibiting $\epsilon$-approximate anticoncentration of Haar random states can be generated using only $\mathcal{O}(\log n)$ physical qubits and linear depth, establishing a precise space-time trade-off and indicating efficient classical simulation. Our analyses is built upon exact formulas for collision probability and higher-order power sums. Our experimental validation on IBM Quantum devices demonstrates sampling up to 200 classical bits using only 20 qubits.
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Quantum Physics · Cross-listed from physics.atom-ph

Sagnac Tractor Atom Interferometer on Photonic Integrated Circuit

Lefeng Zhou, Anne Graf and Georg Raithel

We study the theory of, and propose an experimental design for, a Sagnac tractor atom interferometer based on a photonic integrated circuit (PIC). The atoms are trapped in counter-rotating azimuthal optical lattices, formed by interfering evanescent fields of laser modes injected into circular PIC waveguides. We develop quantum models for the radial and azimuthal dynamics of the interfering atoms in adiabatic frames, which provide computational efficiency. The theory is applied to an exemplary PIC, for which we first compute field modes and atom trapping potentials for $^{87}$Rb. We then evaluate non-adiabaticity, fidelity, and sensitivity of the exemplary PIC.
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Quantum Physics

Strain-engineered nanoscale spin polarization reversal in diamond nitrogen-vacancy centers

Zhixian Liu, Jiahao Sun, Ganyu Xu et al. (+7 more)

The ability to control solid-state quantum emitters is fundamental to advancing quantum technologies. The performance of these systems is fundamentally governed by their spin-dependent photodynamics, yet conventional control methods using cavities offer limited access to key non-radiative processes. Here we demonstrate that anisotropic lattice strain serves as a powerful tool for manipulating spin dynamics in solid-state systems. Under high pressure, giant shear strain gradients trigger a complete reversal of the intrinsic spin polarization, redirecting ground-state population from $|0\rangle$ to $|\pm 1\rangle$ manifold. We show that this reprogramming arises from strain-induced mixing of the NV center's excited states and dramatic alteration of intersystem crossing, which we quantify through a combination of opto-magnetic spectroscopy and a theoretical model that disentangles symmetry-preserving and symmetry-breaking strain contributions. Furthermore, the polarization reversal is spatially mapped with a transition region below 120 nm, illustrating sub-diffraction-limit control. Our work establishes strain engineering as a powerful tool for tailoring quantum emitter properties, opening avenues for programmable quantum light sources, high-density spin-based memory, and hybrid quantum photonic devices.
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All authors Zhixian Liu, Jiahao Sun, Ganyu Xu, Bo Yang, Yuhang Guo, Yu Wang, Cunliang Xin, Hongfang Zuo, Mengqi Wang, Ya Wang

Quantum Physics

Adaptive quantum phase estimation can be better than non-adaptive

Noah Linden and Ronald de Wolf

Quantum phase estimation is one of the most important tools in quantum algorithms. It can be made non-adaptive (meaning all applications of the unitary $U_\phi$ happen simultaneously) without using more applications of $U_\phi$, albeit at the expense of using many more qubits. It is also known that there is no advantage for adaptive algorithms in the case where the phase that needs to be estimated is arbitrary or is uniformly random. Here we give examples of a special case of phase estimation, with a promise on the values that the unknown phase can take, where adaptive methods are provably better than non-adaptive methods by a factor of nearly 2 in the number of uses of $U_\phi$. We also prove some upper bounds on the maximum advantage that adaptive algorithms for phase estimation can achieve over non-adaptive ones.
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Quantum Physics

Spectroscopy and Coherence of an Excited-State Transition in Tm$^{3+}$:YAlO$_3$ at Telecommunication Wavelength

Luozhen Li, Akshay Babu Karyath, Julien Bertrand et al. (+12 more)

We characterize spectroscopic and coherence properties of the 1451.37 nm excited-state zero-phonon line (ZPL) between the $^{3}F_{4}$ and the $^{3}H_{4}$ manifolds of a thulium-doped yttrium aluminum perovskite (Tm$^{3+}$:YAlO$_3$) crystal at temperatures around 1.5 K. We measure the absorption spectrum between the $^{3}H_{6}$ - $^{3}F_{4}$ and $^{3}F_{4}$ - $^{3}H_{4}$ manifolds, the inhomogeneous broadening of the $^{3}F_{4}$ - $^{3}H_{4}$ (excited-state) ZPL, and the lifetimes of the higher-lying and lower-lying excited states. We also investigate level shifts caused by the quadratic Zeeman interaction as well as spectral hole-burning spectra with varying magnetic fields, providing insights into hyperfine interactions. Using again spectral holes but also optical free induction decays (FIDs), we assess optical coherence times, finding a maximum of $4.75 \pm 0.07~\mu s$ at B=2T and low ion concentration in the $^{3}F_{4}$ level. Our results – the first to demonstrate coherence of an excited-state transition in a rare-earth crystal – suggest the possibility of exploiting such transitions for quantum technology.
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All authors Luozhen Li, Akshay Babu Karyath, Julien Bertrand, Mohsen Falamarzi Askarani, Maria Gieysztor, Hridya Meppully Sasidharan, Joshua A. Slater, Aaron D. Marsh, Philip J. T. Woodburn, Charles W. Thiel, Rufus L. Cone, Sara Marzban, Nir Alfasi, Patrick Remy, Wolfgang Tittel

Quantum Physics

Mapping the positions of Two-Level-Systems on the surface of a superconducting transmon qubit

Jürgen Lisenfeld, Alexander K. Händel, Etienne Daum et al. (+3 more)

The coherence of superconducting quantum computers is severely limited by material defects that create parasitic two-level-systems (TLS). Progress is complicated by lacking understanding how TLS are created and in which parts of a qubit circuit they are most detrimental. Here, we present a method to determine the individual positions of TLS at the surface of a transmon qubit. We employ a set of on-chip gate electrodes near the qubit to generate local DC electric fields that are used to tune the TLS' resonance frequencies. The TLS position is inferred from the strengths at which TLS couple to different electrodes and comparing them to electric field simulations. We found that the majority of detectable surface-TLS was residing on the leads of the qubit's Josephson junction, despite the dominant contribution of its coplanar capacitor to electric field energy and surface area. This indicates that the TLS density is significantly enhanced near shadow-evaporated electrodes fabricated by lift-off techniques. Our method is useful to identify critical circuit regions where TLS contribute most to decoherence, and can guide improvements in qubit design and fabrication methods.
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All authors Jürgen Lisenfeld, Alexander K. Händel, Etienne Daum, Benedikt Berlitz, Alexander Bilmes, Alexey V. Ustinov

Quantum Physics

Quantum non-Markovian Hatano-Nelson model

Sumit Kumar Jana, Ryo Hanai, Tan Van Vu et al. (+2 more)

While considering non-Hermitian Hamiltonians arising in the presence of dissipation, in most cases, the dissipation is taken to be frequency independent. However, this idealization may not always be applicable in experimental settings, where dissipation can be frequency-dependent. Such frequency-dependent dissipation leads to non-Markovian behavior. In this work, we demonstrate how a quantum non-Markovian Hatano-Nelson model arises microscopically in a quasi-one-dimensional dissipative lattice. This is achieved using non-equilibrium Green's functions without requiring any approximation like weak system-bath coupling or a time-scale separation, which would have been necessary for a Markovian treatment. The resulting effective system exhibits nonreciprocal hopping, the defining characteristic of the Hatano-Nelson model, as well as uniform dissipation, both of which are frequency-dependent. This holds for both bosonic and fermionic settings. We find solely non-Markovian nonreciprocal features like unidirectional frequency blocking in bosonic settings, and a particular type of non-equilibrium dissipative quantum phase transition in fermionic settings, that cannot be captured in a Markovian theory, nor have any analog in reciprocal systems. Our results lay the groundwork for describing and engineering non-Markovian nonreciprocal quantum lattices.
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All authors Sumit Kumar Jana, Ryo Hanai, Tan Van Vu, Hisao Hayakawa, Archak Purkayastha

Quantum Physics

Open quantum-classical systems: A hybrid MASH master equation

Kasra Asnaashari and Jeremy O. Richardson

We propose a method which combines the quantum-classical mapping approach to surface hopping (MASH) with the dissipative quantum dynamics of the Lindblad master equation. Like conventional surface-hopping methods, our approach is based on classical trajectories coupled to the dynamics of a quantum subsystem. However, instead of evolving the subsystem wavefunction according to the time-dependent Schrödinger equation, we use stochastic quantum trajectories derived from secular Redfield theory. This enables the simulation of open quantum systems coupled simultaneously to Markovian quantum baths and anharmonic non-Markovian classical degrees of freedom. Applications to the spin–boson model and to the cavity-enhanced fluorescence of an electronically nonadiabatic molecule show excellent agreement with fully quantum-mechanical benchmarks.
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Quantum Physics

Variational noise mitigation in quantum circuits: the case of Quantum Fourier Transform

Rafael Gómez-Lurbe, Alexander Bernal, Armando Pérez et al. (+2 more)

We propose using variational quantum algorithms (VQAs) to simulate established quantum algorithms under realistic noise conditions, aiming to surpass the fidelity of theoretical circuits in noisy environments. Focusing on the Quantum Fourier Transform (QFT), we perform numerical simulations for two qubits under both coherent and incoherent noise. To enhance generalization, we further introduce the use of Mutually Unbiased Bases (MUBs) during the optimization. Our results show that the variational circuit can reproduce the QFT with higher fidelity in scenarios dominated by coherent noise. This demonstrates the potential of the approach as an effective error-mitigation strategy for small- to medium-scale quantum systems, particularly in settings where coherent noise strongly impacts performance. Beyond mitigating noise and improving fidelity, the method can be adapted to the noise profile of a specific device, providing a versatile and practical route to enhance the reliability of quantum algorithms in near-term quantum hardware.
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All authors Rafael Gómez-Lurbe, Alexander Bernal, Armando Pérez, Bryan Zaldívar, J. Alberto Casas

Quantum Physics

Shallow instantaneous quantum polynomial-time circuits for generative modeling on noisy intermediate-scale quantum hardware

Oriol Balló-Gimbernat, Marcos Arroyo-Sánchez, Paula García-Molina et al. (+2 more)

Generative modeling is one of the most promising applications of quantum machine learning, yet training and deploying Quantum Generative Models (QGMs) on near-term hardware remains effectively intractable due to prohibitive gradient estimation and implementation costs. We propose a resource-efficient approach based on shallow Instantaneous Quantum Polynomial-time (IQP) circuits that circumvents these bottlenecks by leveraging efficient classical training while retaining the guarantee of sampling hardness. To validate this approach, we formalize graph generation as a hierarchy of physical correlations, allowing us to map abstract data features, such as edge density and bipartiteness, directly to the quantum observables required to learn them. We validate our protocol through demonstrations both on real hardware (from $28$ to $153$ qubits) and simulations ($28$ qubits). Results show that while global structural features exhibit significant degradation beyond $91$ qubits, our models achieve high-precision reproduction of local correlations, even up to $153$ qubits. These findings establish shallow IQP circuits as a robust, scalable candidate for generative tasks on current Noisy Intermediate-Scale Quantum (NISQ) devices.
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All authors Oriol Balló-Gimbernat, Marcos Arroyo-Sánchez, Paula García-Molina, Adan Garriga, Fernando Vilariño

Quantum Physics

Effects of boundary conditions on quantum nanoresonators: decoherence-free subspaces

Humberto C. F. Lemos, Thiago Cordeiro and Adelcio C. Oliveira

The Euler-Bernoulli beam model has been studied classically and semi-classically. The semi-classical quantization is done in an analogous way to the quantization of the electromagnetic field, and we found an effect that is similar to the Casimir effect, which is the photonic Casimir effect. The Casimir force, by unit area, is proportional to the first mode energy divided by the volume of the beam. For the hinged-hinged boundary condition, degenerate states were found. These degenerate pairs form decoherence-free subspaces for dispersive thermal reservoirs. For other boundary conditions, there are also subspaces with lower decoherence rates, which occur for quasi-degenerate states.
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Quantum Physics

Generating function and Bloch representation for quantum Fisher tensor

Felipe P. Abreu and Wei Chen

The Uhlmann relative amplitude between two density matrices is shown to be a generating function, through which the quantum Fisher tensor that contains both the quantum Fisher information matrix and the mean Uhlmann curvature can be obtained via differentiation over system parameters. In the pure state limit, our generating function recovers that of the quantum geometric tensor proposed by Hetényi and Lévay, and also clarifies the fidelity and phase between two quantum states as the generating functions of the quantum metric and Berry curvature, respectively. A generic expression for the quantum Fisher tensor in terms of the Bloch representation of density matrices is derived, which facilitates the calculation of the tensor, mean Uhlmann curvature, and geometric properties derived from the quantum Fisher information matrix. Canonical ensembles of spins are adopted to demonstrate our formalism, which reveals a constant Ricci scalar, a vacuum Einstein equation, and a cosmological constant on the 3D Euclidean manifold of the magnetic field
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Quantum Physics · Cross-listed from math.OC

Convex semidefinite tensor optimization and quantum entanglement

Liding Xu, Ye-Chao Liu and Sebastian Pokutta

The cone of positive-semidefinite (PSD) matrices is fundamental in convex optimization, and we extend this notion to tensors, defining PSD tensors, which correspond to separable quantum states. We study the convex optimization problem over the PSD tensor cone. While this convex cone admits a smooth reparameterization through tensor factorizations (analogous to the matrix case), it is not self-dual. Moreover, there are currently no efficient algorithms for projecting onto or testing membership in this cone, and the semidefinite tensor optimization problem, although convex, is NP-hard. To address these challenges, we develop methods for computing lower and upper bounds on the optimal value of the problem. We propose a general-purpose iterative refinement algorithm that combines a lifted alternating direction method of multipliers with a cutting-plane approach. This algorithm exploits PSD tensor factorizations to produce heuristic solutions and refine the solutions using cutting planes. Since the method requires a linear minimization oracle over PSD tensors, we design a spatial branch-and-bound algorithm based on convex relaxations and valid inequalities. Our framework allows us to study the white-noise mixing threshold, which characterizes the entanglement properties of quantum states. Numerical experiments on benchmark instances demonstrate the effectiveness of the proposed methods.
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Quantum Physics

A Gate-Based Quantum Genetic Algorithm for Real-Valued Global Optimization

Leandro C. Souza, Laurent E. Dardenne and Renato Portugal

We propose a gate-based Quantum Genetic Algorithm (QGA) for real-valued global optimization. In this model, individuals are represented by quantum circuits whose measurement outcomes are decoded into real-valued vectors through binary discretization. Evolutionary operators act directly on circuit structures, allowing mutation and crossover to explore the space of gate-based encodings. Both fixed-depth and variable-depth variants are introduced, enabling either uniform circuit complexity or adaptive structural evolution. Fitness is evaluated through quantum sampling, using the mean decoded output of measurement outcomes as the argument of the objective function. To isolate the impact of quantum resources, we compare gate sets with and without the Hadamard gate, showing that superposition consistently improves convergence and robustness across benchmark functions such as the Rastrigin function. Furthermore, we demonstrate that introducing pairwise inter-individual entanglement in the population accelerates early convergence, revealing that quantum correlations among individuals provide an additional optimization advantage. Together, these results show that both superposition and entanglement enhance the search dynamics of evolutionary quantum algorithms, establishing gate-based QGAs as a promising framework for quantum-enhanced global optimization.
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Quantum Physics

A Triple-Hybrid Quantum Support Vector Machine Using Classical, Quantum Gate-based and Quantum Annealing-based Computing

Juan C. Boschero, Ward van der Schoot and Niels M. P. Neumann

Quantum machine learning is one of the fields where quantum computers are expected to bring advantages over classical methods. However, the limited size of current computers restricts the exploitation of the full potential of quantum machine learning methods. Additionally, different computing paradigms, both quantum and classical, each have their own strengths and weaknesses. Obtaining optimal results with algorithms thus requires algorithms to be tweaked to the underlying computational paradigm, and the tasks to be optimally distributed over the available computational resources. In this work, we explore the potential gains from combining different computing paradigms to solve the complex task of data classification for three different datasets. We use a gate-based quantum model to implement a quantum kernel and implement a complex feature map. Next, we formulate a quadratic unconstrained optimisation problem to be solved on quantum annealing hardware. We then evaluate the losses on classical hardware and reconfigure the model parameters accordingly. We tested this so-called triple-hybrid quantum support vector machine on various data sets, and find that it achieves higher precision than other support vector machines (both quantum and classical) on complex quantum data, whereas it achieves varying performance on simple classical data using limited training. For the complex data sets, the triple-hybrid version converges faster, requiring fewer circuit evaluations.
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Quantum Physics

Adaptive Entanglement-Aware Routing for Satellite Quantum Networks under Orbital and Atmospheric Variability

Dhrumil Bhatt and Vidushi Kumar

The expansion of satellite-based quantum networks requires adaptive routing mechanisms that can sustain entanglement under dynamic orbital and atmospheric conditions. Conventional schemes, often tailored to static or idealised topologies, fail to capture the combined effects of orbital motion, fading, and trust variability in inter-satellite links. This work proposes an adaptive entanglement-aware routing framework that jointly accounts for orbital geometry, atmospheric attenuation, and multi-parameter link evaluation. The routing metric integrates fidelity, trust, and key-rate weights to maintain connectivity and mitigate loss from turbulence and fading. Monte Carlo simulations across multiple orbital densities ($\rho = 10^{-6}$~km$^{-3}$) and environmental regimes, standard atmosphere, strong turbulence, and clear-sky LEO show up to a 275% improvement in key generation rate and a 15% increase in effective entanglement fidelity over existing adaptive methods. The framework achieves sub-linear path-length scaling with network size and remains robust for fading variances up to $\sigma_{\mathrm{fade}}=0.1$, demonstrating strong potential for future global quantum constellations.
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Quantum Physics

Scaling behavior of dissipative systems with imaginary gap closing

Jinghui Pi, Xingli Li and Yangqian Yan

Point-gap topology, characterized by spectral winding numbers, is crucial to non-Hermitian topological phases and dramatically alters real-time dynamics. In this paper, we study the evolution of quantum particles in dissipative systems with imaginary gap closing, using the saddle-point approximation method. For trivial point-gap systems, imaginary gap-closing points can also be saddle points. This leads to a single power-law decay of the local Green's function, with the asymptotic scaling behavior determined by the order of these saddle points. In contrast, for nontrivial point-gap systems, imaginary gap-closing points do not coincide with saddle points in general. This results in a dynamical behavior characterized by two different scaling laws for distinct time regimes. In the short-time regime, the local Green's function is governed by the dominant saddle points and exhibits an asymptotic exponential decay. In the long-time regime, however, the dynamics is controlled by imaginary gap-closing points, leading to a power-law decay envelope. Our findings advance the understanding of quantum dynamics in dissipative systems and provide predictions testable in future experiments.
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Quantum Physics

Revocation and Reconstruction of Shared Quantum States

Prakash Mudholkar, Chiranjeevi Vanarasa, Indranil Chakrabarty et al. (+1 more)

The problem of revocation of quantum states after sharing is interesting and we ask: Is it possible for a dealer to revoke the state once shared, before the reconstruction process? Additional resources like bell states are used to help the dealer to get back the state. In a three-party scenario, we show an independent way to revoke, if, for any reason, the dealer is not sure about the intention of the/any reconstructor. In general, the classical outcomes of the dealer in sharing phase are needed, to be able to reconstruct the state perfectly. When both the shareholders are dishonest, and without the dealer's knowledge, collude to reconstruct, they always have some chance of succeeding. This is addressed by giving more control to the dealer by making him/her) to have a quantum share as well. We give a sharing and revocation protocol with a four-qubit entangled resource shared among three parties (two qubits with the dealer and one each with the shareholders). We further consider a class of four qubit pure entangled states as resource and explicitly find the range of parameters for which the protocol will be successful.
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All authors Prakash Mudholkar, Chiranjeevi Vanarasa, Indranil Chakrabarty, Srinathan Kannan

Quantum Physics

CUNQA: a Distributed Quantum Computing emulator for HPC

Jorge Vázquez-Pérez, Daniel Expósito-Patiño, Marta Losada et al. (+3 more)

The challenge of scaling quantum computers to gain computational power is expected to lead to architectures with multiple connected quantum processing units (QPUs), commonly referred to as Distributed Quantum Computing (DQC). In parallel, there is a growing momentum toward treating quantum computers as accelerators, integrating them into the heterogeneous architectures of high-performance computing (HPC) environments. This work combines these two foreseeable futures in CUNQA, an open-source DQC emulator designed for HPC environments that allows testing, evaluating and studying DQC in HPC before it even becomes real. It implements the three DQC models of no-communication, classical-communication and quantum-communication; which will be examined in this work. Addressing programming considerations, explaining emulation and simulation details, and delving into the specifics of the implementation will be part of the effort. The well-known Quantum Phase Estimation (QPE) algorithm is used to demonstrate and analyze the emulation of the models. To the best of our knowledge, CUNQA is the first tool designed to emulate the three DQC schemes in an HPC environment.
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All authors Jorge Vázquez-Pérez, Daniel Expósito-Patiño, Marta Losada, Álvaro Carballido, Andrés Gómez, Tomás F. Pena

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

Restricted-Geometry Quantum Models Beyond Atoms: Application of the Eckhardt-Sacha approach to NSDI in Diatomic Systems

Lars C. Bannow, Jan H. Thiede, Michał Ogryzek et al. (+2 more)

We present a (1+1)-dimensional quantum model designed to describe nonsequential double ionization (NSDI) in homonuclear diatomic molecules exposed to strong linearly polarized laser fields. Extending the restricted-geometry framework previously developed for atomic systems by Eckhardt and Sacha, our approach captures key features of NSDI, including the characteristic knee structure in double ionization yields. Despite its simplifying assumptions, the model shows good agreement with experimental data and proves particularly suitable for systems with $\sigma$-type orbital symmetry. It offers a computationally efficient tool for exploring multi-electron dynamics in molecular systems.
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All authors Lars C. Bannow, Jan H. Thiede, Michał Ogryzek, Dmitry K. Efimov, Jakub S. Prauzner-Bechcicki

Quantum Physics

Revivals and quantum carpets for the relativistic Schrödinger equation

Benoît Zumer, Florent Daem and Alexandre Matzkin

We investigate wavepacket dynamics for a relativistic particle in a box evolving according to the relativistic Schrödinger (also known as the Salpeter) equation. We derive the solutions for an infinite well – which contrary to the standard relativistic wave equations (such as the Klein-Gordon or Dirac equations) – are well defined, and use these solutions to construct wavepackets. We obtain expressions for the wavepacket revival times and explore the corresponding quantum carpets (the space-time probability density plots) for different dynamical regimes. We further analyze level spacing statistics as the dynamics goes from the non-relativistic regime to the ultra-relativistic limit.
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