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Friday 17 April

Quantum Physics

GreenPeas: Unlocking adaptive quantum error correction with just-in-time decoding hypergraphs

Abbas B. Ziad, Jubo Xu and Hongxiang Fan

Circuit-level decoders are essential for the realisation of low-overhead fault-tolerant quantum computing. However, they rely on complex hypergraphs that are traditionally compiled ahead-of-time. This static approach introduces a significant bottleneck for an emerging class of adaptive circuits, where the structure is modified during execution based on mid-circuit measurement outcomes. Pre-compiling hypergraphs for all possible circuit branches would incur an exponential memory cost, rendering current tools impractical for these workloads. Hence, we introduce GreenPeas, a just-in-time compiler for decoding hypergraphs. By lowering the realised circuit to a space-time error propagation graph, GreenPeas decomposes Stim's backtracking algorithm for error analysis into two sequentially dependent, internally parallelisable stages: (1) mapping physical errors to their corresponding equivalence classes, and (2) aggregating error probabilities within each class. Evaluated on surface and bivariate bicycle code memory circuits without user-annotated repeat blocks, GreenPeas achieves a geometric mean speedup of 13.2x over Stim using a high-end GPU. This speedup carries over to the adaptive regime, unlocking circuit-level decoding of [[4,2,2]]-concatenated surface code memories with adaptive syndrome measurements – a capability previously restricted to less accurate phenomenological decoders – yielding 6.7x lower logical error rate and 4.5x lower decoding latency at a representative outer code distance of 10.
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Quantum Physics

Verifying random matrix product states with autoregressive local measurements

Hyunho Cha, Subin Kim and Jungwoo Lee

Matrix product states (MPS) are a central language for one-dimensional quantum matter and a practical target for near-term quantum simulators and variational algorithms. Yet, while substantial effort has focused on preparing MPS with shallow circuits, scalable methods to verify that a many-body device has actually produced the intended state remain underdeveloped. Direct fidelity estimation (DFE) relies only on local Pauli measurements, but in many-body settings it suffers an exponential classical overhead from the preprocessing needed to sample Pauli strings. We eliminate this obstacle by introducing an autoregressive importance sampler that draws Pauli strings sequentially from efficiently computable conditional distributions, reducing the per-shot classical overhead to linear scaling in the number of qubits. We further develop a grouped extension that constructs qubit-wise commuting measurement settings via a sorting string and simultaneously estimates the entire commuting group from a single setting, significantly reducing estimator variance while preserving efficient postprocessing. Our approach extends naturally to matrix product operators (MPO), enabling scalable verification of tensor-network states and observables in long one-dimensional quantum systems. We utilize random MPS as a natural benchmark for generic 1D entangled states.
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Quantum Physics · Cross-listed from cond-mat.quant-gas

Supersolid Rotation in an Annular Bose-Einstein Condensate coupled to a Ring Cavity

Gunjan Yadav, Nilamoni Daloi, Pardeep Kumar et al. (+2 more)

We theoretically investigate an annularly confined Bose-Einstein Condensate (BEC) coupled to a four-mirror ring cavity supporting traveling-wave optical modes. Under symmetric driving by counter-propagating Laguerre-Gaussian beams carrying equal and opposite orbital angular momenta, the system realizes supersolid phases coexisting with persistent superfluid circulation. Specifically, we obtain a supersolid state if we start with a BEC of winding number $L_p$ as well as supersolid packets with coherent superpositions of two different BEC $L_p$ values. Under asymmetric pumping, realized with Laguerre-Gaussian beams of different orbital angular momenta, chiral symmetry is broken in the system, resulting in asymmetric cavity field amplitudes, directional density modulations, and tunable rotational dynamics of the resulting supersolid lattice. This leads to rotating supersolid density structures for a single winding-number state, and rotating wave packets for an initial superposition of rotational eigenstates. Finally, we probe the presence of Goldstone and Higgs modes which can be observed using minimally destructive measurements of the cavity output spectrum. Our mean-field theory reveals interference-driven rotation without physical stirring, and distinguishes our work from prior static cavity supersolids. Our results establish the ring cavity annular BEC as a versatile platform for generating chiral quantum matter, implementing rotation-sensing devices and generating atomtronic circuits with supersolids.
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All authors Gunjan Yadav, Nilamoni Daloi, Pardeep Kumar, M. Bhattacharya, Tarak Nath Dey

Quantum Physics

Momentum reconstruction from Unruh-deWitt detectors

Jesse Huhtala and Iiro Vilja

We investigate momentum reconstruction for particle processes observed by Unruh-deWitt detector setups. In particular, we derive the probability distributions for particle momenta conditioned on detector clicks in three spatial dimensions. We investigate the statistical properties of such detector setups and discuss their use as models of measurement devices in particle physics.
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Quantum Physics · Cross-listed from physics.acc-ph

Comment on "Angular momentum dynamics of vortex particles in accelerators”

S.S. Baturin

We comment on Ref.[D. Karlovets, D. Grosman, and I. Pavlov, Phys. Rev. Lett. 136, 085002 (2026)], which proposes a BMT-like equation for the mean kinetic orbital angular momentum (OAM) of vortex particles in accelerator fields and draws spin-like conclusions about depolarization, resonances, and control. We show that the proposed closure is not generally valid even at the mean-value level. In the authors' own homogeneous-field model, Eq.(8) already makes $\langle L_z\rangle$ depend on the packet second moment $\langle \rho^2\rangle(\tau)$; for an exact family of breathing Landau/LG packets this yields an explicit oscillation incompatible with Eq.(9) except in the nongeneric matched case. Moreover, the Appendix A assumption that mixed correlators are negligible suppresses the transverse kinetic-OAM components themselves, since those correlators are precisely the building blocks of $L_x$ and $L_y$. We also stress that, even if a closed equation for $\langle \hat{\mathbf L}\rangle$ were available, it would still not constitute a transport equation for a vortex quantum state. Mean-OAM transport does not determine OAM spectra, inter-mode coherences, or fidelity. State-level claims therefore require a mode-resolved density-matrix treatment rather than an Ehrenfest equation for a low-order moment.
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Quantum Physics

Fast, High-Fidelity Erasure Detection of Dual-Rail Qubits with Symmetrically Coupled Readout

Jimmy Shih-Chun Hung, Arbel Haim, Mouktik Raha et al. (+14 more)

Erasure qubits are a promising platform for implementing hardware-efficient quantum error correction. Realizing the error-correction advantages of this encoding requires frequent mid-circuit erasure checks that are fast, high-fidelity, and scalable. Here, we realize erasure detection with a hardware-efficient circuit consisting of a single readout resonator dispersively and symmetrically coupled to both transmons of a dual-rail qubit. We use this circuit to demonstrate single-shot erasure detection in 384 ns with minimal impact on the dual-rail logical manifold, achieving a residual error per check of $6.0(2) \times 10^{-4}$, with only $8(3) \times 10^{-5}$ induced dephasing per check, and an erasure error per check of $2.54(1)\times 10^{-2}$. The high degree of matched dispersive readout coupling ($\chi$-matching) within the dual-rail qubit code space also allows us to realize a new modality: time-continuous erasure detection performed in parallel with single-qubit gates. Here we achieve a median $7.2 \times 10^{-5}$ error per gate with $< 1 \times 10^{-5}$ error induced by erasure detection. This demonstrates a reduction in erasure detection overhead as well as a crucial ingredient for soft information quantum error correction. Together, these results establish symmetrically coupled dispersive readout as a fast, hardware-efficient, and scalable component for erasure-based quantum error correction using transmon dual-rail qubits.
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All authors Jimmy Shih-Chun Hung, Arbel Haim, Mouktik Raha, Gihwan Kim, Ziwen Huang, Ming-Han Chou, Mitch D'Ewart, Erik Davis, Anurag Mishra, Patricio Arrangoiz Arriola, Amirhossein Khalajhedayati, David Hover, Fernando G.S.L. Brandão, Aashish A. Clerk, Alex Retzker, Harry Levine, Oskar Painter

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

Renormalised thermodynamics for Bose gases from low to critical temperatures

Michael H. Heinrich, Alexander Wowchik and Jürgen Berges

We compute thermodynamic properties of dilute Bose gases using non-perturbative approximations of the two-particle irreducible (2PI) effective action. It is shown how to systematically renormalise the self-consistent descriptions beyond conventional Gaussian approximations such as Hartree-Fock-Bogoliubov theory. This allows us to determine the condensate depletion from low to high temperatures, including its critical behaviour at the phase transition. While the universal anomalous dimension at criticality is vanishing for Gaussian approximations, we determine its non-zero value at next-to-leading order of a self-consistent expansion in the number of field components.
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Quantum Physics

How to unitarily map between any two pure states with a single closed-form exponential

Peter T. J. Bradshaw, Marcus Gouveia and Jonte R. Hance

It is well-known that any two pure quantum states (in the same Hilbert space) can be mapped to any other using unitary transformations. However, previous approaches to this problem required two explicit bases for the Hilbert space, one each for the initial and target states, and thus their complexity necessarily scales with the dimension of the Hilbert space. In this Letter, we show how to utilize novel algebraic methods to construct a closed-form exponential unitary transformation which achieves this in general, using only a single unitary generator. This construction is independent of any bases and agnostic to the dimension of the Hilbert space. We highlight the usefulness of this tool for studying relationships between systems of pure states in quantum information theory, as well in elementary analyses of quantum circuits and unitary operators.
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Quantum Physics

Boson correlations are spurious for classical states

Daniel E. Salazar and Fabrice P. Laussy

We show that boson correlations from quantum states with a Glauber-Sudarshan representation of their density matrix which provides a well-behaved probability distribution – including coherent states, thermal states, and all states that can be deemed classical – are a manifestation of the Simpson paradox: they are spurious correlations from statistical (ensemble) averages over uncorrelated measurements made in varying geometries, due to a process of symmetry-breaking as a confounding factor. Bosonic correlations encoded by the wavefunction appear to be formed in the geometry assumed, which however is not that of the statistical ensemble but varies from realization to realization. This calls to distinguish between quantum and statistical averages and sheds new understandings on the fundamental problems of nonclassicality and quantum advantage.
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Quantum Physics

Aziz and Howl's Gravity-Induced Entanglement Channel is Essentially Classical Mechanics

Hanyu Xue, Ziqian Tang, Chen Yang et al. (+3 more)

Aziz and Howl argued that a classical gravitational field can generate quantum entanglement through a quantum-field-theoretic channel mediated by virtual matter propagation. However, their claimed channel is more naturally and accurately understood as semiclassical wavepacket motion in an external gravitational field, rather than as a distinctively quantum-field-theoretic entangling effect. Moreover, the result of their perturbative computation is incorrectly magnified: they selected a discontinuous wavefunction with infinite kinetic energy as the initial state and simultaneously treated it as stationary. Once a correct treatment using Gaussian wavepacket is adapted, the resulting effect will be negligibly small.
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All authors Hanyu Xue, Ziqian Tang, Chen Yang, Zizhao Han, Zikuan Kan, Yulong Liu

Quantum Physics

Yttrium ion as a platform for quantum information processing

Christopher N. Gilbreth, Dmytro Filin, Marianna S. Safronova et al. (+2 more)

Engineering large-scale quantum computers which simultaneously provide high-fidelity quantum operations, low memory errors, low crosstalk, and reasonable resource usage remains an outstanding challenge across quantum computing platforms. In trapped ions, progress has largely focused on alkaline-earth and ytterbium ions, whose simple electronic structures facilitate control over their internal state. Here we investigate singly-ionized yttrium ($^{89}\mathrm{Y}^+$), a two-valence-electron ion whose ground-state manifold hosts a nuclear-spin qubit and which also features a variety of low-lying metastable manifolds, for applications in quantum information processing. Because experimental data are limited, we perform high-resolution laser-induced fluorescence spectroscopy to measure the hyperfine structure of several low-lying levels, and carry out comprehensive electronic structure calculations to determine lifetimes, transition matrix elements, and hyperfine coefficients for manifolds addressable with visible, near-visible, or infrared wavelengths. Using these results, we analyze schemes for qubit storage, initialization, readout, leakage mitigation, and single- and two-qubit gates. These results position $^{89}\mathrm{Y}^+$ as a uniquely capable next-generation trapped-ion qubit, combining field-insensitive nuclear-spin or clock-qubit storage with spectrally isolated transitions for operations.
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All authors Christopher N. Gilbreth, Dmytro Filin, Marianna S. Safronova, Guanming Lao, Eric R. Hudson

Quantum Physics

Long-term Performance Analysis of a Commercial QKD Device Under Real-world Deployment Conditions

Alisson Tezzin, Gustavo M. Uhdre, Oscar Martins et al. (+2 more)

Quantum key distribution (QKD) has reached a commercially viable stage, with several companies offering hardware systems designed for operational deployment. Evaluating the performance of commercial QKD devices under real-world deployment conditions is essential for users seeking to understand the practical limitations and operational reliability of these systems. In this paper, we present a long-term performance analysis of ID Quantique's Clavis XGR deployed within the Hermes Quantum Network, in Brazil. Our study provides a detailed characterization of key operational metrics, such as secret key rate, quantum bit error rate (QBER), visibility, and detection counts, mapping their behavior over extended periods of continuous operation. We analyze the system's stability across two distinct optical links: a 40.3 km indoor spooled fiber and a 3.5 km outdoor deployed underground fiber. Monitored under both unregulated tropical ambient fluctuations and actively controlled thermal stress, our results demonstrate excellent overall baseline resilience, with the system maintaining visibility above 97% and QBER below 1% on average. These findings provide practical insights into the expected behavior and thermal bottlenecks of commercial QKD systems in field deployments, particularly in tropical climates, helping to inform realistic expectations for operational quantum-safe infrastructures.
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All authors Alisson Tezzin, Gustavo M. Uhdre, Oscar Martins, Sabrina Rufo, Vitor G.A. Carneiro

Quantum Physics · Cross-listed from hep-ph

Quantum Tomography and Entanglement in Semi-Leptonic $h\to VV^*$ Decays at Higher Orders

Dorival Gonçalves, Ajay Kaladharan and Alberto Navarro

Angular correlations in Higgs decays to electroweak gauge bosons, $h \to ZZ^*, WW^*$, provide a powerful probe of both new physics effects and quantum information observables. We present a systematic study of semi-leptonic Higgs decays $h \to V V^* \to \ell^+\ell^- q\bar{q}$ and $\ell^\pm \nu_\ell q\bar{q}'$, including finite final state fermion masses, NLO QCD, and NLO electroweak corrections. We show that finite final state quark masses can induce effects that go beyond the two-qutrit description in more inclusive regimes, while remaining controllable with suitable kinematic selections. QCD corrections lead to modest percent-level shifts, whereas electroweak corrections can significantly modify the angular structure, particularly in the $h\to ZZ^*$ channels. We assess the impact of these effects on the reconstructed density matrix and entanglement measures, finding that, although they modify the angular observables, the semi-leptonic channels are better approximated by a two-qutrit description than the fully leptonic decay $h\to e^+e^-\mu^+\mu^-$.
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Quantum Physics

A digitally controlled silicon quantum processing unit

Members of the HRL Quantum Team, Collaborators: Michael Abraham, Edwin Acuna et al. (+253 more)

Commercially-relevant quantum computers will require large numbers of high-performing qubits that can be manufactured, integrated, and controlled at scale. Silicon exchange-only (EO) qubits are a strong candidate modality due to their control-signal simplicity and compatibility with advanced semiconductor manufacturing, but questions remain around the achievability of sufficiently low noise and a scalable control and wiring solution. Here we introduce a quantum processing unit composed of a custom-designed cryogenic CMOS controller, a novel high-density superconducting ribbon cable, and a low-noise EO qubit device. The quantum chip features a three-rail array of 54 exchange-coupled quantum dots, configurable to host up to 18 EO qubits. We integrate and use these components to demonstrate qubit performance for both single-qubit and entangling operations that advances the EO state of the art by an order of magnitude. We further validate this system by implementing a distance-5 repetition code and a quantum error detecting code then make detailed comparisons with simulations. Our approach facilitates a utility-scale quantum computer with manageable operational and capital requirements.
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All authors Members of the HRL Quantum Team, Collaborators: Michael Abraham, Edwin Acuna, Tower S. Adams, Moonmoon Akmal, Matthew R. Alfaro, I. Alvarado, Jacob Amontree, Carter Andrews, Reed W. Andrews, Michael Antcliffe, Andre R. Aséncio, Ryan M. Avila Batres, Cynthia D. Baringer, David W. Barnes, Katherine M. Beech, Russell G. Blakey, Zachery T. Bloom, Aaron J. Bluestone, Jacob Z. Blumoff, Matthew G. Borselli, Koel A. Bose, Brydon Boyd, Jacob T. Boyer, Teresa L. Brecht, Christopher C. Brough, Rex A. Brown, Steven L. Brown, Tyler A. Cain, John B. Carpenter, Stephen Carr, Faustin W. Carter, Mitchell Casanova, Jacob L. Chambers, Matthew D. Chambers, Khamsorn L. Chanthavong, James M. Chappell, Rhian Chavez, Kevin C. Chen, Peter S. Chen, Maxwell D. Choi, Krishna Choudhary, Matthew N. H. Chow, Justin E. Christensen, Aaron M. Chronister, Andrew M. Clapper, Abigail A. Coker, Michael D. Cornelius, Albert E. Cosand, Ian T. Counts, Edward T. Croke, Gregory M. Crosswhite, Adam Dally, Erik S. Daniel, Tuan A. Dao, Dominic Daprano, Tiffany Davis, Neha Deshpande, Rachel S. Dey, D. Scott Diamond, Claire E. Dickerson, J. P. Dodson, James B. Dragan, Marc Dvorak, Lisa F. Edge, Charles R. Elliott, Kenneth R. Elliott, Kevin Eng, Jacob Fast, Colin P. Feeney, David J. Fialkow, Dylan H. Finestone, Micha N. Fireman, Bryan H. Fong, Trevor M. Fowler, Sean Frazier, Kiera L. Fuller, Christina A. C. Garcia, Kacy L. Garstka, Kara C. Garvey, Zachary A. Geiger, Galen R. Gledhill, Caleigh M. Goodwin-Schoen, Joseph L. Goralka, Bradley W. Greene, Hrayr K. Gurgenian, Sieu D. Ha, Wonill Ha, Nathanial R. Hapeman, Brooke M. Hardesty, Jim W. Harrington, Patrick M. Harrington, Thomas R. B. Harris, Ben M. Harrison, Anthony T. Hatke, Robert R. Hayes, Kevin He, Raul Hernandez Garcia, Ryan M. Hickey, Jocelyn Hicks-Garner, Alex Hirman, Donald A. Hitko, David Ho, Holland Y. Ho, Vinh S. Ho, nathan holman, Adam Holmes, Nerys Huffman, Daniel R. Hulbert, Eric B. Isaacs, Clayton A. C. Jackson, Logan Jaeger, Ian Jenkins, Cameron Jennings, Paul C. Jerger, B. Johnson, Aaron M. Jones, Michael P. Jura, Adour V. Kabakian, Raj M. Katti, Tyler Keating, Joseph Kerckhoff, Joseph D. Kern, Isaac Khalaf, Aditya Kher, Jake J. Kim, Erich W. Kinder, Andrey A. Kiselev, William F. Koehl, Patrick W. Krantz, Thaddeus D. Ladd, Pierce G. Laing, Sanaaya Lakdawala, Nathan J. Lang, Robert Lanza, Elias Lawson-Fox, Dustin Le, Kangmu Lee, Nathan R. A. Lee, Jaime Lerma, Mark P. Levendorf, Alwina R. Liu, Henry Lizarraga, Aurelio Lopez, Hoa C. Ly, Torrey T. Lyons, Theodore K. Macioce, Matthew M. Mackey, John K. Maeda, Ryan M. Martin, Daniel S. Matic, Justine W. Matten, Gavin C. Mazur, Max S. McCready, Olivia Means, Kevin E. Millner, Ivan Milosavljevic, Matthew Morris, Susan L. Morton, Samuel Mumford, Bryce D. Murley, Robert G. Nagele, Taro A. Naoi, Cameron R. Nelson, Georgia A. Newman, David B. Nguyen, Tina Niknejad, Rebecca N. Nishide, Liam C. O'Brien, Colin B. E. O'Keefe, Riley P. O'Neil, Andrew E. Oriani, Anthony F. Ortiz, John J. Ottusch, Andrew Pan, Pamela R. Patterson, Uttam Paudel, Julius C. Perez, Christi A. Peterson, Vu T. Phan, Nickolas H. Pilgram, Clifford E. Plesha, Winston Pouse, Eric M. Prophet, Daniel R. Queen, Nicholas Quirk, Kate Raach, Matthew T. Rakher, Matthew D. Reed, Brandon D. Reynolds, Luke D. Robertson, Zechariah Rogers, Yakov Royter, Matthew J. Ruiz, Golam Sabbir, Roshan Sajjad, Christopher D. Sanborn, Rachel H. Sarmiento, Christian J. Schnaible, Cole Scott, Nicholas M. Sebastiani, Eric M. Segall, Alen Senanian, Adalberto Sicairos, Shariq Siddiqui, Kartik Singh, Aaron Smith, Daniel E. Smith, Robert S. Smith, Sarah F. Sontag, Emilio A. Sovero, Kevin C. Staley, Andrea Su, June Suh, Bo Sun, Danny Sun, Christopher M. Swank, Noah Swimmer, Mariano J. Taboada, Bryan J. Thomas, Yessica Torres, Jeremy W. Touve, Alan Tran, Ivan Tran, Chantang Tsen, Skylar Turner, Miguel Valencia, Irma Valles, James R. van Meter, Nicholas D. VanRensselaer, Franklin Vartanian, Daniel Volya, Zachary J. Vrba, Phuong Hong Vu, Annette L. Wagner, John Wallner, Michael P. Walsh, Shuoqin Wang, Tong Wang, Daniel R. Ward, Aaron J. Weinstein, Terry B. Welch, Thomas V. Westrick, Evan T. White, Randall M. White, Samuel J. Whiteley, Gananath Wijeratne, Parker Williams, Jack T. Wilson, Courtney P. Wilt, Deborah E. Winklea, Onnik Yaglioglu, Daniel Yap, Clifford S. YoungSciortino, Daniel Zehnder, Andrew Ziegler

Quantum Physics

Preparation and detection of quasiparticles for quantum simulations of scattering

Mattia Morgavi, Peter Majcen, Marco Rigobello et al. (+2 more)

We introduce a method for the selective preparation and detection of quasiparticle wave packets, based on creation operators that generate dressed, localized excitations on top of interacting vacua of (quasi-)one-dimensional quantum lattice theories. This method exploits maximally localized Wannier functions (MLWFs) constructed from quasiparticle bands at intermediate system sizes, enabling the construction of unitary local dressed creation operators. The algorithm allows for species-resolved wave-packet preparation and detection, enabling the separation of known quasiparticle contributions from unknown resonances. We test this approach with matrix product states (MPS) on pure hardcore Hamiltonian QCD on a ladder lattice, detecting scattering outputs and mass resonances.
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All authors Mattia Morgavi, Peter Majcen, Marco Rigobello, Simone Montangero, Pietro Silvi

Quantum Physics

Towards Ultra-High-Rate Quantum Error Correction with Reconfigurable Atom Arrays

Chen Zhao, Casey Duckering, Andi Gu et al. (+2 more)

Quantum error correction is widely believed to be essential for large-scale quantum computation, but the required qubit overhead remains a central challenge. Quantum low-density parity-check codes can substantially reduce this overhead through high-rate encodings, yet finite-size instances with practical logical error rates often achieve encoding rates only around or below $1/10$. Here, building on a recent ultra-high-rate construction by Kasai, we identify new structural conditions on the underlying affine permutation matrices that make encoding rates exceeding $1/2$ compatible with efficient implementation on reconfigurable neutral atom arrays. These conditions define a co-designed family of ultra-high-rate quantum codes that supports efficient syndrome extraction and atom rearrangement under realistic parallel control constraints. Using a hierarchical decoder with high accuracy and good throughput, we study the performance under a circuit-level noise model with $p=0.1\%$, achieving per-logical-per-round error rates of $1.3_{-0.9}^{+3.0} \times 10^{-13}$ with a $[[2304,1156,\leq 14]]$ code and $2.9_{-1.5}^{+3.1} \times 10^{-11}$ with a $[[1152,580,\leq 12]]$ code. We compare these codes against a heuristic Pareto frontier for finite-blocklength codes relating block length, encoding rate, and logical error rates, and find that our codes lie near the frontier. These results approach the teraquop regime, highlighting the promise of this code family for practical ultra-high-rate quantum error correction.
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All authors Chen Zhao, Casey Duckering, Andi Gu, Nishad Maskara, Hengyun Zhou

Quantum Physics

Squeezing and measurement of a mechanical quadrature via PID feedback

Alberto Hijano and Tero T. Heikkilä

Proportional-Integral-Derivative (PID) control is used for automatically regulating a measurable quantity to a desired setpoint. It is widely used in different types of classical control electronics. Here, we show how extending the feedback theory in quantum systems to include the derivative and integral parts influences both the transient and steady-state behavior of the amplitude and squeezing of a mechanical quadrature in an optomechanical system. We show that, in contrast to standard proportional feedback, derivative feedback affects both the conditional and unconditional squeezing. Furthermore, we demonstrate how feedback may be employed to drive a mechanical quadrature to track a desired reference signal. Our findings offer new routes for an improved quantum state control and measurement precision.
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Quantum Physics

Strain-induced modification of spin-optical dynamics in silicon vacancy centers for integrated quantum technologies

Maximilian Hollendonner, Fedor Dzmitryevich Hrunski, Daniel Scheller et al. (+8 more)

Silicon vacancy (VSi) centers in 4H silicon carbide have emerged as a highly promising platform for semiconductor-based quantum technologies, combining excellent spin and optical properties with an industrial-grade, CMOS-compatible material. As these defects are increasingly integrated into practical quantum devices, they inevitably encounter lattice strain. However, while the impact of strain is well documented for other solid-state defects like NV centers in diamond, its specific influence on key VSi spin dynamics such as initialization fidelity and state lifetimes remain largely unexplored. In this work, we address this critical gap by designing fully optical pulse sequences and incorporating the effective spin-3/2 strain Hamiltonian into our analysis. This combined approach allows us to isolate both axial and transverse strain contributions and systematically characterize their effect on the metastable state transition rates. Specifically, we reveal that strain significantly reduces the transition rates from the energetically lowest metastable state to the ground state quartet, leading to decreased photon emission. Supported by first-principles calculations, our findings provide a deeper understanding of VSi spin-strain dynamics, yielding crucial insights for the robust deployment of these centers in realistic, strain-prone environments.
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All authors Maximilian Hollendonner, Fedor Dzmitryevich Hrunski, Daniel Scheller, Kim Ullerich, Shravan Kumar Parthasarathy, Wolfgang Knolle, Maximilian Schober, Mirjam Neubauer, Durga Bhaktavatsala Rao Dasari, Michel Bockstedte, Roland Nagy

Quantum Physics

Coherence dynamics in Simon's quantum algorithm

Linlin Ye, Zhaoqi Wu and Shao-Ming Fei

Quantum coherence plays a pivotal role in quantum algorithms. We study the coherence dynamics of the evolved states in Simon's quantum algorithm based on Tsallis relative $\alpha$ entropy and $l_{1,p}$ norm. We prove that the coherences of the first register and the second register both rely on the dimension $N$ of the state spaces of the $n$ qubit systems, and increase with the increase of $N$. We show that the oracle operator $O$ does not change the coherence. Moreover, we study the coherence dynamics in the Simon's quantum algorithm and prove that in overall the coherence is in production when $N>4$ and in depletion when $N<4$.
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Quantum Physics · Cross-listed from physics.flu-dyn

Quantum-Inspired Simulation of 2D Turbulent Rayleigh-Bénard Convection

Nis-Luca van Hülst, Mario Guillaume Cecile, Hai-Yen Van et al. (+3 more)

Turbulent thermal convection governs heat transport in systems ranging from stellar interiors to industrial heat exchangers. Two-dimensional Rayleigh-Bénard convection serves as a paradigm for these flows, reproducing key features such as thin boundary layers, large-scale circulation, and sustained plume dynamics. While Matrix Product State (MPS) methods have demonstrated significant compression of isothermal turbulent fields, their application to buoyancy-driven flows with active thermal coupling has remained unexplored. We apply MPS to two-dimensional Rayleigh-Bénard convection with dynamical simulations up to $\mathrm{Ra} = 10^{10}$. An a priori decomposition of DNS snapshots up to $\mathrm{Ra} = 10^{11}$ shows that the bond dimension $\chi$ required to represent the flow fields grows without saturation, in contrast to the plateauing of $\chi$ reported for velocity fields in isothermal 2D turbulence. Crucially, however, dynamical simulations solving the governing equations directly in the compressed MPS format at fixed $\chi$ show that the $\chi$ required to recover statistical observables, such as the Nusselt number, scales significantly more favorably with $\mathrm{Ra}$ than the a priori complexity suggests. At $\mathrm{Ra} = 10^{10}$, a relative error of $1.8\%$ in the mean Nusselt number is achieved with a nearly 9-fold reduction in degrees of freedom, using a $\chi$ comparable to that required at $\mathrm{Ra} = 10^{9}$. Spectral analysis confirms the progressive recovery of spatial and temporal scales with increasing $\chi$. These findings establish MPS as a scalable tool for simulating thermally driven turbulence, suggesting the method may remain viable for investigations of the ultimate regime at substantially higher $\mathrm{Ra}$.
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All authors Nis-Luca van Hülst, Mario Guillaume Cecile, Hai-Yen Van, Tomohiro Hashizume, Eugene de Villiers, Dieter Jaksch

Quantum Physics

All-photonic quantum key distribution beyond the single-repeater bound

Matthew S. Winnel, Sergio Juárez, Chithrabhanu Perumangatt et al. (+2 more)

Quantum protocols require classical signaling, and when classical signals propagate faster than quantum ones, standard rate-loss limits can be surpassed. We introduce an all-photonic measurement-device-independent quantum key distribution protocol that exceeds the single-repeater bound without error correction. When quantum signals travel at two-thirds the classical speed, the key rate scaling approaches $\eta^{2/5}$. We propose a single-rail, temporally multiplexed architecture that extends twin-field-type protocols to multiple nodes and surpasses their key rate without ideal quantum memories.
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All authors Matthew S. Winnel, Sergio Juárez, Chithrabhanu Perumangatt, Taofiq Paraiso, R. Mark Stevenson

Quantum Physics

Single-Satellite Quantum Repeater Performance Analysis

Cameron Paterson, Jasminder S. Sidhu, Thomas Brougham et al. (+2 more)

Satellite optical links may be able to extend quantum communication range beyond that which is possible through optical fibres which are constrained by exponential loss scaling. Quantum repeaters have been proposed to mitigate high channel losses in both fibre and satellite links. Although satellite quantum repeaters could improve entanglement distribution efficiency, the attempt rate is constrained by classical communication latency in the entanglement swapping step. Direct entanglement distribution does not suffer this latency and can therefore compensate for high dual-downlink losses by increasing the source rate. A systematic comparative analysis of repeater-assisted and direct entanglement distribution rates using a satellite is absent from the literature. In addition to entanglement distribution rates, a detailed analysis of the fidelity of entanglement distributed by repeater-assisted satellite links has yet to be performed. Here, we develop an in-depth framework for describing a space quantum link consisting of a single satellite and two ground stations. We compare the entanglement distribution rates of direct and repeater-assisted satellite entanglement distribution, taking into account variations in overpass geometry and quantum memory capacity. The long-term entanglement distribution performance to different pairs of major cities is also considered, and the altitudinal dependence is explored. Furthermore, we develop a Monte Carlo model of the repeater satellite to explore the effect that different memory management policies and capacities have on the waiting time statistics of memory qubits. From these statistics we calculate the fidelity distributions of the entangled pairs established between the ground stations. These results will inform mission design for future space-borne quantum repeater nodes, as well as requirements on space-based memory platforms.
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All authors Cameron Paterson, Jasminder S. Sidhu, Thomas Brougham, Sarah E. McCarthy, Daniel K. L. Oi

Quantum Physics

A unified framework for efficient quantum simulation of nonlinear spectroscopy

Long Xiong, Xiaoyang Wang, Xiaoxia Cai et al. (+1 more)

Nonlinear spectroscopy is a cornerstone of quantum science, providing unique access to multi-point correlations, quantum coherence, and couplings that are invisible to linear methods. However, classical simulation of these phenomena is fundamentally limited by the exponential growth of the Hilbert space, and practical quantum algorithms for the nonlinear regime have remained largely unexplored. Here, we present a unified quantum algorithmic framework for computing $n$-th order nonlinear spectroscopies. By reformulating multi-time responses as a weighted sum of expectation values at finite pump amplitudes via a generalized parameter shift rule, our approach bypasses the costly evaluation of high-order commutators and time-dependent operator expansions. This reformulation enables efficient execution via real-time evolution on current quantum hardware, ensuring inherent noise resilience. We validate the framework on IBM's superconducting quantum processors, successfully obtain higher-order response functions of a 12-qubit XXZ spin-chain. Furthermore, the versatility of our method is demonstrated by resolving quasi-particle excitation spectra in spin-liquids and identifying interaction-induced cross-peaks in atomic systems. Our results establish a practical and scalable pathway for probing complex quantum dynamics on near-term quantum devices, extending the reach of quantum simulation into the nonlinear domain.
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All authors Long Xiong, Xiaoyang Wang, Xiaoxia Cai, Xiao Yuan

Quantum Physics

Gravitationally induced wave-function collapse from dynamical bifurcation

C. A. S. Almeida

We propose an effective non-relativistic framework in which wave-function collapse emerges as a deterministic dynamical instability induced by gravitational self-interaction and regulated by short-distance repulsion. The dynamics is described by a nonlinear Schrödinger equation supplemented by a phenomenological repulsive sector ensuring regularity at high densities. Using a variational Gaussian ansatz, we derive an explicit effective energy functional and show that extended quantum states lose stability beyond a critical mass scale. This loss of stability is associated with a bifurcation in the reduced dynamical system governing the wave-function width, leading to the emergence of stable localized configurations. Within this picture, collapse corresponds to the dynamical selection of one of these localized attractors, driven by infinitesimal asymmetries in the initial state and occurring without stochastic noise or environmental coupling. The mechanism provides a controlled and quantitative realization of gravity-induced localization, extending Schrödinger–Newton-type models while avoiding their pathological short-distance behavior. Possible implications for mesoscopic systems probing the quantum-to-classical transition are briefly discussed.
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Quantum Physics

Characterizing all non-Hermitian degeneracies using algebraic approaches: Defectiveness and asymptotic behavior

Sharareh Sayyad and Grigory A. Starkov

The presence of degeneracies plays a crucial role in describing the behavior of non-Hermitian (NH) systems. In these systems, there are two key types of degeneracies: $n$-bolical degeneracies, which are analogous to Hermitian degeneracies, and various forms of exceptional points, each associated with different orders that correspond to sizes of the Jordan blocks. These types of degeneracies may coalesce at the same energy level, forming multi-block degeneracies. To understand how a multi-block degenerate NH system responds to perturbations, one should address how each types of involved degeneracies disperse. In this work, we systematically characterize the asymptotic behavior of all types of multi-block degeneracies in NH systems using a rigorous mathematical formulation. Through a range of examples, we demonstrate that our algebraic approach can facilitate the analysis of NH degeneracies in various settings relevant to experiments.
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Quantum Physics

Converting non-Hermitian degeneracies of any order: Hierarchies of exceptional points and degeneracy manifolds

Grigory A. Starkov and Sharareh Sayyad

The emergence of various types of degeneracies plays a crucial role in optimizing and engineering different physical phenomena in non-Hermitian physics. In our work, we focus on the derogatory Exceptional Points (EPs), which are characterized by multiple Jordan blocks corresponding to the same eigenvalue. We demonstrate that, under certain infinitesimal perturbations, a derogatory EP can be converted into an EP of different structure without varying the total order of degeneracy. In particular, such conversion can increase the size of the largest Jordan block and, hence, the sensitivity of the eigenspectrum to parameter variation, which is an important feature for practical applications. Furthermore, by analyzing all possible conversions, we introduce hierarchies of degeneracies of the same order that appear when perturbing non-Hermitian systems. We systematically explore hierarchies in the absence of any symmetry and when pseudo-Hermitian symmetry is present. Our study facilitates engineering various degeneracies of non-Hermitian systems, paving the way to extending the implications of non-Hermitian physics.
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Quantum Physics · Cross-listed from cond-mat.quant-gas

Observation of Strong-to-Weak Spontaneous Symmetry Breaking in a Dephased Fermi Gas

Si Wang, Thomas G. Kiely, Dorothee Tell et al. (+9 more)

Symmetry-based classification of quantum phases of matter is one of the most foundational organizing principles in physics; however, an analogous framework for mixed, decohered quantum states has only begun to emerge. A central new concept is strong-to-weak spontaneous symmetry breaking (SW-SSB), a sharp transition in mixed quantum states that is invisible to any observable linear in the density matrix and that has since been predicted across a broad class of open and monitored quantum systems. It also provides a unifying language for phenomena as disparate as the decodability of topological quantum memories and the emergence of classical hydrodynamics from decohered quantum dynamics. Here we report the first experimental observation of SW-SSB, in dephased single-component fermionic matter imaged by a quantum gas microscope. A quantum-classical estimator built on a machine-learned Gaussian reference state gives direct access to the nonlinear Rényi-1 and Rényi-2 correlators that diagnose SW-SSB, and reveals long-range Rényi order in the dephased Fermi liquid. Adding a commensurate superlattice drives the underlying fermions through a metal-to-insulator transition that, after full dephasing, manifests as a sharp SW-SSB phase transition. Our results uncover the symmetry principle behind information-theoretic transitions in open quantum systems, and extend Landau's symmetry paradigm into the regime of real, decohering quantum devices.
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All authors Si Wang, Thomas G. Kiely, Dorothee Tell, Johannes Obermeyer, Marnix Barendregt, Petar Bojović, Philipp M. Preiss, Abhijat Sarma, Titus Franz, Matthew P. A. Fisher, Cenke Xu, Immanuel Bloch

Quantum Physics

Quantum Noise Suppression Beyond the Standard Quantum Limit in a Hybrid Magnonic Optomechanical System

Alolika Roy and Amarendra K. Sarma

We theoretically study how quantum measurement noise can be engineered in a hybrid cavitymagnomechanical platform for precision force sensing. The proposed configuration consists of a driven optomechanical cavity, with a movable mirror on one side plus a fixed semi-transparent mirror on the other side, coupled to a magnon mode, with an OPA placed inside the cavity. We show that the magnon mediated dynamics reshapes the added-noise spectrum leading to improved sensitivity compared to a conventional optomechanical sensor. In particular, by satisfying the coherent quantum noise cancellation (CQNC) criterion, radiation-pressure back-action can be fully suppressed. In addition, a larger OPA pump gain permits operation beyond the standard quantum limit at substantially reduced laser power, thereby mitigating power-related constraints without sacrificing performance. These combined advantages provide a practical pathway to below-SQL weak force detection and can outperform existing approaches based on squeezing in magnomechanics.
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Quantum Physics

Entanglement and photoelectron holography in dissociative photoionization: molecular quantum eraser

Sebastian Hell, Paul Winter, Martin Gärttner et al. (+6 more)

In a double-slit experiment with a bipartite system, the visibility of interference fringes depends on the availability of which-way information. Here, we report a Bell-like state of photoelectron and residual ion in the multiphoton dissociative ionization of the D$_2$ molecule. Evidence for entanglement is provided by the correlated emission directions of photoelectron and ion, which is observed using a COLTRIMS reaction microscope. In the presence of this correlation, the holographic interference fringes contained in the photoelectron momentum distributions are suppressed, indicating the existence of which-way information. The interference pattern persists when a single ionic state is populated and thus no which-way information is available. The experimental observations and conclusions are supported by the numerical solution of the electronic-nuclear time-dependent Schrödinger equation. Our work demonstrates that coincidence spectroscopy of ions and electrons is a powerful method for studying fundamental concepts of quantum information science within the context of ultrafast light-matter interactions.
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All authors Sebastian Hell, Paul Winter, Martin Gärttner, Julian Späthe, Saurabh Mhatre, Dejan B. Milošević, Gerhard G. Paulus, Manfred Lein, Matthias Kübel

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

Exact Steady State of a One-end Driven XXZ Spin Chain with Boundary Field

V. Popkov and T. Prosen

We find an exact nonequilibrium steady state of an open dissipatively driven XXZ spin-1/2 chain with source or sink spin bath at one end and an arbitrary boundary field at the other end.