Quantum Physics · Cross-listed from physics.atom-ph
Fundamental Limits of Large Momentum Transfer in Optical Lattices
Ashkan Alibabaei, Patrik Mönkeberg, Florian Fitzek et al. (+5 more)
Large-momentum-transfer techniques are instrumental for the next generation of atom interferometers as they significantly improve their sensitivity. State-of-the-art implementations rely on elastic scattering processes from optical
lattices such as Bloch oscillations or sequential Bragg diffraction, but their performance is constrained by imperfect pulse efficiencies. Here we develop a Floquet-based theoretical framework that provides a unified description of elastic light-atom scattering across all relevant regimes. Within this formalism, we identify practical regimes that exhibit orders of magnitude reduced losses and improved phase accuracy compared to previous implementations. The model's validity is established through direct comparison with numerical solutions of the Schrödinger equation and through quantitative agreement with recent experimental
benchmark results. These findings delineate previously unexplored operating regimes for large momentum transfer beam splitters and open new perspectives for precision atom-interferometric measurements in fundamental physics,
gravity gradiometry or gravitational wave detection.
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All authors Ashkan Alibabaei, Patrik Mönkeberg, Florian Fitzek, Michael Werner, Alexandre Gauguet, Baptiste Allard, Klemens Hammerer, Naceur Gaaloul
Quantum Physics · Cross-listed from cs.LG
Quantum Generator Kernels
Philipp Altmann, Maximilian Mansky, Maximilian Zorn et al. (+2 more)
Quantum kernel methods offer significant theoretical benefits by rendering classically inseparable features separable in
quantum space. Yet, the practical application of
Quantum Machine Learning (QML), currently constrained by the limitations of Noisy Intermediate-Scale
Quantum (NISQ) hardware, necessitates effective strategies to compress and embed large-scale real-world data like images into the constrained capacities of existing
quantum devices or simulators. To this end, we propose
Quantum Generator Kernels (QGKs), a generator-based approach to
quantum kernels, comprising a set of Variational Generator Groups (VGGs) that merge universal generators into a parameterizable operator, ensuring scalable coverage of the available
quantum space. Thereby, we address shortcomings of current leading strategies employing hybrid architectures, which might prevent exploiting
quantum computing's full potential due to fixed intermediate
embedding processes. To optimize the kernel
alignment to the target domain, we train a weight vector to parameterize the projection of the VGGs in the current data context. Our empirical results demonstrate superior projection and classification capabilities of the QGK compared to state-of-the-art
quantum and classical kernel approaches and show its potential to serve as a versatile framework for various QML applications.
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All authors Philipp Altmann, Maximilian Mansky, Maximilian Zorn, Jonas Stein, Claudia Linnhoff-Popien
Quantum Physics · Cross-listed from hep-ph
Wave-like amplification of near-threshold two-particle reactions: from muon-catalyzed fusion to $\Lambda\bar{\Lambda}$ production at $e^-e^+$ annihilation
Vladimir S. Melezhik
A simple model is proposed to explain the recently found wave-like enhancement of the $\Lambda\bar{\Lambda}$ pair production near the threshold at the $e^-e^+$ annihilation, which allows extracting model-independent scattering parameters and spectral information for the $\Lambda\bar{\Lambda}$ pair from the oscillatory nature of the measured cross section. In particular, it predicts a single bound state of $\Lambda\bar{\Lambda}$ with a binding energy of $\varepsilon_{\Lambda\bar{\Lambda}}=(36\pm5)$MeV. The model is a generalization of the formulas obtained in our earlier work [1] to explain the effect of wave-like amplification found in it near the threshold of fusion reactions screened by a
muon or electron. The analysis allows us to conclude that the effect of wave-like amplification is an integral feature of any two-particle near-threshold reaction. In this regard, it seems promising to investigate, within the framework of our model, the oscillatory nature of the electromagnetic form factors of hyperons and nucleons extracted in experiments on $e^- e^+$ annihilation. A natural further development of the model could be its generalization to processes of producing various
hadron pairs in $e^-e^+$ annihilation.
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Quantum Physics
A room-temperature cavity-magnonic source of correlated microwave pairs
Qiuyuan Wang, Aravind Karthigeyan, Chung-Tao Chou et al. (+1 more)
Correlated microwave
photon sources are key enablers for technologies in
quantum-limited sensing, signal amplification and communication, but the reliance on millikelvin operating temperature limits their scalability for broader applications. Here, at room temperature, we demonstrate strong correlated microwave signals emitted from a hybrid
magnon-
photon platform. Different from traditional parametrically induced
magnons with degenerate frequencies, we achieve non-degenerate excitations by coupling
magnon modes simultaneously with two cavity
photon modes. Through the
magnon-
photon interactions in the corresponding linear and nonlinear regimes, one input microwave
photon splits into a pair of
magnon polaritons that possess distinct frequencies but maintain strong inter-mode correlations. The nonlinear
magnon polariton dynamics empowered by this new parametric platform brings both verified true randomness and robust multi-channel correlations, from which we construct a microwave communication experiment for noise resilient signal transmission with added security. This work establishes cavity magnonics as a versatile and compact platform for generating correlated multi-mode microwave signals, opening new avenues for applications in classical and
quantum domains.
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All authors Qiuyuan Wang, Aravind Karthigeyan, Chung-Tao Chou, Luqiao Liu
Quantum Physics · Cross-listed from hep-th
Kirill Russkov
The Dirac-Bergmann
algorithm for the
Hamiltonian analysis of constrained systems is a nice and powerful tool, widely used for quantization and non-perturbative counting of degrees of freedom. However, certain aspects of its application to systems with first-class constraints are often overlooked in the literature, which is unfortunate, as a naive treatment leads to incorrect results. In particular, when transitioning from the total to the extended
Hamiltonian, the physical information encoded in the constrained modes is lost unless a suitable redefinition of gauge invariant quantities is made. An example of this is electrodynamics, in which the electric field gets an additional contribution to its longitudinal component in the form of the gradient of an arbitrary Lagrange multiplier. Moreover, Dirac's
conjecture, the common claim that all first-class constraints are independent generators of gauge transformations, is somewhat misleading in the standard notion of gauge
symmetry used in field theories. At the level of the total
Hamiltonian, the true gauge generator is a specific combination of primary and secondary first-class constraints; in general, Dirac's
conjecture holds only in the case of the extended
Hamiltonian. The aim of the paper is primarily pedagogical. We review these issues, providing examples and general arguments. Also, we show that the aforementioned redefinition of gauge invariants within the extended
Hamiltonian approach is equivalent to a form of the Stueckelberg trick applied to variables that are second-class with respect to the primary constraints.
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Quantum Physics
From Feynman-Vernon to Wiener Stochastic Path Integral
Antonio Camurati, Felipe Sobrero, Bruno Suassuna et al. (+1 more)
We establish a direct connection between the Feynman-Vernon path integral formalism for open
quantum systems and the Wiener path integral used in classical
stochastic dynamics. By considering a generalized influence functional in the strong decoherence limit, we demonstrate that integrating over the
quantum coherence length leads to a derivation of
stochastic Langevin dynamics. Specifically, we show that the
quantum Feynman measure transforms into the
stochastic Wiener measure. Applying this framework to the Wigner function representation, we show that the system follows a
stochastic path interpretable via classical probability theory. Finally, we address the inverse problem: constructing an equivalent
quantum influence functional from a given classical Langevin equation.
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All authors Antonio Camurati, Felipe Sobrero, Bruno Suassuna, Pedro V. Paraguassú
Quantum Physics
Lower bounds on non-local computation from controllable correlation
Richard Cleve and Alex May
Understanding
entanglement cost in non-local
quantum computation (NLQC) is relevant to complexity, cryptography,
gravity, and other areas. This
entanglement cost is largely uncharacterized; previous lower bound techniques apply to narrowly defined cases, and proving lower bounds on most simple
unitaries has remained open. Here, we give two new lower bound techniques that can be evaluated for any
unitary, based on their controllable correlation and controllable
entanglement. For Haar random two
qubit unitaries, our techniques typically lead to non-trivial lower bounds. Further, we obtain lower bounds on most of the commonly studied two
qubit quantum gates, including CNOT, DCNOT, $\sqrt{\text{SWAP}}$, and the XX interaction, none of which previously had known lower bounds. For the CNOT gate, one of our techniques gives a tight lower bound, fully resolving its
entanglement cost. The resulting lower bounds have parallel repetition properties, and apply in the noisy setting.
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Quantum Physics
Quantum entanglement distribution coexisting with high-rate, broadband classical optical communications over a real-world fiber connecting remote, synchronized nodes
Gina M. Talcott, Ahnnika I. Hess, Laura d'Avossa et al. (+8 more)
Compatibility with existing classical network infrastructure offers a scalable path towards deploying large-scale
quantum networks. Here, we demonstrate O-band polarization-encoded
quantum entanglement distribution over an installed 24.4-km fiber while coexisting with a state-of-the-art fully-loaded C-band classical communications line system and a picosecond-level precision L-band synchronization signal. The classical system carries two 800-Gbps channels while the remainder of the C-band is filled with amplified spontaneous emission, as is standard for such state-of-the-art communications systems. We examine the spontaneous Raman scattering spectrum generated from this broadband C-band light and offer insights into wavelength allocation for O-band
quantum channels. Optimal wavelength selection and narrow filtering enable well-preserved Bell state fidelity when coexisting with 21.4-dBm aggregate launch power across the C-band suitable for 36-Tbps transmission. To the best of our knowledge, this is the first implementation of
entanglement-based
quantum communications between two remote nodes coexisting with independent classical communications traffic. We demonstrate coexistence of
quantum entanglement with ultra-high power levels and record classical bandwidth, offering promise for real-world
entanglement-based networking integrated within high-capacity communications infrastructure.
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All authors Gina M. Talcott, Ahnnika I. Hess, Laura d'Avossa, Scott J. Kohlert, Fei I. Yeh, Jim Hao Chen, Joe J. Mambretti, Tim M. Rambo, Gregory S. Kanter, Jordan M. Thomas, Prem Kumar
Quantum Physics
Fermionic magic resources in disordered quantum spin chains
Pedro R. Nicácio Falcão, Jakub Zakrzewski and Piotr Sierant
Fermionic non-Gaussianity quantifies a
quantum state's deviation from a classically tractable free-fermionic description, constituting a necessary resource for computational
quantum advantage. Here we use fermionic antiflatness (FAF) to measure this deviation across ergodic and many-body localized (MBL) regimes. We focus on the paradigmatic disordered
spin-$1\!/2$ XXZ chain and its
impurity variant with local interactions. Across highly excited eigenstates, FAF evolves from typical-state behavior at weak disorder to strongly suppressed values deep in the MBL regime, with volume-law scaling in the XXZ chain and an area-law bound in the
impurity setting. Rare long-range cat-like eigenstates exhibit a pronounced enhancement of FAF, making it a sensitive diagnostic of mechanisms proposed to destabilize MBL. Starting from product states, we find that in the MBL regime FAF grows slowly in time, approaching saturation via a power-law relaxation. Overall, our results show that MBL suppresses fermionic non-Gaussianity, and the associated complexity beyond free
fermions, while ergodicity restores it, motivating explorations of fermionic non-Gaussianity in other ergodicity-breaking phenomena.
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Quantum Physics
Complexity of Quantum Trajectories
Luca Lumia, Emanuele Tirrito, Mario Collura et al. (+2 more)
Open
quantum systems can be described by unraveling Lindblad master equations into ensembles of
quantum trajectories. Here we investigate how the complexity of such trajectories is affected by conservation laws and other dynamical constraints of the underlying Lindblad
evolution. We characterize this complexity using a data-driven approach based on the intrinsic dimension, defined as the minimal number of variables required to encode the information contained in a data set. Applying this framework to several systems, including dissipative variants of the
quantum top and of the XXZ chain, we find that the intrinsic dimension is sensitive to the structure of their dynamics. The Lindblad
evolution in these systems is typically chaotic; in particular, we report new signatures of autonomous chaos in the
quantum top. At specific parameter values, however, additional constraints arise: the dynamics becomes integrable, exhibits
Hilbert-space fragmentation, or develops a closed BBGKY hierarchy, leading to pronounced minima in the intrinsic dimension. Our approach results in an unsupervised probe of the complexity of dissipative
quantum systems that is sensitive to chaos and ergodicity breaking phenomena beyond the initial transient regime.
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All authors Luca Lumia, Emanuele Tirrito, Mario Collura, Fabian H.L. Essler, Rosario Fazio
Quantum Physics · Cross-listed from cond-mat.stat-mech
Topological Defects from Quantum Reset Dynamics
R. Jafari, Henrik Johannesson and Sebastian Eggert
We analyze mechanisms for universal out-of-
equilibrium dynamics near criticality by exploring the effect of randomized
quantum resetting (QR) under a finite-time quench across a
quantum phase transition. Using the transverse-field Ising chain as a generic model and exploiting its exact solution, QR is found to cause a crossover of the scaling of the topological defect density with the time scale $\tau$ of the quench, from Kibble-Zurek to anti-Kibble-Zurek scaling as $\tau$ increases. This reflects a competition between non-adiabatic quench-driven excitations and QR, giving rise to local minima of the defect densities at optimal annealing times. These times and the corresponding local minima are shown to scale as universal power laws with the rate of QR. Additional results for the scaling of the mean excess energy suggest that a system driven across a
quantum critical point exhibits the same scaling behavior under a linear quench with QR as with uncorrelated noise.
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Quantum Physics
Signatures of coherent initial ensembles on all work moments
Pranay Nayak, Sreenath K. Manikandan, Tan Van Vu et al. (+1 more)
Standard treatments of
quantum work using projective energy measurements erase initial coherence and alter the dynamics, thereby failing to capture the thermodynamic effects of coherent superpositions of energy eigenstates in an ensemble of initial states. In this article, we use an operational work definition that is non-intrusive, applying it to the case of a driven dissipative
qubit, where the
qubit's initial preparation comprises coherent superposition states, while the driving is coherence-less. We derive an
evolution equation for the moment generating function for this work, faithfully capturing the thermodynamic signature of coherent superpositions in the initial ensemble. We demonstrate that different initial ensembles that correspond to the same density
matrix upon ensemble average, while having the same average work, display different work fluctuations. For monotonic driving, we show that fluctuations are maximum for coherence-less initial ensembles. As an application, we consider
quantum bit-erasure in finite time and demonstrate significantly different work statistics for erasing a classical bit of information versus a Haar random initial ensemble. Our results indicate that coherence in the initial ensemble can be utilized as a resource for thermodynamic precision without incurring additional dissipative work costs. We also obtain a generalized fluctuation
theorem that establishes a new
quantum lower bound on the mean dissipated work. This bound, counterintuitively, is also applicable to a "classical" initial ensemble with the same initial density
matrix and is connected to
quantum absolute irreversibility.
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All authors Pranay Nayak, Sreenath K. Manikandan, Tan Van Vu, Supriya Krishnamurthy
Quantum Physics
Robust multiparameter estimation using quantum scrambling
Wenjie Gong, Bingtian Ye, Daniel Mark et al. (+1 more)
We propose and analyze a versatile and efficient multiparameter
quantum sensing protocol, which simultaneously estimates many non-commuting and time-dependent signals that are coherently or incoherently coupled to sensing particles. Even in the presence of control imperfections and readout errors, our approach can detect exponentially many parameters in the system size while maintaining the optimal scaling of sensitivity. To accomplish this, scrambling dynamics are leveraged to map distinct signals to unique patterns of bitstring measurements, which distinguishes a large number of signals without significant sensitivity loss. Based on this principle, we develop a computationally efficient protocol utilizing random global Clifford
unitaries and evaluate its performance both analytically and numerically. Our protocol naturally extends to scrambling dynamics generated by random local Clifford
circuits, local random
unitary circuits (RUCs), and ergodic
Hamiltonian evolution–commonly realized in near-term
quantum hardware–and opens the door to applications ranging from precise noise benchmarking of
quantum dynamics to learning time-dependent
Hamiltonians.
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All authors Wenjie Gong, Bingtian Ye, Daniel Mark, Soonwon Choi
Quantum Physics
Understanding multiscale disorder in superconducting nanowire single photon detectors
Nirjhar Sarkar, Ronan Gourgues, Yueh-Chun Wu et al. (+7 more)
Superconducting nanowire single-
photon detectors are central to applications across
quantum information science. Yet, their performance is limited by the effects of disorder and electrodynamic inhomogeneities that are not well understood. By combining DC transport, dark-count measurements, and bias-dependent microwave transmission spectroscopy in the presence of controlled nanoscale disorder introduced through helium-ion irradiation, we distinguish local instability-driven processes from intrinsic superconducting depairing and kinetic inductance nonlinearities. This approach enables systematic tuning of kinetic inductance, depairing currents, microwave dissipation, and mode structure within a single device. Bias- and temperature-dependent resonance shifts quantify disorder-induced modifications of the superconducting density of states through the nonlinear kinetic inductance, while the emergence of multiple resonant modes reveals the formation of electrodynamically distinct superconducting regions. Comparing depairing under current, field, and temperature isolates the dominant microwave loss mechanisms, separating
vortex, quasiparticle, and two-level-system contributions, thus providing a robust multifunctional foundation for disorder engineering of superconducting nanowire detectors and resonators.
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All authors Nirjhar Sarkar, Ronan Gourgues, Yueh-Chun Wu, Chengyun Hua, Katyayani Seal, Andreas Fognini, Steven Randolph, Eugene Dumitrescu, Gabor B. Halasz, Benjamin Lawrie
Quantum Physics · Cross-listed from cond-mat.mes-hall
Analytical topological invariants for 2D non-Hermitian phases using Morse theory
Cameron Gibson and Evelyn Tang
As energy dissipation and gain are ubiquitous in the real world, such phenomena demand the generalization of Hermitian methods such as the analysis of topological properties for non-Hermitian systems. However, as non-Hermitian systems typically contain more degrees of freedom, this poses a challenge for analytical approaches to understand their
topology and invariants. In this work, we analytically calculate the 2D Zak phase for a 2D non-Hermitian SSH-type
Hamiltonian that supports a rich structure and edge currents. Closed-form expressions for eigenstates and divisions of the phase diagram are obtained, including for regions in the phase diagram where different types of exceptional points exist. We use Morse theory to determine the
topology of exceptional points in momentum space. Although the band structure breaks down at exceptional points, we show that a specific phase-based topological invariant remains well-defined. Furthermore, our work yields an analytic derivation for counting edge states in the Hermitian limit. These results provide new conceptual and analytical tools for the study of complex topological systems.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Weight-four parity checks with silicon spin qubits
Brennan Undseth, Nicola Meggiato, Yi-Hsien Wu et al. (+7 more)
Recent advances in coherent
spin shuttling have made sparse
semiconductor spin qubit arrays an appealing solid-state platform to realize
quantum processors. The dynamic and long-range connectivity enabled by shuttling is also essential for many
quantum error-correction (QEC) schemes. Here, we demonstrate a silicon
spin-
qubit device that comprises a shuttling bus for coherently transporting
qubits that can interact at four isolated locations we call bus stops. We dynamically populate the array and tune all single- and two-
qubit operations using shuttling and
quantum non-demolition (QND)
spin measurements, without access to charge sensing in most of the device. We achieve universal control of the effective five-
qubit processor and select the connectivity required to form a surface-code stabilizer plaquette that supports X- and Z-type parity checks up to weight-four. We use the parity checks to generate multi-
qubit entanglement between all
qubit combinations in the array and report the genuine
entanglement of a five-
qubit Greenberger-Horne-Zeilinger (GHZ) state, constituting the largest such state ever constructed with gate-defined
semiconductor spins. This work opens immediate opportunities to pursue QEC experiments with
spin qubits, and the protocols developed here lay the groundwork for the modular calibration and operation of sparse
spin qubit arrays.
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All authors Brennan Undseth, Nicola Meggiato, Yi-Hsien Wu, Sam R. Katiraee-Far, Larysa Tryputen, Sander L. de Snoo, Davide Degli Esposti, Giordano Scappucci, Eliška Greplová, Lieven M. K. Vandersypen
Quantum Physics
High-gain effects in broadband continuous-wave parametric down conversion sources and measurements with undetected photons
Martin Houde, Franz Roeder, Christine Silberhorn et al. (+2 more)
We study theoretically how high-gain effects affect the measurement outcome of visible signal spectra in undetected
photon measurement schemes. We consider two interferometric configurations: firstly, the SU(1,1) interferometer where the idler incurs loss and additional dispersion in between two identical, lossless, squeezers; secondly, the induced coherence interferometer where the idler incurs loss and additional dispersion in between two identical, lossless, squeezers and where the second squeezer is seeded by the idler and a
vacuum ancilla mode. Furthermore, we consider a distributed loss configuration where the idler incurs loss as it propagates in the nonlinear medium. Motivated by experimental evidence and due to the fact that broadband sources are ideal for these measurement schemes, we use the dispersive data of a third-order dispersion engineered integrated waveguide parametric down conversion (PDC) source presented in New Journal of Physics 26, 123025 (2024) to model the PDC spectra in the three configurations. For each configuration we consider the case of idler-only (i) absorption, (ii) additional dispersion, and (iii) the combined effects. We obtain results which outline the strength and weaknesses of the different configurations at different operation points.
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All authors Martin Houde, Franz Roeder, Christine Silberhorn, Benjamin Brecht, Nicolás Quesada
Quantum Physics
Lisa T. Weinbrenner, Albert Rico, Kenneth Goodenough et al. (+2 more)
In
quantum physics, multiparticle systems are described by
quantum states acting on tensor products of
Hilbert spaces. This product structure leads to the distinction between product states and entangled states; moreover, one can quantify
entanglement by considering the distance of a
quantum state to the set of product states. The underlying optimization problem occurs frequently in physics and beyond, for instance in the computation of the injective tensor norm in multilinear
algebra. Here, we introduce a method to determine the maximal overlap of a pure multiparticle
quantum state with product states based on considering several copies of the pure state. This leads to three types of hierarchical approximations to the problem, all of which we prove to converge to the actual value. Besides allowing for the computation of the
geometric measure of
entanglement, our results can be used to tackle optimizations over
stochastic local transformations, to find
entanglement witnesses for weakly entangled bipartite states, and to design strong separability tests for mixed multiparticle states. Finally, our approach sheds light on the complexity of separability tests.
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All authors Lisa T. Weinbrenner, Albert Rico, Kenneth Goodenough, Xiao-Dong Yu, Otfried Gühne
Quantum Physics · Cross-listed from cond-mat.stat-mech
Riccardo Travaglino, Federico Rottoli and Pasquale Calabrese
We investigate the dynamics of
Entanglement Hamiltonians (EHs) in dissipative free-fermionic systems using a recent operator-based formulation of the quasiparticle picture. Focusing on gain and loss dissipation, we study the post-quench
evolution and derive explicit expressions for the EH at the ballistic scale. In the long-time and weak-dissipation regime, the EH is shown to take the form of a time-dependent Generalized Gibbs Ensemble (t-GGE), with a structure that is universal across different initial states of the quench protocol. Within this framework, the emergence of the t-GGE is fully accounted for by the quasiparticle picture, and we argue that this description remains valid whenever the Lindbladian admits an appropriate coarse-grained representation.
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Quantum Physics
Compact U(1) Lattice Gauge Theory in Superconducting Circuits with Infinite-Dimensional Local Hilbert Spaces
J. M. Alcaine-Cuervo, S. Pradhan, E. Rico et al. (+2 more)
We propose a superconducting-
circuit architecture that realizes a compact U(1)
lattice gauge theory using the intrinsic infinite-dimensional
Hilbert space of phase and charge variables. The gauge and matter fields are encoded directly in the degrees of freedom of the rotor variables associated with the
circuit nodes, and Gauss's law emerges exactly from the conservation of local charge, without auxiliary stabilizers, penalty terms, or
Hilbert-space truncation. A minimal gauge-matter coupling arises microscopically from Josephson nonlinearities, whereas the
magnetic plaquette interaction is generated perturbatively via virtual matter excitations. Numerical diagonalization confirms the emergence of compact electrodynamics and coherent
vortex excitations, underscoring the need for large local
Hilbert spaces in the continuum regime. The required
circuit parameters are within the current experimental capabilities. Our results establish superconducting
circuits as a scalable, continuous-variable platform for analog
quantum simulation of non-perturbative gauge dynamics.
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All authors J. M. Alcaine-Cuervo, S. Pradhan, E. Rico, Z. Shi, C. M. Wilson
Quantum Physics · Cross-listed from physics.optics
Spontaneous four-wave mixing in a thin layer with second-order nonlinearity
Changjin Son and Maria Chekhova
Pairs of entangled
photons are crucial for photonic
quantum technologies. The demand for integrability and multi-functionality suggests 'flat' platforms - ultrathin layers and metasurfaces - as sources of
photon pairs. With the success in demonstrating spontaneous parametric down-conversion (SPDC) from such sources, an alternative process to generate
photon pairs, spontaneous four-wave mixing (SFWM), also starts to attract interest. In materials with nonzero second-order nonlinear susceptibility $\chi^{(2)}$, SFWM can generate
photon pairs both directly, through the third-order nonlinear susceptibility $\chi^{(3)}$, and in a cascaded way, through second harmonic generation (SHG) followed by SPDC. Usually, the cascaded process is more efficient. Here, we show that in a thin layer, direct SFWM dominates, because the wavevector mismatch for SFWM is much smaller than for SHG or SPDC. To demonstrate it, we implement the
photon pair generation via SFWM in a second-order nonlinear material - a thin layer of lithium niobate (LN). The existence of both second- and third-order nonlinear processes offers broader opportunities for
quantum state engineering.
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Quantum Physics
Free encoding capacity: A universal unit for quantum resources
Shampa Mondal, Soumajit Das, Preeti Parashar et al. (+1 more)
A perfect d-dimensional
quantum channel can convey log d-bits of classical information by encoding messages in d-orthogonal
quantum states. Alternatively, for every
quantum state at the senders end, there exist d-encoding operations which produce d-orthogonal
quantum states. Transmitting which via a d-level perfect
quantum channel it is possible to communicate log d-bits of classical information. But what if the set of encoding operations is restricted only within a physically constrained class? Here, we consider such a class of encoding operations to be the set of free operations for any
quantum resource theory and show that the constrained capacity - namely, the free encoding capacity (FEC) emerged as a unit of the corresponding
quantum resource. Moreover, we show that for the pointed resource theories - a resource theory admitting only a single free state - FEC becomes a faithful resource measure also. We also discuss the implications of FEC in the question of resource-theoretic state transformations and the possibility of extending its faithfulness for general
quantum resource theories.
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All authors Shampa Mondal, Soumajit Das, Preeti Parashar, Tamal Guha
Quantum Physics
TopoLS: Lattice Surgery Compilation via Topological Program Transformations
Junyu Zhou, Yuhao Liu, Ethan Decker et al. (+5 more)
Lattice surgery is a leading approach for implementing fault-tolerant logical operations in surface code
quantum computing, but compiling efficient
lattice surgery layouts remains challenging. Existing compilers are largely
circuit-centric and operate directly on gate sequences, limiting their ability to exploit the topological flexibility of merge-split operations and minimize space–time volume. We present TopoLS, a
topology-centric compiler that uses ZX diagrams as an intermediate representation for
lattice surgery compilation. TopoLS combines semantic-preserving ZX-level program transformations, including spider fusion and
topology-aware slicing, with a Monte Carlo Tree Search (MCTS)-based synthesis procedure that constructs pipe-diagram
embeddings by jointly optimizing placement and routing in 3D space–time. To scale to large
circuits, TopoLS further introduces
topology-aware partitioning that decomposes the compilation task into bounded subproblems and limits the routing frontier during
embedding. Across evaluated
benchmarks, TopoLS achieves an average $46\%$ reduction in space–time volume over prior
circuit-centric compilers, with improvements ranging from $25\%$ to $90\%$, and exhibits strong empirical scalability on large
benchmark families. Compared with SAT-based formulations that become intractable on larger instances, TopoLS offers a practical end-to-end solution for optimized
lattice surgery compilation. TopoLS has been integrated into the TQEC ecosystem, enabling downstream
circuit-level simulation and resource estimation workflows.
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All authors Junyu Zhou, Yuhao Liu, Ethan Decker, Justin Kalloor, Mathias Weiden, Kean Chen, Costin Iancu, Gushu Li
Quantum Physics
The Cross-Kernel Margin: A Robustness Measure for Quantum Kernel Methods
Saarisha Govender and Ilya Sinayskiy
Quantum devices in the current Noisy Intermediate-Scale
Quantum (NISQ) era are inherently affected by noise, which can degrade the predictive performance of
quantum machine learning models. In this work, we introduce a new margin-based robustness measure for
Quantum Kernel-Assisted Support Vector Machines (QSVMs), termed the cross-kernel margin. This measure quantifies the stability of a classifier learned under a perturbed kernel relative to the ideal feature space. We derive a posteriori stability bounds for the corresponding cross-kernel inverse squared-margin under kernel perturbations using the Tikhonov-stabilised SVM dual formulation. The local depolarising noise model is then applied to this framework to induce perturbations in the kernel. The resulting bounds are numerically checked using simulations across multiple
datasets and further tested using kernel
matrices obtained from real
quantum hardware and a noisy backend simulator. Furthermore, we empirically compare the degradation of test accuracy under local depolarising noise with the commonly used global depolarising noise model in order to motivate its use in our study. Finally, we present empirical results linking margin-based quantities with the generalisation performance of QSVMs, providing additional motivation for our margin-based robustness analysis.
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Quantum Physics
Scalable Memory Sharing in Photonic Quantum Memristors for Reservoir Computing
Chaehyeon Lim, Hyungchul Park, Beomjoon Chae et al. (+4 more)
Although
photons are robust, room-temperature carriers well suited to
quantum machine learning, the absence of
photon-
photon interactions hinder the realization of
memory functionalities that are critical for capturing long-range context. Recently, measurement-based implementations of photonic
quantum memristors (PQMRs) have enabled tunable non-Markovian responses. However, their
memory remains confined to local elements, in contrast to biological or artificial networks where
memory is shared across the system. Here, we propose a scalable PQMR network that enables measurement-based
memory sharing. Each memristive node updates its internal state using the history of its own and neighbouring
quantum states, thereby realizing distributed
memory. By modelling each node as a photonic
quantum memtransistor, we demonstrate pronounced enhancements in both classical and
quantum hysteresis at the device level, as well as enhanced network-level
quantum hysteresis. Implemented as a
quantum reservoir, the architecture achieves improved Fashion-MNIST classification accuracy and confidence via increased data separability. Our approach paves the way toward high-capacity
quantum machine learning using memristive devices compatible with linear-optical
quantum computing.
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All authors Chaehyeon Lim, Hyungchul Park, Beomjoon Chae, Jeonghun Kwak, Soo-Yeon Lee, Namkyoo Park, Sunkyu Yu
Quantum Physics
Dicke superposition probes for noise-resilient Heisenberg and super-Heisenberg Metrology
Sudha, B.N.Karthik, K.S.Akhilesh et al. (+1 more)
Phase sensing with entangled multi-
qubit states in the presence of noise is a central theme of modern
quantum metrology. The present work investigates Dicke state superposition probes for
quantum phase sensing under parameter encoding generated by one- and two-body interaction
Hamiltonians. Under linear collective-
spin encoding, near-optimal Dicke superposition states are shown to exhibit significantly enhanced robustness against phase damping noise compared with Greenberger–Horne–Zeilinger (GHZ), W-superposition, and balanced Dicke states, while maintaining favorable metrological performance under realistic decoherence channels. For two-body interactions, optimal probe states maximizing the
quantum Fisher information are identified. Their noise resilience and metrological scaling behaviour under phase damping, amplitude damping, and global depolarizing channels are analyzed. The associated near-optimal Dicke superposition states are found to exhibit improved resilience to phase damping, for the system sizes considered. These results establish tailored near-optimal Dicke state superposition probes as versatile and noise-resilient resources for Heisenberg and super-Heisenberg
quantum phase sensing governed by one- and two-body interactions.
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All authors Sudha, B.N.Karthik, K.S.Akhilesh, A.R. Usha Devi
Quantum Physics
High-resolution tunable frequency beamsplitter enabled by an integrated silicon pulse shaper
Chen-You Su, Kaiyi Wu, Lucas M. Cohen et al. (+6 more)
We demonstrate high-fidelity, tunable, and ultrafine-resolution on-chip frequency beamsplitters using a
quantum frequency processor based on an integrated pulse shaper with six spectral channels. Near-ideal Hadamard gate performance is achieved, with fidelity F > 0.9995 and modified success probability P > 0.9621 maintained across frequency spacings from 2-5 GHz and down to as few as four spectral pulse shaper channels. The system's support of frequency spacings as narrow as 2 GHz significantly surpasses prior bulk demonstrations and enables arbitrary splitting ratios via spectral phase or modulation index control. These results establish a scalable and resource-efficient platform for integrated frequency-bin
quantum photonics, opening new directions in
quantum information processing, including densely parallel single-
qubit operations and multidimensional gate implementations.
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All authors Chen-You Su, Kaiyi Wu, Lucas M. Cohen, Saleha Fatema, Navin B. Lingaraju, Hsuan-Hao Lu, Andrew M. Weiner, Joseph M. Lukens, Jason D. McKinney
Quantum Physics · Cross-listed from cond-mat.mes-hall
Quasiperiodic Skin Criticality in an Exactly Solvable Non-Hermitian Quasicrystal
Zhangyuan Chen, Muhammad Idrees, Ying Yang et al. (+2 more)
Critical states in quasiperiodic systems defy the conventional dichotomy between extended and localized states. In this work, we demonstrate that non-Hermiticity fundamentally reshapes this paradigm by giving rise to an exactly solvable quasiperiodic critical phase with no energy selectivity. We introduce a non-Hermitian quasiperiodic
lattice based on a modulated Hatano-Nelson model and uncover a new universality class of quasiperiodic skin criticality, in which all eigenstates share an identical multifractal spatial structure. Through a nonunitary gauge transformation, the system is mapped onto a disorder-free
lattice, enabling exact analytical solutions for the full spectrum and eigenstates. As a consequence, the inverse participation ratio is strictly energy-independent and controlled solely by a global phase. We further show that this criticality persists in multiband
lattices, establishing a general and analytically controlled framework for non-Hermitian quasiperiodic critical phenomena.
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All authors Zhangyuan Chen, Muhammad Idrees, Ying Yang, Xianqi Tong, Xiaosen Yang
Quantum Physics · Cross-listed from physics.atom-ph
Direct observation of the optical Magnus effect with a trapped ion
Philip Leindecker, Louis P.H. Gallagher, Edgar Brucke et al. (+7 more)
We directly observe and spatially map an optical analog of the Magnus effect, where intrinsic
spin-orbit-like coupling of light generates a
spin-dependent transverse displacement of the atom-light interaction profile for a $^{40}$Ca$^+$ ion. Probed on a quadrupole transition using a tightly focused beam, we observe displacements of the maximum in the profile of the effective interaction by several 100 nm originating from intrinsic longitudinal electric field components beyond the paraxial approximation. The tight focus of the beam induces additional transverse polarization gradients, which we characterize through a phase-sensitive measurement and spatial maps for different beam configurations. The results establish the physical basis of polarization-gradient interactions relevant to optical tweezer-based
quantum control.
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All authors Philip Leindecker, Louis P.H. Gallagher, Edgar Brucke, Dominique Zehnder, Luka Milanovic, Matteo Marinelli, Rene Gerritsma, Robert J.C. Spreeuw, Jonathan Home, Cornelius Hempel
Quantum Physics · Cross-listed from hep-th
A novel Hamiltonian formulation of $1+1$ dimensional $\phi^4$ theory in Daubechies wavelet basis: momentum space analysis
Mrinmoy Basak
We employ the wavelet formalism of
quantum field theory to study field theories in the nonperturbative
Hamiltonian framework. Specifically, we make use of Daubechies wavelets in momentum space. These basis elements are characterised by a resolution and a translation index that provides for a natural nonperturbative infrared and ultraviolet truncation of the
quantum field theory. As an application, we consider the $\phi^4$ theory and demonstrate the emergence of the well-known nonperturbative strong-coupling phase transition in the $m^2>0$ sector.
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