Peer Revenue Play credits. Real papers.

Papers

Fresh from arXiv, typeset properly. Highlighted words trade on the market.

Physics Quantum Physics

quant-ph Watch Quantum Physics

Friday 17 October

Quantum Physics

Environment-imposed selection rules for nuclear-spin conversion of H$_2$ in molecular crystals

Nathan Mclane, LeAnh Duckett and Leah G. Dodson

Nuclear-spin conversion in molecular hydrogen is governed by strict symmetry rules that typically require magnetic fields or catalytic surfaces to break. Here we demonstrate that the intrinsic tensor composition of a non-magnetic molecular crystal field can impose and relax these rules without external fields. High-resolution infrared spectra of H$_2$ in crystalline CO$_2$ reveal large rank-2 (quadrupolar) crystal-field splittings of the $m$ sublevels, while nuclear-spin conversion occurs only through $\Delta m = 0$ channels. Replacing CO$_2$ with polar N$_2$O introduces rank-1 (dipole) components that partially open $\Delta m \neq 0$ pathways, while incorporation of paramagnetic NO$_2$ fully lifts the restriction. These results establish a direct correspondence between crystal-field tensor rank and nuclear-spin dynamics, introducing a general symmetry-based framework for designing and controlling spin-isomer populations and quantum-state connectivity in molecular solids.
Read the full abstract

Quantum Physics

Efficient Quantum State Preparation with Bucket Brigade QRAM

Alessandro Berti and Francesco Ghisoni

The preparation of data in quantum states is a critical component in the design of quantum algorithms. The cost of this step can significantly limit the realization of quantum advantage in domains such as machine learning, finance, and chemistry. One of the main approaches to achieve efficient state preparation is through the use of Quantum Random Access Memory (QRAM), a theoretical device for coherent data access with several proposed hardware implementations. In this work, we present a framework that integrates the hardware model of the Bucket Brigade QRAM (BBQRAM) with the classical data structure of the Segment Tree to achieve efficient state preparation. We introduce a memory layout that embeds a segment tree within BBQRAM memory cells by preserving the segment tree's hierarchy and supporting data retrieval in logarithmic time via specialized access primitives. We demonstrate that our method encodes a matrix $A \in \mathbb{R}^{M \times N}$ in a quantum register of $\Theta(\log_2(MN))$ qubits in $\mathcal{O}(\log_2^2(MN))$ time, {requiring a constant number of working qubits (under fixed precision) and $\mathcal{O}(MN)$ memory cells within the BBQRAM architecture.} We further illustrate the method through a numerical example. This framework provides theoretical support for quantum algorithms that assume negligible data loading overhead and establishes a foundation for designing classical-to-quantum encoding algorithms that are aware of the underlying hardware QRAM architecture.
Read the full abstract

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

Unravelling inter-channel quantum interference in below-threshold nonsequential double ionization with statistical measures

S. Hashim and C. Figueira de Morisson Faria

We present a systematic study of interchannel quantum interference in laser-induced nonsequential double ionization (NSDI) within the strong-field approximation. Focusing on the below-threshold intensity regime where the recollision-excitation with subsequent ionization (RESI) pathway dominates, we derive analytical phase conditions governing interference between distinct excitation channels for arbitrary driving fields. To quantify the interplay between channels resulting from a vast number of interfering processes, we introduce statistical metrics based on the Earth Mover's Distance, allowing us to assess the relative weight of each channel's contribution to the two-electron photoelectron momentum distributions (PMDs). We identify key factors that determine whether interchannel interference is appreciable such as comparable channel intensities, strong spatial overlap between the excited-state wavefunctions and the energy difference between contributing channels. We demonstrate that for linearly polarized few-cycle pulses, the typical intrachannel interference features associated with exchange, temporal shifts and combined exchange-temporal interference are retained with interchannel interference. Our findings establish a hierarchy of interference mechanisms in RESI and may provide practical guidelines for enhancing or suppressing interference in different regions of the momentum plane. The toolkit presented in this work is transferable to a wide range of interferometric schemes involving different excitation channels.
Read the full abstract

Quantum Physics

Efficient state estimation on quantum processors

Victor Gonzalez Avella, Abraham Vega Vargas, Tomas Merlo Vergara et al. (+6 more)

We present two scalable and entanglement-free methods for estimating the collective state of an n-qubit quantum computer. The first method consists of a fixed set of five quantum circuits-regardless of the number of qubits-that avoid the use of entanglement as a measurement resource, relying instead on classical communication between selected pairs of qubits. The second method requires only 2n+1 circuits, each of which applies a single local gate to one of the n qubits during the measurement stage. Unlike traditional estimation methods, our approaches do not require any costly post-processing procedure to estimate a quantum state, enabling scalability to relatively large system sizes. We experimentally compare both methods on freely available IBM quantum processors, and observe how the state estimation varies with increasing number of qubits and shots. We further validated our results by estimating the 4-qubit entangled state of two remote ion-trap quantum processors, demonstrating that the optimized 2n+1 tomographic scheme achieves estimates consistent with standard methods while using exponentially fewer measurements.
Read the full abstract

All authors Victor Gonzalez Avella, Abraham Vega Vargas, Tomas Merlo Vergara, Kevin de la Ossa Doria, Jakub Czartowski, Dougal Main, Gabriel Araneda, Aldo Delgado, Dardo Goyeneche

Quantum Physics

Out-of-Equilibrium Dynamics in a U(1) Lattice Gauge Theory via Local Information Flows: Scattering and String Breaking

Claudia Artiaco, João Barata and Enrique Rico

We introduce local information flows as a diagnostic tool for characterizing out-of-equilibrium quantum dynamics in lattice gauge theories. We employ the information lattice framework, a local decomposition of total information into spatial- and scale-resolved contributions, to characterize the propagation and buildup of quantum correlations in real-time processes. Focusing on the Schwinger model, a canonical $(1+1)$-dimensional U(1) lattice gauge theory, we apply this framework to two scenarios. First, in the near-threshold scattering of two vector mesons, we demonstrate that the emergence of correlations at a longer length scale in the information lattice marks the production of heavier scalar mesons. Second, in the dynamics of electric field strings, we clearly distinguish between the confining regime, which evolves towards a steady state with a static correlation profile, and the string-breaking sector. The latter is characterized by dynamic correlation patterns that reflect the sequential formation and annihilation of strings. This information-centric approach provides a direct, quantitative, and interpretable visualization of complex many-body phenomena, offering a promising tool for analyzing dynamics in higher-dimensional gauge theories and experiments on quantum hardware.
Read the full abstract

Quantum Physics · Cross-listed from cs.LG

Near-Equilibrium Propagation training in nonlinear wave systems

Karol Sajnok and Michał Matuszewski

Backpropagation learning algorithm, the workhorse of modern artificial intelligence, is notoriously difficult to implement in physical neural networks. Equilibrium Propagation (EP) is an alternative with comparable efficiency and strong potential for in-situ training. We extend EP learning to both discrete and continuous complex-valued wave systems. In contrast to previous EP implementations, our scheme is valid in the weakly dissipative regime, and readily applicable to a wide range of physical settings, even without well defined nodes, where trainable inter-node connections can be replaced by trainable local potential. We test the method in driven-dissipative exciton-polariton condensates governed by generalized Gross-Pitaevskii dynamics. Numerical studies on standard benchmarks, including a simple logical task and handwritten-digit recognition, demonstrate stable convergence, establishing a practical route to in-situ learning in physical systems in which system control is restricted to local parameters.
Read the full abstract

Quantum Physics

Spectral statistics and energy-gap scaling in $k-$local spin Hamiltonians

Sasanka Dowarah

We investigate the spectral properties of all-to-all interacting spin Hamiltonians acting on exactly $k$ spins, whose coupling coefficients are drawn from a normal distribution with mean $\mu$ and variance $\sigma^2$. For $\mu = 0$, we demonstrate that the random matrix ensemble – Gaussian Orthogonal Ensemble (GOE), Gaussian Unitary Ensemble (GUE), or Gaussian Symplectic Ensemble (GSE) – is determined by the parity of system size $L$ and locality $k$, following standard time-reversal symmetry classification. For couplings with a nonzero mean, we map the Hamiltonians to deformed random matrix ensembles and analyze conditions for an energy gap between the ground state and the first excited state. For $\mu < 0$, we find two distinct regimes: for $k \gg \sqrt{L}$, the gap closes at critical disorder $\sigma_{c} \approx |\mu|$. Near this transition the energy gap $\Delta$ exhibits universal quadratic scaling $\Delta /L \sim (\sigma - \sigma_{c})^{2}$. When $k \ll \sqrt{L}$, $\sigma_{c}$ scales with $|\mu|$, but lacks a sharp transition. Our work introduces a semi-solvable model that captures universal features of random-matrix statistics, and spectral gap formation, providing a foundation for systematic extensions to more general many-body systems.
Read the full abstract

Quantum Physics

Operator Commutativity Screening and Progressive Operator Block Reordering toward Many-body Inspired Quantum State Preparation

Dibyendu Mondal, Debaarjun Mukherjee and Rahul Maitra

In the field of quantum chemistry, the variational quantum eigensolver (VQE) has emerged as a highly promising approach to determine molecular energies and properties within the noisy intermediate-scale quantum (NISQ) era. The central challenges of this approach lie in the design of an expressive ansatz capable of representing the exact ground state wavefunction while concurrently being efficient to avoid numerical instabilities during the classical optimization. Owing to the constraints of current quantum hardware, the ansatz must remain sufficiently compact while retaining the flexibility to capture essential correlation effects. To address these challenges, we propose a systematic dynamic ansatz construction strategy in which the dominant operator blocks are initially identified through commutativity screening, combined with an energy sorting criteria. Subsequently, the ansatz is progressively expanded in a stepwise manner via iterative operator block reordering. To minimize the overhead, the higher order correlation terms are incorporated via reduced lower-body tensor factorization in each operator block, while the adaptive construction strategy ensures that the optimization is guided along the optimal trajectory to mitigate potential numerical instabilities due to the presence of local traps. Benchmark applications to various molecular systems demonstrate that this strategy of progressive operator-block addition achieves accurate energetics with significantly fewer parameters while efficiently bypassing local traps. Moreover, in strongly correlated regions, such as bond dissociation, the method successfully reproduces the ground state, where other contemporary approaches often fail.
Read the full abstract

Quantum Physics

Role of exceptional points in the dynamics of the Lindblad Sachdev-Ye-Kitaev model

Jie-ping Zheng, Jorge Dukelsky, Rafael A. Molina et al. (+1 more)

The out of equilibrium dynamics of the Sachdev-Ye-Kitaev model (SYK), comprising $N$ Majoranas with random all-to-all four-body interactions, minimally coupled to a Markovian bath modeled by the Lindblad formalism, displays intriguing nontrivial features. In particular, the decay rate towards the steady state is a non-monotonic function of the bath coupling $\mu$, and an analogue of the Loschmidt echo for dissipative quantum systems undergoes a first order dynamical phase transitions that eventually becomes a crossover for sufficiently large $\mu$. We provide evidence that these features have their origin in the presence of exceptional points in the purely real eigenvalues of the SYK Liouvillian closest to the zero eigenvalue associated with the steady state. An analytic calculation at small $N$, supported by numerical results for larger $N$, reveals that the value of $\mu \sim 0.1$ at which the exceptional point corresponding to the longest living modes occurs is close to a local maximum of the decay rate. This value marks the start of a region of anomalous equilibration where the relaxation rate diminishes as the coupling to the bath becomes stronger. Moreover, the mentioned change from transition to crossover in the Loschmidt echo occurs at a larger $\mu \sim 0.3$ corresponding with a proliferation of exceptional points in the low energy limit of the Liouvillian spectrum. We expect these features to be generic in the approach to equilibrium in quantum strongly interacting many-body Liouvillians.
Read the full abstract

All authors Jie-ping Zheng, Jorge Dukelsky, Rafael A. Molina, Antonio M. García-García

Quantum Physics

Adaptive time Compressed QITE (ACQ) and its geometrical interpretation

Alberto Acevedo Meléndez, Carmen G. Almudéver, Miguel Angel Garcia-March et al. (+8 more)

Imaginary time evolution (ITE) is a well-established method for ground-state preparation, a fundamental problem in many fields such as materials science, chemistry, and optimization. Quantum ITE (QITE) approximates this evolution on quantum hardware but suffers from high circuit depth and numerous measurements. In this work we introduce adaptive-time compressed QITE (ACQ), a novel algorithm that reduces resource-cost by combining adaptive time steps with circuit compression. This approach leverages geometric insights by characterizing its relationship to geodesic trajectories with a measure that distinguishes trajectories in CPN. Recalling that ITE is a gradient flow on the complex projective plane $\mathbb{CP}^N$, such trajectory measures allow one to measure the deviation from geodesicity of said flow. For Hamiltonians with only two distinct eigenvalues (spectral cardiality), ITE and QITE exactly trace geodesics, this fact motivates an adaptive strategy for systems whose corresponding spectral cardinality is greater than 2, where QITE unitaries are reused until an energy increase signals departure from the ITE path. This is implemented via a line search for energy minimization. Circuit compression is achieved by approximating the sequence of QITE unitaries with a single element of a one-parameter group. Numerical simulations on the transverse field Ising model and the Heisenberg model demonstrate that ACQ achieves comparable fidelity to standard QITE while significantly reducing the number of QITE optimizations and maintaining fixed circuit depth during propagation. Gate-count estimates and an analysis of the fidelity scaling with truncation parameters are provided. A gate count and performance comparison with the state of the art method double bracket QITE is also performed.
Read the full abstract

All authors Alberto Acevedo Meléndez, Carmen G. Almudéver, Miguel Angel Garcia-March, Rafael Gómez-Lurbe, Luca Ion, Mohit Lal Bera, Rodrigo M. Sanz, Somayeh Mehrabankar, Tanmoy Pandit, Armando Pérez, Andreu Anglés-Castillo

Quantum Physics

Flexible Qubit Allocation of Network Resource States

Francesco Mazza, Jorge Miguel-Ramiro, Jessica Illiano et al. (+4 more)

The Quantum Internet is still in its infancy, yet identifying scalable and resilient quantum network resource states is an essential task for realizing it. We explore the use of graph states with flexible, non-trivial qubit-to-node assignments. This flexibility enables adaptable engineering of the entanglement topology of an arbitrary quantum network. In particular, we focus on cluster states with arbitrary allocation as network resource states and as a promising candidate for a network core-level entangled resource, due to its intrinsic flexible connectivity properties and resilience to particle losses. We introduce a modeling framework for overlaying entanglement topologies on physical networks and demonstrate how optimized and even random qubit assignment creates shortcuts and improves robustness and memory savings, while reducing the worst-case hop distance between remote network nodes, when compared to conventional approaches.
Read the full abstract

All authors Francesco Mazza, Jorge Miguel-Ramiro, Jessica Illiano, Alexander Pirker, Marcello Caleffi, Angela Sara Cacciapuoti, Wolfgang Dür

Quantum Physics

Hybrid Path-Transverse Electric Mode Qudit Encoding on an Integrated Photonic Chip

Imogen Forbes, Patrick Yard, Martin Bielak et al. (+5 more)

Hybrid encodings, where multiple degrees of freedom are used to encode quantum information, can increase the size of the Hilbert space with minimal increase to hardware requirements. We show a reprogrammable integrated photonic device, with multimodal components designed to allow for control over the transverse electric modes. We use this device to generate qudit states entangled in the path and transverse electric mode degrees of freedom. We generate and verify a hyperentangled state with a fidelity of $\mathcal{F}_{\text{HE}} = 67.3 \pm 0.2\%$ and a GHZ$_{4}$-style state with a fidelity of $\mathcal{F}_{\text{GHZ}_{4}} = 85.2 \pm 0.4 \%$. We use our hyperentangled state in a single-copy entanglement distillation protocol, resulting in an average $9.1 \%$ increase in the fidelity of the distilled Bell state for up to a $50\%$ probability of bit flip error. By utilising degrees of freedom which are readily compatible with integrated photonics, our work highlights how this hybrid encoding demonstrates a first step in using the transverse electric mode to reduce the footprint of integrated quantum photonic experiments.
Read the full abstract

All authors Imogen Forbes, Patrick Yard, Martin Bielak, Molly A. Thomas, Matthew S. Jones, Stefano Paesani, Massimo Borghi, Anthony Laing

Quantum Physics · Cross-listed from cond-mat.str-el

Subdimensional Entanglement Entropy: from Geometric-Topological Response to Entanglement-Induced Mixed-State Holography

Meng-Yuan Li and Peng Ye

We introduce the subdimensional entanglement entropy (SEE), defined on subdimensional entanglement subsystems (SESs) embedded in the bulk, as an entanglement-based probe of how geometry and topology jointly shape universal properties of quantum matter. By varying the dimension, geometry, and topology of the SES, we show that the subleading term of SEE exhibits sharply distinct responses in a series of stabilizer states representing fixed points of different phases, including cluster states, $\mathbb{Z}_q$ topological orders, and fracton orders. Treating the reduced density matrix of an SES as a many-body mixed state supported on the SES manifold, we further establish a general correspondence between bulk stabilizers and mixed-state symmetries on SESs, separating them into strong and weak classes, and use it to identify strong-to-weak spontaneous symmetry breaking within SESs. Finally, for SESs with nontrivial SEE, we show that a set of weak symmetries acts as transparent patch operators of the corresponding strong symmetries. This motivates the notion of transparent composite symmetry, which remains robust under bipartition-factorized finite-depth quantum circuits that preserve SEE, and implies that a $D$-dimensional SES may holographically encode a $(D+1)$-dimensional topological order. These results establish SEE not only as a sharp probe of geometric-topological response, but also as a route from bulk pure-state entanglement to mixed-state symmetry and holography on subdimensional manifolds.
Read the full abstract

Quantum Physics

A source of heralded atom-photon entanglement for quantum networking

Gianvito Chiarella, Tobias Frank, Leart Zuka et al. (+2 more)

Communication in quantum networks suffers notoriously from photon loss. Resulting errors can be mitigated with a suitable measurement herald at the receiving node. However, waiting for a herald and communicating the measurement result back to the sender in a repeat-until-success strategy makes the protocol slow and prone to errors from false heralds such as detector dark counts. Here we implement an entanglement herald at the sending node by employing a cascaded two-photon emission of a single atom into two optical fiber cavities: The polarization of one photon is entangled with the spin of the atom, and the second photon heralds entanglement generation. We show that heralding improves the atom-photon entanglement in-fiber efficiency and fidelity to 68(3)% and 87(2)%, respectively. We highlight the potential of our source for noise-limited long-distance quantum communication by extending the range for constant fidelity or, alternatively, increasing the fidelity for a given distance.
Read the full abstract

All authors Gianvito Chiarella, Tobias Frank, Leart Zuka, Pau Farrera, Gerhard Rempe

Quantum Physics

A Gauss-Bonnet Theorem for Quantum States: Gauss Curvature and Topology in the Projective Hilbert Space

Shin-Ming Huang

Geometry and topology are fundamental to modern condensed matter physics, but their precise connection in quantum systems remains incompletely understood. Here, we develop an analytical scheme for calculating the curvature of the quantum metric of Bloch bands. Using a gauge-invariant formulation based on eigenprojectors, we construct the full Riemannian geometry of the quantum-state manifold and apply it to a two-dimensional two-band model. We find that the Gauss curvature is constant over regular regions, but the manifold inevitably develops a closed curve of singular points where the metric tensor degenerates. These singularities obstruct the conventional Gauss-Bonnet theorem. By introducing the notion of a front and a signed area form, we derive a generalized Gauss-Bonnet relation that includes a singular curvature term defined along the fold curve. This result establishes a direct, quantized link between the total signed Gauss curvature and the Chern number, providing a unified geometric interpretation of Berry curvature and quantum metric. This framework bridges differential geometry and topological band theory, revealing how singular folds mediate the discrepancy between quantum volume and topological charge.
Read the full abstract

Quantum Physics · Cross-listed from gr-qc

Scalar fields with power-law potentials in quantum cosmology

V. E. Kuzmichev and V. V. Kuzmichev

A homogeneous and isotropic quantum cosmological system (universe) initially filled with a uniform scalar field that has a potential in the power law representation is considered. Depending on the epoch, this scalar field yields barotropic matter in the form of stiff matter, perfect gas, radiation, dust, cosmic strings, domain walls, de Sitter vacuum, or phantom matter. The proposed approach is based on quantum geometrodynamics for the maximally symmetric space. The relevant differential equations for the separate power-law summands of the scalar field potential and the corresponding quantum Hamiltonian constraint equations, which describe the universe that can be viewed as dominated by one form or another of barotropic matter, were obtained. The solutions to these equations have been found in analytical form.
Read the full abstract

Quantum Physics

Observation of the Transition from Reversible to Irreversible Decoherence of Mesoscopic Quantum Superpositions

Ri-Hua Zheng, Jia-Hao Lü, Fan Wu et al. (+4 more)

The decoherence of superpositions of classically distinguishable states (cat states) is crucial for understanding quantum-to-classical transitions and quantum measurements. So far, irreversible decoherence processes of mesoscopic cat states have been demonstrated in several experiments. We here report on an experimental demonstration of the dynamics of such states in a reservoir with controllable degrees of freedom, which allows observation of the transition from reversible to irreversible mesoscopic decoherence. The experiment is performed with a circuit quantum electrodynamics device, where a bus microwave resonator storing a photonic cat state is connected to many nonlinear electronic oscillators. Each of the oscillators that are effectively coupled to the bus resonator serves as one degree of freedom of the reservoir. We characterize the which-path information about the resonator's state with the distinguishability. The experimental results reveal that the amount of spontaneously erasable which-path information progressively decreases with the increase in the number of the reservoir's degrees of freedom. When the reservoir contains sufficiently many oscillators, the which-path information acquired by the reservoir cannot be erased by the system-reservoir interaction at later times, which makes the decoherence irreversible. We further characterize the non-Markovianity of such a controllable open mesoscopic system.
Read the full abstract

All authors Ri-Hua Zheng, Jia-Hao Lü, Fan Wu, Yan Xia, Li-Hua Lin, Zhen-Biao Yang, Shi-Biao Zheng

Quantum Physics

The Geometry of Qubit Decoherence: Linear vs. Nonlinear Dynamics in the Bloch Ball

Alan C. Maioli, Evaldo M. F. Curado, Jean-Pierre Gazeau et al. (+1 more)

We present two complementary approaches to the GKSL equation for an open qubit. The first, based on linearity, yields solutions illustrated by mixed states trajectories in the Bloch ball, including non-random asymptotic fixed points, and exceptional points. The second, exploiting the SU(2) symmetry, leads to a nonlinear dynamical system that separates angular dynamics from radial dissipation. This symmetry-based perspective offers a promising route toward generalisation to open qudits.
Read the full abstract

All authors Alan C. Maioli, Evaldo M. F. Curado, Jean-Pierre Gazeau, Tomoi Koide

Quantum Physics

Optomechanical crystal in light-resilient quantum ground state

Johan Kolvik, Paul Burger, David Hambraeus et al. (+4 more)

Interaction between light and high-frequency sound is a key area in integrated photonics, quantum and nonlinear optics, and quantum science. However, typical suspended optomechanical structures suffer from poor thermal anchoring, making them susceptible to thermal noise arising from optical absorption. Here, we demonstrate a chip-scale, release-free silicon optomechanical crystal cavity operating cryogenically with improved resilience to laser light. Relative to a suspended design, we observe 18 dB suppression of the thermo-optic effect, and the device also sustains near-unity phonon occupation at 35 dB higher intracavity optical energy in continuous-wave operation. Non-exponential decay dynamics from an as yet unidentified thermal process limit transfer of this performance to pulsed operation. Resolving this channel would open a clear path to near-term quantum protocols, such as microwave-to-optical quantum transduction. More broadly, the release-free architecture offers a novel platform for studying thermal noise dynamics with results relevant across silicon optomechanical platforms.
Read the full abstract

All authors Johan Kolvik, Paul Burger, David Hambraeus, Trond H. Haug, Joey Frey, Mads B. Kristensen, Raphaël Van Laer

Quantum Physics

Quantum Worst-Case to Average-Case Reduction for Matrix-Vector Multiplication

Divesh Aggarwal and Dexter Kwan

Worst-case to average-case reductions are a cornerstone of complexity theory, providing a bridge between worst-case hardness and average-case computational difficulty. While recent works have demonstrated such reductions for fundamental problems using deep tools from ad- ditive combinatorics, these approaches often suffer from substantial complexity and suboptimal overheads. In this work, we focus on the quantum setting, and provide a new reduction for the Matrix-Vector Multiplication problem that is more efficient, and conceptually simpler than previous constructions. By adapting hardness self-amplification techniques to the quantum do- main, we obtain a quantum worst-case to average-case reduction with improved dependence on the success probability, laying the groundwork for broader applications in quantum fine-grained complexity.
Read the full abstract

Quantum Physics

Globalizing the Carleman linear embedding method for nonlinear dynamics

Ivan Novikau and Ilon Joseph

The Carleman embedding method is a widely used technique for linearizing a system of nonlinear differential equations, but fails to converge in regions where there are multiple fixed points. We propose and test three different versions of a global piecewise Carleman embedding technique, based on partitioning space into multiple regions where the center and size of the embedding region are chosen to control convergence. The first method switches between local linearization regions of fixed size once the trajectory reaches the boundary of the current linearization chart. During the transition, the embedding is reconstructed within the newly created chart, centered at the transition point. The second method also adapts the chart size dynamically, enhancing accuracy in regions where multiple fixed points are located. The third method partitions the state space using a static grid with precomputed linearization charts of fixed size, making it more suitable for applications that require high speed. All techniques are numerically tested on multiple integrable and chaotic nonlinear dynamical systems demonstrating their applicability for problems that are completely intractable for the standard Carleman embedding method. Simulations of chaotic dynamical systems such as various types of strange attractors demonstrate the power of the adaptive methods, if a sufficiently low tolerance is imposed. Still, the non-adaptive version of the method, with fixed centers and sizes of the linearization charts, can be faster in simulating dynamical systems while providing similar accuracy and may be more appropriate as the basis of algorithms for future quantum computers.
Read the full abstract

Quantum Physics

Generation of multipartite photonic entanglement using a trapped-ion quantum processing node

Marco Canteri, James Bate, Ida Mishra et al. (+3 more)

The ability to establish entanglement between the nodes of future quantum networks is essential for enabling a wide range of new applications in science and technology. A promising approach involves the use of a powerful central node capable of deterministically preparing arbitrary multipartite entangled states of its matter-based qubits and efficiently distributing these states to surrounding end nodes via flying photons. This central node, referred to as a “factory node", serves as a hub for the production and distribution of multipartite entanglement. In this work, we demonstrate key functionalities of a factory node using a cavity-integrated trapped-ion quantum processor. Specifically, we program the system to generate genuinely multipartite entangled Greenberger-Horne-Zeilinger (GHZ) states of three path-switchable photons and verify them using custom-designed entanglement witnesses. These photons can, in the future, be used to establish stored multipartite entanglement between remote matter-based nodes. Our results demonstrate that the well-established techniques for the deterministic preparation of entangled states of co-trapped ion qubits can be used to prepare the same states of traveling photons, paving the way for multipartite entanglement distribution in quantum local area networks.
Read the full abstract

All authors Marco Canteri, James Bate, Ida Mishra, Nicolai Friis, Victor Krutyanskiy, Benjamin P. Lanyon

Quantum Physics

Fragment, Entangle, and Consolidate: Strong Correlation through Bi-fold Quantum Circuits

Arpan Choudhury, Sonaldeep Halder, Rahul Maitra et al. (+1 more)

An accurate description of strong correlation is quintessential for the exploration of emerging chemical phenomena. While near-term variational quantum algorithms provide a theoretically scalable framework for quantum chemical problems, the accurate simulation of multireference effects remains elusive, hindering progress toward the rational design of novel chemical space. In this regard, we introduce a general and customizable scheme to handle strong electronic correlation, based on problem decomposition, entanglement buildup, and subsequent consolidation. Based on a problem-inspired molecular decomposition, the deployment of Hardware Efficient Ansatz to prepare entangled subsystems ensures efficient construction of a multireference state while concurrently adhering to the hardware topology. The dynamic correlation is subsequently introduced through a unitary coupled cluster framework, with static or dynamic ansatz parametrized by a set of inter-fragment generalized operators, and with the product state spanning various subsystems taken as the reference. The hybrid architecture ensures a judicious deployment of separate ansatze structures for capturing various degrees of correlation in a balanced manner, while concurrently retaining the scalability and flexibility provided by them individually. Over a number of numerical applications on a strongly correlated system, the proposed scheme is shown to be highly accurate, flexible, and robust in unlocking the potential to harness quantum advantage for quantum chemistry.
Read the full abstract

All authors Arpan Choudhury, Sonaldeep Halder, Rahul Maitra, Debashree Ghosh

Quantum Physics

High-dimensional Path-Encoded Entanglement Distribution Between Photonic Chips Enabled by Multimode Phase Stabilisation

Molly A. Thomas, Daniel Llewellyn, Patrick W. Yard et al. (+13 more)

The reliable distribution of high-dimensional entangled quantum states, an important resource in quantum technologies, through optical fibre networks is challenging due to the need to maintain coherence across multiple modes. Here we demonstrate the distribution of four-dimensional path-encoded entangled quantum states between photonic chips, enabled by a novel multimode phase stabilisation algorithm. The algorithm utilises the reconfigurability of the integrated photonic circuits to complete one iteration of phase stabilisation in just two measurement rounds for an arbitrary number of modes, and requires no additional hardware to the quantum measurements it enables. As a result, we are able to perform complete quantum state tomography across two chips using the minimum number of local projective measurements to verify the fidelity of the distributed entangled state to be 86% (compared to 8.1% without the phase stabilisation) with an entanglement entropy of 0.995+/-0.002.
Read the full abstract

All authors Molly A. Thomas, Daniel Llewellyn, Patrick W. Yard, Benjamin A. Slater, Caterina Vigliar, Stefano Paesani, Massimo Borghi, Döndü Sahin, John G. Rarity, Leif K. Oxenløwe, Mark G. Thompson, Karsten Rottwitt, Yunhong Ding, Jianwei Wang, Davide Bacco, Jorge Barreto

Quantum Physics

Solving the 3D Heat Equation with VQA via Remeshing-Based Warm Starts

Samuel Donachie, Ulysse Remond, Arthur Mathorel et al. (+1 more)

Quantum computing holds great promise for solving classically intractable problems such as linear systems and partial differential equations (PDEs). While fully fault-tolerant quantum computers remain out of reach, current noisy intermediate-scale quantum (NISQ) devices enable the exploration of hybrid quantum-classical algorithms. Among these, Variational Quantum Algorithms (VQAs) have emerged as a leading candidate for near-term applications. In this work, we investigate the use of VQAs to solve PDEs arising in stationary heat transfer. These problems are discretized via the finite element method (FEM), yielding linear systems of the form Ku=f, where K is the stiffness matrix. We define a cost function that encodes the thermal energy of the system, and optimize it using various ansatz families. To improve trainability and bypass barren plateaus, we introduce a remeshing strategy which gradually increases resolution by reusing optimized parameters from coarser discretizations. Our results demonstrate convergence of scalar quantities with mesh refinement. This work provides a practical methodology for applying VQAs to PDEs, offering insight into the capabilities and limitations of current quantum hardware.
Read the full abstract

All authors Samuel Donachie, Ulysse Remond, Arthur Mathorel, Kyryl Kazymyrenko

Quantum Physics

Distributed Quantum Information Processing: A Review of Recent Progress

Johannes Knörzer, Xiaoyu Liu, Benjamin F. Schiffer et al. (+1 more)

Distributed quantum information processing seeks to overcome the scalability limitations of monolithic quantum devices by interconnecting multiple quantum processing nodes via classical and quantum communication. This approach extends the capabilities of individual devices, enabling access to larger problem instances and novel algorithmic techniques. Beyond increasing qubit counts, it also enables qualitatively new capabilities, such as joint measurements on multiple copies of high-dimensional quantum states. The distinction between single-copy and multi-copy access reveals important differences in task complexity and helps identify which computational problems stand to benefit from distributed quantum resources. At the same time, it highlights trade-offs between classical and quantum communication models and the practical challenges involved in realizing them experimentally. In this review, we contextualize recent developments by surveying the theoretical foundations of distributed quantum protocols and examining the experimental platforms and algorithmic applications that realize them in practice.
Read the full abstract

All authors Johannes Knörzer, Xiaoyu Liu, Benjamin F. Schiffer, Jordi Tura

Quantum Physics

Time evolution of the Husimi and Glauber-Sudarshan functions in terms of complementary Hamiltonian symbols

Mritunjay Tyagi and Simon Friederich

We present a compact, systematic formulation of the dynamics of the Husimi Q- and Glauber-Sudarshan P-phase space distribution functions expressed in terms of their complementary Hamiltonian symbols: Anti-Wick for Q and Wick for P. The resulting evolution equations have a universal leading structure, the classical Liouvillian drift plus terms with higher-order derivatives of the Hamiltonian. For Hamiltonians no higher than quartic in the moduli of the complex phase space variables $\alpha_i$, the higher-order terms reduce to a second-order Fokker-Planck type term with a traceless diffusion matrix, thereby clarifying and recovering recent results for such Hamiltonians within a simple star-product framework. We further derive a transparent Ehrenfest theorem for Wick/Anti-Wick symbols of the operators representing dynamical observables. Using these results, we show that a previously reported nonclassical contribution to the Q-function drift for the anharmonic oscillator is an artifact of the quantization scheme used. Our paper consolidates the formulation of the dynamics of the phase space distribution functions using complementary symbols and provides an efficient route to compute and interpret quantum phase space evolution.
Read the full abstract

Quantum Physics · Cross-listed from physics.optics

Ultracompact Low-Loss Grating Couplers

Shiang-Yu Huang, Jonas Zatsch, Tim Engling et al. (+2 more)

Fiber-to-chip couplers play a crucial role in interfacing on-chip photonic circuits with other optical systems or off-chip devices. Downsizing the couplers via topology optimization addresses the demand for high-density integration and improves the scalability of photonic integrated systems. However, these optimized couplers have yet to reach the performance level demonstrated by their conventional counterparts, leaving room for further improvement. In this work, we apply topology optimization to design single-polarization 1D and dual-polarization 2D grating couplers incorporating bottom reflectors and achieve sub-decibel coupling efficiency. Both types of couplers are fabricated on the silicon-on-insulator platform with dimensions of mere 14 $\mu$m $\times$ 14 $\mu$m and are compatible with standard single-mode fibers at normal incidence. From our experimental characterization, the measured peak coupling efficiency of the topology-optimized 1D and 2D couplers is -0.92(1) dB and -0.86(13) dB, respectively, within the telecom C-band. Our demonstration provides a coupling solution for photonic applications requiring high efficiency and high-density integration, such as spatial division multiplexing and photonic quantum technologies.
Read the full abstract

All authors Shiang-Yu Huang, Jonas Zatsch, Tim Engling, Jeldrik Huster, Stefanie Barz

Quantum Physics

A single optically detectable tumbling spin in silicon

Félix Cache, Yoann Baron, Baptiste Lefaucher et al. (+9 more)

We demonstrate single spin spectroscopy of a fluorescent tumbling defect in silicon called the G center, behaving as a pseudo-molecule randomly reorienting itself in the crystalline matrix. Using high-resolution spin spectroscopy, we reveal a fine magnetic structure resulting from the spin principal axes jumping between discrete orientations in the crystal. Modeling the atomic reorientation of the defect shows that spin tumbling induces variations in the coupling to the microwave magnetic field, enabling position-dependent Rabi frequencies to be detected in coherent spin control experiments. By virtue of its pseudo-molecule configuration, the G center in silicon is a unique quantum system to investigate the mutual interaction between optical, spin and rotation properties in a highly versatile material.
Read the full abstract

All authors Félix Cache, Yoann Baron, Baptiste Lefaucher, Jean-Baptiste Jager, Frédéric Mazen, Frédéric Milési, Sébastien Kerdilès, Isabelle Robert-Philip, Jean-Michel Gérard, Guillaume Cassabois, Vincent Jacques, Anaïs Dréau

Quantum Physics

Elastic Quantum Coupling Between Free Electrons and Photons

Dingguo Zheng and Ofer Kfir

The quantum coupling between free-electrons and photons enables applying quantum optics techniques in electron microscopy. Here, we formulate the elastic electron-photon quantum coupling and its possible implications. Our analysis shows that when an electron traverses the field of an optical cavity, it induces a phase shift onto its confined photonic mode, which can be quantified as a refractive index of a free electron. This principle can be applied to counting electrons in a beam without changing its quantum states. The elastic scattering operator forms an electron-counting dispersive Hamiltonian for electron-photon systems within electron microscope, and it could enable quantum- and sub-shot-noise sensing and imaging at the Å-scale.
Read the full abstract