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Thursday 23 October

Quantum Physics

Realization of Trapped Ion Dynamics in the Strong-Field Regime and Non-Markovianity

Kamran Rehan, Hengchao Tu, Tadeu Tassis et al. (+5 more)

We experimentally investigate trapped ion dynamics in the strong-driving regime, where the Rabi frequency (Omega) is comparable to the vibrational mode frequency (nu). In the conventional weak-driving regime (Omega << nu), the dynamics is well described by effective Hamiltonians for the carrier and motional sidebands, associated with detunings (delta = n nu (n = 0, +/- 1, ...)). In the strong-driving regime (Omega ~ nu), these interactions can no longer be treated independently. We characterize this physics through the reduced dynamics of the qubit, where non-Markovian behavior emerges as an operational probe of the spin-motion coupling. We observe a structured non-Markovian response, with well-defined maxima that follow the generalized resonance condition (delta^2 + Omega^2 = nu^2), reflecting the strong underlying spin-motion hybridization characteristic of the strong-driving regime.
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All authors Kamran Rehan, Hengchao Tu, Tadeu Tassis, Menglin Zou, Zihan Yin, Jing-Ning Zhang, Fernando L. Semiao, Kihwan Kim

Quantum Physics

Robust gigahertz-range ac magnetometry with an ensemble of NV centers in diamond using concatenated continuous dynamical decoupling

Takuya Kitamura, Genko Genov, Alon Salhov et al. (+5 more)

Sub-picotesla level magnetometry has been demonstrated using negatively-charged nitrogen-vacancy (NV) centers in diamond by increasing the number of spins simultaneously used for sensing in an NV ensemble. However, such scale-up often introduces spatial inhomogeneities in detuning and control field amplitudes, which degrade sensitivity. Although several techniques have been utilized to overcome these challenges, including pulsed dynamical decoupling or shaped pulses, these are not generally compatible with the current state-of-the-art techniques for GHz-range AC magnetometry with NV ensembles, which are typically based on Rabi oscillations. In this work we experimentally demonstrate GHz-range AC magnetometry using a large ensemble of NV centers under spatially inhomogeneous drive fields by employing concatenated continuous dynamical decoupling, which is designed for robustness against such imperfections. We compare its performance with the conventional direct Rabi method and show that the robust dressed states in our method extend significantly the measuring range to weaker signals in GHz-range AC magnetometry.
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All authors Takuya Kitamura, Genko Genov, Alon Salhov, Yutaka Kobayashi, Shinobu Onoda, Junichi Isoya, Alex Retzker, Fedor Jelezko

Quantum Physics

Multiplexed ion-ion entanglement over $1.2$ kilometer fibers

Z. B. Cui, Z. Q. Wang, P. Y. Liu et al. (+14 more)

Quantum networks and quantum repeaters represent the promising avenues for building large-scale quantum information systems, serving as foundational infrastructure for distributed quantum computing, long-distance quantum communication, and networked quantum sensing. A critical step in realizing a functional quantum network is the efficient and high-fidelity establishment of heralded entanglement between remote quantum nodes. Multiplexing offers a powerful strategy to accelerate remote entanglement distribution, particularly over long optical fibers. Here, we demonstrate the first multiplexing-enhanced heralded entanglement between two trapped-ion quantum network nodes. By multiplexing $10$ temporal photonic modes, we achieve a 4.59-fold speedup in ion-ion entanglement generation and attain an entanglement fidelity of $95.9\pm1.5\%$ over $1.2$ km of fiber. Employing a dual-type architecture, our system is readily scalable to multiple nodes, thereby establishing a key building block for future large-scale quantum networks.
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All authors Z. B. Cui, Z. Q. Wang, P. Y. Liu, Y. Wang, P. C. Lai, J. X. Shi, Y. D. Sun, Z. C. Tian, H. S. Sun, Y. B. Liang, B. X. Qi, Y. Y. Huang, Z. C. Zhou, Y. K. Wu, Y. Xu, Y. F. Pu, L. M. Duan

Quantum Physics

Restoring Quantum Superiority of Noisy Quantum Illumination

Wei Wu and Jun-Hong An

Quantum illumination uses quantum entanglement as a resource to enable higher-resolution detection of low-reflectivity targets than is possible with classical techniques. This revolutionary technology could transform modern radar. However, it is widely believed that the decoherence induced by the ubiquitous quantum noise destroys the superiority of quantum illumination, severely constraining its performance and application in our present noisy intermediate-scale quantum era. Here, we propose a method to restore the quantum superiority of the quantum illumination in the presence of quantum noises. Going beyond the widely used Born-Markov approximation, we discover that the resolution of noisy quantum illumination is highly sensitive to the energy spectrum of the composite system formed by each of the two light modes and its local quantum noise. When a bound state is present in the energy spectrum, the resolution asymptotically approaches its ideal form. Our result establishes a physical principle to preserve the quantum superiority and paves the way for the realization of high-resolution quantum illumination in noisy situations.
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Quantum Physics

Complete characterisation of state conversions by work extraction

Chung-Yun Hsieh and Manuel Gessner

We introduce a thermodynamic work extraction task that describes the energy storage enhancement of quantum systems. This task induces majorisation-like conditions that provide a necessary and sufficient characterisation of state conversions in general quantum resource theories. When applied to specific resources, these conditions reduce to the majorisation conditions under unital channels and provide a thermodynamic version of Nielsen's theorem in entanglement theory. We show how this result establishes the first universal resource certification class based on thermodynamics, and how it can be employed to quantify general quantum resources based on work extraction.
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Quantum Physics · Cross-listed from cond-mat.supr-con

Phase Separation in Kitaev Chain

Kazuhiro Kuboki

Kitaev chain is a one-dimensional spinless fermion model that has $p$-wave superconducting (SC) states and Majorana zero modes at the edge. Usually this model is analyzed by taking only SC order parameter (OP) into account, but the situation significantly changes when OPs other than the SCOP are included. It turns out that the SC state in the latter case is prone to phase separation for moderate to strong attractiive interactions.
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Quantum Physics

Factorizability of optimal quantum sequence discrimination under maximum-confidence measurements

Donghoon Ha and Jeong San Kim

We consider the discrimination of quantum sequences under maximum-confidence measurements and show that the optimal discrimination of a quantum sequence ensemble can always be factorized into that of each individual ensemble. In other words, the optimal quantum sequence discrimination under maximum-confidence measurements can be achieved just by performing a maximum-confidence discrimination independently at each step of the quantum sequence. We also show that the maximum confidence of identifying a quantum sequence is to achieve the maximum confidence of identifying each state comprising the quantum sequence. We further provide a necessary and sufficient condition for the optimal quantum state discrimination under maximum-confidence measurements.
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Quantum Physics

Non-Markovianity in Quantum Information Processing: Interplay with Quantum Error Mitigation

Suguru Endo, Hideaki Hakoshima and Tomohiro Shitara

Non-Markovian dynamics are typically present in the dynamics of open quantum systems. Despite the rich structure of non-Markovian dynamics, their relevance to quantum information processing (QIP) has been rarely discussed. In this work, we demonstrate that the negativity of the dynamics, a characteristic of non-Markovian dynamics, naturally arises in quantum error correction (QEC) and quantum teleportation. The negativity in open quantum systems is naturally attributed to the information backflow from the environment. We partition the whole Hilbert space into the logical subsystem and the gauge subsystem. The logical subsystem stores the quantum information for QIP, while the gauge subsystem stores the information for recovery of the logical information, i.e., the syndrome measurement outcomes for quantum error correction and Bell measurement outcomes for successful teleportation. We then show that the negativity in quantum information processing appears as a consequence of the feedback operation based on the measurement outcomes of the gauge subsystem. Finally, we show that the negativity of non-Markovianity in QIP reduces the sampling cost of quantum error mitigation (QEM), shedding light on the importance of combination strategies of QEC and QEM in a practical QIP.
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Quantum Physics · Cross-listed from gr-qc

Emergent time and more from wavefunction collapse in general relativity

Sung-Sik Lee

In this paper, we further develop a recently proposed theory of time based on wavefunction collapse in general relativity. It is based on the postulations that quantum states, which violate the momentum and Hamiltonian constraints, represent instances of time, and stochastic fluctuations of the lapse and shift generate the time evolution under which an initial state gradually collapses toward a diffeomorphism-invariant state. Under the wavefunction collapse, the scale factor monotonically increases, thus acting as a clock. The scalar, vector, and tensor gravitons arise as physical excitations, and the arrow of time for their evolution is set by the initial state. In the long-time limit, the tensor gravitons exhibit emergent unitary dynamics. However, the extra modes are strongly damped due to the non-unitary dynamics that suppress the constraint-violating excitations. The vector mode is uniformly suppressed over all length scales, but the decay rate of the scalar is proportional to its wave vector. This makes the latter a viable candidate for dark matter; excitations with large wavelengths survive over long periods, contributing to long-range interactions, while the fast decay of short-wavelength modes renders them undetectable without sufficient temporal resolution. These are demonstrated for the cosmological constant-dominated universe through semi-classical and adiabatic approximations, which are controlled in the limit of large space dimension.
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Quantum Physics

Parametric phase modulation in superconducting circuits

Zhuang Ma, Xianke Li, Hongyi Shi et al. (+4 more)

Parametric modulation, valued for its versatility, is widely employed in superconducting circuits for quantum simulations and high-fidelity two-qubit gates. Conventionally, the qubit coupling strength is determined by the amplitude of the parametric flux pulse, which affects the qubit parameters dramatically. In this paper, we propose and implement a phase-modulation scheme to tune the interaction strength via adjustment of the relative phase between the parametric flux pulses applied to two coupled qubits. We characterize this modulation for sideband couplings, at both sweet and off-sweet spots, achieving a broad range of coupling strengths, as confirmed by both population dynamics and spectroscopy methods. This approach enables phase-controlled modulation of coupling strength, providing a promising candidate for parametrically driven quantum simulations and gate operations.
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All authors Zhuang Ma, Xianke Li, Hongyi Shi, Ruonan Guo, Jianwen Xu, Xinsheng Tan, Yang Yu

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

Kondo breakdown induced by non-Hermitian complex hybridization

Kazuki Yamamoto, Masaya Nakagawa and Norio Kawakami

Recently, a non-Hermitian Anderson impurity model with one-body loss has been studied in [Phys. Rev. B 111, 125157 (2025)}], and it has been demonstrated that the renormalization effect generated by strong correlations counterintuitively changes the nature of dissipation into an emergent many-body dissipation that causes a Kondo breakdown. In a closely related context, it is also known that two-body loss in a non-Hermitian Kondo model triggers the Kondo breakdown. To elucidate the essence of these phenomena, we study the Anderson impurity model with a non-Hermitian complex hybridization as an effective model that provides a simple understanding of the Kondo breakdown. Using the slave-boson mean-field theory, we show that this model can explain the Kondo breakdown with a single complex parameter. Furthermore, we provide the exact Bethe ansatz solutions that support the results obtained by the slave-boson mean-field theory.
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Quantum Physics

Unveiling non-Hermitian band structures with non-Bloch supercells

Jia-Xin Zhong, Jing Lin, Kai Chen et al. (+3 more)

Real-valued band structures are foundational to analyzing periodic systems within the Hermitian description and have been experimentally well-established over recent decades. In contrast, non-Hermitian systems exhibit complex band structures where both energy and momentum have imaginary parts, underpinning phenomena like the non-Hermitian skin effect and anomalous bulk-boundary correspondence that defy conventional Bloch theory. Experimentally mapping these complex bands-relating complex momentum to complex energy-and identifying their associated eigenstates is crucial for understanding these systems but remains a significant challenge. Here, we introduce a non-Bloch supercell framework designed to overcome this challenge by decoupling Bloch phase control from the imaginary part of momentum. Our method combines an exponent-flattening protocol with twisted boundary conditions, enabling system-size-independent control of imaginary momentum while preserving high-resolution Bloch phase sampling. Implemented in programmable one- and two-dimensional acoustic crystals, our approach acquires momentum-resolved complex energy surfaces and biorthogonal eigenmodes by Green's function measurements.Data obtained using this framework accurately predict open-boundary spectra and eigenstates, findings we verify through separate open-geometry experiments. Our work provides a broadly applicable experimental toolkit for exploring non-Hermitian band geometry and topology in diverse engineered classical and quantum platforms.
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All authors Jia-Xin Zhong, Jing Lin, Kai Chen, Jing Lu, Kun Ding, Yun Jing

Quantum Physics

Efficient Floating-Point Arithmetic on Fault-Tolerant Quantum Computers

José E. Cruz Serrallés, Oluwadara Ogunkoya, Dog̃a Murat Kürkçüog̃lu et al. (+5 more)

We propose a novel floating-point encoding scheme that builds on prior work involving fixed-point encodings. We encode floating-point numbers using Two's Complement fixed-point mantissas and Two's Complement integral exponents. We used our proposed approach to develop quantum algorithms for fundamental arithmetic operations, such as bit-shifting, reciprocation, multiplication, and addition. We prototyped and investigated the performance of the floating-point encoding scheme on quantum computer simulations by performing reciprocation on randomly drawn inputs and by solving first-order ordinary differential equations, while varying the number of qubits in the encoding. We observed rapid convergence to the exact solutions as we increased the number of qubits and a significant reduction in the number of ancilla qubits required for reciprocation when compared with similar approaches.
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All authors José E. Cruz Serrallés, Oluwadara Ogunkoya, Dog̃a Murat Kürkçüog̃lu, Nicholas Bornman, Norm M. Tubman, Anna Grassellino, Silvia Zorzetti, Riccardo Lattanzi

Quantum Physics

Quantum mysteries explained in digestible form

Alejandro Hnilo

Years ago, Itamar Pitowski asked two relevant questions: Why microphysical (quantum) phenomena and classical phenomena differ in the way they do? and, what kind of explanation could qualify as a reasonable one? I argue that both questions can be answered by the comparison of quantum phenomena with some features of vectors in real space. In particular, I show how violation of Bell's inequalities, Teleportation, Kochen-Specker and Greenberger-Horne-Zeilinger theorems can be understood in terms of vectors. This does not mean that the difference between quantum and classical phenomena is illusory. This means that vectors are stranger objects that they may seem to be at first sight.
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Quantum Physics · Cross-listed from cs.DC

A Full Stack Framework for High Performance Quantum-Classical Computing

Xin Zhan, K. Grace Johnson, Aniello Esposito et al. (+5 more)

To address the growing needs for scalable High Performance Computing (HPC) and Quantum Computing (QC) integration, we present our HPC-QC full stack framework and its hybrid workload development capability with modular hardware/device-agnostic software integration approach. The latest development in extensible interfaces for quantum programming, dispatching, and compilation within existing mature HPC programming environment are demonstrated. Our HPC-QC full stack enables high-level, portable invocation of quantum kernels from commercial quantum SDKs within HPC meta-program in compiled languages (C/C++ and Fortran) as well as Python through a quantum programming interface library extension. An adaptive circuit knitting hypervisor is being developed to partition large quantum circuits into sub-circuits that fit on smaller noisy quantum devices and classical simulators. At the lower-level, we leverage Cray LLVM-based compilation framework to transform and consume LLVM IR and Quantum IR (QIR) from commercial quantum software frontends in a retargetable fashion to different hardware architectures. Several hybrid HPC-QC multi-node multi-CPU and GPU workloads (including solving linear system of equations, quantum optimization, and simulating quantum phase transitions) have been demonstrated on HPE EX supercomputers to illustrate functionality and execution viability for all three components developed so far. This work provides the framework for a unified quantum-classical programming environment built upon classical HPC software stack (compilers, libraries, parallel runtime and process scheduling).
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All authors Xin Zhan, K. Grace Johnson, Aniello Esposito, Barbara Chapman, Marco Fiorentino, Kirk M. Bresniker, Raymond G. Beausoleil, Masoud Mohseni

Quantum Physics

Electronically-controlled one- and two-qubit gates for transmon quasicharge qubits

Nicholas M. Christopher, Deniz E. Stiegemann, Abhijeet Alase et al. (+1 more)

Superconducting protected qubits aim to achieve sufficiently low error rates so as to allow realization of error-corrected, utility-scale quantum computers. A recent proposal encodes a protected qubit in the quasicharge degree of freedom of the conventional transmon device. Operating such a protected `quasicharge qubit' requires implementing new strategies. Here we show that an electronically-controllable tunnel junction formed by two topological superconductors can be used to implement single- and two-qubit gates on quasicharge qubits. Schemes for both these gates are based on the same dynamical $4\pi$-periodic Josephson effect and therefore have the same gate times and error characteristics. We simulate the dynamics of a topological Josephson junction in a parameter regime with non-negligible charging energy, and characterize the robustness of such gate operations against charge noise. Our results point to a compelling strategy for implementation of quasicharge qubit gates based on junctions of minimal Kitaev chains of quantum dots.
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All authors Nicholas M. Christopher, Deniz E. Stiegemann, Abhijeet Alase, Thomas M. Stace

Quantum Physics

Variational quantum simulation of many-body dissipative dynamics on a superconducting quantum processor

Huan-Yu Liu, Tai-Ping Sun, Zhao-Yun Chen et al. (+7 more)

Open quantum systems host a wide range of intriguing phenomena, yet their simulation on well-controlled quantum devices is challenging, owing to the exponential growth of the Hilbert space and the inherently non-unitary nature of the dynamics. Here we propose and experimentally demonstrate a variational quantum algorithm capable of scalable simulation of non-unitary many-body dissipative dynamics. The algorithm builds on the framework of linear combination of Hamiltonian simulation, which converts non-unitary dynamics into a weighted sum of unitary evolutions. With the further introduction of a simplified quantum circuit for loss-function evaluation, our scheme is suitable for near-term quantum hardware, with the circuit depth independent of the simulation time. We illustrate our scheme by simulating the collective dynamics of a dissipative transverse Ising model, as well as an interacting Hatano-Nelson model, on the superconducting quantum processor Wukong. Our work underlines the capability of noisy intermediate-scale quantum devices in simulating dissipative many-body dynamics and represents a step forward in exploiting their potential for solving outstanding physical problems.
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All authors Huan-Yu Liu, Tai-Ping Sun, Zhao-Yun Chen, Cheng Xue, Chao Wang, Xi-Ning Zhuang, Jin-Peng Liu, Wei Yi, Yu-Chun Wu, Guo-Ping Guo

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

Circuit-based cavity magnonics in the ultrastrong and deep-strong coupling regimes

Takahiro Chiba, Ryunosuke Suzuki, Takashi Otaki et al. (+1 more)

We theoretically study nonperturbative strong-coupling phenomena in cavity magnonics systems in which the uniform magnetization dynamics (magnons) in a ferromagnet is coupled to the microwave magnetic field (photons) of a single LC resonator. Starting from an effective circuit model that accounts for the magnetization dynamics described by the Landau-Lifshitz-Gilbert equation, we show that a nontrivial frequency shift emerges in the ultrastrong and deep-strong coupling regimes, whose microscopic origin remains elusive within a purely classical framework. The circuit model is further quantized to derive a minimal quantum mechanical model for generic cavity magnonics, which corresponds to a two-mode version of the Hopfield Hamiltonian and explains the mechanism of the frequency shifts found in the classical circuit model. We also formulate the relation between the frequency shift and quantum quantities, such as the ground-state particle number, quantum fluctuations associated with the Heisenberg uncertainty principle, and entanglement entropy, providing a nondestructive means to experimentally access to these quantum resources. By utilizing soft magnons in an anisotropic ferromagnet, we further demonstrate that these quantum quantities diverge at the zeros of the magnon band edges as a function of the external magnetic field. This work paves the way for cavity magnonics beyond the conventional strong coupling regime.
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All authors Takahiro Chiba, Ryunosuke Suzuki, Takashi Otaki, Hiroaki Matsueda

Quantum Physics · Cross-listed from cond-mat.supr-con

Fabrication and Structural Analysis of Trilayers for Tantalum Josephson Junctions with Ta$_2$O$_5$ Barriers

Raahul Potluri, Rohin Tangirala, Jiangteng Liu et al. (+6 more)

Tantalum (Ta) has emerged as a promising low-loss material, enabling record coherence times in superconducting qubits. This enhanced performance is largely attributed to its stable native oxide, which may host fewer two-level system (TLS) defects, which are the key contributors to decoherence in superconducting circuits. Nevertheless, aluminum oxide remains the predominant choice for Josephson junction (JJ) barriers in most qubit architectures. Here, we investigate techniques for forming high-quality oxide layers on $\alpha$-phase tantalum films to develop tantalum-oxide JJ barriers. We explore thermal oxidation in a tube furnace, rapid thermal annealing, and plasma oxidation of both room-temperature and heated Ta films, characterize the resulting structures using X-ray techniques and electron microscopy, and propose a mechanistic picture of the oxidation pathways. We find that plasma oxidation provides the smoothest Ta$_2$O$_5$ layers, is compatible with in situ Ta deposition, and offers thickness control through the annealing temperature, advantageous for JJ fabrication. Lastly, we evaluate methods for growing Ta/TaO$_x$/Ta trilayers. All trilayers showed c-axis-oriented columnar growth of the bottom Ta layer, with sapphire substrates producing larger, better-aligned grains yet higher dislocation densities than silicon. Nucleation of c-axis-oriented $\alpha$-Ta on tantalum-oxide required an Nb seed layer, as direct Ta deposition yielded amorphous Ta. These results demonstrate the feasibility of $\alpha$-Ta/Nb/TaO$_x$/$\alpha$-Ta stacks for JJs with clean interfaces.
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All authors Raahul Potluri, Rohin Tangirala, Jiangteng Liu, Alejandro Barrios, Praveen Kumar, Sage R. Bauers, Peter V. Sushko, David P. Pappas, Serena Eley

Quantum Physics

All-Gaussian State Discrimination Beyond the Coherent Helstrom Bound

Angus Walsh, Lorcan Conlon, Biveen Shajilal et al. (+5 more)

A core problem in communications is the optimal discrimination of binary-phase-shift-keyed (BPSK) signals. A longstanding goal has been to reach the fundamental quantum limit, known as the Helstrom bound, for BPSK signals encoded in coherent states. However, due to technical constraints, proposals for reaching the bound remain impractical. In this letter we take an alternative approach: using only Gaussian optics - displaced squeezed states and homodyne detection - we achieve discrimination of BPSK signals with error rates below what can be achieved using coherent states and any quantum measurement.
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All authors Angus Walsh, Lorcan Conlon, Biveen Shajilal, Ozlem Erkilic, Jiri Janousek, Syed Assad, Jie Zhao, Ping Koy Lam

Wednesday 22 October

Quantum Physics · Cross-listed from cs.LG

KARIPAP: Quantum-Inspired Tensor Network Compression of Large Language Models Using Infinite Projected Entangled Pair States and Tensor Renormalization Group

Azree Nazri

Large Language Models (LLMs) like ChatGPT and LLaMA drive rapid progress in generative AI, yet their huge parameter scales create severe computational and environmental burdens. High training costs, energy use, and limited device deployment hinder accessibility. Existing compression - pruning, distillation, low-rank, and quantization - reduces size but ignores complex inter-layer correlations. We propose KARIPAP, a quantum-inspired tensor network compression using Infinite Projected Entangled Pair States (iPEPS) and Tensor Renormalization Group (TRG) contraction. Unlike 1D Matrix Product States, iPEPS captures multi-directional entanglement in attention and deep transformer layers. TRG ensures polynomial-time contraction, making tensorization feasible while preserving key correlation geometry. Experiments on LLaMA-2 7B show up to 93% memory and 70% parameter reduction, with 50% faster training, 25% faster inference, and only 2-3% accuracy loss. Layer-wise entanglement profiling reveals redundancy in deeper layers, confirming their suitability for tensor factorization. KARIPAP demonstrates that modern LLMs occupy low-dimensional entanglement manifolds, enabling scalable, energy-efficient, and quantum-aware AI architectures.
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Quantum Physics

Photon quantum mechanics with a position observable

Margaret Hawton

We second quantize an explicitly Lorentz invariant lagrangian density and derive a theory of photon quantum mechanics. The one photon Hilbert space is the vector space of normalizable positive frequency four-potentials. Observables are described by the Poincare operators augmented with a photon position operator. It is found that the probability amplitude to observe a photon in a bounded region of space, defined as the projection of the four-potential onto the basis of position eigenvectors, equals the inverse Fourier transform of the probability amplitude for a plane wave. A continuity equation that describes photon propagation in free space and an optical circuit is derived.
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Quantum Physics

Exact State Evolution and Energy Spectrum in Solvable Bosonic Models

Valery Shchesnovich

Solvable bosonic models provide a fundamental framework for describing light propagation in nonlinear media, including optical down-conversion processes that generate squeezed states of light and their higher-order generalizations. In quantum optics a central objective is to determine the time evolution of a given initial state. Exact analytic solution to the state-evolution problem is presented, applicable to a broad class of solvable bosonic models and arbitrary initial states. Moreover, the characteristic equation governing the energy spectrum is derived and the eigenstates are found in the form of continued fractions and as the principal minors of the associated Jacobi matrix. The results provide a solid analytical framework for discussion of exactly solvable bosonic models.
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Quantum Physics

On Encoding Matrices using Quantum Circuits

Liron Mor Yosef and Haim Avron

Over a decade ago, it was demonstrated that quantum computing has the potential to revolutionize numerical linear algebra by enabling algorithms with complexity superior to what is classically achievable, e.g., the seminal HHL algorithm for solving linear systems. Efficient execution of such algorithms critically depends on representing inputs (matrices and vectors) as quantum circuits that encode or implement these inputs. For that task, two common circuit representations emerged in the literature: block encodings and state preparation circuits. In this paper, we systematically study encodings matrices in the form of block encodings and state preparation circuits. We examine methods for constructing these representations from matrices given in classical form, as well as quantum two-way conversions between circuit representations. Two key results we establish (among others) are: (a) a general method for efficiently constructing a block encoding of an arbitrary matrix given in classical form (entries stored in classical random access memory); and (b) low-overhead, bidirectional conversion algorithms between block encodings and state preparation circuits, showing that these models are essentially equivalent. From a technical perspective, two central components of our constructions are: (i) a special constant-depth multiplexer that simultaneously multiplexes all higher-order Pauli matrices of a given size, and (ii) an algorithm for performing a quantum conversion between a matrix's expansion in the standard basis and its expansion in the basis of higher-order Pauli matrices.
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Quantum Physics

Quantum Nonlinear Response of Emitter Lattices

Blas Durá-Azorín, Antonio I. Fernández-Domínguez and Alejandro Manjavacas

We theoretically investigate the emergence of quantum nonlinearities in the optical response of lattices of two-level quantum emitters coherently driven by a laser. For subwavelength lattice periods, where the system behaves as a quantum metasurface, we find that a resonant incident plane wave can populate excitonic Bloch states with parallel wavevectors different from the incident field, including those lying outside the light cone. Closely related to resonance fluorescence, the far-field emission from the system in the strong-driving regime is dominated by a broadband background of photons spanning a wide range of frequencies and wavevectors. Moreover, we show that, for periods approaching the driving wavelength, the emitter lattice enters in a bistable regime due to the renormalization of the driving rate, in striking contrast with its classical (bosonic) analog. This bistable behavior enables the selective activation and deactivation of the optical quantum nonlinearities of the system.
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Quantum Physics

Reduced State Embedding for Error Correction in Quantum Cryptography

Amit Kam, Kfir Sulimany, Shai Tsesses et al. (+1 more)

Encoding in a high-dimensional Hilbert space improves noise resilience in quantum information processing. This approach, however, may result in cross-mode coupling and detection complexities, thereby reducing quantum cryptography performance. This fundamental trade-off between correctness and secrecy motivates the search for quantum error-correction approaches for cryptography. Here, we introduce state embeddings that use a k-symbol subset within a d-dimensional Hilbert space, tailored to the channel's error structure. In the framework of quantum error-correction, our reduced-state embedding realizes an explicit erasure-type error-correction within the quantum channel. We demonstrate the advantage of our scheme in realistic quantum channels, producing a higher secure key rate. We validate our approach using a d=25 quantum key distribution (QKD) experimental data, derive closed-form expressions for the key rate and threshold, and determine the optimum at k=5. These findings advance high-dimensional QKD and pave the way to error-correction and modulation for quantum cryptography.
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All authors Amit Kam, Kfir Sulimany, Shai Tsesses, Uzi Pereg

Quantum Physics

Separation of quantum time evolution into holonomic and dynamical parts

Adam Fredriksson and Erik Sjöqvist

The issue of separating Schrödinger-type quantum time evolution into a product of holonomic and dynamical parts in the non-adiabatic non-Abelian case is examined. We identify all special cases in which this kind of separation is possible, and we prove that separability is a gauge invariant property of quantum time evolution. The general analysis is implemented in a three-level system with $\Lambda$-type coupling structures. The typical case where the holonomic and dynamical parts do not separate is illustrated by means of two non-commuting $\Lambda$-type Hamiltonians.
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Quantum Physics

A transmon qubit realized by exploiting the superconductor-insulator transition

C. G. L. Bøttcher, E. Önder, T. Connolly et al. (+8 more)

Superconducting qubits are among the most promising platforms for realizing practical quantum computers. One requirement to create a quantum processor is nonlinearity, which in superconducting circuits is typically achieved by sandwiching a layer of aluminum oxide between two aluminum electrodes to form a Josephson junction. These junctions, however, face several limitations that hinder their scalability: the small superconducting gap of aluminum necessitates millikelvin operating temperatures, the material interfaces lead to dissipation, and the sandwich geometry adds unwelcome capacitance for high-frequency applications. In this work, we address all three limitations using a novel superconducting weak link based on the superconductor-insulator transition. By locally thinning a single film of niobium nitride, we exploit its thickness-driven superconductor-insulator transition to form a weak link employing only atomic layer deposition and atomic layer etching. We utilize our weak links to produce a transmon qubit, '$planaron$', with a measured anharmonicity of $\alpha/2\pi = 235$ MHz; at present, the linewidth is $\kappa/2\pi = 15 \mathrm{\: MHz}$. The high superconducting gap of niobium nitride can enable operation at elevated temperatures in future devices, and the fully planar geometry of the weak link eliminates superfluous material interfaces and capacitances. The investigation of small patches of material near the SIT can shed new light on the nature of the transition, including the role of dissipation and finite-size effects.
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All authors C. G. L. Bøttcher, E. Önder, T. Connolly, J. Zhao, C. Kvande, D. Q. Wang, P. D. Kurilovich, S. Vaitiekėnas, L. I. Glazman, H. X. Tang, M. H. Devoret

Quantum Physics

Morphological computational capacity of Physarum polycephalum

Suyash Bajpai, Aviva Lucas-DeMott, Nirosha J Murugan et al. (+2 more)

While computational capacity limits of the universe and carbon-based life have been estimated, a stricter bound for aneural organisms has not been established. Physarum polycephalum, a unicellular, multinucleated amoeba, is capable of complex problem-solving despite lacking neurons. By analyzing growth dynamics from high-throughput imaging data-more than 500 samples tracked every 30 minutes for one to three days-across two distinct Physarum strains under diverse biological conditions, we map morphological evolution to information processing. We revisit the Margolus-Levitin bound that constrains maximum computation rates, deriving a correction for these driven-dissipative living systems. Time-series data of Physarum's morphology-quantified through area, perimeter, circularity, and fractal dimension-enable the determination of upper bounds on the number of logical operations achievable through its hydromechanical, chemical, kinetic, and quantum-optical degrees of freedom. The time-series trends of these morphological quantities remain statistically robust across 100,000 bootstrap resamples at both 95% and 99.5% confidence intervals. Based on the organism's explored areas and characteristic values of ATP available for work, the computational capacity of Physarum is upper bounded by ${\sim}10^{33}$ logical operations in 24 hours, scaling linearly in the non-equilibrium steady state. This framework enables comparison of the computational capacities of life, exploiting either classical or quantum degrees of freedom.
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All authors Suyash Bajpai, Aviva Lucas-DeMott, Nirosha J Murugan, Michael Levin, Philip Kurian

Quantum Physics

Classical Gravity Cannot Mediate Entanglement by Local Means

Chiara Marletto and Vlatko Vedral

We rebut a recent paper that claims that classical gravity can entangle two massive superpositions by local means. We refute the misconceptions appearing in this paper and confirm that the quantum features are necessary in the gravitational field if it can lead to entanglement by local propagation between distant masses.
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