Quantum Physics · Cross-listed from physics.atom-ph
Rapid multi-mode trapped-ion laser cooling in a phase-stable standing wave
Zhenzhong Xing, Hamim Mahmud Rivy, Vighnesh Natarajan et al. (+3 more)
Laser cooling is fundamental to
quantum computing and metrology using atomic systems. Precise control often requires cooling atoms' motional degrees of freedom to the
quantum ground state, imposing operation time and architectural limitations particularly in large-scale systems. Here we demonstrate how the integrated optical control of interest for scaling trapped-ion systems additionally enables
laser cooling that bypasses limitations of conventional schemes. Leveraging multi-channel integrated delivery of ultraviolet to infrared wavelengths for calcium ion control including in passively phase-stable ultraviolet standing waves (SWs), we experimentally verify a long-standing prediction by Cirac et al., realizing Doppler cooling to below the conventional Doppler limit at a SW node. We also present the first realization of ground-state cooling via electromagnetically induced transparency (EIT) using a “probe" beam delivered as a SW with atoms positioned at a node, predicted to enable multi-mode sub-
recoil-limit
laser cooling. We demonstrate cooling of motional modes spanning an approximately 5 MHz bandwidth from the Doppler temperature to near the ground state within 150~\textmu s, reaching $\bar n \approx 0.05$
phonon number occupancies for the target mode. Direct evaluation against the comparable running-wave (RW) scheme shows the SW implementation's simultaneous advantage in cooling rate, motional mode bandwidth, and final
phonon number, as previously theoretically predicted. Our work leverages capabilities enabled by integrated optical delivery to demonstrate fast cooling of multiple modes to the
quantum ground state, and more broadly how scalable approaches to optical control can enable enhancements in fundamental atomic functionalities.
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All authors Zhenzhong Xing, Hamim Mahmud Rivy, Vighnesh Natarajan, Aditya Milind Kolhatkar, Gillenhaal Beck, Karan K. Mehta
Quantum Physics · Cross-listed from cond-mat.str-el
Hybrid quantum-classical matrix-product state and Lanczos methods for electron-phonon systems with strong electronic correlations: Application to disordered systems coupled to Einstein phonons
Heiko Georg Menzler, Suman Mondal and Fabian Heidrich-Meisner
We present two
quantum-classical hybrid methods for simulating the time-dependence of electron-
phonon systems that treat electronic correlations numerically exactly and optical-
phonon degrees of freedom classically. These are a time-dependent Lanczos and a
matrix-product state method, each combined with the multi-trajectory Ehrenfest approach. Due to the approximations, reliable results are expected for the adiabatic regime of small
phonon frequencies. We discuss the convergence properties of both methods for a system of interacting spinless
fermions in one dimension and provide a
benchmark for the Holstein chain. As a first application, we study the decay of charge density wave order in a system of interacting spinless
fermions coupled to Einstein oscillators and in the presence of quenched disorder. We investigate the dependence of the relaxation dynamics on the electron-
phonon coupling strength and provide numerical evidence that the coupling of strongly disordered systems to classical oscillators leads to delocalization, thus destabilizing the (finite-size) many-body localization in this system.
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Quantum Physics
Quantifying classical and quantum bounds for resolving closely spaced, non-interacting, simultaneously emitting dipole sources in optical microscopy
Armine I. Dingilian, Aarnah Kurella, Cheyenne S. Mitchell et al. (+3 more)
Recent theoretical and experimental work has shown that the
quantum Fisher information associated with estimating the separation between two optical point sources remains finite at small separations, effectively opening new routes to super-resolution imaging of simultaneously emitting sources. Most studies to date, however, implicitly invoke the scalar approximation, which is not appropriate in the context of high-numerical-aperture microscopy. Utilizing parameter estimation theory, here we consider the estimation of separation between two closely spaced dipole emitters, a commonly employed model for single-molecule optical beacons. We consider two limiting cases: one in which the orientations of the emitters are fixed and equal, and another in which both dipoles freely sample all of orientation space over the course of the measurement. We quantify precision limits using
quantum and classical variants of the Fisher information and Cramér-Rao bound. In all cases, the vectorial nature of the emission complicates the analyses, but with appropriate filtering of the collected light in the azimuthal-radial polarization basis, a previously proposed scheme to saturate the
quantum Fisher information via image inversion interferometry can be salvaged.
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All authors Armine I. Dingilian, Aarnah Kurella, Cheyenne S. Mitchell, Dhananjay Dhruva, David J. Durden, Mikael P. Backlund
Quantum Physics
ENTCALC: Toolkit for calculating geometric entanglement in multipartite quantum systems
Piotr Masajada, Aby Philip and Alexander Streltsov
We present entcalc, a Python and MATLAB package for estimating the
geometric entanglement of multipartite
quantum states. The package operates as follows: given a multipartite
quantum state as input, it outputs an estimate of its
geometric entanglement. For pure states, it computes the
geometric entanglement together with an estimation error. For mixed states, it provides both lower and upper bounds on the
geometric entanglement, thereby identifying an interval in which the true value lies. We provide several methods to compute the lower bound, enabling users to balance accuracy against computational cost. We apply entcalc to several representative examples, including for $3\otimes3$ PPT entangled states, mixtures of GHZ and W states, thermal states of selected three-
qubit spin chains, and noisy GHZ and W states. We observe signatures of
quantum phase transitions by quantifying
entanglement in
spin chains. We also demonstrate that
entanglement between non-neighbouring sites can be activated by tuning the external
magnetic field. In all tested cases, the gap between the lower and upper bounds is found to be very small, indicating that entcalc provides highly accurate estimates of the
geometric entanglement for these states.
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Quantum Physics
Julio Del Castillo, Mats Granath and Evert van Nieuwenburg
Quantum Imaginary-Time
Evolution (QITE) is a powerful method for preparing ground states on
quantum hardware. However, executing QITE has costly measurement budgets for general
Hamiltonians. Both fidelity and computational cost are strongly dependent on the definition of suitable local domains and
Hamiltonian partitions. In this work, we introduce the Multiple-Time QITE
algorithm (MT-QITE). We show how using more than one imaginary time substantially improves the fidelity of the resulting ground state as well as the measurement overhead with respect to the previously published QITE
algorithm, while preserving its deterministic character and its independence from ad hoc ansatze. Moreover, unlike QITE and other QITE-based
algorithms, MT-QITE is parallelizable, and we show that even in
Hamiltonians with non-local interactions, partitioning may entail a computational advantage.
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Quantum Physics
Optimized Measurement Schedules for the Surface Code with Dropout
Benjamin Anker and Dripto M. Debroy
Recent work has shown that fabrication defects can be well-handled using a strategy relying on the mid-error-correction-cycle state. In this work we present two improvements to the original prescription. First, we quantify the impact of the choice of a more complete set of gauge operators originally proposed for the hex-grid surface code on the standard square-grid surface code, as well as a new method for excising effectively unused
qubits. Second, we leverage the expressivity of the LUCI framework as an intermediate representation, using integer linear programming to find performant physical
circuits from the large space of valid LUCI
circuits. We show that on the $d = 11$ surface code at $1\%(3\%)$ dropout rate for
qubits and couplers, these optimizations allow for a total improvement of $14.5\%(23.6\%)$ over $4d$ round of syndrome extraction using the SI1000 noise model at $0.1\%$ noise.
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Quantum Physics · Cross-listed from math.OA
Metrics on completely positive maps via noncommutative geometry
Are Austad, Erik Bédos, Jonas Eidesen et al. (+2 more)
We study methods of inducing metrics on unital completely positive maps by employing seminorms arising in noncommutative geometry. Our main approach relies on the development of an infinite-dimensional $C^*$-algebraic analogue of the Choi-Jamiołkowski isomorphism. Under suitable conditions, we show that the induced metrics satisfy the
quantum information theoretic properties of stability and chaining. Moreover, we show how to generate such metrics using constructions native to noncommutative geometry, by for example using external Kasparov products of spectral triples.
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All authors Are Austad, Erik Bédos, Jonas Eidesen, Nadia S. Larsen, Tron Omland
Quantum Physics
Estimating Detector Error Models on Google's Willow
Kregg Elliot Arms, Martin James McHugh, Joseph Edward Nyhan et al. (+2 more)
We consolidate recent theoretical advances in Detector Error Model (DEM) estimation and formalize several
algorithms to learn DEM parameters and structure from syndromes without using a decoder, demonstrating recovery of known DEMs from simulated syndromes with precision limited only by finite-sample effects. We then apply these
algorithms to estimate DEMs from Google's 72- and 105-
qubit chips. Using a likelihood function that is tractable for small DEMs, we show that DEMs estimated directly from syndromes agree more closely with unseen syndromes than DEMs trained to optimize logical performance, whereas the latter outperform the former as priors for decoders in logical
memory experiments. We used a time-series of estimated DEMs to track both global error and specific local errors over the course of a QEC experiment, suggesting applications in online characterization. We employ a sequence of DEM estimation techniques to discover and quantify long-range detector correlations spanning the width of the 105-
qubit chip, for which DEM analysis suggests correlated measurement errors rather than high-weight Pauli errors as the most likely explanation. Finally, we present two artifacts in repetition code syndromes that are not well-modeled by a DEM: correlated flipping of pairs of adjacent detectors in many consecutive rounds of QEC, and signatures consistent with radiation events occurring more frequently than previously reported.
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All authors Kregg Elliot Arms, Martin James McHugh, Joseph Edward Nyhan, William Frederick Reus, James Loudon Ulrich
Quantum Physics
Quantum Approaches to Urban Logistics: From Core QAOA to Clustered Scalability
F. Picariello, G. Turati, R. Antonelli et al. (+10 more)
The Traveling Salesman Problem (TSP) is a fundamental challenge in combinatorial optimization, widely applied in logistics and transportation. As the size of TSP instances grows, traditional
algorithms often struggle to produce high-quality solutions within reasonable timeframes. This study investigates the potential of the
Quantum Approximate Optimization
Algorithm (QAOA), a hybrid
quantum-classical method, to solve TSP under realistic constraints. We adopt a QUBO-based formulation of TSP that integrates real-world logistical constraints reflecting operational conditions, such as vehicle capacity, road accessibility, and time windows, while ensuring compatibility with the limitations of current
quantum hardware. Our experiments are conducted in a simulated environment using high-performance computing (HPC) resources to assess QAOA's performance across different problem sizes and
quantum circuit depths. In order to improve scalability, we propose clustering QAOA (Cl-QAOA), a hybrid approach combining classical machine learning with QAOA. This method decomposes large TSP instances into smaller sub-problems, making
quantum optimization feasible even on devices with a limited number of
qubits. The results offer a comprehensive evaluation of QAOA's strengths and limitations in solving constrained TSP scenarios. This study advances
quantum optimization and lays groundwork for future large-scale applications.
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All authors F. Picariello, G. Turati, R. Antonelli, I. Bailo, S. Bonura, G. Ciarfaglia, S. Cipolla, P. Cremonesi, M. Ferrari Dacrema, M. Gabusi, I. Gentile, V. Morreale, A. Noto
Quantum Physics
Complexity and multi-functional variants of the Quantum-to-Quantum Bernoulli Factories
Francesco Hoch, Taira Giordani, Gonzalo Carvacho et al. (+2 more)
A Bernoulli factory is a model for randomness manipulation that transforms an initial Bernoulli random variable into another Bernoulli variable by applying a predetermined function relating the output bias to the input one. In literature,
quantum-to-
quantum Bernoulli factory schemes have been proposed, which encode both the input and output variables using
qubit amplitudes. This fundamental concept can serve as a subroutine for
quantum algorithms that involve
Bayesian inference and Monte Carlo methods, or that require data encryption, like in blind
quantum computation. In this work, we present a characterisation of the complexity of the
quantum-to-
quantum Bernoulli factory by providing a lower bound on the required number of
qubits needed to implement the protocol, an upper bound on the success probability and the
quantum circuit that saturates the bounds. We also formalise and analyse two different variants of the original problem that address the possibility of increasing the number of input biases or the number of functions implemented by the
quantum-to-
quantum Bernoulli factory. The obtained results can be used as a framework for randomness manipulation via such an approach.
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All authors Francesco Hoch, Taira Giordani, Gonzalo Carvacho, Nicolò Spagnolo, Fabio Sciarrino
Quantum Physics · Cross-listed from cond-mat.str-el
Phase structure of the one-dimensional $\mathbb{Z}_2$ lattice gauge theory with second nearest-neighbor interactions
Yeimer Zambrano, Aleksey Alekseev, Konrad J. Kapcia et al. (+2 more)
We investigate the ground-state phase diagram of a one-dimensional $\mathbb{Z}_2$
lattice gauge theory (LGT) model with hard-core
bosons at half-filling, extending previous studies by including second nearest-neighbor (2NN) interactions. Using
matrix product state techniques within the density
matrix renormalization group, we compute charge gap, static structure factor, pair-pair correlation functions, and
entanglement entropy for various interaction strengths and field parameters. We analyze two representative neatest-neighbor interaction strengths ($V_1$) that correspond to the Luttinger
liquid (LL) and Mott
insulator (MI) phases in the absence of the 2NN interactions. We introduce the 2NN coupling $V_2$ and investigate its impact on the system. Our results reveal very rich behavior. As the 2NN repulsion increases, in the case of small $V_1$, we observe a direct transition from the LL phase to a charge-ordered
insulator (COI) phase with four-site ordering pattern, whereas for large $V_1$, we observe a transition from the MI phase with two-site ordering pattern (previously found with only $V_1$ included), going through an intermediate LL region, and finally reaching the COI regime. Additionally, the inclusion of 2NN interactions enhances charge order and suppresses pair coherence, evidenced by sharp peaks in the structure factor and rapid decay in pair-pair correlators. Our work extends the well-studied phase structure of 1D $\mathbb{Z}_2$ LGT models and demonstrates the interplay between gauge fields, confinement, and extended interactions.
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All authors Yeimer Zambrano, Aleksey Alekseev, Konrad J. Kapcia, Krzysztof Cichy, Agnieszka Cichy
Quantum Physics
Understanding Surface-Induced Decoherence of NV Centers in Diamond
Jonah Nagura, Mykyta Onizhuk and Giulia Galli
Nitrogen vacancy centers (NV) in proximity to diamond surfaces are promising nanoscale
quantum sensors. However, their coherence properties are negatively affected by
magnetic and electric surface noise, whose origin and detailed impact have remained elusive. Using atomistic models of diamond surfaces derived with density functional theory, together with decoherence time calculations with cluster correlation expansion methods, we quantify the effects of surface crystallographic orientation and functionalization, and of the density of unpaired electrons on the NV Hahn-echo time $T_2$. We determine a crossover depth at which $T_2$ ceases to be limited by surface
nuclear spins and recovers the bulk-limited value. We find that for static surface-electron baths, the ratio between the NV depth and the separation between surface electron
spins determines a transition from fast-fluctuating to quasi-static noise, leading to a dependence of $T_2$ on orientation for specific surfaces. We also find that the modulation of $T_2$ by
spin-
phonon relaxations leads to motional-narrowing at sub-microsecond relaxation times. Importantly, our calculations show that it is only when accounting for surface-
spin in-sequence hopping that measured $T_2$ values as a function of depth can be reproduced, thus highlighting the importance of hopping-mediated models to describe the surface
spin noise affecting NV sensors. Overall, our work provides clear guidelines for engineering diamond surfaces to achieve enhanced NV coherence for
quantum sensing and information processing applications.
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Quantum Physics
Further Statistical Study of NISQ Experiments
Gil Kalai, Tomer Shoham and Carsten Voelkmann
We revisit and extend some topics that we studied in our previous works (Rinott, Kalai and Shoham 2022; Kalai, Rinott and Shoham, 2023,2024) regarding the Google 2019 "
quantum supremacy" experiment. We extend our analysis of the prediction based on Google's digital error model (Formula (77)), based on more detailed data provided by Google. We also provide some preliminary analysis for a few other NISQ experiments.
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Quantum Physics
Scalable Optical Links for Controlling Bosonic Quantum Processors
Chuanlong Ma, Jia-Qi Wang, Linze Li et al. (+13 more)
Superconducting
quantum computing has the potential to revolutionize computational capabilities. However, scaling up large
quantum processors is limited by the cumbersome and heat-conductive electronic cables that connect room-temperature control electronics to
quantum processors, leading to significant signal attenuation. Optical fibers provide a promising solution, but their use has been restricted to controlling simple two-level
quantum systems over short distances. Here, we demonstrate optical control of a bosonic
quantum processor, achieving universal operations on the joint
Hilbert space of a transmon
qubit and a storage cavity. Using an array of cryogenic fiber-integrated uni-traveling-carrier photodiodes, we prepare Fock states containing up to ten
photons. Additionally, remote control of bosonic modes over a transmission distance of 15 km has been achieved, with fidelities exceeding 95%. The combination of high-dimensional
quantum control, multi-channel operation, and long-distance transmission addresses the key requirements for scaling superconducting
quantum computers and enables architectures for distributed
quantum data centers.
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All authors Chuanlong Ma, Jia-Qi Wang, Linze Li, Jiajun Chen, Xiaoxuan Pan, Zheng-Hui Tian, Zheng-Xu Zhu, Jia-Hua Zou, Dingran Gu, Luyu Wang, Qiushi Chen, Weiting Wang, Xin-Biao Xu, Chang-Ling Zou, Baile Chen, Luyan Sun
Quantum Physics
Sub-Bath Cooling in Bosonic Systems: Gaussian Constraints and Non-Gaussian Enhancements
Wen-Han Png, Xueyuan Hu and Valerio Scarani
Cooling
quantum systems with finite resources is a central task in
quantum technologies and has been extensively explored in discrete-variable settings. As continuous-variable (CV) platforms play an increasingly important role in
quantum information processing, it becomes crucial to understand the fundamental limitations of cooling bosonic systems. In this work, we develop a general framework for cooling CV systems, identifying both the constraints imposed by Gaussianity and the advantages enabled by non-Gaussian interactions. We derive a reachable bound on the cooling performance of Gaussian operations that applies to arbitrary cooling architectures. By optimizing over all protocols saturating this bound, we further identify the most efficient scheme, which minimizes dissipated energy for a given number of ancilla modes. Beyond Gaussian operations, we show that $p$-excitation exchange exploits non-Gaussian resources to achieve a $p$-fold enhancement of the cooling limit. Our results establish the fundamental limits of CV heat-bath algorithmic cooling and reveal the crucial role of non-Gaussianity in surpassing Gaussian cooling barriers.
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Quantum Physics
Advantage in distributed quantum computing with slow interconnects
Evan E Dobbs, Nicolas Delfosse and Aharon Brodutch
The main bottleneck for distributed
quantum computing is the rate at which
entanglement is produced between
quantum processing units (QPUs). In this work, we prove that multiple QPUs connected through slow interconnects can outperform a monolithic architecture made with a single QPU. We consider a distributed
quantum computing model with the following assumptions: (1) each QPU is linked to only two other QPUs, (2) each link produces only one Bell pair at a time, (3) the time to generate a Bell pair is $\tau_e$ times longer than the gate time. We propose a distributed version of the CliNR partial error correction scheme respecting these constraints and we show through
circuit level simulations that, even if the
entanglement generation time $\tau_e$ is up to five times longer than the gate time, distributed CliNR can achieve simultaneously a lower logical error rate and a shorter depth than both the direct implementation and the monolithic CliNR implementation of random Clifford
circuits. In the asymptotic regime, we relax assumption (2) and we prove that links producing $O(t/\ln t)$ Bell pairs in parallel, where $t$ is the number of QPUs, is sufficient to avoid stalling distributed CliNR, independently of the number of
qubits per QPU. This demonstrates the potential of distributed CliNR for near-term multi-QPU devices. Moreover, we envision a distributed
quantum superiority experiment based on the conjugated Clifford
circuits of Bouland, Fitzsimons and Koh implemented with distributed CliNR.
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Quantum Physics
A Cryogenic Muon Tagging System Based on Kinetic Inductance Detectors for Superconducting Quantum Processors
Ambra Mariani, Laura Cardani, Mustafa Bal et al. (+16 more)
Ionizing radiation has emerged as a potential limiting factor for superconducting
quantum processors, inducing quasiparticle bursts and correlated errors that challenge fault-tolerant operation. Atmospheric
muons are particularly problematic due to their high energy and penetration power, making passive shielding ineffective. Therefore, monitoring the real-time
muon flux is crucial to guide the development of alternative error-correction or mitigation strategies. We present the design, simulation, and first operation of a cryogenic
muon-tagging system based on Kinetic Inductance Detectors (KIDs), developed as a stand-alone cryogenic particle-tagging module for superconducting
quantum processors. The system consists of two KIDs arranged in a vertical stack and operated at $\sim$20 mK. Monte Carlo simulations based on Geant4 guided the prototype design and provided reference expectations for
muon-tagging efficiency and accidental coincidences due to ambient $\gamma$-rays. We observed a
muon-induced coincidence rate among the top and bottom detectors of (192 $\pm$ 9)$\times10^{-3}$ events/s, in excellent agreement with the Monte Carlo prediction. The prototype achieves a
muon-tagging efficiency of about 90% with negligible dead time. These results demonstrate the feasibility of operating a
muon-tagging system at millikelvin temperatures and represent a key step toward the integration of cryogenic veto systems with multi-
qubit chips to mitigate
muon-induced errors.
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All authors Ambra Mariani, Laura Cardani, Mustafa Bal, Nicola Casali, Ivan Colantoni, Angelo Cruciani, Giorgio Del Castello, Daniele Delicato, Francesco De Dominicis, Matteo del Gallo Raccagiovine, Matteo Folcarelli, Sabrina Garattoni, Anna Grassellino, Mehmood Khan Yasir Raja, Valerio Pettinacci, Alberto Ressa, Tanay Roy, Marco Vignati, David van Zanten
Quantum Physics
Pulsed learning for quantum data re-uploading models
Ignacio B. Acedo, Pablo Rodriguez-Grasa, Pablo Garcia-Azorin et al. (+1 more)
While
Quantum Machine Learning (QML) holds great potential, its practical realization on Noisy Intermediate-Scale
Quantum (NISQ) hardware has been hindered by the limitations of variational
quantum circuits (VQCs). Recent evidence suggests that VQCs suffer from severe trainability and noise-related issues, leading to growing skepticism about their long-term viability. However, the possibility of implementing learning models directly at the pulse-control level remains comparatively unexplored and could offer a promising alternative. In this work, we formulate a pulse-based variant of data re-uploading,
embedding trainable parameters directly into the native system's dynamics. We
benchmark our approach on a simulated superconducting transmon processor with realistic noise profiles. The pulse-based model consistently outperforms its gate-based counterpart, exhibiting higher test accuracy and improved generalization under equivalent noise conditions. Moreover, by systematically increasing noise strength, we show that pulse-level implementations retain higher fidelity for longer, demonstrating enhanced resilience to decoherence and control errors. These results suggest that pulse-native architectures, though less explored, may offer a viable and hardware-aligned path forward for practical QML in the NISQ era.
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All authors Ignacio B. Acedo, Pablo Rodriguez-Grasa, Pablo Garcia-Azorin, Javier Gonzalez-Conde
Quantum Physics
All-photonic entanglement swapping with remote quantum dots
Mattia Beccaceci, Giuseppe Ronco, Fabrizio Cienzo et al. (+16 more)
Entanglement swapping is a protocol that details how to create
entanglement between previously uncorrelated particles. Its all-photonic version - mediated by the interference of
photon pairs generated by separate
quantum systems-finds disparate applications in
quantum networks. So far, all-photonic
entanglement swapping between remote systems has been implemented only using sources that operate probabilistically. However, the scaling up of
quantum networks requires deterministic
quantum emitters that do not suffer from a trade-off between degree of
entanglement and photonpair generation rate. Here, we demonstrate all-photonic
entanglement swapping using
photon-pairs generated by two separate GaAs
quantum dots. The emitters are deterministically embedded in hybrid
semiconductor-piezoelectric devices that make the entangled-
photons from two dissimilar
quantum dots nearly identical.
Entanglement swapping is demonstrated with a fidelity as high as 0.71(2), more than 10 standard deviations above the classical limit. The experimental data are quantitatively explained by a theoretical model that also suggests how to boost the protocol performances. Our work opens the path to the exploitation of
quantum dot entangled-
photon sources in
quantum repeater networks.
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All authors Mattia Beccaceci, Giuseppe Ronco, Fabrizio Cienzo, Pierpaolo Bassetti, Alessandro Laneve, Francesco Basso Basset, Tobias M. Krieger, Qurin Buchinger, Francesco Salusti, Barbara Souza Damasceno, Silke Kuhn, Saimon F. Covre da Silva, Sandra Stroj, Klaus D. Jöns, Sven Höfling, Tobias Huber-Loyola, Armando Rastelli, Michele B. Rota, Rinaldo Trotta
Quantum Physics
Efficient simulation of low-entanglement bosonic Gaussian states in polynomial time
Tong Liu, Hui-Ke Jin, Tao Xiang et al. (+1 more)
Bosonic Gaussian states are ubiquitous in
quantum optics and condensed matter physics. While they are efficiently handled within the Gaussian formalism, sampling requires calculating amplitudes in the
boson occupation basis. This step, however, is hindered by a significant bottleneck due to the hafnian. We present an efficient
algorithm that converts pure bosonic Gaussian states into
matrix product states (MPSs), thereby establishing a versatile tool for probing bosonic Gaussian systems in settings where direct Gaussian-formalism-based calculations become inefficient. Our method combines a Gaussian singular value decomposition with a projected-creation-operator mapping that constructs local MPS tensors without computing hafnians. Benchmarking on covariance
matrices from the Jiuzhang 2.0 and Jiuzhang 4.0 Gaussian
boson sampling experiments demonstrates substantial speedups over previous tensor-network approaches in the low-
entanglement regime relevant to lossy devices. The method provides a scalable classical simulation framework for bosonic Gaussian states with limited
entanglement. In this regime, a target accuracy can be achieved with a bond dimension that remains computationally tractable, thereby extending the applicability of MPS-based methods to a broad range of bosonic systems.
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All authors Tong Liu, Hui-Ke Jin, Tao Xiang, Hong-Hao Tu
Quantum Physics · Cross-listed from cond-mat.mes-hall
Edge states of a Bi$_2$Se$_3$ nanosheet in a perpendicular magnetic field
Stan P. J. Koenis, Lucas Maisel Licerán and Henk T. C. Stoof
Conventional wisdom dictates that the conducting edge states of two-dimensional topological
insulators of the Bi$_2$Se$_3$ family are protected by time-reversal
symmetry. However, theoretical bulk calculations and a recent experiment show that the edge states persist in the presence of large external
magnetic fields. To address this apparent contradiction, we have developed an analytical description for the edge-state wave function of a semi-infinite sample in a perpendicular
magnetic field. Our description relies on the usual bulk Landau levels, together with additional states arising due to the presence of the hard wall, which are unnormalizable in the infinite system. The analytical wave functions agree extremely well with numerical calculations and can be used to directly analyze the behavior of the edge states in a
magnetic field.
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Quantum Physics
Topology-Guided Quantum GANs for Constrained Graph Generation
Tobias Rohe, Markus Baumann, Michael Poppel et al. (+3 more)
Quantum computing (QC) promises theoretical advantages, benefiting computational problems that would not be efficiently classically simulatable. However, much of this theoretical speedup depends on the
quantum circuit design solving the problem. We argue that QC literature has yet to explore more domain specific ansatz-
topologies, instead of relying on generic, one-size-fits-all architectures. In this work, we show that incorporating task-specific inductive biases – specifically
geometric priors – into
quantum circuit design can enhance the performance of hybrid
Quantum Generative
Adversarial Networks (QuGANs) on the task of generating geometrically constrained K4 graphs. We evaluate a portfolio of
entanglement topologies and loss-function designs to assess their impact on both statistical fidelity and compliance with
geometric constraints, including the Triangle and Ptolemaic inequalities. Our results show that aligning
circuit topology with the underlying problem structure yields substantial benefits: the Triangle-
topology QuGAN achieves the highest
geometric validity among
quantum models and matches the performance of classical Generative
Adversarial Networks (GAN). Additionally, we showcase how specific architectural choices, such as entangling gate types, variance regularization and output-scaling govern the trade-off between
geometric consistency and distributional accuracy, thus emphasizing the value of structured, task-aware
quantum ansatz-
topologies.
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All authors Tobias Rohe, Markus Baumann, Michael Poppel, Gerhard Stenzel, Maximilian Zorn, Claudia Linnhoff-Popien
Quantum Physics
Insensitivity points and performance of open quantum interferometers under number-conserving & non-conserving Lindblad dynamics
Tommaso Favalli, Žan Kokalj and Andrea Trombettoni
We investigate the phase sensitivity of a linear two-mode atom interferometer subject to environmental noise, modeled within the framework of open
quantum systems with both number-conserving and non-conserving Lindblad operators. Considering several input states, we first study the cases N=1,2 (N number of particles) and perform numerical simulations for N>2. The sensitivity as a function of the holding time can display divergence points where phase estimation becomes impossible, to which we refer as insensitivity points. We characterize their behavior as the input state, particle number, and noise operator are varied, and we find that their positions are independent of the noise intensity. Moreover, while our fixed measurement scheme may favor number-conserving noise at small N (i.e., having better sensitivity), the Cramér-Rao bound reveals that particle non-conserving noise yields strictly lower achievable sensitivity for all particle numbers.
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Quantum Physics
Quantum-Amplified M/G/1/K Simulation: A Comparator-Controlled Framework for Arbitrary Service Distributions
Or Peretz, Michal Koren and Nir Perel
Finite-capacity single-server queues with general service-time distributions form the backbone of numerous real-world systems, yet classical simulation of performance metrics such as blocking probabilities and delay becomes computationally prohibitive as service variability or required precision increases. This work presents the first coherent
quantum circuit for simulating an M/G/1/K queue under arbitrary service-time laws. The
circuit encodes the service distribution through a logarithmic-depth ladder of $R_y$ rotations and enforces buffer constraints via a comparator-controlled phase gate, while preserving the quadratic speed-up of amplitude amplification. Grover iterations center on estimating the expected number of customers in the system, yielding provable $O(\sqrt{N})$ variance reduction and closed-form confidence bounds, where $N$ denotes the number of shots. Empirical evaluations on IBM
quantum simulators across four service distributions and three traffic intensities demonstrate fidelity above 0.99 with four
qubits and above 0.76 with ten
qubits, with Jensen-Shannon divergence below 0.11. Waiting-time estimation errors decrease by an order of magnitude as system load approaches capacity and remain within 3% in high-traffic regimes using registers of up to 63
qubits. These results establish the first end-to-end
quantum simulation framework for finite-buffer, non-Markovian queueing systems and provide a concrete foundation for
quantum-accelerated performance analysis in service-oriented architectures.
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Quantum Physics
Sensitivity threshold defines the optimal spin subset for ensemble quantum sensing
Suwan I. Kang, Minhyeok Kim, Sanghyo Park et al. (+3 more)
Inhomogeneous broadening and spatial gradients in control fields inevitably produce large variations in characteristic sensitivity across
spin ensembles. We derive an analytic expression for the sensitivity of an inhomogeneous ensemble and introduce a sensitivity threshold that identifies the optimal subset of
spins. For both pulsed and continuous-wave magnetometry, the optimal ensembles deliver up to an eightfold improvement over conventional schemes relying on nominally uniform regions of the ensemble. We demonstrate phase-only digital holography to implement the optimal ensembles and show that the measured illumination non-uniformity limits the sensitivity by 14%. This approach enables optimal
quantum sensing in optically scattering environments.
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All authors Suwan I. Kang, Minhyeok Kim, Sanghyo Park, Heonsik Lee, Keunyoung Lee, Donggyu Kim
Quantum Physics
Hybrid Quantum Annealing Approach for High-Dimensional and Multi-Criteria Constrained Quadratic Optimization in Arctic Ship Routing
Tara Kit, Kimsay Pov, Myeongseong Go et al. (+5 more)
The opening of Arctic sea routes presents unprecedented opportunities for global trade but poses significant operational and computational challenges due to the dynamic nature of sea ice conditions. This study formulates a multi criteria Arctic route optimization problem that integrates Copernicus Marine Environment Monitoring Service (CMEMS) variables into a Constrained Quadratic Model (CQM) and solves it using D Wave's hybrid
quantum classical solver. We
benchmark the feasibility and scalability of this approach against classical Mixed Integer Quadratic Programming (MIQP) solvers such as Gurobi and CPLEX. Results show that the CQM formulation achieves feasible solutions with stable runtimes as quadratic density increases, demonstrating 10 to 100 times faster convergence and reduced computational time compared with classical solvers, while also improving route smoothness by approximately 10 percent and reducing total length by approximately 1 percent. This reflects the effectiveness of the hybrid
quantum annealing approach for Arctic routing problems.
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All authors Tara Kit, Kimsay Pov, Myeongseong Go, Leanghok Hour, Arim Ryou, Kiwoong Kim, Tae-Kyung Kim, Youngsun Han
Quantum Physics
Encoding parameters by measurement: Forgetting can be better in quantum metrology
Shuva Mondal, Priya Ghosh and Ujjwal Sen
We introduce
quantum parameter estimation with the encoding being via a
quantum measurement. We quantify the precision for estimating parameters characterizing a general two-outcome
qubit measurement, considering two cases: when the outcomes of the encoding measurement are recorded and when the same are ignored. We find that in a large variety of such estimation scenarios, forgetting the outcomes yields higher precision. We derive a necessary criterion under which remembering the measurement outcomes provides better precision in comparison to the outcome-forgotten strategy. Furthermore, we establish a necessary and sufficient criterion for the simultaneous estimation of multiple parameters encoded by an arbitrary
quantum process, including those involving measurements, using
qubit probes, and find when the
quantum Cramér$-$Rao bound is valid and achievable. For simultaneous estimation of two parameters characterizing the measurement, we find that the achievable
quantum Cramér$-$Rao bound can be a valid precision bound only when the measurement direction depends on the parameters of interest.
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Quantum Physics · Cross-listed from physics.atom-ph
Rapid all-optical loading of trapped ions using a miniaturised atom source
Lorenzo Versini, Tim F. Wohlers-Reichel, Catherine E. J. Challoner et al. (+8 more)
We characterise an efficient optically-heated neutral atom source for ion trapping. We observe loading rates of up to $24(3)\,\mathrm{s}^{-1}$ with heating powers below $85\,\mathrm{mW}$, and demonstrate loading of a single ion in under $30\,\mathrm{s}$ with $41.4(4)\,\mathrm{mW}$ of optical power in a room-temperature ion trap system with an ionisation probability of $1.50(5)\times10^{-5}$. We calibrate a thermal model for the source's internal temperature by imaging the fluorescence of a collimated flux of neutral calcium that effuses from the oven at various optical heating powers. We show that the thermal performance of this oven is mainly limited by radiative losses. We explore the effect of second-stage photo-ionisation
laser power on the loading rate, and identify a path beyond the loading rates reported in this study. We predict that this source is also well-suited to a wide range of metals used in ion-trapping.
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All authors Lorenzo Versini, Tim F. Wohlers-Reichel, Catherine E. J. Challoner, Thomas Hinde, Arjun D. Rao, William J. Hughes, Peter Drmota, Thomas H. Doherty, Laurent J. Stephenson, Jacob A. Blackmore, Joseph F. Goodwin
Quantum Physics
Tianyan: Cloud services with quantum advantage
Tianyan Quantum Group
Tianyan
Quantum Cloud Platform offers cloud services demonstrating
quantum advantage capabilities with a Zuchongzhi 3.0-like superconducting
quantum processor. This cloud-accessible superconducting
quantum prototype, named Tianyan-287, features 105
qubits and achieves high operational fidelities, with single-
qubit gates, two-
qubit gates, and readout fidelity at 99.90%, 99.56%, 98.7%, respectively. For a specific
benchmark task involving random
circuit sampling on a 74-
qubit system over 24 cycles, the platform completes one million samples in just 18.4 minutes. In contrast, state-of-the-art classical supercomputers would require approximately 16,000 years to complete the equivalent calculation. To facilitate this, the platform provides access via Cqlib, an open-source SDK designed for working with
quantum systems at the level of extended
quantum circuits, operators, and primitives. The cloud service aims to democratize access to high-performance
quantum hardware, enabling the community to validate and explore practical
quantum advantages.
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Quantum Physics
The relativistic reason for quantum probability amplitudes
Karol Sajnok, Kacper Dębski and Andrzej Dragan
We show that the
quantum-mechanical probability distribution involving complex probability amplitudes can be derived from three natural conditions imposed on a relativistically invariant probability function describing the motion of a particle that can take multiple paths simultaneously. The conditions are: (i) pairwise Kolmogorov additivity, (ii) time
symmetry, and (iii) Bayes' rule. The resulting solution, parameterized by a single constant, is the squared modulus of a sum of complex exponentials of the relativistic action, thereby recovering the Feynman path-integral formulation of
quantum mechanics.
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