Quantum Physics
Efficient and SPAM-Robust Ansatz-Free Lindbladian Learning
Savar D. Sinha
Describing the dynamics of open systems is essential for fault-tolerant
quantum computation. Under Markovian assumptions, we can characterize dissipative dynamics via the Lindbladian. Using Bell sampling, we provide an efficient, ansatz-free Lindbladian learning
algorithm with
polynomial-time classical postprocessing. Motivated by the prevalence of state preparation and measurement (SPAM) noise on near-term devices, we also introduce the first efficient SPAM-robust protocol capable of learning the gauge-independent components of sparse Lindbladians to arbitrary precision in the presence of constant-order SPAM error. In doing so, we provide the first rigorous characterization of the gauge degrees of freedom in noisy Lindbladian learning, precisely identifying which components remain learnable under SPAM noise.
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Quantum Physics
Time-spectral control of accidental coincidences in daylight entanglement-based free-space QKD
Jiyoung Moon, Yonggi Jo, Zaeill Kim et al. (+2 more)
Daylight
entanglement-based free-space
quantum key distribution (QKD) is limited by accidental coincidences from receiver-admitted background light. We develop and experimentally validate a receiver-level framework anchored to measured singles rates and reference coincidence components, linking receiver bandwidth, accepted temporal width, and background-noise density to Bob singles, sifted-key rate, error rate, and
quantum bit error rate (QBER) in telecom-wavelength BBM92 QKD. Indoor sweeps show that the sifted-key rate saturates near the source-matched bandwidth, whereas broader bandwidth or higher background mainly increases accidental contamination. Increasing the accepted temporal width leaves Bob singles nearly unchanged but directly raises QBER by enlarging the random-overlap probability. A two-dimensional design map calculated from the model shows that the temporal-window margin contracts rapidly with increasing background-to-signal ratio, while the bandwidth margin remains comparatively broad near source-matched filtering. A rooftop experiment demonstrates daylight operation over a 10 m free-space link, yielding a mean sifted-key rate of 2,811 cps and a mean QBER of 4.43%. This framework provides a quantitative basis for choosing receiver bandwidth and temporal acceptance to meet a target QBER under specified background conditions.
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All authors Jiyoung Moon, Yonggi Jo, Zaeill Kim, Yong Sup Ihn, Nam Hun Park
Quantum Physics
Canonical regularization of the stationary Coulomb problem and an Aufbau-like spectral ordering
Anand Aruna Kumar
The stationary hydrogen atom has Coulomb degeneracy across orbital levels, whereas the Aufbau/Madelung ordering is an empirical, many-electron rule established in atomic physics. We examine the hydrogen atom through a regularized de Broglie–Bohm representation, in which stationary amplitude current constraints generate separable Sturm–Liouville branches. In this formulation, the radial, orbital, and
magnetic sectors acquire canonical Langer-like inverse square corrections. The modified boundary value problems allow analytical solutions and produce a hydrogen-like spectrum with regularized radial and angular indices. Consequently, radial Coulomb quantization acquires an orbital dependent shift, lifting the Coulomb degeneracy and producing a spectral ordering that follows the Aufbau/Madelung sequence. On this basis, we construct the ordering of the regularized de Broglie–Bohm states and show that the spectral structure retains the standard degenerate Rydberg sequence in the l=0 sector. The separated amplitudes are represented by generalized special function branches, including the associated Laguerre, Legendre, and Bessel functions with non-integral parameters arising from regularized separation. Therefore, the treatment is intended as an analytical examination of spectral ordering in a regularized one center Coulomb problem rather than as a replacement for the many electron atomic structure theory. Keywords: de Broglie–Bohm representation; Coulomb spectrum; canonical regularization; Langer correction; Sturm–Liouville equations; Aufbau principle; Madelung ordering; associated Legendre functions; associated Laguerre functions; Bessel functions.
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Quantum Physics
Pulse-optimised circuit elements for scalable and noise-resilient quantum chemistry
Henrik Gothen, Christopher K. Long, Djamila Hiller et al. (+4 more)
Useful chemistry calculations on near-term
quantum processors are hindered by current algorithmic runtimes. We develop a methodology to significantly reduce these runtimes. Typically, variational
quantum eigensolver (VQE)
algorithms are implemented as sequences of primitive gates. Our methodology instead relies on gradient-ascent pulse engineering to construct hardware-tailored pulses for the direct implementation of VQEs. As problem sizes increase, it quickly becomes intractable to optimise a pulse that implements an entire VQE ansatz
circuit. However, leading VQEs are constructed in a modular fashion. A problem-tailored VQE is assembled from parameterised
circuit elements that simulate hopping between two or four electronic
spin orbitals. We show that these
circuit elements can be implemented more efficiently using hardware-tailored pulses. We numerically demonstrate our methodology on a silicon
spin-
qubit quantum processor. We find that common
circuit elements, known as single- and double-
qubit excitations, can be implemented in less than 289 ns and 927 ns, respectively. Compared with conventional gate-based implementations, our pulse-accelerated
qubit excitations provide a scalable approach for faster and therefore more noise-robust
quantum chemistry simulations by reducing VQE runtimes by up to a factor of 15.3.
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All authors Henrik Gothen, Christopher K. Long, Djamila Hiller, Yunming Qian, Crispin H. W. Barnes, Normann Mertig, David R. M. Arvidsson-Shukur
Quantum Physics
Effects of Josephson Junction Non-idealities on Adiabatic Quantum Flux Parametron Circuits
Daryoush Shiri, Likai Yang, Mohamed A. Hassan et al. (+2 more)
Adiabatic
quantum flux parametron (AQFP) gate is a promising approach to scale up the cryogenic microwave electronics for superconducting
qubit multiplexed control. However, the performance of these
circuits depends on the quality of the Josephson junctions which are ideally
superconductor-
insulator-
superconductor (SIS) type following the ideal sinusoidal relation between current and
quantum phase. We demonstrate how the non-sinusoidal current-phase relation in
Superconductor-Normal metal-
Superconductor (SNS) and weak link (WL) junctions affects the speed, delay, and margin of the AQFP gates. The JJ models are defined in the Keysight ADS simulator using symbolically defined device (SDD) method.
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All authors Daryoush Shiri, Likai Yang, Mohamed A. Hassan, Philip Krantz, Eric T. Holland
Quantum Physics · Cross-listed from cond-mat.mtrl-sci
Robust Spin Splitting and Strain-Controlled Optical Response in Monolayer CrC2N4 for Valleytronic and Optoelectronic Applications
Md. Samrat, Vivek Chowdhury, Sake Wang et al. (+1 more)
Monolayer CrC2N4 recently emerged as a promising two-dimensional
semiconductor, yet its
spin-orbit-coupled (SOC) physics and strain-tunable optical response remained largely unexplored. Here, we investigated the electronic, valley, charge-transfer, and optical properties of pristine and biaxially strained monolayer CrC2N4 using first-principles calculations. The monolayer exhibited a direct band gap at the K/K' valleys. SOC produced valley contrasting out-of-plane
spin polarization, yielding a moderate valence band
spin splitting of 51.9 meV and a small conduction band
spin splitting of 1.7 meV. Orbital-resolved analysis showed that the edge states were mainly governed by Cr-d and N-p hybridization, while Bader analysis indicated polar-covalent bonding through charge transfer toward N atoms. Biaxial strain in the range of -4% to +4% tuned the band gap from 1.987 to 1.421 eV and drove an indirect-to-direct gap transition near -1% strain. Tensile strain enhanced the Berry
curvature and red-shifted the optical response toward the visible-near-infrared region. These results suggested monolayer CrC2N4 as a promising platform for strain-engineered valleytronic and optoelectronic device applications.
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All authors Md. Samrat, Vivek Chowdhury, Sake Wang, Ahmed Zubair
Quantum Physics
Induced Resource Theories and Harvesting via Quantum Probes
Ron Nyström, Simone Cepollaro, Nicola Pranzini et al. (+3 more)
We consider scenarios in which a
quantum system with a well-defined resource theory is used as a probe to interact with an environment, such as a
quantum field, for which a resource-theoretic description is absent or incomplete. We clarify if and how the harvesting of a resource in the probe can tell us about the state of the environment. This is particularly ambiguous when the probe-environment interaction is not a free operation, or the concept of such free operations cannot be defined altogether. We propose a framework and precise conditions under which it becomes possible to interpret resource generation on the probe as evidence of resources in the environment, thereby introducing an effective notion of resources for the latter. Our results clarify in which sense resources can be said to be harvested from the environment and provide a systematic way to analyse such processes beyond fully controlled resource-theoretic settings. More generally, this work may provide a step towards a more general understanding of the interplay of different
quantum resources.
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All authors Ron Nyström, Simone Cepollaro, Nicola Pranzini, Stefano Cusumano, Alioscia Hamma, Esko Keski-Vakkuri
Quantum Physics
Breaking the bicycle frame: Coset-based quantum LDPC codes
Arda Aydin, Itzhak Tamo and Alexander Barg
Generalizing the construction of two-block group
algebra (2BGA) codes, we introduce a family of two-block
quantum LDPC codes constructed using the action of a group on the cosets of its subgroup. This replaces the regular group actions of the earlier two-block constructions and significantly expands the search space, yielding new
quantum LDPC codes outside the 2BGA family. Through a computer search, we identify several new
quantum LDPC codes, including weight-6 codes with parameters $[[48,8,6]]$, $[[96,8,10]]$, and $[[224,12,16]]$, as well as weight-8 codes with parameters $[[84,16,8]]$, $[[112,16,10]]$, $[[128,16,12]]$, and $[[168,16,15]]$. Furthermore, we introduce a maximally packed syndrome extraction schedule of depth $w+2$, including initialization and measurement steps, for any code with a maximum stabilizer weight of $w$ from our family. Under a standard
circuit-level noise model, our codes, when decoded using BP-OSD, perform competitively with BB codes, achieving thresholds of $\approx0.65\%$ for the weight-6 family and $\approx0.35\%$ for the weight-8 family. Finally, we introduce a group-theoretic framework to generate sequences of graph-based covers of 2BGA codes, recovering and extending recent results on code constructions of this type.
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Quantum Physics · Cross-listed from physics.app-ph
Coupled-Mode Equations with Arbitrary Mode Combinations for Kinetic-Inductance Superconducting Traveling-Wave Parametric Devices: Theory and Experimental Validation
F. Patricio Mena, Camilo Espinoza, Ryan O. Berriel et al. (+2 more)
The coupled-mode equations (CMEs) have proven very successful in describing parametric processes in nonlinear optics. More recently, the same formulation has been used to model microwave superconducting parametric amplifiers and frequency multipliers. However, when applied to the microwave regime, not all assumptions remain valid and losses play a more dramatic role. Here, we revisit the CMEs applied to traveling-wave superconducting amplifiers to include losses and provide a formulation that enables their systematic derivation for any combination of traveling waves. As examples, we discuss the impact of unwanted harmonics and intermodulation products on parametric amplification, as well as harmonic generation. We verify that, if not properly accounted for, device performance can deviate considerably from the ideal case. Furthermore, using a superconducting CPW-based artificial transmission line and combining an independent experimental determination of its nonlinear parameter $I'_*$ with simulations of its linear properties, we obtain a parameter-free validation of this formulation. The nonlinear parameter was determined to be $I'_* \approx 27$ mA which, surprisingly, scales with the theoretical depairing current and not with the much smaller critical current of the device. For the validation, we measured multiple-harmonic generation and found excellent agreement between theory and experiment. The fact that $I'_* \gg I_C$ has direct implications for device design.
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All authors F. Patricio Mena, Camilo Espinoza, Ryan O. Berriel, Ricardo Finger, David J. Thoen
Quantum Physics
Post-Selection Probability and Fidelity of Bidirectional Teleportation
Ning Sun, Lei Feng and Pengfei Zhang
Understanding the scrambling of
quantum information is central to many areas of
quantum physics, including
quantum thermalization,
entanglement growth, and
quantum information processing. Insights from these studies have, in turn, inspired the development of novel
quantum protocols and
algorithms. Recently, a bidirectional teleportation protocol was proposed to implement a digital SWAP operation between
qubits by leveraging chaotic
Hamiltonian evolution combined with measurement and post-selection. In this work, we provide a comprehensive study of two central quantities that characterize the protocol, the post-selection probability and the fidelity, taking into account possible errors in time-reversed dynamics. We show that these quantities can be expressed in terms of standard diagnostics in
quantum dynamics, including the Loschmidt echo and its subsystem variant. The results unveil (1) the initial-state dependence of the fidelity and (2) the stability of the post-selection probability in integrable models. Our findings offer practical guidance for the implementation of the protocol on realistic
quantum devices.
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Quantum Physics · Cross-listed from cond-mat.supr-con
Hybrid Ferromagnet-SNSPDs: Single photon induced order-to-disorder transition in ferromagnets coupled to thin film superconductors
Leif Bauer, Daien He, Sathwik Bharadwaj et al. (+1 more)
The development of midwave and longwave infrared single
photon detectors is crucial for their emerging applications in spectroscopy, remote sensing,
exoplanet detection, and free space
quantum communications. However, existing sensors need to be operated at extremely low temperatures (0.08-0.9K) to reduce dark noise and hence require the use of advanced cryogenics such as dilution refrigerators or $^3$He cryogens, significantly limiting applications. Here we propose a
vortex-engineering approach based on a hybrid phase transition in a ferromagnet/
superconductor bilayer to increase the operating temperature of infrared single
photon detectors up to 3.75K. We show that the introduction of a
ferromagnetic layer produces a local
magnetic field which impedes
vortex crossing in the
superconductor, reducing dark noise. When a single
photon is incident, the
photon-induced hotspot causes an order-to-disorder transition in the ferromagnet, leading to a
vortex-induced phase transition in the superconducting layer. By engineering the ferromagnet's Curie temperature to be close to the device's operating temperature, single
photon sensitivity can be achieved at increased operating temperatures. We predict at midwave/longwave infrared wavelengths (3-14$\mu$m) the operating temperature can be raised to 3.25-3.75K, enabling significantly simpler cooling systems.
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All authors Leif Bauer, Daien He, Sathwik Bharadwaj, Zubin Jacob
Quantum Physics
Isabella Goetting, Ashish Kalakuntla, Mikhail Shalaev et al. (+6 more)
Distributed
entanglement across multi-node
quantum networks is essential for a wide range of
quantum technologies, including modular
quantum computers, distributed sensing and metrology, and multi-party secure communication protocols. Such large-scale
quantum networks will require photonic interconnects to generate and sustain entangled states across localized nodes. Previously, three-node distributed Greenberger-Horne-Zeilinger (GHZ) states have been generated between solid-state
qubits and atomic ensembles, but not yet in the platform of individual atomic
qubits, which can be replicated, detected, and individually controlled with high fidelity. Here we report the first fully-distributed GHZ state of
qubits across a three-node
quantum network of single atomic
memories, using photonic interconnects. We achieve a bounded fidelity of $0.841(17) \leq \mathcal{F} \leq 0.881(17)$ at an
entanglement generation rate of 0.095(5)/sec and measure a clear violation of Mermin's inequality while closing the detection loophole for the first time in a fully-distributed multipartite entangled state.
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All authors Isabella Goetting, Ashish Kalakuntla, Mikhail Shalaev, Harriet Bufan Shi, Ana Ferrari, Sagnik Saha, George Toh, Saki Male, Christopher Monroe
Quantum Physics
Fermionic Hamiltonian engineering with local control
Özgün Kum, Matthias Zipper, Ludwig Mathey et al. (+1 more)
Quantum simulators enable the exploration of complex
quantum phenomena in condensed-matter systems by reproducing their dynamics on controllable
quantum devices. However, experimental constraints often restrict the class of
Hamiltonians that can be realized natively.
Hamiltonian engineering addresses this limitation by expanding the set of accessible
Hamiltonians beyond the fixed system
Hamiltonian provided by the hardware. We introduce a framework for fermionic
Hamiltonian engineering based on interleaving free
evolution under the system
Hamiltonian with a sequence of experimentally feasible local fermionic
unitaries. The required sequences and free-
evolution times are obtained efficiently through a linear program, enabling effective time
evolution under a broad class of target
Hamiltonians. In particular, our method realizes arbitrary complex tunnelling coefficients, constrained only by the connectivity of the underlying system
Hamiltonian. We further show that
symmetries of the
Hamiltonian engineering task can be exploited to drastically reduce the cost of computing the control parameters, and that errors arising from finite pulse times can be compensated within the same framework through a simple modification of the linear program. To our knowledge, this is the first automated method to realize locally tunable artificial gauge fields in interacting fermionic
lattice models using only site-resolved energy shifts as control. We demonstrate the method by engineering the dynamics of interacting
Fermi-Hubbard chains with complex tunnelling coefficients and of the Harper-Hofstadter model on a 1088-mode
lattice. For the latter, we exploit the translation
symmetry of the model to determine a provably optimal six-pulse sequence that produces the desired dynamics independent of the
lattice size.
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All authors Özgün Kum, Matthias Zipper, Ludwig Mathey, Martin Kliesch
Quantum Physics · Cross-listed from nucl-th
Quantum Resources and Wigner Symmetry in Nucleon-Nucleon Scattering from Effective Field Theory
Ian Low, Thomas R. Richardson and Sokratis Trifinopoulos
We study
quantum resources in the
spin degrees of freedom, such as
entanglement, stabilizer magic, and non-local magic, in low-energy nucleon-nucleon scattering through next-to-leading order in pionless effective field theory. Treating each nucleon
spin as a
qubit, we calculate the corresponding resource-generating powers of the scattering operator at generic center-of-mass momentum and scattering angle $\Theta$. The analysis retains $S$- and $P$-wave channels generated by two-derivative contact interactions. When the microscopic physics exhibits Wigner's $SU(4)$
spin-
flavor symmetry, the
neutron-
proton amplitude becomes proportional to the
spin-space identity operator and therefore generates no new resources after scattering, extending an observation previously made for leading-order $S$-wave scattering. The same-nucleon channel remains resource-generating because constraints from identical particles project out part of the
Hilbert space. These results show how enhanced
symmetries, partial-wave structure, and resource generation are intertwined in low-energy two-body scattering.
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Quantum Physics · Cross-listed from hep-th
When Renormalisation Remembers: UV/IR Mixing as an Entanglement Bridge
Steven Abel
Renormalisation is traditionally understood to be a Wilsonian memoryless process in which ultraviolet (UV) degrees of freedom gradually decouple, leaving an autonomous infrared (IR) description. However this need not be the case: in UV/IR mixed theories correlations between widely separated scales can persist. In this work I recast UV/IR mixing as a
Hilbert-space phenomenon, realised as correlations across renormalisation scales. This formulation is implemented using the Born-Reciprocal Tensor Network (BRTN), a new configuration of tensor network that is globally symmetric under phase-space reciprocity. On this network I prepare the
vacuum and reproduce the expected radiative corrections. The resulting renormalisation geometry exhibits
memory, with a bridge linking reciprocal representations of IR physics, whose cross-bridge
entanglement provides a precise criterion for the viability of an effective description. I analyse when this criterion is met, and show that there is a large-volume limit, with the fundamental scale held fixed, in which the obstruction to a local description scales away: Wilsonian behaviour is restored and renormalisation forgets. The BRTN therefore provides a concrete and calculable platform for UV/IR mixing.
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Quantum Physics
Projected logical ensembles in surface codes via the random-matrix theory of quantum dots
Mircea Bejan, Jan Behrends, Max McGinley et al. (+1 more)
Measurements underpin active
quantum error correction (QEC) and have been recognized as a source of novel measurement-induced many-body phenomena. Here, we study the statistical properties of post-measurement logical states arising in QEC on topological codes subject to deterministic transversal
unitary gates. Upon syndrome extraction followed by maximum-likelihood decoding, a Born-weighted ensemble arises which we dub the "projected logical ensemble" (PLE). Focusing on surface codes subject to uniform single-
qubit Pauli-$X$ rotations, we characterize the measurement-induced randomness of the PLE. To this end, we show that for a code with a single logical
qubit, the PLE is isomorphic to an ensemble of scattering
matrices describing mesoscopic
quantum dots obtained from a 2D Majorana network model with suitable boundary conditions. We uncover regimes where these
quantum dots are chaotic such that their scattering
matrices are well-described by random
matrix theory. In these regimes, the PLE approaches a universal ensemble that is maximally random up to
symmetry and decoder-induced constraints. The
symmetry constraints, set by stabilizer and logical operator weights, realize Altland-Zirnbauer classes D or DIII, which we both illustrate. Our results establish a fundamental connection between emergent universality concepts in mesoscopic physics,
quantum many-body systems, and QEC.
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All authors Mircea Bejan, Jan Behrends, Max McGinley, Benjamin Béri
Quantum Physics
Grid-state deformation in a no-jump non-Hermitian bosonic dimer
B. M. Rodriguez-Lara, H. Ghaemi-Dizicheh, S. Dehdashti et al. (+3 more)
We study the no-jump
evolution of ideal grid states in a lossy bosonic dimer with differential decay. The effective non-Hermitian quadratic dynamics induces a complex
symplectic flow in phase space that deforms both the primitive
lattice vectors and the origin seed. The average decay rate controls common attenuation, while coherent hopping and differential decay control the reduced dimer deformation. The reduced sector contains
elliptic,
parabolic, and
hyperbolic regimes with imaginary spectra, an exceptional point, and real spectra, producing oscillatory, linear, and exponential
lattice deformations. Although projected
lattice areas can change, the deformation comes from a determinant-one complex
symplectic flow on the full four-dimensional phase space. For a Gaussian regularization of the origin seed, we derive the associated complex width
matrix and identify the positivity conditions that preserve Gaussian form. For an initial two-mode qunaught product state, the lossless limit recovers the standard beam-splitter generation of a square GKP$+$ Bell pair, while the no-jump dynamics produces its non-Hermitian deformation with a postselection cost set by the no-jump probability.
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All authors B. M. Rodriguez-Lara, H. Ghaemi-Dizicheh, S. Dehdashti, A. Hanke, A. Touhami, J. Nötzel
Quantum Physics
Bath memory as a precision resource in quantum transport
José Molina, Sheikh Parvez Mandal, Mahasweta Pandit et al. (+1 more)
Structured baths can reshape transport fluctuations in mesoscopic
quantum devices, yet a predictive criterion for when this enhances precision has been lacking. We propose a route towards such precision advantages by utilizing bath
memory in coherent fermionic transport through a noninteracting
quantum-dot chain. Using the Landauer-Büttiker formalism, we derive a dual impedance-matching condition that synchronizes the conductor mode splitting, boundary dissipation, and bath bandwidth, and sustains constructive multimode interference across the transmission window. The analytical predictions for the optimal bath bandwidths show excellent agreement with exact nonequilibrium Green's function calculations of the transport for Lorentzian, Gaussian, and Newns spectral densities. The prescription yields an optimal bath bandwidth at which the current Fano factor is minimized and the thermodynamic and kinetic precision coefficients are simultaneously enhanced beyond their Markovian limits. The
alignment of the optimal precision regime with the experimentally accessible current Fano factor minimum thus provides a practical strategy for designing precision-enhanced transport in mesoscopic platforms such as
semiconductor quantum-dot arrays and ultracold fermionic channels.
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All authors José Molina, Sheikh Parvez Mandal, Mahasweta Pandit, Javier Prior
Quantum Physics
How Many Shots Are Enough for a Quantum Circuit?
Giuseppe Bisicchia, Alessandro Bocci, Ernesto Pimentel et al. (+1 more)
Quantum algorithms require repeated
circuit executions, known as shots, to estimate output distributions accurately. Determining the minimal number of shots needed to meet a target accuracy is crucial to reduce costs and resource usage, especially on today's noisy and expensive
quantum hardware. In this paper, we address the shot optimisation problem in a black-box setting, where no assumptions are made about the structure of the
quantum circuit or the noise model of the backend. We introduce IncrementalExecution, a novel online framework that dynamically determines when to stop executing shots based on the principle of point of diminishing returns: the point at which additional shots no longer significantly alter the empirical distribution of a fixed
circuit. The framework supports customisable policies for shot management, enabling flexible trade-offs between execution cost and result fidelity within static execution scenarios. We assess our proposal through an extensive experimental evaluation spanning 33,750 framework configurations across 180 unique static
quantum circuit-backend combinations, for a total of 7.3M independent experiments. Unlike prior work that relies on problem-specific knowledge or
algorithm-dependent assumptions (e.g., variational or adaptive workflows), our approach is applicable to a large set of static
circuits and immediately deployable on current
quantum cloud platforms.
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All authors Giuseppe Bisicchia, Alessandro Bocci, Ernesto Pimentel, Antonio Brogi
Quantum Physics
Diagonal-Budgeted Trotterization for Efficient Quantum Hamiltonian Simulation
Srikar Chundury, Blake Burgstahler, Jiajia Li et al. (+2 more)
Efficient classical simulation of
quantum Hamiltonian dynamics is often bottlenecked by exponential state growth and the overhead of generic sparse linear
algebra. We introduce diagonal-budgeted Trotterization, a structure-aware strategy that decomposes
Hamiltonians into factors preserving diagonal sparsity while tightly controlling fidelity loss. Our implementation, HamSim, utilizes a compact diagonal-sparse data layout and specialized C++/CUDA kernels to bypass the overheads of generic formats like CSR. By leveraging SIMD vectorization, multithreading, and GPU acceleration, HamSim achieves high performance across heterogeneous architectures.
Benchmarks on the HamLib suite show that HamSim significantly outperforms Qiskit-Aer. On CPUs, HamSim attains speedups of $182$–$1,269\times$ on optimization instances (TSP, MaxCut) and $4.8$–$841\times$ on physical models (TFIM, Heisenberg). On GPUs, it achieves up to $178\times$ speedup for $12$–$16$
qubit problems. Unlike traditional Trotterization, HamSim maintains near-perfect fidelity without requiring exponential steps. This demonstrates that diagonal-aware numerical kernels provide a scalable foundation for high-fidelity classical
Hamiltonian simulation.
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All authors Srikar Chundury, Blake Burgstahler, Jiajia Li, In-Saeng Suh, Frank Mueller
Quantum Physics
The Optimal Rate Function in Covariant Quantum State Tomography
Arick Grootveld, Alexander Maloney, Jason Pollack et al. (+1 more)
The problem of
quantum tomography is to estimate an unknown
quantum state $\rho$ from a measurement of $n$ copies of $\rho$. One can ask which
tomography protocol, i.e.\ which choice of multi-copy measurement, gives the best possible estimate of $\rho$. To do so, we characterize
tomography protocols by their rate function, which governs the exponential rate at which a protocol assigns probability to a particular estimate $\sigma$ of the true state $\rho$. This rate function is a
quantum mechanical generalization of the classical relative
entropy between the true state and its estimate, and depends on the choice of protocol. It is bounded by the
quantum relative
entropy, and we show that this bound is sharp: for any $\rho$ and $\sigma$ we construct a family of protocols whose rate functions converge to the
quantum relative
entropy $D(\sigma\|\rho)$. We consider the family of covariant
tomography protocols; these are the basis independent state estimation schemes that assume no prior information about $\rho$ and $\sigma$. Keyl described a specific
tomography protocol based on Schur sampling, and conjectured that among all covariant
tomography protocols it has the largest possible rate function for all $\sigma$ and $\rho$. We prove this
conjecture. The resulting rate function is an annealed version of
quantum relative
entropy, due to the cost of learning the eigenbasis in covariant
quantum state
tomography.
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All authors Arick Grootveld, Alexander Maloney, Jason Pollack, Peixue Wu
Quantum Physics
Counterdiabatic Raman Atom Optics for Compact High-Sensitivity Gravimetry
Asad Ali, Hamid Arian Zad, Saif Al-Kuwari et al. (+6 more)
Large-momentum-transfer (LMT) atom interferometry provides a route toward enhanced inertial sensitivity in compact
quantum sensors, but its scalability is limited by the accumulation of pulse-transfer errors across long Raman pulse sequences. We investigate theoretically the use of stimulated Raman shortcut-to-adiabatic passage (STIRSAP) for high-fidelity LMT atom optics in a Mach–Zehnder interferometer geometry. The counterdiabatic correction is encoded directly into the Raman pulse envelopes, eliminating the need for auxiliary microwave or
radio-frequency control fields. Numerical simulations based on an effective Raman model show that $1~\mu\mathrm{s}$ STIRSAP pulses achieve single-pulse transfer fidelities of $F_\pi = 0.99902$ while maintaining negligible pulse-time overhead even at high momentum order. We analyze the resulting tradeoff between interferometric phase enhancement and compound contrast decay and identify an unconstrained shot-noise optimum near $n\approx270$. The analysis further shows that practical operation at extreme LMT order is constrained by wave-packet separation, vibration noise, Doppler detuning, and accumulated systematic effects rather than by pulse duration itself. These results establish superadiabatic Raman control as a promising approach for scalable high-fidelity atom optics and clarify the physical limitations governing compact high-order atom interferometers.
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All authors Asad Ali, Hamid Arian Zad, Saif Al-Kuwari, Muhammad Irtiza Hussain, Muhammad Talha Rahim, Hashir Kuniyil, Tim Byrnes, James Q. Quach, Saeed Haddadi
Quantum Physics
Experimental quantum state learning with pairs of photons
C. Pria Dobney, Johan Henaff, Allen Kasum et al. (+5 more)
Tomography allows one to estimate the density
matrix describing the state an ensemble of
quantum systems are prepared in (for example, polarization
tomography determines the polarization state of a beam of identically prepared
photons). In general, it is not possible to uniquely decompose the density
matrix into its pure state components. Agarwal et al. proposed a protocol which, for a mixture composed of any two pure states of a
qubit (with arbitrary probabilities), allows an observer to infer not only the density
matrix but the identity of those specific pure states and their weights - the additional requirement being that the
qubits arrive in pairs, where both
qubits in each pair are in the same state. We experimentally demonstrate this learning-from-pairs concept using
photons in the polarization degree of freedom. We use
tomography to measure a sequence of single
photons and make use of their time-of-arrival information to 'pair up' the
photons after the measurement. From here we are able to infer the
photons' polarization states and their respective probabilities, and we demonstrate this for various different choices of polarization states and ratios. Finally, we investigate our ability to discriminate between two equal mixtures of distinct pairs of orthogonal polarization states. We find that on the order of approx. 10e4
photons is typically enough to achieve
tomography fidelities of approximately 0.9999. This is sufficient to discriminate between two different preparations of the same mixed state, differing by angles of less than 5 degrees between the pure states used in the two preparations.
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All authors C. Pria Dobney, Johan Henaff, Allen Kasum, Rui Jie Tang, Haru Mukumoto, Mark Hillery, Berthold-Georg Englert, Aephraim Steinberg
Quantum Physics
Controlled Quantum Metrology with Anisotropic Heisenberg Spin Interactions under Intrinsic Decoherence
S.K. Singh, Jia-Xin Peng, Y-J Zhu et al. (+1 more)
We theoretically investigate
quantum parameter estimation in a two-
qubit anisotropic Heisenberg
spin system with Dzyaloshinskii-Moriya (DM) interaction in the presence of intrinsic decoherence described by the Milburn model. Using the
Quantum Fisher Information (QFI), we study the estimation of both the uniform
magnetic field and the DM interaction strength. Analytical expressions for the time-evolved density
matrix are obtained and used to explore the effects of exchange anisotropy, intrinsic decoherence, and probe-state preparation on the achievable estimation precision. Our results show that suitable tuning of the anisotropic exchange coupling and the initial entangled state can considerably enhance the estimation performance, with different optimal parameter regimes emerging for
magnetic-field and DM-interaction sensing. To better understand the role of
quantum resources in metrology, we also examine the behaviour of concurrence,
quantum coherence, and von Neumann
entropy. Overall, our findings demonstrate that anisotropic Heisenberg
spin systems with DM interaction provide a promising and flexible platform for high-precision
quantum metrology even in the presence of intrinsic decoherence.
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All authors S.K. Singh, Jia-Xin Peng, Y-J Zhu, Mohammad Khalid
Quantum Physics
Analytic Benchmarks for Coherence-to-Entanglement Conversion under Post-Gate Noise in CNOT-Based Protocols
Asad Ali, H. Kuniyil, M.I. Hussain et al. (+3 more)
Coherence-to-
entanglement conversion transforms single-
qubit superposition into a practical two-
qubit resource, but noise limits this process in near-term
quantum hardware. We derive closed-form
benchmarks for a minimal CNOT primitive in which a coherent
qubit and an incoherent ancilla generate
entanglement before undergoing phase damping, global depolarizing, amplitude damping, or independent local depolarizing noise. Using the $\ell_1$-norm of coherence and negativity, we prove the noiseless law $\mathcal{N}_0=C_{\ell_1}/2$, valid for arbitrary mixed inputs, and obtain exact negativities, survival fractions, and
entanglement-sudden-death thresholds. For all $X$-state-preserving channels, a master relation shows that
entanglement loss results from the competition between coherence suppression and partial-transpose spectral shifts. Phase damping yields $\eta=1-p$ without finite-noise sudden death; global depolarization gives coherence-dependent sudden death; amplitude damping adds an excited-population penalty and sudden death only for $\theta>\pi/4$; while local depolarization is most destructive at equal depolarizing strength. The initial survival slopes, $-1$, $-3/2$, $-2$, and $-3$, act as compact noise fingerprints. Since concurrence satisfies $C=2\mathcal{N}$ for the generated states, all robustness rankings remain unchanged. Mapping channel parameters to $T_1$, $T_\varphi$, and average gate fidelity connects the theory to hardware-level performance.
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All authors Asad Ali, H. Kuniyil, M.I. Hussain, M.T. Rahim, Abdallah Slaoui, Saif Al-Kuwari
Quantum Physics
Morphology-resolved scrambling in a chaotic quantum billiard
Pranaya Pratik Das
Chaotic
quantum systems can retain spatial
memory through scarred eigenstates, but whether these static structures control scrambling remains unclear. This work establishes a morphology-resolved connection between scarred eigenstates and eigenstate-resolved OTOCs in a peanut-shaped
quantum billiard. Scalar localisation diagnostics, including differential
entropy and continuum participation ratios, detect anomalous concentration but discard spatial architecture. A scale-normalised density overlap, in contrast, directly compares probability density profiles, revealing families of orthogonal eigenstates with nearly identical spatial morphology. Comparing the complete OTOC time traces of these orthogonal eigenstates reveals that morphological recurrence has dynamical content: moderate density overlap yields no universal prediction, whereas strongly recurring morphologies exhibit nearly identical OTOC growth and saturation. Thus, scarred structures act as spatial templates for operator growth, not merely static violations of ergodicity. This morphology-resolved framework turns eigenstate shape into a quantitative predictor of scrambling and provides a scale-controlled diagnostic of weak ergodicity breaking in
quantum chaos.
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