Quantum Physics · Cross-listed from physics.optics
Saturable absorption in defect-rich diamond nanophotonics
Christopher Coutts, Nicholas J. Sorensen, Elham Zohari et al. (+3 more)
Diamond is a leading
quantum photonics platform due to its ability to host
qubits based on
crystal defects such as nitrogen-vacancy centres. Fabricating nanophotonic devices from defect-rich diamond, which underpins many
quantum sensing technologies, promises enhanced performance and integrability of diamond
quantum sensors. Here we demonstrate microdisk cavities fabricated from defect-rich diamond that support optical modes with high quality factor (${Q}\sim7\times10^4$ at $1042\,$nm) and show that they exhibit saturable absorption. Power-dependent spectroscopy measurements spanning 979$\,$nm to 1604$\,$nm are used to observe intensity-dependent cavity loss and extract wavelength-dependent absorption coefficients and saturation intensities. At 1047$\,$nm, we observe saturation and measure a saturation intensity of $2.1\,(8)\,$MW/cm$^2$ and an absorption coefficient of $0.53\,(2)\,$cm$^{-1}$. These results provide insight into defect-mediated optical loss in diamond nanophotonics and suggest strategies to harness defect-induced nonlinearities in future diamond photonic devices.
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All authors Christopher Coutts, Nicholas J. Sorensen, Elham Zohari, Sean McNaney, Sigurd Flågan, Paul E. Barclay
Quantum Physics
QuaNTUM: A Modular Quantum Communication Testbed for Scalable Fiber and Satellite Integration
Julien Chénedé, Tjorben Matthes, Josefine Krause et al. (+2 more)
Secure communication is essential for modern society, from financial transactions to critical infrastructure. As classical encryption faces threats from advancing computational power,
quantum communication provides a fundamentally secure alternative based on physical laws. We present
QuaNTUM (
Quantum Network at the Technical University of Munich), a modular and extensible
quantum communication testbed enabling scalable experiments across fiber-based campus networks and satellite-ground links. The terrestrial network connects research institutions in Garching near Munich via single-mode fibers in a
star topology with polarization-maintaining components, multiplexers, and time-synchronized analysis modules. Active polarization control and real-time feedback support stable
qubit transmission for high-fidelity
quantum key distribution and
entanglement distribution. A key feature is the integration of deterministic solid-state single-
photon sources, including defects in hexagonal boron nitride and excited erbium atoms, with initial deployments on small satellites to bridge terrestrial and free-space channels. As an open-access platform,
QuaNTUM enables protocol development, device benchmarking, and hybrid network research, providing a foundation for scalable
quantum communication and future global
quantum networks.
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All authors Julien Chénedé, Tjorben Matthes, Josefine Krause, Asli Cakan, Tobias Vogl
Quantum Physics
Many-Body Entanglement Properties of Finite Interacting Fermionic Hamiltonians
Irakli Giorgadze, Grayson Welch, Haixuan Huang et al. (+2 more)
We analyze many-body
entanglement in interacting fermionic systems by using the $M$-body reduced density
matrix. We demonstrate that if a particle number conserving fermionic
Hamiltonian contains only up to $M$-body interaction terms, then its $N$-particle ground state cannot be maximally $M$-body entangled. As a key step in the proof, we show that the energy expectation value of a maximally $M$-body mixed state is equal to the spectral mean of the
Hamiltonian on the corresponding $N$-particle subspace. We further demonstrate that the many-body
entanglement structure of a ground state can place quantitative constraint on the interaction strength of its parent
Hamiltonian. We illustrate the
theorem and its implications in Hubbard and extended SYK models. Going beyond ground states, we analyze
entanglement generation under
unitary dynamics from Slater-determinant initial states in these models. We determine early-time growth and estimate
entanglement saturation times. Finally, we derive explicit
symmetry-refined saturation upper bounds for $M$-body
entanglement in the presence of an Abelian
symmetry.
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All authors Irakli Giorgadze, Grayson Welch, Haixuan Huang, Elio J. König, Jukka I. Väyrynen
Quantum Physics
Focusing Surface-Acoustic-Wave Resonators on Thin-Film Lithium Niobate with Transverse-Mode Suppression
Ryo Sasaki, Ryusuke Hisatomi, Rekishu Yamazaki et al. (+3 more)
Surface-acoustic-wave (SAW) resonators are a promising platform for constructing hybrid
quantum systems, where confined acoustic waves enable strong interaction with various physical systems. Focusing SAW resonators, reducing mode volume while suppressing diffraction losses, have recently been investigated for application in such hybrid systems. However, the resonator leads to additional transverse-mode resonances, which cause undesired responses. In this work, we develop focusing SAW resonators on a thin-film lithium niobate on sapphire. A film thinner than the SAW wavelength allows a highly confined acoustic-wave mode to be localized on the substrate surface. By using contoured electrodes following a two-dimensional Gaussian beam shape, we make the SAW mode focused to nearly a diffraction-limited and confirm it via optical imaging. Furthermore, by engineering the spatial mode overlap of the interdigital transducer electrodes, we suppress the excitation of higher-order transverse modes.
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All authors Ryo Sasaki, Ryusuke Hisatomi, Rekishu Yamazaki, Yuya Yamaguchi, Yasunobu Nakamura, Atsushi Noguchi
Quantum Physics · Cross-listed from hep-ph
Quantum tomography of $H \to ZZ, WW$ beyond leading order
J. A. Aguilar-Saavedra and Pier Paolo Giardino
We revisit
quantum tomography of $H \to ZZ$ and $H \to WW$ in the presence of higher-order corrections. We verify that neither the use of an effective
spin analysing power (only for $ZZ$) or a
photon veto are sufficient to render the naively-constructed
spin density operators physical. A subtraction of higher-order corrections is thus necessary to perform consistent
quantum tomography. Such corrections are small when compared to expected experimental uncertainties with current data. As a by-product, we point out the striking possibility to observe parity-violating effects in $H \to WW$.
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Quantum Physics
Infinite Distance Extrapolation: How error mitigation can enhance quantum error correction
George Umbrarescu, Oscar Higgott and Dan E. Browne
Quantum error mitigation (QEM) and
quantum error correction (QEC) are two research areas that are often considered as distinct entities, and the problem of combining the two approaches in a non-trivial way has only recently started to be explored. In this paper, we explore a paradigm at the intersection of the two, based on the error mitigation technique of Zero-Noise Extrapolation (ZNE), that uses the distance of an error correcting code as a noise parameter. This is distinct from some alternative approaches, as QEC is here used as a subroutine inside the QEM framework, while other proposals use QEM as a subroutine inside QEC experiments. Intuitively, we exploit the fact that a reduction in the physical noise level is analogous to an increase in the code distance, as both of them result in a decrease in the logical error rate. As such, the extrapolation to zero noise in the case of ZNE becomes comparable to the extrapolation to infinite distance in the case of this method. We describe how to calculate expectation values from a fault-tolerant computation, and we gain some analytical intuition for our ansatz choice. We explore the performance of the considered method to reduce the errors in a range of expectation values for a realistic
circuit-level noise model and realistic device imperfections on the rotated surface code, and we particularly show that the performance of the method holds even in the case of non-stabiliser input states.
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Quantum Physics
A First-Principles Thermodynamic Uncertainty Relation for Shortcuts to Adiabaticity
Guillermo Perna, Federico Centrone and Esteban Calzetta
We study the fundamental limitations of implementing time-dependent
Hamiltonian protocols when ”time” is provided by a
quantum clock rather than an external classical parameter. For a parametric harmonic oscillator controlled through a shortcut-to-adiabaticity (STA) schedule and coupled to a minimal clock degree of freedom, tracing out the clock yields an effective reduced dynamics that is a mixture of
unitary Gaussian trajectories. Within a noise-dominated regime, we compute the energetic deviation from the target STA outcome and its fluctuations, together with the fidelity to the target
evolution and the purity loss of the reduced state, for
vacuum and coherent initial states. Combining these observables produces a thermodynamic-uncertainty-type tradeoff that links achievable precision to an irreducible loss of purity set by the clock precision and the protocol sensitivity.
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Quantum Physics
Higher-Order Quantum Objects are Strong Profunctors
Matt Wilson and James Hefford
We explore the sense in which the existing constructions for higher-order maps on
quantum theory based on causality constraints and compositionality constraints respectively, coincide. More precisely, we construct a functor F : Caus(C) -> StProf(C1) from higher-order causal categories to the category of strong profunctors over first-order causal processes that is lax-lax duoidal, full, faithful, and strongly closed whenever C is additive. When C = CP this
embedding is furthermore strong on the sequencer for duoidal categories, expressing the possibility to interpret one-way signalling (but not general non-signalling) constraints in terms of the coend calculus for profunctors. We conclude that insofar as compositional constraints can be used to express causality constraints, the profunctorial approach generalises higher-order
quantum theory to a construction over general symmetric monoidal categories.
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Quantum Physics
Imaginary time evolution and ground state preparation using unitary multi-copy protocols
Tal Schwartzman, Torsten V. Zache, Hannes Pichler et al. (+1 more)
Efficient low-energy state preparation is a key objective in
quantum computation and
quantum simulation.
Quantum imaginary-time
evolution replaces real-time dynamics with imaginary-time dynamics, exponentially suppressing higher-energy eigenstates. We introduce deterministic
unitary protocols that approximate imaginary-time
evolution for ground state preparation. The protocols require multiple copies of the system, real-time
evolution under the system
Hamiltonian, and controlled-SWAP operations (or more general SWAP-generated
unitaries). Our analysis focuses on two concrete
circuit families: a tree architecture with provable
polynomial-in-depth convergence but rapidly growing width, and a compact "hedge" architecture that achieves comparable accuracy with only
polynomial width in a heuristic construction supported by numerics. Numerical evidence indicates that mid-
circuit post-selection can accelerate convergence with practical success probabilities. Separately, we demonstrate that
circuit volume can be traded for the shot complexity of post-
circuit observable estimation in the ground state preparation setting. Finally, we outline concrete platform-specific implementations in which multi-copy registers and SWAP-mediated couplings are natural, illustrating how these hybrid analog-digital
circuits can complement existing state-preparation methods in the near term.
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All authors Tal Schwartzman, Torsten V. Zache, Hannes Pichler, H. R. Sadeghpour
Quantum Physics
Kraus map closed-form solution for general master equation dynamics
Shahrukh Chishti, Francisco Andrés Cárdenas-López and Felix Motzoi
The Kraus representation of
quantum channels allows for a precise emulation of the complex dynamics that take place on
quantum processors, whether for benchmarking
algorithms, predicting the performance of error correction and mitigation, or in the myriad other uses of compiled digital sequences. Nonetheless, starting from first principles to obtain continuous
quantum master equations involves various approximations such as weak coupling to the environment. Further, converting these equations to Kraus operators cannot generally be obtained in closed-form due to the complicated commutator structure of the problem. In our work, we bridge this gap by providing a general closed form formulation for arbitrarily strong driving while remaining linear in the dissipator. The Kraus solution is expressed as a Riemann sum where higher terms can converge quickly to high precision, which we demonstrate numerically. Such a formulation is highly relevant to
quantum computing and gate-based models, where effective models are highly sought for large rotation gate angles, even under the influence of underlying non-trivial noise mechanisms.
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Quantum Physics
Auxiliary-Field Quantum Monte Carlo on Quantum Hardware via Unitary Dilation
Xiantao Li
We present near-term
quantum algorithms for auxiliary-field
quantum Monte Carlo (AFQMC), viewed as imaginary-time projection for ground-state calculation as an ensemble of one-body propagators driven by
stochastic fields $\Omega$. Starting from the Feynman-Kac formula, we convert each trajectory into a sequence of piecewise-constant one-body generators using
stochastic Magnus expansions up to second order, and embed the resulting nonunitary slices into
unitaries with a small ancilla overhead. This lifts the projector dynamics to a
unitary evolution, enabling coherent
circuit execution in the regime $\|\Omega \| \tau=O(1)$ and reducing the need for frequent mid-
circuit measurement. We further derive an equivalent linear-combination-of-
unitaries (LCU) form that yields system-only, shallower
circuits by trading ancilla cost for additional trajectory sampling.
Benchmarks on the Hubbard model verify the accuracy of the dilation and Magnus expansions classically and demonstrate multi-step executions on IBM
quantum hardware.
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Quantum Physics
Generative modeling with Gaussian Boson Sampling: classically trainable Bosonic Born Machines
Zoltán Kolarovszki, Bence Bakó, Michał Oszmaniec et al. (+2 more)
Quantum generative modeling has emerged as a promising application of
quantum computers, aiming to model complex probability distributions beyond the reach of classical methods. In practice, however, training such models often requires costly gradient estimation performed directly on the
quantum hardware. Crucially, for certain structured
quantum circuits, expectation values of local observables can be efficiently evaluated on a classical computer, enabling classical training without calls to the
quantum hardware in the optimization loop. In these models, sampling from the resulting
circuits can still be classically hard, so inference must be performed on a
quantum device, yielding a potential computational advantage. In this work, we introduce a photonic
quantum generative model built on parametrized Gaussian
Boson Sampling
circuits. The training is based on the efficient classical evaluation of expectation values enabled by the Gaussian structure of the state, allowing scalable optimization of the model parameters through the maximum mean discrepancy loss function. We demonstrate the effectiveness of the approach through numerical experiments on photonic systems with up to 805 modes and over a million trainable parameters, highlighting its scalability and suitability for near-term photonic
quantum devices.
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All authors Zoltán Kolarovszki, Bence Bakó, Michał Oszmaniec, Changhun Oh, Zoltán Zimborás
Quantum Physics
Exact quantum scars from kinetic frustration for cross-platform realizations
Zhuoli Ding, Ruben Verresen and Zoe Z. Yan
Quantum many-body scars are nonthermal states exhibiting persistent revivals in an otherwise ergodic, nonintegrable
quantum system. Here we leverage the phenomenon of kinetic frustration – the destructive interference of multiple
quantum paths – to create exact scars. The simplicity makes these models directly suitable for implementation on multiple existing
quantum simulation platforms. In particular, we show how frustrated hardcore
bosons in cold atom Bose-Hubbard simulators and polar molecule or Rydberg atom tweezer arrays have persistent oscillations whose lifetimes can be tuned with experimentally accessible parameters, like the Hubbard interaction or a Floquet drive. Second, we propose an experimentally realizable scar within a non-integrable
Fermi-Hubbard model where the frustration arises from the fermionic exchange statistics, which admits a one-to-one mapping with the bosonic model in the scar subspace. Finally, we introduce a practical heuristic based on the energy distribution of eigenstates for systematically predicting and optimizing
quantum many-body scar lifetimes. Their cross-platform realizability and long lifetimes make them well-suited for benchmarking coherence and exploring nonergodic dynamics in current and near-term
quantum devices.
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Quantum Physics
DysonNet: Constant-Time Local Updates for Neural Quantum States
Lucas Winter and Andreas Nunnenkamp
Neural
quantum states (NQS) provide a flexible variational framework for many-body wavefunctions, but suffer from high computational cost and limited interpretability. We introduce DysonNet, a broad class of NQS that couples strictly local nonlinearities through global linear layers. This structure is analogous to a truncated Dyson series which gives an intuitive interpretation of local wavefunction updates as scattering from static
impurities. By resumming the scattering series, single-
spin-flip updates can be computed in $\mathcal{O}(1)$ time, independent of system size, using an
algorithm we call ABACUS. Implementing DysonNet with the state-space model S4, we obtain up to $230\times$ speedups over
Vision-
Transformers for computing the local
estimator. This corresponds to an asymptotic $\mathcal{O}(N^2)$ improvement in training-time scaling, reaching $\mathcal{O}(N \log^2 N)$ total training complexity in area-law phases.
Benchmarks on the 1D long-range Ising model and frustrated $J_1$-$J_2$ chains show that DysonNet matches state-of-the-art NQS accuracy while removing the dominant local-update overhead. More broadly, our results suggest a route to scalable NQS architectures where physical interpretability directly enables computational efficiency.
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Quantum Physics
Rhenium as a material platform for long-lived transmon qubits
Yanhao Wang, Suhas Ganjam, Ishan Narra et al. (+2 more)
Dielectric loss at the interfaces of superconducting films has long been recognized as limiting the performance of state-of-the-art superconducting
circuits. Notably, the presence of a native
oxide layer on the film is hypothesized to contribute to dielectric loss at the metal-air interface. Here, we explore rhenium as a candidate for the film, motivated by its remarkable property to suppress native
oxide formation. We demonstrate rhenium on sapphire as a promising material platform for superconducting
circuits through the realization of transmons with mean relaxation times $T_1$ up to 407 microseconds at 5 GHz. Our transmons are supplemented with a loss characterization study, in which we separate the dominant loss mechanisms and construct a loss budget that agrees with our $T_1$ measurements. Further characterization may establish rhenium as a leading candidate for maximizing decoherence time.
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All authors Yanhao Wang, Suhas Ganjam, Ishan Narra, Luigi Frunzio, Robert J. Schoelkopf
Quantum Physics · Cross-listed from cond-mat.mes-hall
Hall conductance in a weakly time-reversal invariant open system
Alexander Fagerlund, Christopher Ekman and Rodrigo Arouca
The
quantum Hall effect and the
quantum anomalous
Hall effect both require time-reversal invariance to be broken. We show that non-
equilibrium effects can cause
Hall physics to arise even when the system is weakly time-reversal symmetric and no
magnetic field is applied. In our model, this occurs due to a fermionic subsystem breaking time-reversal invariance even if the system as a whole does not. The
fermions receive a TRI-breaking self-energy, caused by interactions with bosonic degrees of freedom in the system and with an external reservoir. As a result, the
fermions develop a non-quantized
Hall conductance. We demonstrate that, unlike in the
equilibrium case, the presence of a mass term is insufficient for the
Hall conductance to appear, and wave-function renormalization effects have to be included.
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Quantum Physics
Engineering Higher-order Effective Hamiltonians
Jiahui Chen and David Cory
Advancing
quantum technologies requires precise and robust coherent control of
quantum systems. Robust higher-order
Hamiltonian engineering is essential for high-precision control and for accessing effective dynamics absent at zeroth order. Here, we introduce a systematic methodology for achieving the precision, robustness, and complexity required for
quantum control through the engineering of higher-order processes and effective
Hamiltonians. We identify the minimal subspace of achievable effective
Hamiltonian at each order and provide universal cost functions for achieving desired targets. Examples include robust sequences for decoupling, three-body interactions and detuning/interaction correlations.
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Quantum Physics · Cross-listed from nlin.SI
Bootstrapping Fermionic $R$-operators with Decorated Yang–Baxter Equations
Zhao Zhang
Integrating infinitesimal generators of local dynamics into finite-time
evolution is generally harder than extracting a local
Hamiltonian from a
quantum gate. For integrable one-dimensional
quantum many-body systems, a recent bootstrap constructs one-parameter $R$-operators directly from
Hamiltonians satisfying the Reshetikhin condition. However, this condition excludes important models such as the Hubbard model, whose two-parameter $R$-operator was obtained by Shastry from a decorated Yang–Baxter equation obeyed by its free-
fermion sector. Here, I extend the bootstrap to $R$-operators constrained by both the ordinary and decorated Yang–Baxter equations. Their joint expansion produces additional
Hamiltonian-level necessary conditions and recursively determines candidate higher-order terms, subject to two-site locality at each order. I also derive compatibility and factorization conditions under which a Shastry-type superposition solves the $RLL$ relation after deformation by an onsite involution. The construction recovers the known two-parameter $R$-operators of the Hubbard and XYh models, whose full Yang–Baxter equations follow from established tetrahedral Zamolodchikov
algebra relations. For a superconducting Hubbard deformation, the scalar equations are generically incompatible, placing it outside this framework. These results provide diagnostics and a systematic route to fermionic $R$-operators beyond the Reshetikhin condition. General sufficiency, convergence of the formal bootstrap, and sufficiency of the deformation conditions remain open.
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Quantum Physics
Low $T$-count preparation of nuclear eigenstates with tensor networks
Joe Gibbs, Lukasz Cincio, Chandan Sarma et al. (+2 more)
We present an efficient protocol leveraging classical computation to support Initial State Preparation for strongly correlated fermionic systems, a critical bottleneck for fault-tolerant
quantum simulation. Focusing on
nuclear shell model eigenstates, we first demonstrate that the Density
Matrix Renormalization Group
algorithm can efficiently approximate target states as
Matrix Product States, capitalizing on the favourable
entanglement structure of these fermionic systems. These high-fidelity approximations are then leveraged as a classical resource in a variational
circuit optimization scheme to compile shallow
quantum circuits. We establish concrete resource estimates by decomposing the resulting
circuits into the industry-standard Clifford$+T$ gateset, exploring the benefits of specialized $U3$ synthesis techniques. For all
nuclear systems tested, on up to 76
qubit Hamiltonians, we consistently find low $T$-count
circuits preparing the
nuclear eigenstates to high fidelity with $\sim 2\times 10^4$ total $T$ gates. This low number gives confidence these eigenstates can be prepared on early fault-tolerant
quantum computers. Our work establishes a viable path toward practical ground state preparation for
nuclear structure and other fermionic applications.
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All authors Joe Gibbs, Lukasz Cincio, Chandan Sarma, Zoë Holmes, Paul Stevenson
Quantum Physics
Feasibility of satellite-augmented global quantum repeater networks
Manik Dawar, Clement Paillet, Nilesh Vyas et al. (+3 more)
A large scale
quantum network requires the distribution of high-fidelity end-to-end
entanglement. To overcome the range limitations inherent to terrestrial fiber, a leading architecture has emerged: satellite-based sources transmitting
entanglement to
quantum repeaters on the ground. By bridging the gap between abstract analytical frameworks and computationally heavy numerical simulations, this paper provides the first quantitative answer to the question of such a network's achievable performance with current and near-term space technology, while accounting for
entanglement swapping and purification. This is achieved by integrating a detailed physical model of a satellite-to-ground link into an analytical
entanglement resource estimation framework for
quantum repeaters, enabling an optimization of the end-to-end
entanglement rate. Our analysis, performed across leading
quantum hardware platforms, shows that Low Earth Orbit satellite constellations combined with
quantum repeaters employing Neutral Atom or Nitrogen and Silicon Vacancy
qubits, could enable a global
quantum network, distributing
entanglement over distances up to 20,000 km, sufficient for connecting any two points on Earth. This work highlights the major bottlenecks in space and
quantum hardware technologies, which need to be addressed, thereby guiding informed investments necessary for enabling a large scale
quantum network.
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All authors Manik Dawar, Clement Paillet, Nilesh Vyas, Andrew Thain, Rodrigo Henriques Guilherme, Ralf Riedinger
Quantum Physics · Cross-listed from cond-mat.quant-gas
Realizing the Emery Model in Optical Lattices for Quantum Simulation of Cuprates and Nickelates
Hannah Lange, Liyang Qiu, Robin Groth et al. (+7 more)
The microscopic origin of high-temperature superconductivity in cuprates remains one of the central open questions in condensed matter physics. Growing experimental and theoretical evidence suggests that the bare single-band
Fermi-Hubbard model may not fully capture properties of cuprates such as superconductivity, motivating us to revisit the canonical three-band model of the copper-
oxide planes - the Emery model - from which the single-band counterpart was originally derived. Here, we propose and analyze a
quantum simulation scheme for realizing the Emery model in regimes relevant to cuprates and infinite-layer nickelates with today's ultracold atom
quantum simulation platforms, enabling the exploration of the three-band physics on system sizes that are challenging for current numerical methods. Specifically, we show that a two-dimensional optical
lattice with a superimposed pattern of repulsive potentials can be designed to study low-temperature properties for variable parameter regimes of the Emery model relevant to cuprates as well as infinite-layer nickelates. Our results pave the way for real material simulations with ultracold atom
quantum simulators and a better understanding of the physics of unconventional
superconductors.
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All authors Hannah Lange, Liyang Qiu, Robin Groth, Andreas von Haaren, Luca Muscarella, Titus Franz, Immanuel Bloch, Fabian Grusdt, Philipp M. Preiss, Annabelle Bohrdt
Quantum Physics
Quantum-to-classical correspondence in Krylov complexity
Gastón F. Scialchi, Augusto J. Roncaglia and Diego A. Wisniacki
We study
quantum-to-classical correspondence of the Krylov space for evolutions driven by
unitary maps with a classical limit. This entails a proper definition of corresponding
quantum and classical operators, inner products and initial states. We prove that with these definitions the purely classical Krylov space is indeed obtained as the asymptotic $\hbar\to 0$ expansion of the
quantum Krylov space, and provide several examples of such correspondence. We use these examples to analyze some general aspects about the
evolution of the Krylov complexity as they relate to the phase-space representation for the Krylov states. Additionally, we discuss alternative definitions to obtain the correspondence and why they fail. This paper constitutes a first step in understanding complexity and ergodicity of
unitary evolution through the Krylov perspective as they relate to classical dynamical notions.
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Quantum Physics
Mitigating crosstalk errors for simultaneous single-qubit gates on a superconducting quantum processor
Jaap J. Wesdorp, Eric Hyyppä, Joona Andersson et al. (+61 more)
Single-
qubit gates on superconducting
quantum processors are typically implemented using microwave pulses applied through dedicated control lines. However, these microwave pulses may also drive other
qubits due to crosstalk arising from capacitive coupling and wavefunction overlap in systems with closely spaced transition frequencies. Crosstalk and frequency crowding increase errors during simultaneous single-
qubit operations relative to isolated gates, thus forming a major bottleneck for scaling superconducting
quantum processors. In this work, we combine model-based
qubit frequency optimization with pulse shaping to demonstrate crosstalk error mitigation in single-
qubit gates on a 49-
qubit superconducting
quantum processor. We introduce and experimentally verify an analytical model of simultaneous single-
qubit gate error caused by microwave crosstalk that depends on a given pulse shape. By employing a model-based optimization strategy of
qubit frequencies, we minimize the crosstalk-induced error across the processor and achieve a mean simultaneous single-
qubit gate fidelity of 99.96% for a 16-ns gate duration, approaching the mean individual gate fidelity. To further reduce the simultaneous error and required
qubit frequency bandwidth on high-crosstalk
qubit pairs, we introduce a crosstalk transition suppression (CTS) pulse shaping technique that minimizes the spectral energy around transitions inducing leakage and crosstalk errors. Finally, we combine CTS with model-based frequency optimization across the device and experimentally show a systematic reduction in the required
qubit frequency bandwidth for high-fidelity simultaneous gates, supported by simulations of systems with up to 1000
qubits. By alleviating constraints on
qubit frequency bandwidth for parallel single-
qubit operations, this work represents an important step for scaling towards larger
quantum processors.
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All authors Jaap J. Wesdorp, Eric Hyyppä, Joona Andersson, Janos Adam, Rohit Beriwal, Ville Bergholm, Saga Dahl, Simone Diego Fasciati, Alejandro Gomez Friero, Zheming Gao, Daria Gusenkova, Andrew Guthrie, Johannes Heinsoo, Tuukka Hiltunen, Keiran Holland, Amin Hosseinkhani, Sinan Inel, Joni Ikonen, Shan W. Jolin, Kristinn Juliusson, Seung-Goo Kim, Anton Komlev, Roope Kokkoniemi, Otto Koskinen, Joonas Kylmälä, Alessandro Landra, Julia Lamprich, Magdalena Lehmuskoski, Nizar Lethif, Per Liebermann, Tianyi Li, Aleksi Lintunen, Fabian Marxer, Kunal Mitra, Jakub Mrożek, Lucas Ortega, Miha Papič, Matti Partanen, Alexander Plyushch, Stefan Pogorzalek, Michael Renger, Jussi Ritvas, Sampo Saarinen, Indrajeet Sagar, Matthew Sarsby, Mykhailo Savytskyi, Ville Selinmaa, Ivan Takmakov, Brian Tarasinski, Francesca Tosto, David Vasey, Panu Vesanen, Jeroen Verjauw, Alpo Välimaa, Nicola Wurz, Hsiang-Sheng Ku, Frank Deppe, Juha Hassel, Caspar Ockeloen-Korppi, Wei Liu, Jani Tuorila, Chun Fai Chan, Attila Geresdi, Antti Vepsäläinen
Quantum Physics
Universality of Classically Trainable, Quantum-Deployed Boson-Sampling Generative Models
Andrii Kurkin, Ulysse Chabaud, Zoltán Kolarovszki et al. (+3 more)
Recent work on the instantaneous
quantum polynomial-time (IQP)
quantum-
circuit Born machine (QCBM) highlights a promising paradigm for generative modeling: train classically, deploy quantumly. In this setting, the training objective can be evaluated efficiently on a classical computer, while sampling from the resulting model may still be classically intractable. Furthermore, in the IQP-QCBM framework, extending the model family with ancillary
qubits has been proven to yield universality. This paper asks whether similar results hold for linear-optical generative models. To this end, we introduce the
Boson Sampling Born Machine (BSBM). Our analysis retraces analogous steps as were found for IQP-QCBMs with twists. Using recent results that enable classical approximation of broad classes of expectation values in linear optics, we show that BSBMs can be trained classically for wide families of loss functions. Next, we argue that "basic" BSBMs are not universal generative models, and that universality can be achieved by expanding the model while preserving efficient classical training and sampling hardness. In our approach, we introduce and analyze the role of constant-function postprocessing, generalizing the construction for IQP-QCBMs, which under suitable conditions can lead to universality while preserving the hardness of classically simulating the models. We showcase a family of BSBMs, characterized by a single hyperparameter, that allows for a monotonic increase in expressivity toward universality while retaining the capacity to represent ostensibly hard distributions. Furthermore, we discuss the possible modalities for the efficient classical training, in the sense of efficient estimation of gradients of the loss function.
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All authors Andrii Kurkin, Ulysse Chabaud, Zoltán Kolarovszki, Bence Bakó, Zoltán Zimborás, Vedran Dunjko
Quantum Physics
Permutation-invariant codes: a numerical study and qudit constructions
Liam J. Bond, Jiří Minář, Māris Ozols et al. (+2 more)
We investigate Permutation-Invariant (PI)
quantum error-correcting codes encoding a logical qudit of dimension $\mathrm{d}_\mathrm{L}$ in PI states using physical qudits of dimension $\mathrm{d}_\mathrm{P}$. We extend the Knill–Laflamme (KL) conditions for $d-1$ deletion errors from
qubits to qudits and investigate numerically both
qubit ($\mathrm{d}_\mathrm{L} = \mathrm{d}_\mathrm{P} = 2$) and qudit ($\mathrm{d}_\mathrm{L} > 2$ or $\mathrm{d}_\mathrm{P} > 2$) PI codes. We analyze the scaling of the block length $n$ in terms of the code distance $d$, and compare to existing families of PI codes due to Ouyang, Aydin–Alekseyev–Barg (AAB) and Pollatsek–Ruskai (PR). Our three main findings are: (i) We
conjecture that
qubit PI codes correcting up to $d-1$ deletion errors have block length $n(d) \geq (3d^2 + 1) / 4$, which implies an upper bound $d \leq \sqrt{12n-3}/3$ on their code distance, and that PR codes can saturate this bound. (ii) For qudit PI codes encoding a single qudit we numerically observe that increasing $\mathrm{d}_\mathrm{P}$ results in $n$ monotonically decreasing and approaching the
quantum Singleton bound $n(d) \geq 2d-1$. (iii) We propose a semi-analytic extension of the
qubit AAB construction to qudits that finds explicit solutions by solving a linear program. Our results therefore provide key insights into lower bounds on the block length scaling of both
qubit and qudit PI codes, and demonstrate the benefit of increased physical local dimension in the context of PI codes.
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All authors Liam J. Bond, Jiří Minář, Māris Ozols, Arghavan Safavi-Naini, Vladyslav Visnevskyi
Quantum Physics · Cross-listed from cond-mat.stat-mech
Violating the All-or-Nothing Picture of Local Charges in Non-Hermitian Bosonic Chains
Mizuki Sanatani and Naoto Shiraishi
We present explicit counterexamples to a widespread empirical expectation that local commuting charges display all-or-nothing behavior. In the class of bosonic chains with symmetric nearest-neighbor hopping and arbitrary on-site terms (including non-Hermitian terms), we exhibit systems that possess k-local charges for some but not all k. Concretely, we construct non-Hermitian models with a 3-local charge but no other nontrivial local charges and models with k-local charges for all k except k = 4. These results show that the Grabowski–Mathieu integrability test based on 3-local charges is not universally applicable. We further give necessary and sufficient conditions for the existence of k-local charges in this class, yielding an exhaustive classification and uncovering additional integrable models.
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Quantum Physics · Cross-listed from cond-mat.str-el
Generalized Reduced-Density-Matrix Quantum Monte Carlo Gives Access to More
Zhiyan Wang, Zhe Wang, Bin-Bin Mao et al. (+1 more)
For a long time, people have been focusing on how to extract more information, such as off-diagonal observables, from the
quantum Monte Carlo (QMC) simulation of the partition function, but there have been numerous difficulties, and many of them are insurmountable. In this article, we point out that all the difficulties stem from the starting point of the simulation: calculating a partition function. We introduce a paradigm shift: when we transform the simulated object from a partition function to a generalized reduced density
matrix (GRDM), the difficult problem of measurement can be readily solved. By designing the GRDM, both equal-time and nonequal-time off-diagonal observables have been measured easily in QMC with a
polynomial computation complexity. As a demonstration, the GRDM enables direct access to nonequal-time correlators for dynamical spectra as well as Rényi-1 correlators that reveal strong-to-weak
symmetry breaking in the mixed state, capabilities that lie beyond the reach of prior methods. This establishes a unified framework for
holographic characterization within QMC.
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All authors Zhiyan Wang, Zhe Wang, Bin-Bin Mao, Zheng Yan
Quantum Physics
Gauge-invariant QMETTS with mutually unbiased physical bases for $Z_2$ lattice gauge theories at finite temperature and density
Reita Maeno
In
quantum computations of gauge theories at finite temperature and finite density, enforcing Gauss's law for all states contributing to the thermal ensemble is a nontrivial challenge. In this work, we adopt the
Quantum Minimally Entangled Typical Thermal States (QMETTS)
algorithm for $Z_2$ gauge-constrained systems and propose a method for computing finite-temperature and finite-density expectation values without eliminating gauge degrees of freedom. To preserve gauge invariance while maintaining efficient sampling, we introduce measurement bases that are gauge invariant and mutually unbiased within the physical subspace. We show that such measurement bases can be constructed efficiently for $Z_2$
lattice gauge theories in general dimensions and arbitrary boundary conditions by exploiting the correspondence between $Z_2$
lattice gauge theories and the stabilizer formalism. Furthermore, since expectation-value estimation on
quantum hardware is inherently affected by shot noise, we explicitly incorporate shot noise into the analysis. We find that the single-shot strategy is near optimal under a fixed total number of
circuit executions in terms of the variance. This result indicates that it is generally more efficient to generate more QMETTS samples than to accurately estimate the expectation value for each individual pure state. We validate the proposed method numerically in a $(1+1)$-dimensional $Z_2$
lattice gauge theory coupled to staggered
fermions. Our results provide a gauge-invariant framework for finite-temperature and finite-density calculations on
quantum devices.
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Quantum Physics
Variational Adaptive Gaussian Decomposition: Scalable Quadrature-Free Time-Sliced Thawed Gaussian Dynamics
Rahul Sharma and Amartya Bose
Time-slicing has emerged as a strategy for incorporating semiclassical propagation into real-time path integral formulation and recovering full
quantum dynamics. A central step is the decomposition of a time-evolved wave function into a superposition of Gaussian wave packets (GWPs). Here we introduce a quadrature-free variational framework for GWP decomposition, reformulating it as an optimization problem in which the GWP parameters are chosen to maximize the overlap with the time-evolving wave function. An autoencoderdecoder neural network is used for this optimization, with the representation being adaptively reoptimized during propagation. Each wave packet in this decomposition represents a localized patch of the underlying semiclassical
manifold, while retaining full correlations between all degrees of freedom. This variational adaptive Gaussian decomposition (VAGD) approach yields a compact Gaussian expansion, providing a scalable route to time-sliced semiclassical
quantum dynamics. While general, applying VAGD to facilitate time-slicing of thawed Gaussian dynamics allows a route to improving the semiclassical treatment to the full
quantum mechanical result in a systematic manner.
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Quantum Physics · Cross-listed from cond-mat.supr-con
Supercurrents in Josephson junctions with chiral molecular potentials
Oleg Kuliashov, Alberto Cappellaro, Oded Millo et al. (+3 more)
The influence of
chiral molecular potentials on phase-coherent transport in superconducting Josephson junctions is investigated. Within a Bogoliubov–de Gennes tight-binding framework, a superconducting–normal–superconducting (SNS) junction functionalized by adsorbed
chiral molecules is modeled, where electrostatic gradients generated by the molecules induce
spin–orbit coupling in the normal region. The
equilibrium charge current–phase relation is found to remain largely insensitive to molecular chirality in symmetric, zero-field configurations. In contrast, the
spin-polarized
equilibrium current exhibits a pronounced chirality-dependent response, with opposite enantiomers producing distinct and anisotropic
spin-polarized Josephson currents. The resulting handedness contrast can be enhanced through control parameters such as molecular orientation and the strength of the induced
spin–orbit coupling. The temperature dependence of these currents further shows that the chirality-dependent signatures persist across a range of temperatures well below the superconducting critical temperature. These results identify Josephson interferometry as a phase-sensitive setting for probing chirality-dependent
spin structure and highlight
spin-polarized superconducting transport as a controlled route toward integrating
chiral molecular functionality into superconducting spintronic devices.
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All authors Oleg Kuliashov, Alberto Cappellaro, Oded Millo, Yossi Paltiel, Mikhail Lemeshko, Ragheed Alhyder