Quantum Physics · Cross-listed from cond-mat.supr-con
Anomalous Microwave Response in YBCO Resonators beyond the Two-Level-System Model
Kaiwen Zheng, Nathan J. Johnson, Nathan T. Thobaben et al. (+5 more)
We report the microwave response of coplanar-waveguide (CPW) resonators fabricated from $\mathrm{YBa_2Cu_3O_{7-\delta}}$ (YBCO) thin films over temperatures from approximately $70~\mathrm{mK}$ to $40~\mathrm{K}$. The resonators exhibit internal quality factors $Q_\mathrm{i}$ in the range of $4\times10^3$ to $10^4$ at 70 mK, which increase to a maximum of approximately $8\times10^3$ to $1.2\times10^4$ near $6~\mathrm{K}$. At low temperatures, both $Q_\mathrm{i}$ and the fractional shift of the resonance frequency $\Delta f_\mathrm{r}/f_\mathrm{r}$ increases with temperature, qualitatively resembling behavior commonly associated with two-level-system (TLS) defects. However, neither response saturates on the temperature scale set by the resonator frequency, and the loss exhibits no observable microwave-power dependence. We show that low-temperature frequency upturn may be better described by an additional paramagnetic response associated with defect-induced local moments or Andreev bound states, while the low-temperature loss follows an approximately logarithmic temperature dependence whose microscopic origin remains unresolved. These measurements establish the millikelvin performance of patterned YBCO resonators and show that their low-temperature response cannot be understood within the conventional TLS framework alone.
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All authors Kaiwen Zheng, Nathan J. Johnson, Nathan T. Thobaben, Sidharth Duthaluru, Haochen Shen, Denae T. Cherry, David S. Wisbey, Kater W. Murch
Quantum Physics
Area-Information Trade-Offs in Acceleration Radiation from Atoms Falling into Black Holes
Yusef Maleki, Gustavo Valdivia-Mera, Carlos R. Ordonez et al. (+2 more)
We develop a
geometric theory of information processing in the Horizon-brightened acceleration radiation (HBAR) channel, in which the radiative horizon-area change provides an
entropy budget for the information carried by the radiation field. Building on the
quantum-optical description of atom–field interactions near the horizon and the resulting HBAR thermodynamic correspondence, we derive area-cost laws in the near-steady, thermally saturated regime. The accessible classical information and the mutual information generated between the radiation field and its environment are bounded by the associated radiative horizon-area budget. Reliability is incorporated through Fano's inequality, which translates a prescribed decoding error probability into an area requirement. We further derive Fisher-information speed limits that constrain the statistical
evolution of the radiation field and place a lower bound on the duration required for correlation generation. Together, these results establish a bits-per-area principle linking black-hole thermodynamics, information geometry, and
quantum information in the HBAR framework.
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All authors Yusef Maleki, Gustavo Valdivia-Mera, Carlos R. Ordonez, Horacio E. Camblong, Marlan O. Scully
Quantum Physics
Geometry-Induced Domain-Wall Pinning and $\mathbb{Z}_2$ Asymmetry in Nominally One-Dimensional Rydberg Arrays
Elijah Pelofske, Frank Barrows and Cristiano Nisoli
We perform experiments on QuEra's neutral-Rydberg-atom Aquila
quantum computer, using quasi-adiabatic
evolution on 1-dimensional models. We use hardware fine-tuning to balance the measurement statistics close to zero net magnetization, in two different
geometric outlines: one closed triangular model with 33 atoms, and one 47-atom square with an engineered atom vacancy. The nature of this processor restricts the geometry of the atoms that can be programmed to a 2D plane. We report effects not predicted by the pure 1D Ising model, such as pinning of domain walls and $\mathbb{Z}_2$
symmetry breaking due to the interplay between van der Waals interactions and the Rydberg blockade. In particular, atoms around the atom vacancy and at the vertices of the 1-dimensional square-outline model strongly prefer the Rydberg state, leading to the effective model having a
ferromagnetic bond across the vacancy.
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Quantum Physics
Restrictions on non-Clifford fault tolerance and ruling out beyond-SQL quantum metrology
Constantin Cedillo Vayson de Pradenne, Ishaan Kannan, Harald Putterman et al. (+1 more)
Quantum metrology promises a quadratic speedup over the standard
quantum limit (SQL), but signal-aligned noise is expected to preclude this advantage in realistic settings. A potential route around known no-go results is to encode the sensors in a
quantum code where the physical signal acts transversally as a logical gate. Understanding restrictions on transversal non-Clifford gates is therefore central to both
quantum metrology and fault-tolerant
quantum computation. Here, we prove such restrictions and apply them to transversal sensing. For any stabilizer code of distance $d\ge 3$ supporting a transversal logical action in level $D$ of the Clifford hierarchy, every stabilizer generating set must contain a check of weight at least $2^D$. Moreover, any $r$-level concatenated realization satisfies $r\leq \lfloor \log_2 n/D\rfloor$, forcing $r=1$ and ruling out concatenation when applied to beyond-SQL metrology. We then show that transversal single-
qubit rotations by a small angle $\theta$ can only induce a nontrivial logical action on an $n$-
qubit code if its checks include irreducible stabilizers of weight $\Omega(1/(n|\theta|^2))$. Here, many single-
qubit errors commute with every stabilizer or logical Pauli below this weight and are only detected by a high-weight check, so their syndromes cannot be fault-tolerantly reconstructed from low-weight normalizer measurements. Since beyond-SQL transversal sensing requires $|\theta| = o(n^{-1/2})$, the weight of checks required for syndrome extraction diverges with $n$. Finally, we prove a broader metrological no-go
theorem that avoids the assumptions of the
quantum Cramér-Rao bound: constant-strength signal-aligned noise rules out any asymptotic advantage over the SQL in AC or DC sensing, even with biased
estimators, nonstabilizer or approximate encodings,
quantum memory, intermediate measurements, or adaptive control.
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All authors Constantin Cedillo Vayson de Pradenne, Ishaan Kannan, Harald Putterman, Jordan Cotler
Quantum Physics
Quantum vs Classical Erasure: Equal Bounds but Unequal Costs
Jan Neuser, Jake Xuereb, Pharnam Bakhshinezhad et al. (+1 more)
Irreversibility has a fundamental thermodynamic cost, erasing information inevitably generates heat. This connection is quantified by the Landauer bound, which gives the minimum dissipation needed to erase a single bit of information. While this bound applies in both classical and
quantum settings, it is saturated only in idealised limits of infinite resources. Here, we provide a unified first principles description of finite-resource erasure in both classical and
quantum systems. We begin by proving the communal folklore that in the idealised regime the erasure cost of a bit encoded in a
quantum or classical system is the same. Despite this, we show that their practical implementation differs substantially: achieving comparable erasure quality in
quantum systems requires more control, larger accessible energy gaps and longer operation times. Classical protocols can achieve the erasure of a comparable
quantum protocol under far weaker constraints which we expose in trade-off relations. Our results explain why practical erasure schemes fall short of Landauer's bound and show that classical systems enjoy several fundamental thermodynamic advantages.
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All authors Jan Neuser, Jake Xuereb, Pharnam Bakhshinezhad, Marcus Huber
Quantum Physics
Quantum State Discrimination With Stabilizer Circuits
Benjamin Stratton, Santiago Zamora and Rafael Chaves
The task of
quantum state discrimination provides an operational characterization of distinguishability and plays a central role in
quantum information science. Since the achievable success probability in
quantum state discrimination depends both on the states being discriminated and on the allowed measurements, it is natural to study discrimination under physically motivated measurement constraints. Here, we investigate minimum-error
quantum state discrimination under measurements implementable by both fixed and adaptive stabilizer
circuits. Given arbitrary stabilizer-state ancillas, we show that fixed stabilizer
circuits provide no additional discrimination power, whereas adaptive
circuits do. Then, focusing on single-
qubit systems, we derive analytical expressions for the success probability across both
circuit classes when supplied with arbitrary (non-stabilizer) ancillas, showing that the performance gap between fixed and adaptive
circuits persists. We further consider discrimination with non-stabilizerness incorporated directly into the measurement operators. Here, the optimization is formulated as a semidefinite program, and analytical bounds interpolating between the stabilizer and Helstrom limits for
qubits are derived. Finally, we illustrate applications in
quantum random access codes and bounds on
unitary synthesis fidelity with a finite number of magic states.
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Quantum Physics · Cross-listed from hep-ph
Suppressed Quantum Effects of Weakly Coupled Waves
Yunjia Bao, Dhong Yeon Cheong, Nicholas L. Rodd et al. (+3 more)
Precision experiments increasingly target weakly coupled waves, including
axion dark matter and gravitational radiation. Such waves are commonly described as classical fields, yet they could exist in
quantum states with no classical counterpart. We exhibit two severe obstructions to detecting nonclassical effects, both independent of the mode occupancy. First, realistic detectors cannot resolve the fundamental modes of a field; instead they couple to coarse-grained "effective" modes, which often washes out nonclassical effects. Second, all nonclassical effects are suppressed by extra powers of the weak coupling, making them much harder to detect than the waves themselves. We prove this in general, and explicitly show how the suppression arises for quadrature and number statistics,
entanglement, and decoherence. The suppression can in principle be overcome given suitable
quantum resources, such as highly squeezed detector states, but the required parameters are far beyond current experimental capabilities. We use the
axion cavity haloscope as an explicit example, although our conclusions apply to many ultralight dark matter searches, and rule out proposals to establish the quantization of
gravity from observations of gravitational waves.
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All authors Yunjia Bao, Dhong Yeon Cheong, Nicholas L. Rodd, Joey Takach, Lian-Tao Wang, Kevin Zhou
Quantum Physics
LLM-Guided Initialization for Accelerated Hybrid Quantum-Classical Medical Image Classification
Riza Alaudin Syah, Irwan Alnarus Kautsar and Haza Nuzly Bin Abdull Hamed
Variational
quantum algorithms often encounter barren plateaus, where cost gradients decay rapidly with increasing
circuit depth, undermining the trainability of parameterized
quantum circuits. This paper evaluates AdaInit (Adaptive Initialization), proposed by Zhuang and Cunningham, which uses large
language models to propose initial parameters for
quantum neural networks. We study a simplified single-query AdaInit variant paired with GPU-accelerated simulation in NVIDIA CUDA-Q and apply it to binary classification on the DMR-IR mammography
dataset. AdaInit delivers 14.6 times higher gradient variance at initialization than random initialization (0.0095 vs. 0.0006), producing 160 times faster convergence (1.1s vs. 176 s) while maintaining the same classification accuracy of 61.4 percent. We provide theoretical analysis grounded in the geometry of parameterized
circuit landscapes and show empirically that LLM-guided initialization places the optimizer in trainable regions of parameter space. Beyond performance, our results indicate that a single LLM query can yield informative parameters without iterative refinement, suggesting a low-overhead path to improved trainability. The findings validate AdaInit in a medical imaging setting and demonstrate its compatibility with GPU-accelerated
quantum backends for practical speedups.
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Quantum Physics
Practical Quantum Topological Data Analysis with Applications to High-Dimensional Feature Extraction and Time Series Analysis
Jason Iaconis, Sayonee Ray, Samwel Sekwao et al. (+2 more)
Topological data analysis (TDA) provides a powerful framework for extracting information about the shape of complex, unstructured data, but the classical cost of computing high dimensional topological features limits its application.
Quantum algorithms for TDA offer a route around this bottleneck, yet existing approaches typically focus on exact or high precision Betti number estimation, making the regime for practical
quantum advantage appear narrow. Here, we instead frame
quantum TDA as a feature-extraction method for downstream data analysis by extracting low-order spectral information from the combinatorial
Laplacian as a proxy for high-dimensional
topology. We support this perspective from both the application and algorithmic sides. First, we show that higher-order TDA features improve predictive performance in two time-series applications: functional MRI analysis for neurodegenerative disease classification and financial time-series analysis for identifying
market instability. Second, we develop a moment-based
quantum algorithm and show that low-order moments, including the relative trace, are strongly correlated with high-dimensional Betti information, even when the relative Betti number is small. Finally, we present
circuit constructions, resource estimates,
quantum-classical crossover projections, and experimental results from a Barium development system similar to the forthcoming IonQ Tempo line, extracting
Laplacian-derived observables from graph instances and quantitatively comparing them with exact Betti information. Together, these results establish
quantum TDA as a practical approach for extracting topological features from classically challenging data
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All authors Jason Iaconis, Sayonee Ray, Samwel Sekwao, Claudio Girotto, Martin Roetteler
Quantum Physics
Fault-Tolerant Logical Operations and Efficient State Preparation in Modular Quantum Architectures with Noisy Interfaces
S. Siddardha Chelluri, Riccardo Mengoni, Tom Darras et al. (+3 more)
Modular
quantum computing is a leading paradigm for scaling
quantum computation beyond the resource limitations of monolithic devices. In this architecture, multiple
quantum processing units (QPUs), employing identical or distinct
qubit modalities, are interconnected via shared
entanglement. Here, we investigate how errors at module interfaces and within individual QPUs affect fault-tolerant computation when
qubits are encoded using the rotated surface code. Going beyond the logical-
memory benchmark, we perform
circuit-level simulations of fault-tolerant nonlocal CNOT gates implemented via
lattice surgery between QPUs connected by noisy Bell pairs, and analyze the resulting logical error rates. Our results show that interfaces can tolerate noise up to an order of magnitude higher than intra-QPU noise, with only a minor reduction in the fault-tolerance threshold. We further develop an efficient protocol for preparing distributed fault-tolerant logical GHZ states, reducing ancilla overhead, time, and nonlocal Bell-pair consumption. We show that ancilla minimization in this setting is equivalent to a vertex-cover problem on an associated graph, and introduce a
polynomial-time heuristic
algorithm for finding low-overhead solutions. Our results provide quantitative evidence that distributed
quantum error correction can enable scalable, fault-tolerant
quantum computation in modular architectures.
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All authors S. Siddardha Chelluri, Riccardo Mengoni, Tom Darras, Julien Laurat, Eleni Diamanti, Ioannis Lavdas
Quantum Physics
Tulja Varun Kondra, Raphael Brinster, Hermann Kampermann et al. (+2 more)
The manipulation of
quantum entanglement is fundamentally irreversible: some mixed entangled states require pure
entanglement for their preparation, although no pure
entanglement can be recovered from them by local operations and classical communication. This irreversibility is known to persist even under the maximal class of operations that do not generate
entanglement, revealing a fundamental distinction between
entanglement theory and thermodynamics. We construct cases for which any attempt to restore reversibility necessarily incurs an error that increases exponentially with the number of copies. Technically, we demonstrate a strict separation between the exponential strong-converse distillable
entanglement and the exponential strong-converse
entanglement cost. Our result resolves a
conjecture posed by Lami and Regula (Nat. Phys. 19, 184-189 (2023)) and strengthens it by showing that the irreversibility of
entanglement persists even at the level of polynomially (in the number of copies) growing error. We further derive a semidefinite-programming lower bound on the exponential strong-converse cost under non-entangling operations. Finally, for the class of completely PPT-preserving operations, we construct analytically solvable families of antisymmetric states exhibiting the exponential strong-converse irreversibility. Remarkably, to our knowledge, no analogous separation between exponential strong converse cost and the analogous distillable
entanglement is currently known even under the more restrictive class of LOCC operations.
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All authors Tulja Varun Kondra, Raphael Brinster, Hermann Kampermann, Dagmar Bruß, Nikolai Wyderka
Quantum Physics
Sharp Bounds on Ground State Energy of the SYK Model
Arpon Basu, Pravesh K. Kothari and Siddhant Midha
We study the Sachdev-Ye-Kitaev (SYK)
Hamiltonian $H_{\operatorname{SYK}}$ on $n$ Majorana modes with $k$-body interactions, and prove that $\mathbb{E}\|H_{\operatorname{SYK}}\|_{\operatorname{op}} = (1 - o(1))\cdot\sqrt{2n}/k$ for super-constant $k\leq o(\sqrt{n})$, where the expectation is over the disorder variables in the
Hamiltonian. This confirms predictions due to Garcia-Garcia, Jia and Verbaarschot and answers a question posed by Feng, Tian, and Wei. Our results extend to the sparse SYK
Hamiltonian. As a corollary, we obtain that a dissipative
quantum algorithm due to Basso, Chen, and Dalzell provably computes the ground state energy of the SYK
Hamiltonian up to an $O(1)$-multiplicative factor for all $k < \sqrt{n}/4$. Our key technical idea is identifying an explicit, deterministic linear operator $\mathsf{x}$ such that a fixed quadratic form of $\mathsf{x}^{2\ell}$ exactly equals the expected trace moments of the SYK
Hamiltonian for every $n$ and $k$. This linear operator can be naturally viewed as a twisted model of
bosons on the space of hyperedges of a hypergraph. The problem thus reduces to identifying the spectral edge of $\mathsf{x}$, which we show is dominated by the spectrum of a natural ${n \choose k}$-dimensional
matrix from the Johnson scheme and is straightforward to compute using known results. To show that our bound is sharp, we construct a witness state with a large quadratic form on $\mathsf{x}$ and transform it into a certificate of a lower bound on the largest quadratic form on $H_{\operatorname{SYK}}$. Beyond the SYK
Hamiltonian, our techniques also apply to the $k-$local
quantum spin glass (QSG), yielding $\mathbb{E}\|H_{k-\operatorname{QSG}}\|_{\operatorname{op}}\leq O(\sqrt{n/k})$ for all $1\leq k\leq\sqrt{n}/2$, thus improving the standard $O(\sqrt{n})$ bound by a factor of $\sqrt{k}$.
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Quantum Physics
Robust quantum state certification and uncertainty principles for total influence
Andrea Coladangelo, Jerry Li and Joseph Slote
We show that nonadaptive single-
qubit Pauli measurements suffice to test whether an unknown $n$-
qubit state $\rho$ is $\varepsilon$-close to or $O(\varepsilon)$-far from an ideal target state $|\psi\rangle$, for all but a $2^{-\Omega(n)}$ fraction of target states. The test uses $O(\varepsilon^{-2}\log(1/\delta))$ copies of $\rho$ to achieve confidence $1-\delta$, which is information-theoretically optimal even among protocols with arbitrary joint measurements. The main technical innovation is an uncertainty principle for weighted generalizations of the total influence of Boolean functions. As a simple example, the unweighted variant states that $\mathbf{Inf}[f]+\mathbf{Inf}[\widehat{f}] = \Omega(n)$, which is a natural hypercube analogue of the Heisenberg uncertainty principle (here $\widehat{\,\cdot\,}$ denotes the $2^{-n/2}$-normalized
Fourier transform). The weighted case generalizes $\mathbf{Inf}[\,\cdot\,]$ and $\mathbf{Inf}[\,\widehat{\,\cdot\,}\,]$ to Dirichlet energies associated with Glauber dynamics for certain dual measures on the cube.
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Quantum Physics
Reconfigurable Optical Platform for One-way Quantum Communication Complexity
Francesco Mazzoncini, Hugo Defienne, Romain Alléaume et al. (+1 more)
Demonstrating a practical
quantum advantage remains a central goal in
quantum information science. While
quantum computational supremacy is still technologically demanding, communication complexity offers a promising route to showcase
quantum advantage with current photonic platforms. Here we introduce a reconfigurable optical platform for one-way
quantum communication complexity based on multimode fibers and wavefront shaping. We experimentally validate it by implementing a genuine one-way
quantum communication complexity problem for which an exponential
quantum–classical communication separation is known. Complementary numerical simulations show that the same reconfigurable decoding architecture can support more general one-way communication tasks with comparable performance, while also offering a route to higher-dimensional implementations without increasing hardware complexity. Together, these results establish multimode-fiber wavefront shaping as a versatile hardware platform for one-way
quantum communication complexity and provide a concrete roadmap toward more demanding protocols, where stronger
quantum–classical separations could enable practical demonstrations of
quantum advantage.
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All authors Francesco Mazzoncini, Hugo Defienne, Romain Alléaume, Sylvain Gigan
Quantum Physics
Classical and Quantum MacWilliams Transforms as Spin Kinematics
Eric Kubischta and Ian Teixeira
Spin is the hidden engine behind the zoo of MacWilliams transforms in weight enumerator theories - not only for
qubits and qudits, but even for classical codes. From nothing more than a split into trivial and nontrivial errors, we kinematically derive the MacWilliams transform as a Wigner-$D$ rotation between two canonical bases. Within each classical and
quantum theory, changing the length $n$ leaves the rotation untouched: the same element simply reappears at
spin $n/2$. And at fixed $n$, changing the rotation axis simply moves between the various classical and
quantum theories.
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Quantum Physics
OQRAM: Oblivious Quantum Random Access Memory for Securing Delegated Quantum Queries
Shifan Xu, Yizhuo Tan, Yongshan Ding et al. (+1 more)
Quantum query is a basic subroutine in many
quantum algorithms, and
Quantum Random Access
Memory (QRAM) provides a natural way to realize such coherent query access. In delegated settings, however, a standard QRAM query interface can expose sensitive information to the server. This paper introduces oblivious QRAM, a cryptographic abstraction for
privacy-preserving delegated coherent query access. The protocol consists of an offline refresh phase and an online protected query phase. The database is stored in an encrypted and shuffled layout, and each query is protected by coherent address masking using either a
quantum-secure pseudorandom permutation (qPRP) based method or a
quantum one-time pad (qOTP) based method. In the adopted client model, the online protection adds only modest
quantum overhead beyond the query register, avoiding the exponential
quantum resources that would otherwise be required by an equivalent local QRAM construction. The qPRP-based variant also supports multi-query use by distributing database refresh across multiple queries to reduce classical communication. To address malicious servers, decoy checks are further incorporated to strengthen
privacy protection and enable probabilistic tampering detection. Compared with fully blind
quantum computing, this framework provides a lighter abstraction tailored to private delegated QRAM access, significantly reducing
quantum resource requirements on both the client and server sides and achieving an exponential reduction in
quantum communication.
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All authors Shifan Xu, Yizhuo Tan, Yongshan Ding, Jakub Szefer
Quantum Physics
Training Quantum Dragons
Muhammad Yusf, Bhola Devkota, Mark A. Novotny et al. (+1 more)
The Non-
Equilibrium Green's Function (NEGF) is the standard formalism for nano-scale electron transport. By recasting the NEGF scattering problem as a linear system of equations whose solution encodes the transmission and reflection amplitudes, we present the first
quantum computerized implementation of NEGF. We apply both the Harrow–Hassidim–Lloyd and Variational
Quantum Linear Solver
algorithms to compute the transmission coefficient $T(E)$ of
quantum dragon nanodevices within the single-band tight-binding model.
Quantum dragon devices exhibit perfect transmission across the full conducting band regardless of internal disorder. The problem maps onto compact
circuits of 3 and 4 total physical
qubits for the 2-site and 6-site dragon devices, respectively. A similarity transformation block-diagonalizes the NEGF linear system reducing the Pauli decomposition of the block-encoded
matrix. We demonstrate the feasibility of
quantum computation by performing ideal and noise-aware simulations and computations on physical IBM
quantum processor.
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All authors Muhammad Yusf, Bhola Devkota, Mark A. Novotny, Gautam Rupak
Quantum Physics
Improved Methods for Determining Quantum Error Correcting Code Performance and Fault Tolerance
Michael Mullan, Matthew Weippert and Winton Brown
One of the central challenges in
quantum error correction is determining the performance of a code in the low-error regimes needed to implement utility-scale computations. While performance at these error rates is not amenable to direct Monte Carlo simulation, it can be extrapolated from simulations at higher logical error rates, assuming the logical error rate scales predictably with increasing distance or decreasing physical error rate. However, the expected scaling depends sensitively on the minimum weight of uncorrectable error patterns. In many cases, the minimum weight is unknown since it depends not only on the theoretical code distance, but also on details of the implementation. Markov chain Monte Carlo (MCMC) methods, as adapted to
quantum error correction by Bravyi and Vargo, provide a way to estimate logical failure rates in these low-error regimes via simulation. While offering significant gains over Monte Carlo, the described Metropolis
algorithm makes small changes to the current logical failure patterns which results in slow convergence. In this paper, we argue that typical failure patterns include a large number of easily correctable errors that coexist alongside a malignant core. This observation motivates two new approaches to better evaluate code performance. First, we describe a
pruning algorithm designed to obviate these correctable errors and focus on the problematic low-weight core. Second, we develop a novel family of Metropolis-Hastings
algorithms, referred to as subregion MCMC. This technique is parameterized by the fraction of the error pattern that is resampled at each step, effectively interpolating between Monte Carlo and single step MCMC. We show that a judicious choice of this parameter results in far faster convergence than prior work.
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Quantum Physics
Parity-Based Time-Bin Encoding Enabling SWAP Between Polarization and Time-Bin Qubits
Adam Sultan and Connor Kupchak
Multi-degree-of-freedom photonic
quantum processing requires routing between degree-of-freedom (DOF)
qubit encodings on a single
photon. A SWAP between polarization and time-bin
qubits is an advantageous primitive for such architectures, however conventional early/late time-bin encoding does not support bidirectional logical time-bin flips from late to early which limits the ability to implement certain
quantum operations. We introduce a parity-based time-bin encoding in which logical $\ket{0}_T$ and $\ket{1}_T$ correspond to even and odd multiples of a spacing $\Delta t$, so that a physical delay of $\Delta t$ implements $\ket{0}_T \leftrightarrow \ket{1}_T$. This encoding is the enabling ingredient that makes a polarization-controlled delay line implement $\mathrm{CNOT}_{P \rightarrow T}$ and aligns naturally with periodic refractive index modulation for $\mathrm{CNOT}_{T \rightarrow P}$. Composing three such CNOT operations sequentially results in a deterministic SWAP between polarization and time-bin degrees of freedom. We quantify how EOM voltage errors reduce the polarization-flip probability and estimate timing requirements from modulator switching, detector resolution, and timing jitter, giving an illustrative timing budget.
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Quantum Physics
Hardware-efficient erasure-error detection with an integer fluxonium
Junyoung An, Helin Zhang, Jeffrey M. Gertler et al. (+13 more)
Erasure-error detection can improve the efficiency of
quantum error correction by revealing the times and locations of their error events. In this work, we demonstrate erasure conversions and mid-
circuit erasure detections in a single integer fluxonium, in which the states $\mathrm{|g\rangle, |f\rangle}$ encode the logical states and $\mathrm{|e\rangle}$ encodes the erasure state. The integer fluxonium suppresses direct $|\mathrm{f} \rangle \rightarrow |\mathrm{g} \rangle$ transitions and allows the dominant $|\mathrm{f} \rangle \rightarrow |\mathrm{e}\rangle$ transitions to be converted into detectable erasures. Furthermore, we identified a design space that nullifies the resonant-frequency shift between the two logical states, enabling ancilla-free mid-
circuit erasure checks using the same resonator employed for final readout. By discarding the detected erasure events, we achieved an 8.4-fold increase in the $|\mathrm{f}\rangle$ state lifetime, a 1.38-fold increase in the Hahn-echo time, and a reduction of single-
qubit gate error from 0.061(2)% to 0.030(5)%. Our results establish integer fluxonium as a hardware-efficient platform for erasure-error detection and conversion, while identifying the improvements required to realize an effective erasure
qubit with high erasure bias.
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All authors Junyoung An, Helin Zhang, Jeffrey M. Gertler, Kate Azar, Renée DePencier Piñero, Michael Gingras, Junghyun Kim, Bethany M. Niedzielski, Ilan T. Rosen, Mollie E. Schwartz, Joel Î-j. Wang, Terry P. Orlando, Jeffrey A. Grover, Max Hays, Kyle Serniak, William D. Oliver
Quantum Physics
The Keyl-Werner algorithm is not optimal for spectrum estimation
Angelos Pelecanos, Jack Spilecki, Ewin Tang et al. (+1 more)
We give an
algorithm which, given $n = O(d^2 \cdot (\log\log(d)/\log(d))^2)$ copies of $\rho$, estimates the
eigenvalues of $\rho$ to constant error in total variation distance. Thus, we can learn the
eigenvalues of a
quantum state with fewer copies than the $\Theta(d^2)$ needed to run full state
tomography. This is the first improvement to spectrum estimation over the influential Keyl-Werner
algorithm, which uses $n = \Theta(d^2)$ copies, thereby resolving a question raised by Keyl and Werner in 2001 and refuting a 2016
conjecture of Wright. Our main technical tool is a new
tomography guarantee, where the error of
tomography in a particular direction $|w\rangle$ scales with $\langle w | \rho |w\rangle$ for all directions simultaneously. From this stronger "relative-error" bound, we recover better
algorithms for principal component analysis in Bures distance and
tomography in $\chi^2$-divergence as corollaries.
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All authors Angelos Pelecanos, Jack Spilecki, Ewin Tang, John Wright
Quantum Physics
Ky Fan majorization for binary tensor products
Michael M. Wolf and Yuanheng Zhou
We provide a short proof of a Ky Fan-type majorization relation for the singular values of a sum of binary tensor products of
matrices. This generalizes Alhejji's result (arXiv:2410.18254) from two summands to arbitrary sums and from positive
matrices to arbitrary
matrices. As an application, we show a majorization relation between the singular values of a completely positive map and those of its Kraus operators.
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Quantum Physics
Razvan Stanescu and Hugo Ribeiro
High-fidelity
quantum control relies on accurate models of driven dynamics. We examine this re- quirement for single-
qubit gates in superconducting transmons by comparing control pulses derived from the standard Duffing approximation and from a
Hamiltonian constructed by diagonalizing the transmon eigenbasis. Using the same correction-pulse construction for both models, we show that correction fields derived from the Duffing approximation can substantially reduce the gate error pre- dicted by that model while remaining less effective when combined with an independently calibrated baseline pulse in the diagonalized-transmon model. In the fast-gate regime, such transferred correc- tions can even fail to improve over the uncorrected diagonalized-transmon baseline. We show that small model-dependent differences in both the energy spectrum and the representation of the drive operator can compound during driven
evolution, resulting in different predicted error generators and correction pulses. A mismatch in the accumulated AC Stark phase provides one illustrative di- agnostic of this dynamical model dependence. We further demonstrate that the model
Hamiltonian informs the choice of control framework: Omitting relevant leakage pathways or higher-order error channels can lead to an overly restricted correction strategy. Including these channels motivates an extended correction framework that improves the gate performance using the same physical control resources.
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Quantum Physics
Native CCZ Gate with Fluxonium Qubits and a Microwave-Driven Coupler
Grigoriy S. Mazhorin, Tatyana A. Chudakova, Alena S. Kazmina et al. (+12 more)
Native multi-
qubit gates could reduce the overhead associated with decompositions into single- and two-
qubit operations, but whether they can simultaneously provide high fidelity, simple control and robustness against parasitic interactions in scalable architectures remains unclear. Here we experimentally realize a 65-ns native controlled-controlled-phase operation, locally equivalent to the Toffoli gate, with a fidelity of 99.39(5)% in a three-
qubit processor unit based on fluxonium
qubits coupled via a microwave-driven transmon coupler. The implemented operation would require CZ fidelities of approximately 99.94% if realized through a conventional decomposition. The gate is implemented with a single control pulse, that relies on a simple calibration procedure yielding coherence-limited performance. This processor unit naturally extends to scalable two-dimensional layouts with low parasitic interactions. Altogether, these results establish native multi-
qubit gates as a viable hardware-efficient primitive for scalable superconducting
quantum processors.
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All authors Grigoriy S. Mazhorin, Tatyana A. Chudakova, Alena S. Kazmina, Nikolai G. Berezkin, Arina V. Zotova, Artyom M. Polyanskiy, Nikolay N. Abramov, Mikhail A. Tarkhov, Alexander M. Mumlyakov, Igor V. Trofimov, Elizaveta A. Krivko, Nikita Yu. Rudenko, Maxim V. Chichkov, Vladimir I. Chichkov, Ilya A. Simakov
Quantum Physics
Adaptive Multi-Backend Simulation of Near-Clifford Quantum Circuits via Spatial Stabilizer-Frame Partitioning
Héctor J. García
We present an exact amplitude simulator for Clifford+T
quantum circuits that combines a Feynman path sum across a balanced
qubit bipartition with stabilizer-frame simulation on each half. The construction extends prior stabilizer-based Schrödinger-Feynman methods in three directions: recursive multilevel bipartition into a binary tree, automatic fallback to dense state-vector simulation when a leaf's stabilizer frame would exceed its
memory ceiling, and a cost-model-driven partition selector that replaces the standard cut-count minimization heuristic. We show cut-count minimization is an unreliable proxy in practice: a globally cleaner partition can reduce cross-cut count yet increase wall-clock time, because it imbalances T-gate density across halves and inflates per-half stabilizer-frame size. Our cost model substitutes the stabilizer-frame bound 2w for the dense 2n ceiling per side and explicitly models per-amplitude readout cost; isolating that term uncovered a quadratic-asymptotic inefficiency in the leaf simulator's end-of-path amplitude extraction, fixed by replacing it with an existing O(F * s * n) single-amplitude inner product. On a structured hierarchical n=16
benchmark the recursive simulator beats monolithic stabilizer-frame simulation by 92x to 17,645x, wins by 79x per path against a dense half-state-vector baseline under an identical cut, and beats a production state-vector simulator end to end by up to 47.9x (median ~5x). On
adversarial random Clifford+T
circuits the dense state vector wins past a crossover near n/2 cross-cut gates – the regime the cost model identifies. The dominant cost, the cross-cut Feynman sum, is embarrassingly parallel with constant inter-worker communication, unlike recent
matrix-product-state stabilizer-tensor methods whose inner contraction loop is sequential.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
A Degenerate Singlet-Triplet Qubit with All-Electrical Orthogonal Control
Phuong X. Nguyen, Konstantinos Tsoukalas, Jann H. Ungerer et al. (+8 more)
Singlet-triplet
qubits offer an attractive encoding for
semiconductor quantum computing, combining ancilla-free readout, reduced sensitivity to common-mode noise, and baseband voltage control. However, the Zeeman energy difference $\Delta E_\mathrm{Z}$ is typically fixed by local
magnetic field gradients or $g$-factor inhomogeneities, leaving the exchange interaction $J$ as the only dynamically tunable parameter. This always-on $\Delta E_\mathrm{Z}$ precludes orthogonal control of the
qubit's rotation axes and introduces unwanted state rotations during idling. Here we demonstrate all-electrical orthogonal control of a degenerate singlet-triplet (DST)
qubit formed by two hole
spins in a germanium double
quantum dot. Exploiting the electrically tunable anisotropic $g$-factors of the two
spins, we identify a regime where both $\Delta E_\mathrm{Z}$ and $J$ vanish, making the $S$ and $T_0$ states degenerate at the idle point. By applying only baseband voltage pulses, we independently control both $J$ and $\Delta E_\mathrm{Z}$, enabling fully orthogonal $Z$- and $X$-axis rotations. Randomized benchmarking yields an average physical single-
qubit gate fidelity of 99.53% for a gate duration of approximately 100 ns. Finally, we electrically tune the degenerate point across a wide range of
magnetic field orientations, enabling operation in a regime of enhanced coherence time and offering a route towards multi-
qubit scaling under a shared global
magnetic field.
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All authors Phuong X. Nguyen, Konstantinos Tsoukalas, Jann H. Ungerer, Julian Santen, Valentin John, Stefan D. Oosterhout, Lucas Stehouwer, Stefano Bosco, Giordano Scappucci, Menno Veldhorst, Amir Yacoby
Quantum Physics
Optimising Trotter-Suzuki Simulations of Markovian Open Quantum Systems via Classical Search
S. M. Pillay, I.J. David, I. Sinayskiy et al. (+1 more)
Simulating an open
quantum system on a digital
quantum computer often involves the use of Trotter-Suzuki (TS) Product Formulas (PF) to approximate the system's time
evolution. Precise estimates for the required number of Trotter steps (and hence the overall gate count) can be crucial for minimising the computational cost of these methods. Building on established theoretical guarantees, we derive analytic bounds for the First- and Second-Order Deterministic and Randomised TS-PF, directly relating the number of Trotter steps to the model parameters,
evolution time and precision. These bounds enable concrete resource estimation for each method. We then present a computationally efficient classical
algorithm that uses diamond norm estimates of individual Liouvillian terms and a binary search to significantly reduce the Trotter steps required for a target precision. Our numerical results on two prototypical models - an XX-
Spin Chain with boundary driving and local dephasing, and a Transverse-Field Ising Model - show that the theoretical (analytic) bounds are often overly conservative, whereas the empirical (optimised) bounds yield a significantly smaller number of Trotter steps for the same precision. Among the methods investigated, the Second-Order Randomised TS-PF typically achieves the lowest resource demands, especially for larger systems. These findings emphasise the significance of empirical bounding strategies in achieving more resource-efficient simulations of Markovian open
quantum systems.
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All authors S. M. Pillay, I.J. David, I. Sinayskiy, F. Petruccione
Quantum Physics
Quasi-polar Decomposition of Quantum Neural Networks via Adaptive Non-local Observables
Shih-Hao Ho, Yan Li, Huan-Hsin Tseng et al. (+3 more)
We use Diagonal Adaptive Non-local Observables (DANO) as a canonical decomposition for studying Variational
Quantum Circuit model
evolution. Separating each learned observable into a diagonal spectrum and a
unitary basis gives a quasi-polar description: the spectral weights are viewed as radial coordinates, while the
unitary circuit serves as angular coordinates through Lie group identifications. This turns the training process into a trajectory in spectral and Lie-
algebra space. Experiments on two classification tasks show that DANO radial spectral expansion correlates with accuracy. DANO angle coordinates reveal a dominant accuracy-correlated component. The framework provides a different perspective to characterize
quantum model behavior.
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All authors Shih-Hao Ho, Yan Li, Huan-Hsin Tseng, Hsin-Yi Lin, Samuel Yen-Chi Chen, Shinjae Yoo
Quantum Physics · Cross-listed from cond-mat.stat-mech
Temporal Interference from Topological Transitions in Monitored Quantum Dynamics
Qingyuan Wang, Ruoyu Yin and Eli Barkai
Quantum interference is conventionally associated with the coherent superposition of alternative paths. We uncover a temporal form of interference in repeatedly monitored
quantum systems, appearing directly as fringes in the
stochastic first-detection-time distribution. This temporal interference becomes remarkably long lived near a class of topological transitions at which a pair of dark states emerges in
Hilbert space. Close to such transitions, the corresponding quasi-dark modes dominate the late-time dynamics and interfere coherently, producing pronounced temporal fringes whose decay becomes arbitrarily slow upon approaching the transition. We identify the
symmetry conditions underlying this mechanism and relate the fringe frequency to the energy spectrum of the
Hamiltonian. Our results reveal how the interplay of
quantum superposition, topological transitions, and dark-state formation generates long-lived temporal interference in monitored
quantum dynamics.
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Quantum Physics
Unconventional Thermalization of a Three-Wave-Mixing Model
Evangelos Varvelis, Miriam Resch and Joachim Ankerhold
Understanding the boundaries between
quantum thermalization and localization in many-body systems remains a central frontier of condensed matter and
quantum information science. In this work, we investigate the dynamics and spectral properties of a generic model with long-range three-body-interaction, namely, a system with non-local three-wave-mixing. This model has been realized recently with a microwave Fabry-Perot cavity terminated on one end by a superconducting
qubit mirror. Utilizing exact diagonalization techniques, we uncover a striking paradox: the global energy level spacing statistics show integrability, even though all dynamic observables and inverse participation ratios of the eigenstates indicate ergodicity and delocalization. We show that this behavior is a hallmark of strong
Hilbert space fragmentation driven by kinematic constraints rather than an explicit global
symmetry. Inside these sectors, dynamics scramble rapidly, as evidenced by the out-of-time-ordered correlator (OTOC), while global transport is heavily bottlenecked, resulting in a logarithmic relaxation to
equilibrium. This picture is further confirmed by fluctuations in eigenstate
entanglement entropy at the same energy. Finally, we demonstrate that the late time OTOC average scales with system size, providing a distinct experimentally accessible signature of the underlying three-body kinetic bottlenecks.
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