Quantum Physics
SSP-QST: Spectral Subspace Purification for Photonic Quantum State Tomography
Anuvab Sen and Saibal Mukhopadhyay
Photonic
quantum sensing often uses low-rank entangled probes such as Greenberger-Horne-Zeilinger (GHZ), Bell, and NOON states. Although these probes are ideally rank-1, practical
quantum state
tomography (QST) can produce density-
matrix estimates with many small finite-shot and noise-induced eigenmodes. This
eigenvalue contamination can increase the estimated
entropy of the reconstruction and reduce the
quantum Fisher information (QFI) available for downstream sensing, while fixed rank-1 purification can discard valid signal modes when real probes acquire additional signal modes. We introduce Spectral Subspace Purification for
Quantum State
Tomography (SSP-QST), a rank-adaptive post-processing layer for least-squares
quantum state
tomography (LS-QST). SSP-QST eigendecomposes the least-squares estimate, computes a Weyl-motivated noise floor from the measured spectrum and shot count, removes eigenmodes below this floor, and renormalises the retained subspace. It requires no rank prior, no iterative optimisation, and only one eigendecomposition. In Qiskit Aer simulations, SSP-QST achieves the highest fidelity among the tested non-iterative baselines across the evaluated probe ranks, with a maximum fidelity gain of $+0.584$. It also improves shot efficiency by at least $8\times$ within the tested range. These results show that SSP-QST can make photonic QST more reliable under finite-shot noise while providing a lightweight reconstruction primitive for feedback-oriented
quantum sensing pipelines.
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Quantum Physics
Efficient Unclonable Encryption from Pauli Eigenstates
Seyoon Ragavan
We give, to our knowledge, the first plain-model, one-time information-theoretically secure, efficient unclonable encryption scheme for one classical bit. Previous work by Bhattacharyya and Culf (Nature Physics, 2026) and Bhattacharyya, Broadbent, and Culf either only showed $1/\mathsf{poly}(\lambda)$ security loss or required inefficient encryption/decryption operations. We avoid both of these caveats; in doing so, we obtain (to our knowledge) the first plain-model construction of many-time secure $1 \to 2$ unclonable encryption for arbitrary
polynomial-length messages, assuming the existence of pseudorandom function-like states (Bartusek and Goldin). The key is a uniformly random non-identity phase-free Pauli on $n$
qubits, and bit $a$ is encrypted as a random $(-1)^a$ eigenstate of that Pauli. The scheme is exponentially secure; we prove that the probability that both receivers recover the bit is at most $\frac{1}{2}+\frac{1}{2}\sqrt{{2^n}/({4^n-1})} = \frac{1}{2} + O\left(2^{-n/2}\right).$ By a lower bound due to Broadbent, Culf, and Rochette, this is the best probability bound achievable with $n$-
qubit ciphertexts (up to the constant hidden in the $O(\cdot)$). The main conceptual idea is to leverage, in a precise spectral sense, the balanced commutation-anticommutation structure of the Pauli group. The proof is intricate but completely elementary and makes use of standard spectral bound techniques. The main technical workhorse is a standalone linear-algebraic lemma that informally relates the positivity of two different operators, each capturing the intuition that if the two receivers can individually decrypt unusually often then they must also disagree often. GPT-5.6 Sol Ultra found this proof in an extended conversation with the author and drafted a preliminary version of this paper. The author is fully accountable for the correctness of this paper.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Observation of room temperature intrinsic nonlinear thermoelectric effects in low-dimensional semimetals
Kurea Nakagawa, Krishnaraajan Sundararajan, Cédric A. Cordero-Silis et al. (+2 more)
Nonreciprocal control of thermoelectric responses offers a promising strategy for next-generation thermal-management and energy-conversion. While nonlinear electrical transport has recently emerged as an intrinsic property of low-
symmetry quantum materials, their thermoelectric counterparts have not been demonstrated. Here, exploiting harmonic detection with gradient-reversal techniques, we report intrinsic nonlinear thermoelectric responses up to room temperature in the low-
symmetry type-II Weyl semimetals $T_\mathrm{d}$-WTe$_2$ and TaIrTe$_4$ in the absence of
magnetic fields or
magnetic materials. We resolve all
symmetry-allowed components of the second-order thermoelectric tensor, including the nonlinear Seebeck, nonlinear Nernst, and nonlinear mixed-directional thermoelectric effects and demonstrate that both Berry-
curvature-related and scattering-induced contributions govern the different nonlinear thermoelectric responses. Our results show that nonlinear thermoelectricity arises intrinsically from reduced
crystal symmetry and that engineering effects related to scattering in these materials provides a versatile platform for exploring higher-order heat-to-charge current conversion beyond the linear response.
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All authors Kurea Nakagawa, Krishnaraajan Sundararajan, Cédric A. Cordero-Silis, Bart J. van Wees, Marcos H.D. Guimarães
Quantum Physics
Spectral Gap of the Davies Generator for the Mean-Field Heisenberg Model
Joao Basso, Thiago Bergamaschi, Lin Lin et al. (+2 more)
The mean-field Heisenberg ferromagnet is a
quantum spin model on the complete graph with isotropic
spin-1/2 interactions. This non-commuting
Hamiltonian is permutation and $\mathsf{SU}(2)$ invariant, and its Gibbs states undergo an $\mathsf{SU}(2)$
symmetry breaking phase transition at inverse temperature $\beta=2$. We consider the associated Davies generator, a canonical model of open-system thermalization, and prove tight asymptotic estimates for its spectral gap at all noncritical temperatures. For fixed $\beta<2$, the gap as a function of number of
qubits $n$ is $\Theta(1)$, while for fixed $\beta>2$ the gap is $\Theta(n^{-1})$. The matching upper bound of the spectral gap is witnessed by the total magnetization order parameter, suggesting that the low-temperature ($\beta>2$) slowdown is associated with broken continuous
symmetry. Two key ingredients in our approach are a comparison argument, which introduces auxiliary generators to bound dissipation on nontrivial representations of the
symmetry groups $\mathsf{SU}(2)$ and $\mathsf{S}_n$, and a decomposition of the space of observables into spherical tensor operators to reveal a form of monotonicity.
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All authors Joao Basso, Thiago Bergamaschi, Lin Lin, Michael Ragone, Kevin D. Stubbs
Quantum Physics
Tunable nonlinear electromechanics at the zero-point motion scale
Christoffer B. Møller, Roger Tormo-Queralt, Elsa Vázquez-Rodríguez et al. (+13 more)
Nonlinearity at the scale of zero-point motion opens new possibilities for the control and readout of nanomechanical systems, but achieving this remains a formidable challenge. Here we demonstrate that ultrastrong coupling (USC) between a nanotube mechanical oscillator and a double-
quantum-dot electronic two-level system enables a mechanical Kerr (Duffing) nonlinearity at the zero-point motion scale. In the dispersive regime, this large coupling yields a mechanical anharmonicity of $\alpha = 1.4\%$ - three orders of magnitude larger than in previous work - while preserving the predominantly mechanical nature of the lowest energy states. We further demonstrate a purely quadratic cavity-based continuous readout of the mechanical motion. This continuous nonlinear optomechanical readout is enforced by a double-
quantum dot
symmetry, which can be broken by gate tuning to introduce a large linear transduction. These results establish a tunable USC platform that enables strong mechanical anharmonicity and nonlinear continuous readout at the zero-point motion scale.
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All authors Christoffer B. Møller, Roger Tormo-Queralt, Elsa Vázquez-Rodríguez, Victor Román-Rodríguez, Marta Cagetti, Eneko Mateos-Madinabeitia, Janine C. Franz, Stefan Forstner, Sergio L. De Bonis, Luca Ornago, Maria El Abbassi, Seoho Jung, Andrew N. Cleland, David A. Czaplewski, Fabio Pistolesi, Adrian Bachtold
Quantum Physics
Deleterious effect of photon-phonon coupling on microcavities in their application as quantum sources
Y. Sacha C. L. S., G. C. Rickli, J. Dipold et al. (+4 more)
In
quantum systems, the contact with the environment is detrimental to the purity of the state, thus limiting the practical use of entangled sources in
quantum information applications. This loss of purity is observed in the form of additional noise in the
tomography of the state. We investigate this noise dependence in $Si_3N_4$ micro-cavities, previously used for
quantum state generation, and demonstrate that the dependence of this noise on the temperature is compatible with a coupling of the photonic chips to a thermal reservoir. The control of this noise source is a necessary condition for the efficient implementation of these devices as sources of entangled states in
quantum networks.
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All authors Y. Sacha C. L. S., G. C. Rickli, J. Dipold, R. A. Kögler, Paulo Nussenzveig, Nathália B. Tomazio, M. Martinelli
Quantum Physics · Cross-listed from hep-th
Page transition for the complexity of an evaporating black hole
Violet Concepcion, Yasunori Nomura, Kyle Ritchie et al. (+1 more)
Recent results demonstrate that there exists a sharp, Page-like transition for the complexity of subsystems of Haar-random states as their fractional subsystem size surpasses one half. They further demonstrate that this transition also occurs for the
holographic complexity of boundary subregions of eternal AdS black holes, assuming the Complexity$=$Volume (CV) proposal for subregions. We interpret this transition as a crossover from spectrum-dominated to basis-dominated subsystem complexity, reflecting the breakdown of approximate thermality beyond half-system size. We then apply this
reasoning to an evaporating AdS black hole coupled to a bath, modeled by a
quantum circuit undergoing random
evolution on an interior subsystem of diminishing size. Using the basis-spectrum decomposition of subsystem complexity, we argue for a similar Page-like transition in the radiation complexity. We then show, using CV for subregions, that the same transition appears in the
holographic complexity of the radiation subsystem through the emergence of an island, whose volume gives the dominant contribution. We argue that the island volume contribution resolves an apparent complexity paradox analogous to the information paradox.
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All authors Violet Concepcion, Yasunori Nomura, Kyle Ritchie, Samuel Weiss
Quantum Physics
Quantum Cellular Automata from Kramers-Wannier Dualities and Modular Relations
Carolyn Zhang and Po-Shen Hsin
Recent work has constructed higher-dimensional analogs of non-invertible
symmetries similar to 1+1d Kramers-Wannier duality. Although their continuum descriptions often treat purely gravitational topological terms as inessential counterterms, these terms can have an essential
lattice manifestation: they distinguish states prepared by finite-depth
quantum circuits (FDQCs) from those entangled by nontrivial
quantum cellular automata (QCAs). Motivated by this mismatch, we show that QCAs associated with gravitational topological responses arise in several related settings: (1)
lattice realizations of projective $\mathrm{SL}(2,\mathbb{Z}_N)$ relations generated by topological operations on
symmetries; (2) squares of dualities that generalize the relation between fermionization and Kramers-Wannier duality; (3)
lattice implementations of QCAs through higher-form gauging; and (4) invertible phases protected by generalized time-reversal
symmetries. We derive new projective $\mathrm{SL}(2,\mathbb{Z}_N)$ relations whose projective phases are gravitational topological responses constructed from Stiefel-Whitney classes. We furthermore give a general protocol for preparing the associated QCA-entangled states using finite-depth
unitary circuits, measurements, and error correction. These results unify the study of gravitational topological responses in field theories, higher dimensional dualities, and
quantum cellular automata.
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Quantum Physics · Cross-listed from physics.optics
How to calculate the Wigner angle
C. J. McKinstrie and M. V. Kozlov
Lorentz transformations in time and two space dimensions consist of boosts and rotations, and combinations thereof. In general, the combination of two boosts is not another boost: It is a boost followed by a rotation. The rotation angle is called the Wigner angle. Although it is straightforward to determine the energy and direction of the combined boost, it is difficult to determine the Wigner angle. In this article, the vector,
matrix and spinor derivations of formulas for the Wigner angle are reviewed, and the underlying mathematics and physics are discussed briefly. Although the derivations are different, the results they produce are equivalent, as they should be. Like many physics problems, if one looks at the problem in the right way, it is not difficult to solve.
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Quantum Physics
Electron shuttling as a probe for charge defects
Hamza Jnane, Tamas Daniel Lipovics and Guido Burkard
Silicon
spin qubits are a leading platform for scalable
quantum computing, but their performance is limited by charge noise, widely attributed to two-level fluctuators (TLFs). The location of individual TLFs is generally unknown, and existing methods to localise them does not scale favourably with device size. Here, we show that electron shuttling turns a single mobile
spin into a scanning probe of individual defects. We show that by shuttling a
spin over a range of distances and tracking its coherence loss, one can localise defects along the channel and constrain their switching rate and fluctuation amplitude. Because one shuttled electron sweeps an extended region, the approach scales more favourably than previous methods. Our protocol requires no additional hardware and provides a practical route to mapping the charge-defect landscape of large silicon devices, enabling defect-aware calibration and avoidance in shuttling-based architectures.
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Quantum Physics
Resolving topological order crossovers on NISQ hardware
Ruizhe Shen, Yin Zhong and Ching Hua Lee
Topological phases of matter provide a promising route toward robust
quantum information processing, but on present-day noisy intermediate-scale
quantum devices the experimentally relevant question is whether signatures of topological crossovers remain resolvable under realistic imperfections. Here, we address this question in the Wen–plaquette model through a two-stage strategy on the IBM
Quantum hardware. We first use a tractable system to systematically characterize crossover signatures. Using variationally compiled
equilibrium and quench-generated states, we resolve crossovers between stabilizer-dominated and trivial or disorder-dominated regimes through local plaquette stabilizers and a Wilson loop, and quantify their robustness against static disorder, deliberately amplified
circuit noise, and effective non-Hermitian fields. The quench dynamics further reveal that plaquette-sector signatures remain substantially more stable deep in the strong-stabilizer regime than near the finite-size crossover. Building on the properties established in the small system, we extend the implementation to a physical two-dimensional IBM processor using a layered representative-
qubit construction. The resulting
lattice-averaged plaquette response exhibits only weak degradation under intentionally amplified local coherent perturbations. Together, these results connect controlled finite-size characterization with a scalable hardware implementation, providing a practical route for preparing and probing topological signatures on near-term
quantum processors.
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Quantum Physics
Mixed-state topological order and error-correction thresholds in non-Abelian codes: rigorous results
Sun Woo P. Kim and Max McGinley
We present a versatile and mathematically rigorous technique for bounding recovery thresholds in topological codes subject to noise. Our method captures the effect of applying an arbitrary (possibly non-Pauli) local noise channel to the code state of a broad class of two-dimensional codes, including surface codes, non-Abelian
quantum doubles, and string-net codes. In each case, we prove that for noise strengths up to some explicit constant value, any initially encoded logical information can be recovered to high precision, and that the noise-corrupted state exhibits key hallmarks of mixed-state topological order: long-range
entanglement and emergent higher-form
symmetries. We also describe how these methods can be adapted to higher dimensions and correlated noise models.
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Quantum Physics · Cross-listed from cond-mat.str-el
Counting Edge Modes with the Higher Berry Curvature: A Bulk Topological Order Parameter for Quantum Spin Chains
Adam J. McRoberts, Joe Crossley, Chris Hooley et al. (+1 more)
We show that the higher Berry
curvature (HBC) can be used to count the gapless edge modes created by an
entanglement cut, and thus defines an integer-valued topological order parameter for
quantum spin chains. Given an individual
spin-chain
Hamiltonian, we construct an extending family by interpolating to a reference product Néel state, and show that the integral of the HBC over this extension is equal to the ordinary Berry phase of half of the chain swept out in response to an infinitesimal field. It thus counts the
spin of the gapless edge modes exposed by the cut, and a change in its integer value signals a phase transition. We illustrate this with several examples: $S=1/2$, $S=1$, and $S=3/2$
spin-Peierls chains, which undergo `singlet flop' transitions between different patterns of dimerisation; the bilinear-biquadratic chain, which clarifies the connection to the strict
symmetry-protected topological phases classification; and the staggered $J_1$–$J_2$ chain, which has both nearest-neighbour and third-neighbour patterns of singlets depending on the signs of the interactions.
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All authors Adam J. McRoberts, Joe Crossley, Chris Hooley, Joe H. Winter
Quantum Physics
Frame-Dependent Traces and the Third-Particle Paradox
Alessandro Palumbo and Luca Apadula
The Paradox of the Third Particle arises when comparing subsystem descriptions across
Quantum Reference Frame (QRF) perspectives. We isolate two distinct origins of the Paradox: the QRF covariance of the partial trace and the failure of the physical
Hilbert space to inherit the kinematical tensor-product structure. We give an explicit counterexample to the Relational Trace (RT) resolution: an uncorrelated product state for which the RT statistical condition trivialises. We then introduce a new statistical consistency condition comparing subsystem discarding between external and internal QRFs, together with an associated frame-dependent map, the Perspective Relational Trace (PRT). We argue that our condition captures the operational content of the Paradox: rather than imposing consistency on the whole state space, we characterise exactly the states on which it holds in the Perspective-Neutral (PN) and
Quantum-Information (QI) approaches. This separates three levels of description: a PN subsystem of a PN whole, where consistency fails on a characterised set that includes product states; a QI subsystem of a QI whole, where it holds for all states; and a QI subsystem obtained from a PN whole by kinematical partial trace, where the full weakly invariant
algebra is recovered, yet consistency holds only on a proper subset. These results show that the PN approach can consistently describe only a closed, isolated system, while the QI approach can accommodate arbitrary subsystems. Tracing out a subsystem from a globally PN state yields a charge-superselected
algebra, reproducing in a minimal QRF model the boundary-charge structure of edge modes. We understand the Paradox not as a genuine contradiction, but as the consequence of comparing inequivalent physical layers without tracking which information is externally and which internally accessible.
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Quantum Physics
Non-Invertible Symmetries Mixing with Witt Non-Trivial Quantum Cellular Automata
Kansei Inamura, Oskar Wojdel, Lukasz Fidkowski et al. (+1 more)
Self-dualities and the stacking of
symmetry-protected topological (SPT) phases are basic operations on
quantum many-body systems. For a $\mathbb{Z}_p$ one-form
symmetry in 3+1d these correspond to the Kramers-Wannier-Wegner duality $S$, which is the gauging operation underlying non-invertible duality
symmetries, and the stacking of a 1-form
symmetry SPT $T$. In the continuum, they form a central extension of $PSL(2,\mathbb{Z}_4)$ for $p=2$, and of $SL(2,\mathbb{Z}_p)$ for odd
primes $p$, whose central elements are invertible theories with purely gravitational response. These central extensions are governed by a twisted, graded generalization of the Witt group of abelian anyon theories, which we determine. For $p=2$ the resulting group is the single-
qubit Clifford group, with duality and entangler acting as the Hadamard and phase gates. We realize this entire structure microscopically as
quantum cellular automata (QCA) acting on a certain local operator
algebra associated with a
spin lattice Hilbert space on a cubic
lattice. Specifically, our local operator
algebra is built by starting with all local operators commuting with a $\mathbb{Z}_p$ 1-form
symmetry, and taking the quotient by all the (local) 1-form
symmetry generators. The central elements can always be extended to the full tensor product
algebra with a uniquely defined QCA class. For $p=2$ they are generated by the non-trivial semion QCA, and for odd
prime $p$ they are generated by the non-trivial $\mathbb{Z}_p$ Clifford QCA. Consequently the
lattice fusion rules reproduce the continuum ones only up to these QCAs and
lattice translations, giving rise to fusion rules refined by QCAs.
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All authors Kansei Inamura, Oskar Wojdel, Lukasz Fidkowski, Sakura Schafer-Nameki
Quantum Physics
Non-Clifford quantum cellular automata from invertible topological quantum field theories
Meng Sun, Zongyuan Wang, Bowen Yang et al. (+2 more)
Quantum cellular automata (QCAs) describe locality-preserving
quantum dynamics and connect
quantum information, many-body physics, and topological
quantum field theory (TQFT). Constructing a QCA from a TQFT, however, is challenging. Although a topological action can produce a commuting
Hamiltonian realizing the desired ground state, it does not by itself specify an
automorphism of the full local operator
algebra. In this work, we develop a unified algebraic construction that extends the commuting generators of the
Hamiltonian to a complete separator-flipper
algebra on the full tensor-product
Hilbert space, providing a microscopic definition of the corresponding QCA. In three spatial dimensions, our formalism unifies all previously known QCA constructions associated with the $\mathbb Z_8\times\mathbb Z_2$ subgroup of the Witt group, including the $U(1)_2$ and $U(1)_4$ QCAs. The same algebraic structure directly yields new infinite families of generalized $U(1)_2$ and $U(1)_4$ non-Clifford QCAs in dimensions $d=4k-1$. We also reformulate the 4-dimensional $w_2w_3$ QCA and use it to develop a general construction of QCAs from TQFTs associated with arbitrary products of Wu classes. This construction includes two infinite families. The first consists of $w_2^nw_3^m$ QCAs in dimension $d=2n+3m-1$, while the second consists of $w_2w_{4k-1}$ QCAs in dimension $d=4k$. As a contrasting result, we explicitly construct finite-depth
quantum circuits for the 5-dimensional $w_3^2$ and $w_2^3$ QCAs, thereby proving that they are trivial, in agreement with the cobordism classification. Overall, these results convert invertible TQFTs into microscopic QCAs, provide a scalable route to higher-dimensional constructions beyond the Clifford setting, and open a systematic approach to classifying their stable structures and boundary anomalies.
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All authors Meng Sun, Zongyuan Wang, Bowen Yang, Nathanan Tantivasadakarn, Yu-An Chen
Quantum Physics · Cross-listed from physics.flu-dyn
Explainable quantum-compressed machine learning for complex fluid flows
Xiao Xue, Maida Wang, Mingyang Gao et al. (+2 more)
Machine-learning surrogates of physical systems face a paradox: explainable models facing the challenge of expressivity to capture complex nonlinear flows, whereas expressive deep surrogates match high-fidelity simulations only through massive parameterisations that turn the learned dynamics into a black box. Here, we introduce
quantum-compressed machine learning (QCML), which resolves this tension by compressing the latent propagator of a flow surrogate from $524{,}288$ trainable parameters to no more than $8$. This parameter reduction brings the learned dynamical law to the parameter scale of a physical constitutive relation rather than a black-box neural network, making the surrogate directly interpretable and controllable without sacrificing expressivity. The compression is realised by a structured
quantum circuit whose
unitary propagator constrains the latent spectrum to the unit circle exactly and by construction, replacing exponential error growth with linear accumulation over autoregressive rollouts. Classical regularisation only approximates this constraint: even a
quantum-inspired classical baseline penalised towards unitarity collapses within one Lyapunov time on turbulent channel flow, whereas QCML remains stable over the full rollout. Shared phase and coupling angles parameterising the
circuit correspond directly to modal frequencies and inter-mode interactions, giving the learned dynamics a physical interpretation in spectral space. On two patient-specific cardiovascular
benchmarks, the structured QCML propagator matches the predictive accuracy of its classical counterpart on surface pressure spectra, pressure drop, and wall shear stress. These results establish QCML as a working component of scientific machine learning and a concrete contribution towards practical
quantum advantage in real-world prediction.
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All authors Xiao Xue, Maida Wang, Mingyang Gao, Minh Chung, Peter V. Coveney
Quantum Physics
Hash-QNeRF: Multiresolution Hash Encoding for Quantum Neural Radiance Fields
Digonto Biswas, Tana Ballove, Anjan Bandyopadhyay et al. (+2 more)
Neural Radiance Fields (NeRF) have revolutionized novel view synthesis, yet their classical implementations remain computationally intensive for high-fidelity rendering. QNeRF recently demonstrated the feasibility of training NeRF on gate-based
quantum computers by combining amplitude
embedding, parameterized
quantum circuits (PQCs), parity-based measurements, and volumetric rendering. However, QNeRF relies on classical sinusoidal positional encoding for spatial coordinates, which scales poorly with scene complexity and resolution. In this work, we replace the sinusoidal positional encoding for spatial coordinates with the multiresolution hash encoding from Instant-NGP while keeping the view-direction encoding, amplitude MLP,
quantum circuit, parity measurement, output scaling, and volumetric rendering pipeline unchanged. This hybrid design, Hash-QNeRF, retains the
quantum radiance prediction step while benefiting from the fast convergence and
memory efficiency of learnable hash grids. On a synthetic Blender scene, we achieve a final training loss of 0.003534, corresponding to approximately 24.5 dB PSNR on the fitted batch. Noise resilience experiments using Qiskit FakeKyiv and FakeTorino backends yield state fidelities of 0.93 to 0.98, indicating that hash encoding does not degrade the
quantum circuit's noise tolerance.
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All authors Digonto Biswas, Tana Ballove, Anjan Bandyopadhyay, Sutanu Mangal, Arun Kumar Pati
Quantum Physics
Tunable Mpemba Effect in a Prethermal Many-Body Spin Network
Chaitali Shah, Cooper M. Selco, Leo Joon Il Moon et al. (+1 more)
Relaxation in an interacting system is determined not only by its initial distance from
equilibrium, but also by the relaxation modes populated by the initial state. Here we experimentally observe and control the Mpemba effect, in which a state farther from
equilibrium overtakes one initially closer, in an extended, disordered $^{13}$C
nuclear-
spin network in diamond. Field cycling allows us to prepare distinct spatial polarization profiles by independently controlling hyperpolarization and defect-mediated relaxation. We then track their
evolution under Floquet driving, which stabilizes a long-lived prethermal regime. We observe reproducible Mpemba crossings and tune the crossing time over several orders of magnitude, from late-time thermalization into the prethermal plateau. Semiclassical simulations show that randomly positioned paramagnetic defects create fast-relaxing regions and defect-poor regions that support the slowest collective relaxation mode. The Mpemba crossings are set by the initial state overlap with this mode. Our results demonstrate anomalous relaxation within a prethermal many-body regime and identify disorder, transport, and mode-selective state preparation as resources for controlling relaxation in extended
spin networks.
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All authors Chaitali Shah, Cooper M. Selco, Leo Joon Il Moon, Ashok Ajoy
Quantum Physics
Quantum Correlations in Frustrated Three-Body Systems
Chaitali Shah and Apoorva D. Patel
Many physical systems are not understood from first principles due to the presence of multi-body
quantum correlations. The effort taken to simulate such systems on classical computers increases exponentially with increase in the system size. We study simple, yet non-trivial, three-body frustrated systems that can help in understanding the underlying
quantum correlations. First we investigate the ground state of helium-like atoms using the variational method and physically meaningful ansatze, revealing how
quantum entanglement arises due to frustration in the system. Next we consider another frustrated three-body system, the hydrogen molecular ion, and analytically demonstrate the nature of its wavefunction arising from the
quantum tunneling phenomenon. Finally, we extend these results to model yet another physically important system, the hydrogen bond.
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Quantum Physics
Identifying the Sign of Coherent Over-Rotations with Logarithmically Many Pauli Settings
Yi Pan, Meng Hsiu Tsai, Weihang You et al. (+6 more)
Calibrating a
quantum processor means estimating gate-error parameters from data, and a hard-to-estimate parameter is usually assumed to leave a weak signature more repetitions will resolve. Coherent over-rotations break that premise. For commuting single- and two-
qubit transverse over-rotations with known support on a computational-basis input, the passive histogram is exactly invariant under a sign group acting on the coherent angles, of order $2^{n+1}$ for the complete family with $n\ge2$, so no
estimator resolves the signs uniformly at any sample size. A calibration correction applied with the wrong sign doubles the rotation error it should remove. At zero angle the obstruction turns continuous, with a Fisher information singular along every coherent direction and an infinite Cramér-Rao bound. At generic angles the continuous defect clears for $5\le n\le8$, the range we compute, while the sign degeneracy persists. Covering the support removes both, and a design resolves the signed angles uniformly exactly when it covers. On $|\eta_R|<\pi/2$, $0\le p_R<1/2$ coverage is constructive, returning each angle in closed form from a ratio of two measured expectations without knowing the rates. A twirl-free code of $\lceil\log_2(n+1)\rceil$ added product-Pauli settings covers, and for the complete family none uses fewer. Conditioning then sets the Cramér-Rao cost, and we give its floor explicitly. Exact computation matches the theory, the passive fit stays at chance on the signs at every budget while the closed-form inverse recovers them, and angle magnitudes on two IBM Heron processors follow the conditioning ordering.
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All authors Yi Pan, Meng Hsiu Tsai, Weihang You, Hanqi Jiang, Junhao Chen, Wei Zhang, Isaac Lyngaas, Yingfeng Wang, Tianming Liu
Quantum Physics
Flow-based Phase-space Tomography of Continuous-variable Quantum States
Owen Dugan, Rumen Dangovski, Peter Y. Lu et al. (+1 more)
Continuous-variable
quantum state
tomography is limited by the cost of resolving non-Gaussian structure in high-dimensional phase space. We introduce QST-Flow, a
quantum state
tomography framework via flow-based generative modeling that represents experimentally accessible phase-space quasiprobability distributions with normalized, samplable neural densities rather than a truncated density
matrix. The framework has two variants: QST-QFlow models the positive Husimi-$Q$ function with a single normalizing flow, while QST-WFlow models sign-changing Wigner functions as a trainable difference of two normalized flows. This construction preserves quasiprobability normalization and enables exact density evaluation, direct sampling, and importance-sampled learning from finite phase-space measurements without a fixed grid.
Benchmarks on non-Gaussian cat, binomial, Gottesman-Kitaev-Preskill, number, and Fock states show accurate single-mode reconstructions, extension to multimode states, robustness on noisy Wigner data, and improved reconstruction error compared with prior machine-learning
tomography methods. QST-Flow opens a promising route toward scalable, measurement-efficient phase-space
tomography of nonclassical bosonic systems.
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All authors Owen Dugan, Rumen Dangovski, Peter Y. Lu, Di Luo
Quantum Physics · Cross-listed from cond-mat.quant-gas
Complexity transition in the Dicke model of light-matter interaction
Yicheng Zhang and Erhai Zhao
Tuning the coupling strength $g$ of an interacting
quantum system may drive a sudden change in its ground-state or thermal properties. To identify and grasp non-analytical, or even discontinuous, transitions in far-from-
equilibrium dynamics proves more challenging. Recently Krylov complexity $C_K$ has offered fresh insights about operator growth, thermalization, and chaos in
quantum dynamics. Yet it remains unclear if, and how, changing $g$ can trigger a sharp transition in the complexity measures. Here we present evidence for such a transition by mapping out the complexity phase diagram of the paradigmatic Dicke model describing two-level atoms coupled to a cavity
photon mode. Two qualitatively different regimes of dynamics are identified and characterized. At the transition, the slope of $C_K$ changes suddenly to coincide with a jump in the Krylov
entropy. We elucidate the nature of the regime change from the wave packet dynamics in Krylov space, where a particle is confined by a roughly linear potential but hops as if it lives in a Rindler reference frame. The competition between confinement, which leads to bouncing, and deconfinement by Rindler hopping, which leads to the destruction of wave packet analogous to gravitational spaghettification, is sensitive to the disorder in Lanczos coefficients. The framework outlined here can be applied to other
quantum many-body systems.
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Quantum Physics
A geometric framework for spin relaxation
Sophia N. Fricke, Kieren A. Harkins, Ashok Ajoy et al. (+1 more)
Spin relaxation is conventionally described by two independent phenomenological rates - longitudinal ($R_1$) and transverse ($R_2$) - whose separation obscures a deeper structural unity. Here we develop a
geometric framework in which dissipation is represented by a single covariant relaxation tensor acting in Liouville space, from which $R_1$ and $R_2$ emerge as complementary projections. This tensor structure is not merely formal but is experimentally accessible through pulse sequences that probe noncommuting directions in
spin space. Using hyperpolarized $^{13}C$
spins in diamond with nitrogen-vacancy centers, we show that commuting pulse trains yield effective relaxation
matrices that are approximately diagonal, while noncommuting sequences produce off-diagonal components that vary with transmitter frequency offset and pulse ordering, providing evidence that relaxation is a directional process governed by a tensor rather than a pair of scalar rates. Complementary measurements of
geometric phase demonstrate that noncommuting dynamics introduce ordering-dependent effects that are separable from dissipation, consistent with the interpretation of relaxation as
geometric transport on the state
manifold. This framework unifies Bloch, Redfield, and Lindblad descriptions within a coordinate-independent formulation and provides a natural
language for relaxation in driven, anisotropic, and non-
equilibrium spin systems.
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All authors Sophia N. Fricke, Kieren A. Harkins, Ashok Ajoy, Jeffrey A. Reimer
Quantum Physics
James Benstead, Michael Garn, Neil Gaspar et al. (+5 more)
To maximize the value of fault-tolerant
quantum computers, it is essential to develop concrete applications beyond well-established domains such as chemistry and condensed-matter physics. Here we construct and compile
quantum algorithms to simulate the structure of atomic
nuclei – a topic that has received relatively little
attention from the
quantum computing community despite its similarities to the electronic structure problem in chemistry – via effective shell-model
Hamiltonians and no-core-shell-model
Hamiltonians with three-body interactions derived from
chiral effective field theory. Furthermore, we provide
quantum resource estimates, in terms of Toffoli gate and
qubit counts, for these
algorithms, which, to our knowledge, are the first such estimates for fault-tolerant
quantum simulation of atomic
nuclei. Notably, the estimates for $^{32}$Mg and $^{219}$At shell-model
Hamiltonians are comparable to recent estimates of Femoco simulations, a standard
benchmark in chemistry. For no-core-shell-model
Hamiltonians suitable for light
nuclei (up to $^{40}$Ca or so), we find that resource requirements are significantly higher, suggesting that more bespoke strategies are required to make such simulations practicable. Throughout this work, we draw upon the similarities between
nuclear and electronic structure problems, while also highlighting challenges that are specific to the former. We hope this work will spur long-term collaborations between the
nuclear and
quantum computing community with the ultimate goal of realizing useful
nuclear simulations on
quantum computers.
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All authors James Benstead, Michael Garn, Neil Gaspar, Sean Greenaway, Angus Kan, Lloyd La Ronde, Chandan Sarma, Paul Stevenson
Quantum Physics
Enhancing Entanglement Purification with Shared Randomness
Allen Zang, Bikun Li, Xinan Chen et al. (+4 more)
Entanglement purification protocols (EPPs) are essential for improving
entanglement fidelity to support fault-tolerant distributed
quantum information processing. Practical
entanglement sources are often heterogeneous and source labels may be unavailable at the EPP layer. We show that classical shared randomness, together with buffer
memories, can enhance
entanglement purification when source labels are unavailable, without state characterization or EPP
circuit optimization. The strategy is to accumulate multiple
entanglement distribution rounds and then use shared randomness to shuffle all the stored entangled states before packaging them as inputs to the EPP. For any $n$ Werner sources and any fixed $n$-to-1 bilocal Clifford EPP, we prove that accumulating and shuffling improves the expected success probability and the success-weighted output Bell fidelity over the baseline without accumulating and shuffling, for every $n$, for every finite number of accumulation rounds and in the asymptotic limit, and the improvement increases monotonically with the number of accumulation rounds.
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All authors Allen Zang, Bikun Li, Xinan Chen, Eric Chitambar, Liang Jiang, Martin Suchara, Tian Zhong
Quantum Physics
Strategic Plan for Neutral Atom Quantum Computation
Adrian J. Menssen, Tout Wang, Michael Gullans et al. (+53 more)
We present a strategic plan for neutral atom
quantum computation, bringing together hardware development and theory advancements to achieve the goal of practical
quantum advantage. The concept of practical
quantum advantage is defined, along with how to verify claims of advantage, and approaches to designing
quantum algorithms that deliver practical advantage. Future directions for neutral atom
quantum processor hardware are described: scaling-up system size,
Qubit encodings and atomic platforms, going further below threshold with neutral-atom logical-
qubit performance, continuous reloading of
qubits, and fast readout. We also explore opportunities for scalable integrated photonic control technologies. Alongside hardware advancements, new developments in
quantum error correction and compilation of
quantum circuits are proposed. Finally, we examine the opportunity of networking multiple neutral atom
quantum processors together to perform distributed
quantum computing and overcome possible limitations of a single system.
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All authors Adrian J. Menssen, Tout Wang, Michael Gullans, Tom Manovitz, Jacob M. Taylor, Jason Cong, Josiah Sinclair, Ziv Aqua, Daniel J. Blumenthal, J. Pablo Bonilla Ataides, Johannes Borregaard, Antoine Browaeys, Paola Cappellaro, Soonwon Choi, Alexandre Cooper, Robin Côté, Jacob P. Covey, Alexandre Dauphin, Ivana Dimitrova, Matt Eichenfield, Dirk Englund, Jacob Freedman, Akihisa Goban, Brandon Grinkemeyer, Andi Gu, Ruonan Han, Dominik Hangleiter, Aram W. Harrow, Liang Jiang, Eun-ah Kim, Felix W. Knollmann, Aleksander Kubica, Thierry Lahaye, Lucas Lassabliere, Joonho Lee, Bingzhao Li, Mo Li, Wan-Hsuan Lin, Mikhail D. Lukin, Varun Menon, Thomas Propson, Akbar Safari, Mark Saffman, Pascal Scholl, Alexander Schuckert, Giulia Semeghini, Jonathan Simon, David Spierings, Daniel Bochen Tan, Shai Tsesses, Vladan Vuletic, Hanrui Wang, Hanyu Wang, Susanne Yelin, Johannes Zeiher, Hengyun Zhou
Quantum Physics
Unconditional Unclonable Encryption
Prabhanjan Ananth, Seyoon Ragavan and Amit Sahai
We give an unconditional construction of information-theoretically secure one-time private-key unclonable encryption for one-bit messages, with efficient encryption and decryption and exponentially small unclonable-indistinguishability advantage. Previous work by Bhattacharyya and Culf (Nature Physics, 2026) and Bhattacharyya, Broadbent, and Culf (arXiv:2603.08916) either only showed inverse-
polynomial security loss or required inefficient encryption/decryption operations; we avoid both of these caveats. Our scheme's key is a uniformly random non-identity phase-free Pauli on $n$
qubits, and bit $a$ is encrypted as a random $(-1)^a$ eigenstate of that Pauli. We prove that the probability that both receivers recover the bit is at most $1/2 + O\left(2^{-n/2}\right).$ By a lower bound due to Broadbent, Culf, and Rochette (arXiv:2510.00903), this is the best probability bound achievable with $n$-
qubit ciphertexts while ensuring perfect correctness (up to the constant hidden in the $O(\cdot)$).
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Quantum Physics
Alexander Makarovskiy, Adam Taylor, Zhenghao Li et al. (+5 more)
Variational
quantum algorithms are a leading approach to near-term
quantum computing, but their scalability can be limited by barren plateaus and the sampling cost of resolving small changes in the loss landscape. Here, we study the trainability of passive linear-optical
quantum circuits and introduce a framework based on the ratio of sample variance to
circuit variance. This ratio determines the number of
circuit samples required to resolve local loss differences and gradients to proportional accuracy. We apply this framework to
photon-number observables and identify both trainable and non-trainable regimes. Supported by analytic results and a numerically observed
polynomial decay of the
circuit variance, we find that fixed-order
photon-number
polynomials require only polynomially many samples as the system size grows, whereas high-order
polynomials and observables based on output probabilities generally require exponentially many samples. Within the trainable regime, we further identify classes of observables in which
quantum estimation achieves a
polynomial speed-up over multiple classical methods. Within this family, neural network observables provide one practical construction that allow measurement outcomes to be efficiently processed into the desired
polynomial. These results establish photonic variational
quantum computing as a promising platform for near-term applications.
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All authors Alexander Makarovskiy, Adam Taylor, Zhenghao Li, Michael Hanks, Aubrey Clark, M. S. Kim, Ian Walmsley, William Clements
Quantum Physics
Beyond Calabrese-Cardy Scaling: Exceptional-Point Sensitivity and the de Sitter RT Surface
Kuang-Hung Chou
Entanglement entropy at one-dimensional criticality typically follows the Calabrese–Cardy scaling. In the non-Hermitian critical free-
fermion systems studied here, however, we find an additional interval-independent contribution $S_{\rm res}=\log(\Delta L)$, which allows the
entropy to remain sensitive to energy gaps below the finite-size scale, $\Delta<1/L$. We refer to this term as the residual
entropy. To understand its infrared origin, we examine finite-depth non-
unitary continuous multiscale
entanglement renormalization on a ring. Terminating the critical RG
circuit before its singular final layer leaves a two-site infrared state. We calculate the
entropy of this state directly from its correlation
matrix and recover the same \(\log(\Delta L)\) dependence. The emergent de Sitter geometry provides a complementary
geometric interpretation: extremal curves associated with different boundary intervals extend to a common infrared endpoint, suggesting how the
entanglement retained there can contribute equally to their
entropies.
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