Quantum Physics
Coherence-Enhanced Spatial Quantum Thermometry
Asghar Ullah, Giovanni Scala, Luis L. Sánchez-Soto et al. (+1 more)
Quantum thermometry has long promised sensitivities beyond classical limits, with recent paradigms highlighting
quantum coherence as a primary driver of precision. While most studies focus on single-point temperature estimation, we extend this framework to the spatial domain. We consider a probe two-level system (TLS) coupled asymmetrically to an auxiliary TLS that equilibrates with a sample of profile T(x). The reduced probe state carries coherence in its local basis that remains temperature-sensitive after the probe transition has frozen out, yielding a Fisher information exponentially larger at low temperature than that of a population-only readout. An exact reparametrization converts the thermometric precision into a local spatial distinguishability bound; its validity requires only a sufficiently small relative temperature uncertainty and yields an explicit measurement budget. Adding auxiliary TLSs shifts the array's optimal working temperature upward and broadens the useful sensing window, with the achievable gain ultimately limited by the total available coupling rather than by an indefinite scaling law. Finally, we propose a microwave Mach-Zehnder interferometer that maps the temperature-dependent probe coherence onto an experimentally accessible quadrature observable.
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All authors Asghar Ullah, Giovanni Scala, Luis L. Sánchez-Soto, Özgür E. Müstecapl{\i}oğlu
Quantum Physics
Influence of fast and slow laser phase noise on the fidelity of the Mølmer-Sørensen trapped-ion gate
Nikita Semenin, Ksenia Khabarova and Nikolay Kolachevsky
High-fidelity two-
qubit entangling gates are essential for the realization of useful
quantum algorithms on
quantum processors. The Mølmer-Sørensen (MS) gate has become a common choice for trapped-ion
quantum computing due to its resilience to ion temperature and its demonstrated record fidelities. However, the spectral impurity of the driving
laser field impacts gate performance, with phase noise influencing the
qubit dynamics through mechanisms operating on different timescales. In this work, we present a comprehensive theoretical analysis of
laser phase noise in the MS gate, identifying two spectral ranges which influence the gate fidelity the most: "fast" noise at frequencies near the motional mode spectrum, and "slow" noise at frequencies on the order of the inverse gate time. We derive the noise
Hamiltonians for two common
laser beam geometries and obtain analytical expressions for the average gate error in terms of the
laser noise power spectral density and gate parameters. For slowly varying noise spectra, we provide simplified error estimates. In addition, we validate our findings against previously published numerical simulations.
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Quantum Physics
The logical set of Gaussian states under the stabilizer subsystem decomposition
Santiago Zamora, Kaustav Chatterjee, Ulrik Lund Andersen et al. (+4 more)
Universal
quantum computation requires non-Gaussianity in continuous-variable systems and non-stabilizerness in discrete-variable systems. Yet, mapping a Gaussian state into the Gottesman-Kitaev-Preskill logical subspace can yield a logical
qubit with non-stabilizer resources. To understand how the continuous-variable structure determines logical resources, we characterize the image of single-mode Gaussian states under the corresponding extraction channel, known as the stabilizer subsystem decomposition. Specifically, we derive series expressions for the logical Bloch vector and characterize the resulting reachable set, enabling an analytical treatment of the robustness of magic of the logical states. We then turn this
geometric framework into a certification tool: by mapping a discrete-variable
qubit witness to a continuous-variable squeezing witness, we obtain lower bounds on the squeezing of prepared states. We also show that ensembles of Gaussian states can violate both stabilizer and classical bounds. Violation of the stabilizer bound certifies the relational resource known as "set-magic," while violation of the classical bound enables the certification of cryptographic randomness at moderate squeezing.
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All authors Santiago Zamora, Kaustav Chatterjee, Ulrik Lund Andersen, Thiago Lucena de Macedo Guedes, Jonatan Bohr Brask, A. de Oliveira Junior, Rafael Chaves
Quantum Physics
Transparent Domain Walls through Information Convex Sets
Jintae Kim, Amanda Gatto Lamas, Jacopo Gliozzi et al. (+3 more)
In $(2+1)$-dimensional topologically ordered many-body states, transparent (topologically deformable) domain walls are invisible to local topological probes, yet can modify the ground state degeneracy (GSD) and transmute anyons transported across them. Here, we develop an
entanglement-bootstrap framework using information convex sets (ICSs) on local and noncontractible annuli to extract information about transparent domain walls directly from ground state wavefunctions at fixed points of Abelian topological phases on a
torus, without taking categorical defect data as input. We derive fusion rules governing the action of anyons on extreme points of ICSs on noncontractible annuli and determine their
quantum dimensions. Extreme points invariant under transport around the complementary cycle correspond one-to-one to minimum
entropy states (MESs), and their number equals the GSD. The maximal topological
entanglement entropy (TEE), $\gamma_{\rm LW}^{\max}=\log(\mathcal{D}/d_\alpha)$, probes the net effect of walls crossing the chosen annulus, where $\mathcal{D}$ is the total
quantum dimension and $d_\alpha$ is the
quantum dimension of an extreme point of its ICS. In contrast, the maximal
entanglement asymmetry is $\Delta S_X^{\max}=\log\mathrm{GSD}$. This value is the same for both annulus orientations and reflects the combined effect of the transparent domain walls. We further show that transparent domain walls can give rise to
symmetries supported jointly on the two chosen fundamental cycles that cannot be decomposed into a product of two $1$-form
symmetry operators, one supported on each cycle. By relating their action on MESs to anyon tunneling and the fusion rules, we clarify how these
symmetries connect distinct ground states. Additionally, we apply the framework to Wen's plaquette model, the anisotropic dipolar toric code, and the rank-2 toric code.
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All authors Jintae Kim, Amanda Gatto Lamas, Jacopo Gliozzi, Bowen Shi, Taylor L. Hughes, Jong Yeon Lee
Quantum Physics
Deterministic photon waveform adaptation for quantum connectivity
Jeffrey Mohan, Jérémy Berroir, Filippo Borselli et al. (+9 more)
The scalability of
quantum technologies will depend on the ability to interconnect independent
quantum systems through photonic channels. However, heterogeneous
quantum platforms emit and absorb
photons with widely differing properties, severely limiting inter-node interference and modular connectivity. Here we demonstrate a cold-atom optical
quantum memory that simultaneously achieves near-unity storage-and-
retrieval efficiency and deterministic temporal adaptation of single
photons between arbitrary and programmable input and output pulse waveforms. Operating at high optical depth and within a fully integrated architecture, the system can store
photons with durations spanning over three orders of magnitude and reshape them arbitrarily without compromising efficiency, achieving compatibility with many current platforms. By augmenting the role of a
quantum memory from a passive storage element to an active programmable photonic interface, our results establish a key building block for scalable
entanglement-based
quantum networks and modular
quantum computing architectures.
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All authors Jeffrey Mohan, Jérémy Berroir, Filippo Borselli, David Libault, Ferhat Loubar, Kilian Müller, Jed Rowland, Félix Hoffet, Eleni Diamanti, Tom Darras, Tommaso Mazzoni, Julien Laurat
Quantum Physics
Sustained growth of quantum circuit complexity in many-body Hamiltonian dynamics
Wonjun Lee, Saúl Pilatowsky-Cameo and Soonwon Choi
The
quantum circuit complexity of an evolving many-body
quantum system is believed to exhibit a sustained growth, maintained for timescales much longer than the onset of thermalization. Most previous works have focused on models which violate energy conservation, such as random
unitary circuits. Here we study generic, local time-independent
Hamiltonian dynamics. We unconditionally prove that for generic local
Hamiltonians and typical initial product states at high effective temperature, the robust
quantum circuit complexity must grow over a very long period of time, attaining an exponentially large value at late times. Our approach relies on two structural properties that we prove rigorously: (i) generic local
Hamiltonians satisfy generalized spectral no-resonance conditions of arbitrary order, and (ii) typical high-temperature product states are effectively supported in exponentially many energy eigenstates. These properties have been widely assumed without proof in prior works. As corollaries of this result, we show the late-time state displays robust volume-law
entanglement that is irremovable by
polynomial-size
circuits, and we establish a no fast-forwarding result for generic local
Hamiltonians. We also lower bound the complexity of preparing sufficiently large subsystems with local
quantum channels, in contrast with sufficiently small thermalizing regions which we show retain low complexity at late times.
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Quantum Physics · Cross-listed from cond-mat.str-el
Fermion bag study of the two-dimensional Majorana-Hubbard model at finite hopping
Mark Johnston and Emilie Huffman
The two-dimensional Majorana-Hubbard model on the $\pi$-flux
lattice involves the most local interaction possible for a
lattice with one relativistic Majorana
fermion per site. It is predicted to host a Majorana semimetal, dimerized phases, and
quantum-critical points with an emergent $U(1)$
symmetry. Nonperturbative
lattice calculations for this model remain a major open challenge, thus far limited to zero hopping. We present
quantum Monte Carlo simulations of the model at finite hopping using
Hamiltonian fermion bags, which are based on a continuous-time expansion with Boltzmann weights that are Pfaffians of a
matrix with dimension set by the expansion order, and which give the sign of the weight. Sign problem severity is related to the location of zero modes in the free hopping
matrix, and for couplings $g\lesssim 0.7$ and $\beta=L/4$ it is fully removed for
lattices up to at least $256$ sites. The interaction strength $g>0$ enhances the $U(1)$-charged dimerization doublet and suppresses the neutral channel, as the renormalization group predicts. Increasing the interactions also restores $U(1)$
symmetry in the component of the dimerization correlator that is even under bond orientation exchange, but not its odd component. There is a persistent anisotropy in the odd component with the
symmetry of a
lattice nematic. The data is consistent with a gapless semimetal throughout the sign-problem-minimal window.
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Quantum Physics
Bosonic Error Correction with Fluxonium
Shantanu R. Jha, Shoumik D. Chowdhury, Gabriele Rolleri et al. (+22 more)
Bosonic
quantum error correction (QEC) offers a hardware-efficient route to fault-tolerant
quantum computing. To date, however, superconducting
circuit implementations of bosonic codes have utilized centimeter-scale 3D microwave cavities controlled by fixed-frequency transmon
qubits, with logical lifetimes limited by transmon bit-flip errors. Here, we realize bosonic QEC in a fully planar architecture by pairing a heavy fluxonium, whose $451 \pm 70~\mu\mathrm{s}$ bit-flip lifetime exceeds that of any control
qubit in previous demonstrations, with an on-chip Archimedean spiral resonator several orders of magnitude smaller in mode volume than prior 3D cavities. We prepare finite-energy Gottesman-Kitaev-Preskill (GKP) states and stabilize them using measurement-free error correction with rapid fluxonium reset, extending the logical lifetime by a factor of $1.59 \pm 0.05$. These results provide the first demonstration of resonator control using a weakly coupled fluxonium and, with it, the first realization of standalone bosonic QEC in a fully planar superconducting
circuit architecture.
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All authors Shantanu R. Jha, Shoumik D. Chowdhury, Gabriele Rolleri, Anaida Ali, Lev-Arcady Sellem, Réouven Assouly, David Pahl, Lukas Pahl, Junyoung An, Farid Hassani, Hung-Yu Tsao, Chia-Chin Tsai, Aranya Goswami, Gabriel D. Cutter, Jeremie Boudreault, Jeffrey M. Gertler, Michael A. Gingras, Bethany M. Niedzielski, Jeffrey M. Knecht, Mollie E. Schwartz, Kyle Serniak, Jeffrey A. Grover, Baptiste Royer, Max Hays, William D. Oliver
Quantum Physics
Quantum Advantage for Distributed Symmetry Breaking
Maxime Flin, Longcheng Li and Jukka Suomela
We present a distributed
quantum algorithm that $3$-colors cycles in $O(1)$ rounds, with high probability. It follows that all locally checkable labeling problems (LCLs) that have round complexity $O(\log^* n)$ in the classical LOCAL model can be solved in $O(1)$ rounds in the
quantum-LOCAL model, with high probability; this includes problems such as maximal independent set and maximal matching in bounded-degree graphs. This presents the first natural examples of graph problems with an asymptotic distributed
quantum advantage for the LOCAL model; all prior examples that separate LOCAL and
quantum-LOCAL are artificial problems constructed merely for the sake of demonstrating
quantum advantage.
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Quantum Physics
Error Suppression in Distributed Quantum Computing with Heterogeneous-Distance Lattice Surgery
Daniel Dilley, Anastashia Jebraeilli, Rayat Roy et al. (+3 more)
Distributed
quantum computing requires fault-tolerant operations across inter-QPU links that can be substantially noisier than local gates. Uniformly increasing code distance provides additional protection but also enlarges data patches used for local storage and computation. Here, we introduce distributed heterogeneous-distance
lattice surgery using an eight-data-patch ancilla-mediated (8-DAM) architecture, which will be useful for near-term
quantum devices with less
qubit overhead. In this architecture, the central ancilla spanning the inter-QPU boundary is enlarged while the data patches retain distance $d$. The protocol uses traveling stabilizers to suppress hook errors during merge and split operations between these unequal-distance patches.
Circuit-level simulations of rotated surface codes at fixed local depolarizing noise show that logical-readout error rates depend only weakly on link noise. The resulting advantage over conventional
lattice surgery grows as link errors increase. Comparisons with uniform distance implementations demonstrate comparable logical error suppression with reduced physical-
qubit overhead. We also demonstrate how 8-DAM layouts support two simultaneous distributed logical CNOT operations between four logical data
qubits using a single enlarged ancilla. At higher link noise, this construction yields lower logical error rates and more stability than two independent distributed logical CNOTs. These results support selective ancilla enlargement as a resource-efficient approach to fault-tolerant distributed
quantum computing.
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All authors Daniel Dilley, Anastashia Jebraeilli, Rayat Roy, Shobhit Gupta, Alvin Gonzales, Zain Saleem
Quantum Physics · Cross-listed from physics.atom-ph
Optical Ion Clock with Engineered Immunity to Motion-Induced Frequency Shifts
Mark Lide, Wesley Hardin and Christian Sanner
Spectroscopic frequency shifts due to residual motion of the probed atoms significantly contribute to the uncertainty budgets of state-of-the-art optical ion clocks. For clock transitions with second-order Doppler and quadratic Stark shifts of opposite signs, it is possible to configure electrodynamic ion confinement such that these two shift effects become anticorrelated causing zero net shift. Here, we introduce a spectroscopic interrogation protocol which leads, for systems with unknown and varying motional energy gain rates, to first-order auto-suppression of corresponding frequency shifts without requiring the opposite-sign configuration. We experimentally demonstrate the proposed method on a new ytterbium ion optical clock probing the 467 nm electric octupole (E3) transition, where the combined fractional uncertainty contribution from motion-induced frequency shifts is reduced from $1.3\times10^{-18}$ to $0.3\times10^{-18}$. An interleaved optical frequency ratio measurement against ytterbium's electric quadrupole transition (E2) at 435 nm delivers an E3/E2 frequency ratio of $0.932 \,829 \,404 \,530 \,965 \, 340 (39)$. Combined with previously published ratio data this leads to a limit for a potential fractional temporal variation of the fine-structure constant of $2.4 (2.7) \times 10^{-19}/$yr, in agreement with existing bounds.
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Quantum Physics
On the Complexity of Finding Decoherence Free Subspaces
Evan Borras
Decoherence free subspaces are a steady-state structure of the open
quantum system which preserves
quantum coherence between the states lying with in it and thus has found a variety of applications throughout
quantum information science and technology. In this paper we study the computational complexity of deciding whether an open
quantum system admits a decoherence free subspace or not. More specifically we study this problem with in the context of Markovian open
quantum systems, governed by the time-independent Lindblad master equation. Along the way we introduce the $k$-Local Lindbladian problem, which captures the difficulty of computing purity decay rates under Lindbladian dynamics. We show that both problems are hard for the complexity class
Quantum Merlin Arthur (QMA) when the locality $k \geq 5$, with the first under perfect completeness and the second being complete for QMA. Our hardness construction generalizes Kitaev's clock
Hamiltonian construction to the open
quantum system setting by encoding the execution of a
quantum circuit into the steady subspace of a Lindbladian containing both pure and mixed history states. This subspace is then mixed depending on the output of the encoded
circuit. Our results suggest that deciding whether a generic Markovian open
quantum system admits a decoherence free subspace is intractable even for
quantum computation.
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Quantum Physics
Quantum Broadcast Channels with Mutually Confidential Messages
Paula Belzig, Sukanya Ghosal, Farzin Salek et al. (+1 more)
We study the transmission of two independent confidential classical messages over a
quantum broadcast channel, one for each receiver. Each message must remain secret from the other receiver, including when that receiver knows its own message. For classical inputs and
quantum outputs, we establish the classical Marton-type inner bound under average reliability and conditional strong secrecy. The encoder selects pairs of codewords from independently generated codebooks using normalized likelihood weights. We prove reliability through a change-of-distribution argument. Our main technical result is a bipartite classical-
quantum resolvability
theorem that accounts for the dependence created by pair selection and establishes secrecy for the same encoder. We also obtain a multi-letter capacity characterization and extend it to arbitrary
quantum inputs under secrecy against the other receiver, together with the Stinespring environment. We recover confidential capacity regions for deterministic classical and degraded classical-
quantum channels, with an explicit evaluation for the classical Blackwell channel. For coherent isometric extensions of injective deterministic classical broadcast channels, we show that the confidential classical capacity region equals that of the corresponding classical channel. We then compare confidential classical communication with
quantum transmission. For the coherent isometric extension of the Blackwell channel, we determine the unassisted
quantum-capacity region and show that some achievable confidential classical rate pairs lie outside it. The Platypus channel provides another example of this separation.
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All authors Paula Belzig, Sukanya Ghosal, Farzin Salek, Graeme Smith
Quantum Physics
Explicit channels with unbounded gains in classical communication using entangled inputs
Hao-Chung Cheng, Benjamin Lovitz and Peixue Wu
A seminal result of Hastings showed that entangled inputs can increase the rate of classical communication through independent uses of a
quantum channel. While Hastings' result proved the existence of this phenomenon, it has remained a major open problem to find an explicit channel with this property. In this work, we resolve this question by constructing an explicit family of finite-dimensional
quantum channels for which entangling inputs between pairs of channel uses yields an unbounded separation from the achievable communication rate using unentangled inputs. We describe a
circuit implementation of our channel that uses only deterministic qudit Clifford
unitaries and a single binary measurement with classical feedforward. The key ingredient is a careful design of the measurement and feedforward process that yields the required one-copy and two-copy output
entropy bounds from moment estimates alone, without requiring strong convergence of the underlying
unitary family.
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Quantum Physics
A Syndrome-Extraction Framework for Distributed Lattice Surgery on Arbitrary Rotated Surface-Code Layouts
Pritesh Thakur, Daniel Dilley and Zain Saleem
Modular
quantum computing is a leading candidate for scalable fault tolerant
quantum computation. Performing
lattice surgery using surface code patches in different modules requires the use of inter-module gates that are noisier than the local gates. Furthermore, the merged layouts during
lattice surgery expose multiple directions in which hook errors can propagate to reduce the fault distance. Existing hook-avoiding schedules either increase the syndrome extraction
circuit depth, or let link faults propagate into the bulk. We introduce a seam construction that allows splitting arbitrary merged rotated surface code layout into smaller patches with uniform boundaries that could keep their own hook-avoiding N/Z shaped schedule. At the seam we split weight-4 stabilizers into pairs of weight-2 stabilizers that are mediated through a Bell pair and measured within a CNOT depth of 4. We also employ the use of three-
qubit $\texttt{CXX}$ gates for parity measurements at the seam to maintain the minimum CNOT depth. Through this seam construction, we confine the elevated link error at the seam, protecting the bulk from higher error rate. We also give a custom detector annotator for such seams that can find the detector space, annotate detectors in the Stim
circuits, and ensure that they are suitable to use by matching decoders. We implement our construction for the logical Pauli-$XX$ measurement
lattice surgery and a cross-shaped spatial junction and sweep the link error rate from $10$ times to $100$ times a fixed bulk error rate, and show that our construction gives the lowest and most stable logical error rate than the other frameworks compared here.
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Quantum Physics
Good Quantum Locally Testable Codes from Product Expansion
Mitali Bafna, Anqi Li and Quynh T. Nguyen
We construct
quantum locally testable codes (LTCs) with constant rate, distance, soundness and locality under a variant of a product expansion
conjecture of Bafna and Vyas about Reed-Solomon codes. In particular, we use the high-dimensional expansion framework of Dinur, Lin and Vidick for constructing
quantum LTCs, instantiated with the non-Abelian cubical complexes of Rungtanapirom, Stix and Vdovina. Our code is obtained by equipping the complex with carefully chosen Reed-Solomon local codes whose
symmetries are compatible with those of the complex.
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Quantum Physics
Evidence for a low-gap component in compressively stressed niobium thin films
J.T. Paustian, Maciej Olszewski, Kiichi Okubo et al. (+6 more)
We investigate the microwave response of sputtered Nb films as a function of biaxial film stress. Although transport measurements show nearly identical superconducting transition temperatures ($9.0\pm 0.2$K) and residual resistance ratios across the stress series, microwave resonators fabricated from highly compressive films exhibit enhanced temperature-dependent loss and larger frequency shifts. A conventional single-phase Mattis–Bardeen description yields an effective critical temperature of $2.4\pm0.2$K, approximately a factor of four below the value obtained from transport measurements. We show that the data are instead consistent with the presence of an additional low-gap component whose contribution increases with compressive stress. These results show that conventional transport measurements can overlook stress-dependent low-energy excitations that strongly influence microwave performance.
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All authors J.T. Paustian, Maciej Olszewski, Kiichi Okubo, Lingda Kong, B.L.T. Plourde, Ethan G. Arnault, Caleb W. Fink, Valla Fatemi, Ivan V. Pechenezhskiy
Quantum Physics
A quantum lower bound for path finding in welded trees
Joseph Carolan, Andrew M. Childs, Matthew Coudron et al. (+1 more)
In the welded tree problem, an
algorithm is tasked with navigating a graph formed from two binary trees joined at the leaves through a “weld” of connecting edges. A
quantum walk can navigate from root to root exponentially faster than any classical
algorithm. However, known efficient
quantum algorithms cannot find a path between the roots, as recording the path destroys constructive interference and thus the speedup. We prove that this is inherent: any
quantum algorithm needs exponentially many queries to find a path between the roots of an independently matched welded tree graph. This provides an example of a problem that a
quantum computer can solve exponentially faster than any classical
algorithm by exploring exponentially many paths in superposition, but where it is provably intractable to find any such path. The proof uses compressed permutation oracles to record the progress of a
quantum algorithm as it queries the graph. We show that the compressed database remains path-free up to a small error. By controlling such errors and bounding the progress of the
algorithm with each compressed oracle query, we show that $\Omega(2^{n/12})$ queries are required to find a path in a height-$n$ tree with constant success probability.
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All authors Joseph Carolan, Andrew M. Childs, Matthew Coudron, Amin Shiraz Gilani
Quantum Physics
When are bosonic Gaussian states classical to learn?
Senrui Chen, Antonio Anna Mele, Francesco Anna Mele et al. (+1 more)
A fundamental question in physics is: When does classical behavior emerge from
quantum systems? Bosonic Gaussian states provide a natural setting to explore this
quantum-classical boundary, as they capture both the classical field behavior and the intrinsic
quantum nature of light. Here, we address this problem from a learning-theoretic perspective by asking: When are bosonic Gaussian states classical to learn? That is, under what conditions (if any) can an n-mode bosonic Gaussian state be learned with as few samples, and with operations as simple, as are needed to learn a classical 2n-variate Gaussian distribution? We establish a smooth crossover in learnability governed by the state's thermal fluctuations: - Cold Gaussian states are non-classical to learn: When the covariance
matrix satisfies $\Sigma\le(\frac12+O(\frac1n))I$, i.e. close to the
vacuum covariance,
tomography under single-copy (i.e., non-entangled) measurements fundamentally requires $\Omega(n^3)$ copies, strictly exceeding the sample complexity $\Theta(n^2)$ of learning classical Gaussian distributions. We show that this hardness persists even when few-copy entangled measurements are allowed. - Warm Gaussian states are classical to learn: When thermal fluctuations exceed the
vacuum noise, parameterized by $\Sigma\ge(\frac12+\nu)I$ for any parameter $\nu>0$, we prove that single-copy
tomography requires $N=\Theta\left(n^2\min(n,1+\nu^{-1})\right)$ copies. This bound is tight and is achieved by simple, non-adaptive, unentangled heterodyne measurements. Crucially, for $\nu=\Omega(1)$, the sample complexity drops to $\Theta(n^2)$, matching the classical case. Our results tightly characterize a
quantum-to-classical crossover in the learnability of bosonic Gaussian states, reveal a novel connection between fundamental physics and statistical learning theory, and have implications for real-world sensing experiments.
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All authors Senrui Chen, Antonio Anna Mele, Francesco Anna Mele, John Preskill
Quantum Physics
Parallelizable and addressable transversal non-Clifford gates on good quantum LDPC codes
Yiming Li, Zimu Li, Zhengyi Han et al. (+1 more)
We achieve linearly many parallelizable and addressable transversal logical multi-controlled-Z gates with asymptotically optimal parameters simultaneously on good
quantum low-density parity check codes and
quantum locally testable codes, by applying the gate framework of [arXiv:2604.01874] to good qLTC constructions. To establish the nontriviality of the logical operation, we express the cup product pairing as a coefficient in a product of Moore determinants and prove that this coefficient is nonzero. Moreover, we use the
symmetries of the covering spaces to construct large spaces of cocycles. By multiplying the initial cocycles by suitable elements of these spaces, we obtain linearly many independent pairings, each selected by a corresponding cycle constructed using Poincaré duality. This yields linearly many parallelizable and addressable multi-controlled-Z gates and enables asymptotically constant-overhead magic state
distillation with good qLDPC codes.
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All authors Yiming Li, Zimu Li, Zhengyi Han, Zi-Wen Liu
Quantum Physics · Cross-listed from cs.CC
Strong Selective and List-Decoding Direct Product Theorems for Quantum Query Complexity
Paul Beame, Niels Kornerup and Michael Whitmeyer
Quantum strong direct-product
theorems for specific functions have been known for nearly two decades. These have been extended to general results for function computation and state generation. The proofs of these results use a version of the multiplicative adversary method that does not naturally extend to relations. Standard strong direct-product
theorems apply when
algorithms must correctly answer every given question. Prior work extended them to equivalent threshold direct-product
theorems, which require answers to all questions but only require that most answers are correct. We focus on two further generalizations. Strong selective direct-products apply to
algorithms that adaptively choose, based on what they learn from queries, which questions from a large list to answer. This generalization is relational and useful for proving time-space tradeoffs. We prove a
quantum strong selective direct-product
theorem for all functions using a relational formulation of the multiplicative adversary method by Jeffery and Zur which we prove, via an equivalent formulation, satisfies a strong selective direct product property and captures any query lower bound for functions proven by negative-weights adversaries. The second generalization is list-decoding direct product problems introduced by Ben-David and Blais for classical query complexity. These allow an
algorithm to produce a large list of possible output vectors such that one of them is fully correct. They proved that such
theorems hold for randomized complexity of all Boolean functions. We prove a
quantum analogue of this
theorem for all partial Boolean-valued functions. We show that strong list-decoding direct-product
theorems are implied by a special case of multiplicative adversaries which we show, via a new reduction, can be obtained from negative-weights adversaries for any Boolean-valued function.
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Quantum Physics
Strong matchgate designs in nearly optimal depth
Maxwell West, M. Cerezo and Martin Larocca
Understanding the resources required to generate approximately random
unitaries over various groups is a natural goal of
quantum information theory. With respect to one notion of approximation, that of a design, it is known that the full
unitary group can be approximated in logarithmic depth by one-dimensional
circuits of nearest-neighbour 2-local gates. On the other hand, remarkably,
circuits with this connectivity cannot form designs over the matchgate group in sublinear depth. Here we show that this dramatic slowdown can disappear when using a general
qubit connectivity graph $\mathsf{G}$ of routing number ${\rm rt}(\mathsf{G})$. Indeed, in this setting one can obtain (strong) $\varepsilon$-approximate relative error matchgate $k$-designs in depth $\mathcal{O}(k^2{\rm rt}(\mathsf{G})\log n\log(n/\varepsilon))$. For all-to-all connectivity, ${\rm rt}(\mathsf{G})=2$. Our construction is conceptually simple, involving a random walk on the matchgate group, and no ancillae. As a technical byproduct, we improve upon the state of the art for fermionic routing, obtaining an $\mathcal{O}({\rm rt}(\mathsf{G})\log n)$ depth router. Additionally, for $k=3$, we obtain an exact strong matchgate design in $\mathcal{O}({\rm rt}(\mathsf{G})\log n)$ depth, again without ancillae. Under all-to-all connectivity, our fermionic router and 3-designs are optimal. Notably, our results imply that
quantum algorithms for fermionic
tomography which require drawing from a matchgate 3-design may be exponentially sped up on
quantum computers with all-to-all connectivity, relative to their strictly one-dimensional counterparts.
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Quantum Physics
Fluctuation-Driven Nonlinear Amplification of Quantum Statistics
Yuewei Song, Zhenghe Zhou, Shuai Wan et al. (+9 more)
Photon statistics have moved to the forefront of modern optics, as intensity fluctuations and correlations shape multiphoton interactions and reveal information beyond mean-intensity measurements. Developing high-quality
photon sources with pronounced correlations is a fundamental necessity in these fields. Here we demonstrate fluctuation-driven nonlinear statistical amplification of
quantum light in spontaneous four-wave mixing using filtered amplified spontaneous emission (ASE). Extending the coherent-pump framework to fluctuating fields, we show how nonlinear weighting of pump intensity combines with bosonic bunching to amplify
quantum statistics and reshape temporal correlations. In a SiN microring, ASE pumping increases the zero-delay unconditional second-order correlation from 2.01 to 7.58 and extends the Hanbury Brown–Twiss correlation time by a factor of approximately 2.4. The super-bunched
quantum source nevertheless retains heralded single-
photon behaviour with $g_H^{(2)}(0)\simeq0.04$, while the same ASE pump supports time–energy
entanglement in a silicon waveguide with a raw Franson visibility of 89.84%. These results establish driving-field statistics as a design dimension for
quantum light, broadening the horizons for research into higher-order correlations and nonlinear physics.
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All authors Yuewei Song, Zhenghe Zhou, Shuai Wan, Hecheng Wang, Jinpeng Li, Bowen Liu, Yinhai Li, Chunhua Dong, Guangcan Guo, Chong Wang, Zhiyuan Zhou, Baosen Shi
Quantum Physics
Error Correction Properties of Covariant Bosonic Encodings
Frederic St-Amand, Jean-Philippe Burelle and Baptiste Royer
Bosonic codes offer a promising approach towards hardware-efficient fault tolerance, with many leading examples such as the cat-code and the GKP code organized by an underlying
symmetry. We build on a representation-theoretic framework of
quantum error correction to formalize the construction and analysis of multimode symmetric bosonic codes, focusing on finite groups. Here, a physical, logical representation and an initial seed state together define the codewords via a covariant encoding. A central observation is that QEC
matrices can be viewed as morphisms of group representations, so that Schur's lemma forces entire irreducible representations blocks to vanish. We recover several known single- and two-mode bosonic codes and build and analyze the error protection capabilities of three codes based on finite subgroups of SU(2). We analyze the impact of reducibility of the logical representation and optimize the seed state using the near-optimal fidelity.
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Quantum Physics
Geometric construction of optimal probe states in distributed multiparameter quantum sensing
Hongzhen Chen, Lingna Wang and Haidong Yuan
The choice of probe state is essential to achieving ultimate precision in
quantum metrology. For a single parameter, an equal superposition of eigenstates associated with the largest and smallest generator
eigenvalues is optimal. This simple construction does not directly extend to multiple parameters, since different parameters can favor different probes. We address this difficulty through a
geometric construction of optimal probes for multiparameter estimation in distributed
quantum sensing. In this setting, the generators commute and share a common eigenbasis, so each common eigenstate defines a vector of generator
eigenvalues. We show that the minimal enclosing balls and ellipsoids of these vectors determine the maximal total
quantum Fisher information and the minimum weighted covariance, respectively. In both cases, the boundary of the convex body identifies the common eigenstates that can contribute to an optimal probe, and its center and shape specify the conditions that their populations must satisfy. Our results provide a systematic approach to designing optimal probe states, linking the structure of the parameter generators directly to the attainable precision in multiparameter
quantum metrology.
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Quantum Physics
Hamiltonian Learning and Certification via Eigenphase Engineering
Myeongjin Shin and Yu Tong
We study
Hamiltonian learning for $k$-local
Hamiltonians at large step size, using only forward time
evolution at integer multiples of a fixed time interval. For this task, our new technique, which we call eigenphase engineering, achieves Heisenberg-limited learning with a step size that is optimal up to logarithmic factors. For an unknown $s$-sparse
Hamiltonian with fixed locality $k$ and $\|H\|_{\mathrm{op}}\leq\Lambda$, our
algorithm recovers its Pauli coefficients to $\ell_2$ error $\varepsilon$ in total
evolution time $\widetilde{O}(s/\varepsilon)$, matching the best known scaling while allowing a near-maximal step size $\widetilde{\Theta}(1/\Lambda)$. As an independent contribution, we strengthen the
evolution-time lower bound to $\Omega_k(s^{1-1/(2k)}/\varepsilon)$, placing our
algorithm within a factor $\widetilde{O}(s^{1/(2k)})$ of being optimal. Eigenphase engineering departs from approaches based on dynamical approximation or iteratively canceling the unknown
Hamiltonian. It encodes energy expectation differences in the perturbative response of eigenphases of the actual controlled
evolution, then extracts them through phase estimation and high-order extrapolation. This avoids the need for polynomially shorter
evolution steps as the target precision improves. A version using only single-
qubit operations retains the same step size with a $\sqrt n$ overhead in $\ell_2$ learning time for $n$
qubits. The method also enables tolerant
Hamiltonian certification and estimation of a specified Pauli coefficient, the latter without locality or sparsity assumptions. These results establish eigenphase engineering as a versatile approach to extracting
Hamiltonian information with infrequent control.
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Quantum Physics
A New Method For Manipulating Circuits, Application To Quantum Adders
William Schober and Scott Wesley
We use a new technique for manipulating controlled
quantum circuits to convert between two distinct types of
quantum adders, one based on the
Quantum Fourier Transform and the other based on the Ripple-Carry technique from classical reversible logic. This conversion takes the form of an explicit gate-level transpilation. We also present a new
quantum adder with a natural interpretation as a kind of Carry-Lookahead adder that uses no ancillas.
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Quantum Physics
Preselection-Encoded Weak-Value Interferometry for Optical-Rotation Metrology
Jinyi Li, Juncheng Zheng, Yuntao Shen et al. (+5 more)
Optical-rotation measurements benefit from increasing probe
photon number, but their performance is often limited by detector saturation and technical noise in the optical readout. Here we introduce preselection-encoded weak-value amplification (PeWVA), in which the quantity of interest is encoded into the probe polarization before a downstream weak-value interferometer. This architecture separates the sensing interaction from the postselected readout, enabling tunable local amplification while reducing the optical power delivered to the detector. We show that PeWVA does not create additional Fisher information, but offers a practical advantage when detector dynamic range is limiting. We further analyze coherent stray light and phase fluctuations and show reduced conversion of these perturbations into equivalent rotation errors. Using an $^{87}$Rb $M_x$ atomic magnetometer, we experimentally demonstrate substantially improved
magnetic-field readout under detector-constrained conditions using both transimpedance-amplified and avalanche photodiodes. The experiments also confirm reduced susceptibility to vapor-
cell-induced stray light and interferometer phase modulation. These results establish PeWVA as a detector-constrained optical readout strategy with enhanced tolerance to representative interferometric perturbations.
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All authors Jinyi Li, Juncheng Zheng, Yuntao Shen, Jiashu Liang, Sicheng Pan, Dongmei Li, Hengyan Wang, Wenqiang Zheng
Quantum Physics
Power-resolved ergotropy and correlation redistribution in a bosonic quantum battery driven by a coherent-state superposition
João P. d'El-Rey, Tailan S. Sarubi, Ana C. S. Costa et al. (+1 more)
Ergotropic-gap relations provide thermodynamic witnesses of
quantum correlations, but a direct dynamical connection between extractable work and
entanglement remains largely unexplored in multipartite non-Gaussian bosonic systems. Here, we establish such a connection for a bosonic
quantum battery driven by a coherent-state superposition. Despite the infinite-dimensional
Hilbert space, the dynamics retains a simple structure that allows the relevant work and correlation measures to be obtained exactly. We show that the loss of extractable work associated with the passive part of the battery energy is directly determined by charger-battery
entanglement. This relation also separates the charging power into contributions from energy transfer and from changes in the battery spectrum. At the same time, multipartite correlations are progressively redistributed from charger-involving correlations to correlations internal to the battery. For a symmetric resonant protocol, complete energy transfer coincides with charger-battery disentanglement and fully extractable stored energy, while multipartite correlations within the battery are maximal. Individual
cells, however, remain mixed, revealing a distinction between global and local work extraction. We further show that, in the regime considered, locally extractable work is entirely supported by energetic coherence. Under single-
photon loss, the analytical structure survives, but environmental correlations separate charger disentanglement from maximal work extraction.
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All authors João P. d'El-Rey, Tailan S. Sarubi, Ana C. S. Costa, José G. G. de Oliveira Jr
Quantum Physics
Magnetic tuning of quantum backflow: Schrödinger vs Pauli system
Tomasz Radozycki
Probability backflow is investigated for charged
spin-$1/2$ particles in a uniform
magnetic field within the lowest-Landau-level approximation. While single-mode Landau states exhibit only weak and non-robust backflow, it is shown that two-mode interference gives rise to negative probability flux in both the Schrödinger and Pauli formulations. In the Pauli case, orbital and
spin contributions to the probability current are explicitly separated, revealing an additional interference mechanism. The external
magnetic field is found to act as a tunable control parameter, simultaneously controlling the spatial overlap of orbitals and the
spin dynamics. In the two-mode regime, a resonance between orbital mismatch and
spin precession leads to an enhancement of backflow. Finite spatial resolution is shown not to suppress the effect within realistic coarse-graining. These results establish
magnetic-field-controlled
quantum backflow in Landau systems as an accessible interference phenomenon.
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