Quantum Physics
Photon localization: a comparative study
Laura M. Ferguson, Ricardo Rojas Castellano, Kayla Oltman et al. (+2 more)
We compare different measures for
photon localization in terms of two-dimensional Gaussian wave packets of width Delta k. We find that all measures start to coalesce if the wave packet has evolved for times which are larger than a few (c Delta k)^{-1}. However, the Landau-Peierls wave function yields the largest initial offset <r>-ct > 0 while the
Fourier transform of the helicity components of the k-space wave packet yields the smallest offset <r>-ct > 0 and converges towards <r>-ct = 0 fastest. We also discuss local detection of
photons through a model detector consisting of ions in ion traps. The position-dependent detection probabilities are inferred from the scattering
matrix. We find that the local detection probability for
photons can be expressed in terms of three of the proposed localization measures, viz. the Landau-Peierls wave function, the energy wave function, and the Hawton density. Those three proposals also remain close throughout the time
evolution of the single-
photon wave packet. However, the energy wave function is locally related to electromagnetic fields and is therefore confirmed as the preferred tool for the description of local
photon densities. The novelty of our observations concerns three items, viz. the possibility to express local
photon absorption
matrix elements through three of the proposed localization measures, the coalescence of all the proposed localization measures after sufficiently long
evolution of a
photon wave packet, and the striking differences of some of the proposed localization measures during early wave packet
evolution.
Read the full abstract ▾
All authors Laura M. Ferguson, Ricardo Rojas Castellano, Kayla Oltman, Joshua G. Fenwick, Rainer Dick
Quantum Physics
Quantum Contextuality and Entanglement-Free Grover Search in a Trapped-Ion Optical Qudit
Tarun Dutta, Jasper Phua Sing Cheng, Alex Jin et al. (+3 more)
Quantum computational advantage is generally attributed to coherent interference and other non-classical resources, yet their respective roles remain difficult to disentangle in experimental platforms where multipartite
entanglement is inherently present. High-dimensional
quantum systems provide an attractive route for investigating these resources while simultaneously reducing hardware overhead for
quantum information processing. Here we realize a programmable four-dimensional optical qudit encoded in a single trapped $^{138}\mathrm{Ba}^{+}$ ion and demonstrate universal coherent control through phase-programmable optical rotations. Using this platform, we implement an
entanglement-free realization of Grover's
quantum search
algorithm, achieving target-state identification probabilities of up to $94.5\pm2.0\%$. Within the same processor, we further demonstrate state-dependent
quantum contextuality through a Clauser–Horne–Shimony–Holt (CHSH)-type noncontextuality inequality, obtaining a maximum violation of $S = 2.816 \pm 0.082$, in close agreement with the Tsirelson bound. By integrating programmable
quantum computation and contextuality measurements within a single multilevel trapped-ion platform, our work establishes a versatile architecture for investigating the relationship between coherent interference and contextuality in
quantum information processing and provides a scalable route toward high-dimensional
quantum technologies.
Read the full abstract ▾
All authors Tarun Dutta, Jasper Phua Sing Cheng, Alex Jin, Sergi Ramos-Calderer, José Ignacio Latorre, Manas Mukherjee
Quantum Physics
Poled-fibre phase modulator for efficient high-dimensional quantum measurements
Nayda Guerrero, Nelson Villalba, Gustavo H. dos Santos et al. (+10 more)
Efficient detection of
quantum states underpins advanced device-independent
quantum-information protocols that provide the ultimate level of security for tasks including
quantum random number generation and
quantum key distribution (QKD). High-dimensional encoding is a natural route to boost the performance of such protocols, offering enhanced noise resilience and higher information capacity, yet their practical implementation remains challenging. A key experimental bottleneck in higher dimensions is the typical need of active modulators for basis selection, which incur substantial optical losses and polarization-sensitive operation. Poled optical fiber phase modulators (PFPMs) are a fiber-native electro-optic technology that naturally addresses these challenges, combining sub-dB insertion loss, intrinsic polarization independence, and direct compatibility with standard telecommunications fiber. Here we report the first use of a PFPM for active
quantum-state measurements in a fully fiber-integrated platform. Basis selection in our receiver for four-dimensional qudits is achieved using a single PFPM, substantially simplifying the receiver architecture. As a
benchmark, we perform a four-dimensional QKD session and obtain a finite secret-key rate per pulse that, to the best of our knowledge, surpasses all previously reported QKD demonstrations. Our results establish poled-fiber electro-optic modulation as a broadly applicable platform for high-efficiency detection in fiber-integrated
quantum information processing.
Read the full abstract ▾
All authors Nayda Guerrero, Nelson Villalba, Gustavo H. dos Santos, Carlos J. Salazar, Cristobal Melo, Fernando Castillo, Jaime Cariñe, Guilherme B. Xavier, Esteban S. Gómez, Stephen P. Walborn, João Pereira, Gabriel Saavedra, Gustavo Lima
Quantum Physics
Reconstructing non-Abelian braiding and fusion without anyon transport
Lucy Byles, Matthew D. Horner, Benjamin T. H. Varcoe et al. (+1 more)
Non-Abelian anyons offer a route to fault-tolerant and universal
quantum computing, but experimental access to their defining braiding and fusion data remains limited by the resource overhead of implementing extended anyonic processes on
quantum hardware. Here we introduce and experimentally realise a measurement-only protocol based on temporally ordered ribbon operations that reconstructs the non-Abelian braiding and fusion primitives of the
quantum double model $D(S_3)$ without physical anyon transport. We implement a reduced two-qutrit version of the protocol on Quantinuum's H2 trapped-ion processors, realising ancilla-assisted ribbon operations and anyonic charge projections in a
qubit encoding. We reconstruct the squared braiding phases and fusion amplitudes using an adapted Hadamard test and post-selected measurements, respectively. The associated braiding and fusion transformations reproduce their ideal actions with average normalised output-state fidelities of $\overline{\mathcal{F}}_{R}=0.9988$ and $\overline{\mathcal{F}}_{F}=0.9987$. Combining these primitives produces a non-Clifford braid and a non-stabilizer resource state, supporting measurement-only anyonic encodings as building blocks for larger topologically encoded
quantum processors.
Read the full abstract ▾
All authors Lucy Byles, Matthew D. Horner, Benjamin T. H. Varcoe, Jiannis K. Pachos
Quantum Physics
Contextuality in Sequential State Discrimination
Nyan Raess and Farid Shahandeh
Generalized contextuality is known to be required in optimal strategies for
quantum state discrimination protocols. More recently, sequential discrimination tasks have been studied; n players attempt to determine in which state a
qubit was prepared, in such a way that they all have a finite probability of success. We consider the extent to which contextuality plays a role in sequential versions of both unambiguous and minimum error discrimination. In the standard nonsequential case where n = 1, we use the COPE formalism to demonstrate that the presence of contextuality is guaranteed not only for the optimal measurement, but for a specific set of nonoptimal measurements as well. In the sequential case n > 1, we show that the presence of contextuality depends on which states are prepared, and on the protocol (unambiguous or minimum error) selected.
Read the full abstract ▾
Quantum Physics · Cross-listed from cond-mat.str-el
Santanu Singh, Surajit Bera, Chenyuan Li et al. (+2 more)
The subsystem-size dependence of ground-state
entanglement entropy and its crossover to thermal
entropy as a function of temperature are well understood for one-dimensional (1D) gapless systems described by conformal field theory (CFT), and for free
fermions with a
Fermi surface in any dimension. However, little is known about the
entanglement entropy for gapless fermionic systems without quasi-particles, such as a strange metal. Here we study the
entanglement entropy of
fermions in a solvable large-$N$ 1D
lattice model akin to the Yukawa-Sachdev-Ye-Kitaev (Yukawa SYK) model. In this model, two
Fermi points are coupled to scalar
bosons via spatially random Yukawa interactions, providing a solvable model of a strange metal when the
bosons become critical at a
quantum critical point. We exactly compute the second Rényi
entropy of
fermions in a spatial subregion in this model. Our results unravel crucial role of intra-subregion
entanglement between fermionic and bosonic degrees of freedom along with the inter-subregion
entanglement in understanding the ground states of such strongly coupled
fermion-
boson systems. We show that the crossover from thermal
entropy to
entanglement entropy, is captured by a single scaling ansatz, that collapses the second Rényi
entropy of
fermions for different subregion sizes and temperatures into a single universal curve. We further show that the universal scaling curve for the critical strange metal is well described by the standard CFT formula, albeit with an effective central charge substantially larger than the non-interacting value.
Read the full abstract ▾
All authors Santanu Singh, Surajit Bera, Chenyuan Li, Subir Sachdev, Sumilan Banerjee
Quantum Physics
An optical-fibre-integrated buffer for packet-switched quantum networks
Daniel Spegel-Lexne, Joakim Argillander, Martin Clason et al. (+5 more)
Packet-switched
quantum networks require buffers that can delay
qubit payloads while routing information is read out in real time. Previous approaches have not provided this functionality in a fully fibre-integrated architecture compatible with telecom infrastructure. Here we demonstrate an optical-fibre-integrated buffer, based on a recirculating loop and a fibre storage line, in which the storage time of a polarisation-encoded
qubit payload is determined by readout of an attached packet header. The key component behind this achievement is an ultra-low-loss poled fibre phase modulator, which provides fast, polarisation-insensitive switching directly in fibre and allows header and payload to be processed within the buffer. We demonstrate storage and
retrieval of polarisation-encoded
qubit payloads for storage times up to 47 $\mu$s, with an average
quantum bit error rate of 1.8% together with stable operation over several hours. These results establish a practical fibre-based architecture for packet-level
quantum network buffering that can easily integrate into the current telecommunication infrastructure opening up new paths for deployment of the
quantum internet.
Read the full abstract ▾
All authors Daniel Spegel-Lexne, Joakim Argillander, Martin Clason, Åsa Claesson, Kenny Hey Tow, Gustavo Lima, João M. B. Pereira, Guilherme B. Xavier
Quantum Physics · Cross-listed from physics.optics
Structured light under turbulence
Guilherme S. Barros, Lucas C. Céleri, Antonio Zelaquett Khoury et al. (+3 more)
Structured light has emerged as a promising resource for high-capacity and secure free-space optical communication, where atmospheric
turbulence remains a major source of signal degradation. In this work, we investigate the resilience of different transverse mode structures of an optical beam with respect to the random action of
turbulence. A spatial light modulator (SLM) is programmed to apply amplitude and phase modulation corresponding to the desired transverse mode, which is then transmitted through a turbulent medium emulated by a second SLM. Laguerre-Gauss, Hermite-Gauss, and Airy beams are investigated through the resulting intensity distributions measured with a camera. Different figures of merit are used to evaluate and compare the resilience of these modes.
Read the full abstract ▾
All authors Guilherme S. Barros, Lucas C. Céleri, Antonio Zelaquett Khoury, André L. S. Santos Junior, Rafael M. Gomes, Guilherme L. Zanin
Quantum Physics
Revealing Noise in Axial Motion Through Quantum Noise Spectroscopy on a Trapped Ion Processor
Vivian Maloney, Matthew Chow, Leigh Norris et al. (+7 more)
Native noise processes in
quantum processors are often difficult to isolate because multiple error mechanisms contribute to the same measured loss of coherence. Here we use dephasing-robust
quantum noise spectroscopy to identify and characterize control noise induced by axial motion in an individually-addressed trapped-ion processor. When the ion motion is transverse to the addressing beam, thermal axial motion couples to the beam profile and produces effective amplitude control noise. We show that this noise is governed primarily by the local beam
curvature and appears as a low-frequency contribution to the reconstructed control-noise spectrum. By varying the ion position within the beam profile, we separate
curvature-dependent axial-motion noise from
curvature-independent native control noise and extract motional parameters that are otherwise difficult to access on this platform. We also apply the protocol in parallel to a four-ion register, demonstrating a spectroscopic method for simultaneous characterization of position-dependent control noise across multiple
qubits. The results of this study identify beam inflection points as operating regions that suppress axial-motion-induced noise at the cost of reduced Rabi rate, as found in PRX
Quantum 3, 010334 (2022).
Read the full abstract ▾
All authors Vivian Maloney, Matthew Chow, Leigh Norris, Melissa Revelle, Daniel Lobser, Brian McFarland, Edward Tortorici, Christopher Yale, Susan Clark, Gregory Quiroz
Quantum Physics · Cross-listed from cond-mat.str-el
Breakdown of Monotonic Impurity Entropy Flow in $\mathscr{PT}$-Symmetric Multichannel Kondo Systems
Pradip Kattel, Abay Zhakenov and Natan Andrei
We study a $\mathscr{PT}$-symmetric non-Hermitian multichannel Kondo model consisting of a pair of
spin-$\frac12$
impurities coupled to $n$ conduction-electron channels through complex-conjugate Kondo couplings. The
impurity renormalization-group (RG) flow is characterized by the Kondo scale $T_K$ and a dimensionless non-Hermiticity parameter $\alpha$. As $\alpha$ increases, the exact Bethe Ansatz solution exhibits four
impurity phases: overscreened Kondo, zero mode, Yu–Shiba–Rusinov (YSR), and local moment. The Kondo, zero-mode, and local-moment phases are $\mathscr{PT}$-unbroken, whereas the YSR phase spontaneously breaks $\mathscr{PT}$
symmetry. Using a generalized thermodynamic Bethe Ansatz, we determine the
impurity free energy and Affleck–Ludwig $g$-function throughout the $\mathscr{PT}$-unbroken phases. In the Kondo phase, the defect RG flow connects the ultraviolet and infrared conformal fixed points, with the
impurity entropy flowing from $2\ln2$ to $2\ln\left[2\cos\left(\frac{\pi}{n+2}\right)\right]$, in agreement with defect conformal field theory. In the zero-mode phase, zero-energy
impurity strings reorganize the spectrum into multiple excitation towers, while in the local-moment phase, the RG flow becomes cyclic, returning to the unscreened local-moment fixed point. We
conjecture that RG irreversibility, and hence a generalized Affleck–Ludwig $g$-
theorem, survives throughout the Kondo phase. Our exact solution nevertheless shows that a real spectrum and defect
entropies consistent with defect CFT do not guarantee RG irreversibility: the
impurity entropy is non-monotonic in both the zero-mode and local-moment phases.
Read the full abstract ▾
Quantum Physics
Quantum Many-Body Metrology of Rotation Sensing with Strong Interactions
Yong-Jun Baak and Uwe R. Fischer
We study the ultimate
quantum limit of rotation sensing with a few strongly interacting
bosons confined in a quasi-one-dimensional ring trap with two weak links. It is demonstrated that a self-consistent many-body solution of the problem is required to correctly predict the ultimate sensitivity of this strongly correlated many-body gyroscope to rotation. For both small rotation velocities and small particle numbers, the many-body
quantum Fisher information becomes maximal for large interaction couplings, showing the potential of strongly interacting miniaturized many-body sensors to precisely estimate slow rotations with high spatial resolution.
Read the full abstract ▾
Quantum Physics
Guiding Compiler Optimizations for Neutral Atom Quantum Computers Through Visualizations
Yannick Stade and Robert Wille
The scale of Neutral Atom (NA)
quantum computers requires automated compilation tools. Designing the required heuristic methods demands a deep understanding of complex hardware trade-offs, for which visualizations can provide crucial insights. This work introduces NAViz, the first publicly available app to visualize
quantum computations on NA devices in real-time. A case study demonstrates how NAViz was instrumental in identifying and resolving inefficiencies in an existing compilation strategy, leading to a new, more performant one. The tool is available as part of the Munich
Quantum Toolkit (MQT) at https://github.com/munich-
quantum-toolkit/naviz.
Read the full abstract ▾
Quantum Physics
Graph-Aware Exact Branch-and-Bound with Device Profiles for Static Qubit Allocation
Kamer Kaya
Static
qubit allocation maps a
circuit's logical
qubits to a sparse physical device while minimising an interaction-weighted physical-distance cost function, yielding a rectangular quadratic assignment problem. Existing work combines strong lower bounds with distributed branch-and-bound. We integrate graph-aware exact reductions with an engineering bundle for a lightweight assignment-bound path: unavoidable assigned-cost filtering, incrementally maintained root-orbit and prefix-stabilizer
symmetry pruning, conditioned parent-LAP screening, and
circuit-independent physical device profiles. On 21 relatively easy Melbourne instances and six Boeblingen instances completed by the GLB baseline, the final single-thread configuration provides
geometric-mean speedups of 2.98x and 13.27x, respectively. With 60 threads on one shared-
memory server, all instances in the final Boeblingen–Cairo experiment are certified optimal within half an hour, excluding one-time device-artifact construction. These results show that graph-aware node processing and engineering the search process substantially reduce the resources required for exact allocation.
Read the full abstract ▾
Quantum Physics
Unifying quantum measurement constructions via a relative-entropy minimum change principle
Nana Liu and Mark M. Wilde
The minimum change principle provides an information-theoretic characterization of the Bayes reversal channel in classical probability theory and has recently been proposed as a framework for extending Bayes' rule to
quantum information theory. Using
quantum relative
entropy, we investigate a minimum change principle for the setting of
quantum statistical inference. Specifically, we consider a forward process based on a classical-to-
quantum preparation channel and a reverse process based on a
quantum-to-classical measurement channel. We establish a closed-form characterization of measurements that are optimal for this principle, and this optimal measurement can be found via a dual formulation involving a single unconstrained Hermitian variable. This perspective allows us to recover some notable measurements within the same framework, including pretty good measurements and
Fermi-Dirac thermal measurements, and we use it to discover a novel family that we call softmin thermal measurements. We further show that softmin thermal measurements arise as optimal solutions to
entropy-regularized semidefinite optimization problems, demonstrating that they play a role for measurements analogous to that of thermal states in statistical mechanics. Finally, we prove an additivity property for the relative-
entropy minimum change principle and investigate the performance of
Fermi-Dirac thermal measurements for
quantum hypothesis testing.
Read the full abstract ▾
Quantum Physics · Cross-listed from physics.pop-ph
From Understanding to Resonance: A Case Study of Quantum Music and Embodied Science Communication
Kenji Noda, Akira Koga, Stefan Heusler et al. (+1 more)
One challenge in science communication is how to engage audiences with highly abstract scientific fields often perceived as distant, technical, or inaccessible. This Practice Insight examines two initiatives implemented during the International Year of
Quantum Science and Technology:
Quantum Fest in Japan and
Quantum100 in Germany, which together attracted more than 4000 participants. Both employed immersive artistic experiences, including music, visual art, and
embodied participation to create alternative entry points into
quantum physics. Drawing on post-event surveys, participant comments, and stakeholder interviews, this paper identifies three dimensions of resonance: sensory immersion and reduced psychological barriers, collective
embodied meaning-making, and emerging transformative engagement. The findings suggest that resonance-driven approaches can complement explanation-centred science communication by enabling audiences to form meaningful relationships with science before, alongside, or beyond conceptual understanding.
Read the full abstract ▾
All authors Kenji Noda, Akira Koga, Stefan Heusler, Koji Hashimoto
Quantum Physics
Non-Relativistic Quantum Electrodynamics of Atoms in a Rotating Ring Cavity
Jarrod T. Reilly and Murray J. Holland
In this paper, we derive a
quantum optics model in a non-inertial rotating ring cavity from first principles. We begin with the Dirac equation in curved spacetime, add minimal coupling to the electromagnetic field, and then find the Dirac
Hamiltonian for the generalized Born metric. We then formally take the non-relativistic limit by way of Foldy-Wouthuysen transformations and project onto a fermionic Fock space for the atoms' electrons,
protons, and
neutrons. Focusing on a protium atom, we next move from a minimal coupling gauge to a multipole expansion gauge by taking a Power-Zienau-Woolley transformation under the dipole and long-wavelength approximations. Here, we find additional terms from the rotation of the system including a rotation-induced hyperfine shift of the atomic transition which could possibly be observed experimentally even for small rotation rates. Making the electric dipole, two-level, and rotating-wave approximations, we arrive at a Jaynes-Cummings-like
Hamiltonian which also accounts for rotational effects, such as the rotation-induced hyperfine shift and the Sagnac shift for the cavity's counterpropagating modes.
Read the full abstract ▾
Quantum Physics
Realified tensor networks: quantum circuit simulation on real-valued matrix accelerators
Yusheng Zhao, Xiwei Pan, Enji Xiong et al. (+2 more)
Tensor-network contraction simulates
quantum circuits, but modern
matrix accelerators (NPUs, TPUs) expose only real GEMM pipelines, so the complex networks of
quantum simulation must be reconstructed in software. We resolve the mismatch by a realification rewrite that maps any complex tensor network to a real one. At each merge of two complex tensors, a rank-3 structure tensor realizes Gauss's three-multiplication (3M) formula; contractions with one or no complex operand need only two or one real products. We prove a tight cost law: overhead $1 + 2m + r$ in real multiplications, where $m$ and $r$ are the volume fractions of two- and one-complex-operand contractions, never exceeding $3\times$ relative to real contraction, with every intermediate at most doubled in size. On 67
circuits (random, Clifford+$T$, QAOA, VQE), the law holds across the real-to-complex range and complex-gate placement, not count, governs cost. Contraction orders transfer from the complex network with a relative arithmetic-cost gap below $5\times 10^{-4}$ on 66 of 67
circuits; the exception closes under a few steps of low-temperature simulated annealing. On an Ascend 910 NPU the rewrite beat both the four-real-GEMM baseline and a per-GEMM Gauss lowering on all twelve random
circuits and on 52 of 55 structured
cells (three
cells slower by at most 12%); the four-GEMM baseline was slower by a median $1.7\times$ (random) and $1.4\times$ (structured). Realification makes complex tensor-network contraction native to real-only
matrix engines.
Read the full abstract ▾
All authors Yusheng Zhao, Xiwei Pan, Enji Xiong, Chengkai Zhu, Jinguo Liu
Quantum Physics
A lower bound on the classical simulation cost of star-network correlations
Martin J. Renner
It is well established that
quantum strategies outperform classical ones in several communication tasks. We study the
quantum communication complexity of correlations arising from joint measurements on
quantum systems distributed across a
star network, where several parties each send a
quantum system to a central node. We introduce an exclusion task that can be solved perfectly when each party sends a
quantum $d$-level system, but would require a large classical message otherwise. In fact, the task cannot be solved with certainty if each of the $n$ parties sends a classical message with less than $n^{(d-1)}$ symbols. This implies an advantage of using
quantum over classical messages in that scenario that scales with both, the dimension of the
quantum system and the number of systems measured simultaneously. As an application, this shows that no finite-size classical description of a
qubit suffices to reproduce the statistics of a joint measurement on sufficiently many
qubits.
Read the full abstract ▾
Quantum Physics
A quantum game of telephone
Arefur Rahman, Matthew L. Stevens, Cory M. Nunn et al. (+3 more)
Characterizing multinode
quantum networks without ubiquitous local
entanglement sources presents significant experimental challenges. We introduce “
quantum telephone,” an iterative ancilla-assisted process
tomography protocol that leverages intermediate detection events, effectively treating previously probed links as ancillae for subsequent links. Implemented on a deployed multinode fiber network, we observe noisy intermediate channels create fundamental parameter degeneracies that compound inference errors under sequential estimation. Counterintuitively, the inclusion of downstream near-
unitary channels provides boundary constraints that, when used in tandem with global inference, can resolve ambiguity in channels earlier in the sequence. By compensating for localized information loss, this approach obviates the strict full-rank requirements of standard ancilla-assisted process
tomography, even when intermediate states become completely depolarized. Overall,
quantum telephone offers a hardware-efficient and information-maximizing path toward characterizing complex
quantum networks with limited resources.
Read the full abstract ▾
All authors Arefur Rahman, Matthew L. Stevens, Cory M. Nunn, Daniel E. Jones, Brian T. Kirby, Joseph M. Lukens
Quantum Physics
Separating quantum circuits from classical LLMs
Srinivasan Arunachalam, Arkopal Dutt, Hari Krovi et al. (+1 more)
Modern large
language models -
transformers and
diffusion language models - are built around two canonical algorithmic tasks: prediction and generation. We prove unconditional separations between low-depth
quantum computation and the corresponding bounded-resource classical
language-model architectures in both regimes. Concretely, we exhibit the following: 1. Distributional separation. We give a distribution that is sampleable by $\textsf{QNC}^0$
circuits (i.e., a family of constant-depth
quantum circuits consisting of bounded fan-in gates) that no constant-round
diffusion language model ($\textsf{DLM}$) with shallow scheduling and denoising can sample within constant distance, even when allowed sublinear chain-of-thought and output-
token revision/remasking events, the very features modern $\textsf{DLM}$s rely on. 2. Functional separation. We exhibit a function computable in $\land \circ \textsf{QNC}^0[\log\log n]$ (i.e., a family of O$(\log\log n)$-depth $\textsf{QNC}^0$
circuits, where $n$ is the input length, followed by a single classical $\mathsf{AND}$ gate) such that any constant-depth decoder-only
transformer computing the function must be large: it would have to have width $n^{\Omega(1)}$. Together, our work initiates the study of
quantum advantage in the era of large
language models.
Read the full abstract ▾
All authors Srinivasan Arunachalam, Arkopal Dutt, Hari Krovi, Rik Sengupta
Quantum Physics · Cross-listed from physics.optics
Joint spectral characterization of SPDC photon pairs near 2 $\mu$m in (Al)GaAs-on-insulator waveguides
Alexandre Z. Leger, Emil Z. Ulsig, Samuel E. Fontaine et al. (+8 more)
Integrated
photon-pair sources are a core component of chip-based
quantum computing, communication, and metrology. Although such sources have been demonstrated at conventional telecom wavelengths, the 2 $\mu$m band remains comparatively less explored, despite offering advantages for free-space
quantum communication, low-loss transmission in emerging fiber networks, and integration with silicon photonic platforms. In this paper, we demonstrate spontaneous parametric down-conversion (SPDC) in straight GaAs- and AlGaAs-on-
insulator waveguides. This platform offers strong second-order nonlinearity and geometry-tunable dispersion, which are advantageous for efficient on-chip pair generation. Measurements of the joint spectral intensity and heralded second-order correlation function show broadband emission around 2 $\mu$m with strong spectral anti-correlations. To our knowledge, this is the first direct joint-spectral characterization of an integrated SPDC source in this wavelength regime.
Read the full abstract ▾
All authors Alexandre Z. Leger, Emil Z. Ulsig, Samuel E. Fontaine, Dileep V. Reddy, Eric J. Stanton, Lynden K. Shalm, Richard P. Mirin, Martin J. Stevens, J. E. Sipe, Nicolas Volet, Deny R. Hamel
Quantum Physics
Real-time decoding of quantum error correction codes using high-performance computing
Lingling Lao, Qiang Wang, Yuanqi Liu et al. (+10 more)
Quantum error correction (QEC) is indispensable for building scalable fault-tolerant
quantum computers. Effective QEC demands stringent real-time decoding: the decoder must process syndrome measurements and determine corrections within a time scale–typically on the order of microseconds, to avoid data backlog. Scaling to large number of logical
qubits further necessitates significant computational resources. In this work, we propose an architecture, called THQLink, for real-time decoding of
quantum error correction codes using high-performance computing (HPC) resources. The network connecting the HPC and the control system of
quantum processing unit (QPU) is built on TH-Express and can be adapted to different
quantum technologies and their associated control stacks. We report a round-trip latency of 2.944 $\mu$s on average, with an incremental overhead of 130 ns per additional hop. Using a parallel window strategy, we demonstrate real-time decoding (1 $\mu$s per QEC round) of the surface code up to distance 19 using a matching-based decoder on CPUs. Our work presents a scalable framework for real-time decoding in fault-tolerant
quantum computing. It can be readily applied to
quantum-centric supercomputers that feature tight integration between QPU and HPC resources, thereby enabling efficient support for hybrid
quantum-classical
algorithms and computation-intensive workloads offloaded from the QPU.
Read the full abstract ▾
All authors Lingling Lao, Qiang Wang, Yuanqi Liu, Yantong Liu, Haowen Wang, Yitao Chen, Yankang Zhao, Zhenwei Wu, Wei Zhang, Yong Dong, Yingwen Liu, Mingche Lai, Junjie Wu
Quantum Physics
Exact Tradeoff Between Quantum Error Correction and Quantum Darwinism: An Information-Theoretic No-Go Theorem
Arghya Maity, Kelvin Onggadinata and Teck Seng Koh
Quantum error correction (QEC) and
Quantum Darwinism describe opposing consequences of system-environment interactions: QEC seeks to preserve logical
quantum information, whereas
Quantum Darwinism explains the emergence of objective classical information through its proliferation into the environment. Despite their common physical origin, no direct quantitative connection between these paradigms has previously been established. We introduce an exactly solvable block-environment model based on the logical GHZ block of the Shor [[9,1,3]]
quantum error-correcting code, collectively coupled to N environment
qubits. The logical fidelity, Holevo information, and Darwinistic redundancy are obtained systematically for arbitrary environment size and imperfect recovery efficiency. Eliminating the common decoherence parameter yields an exact tradeoff relating Darwinistic redundancy directly to the post-recovery logical fidelity, demonstrating that the emergence of redundant classical records occurs at the expense of logical
quantum information. We further prove a model-independent no-go
theorem showing that the logical fidelity exceeds a critical threshold precludes the emergence of Darwinistic redundancy, irrespective of the microscopic
Hamiltonian or environment structure. The solvable model saturates this general bound, establishing the first quantitative information-theoretic connection between logical
quantum information protection and the emergence of redundant classical records.
Read the full abstract ▾
Quantum Physics
High-level quantum structured programs as quantum registers compositions
David Chamizo, Jose Garcia-Alonso and Juan M. Murillo
Current
quantum programs are mainly designed at the level of
quantum gates acting on individual
qubits; on a large scale and for complex problems this may involve a high cognitive load on the programmer, making the program specification nontrivial and error-prone. In this context, providing
quantum programming with higher abstraction mechanisms will assist in making this task more manageable and robust against design errors. In this work, a conceptual framework is addressed following the notion of the whole
quantum computation as a structure composed of
quantum registers representing each an undivided entity. Thus, computation progresses through semantically well-defined transformations that act on, or entangle,
quantum registers, thereby modifying the global state. Ultimately, the program reaches the desired state by following a specific composition strategy. With this in mind, high-level syntax is presented through an algebraic formalism that bridges them with their low-level semantics. Proposed syntax is based on certain well-know operations used on
quantum algorithms that apply phase shifts upon logical condition satisfaction or leverage on parallel evaluation. Based solely on the formalized operations, a
quantum satisfiability modulo theories (SMT) solver can be designed. At its core, this work contributes to establishing some methodological principles towards realizing a high-level
quantum structured programming.
Read the full abstract ▾
Quantum Physics
Few-photon degenerate parametric resonance in a two-tone driven microwave resonator
Orjan Ameye, Jakob Koenig, Clinton Potts et al. (+2 more)
Multi-tone external driving offers a route to parametric physics without directly modulating the device. However, the validity of the parametric response in the few-
photon regime remains underexplored. Here, we apply two coherent microwave tones to a Josephson-junction Kerr oscillator and stimulate degenerate parametric downconversion via four-wave mixing. Using transmission spectroscopy, we observe that the response retains the qualitative semiclassical Kerr parametric oscillator structure, including its instability lobe and bistable phase-space
topology. Interestingly, we demonstrate that a conventional single-mode reduction fails to capture the system quantitatively: the predicted AC Stark shift is severely underestimated, and the reported distributions might not be fully physical when the single-
photon Kerr shift $K$ exceeds the cavity linewidth $\kappa$. Instead, we show that a full three-tone
quantum description accurately reproduces the experimental observables. There,
quantum fluctuations of the drive tones become dynamically dominant over dissipation, and all three interacting tones operate in a deep few-
photon limit where the expected semiclassical macroscopic lobes undergo fundamental renormalization due to profound mixing with
quantum variance. Our results establish two-tone-driven Kerr oscillators as potential parametric amplifiers and open new horizons to explore the
quantum-to-classical crossover in driven-dissipative
circuits.
Read the full abstract ▾
All authors Orjan Ameye, Jakob Koenig, Clinton Potts, Oded Zilberberg, Gary Steele
Quantum Physics
Maximally Sliced States as a Framework for Quantum Coherence, Entanglement, and Teleportation under Decoherence
Anushree Pandey and Sovik Roy
Quantum resources such as coherence and
entanglement need not exhibit the same dependence on the parameters of a
quantum state, and their responses to decoherence can differ qualitatively. We investigate these relationships using the three-
qubit maximally sliced (MS) state, whose continuous parameter provides a tunable framework for redistributing
quantum correlations. We analyze the relative
entropy of coherence, (l_1)-norm coherence, and basis-independent coherence of the tripartite MS state and compare their behavior with the three-tangle. We then trace out one
qubit and study the coherence, concurrence, and optimal teleportation fidelity of the resulting bipartite state. For this reduced state, the concurrence coincides with the (l_1)-norm coherence because both are determined by the same off-diagonal
matrix element. We further examine the effects of amplitude damping and dephasing. Amplitude damping changes both populations and coherence, whereas dephasing preserves populations while suppressing phase coherence. The three coherence measures respond differently to these channels, showing that no single coherence measure fully characterizes decoherence. Under dephasing, the concurrence remains directly related to the (l_1)-norm coherence, yielding the teleportation-fidelity relation (f^T=(2+C)/3). Our results demonstrate that the MS family provides a useful tunable setting for distinguishing basis-dependent and basis-independent coherence and for studying the different responses of
entanglement and teleportation resources to dissipative and phase-decoherence processes.
Read the full abstract ▾
Quantum Physics
Geometric-Symmetry Logical Gate and Local-Probe Selectivity in the Three-Leg AKLT Ladder
Jingnuo Han and Youning Li
Symmetry-protected topological (SPT) phases provide a platform for encoding
quantum information in protected boundary degrees of freedom. Here we study the three-leg Affleck-Kennedy-Lieb-Tasaki (AKLT) ladder as an exactly solvable SPT system with on-site
symmetry $SO(3)\times \mathbb{Z}_2$. Using an exact
matrix product state construction, we characterize the
symmetry action on the edge encoding space and the accessibility of this space by local operators. We find that the continuous $SO(3)$
symmetry induces boundary rotations, while the leg-exchange
symmetry generates a geometry-dependent logical permutation of edge
qubits. Furthermore, by introducing a distinguishability measure motivated by the Knill–Laflamme condition, we derive a
symmetry-resolved decay law for local accessibility. The decay is controlled by a selection rule raised from the Wigner–Eckart
theorem, whereby a rank-$\ell$ local operator couples only to the $\mathcal L=\ell$ transfer-
matrix sector, with a decay length determined by the corresponding correlation length. We further identify a finite-size channel that is independent of the probe operator position. These results establish a quantitative connection between SPT
symmetry,
lattice geometry, and the protection of boundary-encoded
quantum information.
Read the full abstract ▾
Quantum Physics
When Complementary Measurements Count the Same Classical Bit Twice: Counterexamples to CQC, ECQC, and Complementarity-Based Certification
Jinbo Wang, Qihang Wang and Kun Chen
Mutually unbiased measurements are commonly expected to expose independent facets of a
quantum state: a correlation that is classical in one basis should disappear in a complementary basis. In higher dimensions, however, this intuition becomes particularly subtle because correlations recovered in different settings need not represent different information. To expose this loophole, we propose a two-branch classical null test: before the setting is chosen, a shared bit selects one of two orthogonal product preparations, producing a rank-two classical–classical state, and the candidate protocol then runs unchanged. Different settings can read the same bit through different outcome patterns. This two-branch classical architecture disproves the complementary-
quantum correlation (CQC)
conjecture in every dimension $d\geq3$. A distinct rank-two classical–classical state disproves its complete-basis extension (ECQC) at $d=7$, with an overrun that grows without bound along
prime dimensions. Its qutrit CQC instance also gives classical false positives for a proposed
quantum-correlation measure and a proposed one-sided semi-device-independent steering criterion, and refutes a conditional-probability
conjecture. The failures identify the missing requirement: information read in different settings must be nonredundant. In experiments and applications, the same low-overhead architecture can serve as a calibration test before a multibasis score is assigned
quantum meaning.
Read the full abstract ▾
Quantum Physics
CNOT-Distance is NP-complete under all-to-all connectivity
Antonio Acuaviva, Arturo Acuaviva and Pablo Acuaviva
Given $A\in\operatorname{GL}(N,2)$ and an integer $K$, we ask whether $A$ can be implemented by at most $K$ CNOT gates on fixed labelled wires with all-to-all connectivity. We prove that this problem is NP-complete. From a finite simple graph $G=(V,E)$, we construct an upper-unitriangular
matrix $A_G\in\operatorname{GL}(2|V|+|E|+1,2)$ satisfying $\ell_{\mathrm{CNOT}}(A_G)=2|V|+2|E|+\tau(G)$, where $\tau(G)$ is the minimum vertex-cover size. Each target
matrix has $O(N)$ nonzero entries and row Hamming weight at most four. The lower bound unfolds an arbitrary CNOT
circuit into an XOR directed acyclic graph and applies projection–contraction operations, allowing cancellation and unrestricted reuse of intermediate parities. For this family, the optimum is unchanged by any finite number of clean or borrowed ancillary wires that must be restored. A
polynomial-time decoder further yields NP-hardness of approximation within every fixed additive constant and, through an L-reduction from Minimum Vertex Cover on cubic graphs, APX-hardness of the associated CNOT-
circuit optimisation problem.
Read the full abstract ▾
Quantum Physics
Impossibility of Perfectly Complete Many-Round Key Agreement in the QROM
Longcheng Li, Qian Li, Xingjian Li et al. (+1 more)
This paper proves that it is impossible to construct perfectly complete
quantum key agreement protocols (QKA) from quantumly secure one-way functions (OWFs) in a black-box manner. Specifically, consider any protocol in which Alice and Bob exchange only classical messages, make at most $q_{\mathsf{A}}$ and $q_{\mathsf{B}}$
quantum queries, respectively, to a Boolean-valued random oracle, and agree on a shared key with certainty. This paper shows that there exists an eavesdropper, given the classical messages, that can recover the shared key with certainty using $O((q_{\mathsf{A}}+q_{\mathsf{B}})^5)$ classical oracle queries. The bound is independent of the number of rounds, transcript length, key length, and oracle-domain size. Previous results only applies to two-round key agreement (Li et al. CRYPTO 26) or relies on unproven
conjectures (Austrin et al. CRYPTO 22). GPT-5.6 Sol Ultra found this proof in a one-shot conversation and drafted a preliminary version of this paper. The authors are fully responsible for the correctness, writing and discussions of this paper.
Read the full abstract ▾
All authors Longcheng Li, Qian Li, Xingjian Li, Qipeng Liu