Quantum Physics
Permutation Invariant Optimization Problems in Quantum Information Theory: A Framework for Channel Fidelity and Beyond
Bjarne Bergh and Marco Parentin
Exploiting permutation invariance to reduce the exponential scaling of semidefinite programs in
quantum information has emerged as a powerful computational technique. In this work, we develop a systematic framework for using this reduction via Schur-Weyl duality for optimization problems, and establish methods that allow one to work fully inside the permutation invariant subspace while performing operations such as (partially) applying channels and taking (partial) traces, or computing expressions like the
quantum relative
entropy. We then apply our techniques to the problem of computing efficient lower bounds on the channel fidelity over $n$ parallel uses of a
quantum channel. The
algorithm, which we call symmetric seesaw method, exploits permutation-invariant codes to yield improved lower bounds on the channel fidelity over $n$ uses of the depolarizing and amplitude-damping channel in the regime of tens of channel uses, and was used in [arxiv:2604.27042] to demonstrate non-asymptotic superactivation of
quantum capacity for $n = 17$. An implementation of our methods, aimed at being suitable for various
quantum information theoretic optimization problems, is also available as an open-source Python package.
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Quantum Physics
Some applications of Choi polynomials of linear maps
Minh Toan Ho, Thanh Hieu Le, Cong Trinh Le et al. (+1 more)
This paper investigates the properties of Choi
polynomials and their fundamental role in the theory of positive linear maps between
matrix algebras. By focusing on Hermitian symmetric biquadratic forms, we establish a connection between the positivity of these forms and the structure of positive maps. We specifically explore the construction of indecomposable positive maps in
matrix algebras, and their application as
entanglement witnesses. Our analysis extends to the detection of Positive Partial Transpose (PPT) entangled states and the classification of edge PPT states in $M_m(\mathbb{C}) \otimes M_n(\mathbb{C})$. Our results provide a refined framework for identifying non-separable states that escape the standard PPT criterion, contributing to the broader understanding of
entanglement distillation and
quantum information theory.
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All authors Minh Toan Ho, Thanh Hieu Le, Cong Trinh Le, Hiroyuki Osaka
Quantum Physics
Efficient mapping of multi-constraint satisfaction problems to Rydberg platforms
Robert Gloeckner, Shahram Panahiyan, Frederik Koch et al. (+2 more)
We present a hardware-native gadget framework for solving constraint satisfaction problems on Rydberg
quantum computing architectures. Our approach introduces a compact $xor_1$ gadget that enforces exactly-one constraints, ubiquitous in combinatorial optimization, directly through
geometric embedding and blockade interactions. A key advantage of the $xor_1$ gadget is its fixed, problem-size-independent detuning requirements: enforcing constraints through blockade interactions eliminates the need for large penalty terms, thereby substantially reducing the detuning range compared to Quadratic Unconstrained Binary Optimization (QUBO) formulations and improving experimental feasibility. By tailoring the construction to the
geometric connectivity of Rydberg atom arrays, the framework bypasses the all-to-all physical couplings often assumed in logical encodings. This enables
embeddings compatible with planar layouts and avoids highly connected arrangements. We develop scalable implementations that reduce atom count and connectivity overhead while avoiding extensive classical preprocessing, making them compatible with near-term neutral-atom hardware. As illustrations, we apply our framework to the gate-assignment and $N$-queens problems, highlighting its practicality, resource efficiency, and hardware compatibility. In these examples, we observe reductions in detuning range of up to $99\%$ and savings in atom count and connectivity overhead of up to $54\%$ compared to the QUBO method. These results establish a route toward implementing large-scale combinatorial optimization on Rydberg platforms beyond the limits of existing encodings.
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All authors Robert Gloeckner, Shahram Panahiyan, Frederik Koch, Dieter Jaksch, Joseph Doetsch
Quantum Physics
Ground-state energy of a particle in a space with minimal length and minimal momentum
Arsen Panas and Volodymyr Tkachuk
In this article, we derived a rigorous lower bound on the ground-state energy for a class of one-dimensional
quantum systems in deformed space with minimal coordinate and momentum uncertainties, representing the absolute minimum energy that is physically attainable. We considered a harmonic oscillator in such a space and calculate its ground-state energy. We generalized the problem to an a sufficiently broad class of potentials, deriving an equation for the coordinate uncertainty corresponding to the minimal energy, which can be solved numerically. Using a linear approximation in the deformation parameters, we obtained a general expression for the ground-state energy. We determined the domain of existence of solutions for the anharmonic oscillator potential with respect to the deformation parameters.
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Quantum Physics
Congestion-free routing on quantum chips
Mithilesh Kumar, Yusuf Tahir, Varun Daiya et al. (+2 more)
Limited connectivity makes nonlocal
quantum gates expensive on near-neighbor hardware, where compilation typically relies on SWAP transport, inheriting both depth overhead and path congestion. We present a swap-free routing framework in which higher levels of a qudit act as orthogonal spectral buses that transport control information without moving the computational state. We show that exact congestion relief in nearest-neighbor architectures requires local
Hilbert-space expansion. In this model, a nonlocal operation over a path of length $L$ requires $2L+1$ logical routing primitives, compared to the $3L$ baseline. Overlapping routes remain distinguishable through bus labels encoded in the same physical qudits. This routing
algebra extends to Boolean fan-in at a common target: multiple controls arriving on distinct buses trigger a local
unitary based on an arbitrary Boolean function of bus digits, yielding multi-control operations of depth $2L + D_g + O(1)$ for fan-in size $K$ and target-synthesis cost $D_g$. We prove decodability, reversibility, and correctness for CNOT and Boolean fan-in, along with a state-count lower bound $d \geq 2^{K+1}$ for exact overlap routing. Cirq simulations confirm single-control correctness and zero crosstalk. Compiler-level
benchmarks on QFT, QAOA, and mirror-interaction
circuits verify the predicted congestion law and transport reduction. Noisy QuTiP simulations show that the architectural advantage depends on higher-level coherence and speed. These results identify spectral qudit routing as a congestion-relief architecture that separates nonlocal control delivery from local target-side aggregation, providing a minimal mechanism for overcoming
qubit routing limitations.
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All authors Mithilesh Kumar, Yusuf Tahir, Varun Daiya, Sanjana Mattaparthi, Aarav Shaurya
Quantum Physics
Geometric-Phase (Pancharatnam-Berry) Correction for Time-Bin Photonic Qudits: A Calibration and Feed-Forward Algorithm
Ryan Rae-Cheng Wee and Josef Bruzzese
We develop a
geometric-phase framework for time-bin photonic qudits and propose a practical calibration and feed-forward
algorithm for separating and compensating
geometric (Pancharatnam-Berry), dynamical, and technical phase contributions. Working directly in the time-bin basis, we use a parallel-transport gauge so that
geometric phases appear as experimentally identifiable interferometric offsets, while all phase contributions enter a bin-resolved diagonal transformation. We model state preparation by cascaded unbalanced Mach-Zehnder interferometers and give closed-form amplitudes for arbitrary splitting ratios and phases, noting that single-port monitoring requires post-selection and renormalization. We then give an interferometric
tomography recipe based on adjacent-bin scans, with a
Fourier-basis cross-check, and a multi-mode numerical case study that separates total, dynamical, and
geometric phases and demonstrates feed-forward compensation. The protocol uses standard components, including tunable UMZIs, phase shifters or EOMs, and single-
photon detectors, together with routine phase sweeps. It is intended for small to moderate dimensions, approximately d up to 10, and provides a scalable route toward phase-stable high-dimensional temporal encoding for
quantum communication and photonic processing.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Large quantum dot energy level shifts in anomalous photon-assisted tunneling
Jared Benson, C. E. Sturner, A. R. Huffman et al. (+12 more)
Orbital energy splittings are important
quantum dot parameters for the operation of hole
spin qubits. They are known to depend on the lateral confinement of the
quantum dots. However, when changing top, plunger gate voltages, which are the typical control parameter for
qubit applications, such energy splitting changes are typically negligible, both as measured in experiment and as assumed in effective theories. Here, we study the singlet-triplet (ST) splittings, which depend on the orbital splittings, of a double
quantum dot (DQD) in a Ge/SiGe
heterostructure using
photon-assisted tunneling (PAT) and pulsed-gate spectroscopy. We find that the ST splittings have a surprising, strong dependence on the top gate voltages, leading to anomalous PAT measurements. We combine data from both measurements in a model that well describes the linear gate-voltage dependence of the ST splittings. Finally, we show that the ST splittings of the two dots exhibit similar linear gate-voltage dependences when the device is retuned such that their ratio is significantly different.
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All authors Jared Benson, C. E. Sturner, A. R. Huffman, Sanghyeok Park, Valentin John, Brighton X. Coe, Tyler J. Kovach, Stefan D. Oosterhout, Lucas E. A. Stehouwer, Francesco Borsoi, Giordano Scappucci, Menno Veldhorst, Benjamin D. Woods, Mark Friesen, M. A. Eriksson
Quantum Physics
Simulating dynamics of RLC circuits with a quantum differential-algebraic equations solver
Arkopal Dutt, Anirban Chowdhury, Kristan Temme et al. (+1 more)
We introduce a
quantum algorithm for simulating the dynamics of electrical
circuits consisting of resistors, inductors and capacitors (aka RLC
circuits) along with power sources. Given oracle access to the connectivity of the
circuit and values of the electrical elements, our
algorithm prepares a
quantum state that encodes voltages and current values either at a specified time or the history of their
evolution over a time-interval. For an RLC
circuit with $N$ components, our
algorithm runs in time $\textsf{polylog}(N)$ under mild assumptions on the connectivity of the
circuit and values of its components. This provides an exponential speed-up over classical
algorithms that take $\textsf{poly}(N)$ time in the worst-case. Our
algorithm can be used to estimate energy across a set of components or dissipated power in $\textsf{polylog}(N)$ time, a problem that we prove is BQP-hard and therefore unlikely to be efficiently solved by classical
algorithms. The main challenge in simulating the dynamics of RLC
circuits is that they are governed by differential-algebraic equations (DAEs), a coupled system of differential equations with hidden algebraic constraints. Consequentially, existing
quantum algorithms for ordinary differential equations cannot be directly utilized. We therefore develop a
quantum DAE solver for simulating the time-
evolution of linear DAEs. For RLC
circuits, we employ modified nodal analysis to create a system of DAEs compatible with our
quantum algorithm. We establish BQP-hardness by demonstrating that any network of classical harmonic oscillators, for which an energy-estimation problem is known to be BQP-hard, is a special case of an LC
circuit. Our work gives theoretical evidence of
quantum advantage in simulating RLC
circuits and we expect that our
quantum DAE solver will find broader use in the simulation of dynamical systems.
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All authors Arkopal Dutt, Anirban Chowdhury, Kristan Temme, Hari Krovi
Quantum Physics · Cross-listed from hep-th
Schwinger-Keldysh Path Integral for Gauge theories
Greg Kaplanek, Maria Mylova and Andrew J. Tolley
We develop the Schwinger-Keldysh path-integral formalism for open non-Abelian gauge theories that are gauge-fixed via the BRST method in covariant gauges. We focus on generic initial states, pure and mixed, specified at finite times suitable for non-
equilibrium processes. We pay particular
attention to the handling of the indefinite
Hilbert space, the construction of BRST-invariant Schrodinger picture wavefunctionals, density
matrices and inner product, the implementation of the Hata-Kugo prescription, and the role of boundary terms at both the initial and final times. We highlight the advantages of the Nakanishi-Lautrup field representation in dealing with initial/final conditions. The resulting Schwinger-Keldysh path integral is manifestly invariant under a diagonal (retarded) BRST
symmetry for arbitrary physical initial states, whether pure or mixed. From this, we obtain the corresponding Ward-Takahashi-Slavnov-Taylor identities, valid perturbatively. Non-perturbatively the Gribov ambiguity is expected to break or modify the BRST
symmetry. The naive advanced BRST
symmetry is shown to be explicitly violated by the in-in boundary conditions. We show that the Feynman-Vernon influence functional derived by integrating out charged matter and/or hard
gluon modes remains (perturbatively) BRST invariant. When the Open EFT action is expanded to second order in advanced fields it exhibits an exact
symmetry under a contraction of the original BRST
symmetry. This Keldysh BRST
symmetry is equivalent to the BRST associated with the retarded gauge transformations together with a linearly realized BRST transformation of the advanced fields. These govern the structure of the leading terms in an Open EFT. We illustrate this with the explicit example of Hard Thermal Loop Effective Theory, and construct the general form of the Open EFT in a
Higgs phase when all gauge
symmetries are spontaneously broken.
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Quantum Physics
The most discriminable quantum states in the multicopy regime
Maria Kvashchuk, Polina Chernyshova, Lucas E. A. Porto et al. (+3 more)
This work investigates which sets of
quantum states give rise to the highest achievable success probability in minimum-error state discrimination if multiple copies of the unknown state are given. Specifically, we consider uniformly distributed ensembles of the form $\left\{\frac{1}{N},\rho_i^{\otimes k}\right\}_{i=1}^N$, where $N$ states in dimension $d$ are provided in $k$ identical copies, and derive universal limits in this scenario. For pure state ensembles, we prove that whenever $N$ is large enough to support a state $k$-design, these designs will exactly give rise to the maximally discriminable sets. We further show that when $N$ exceeds the size required for a $k$-design, mixed states can outperform all pure state ensembles. We then recognise that the problem of most discriminable classical states in the multi-copy regime is in one-to-one correspondence to the concept of the multiplicative Bayes capacity of independent uses of classical channels, a concept that emerges naturally in the context of classical information leakage. This connection allows us to completely solve the classical analogue of our problem when $N\geq \binom{d + k - 1}{k}$, and to prove that
quantum systems offer a quadratic advantage (in number of copies $k$) over classical ones. Then, we prove that this
quantum over classical advantage is strongly reduced when one is restricted to real
quantum states, more precisely, when $N \geq k + 1$, pure real
qubits only offer a constant advantage over classical bits. Finally, we introduce computational techniques to find sets of most discriminable ensembles and to obtain rigorous universal upper bounds on the maximal success probability for multi-copy state discrimination in cases that are analytically intractable.
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All authors Maria Kvashchuk, Polina Chernyshova, Lucas E. A. Porto, Ties-A. Ohst, Lucas B. Vieira, Marco Túlio Quintino
Quantum Physics
En Route to a Standard QMA1 vs. QCMA Oracle Separation
David Miloschewsky, Supartha Podder and Dorian Rudolph
We study the power of
quantum witnesses under perfect completeness. We construct a classical oracle relative to which a
language lies in $\mathsf{QMA}_1$ but not in $\mathsf{QCMA}$ when the $\mathsf{QCMA}$ verifier is only allowed polynomially many adaptive rounds and exponentially many parallel queries per round. Additionally, we derandomize the permutation-oracle separation of Fefferman and Kimmel, obtaining an in-place oracle separation between $\mathsf{QMA}_1$ and $\mathsf{QCMA}$. Furthermore, we focus on $\mathsf{QCMA}$ and $\mathsf{QMA}$ with an exponentially small gap, where we show a separation assuming the gap is fixed, but not when it may be arbitrarily small. Finally, we derive consequences for approximate ground-state preparation from sparse
Hamiltonian oracle access, including a bounded-adaptivity frustration-free variant.
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Quantum Physics
Contextuality, Locality, and the Interpretation of Bell Violations
Partha Ghose
The ontological significance of violations of Bell inequalities is reexamined. Bell's
theorem admits several mathematically equivalent formulations that support different interpretive perspectives. The present paper develops one such perspective using contextuality, the sheaf-theoretic framework of Abramsky and Brandenburger, and Nelson's
stochastic mechanics.
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Quantum Physics
Digital Simulation of Non-Hermitian Knotted Bands on Quantum Hardware
Truman Yu Ng, Yuzhu Wang, Wei Jie Chan et al. (+3 more)
Knots and links represent a fundamental motif of non-local connectivity that permeates the physical sciences from string theory to
protein folds. While spectral braiding has been explored in two-band non-Hermitian models across various platforms, its direct simulation and characterization on programmable
quantum hardware, particularly beyond two strands, remains a formidable challenge due to the limitations of variational optimization in these systems. Here, we introduce a family of non-Hermitian multi-band twister models and implement a non-variational protocol to characterize their complex braided band structures on a programmable superconducting
quantum processor. By mapping the winding of eigenstates to the spectral
topology, we devise an efficient measurement strategy that extracts braid information, including braid words and
knot invariants like the Alexander and Jones
polynomials, without requiring full spectral
tomography or repeated optimization. We experimentally demonstrate the reconstruction of complicated
knots and links such as the
Hopf chain and Solomon's
knot. Our approach provides a general framework for investigating exotic non-Hermitian
topology on near-term
quantum devices, opening a route to simulate more sophisticated topological structures in
knot theory.
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All authors Truman Yu Ng, Yuzhu Wang, Wei Jie Chan, Ruizhe Shen, Tianqi Chen, Ching Hua Lee
Quantum Physics
Cavity-mediated coherence protection and one-axis twisting for spins in solids
Rikuto Fukumori, Chengyi Luo, Alexey Tiranov et al. (+3 more)
Long-range interactions between emitters give rise to collective phenomena, including superradiance,
spin squeezing, and coherence protection, that are important to both fundamental physics and
quantum technologies. Despite progress in cold atoms, coherent cavity-mediated all-to-all interactions have not yet been realized in a solid-state ensemble. Here we demonstrate such interactions in a $^{171}$Yb$^{3+}$:CaWO$_4$
crystal coupled to a microwave resonator, observing superradiant emission on resonance and
unitary one-axis twisting dynamics in the dispersive regime. The same interaction also opens a many-body energy gap that suppresses inhomogeneous dephasing, extending the ensemble Ramsey coherence time from tens of microseconds to milliseconds without decoupling pulses. These results establish a solid-state platform for collective many-body physics with direct implications for
quantum technologies. Specifically, the observed one-axis twisting dynamics opens a path towards
spin squeezing for
entanglement-enhanced
quantum metrology, and the extended coherence due to gap-protection is relevant for both microwave
photon storage and precision measurement.
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All authors Rikuto Fukumori, Chengyi Luo, Alexey Tiranov, Karolina Waszkowska, Philippe Goldner, Andrei Faraon
Quantum Physics
Strict Hierarchy for Quantum Channel Certification to Unitary
Kean Chen, Qisheng Wang and Zhicheng Zhang
We consider the problem of
quantum channel certification to
unitary, where one is given access to an unknown $d$-dimensional channel $\mathcal{E}$, and wants to test whether $\mathcal{E}$ is equal to a target
unitary channel or is $\varepsilon$-far from it in the diamond norm. We present optimal
quantum algorithms for this problem, settling the query complexities in three access models with increasing power. Specifically, we show that: (i) $\Theta(d/\varepsilon^2)$ queries suffice for incoherent access model, matching the lower bound due to Fawzi, Flammarion, Garivier, and Oufkir (COLT 2023). (ii) $\Theta(d/\varepsilon)$ queries suffice for coherent access model, matching the lower bound due to Regev and Schiff (ICALP 2008). (iii) $\Theta(\sqrt{d}/\varepsilon)$ queries suffice for source-code access model, matching the lower bound due to Jeon and Oh (npj
Quantum Inf. 2026). This demonstrates a strict hierarchy of complexities for
quantum channel certification to
unitary across various access models.
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Quantum Physics
A Gaussian asymmetry measure
Riccardo Travaglino and Pasquale Calabrese
The study of
Entanglement Asymmetry has emerged in recent years as a powerful tool to characterise the
symmetry properties of
quantum states in relation to a given charge operator through the lens of
entanglement. While extremely powerful and general, the standard definition of asymmetry introduces significant non-Gaussian features in free-fermionic systems, leading to certain analytical limitations. In this work, we introduce an asymmetry measure that remains strictly within the Gaussian
manifold and analyse its properties. In particular, we show that it quantifies the minimal distance between a Gaussian state and the
manifold of symmetric Gaussian states. We further demonstrate that this measure captures the established dynamical signatures of
entanglement asymmetry, such as the Mpemba effect,
symmetry restoration, and the lack thereof. The Gaussian structure allows these novel asymmetry measures to be computed exactly using correlation
matrix techniques, and to be described asymptotically through the quasiparticle picture. We also comment on the possibility of using charge fluctuations to characterise the asymmetry of a Gaussian state.
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Quantum Physics
Convex combinations of bosonic pure-loss channels
Giuseppe Catalano, Marco Fanizza, Francesco Anna Mele et al. (+2 more)
The pure-loss channel is a fundamental noise model for bosonic
quantum platforms, characterised by a single parameter, the transmissivity. In realistic scenarios such as free-space
quantum communication, the transmissivity fluctuates from one channel use to another, and the channel is a convex combination of pure-loss channels, known as a fading channel. Despite its practical relevance, its
quantum Shannon theory has remained largely unexplored. Here we investigate degradability, anti-degradability,
entanglement breakingness, and capacities of the fading channel. We prove that
entanglement distribution and
quantum key distribution can be achieved at a strictly positive rate over any fading channel that is not completely noisy. When the transmissivity takes a finite set of values, we determine the energy-unconstrained two-way
quantum and secret-key capacities exactly, as the averages of those of the pure-loss components. We prove that thermal states, optimal for a broad class of static bosonic Gaussian channels, do not in general achieve the
entanglement-assisted classical capacity of fading channels: for a binary fading model we derive the capacity-achieving state in closed form, and we exhibit channels for which non-Gaussian Fock-diagonal states strictly outperform every Gaussian encoding. For the
quantum capacity, we give a simple sufficient condition on the transmissivity distribution under which weak thermal inputs yield a strictly positive rate. Outside this condition, we numerically identify parameter regions where no thermal input compatible with the energy constraint has a positive coherent information, while optimized non-Gaussian inputs do. For general fading distributions, we design an iterative variational
algorithm to optimize the coherent and mutual information. Our work advances the study of
quantum communication in the non-Gaussian regime.
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All authors Giuseppe Catalano, Marco Fanizza, Francesco Anna Mele, Giacomo De Palma, Vittorio Giovannetti
Quantum Physics
MLMC-qDRIFT: Multilevel Variance Reduction for Randomized Quantum Hamiltonian Simulation
Pegah Mohammadipour and Xiantao Li
Simulating
quantum dynamics is one of the central applications of
quantum computing. For
Hamiltonians written as a sum of many terms, deterministic Trotter–Suzuki product formulas can require applying a large number of term-wise evolutions at each time step, leading to high
circuit costs for large or dense systems. Randomized methods such as qDRIFT offer an alternative: each step samples only one
Hamiltonian term, giving a
circuit depth with no explicit dependence on the number of terms. However, when qDRIFT is used for observable estimation, high precision requires many independent random
circuit realizations, resulting in a total gate complexity that scales as $\mathcal{O}(\varepsilon^{-3})$. We introduce a multilevel Monte Carlo framework for qDRIFT that reduces this sampling overhead. The method constructs a hierarchy of qDRIFT
estimators with increasing
circuit depths and couples adjacent levels by sharing their random
Hamiltonian-term samples. This coupling makes the variance of the level differences decay with depth, allowing most samples to be taken on cheaper, coarse
circuits and only a few on expensive, fine
circuits. We prove that the resulting MLMC-qDRIFT
estimator reduces the total gate complexity for fixed-precision observable estimation from the standard qDRIFT scaling $\mathcal{O}(\varepsilon^{-3})$ to $\mathcal{O}(\varepsilon^{-2}\log^2(1/\varepsilon))$, while preserving qDRIFT's lack of explicit dependence on the number of
Hamiltonian terms. Numerical experiments for
spin-chain dynamics confirm the predicted variance decay and demonstrate the practical gate-count savings of the multilevel construction.
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Quantum Physics · Cross-listed from cond-mat.supr-con
Non-local Tunneling Spectroscopy of Inelastic Quasiparticle Relaxation in Superconducting 1-D Wires
Kevin M. Ryan, Detlef Beckmann and Venkat Chandrasekhar
Non-local conductance experiments using tunnel junctions can provide valuable spectroscopic information on both the transport and relaxation of quasiparticles in
superconductors, as these techniques directly probe the quasiparticle charge and energy imbalance even at mK temperatures. In this work, we employ mesoscopic three terminal Cu and Al NIS devices to study non-local quasiparticle transport over length-scales on the order of the superconducting coherence length in this regime. Via a dual-bias scheme, which utilizes detector biases both above and below the superconducting gap, we are able to extract the effect of quasiparticle energy imbalance via its impact on the self consistent pair potential by
symmetry considerations. We observe non-local conductance features due to pair-breaking which are anti-symmetric with respect to the polarity of the voltage bias, with a sharp onset during single electron tunneling at energies around $3\Delta$. We compare these findings with quasiclassical simulations including inelastic effects to obtain estimates of the energy dependent inelastic scattering time. In addition, we demonstrate kinetic effects due to a large applied supercurrent which can also be captured in this formalism and decomposed with respect to the particle-hole
symmetry and supercurrent direction, and discuss further opportunities for the advancement of this method.
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Quantum Physics
Protein folding on a 64 qubit trapped-ion hardware via counterdiabatic quantum optimization
Alejandro Gomez Cadavid, Pavle Nikačević, Pranav Chandarana et al. (+11 more)
We report the largest trapped-ion hardware demonstration of
lattice protein-folding optimization to date, using bias-field digitized counterdiabatic
quantum optimization (BF-DCQO) on a fully connected 64-
qubit Barium development system similar to the forthcoming IonQ Tempo line. Six peptide sequences with 14-16 amino-acid residues are encoded using a coarse-grained tetrahedral
lattice model, yielding higher-order
spin-glass
Hamiltonians with long-range interactions involving up to five-body terms and mapped to 46-61
qubits. The resulting instances are demanding for near-term
quantum hardware because low-energy configurations must satisfy backbone-geometry constraints while optimizing dense residue-contact interactions. BF-DCQO uses a non-variational bias-feedback mechanism, where low-energy samples from each round define longitudinal fields that guide subsequent
quantum evolutions. Across the studied instances, BF-DCQO shifts raw sampled energy distributions toward lower energies than uniform random sampling, with the strongest improvements appearing in residue-contact variables. To preserve this signal, we introduce a consensus-based post-processing pipeline that combines
quantum-learned contact information with feasible backbone geometries. The resulting hybrid workflow reaches the classical reference energy in multiple instances and improves over the corresponding random-seeded pipeline. These results show that BF-DCQO can generate structured samples for dense
protein-folding
Hamiltonians at previously unexplored trapped-ion scales.
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All authors Alejandro Gomez Cadavid, Pavle Nikačević, Pranav Chandarana, Sebastián V. Romero, Enrique Solano, Narendra N. Hegade, Miguel Angel Lopez-Ruiz, Claudio Girotto, Hanna Linn, Hakan Doga, Evgeny Epifanovsky, Panagiotis Kl. Barkoutsos, Ananth Kaushik, Martin Roetteler
Quantum Physics
Fluctuations of path-dependent thermodynamic quantities in open quantum systems via two-point system-only measurements
Alessandra Colla, Andrea Smirne, Heinz-Peter Breuer et al. (+1 more)
We propose a method to evaluate general thermodynamic fluctuations in open
quantum systems, based on performing a two-point measurement scheme on the system using dynamics-dependent thermodynamic observables. Our approach allows one to obtain exact equalities for fluctuations of path-dependent thermodynamic quantities such as work and heat, and to isolate correction factors to Jarzynski's equality, requiring only access to the system degrees of freedom. This framework is flexible and can be applied to the limiting case of closed systems, recovering previous, yet seemingly contradictory, results from the literature. Moreover, the formalism admits a straightforward extension to strongly coupled open
quantum systems. We investigate the effect of specific dynamical classes on the fluctuation relations, and show that the pure decoherence case is particularly special, as it deterministically does not contain any heat contribution and thus constitutes a class of open system dynamics for which the Jarzynski equality for work fluctuations is identically true at any coupling strength. Finally, we look explicitly at the shape and size of the correction factors to Jarzynski's equality for a
qubit undergoing phase covariant dynamics, both in the weakly-coupled regime and in the deep non-Markovian regime.
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All authors Alessandra Colla, Andrea Smirne, Heinz-Peter Breuer, Bassano Vacchini
Quantum Physics
Quantum Feature Selection with Higher-Order Binary Optimization on Trapped-Ion Hardware
Carlos Flores-Garrigós, Anton Simen, Qi Zhang et al. (+6 more)
We present a
quantum feature-selection framework based on a higher-order unconstrained binary optimization (HUBO) formulation that explicitly incorporates multivariate dependencies beyond standard quadratic encodings. In contrast to QUBO-based approaches, the proposed model includes one-, two-, and three-body interaction terms derived from mutual-information measures, enabling the objective function to capture feature relevance, pairwise redundancy, and higher-order statistical structure within a unified energy model. To suppress trivial all-selected solutions, we further include structured linear penalties that promote sparsity while preserving informative variables. The resulting HUBO instances are optimized with digitized counterdiabatic
quantum optimization on IonQ Forte and compared against noiseless
quantum simulation as well as two classical dimensionality-reduction baselines: SelectKBest based on mutual information and principal component analysis (PCA). We evaluate the proposed workflow on two
benchmark classification
datasets, namely the Gallstone
dataset and the Spambase
dataset, and analyze both predictive performance and selected-subset structure. The results show good qualitative agreement between hardware executions and noiseless simulations, supporting the feasibility of implementing higher-order feature-selection
Hamiltonians on current trapped-ion processors. In addition, the
quantum approach yields competitive classification performance while producing compact and informative feature subsets, highlighting the potential of higher-order
quantum optimization for machine-learning preprocessing tasks.
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All authors Carlos Flores-Garrigós, Anton Simen, Qi Zhang, Enrique Solano, Narendra N. Hegade, Sayonee Ray, Claudio Girotto, Jason Iaconis, Martin Roetteler
Quantum Physics
Gouy phase engineering of self-splitting quantum correlations
A. L. S. Santos Junior, M. Damaceno, A. C. Barbosa et al. (+4 more)
In this work, we demonstrate the effect of self-splitting spatial
quantum correlations induced by Gouy phase engineering. In the process of spontaneous parametric down conversion the pump beam is structured with a mode superposition that produces a dynamical splitting and recombination of the light beam. This structure is transferred to the
quantum correlations between signal and idler
photons. As a result the joint two-
photon probability distribution propagates like a self-splitting and recombining light beam, implementing a Mach-Zehnder-like interferometer. We observe heralded single-
photon interference and two-
photon NOON state interference. These results open new avenues for applications in
quantum metrology.
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All authors A. L. S. Santos Junior, M. Damaceno, A. C. Barbosa, N. A. Ribeiro, G. B. Alves, P. H. Souto Ribeiro, A. Z. Khoury
Quantum Physics
Classical simulation of free-fermionic dynamics and quantum chemistry with magic input
Changhun Oh, Michał Oszmaniec, Oliver Reardon-Smith et al. (+1 more)
Establishing the precise computational boundary between classically tractable fermionic systems and those capable of genuine
quantum advantage is a central challenge in
quantum simulation. While injecting non-Gaussian “magic" inputs into free-
fermion circuits is widely expected to generate intractable complexity, we identify a physically motivated intermediate regime. We prove that for block-product paired non-Gaussian fermionic states, essential
quantum simulation primitives – transition amplitudes, overlaps, and arbitrary-weight number correlators – can be efficiently approximated to additive error under free-fermionic dynamics. This tractability stems from an algebraic reduction that compresses exponentially large multiparticle interference into a single coefficient of a multivariate Pfaffian
polynomial. Because these classical
estimators match the intrinsic $O(1/\sqrt{K})$ statistical uncertainty of
quantum hardware utilizing $K$ measurement shots, they constitute a practical
benchmark. Building on this foundation, we construct an additive-error
estimator for high-weight Wilson observables in the noninteracting quench of recent trapped-ion experiments, providing a rigorous classical
benchmark. Extending this to
quantum chemistry, we demonstrate that core overlap-based subroutines for antisymmetrized products of strongly orthogonal geminals admit efficient additive-error Pfaffian-kernel
estimators. Ultimately, these results sharpen the boundary of
quantum advantage, establishing that the paired-electron scaffold is dequantized and clarifying where
quantum resources are indispensable.
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All authors Changhun Oh, Michał Oszmaniec, Oliver Reardon-Smith, Zoltán Zimborás
Quantum Physics
HyPulse: A Pulse Synthesis Framework for Hybrid Qubit-Oscillator Gates on Trapped-Ion Platform
Masoud Hakimi Heris, Yuan Liu and Frank Mueller
As hybrid
qubit-oscillator
algorithm development and trapped-ion hardware demonstrations advance in parallel, there is a lack of a compilation layer connecting the two at the pulse level in the vertical software stack. While
qubit gate control and pulse synthesis are well-established, the translation of hybrid
qubit-oscillator primitives to the pulse level has not been systematically addressed. This gap is further compounded by the inherently continuous parametric nature of such gates. Each distinct parameter value defines a physically unique operation requiring independent pulse optimization, making static pre-compilation strategies inapplicable. To fill this gap, we present HyPulse, a hardware-aware pulse synthesis and generation framework, which contributes a two-phase architecture decoupling pulse discovery from
circuit assembly. An offline optimization engine populates a content-addressed cache of high-fidelity primitives: If a pulse for a given gate, parameter, and device specification already exists in the library, it is retrieved instantly; otherwise the optimizer synthesizes, hashes, and caches it automatically. An online assembler then constructs
circuit-specific pulse programs ready to drive trapped-ion hardware control systems via DAX/ARTIQ (Duke) and JaqalPaw/QSCOUT (Sandia), trapped-ion pulse execution backends.
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Quantum Physics
Fault-Tolerant Resource Comparison of Qudit and Qubit Encodings for Diagonal Quadratic Operators
Samuel Godwood, Doğa Murat Kürkçüoğlu, Gabriel N. Perdue et al. (+2 more)
Finite local
Hilbert-space truncations arise naturally in
quantum simulations of
lattice field theories and motivate qudit encodings, but their fault-tolerant advantage over
qubit encodings remains unclear. We compare the non-Clifford cost of implementing quadratic diagonal evolutions, exemplified by $U=e^{-it\phi_x^2}$ in a uniform field-amplitude discretization of a real scalar field, using either one logical $d$-level qudit or $n_b=\lceil \log_2 d\rceil$ logical
qubits. We analyze two standard settings: product-formula simulation and LCU/block encoding, taking the resource metric to be the number of non-Clifford gates after synthesis into a discrete logical gate set. Because tight synthesis bounds for general single-qudit rotations are not known, we express the qudit constructions in terms of embedded two-level $SU(2)$ rotations and derive explicit finite-$d$ break-even conditions for their synthesis cost; these serve as compiler targets for when qudit encodings can outperform the
qubit baseline. Within the constructive models studied here, product-formula implementations would require an exponentially stronger per-primitive synthesis advantage for qudits to win asymptotically, while in the LCU setting the
qubit encoding is asymptotically cheaper in $d$. Nevertheless, the finite-$d$ threshold analysis identifies low dimensional regions in which qudits can yield meaningful constant-factor savings, particularly for LCU-based implementations. As a secondary analysis of the LCU construction, we use an idealized negligible-overhead
qubit-qudit code-switching model to give an absolute $T$-count comparison, and reinterpret the savings as an allowable per-switch overhead budget.
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All authors Samuel Godwood, Doğa Murat Kürkçüoğlu, Gabriel N. Perdue, Marina Maneyro, Alessandro Roggero
Quantum Physics
Chip-to-chip entanglement distribution over 80-km multicore fiber link
Damien Roux, Giulia Guarda, Mujtaba Zahidy et al. (+7 more)
Long-range
quantum entanglement is essential for building large-scale
quantum networks and unconditionally secure cryptographic systems based on
quantum key distribution (QKD). While photonic integrated
circuits offer a highly scalable platform, the fragility of phase coherence between spatial modes has prevented the distribution of path-encoded
entanglement over long distances. Here, we report chip-to-chip distribution of path-encoded entangled states over 80 km between fully integrated silicon photonic transmitter and receiver chips. Telecom-band entangled
photon pairs are generated via spontaneous four-wave mixing in on-chip spiral waveguides and distributed between chips over a dual-core, actively stabilized fiber link. Upon distribution, we measure a Bell state fidelity of $85.7 \pm 0.2 \%$. Implementing the BBM92 protocol with the same source, we obtain a secure key rate of 2.03 bit/s in the infinite-key regime. These results establish silicon photonic chips as a viable platform for long-distance path-encoded
entanglement-based
quantum key distribution, paving the way toward scalable, device-independent
quantum networks.
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All authors Damien Roux, Giulia Guarda, Mujtaba Zahidy, Yunhong Ding, Siyan Zhou, Domenico Ribezzo, Battulga Munkhbat, Francesco Da Ros, Davide Bacco, Caterina Vigliar
Quantum Physics
Optical squeezing mediated by levitated oscillators at their quantum ground state
A. Pontin, Q. Deplano, F. Marino et al. (+1 more)
We demonstrate optical squeezing below the shot-noise level generated through the interaction of an optical cavity field with two center-of-mass modes of a levitated nanoparticle, simultaneously cooled to occupation numbers well below unity. By analyzing the quadrature fluctuations of the cavity output through heterodyne detection, we resolve the full spectral covariance
matrix of the optical field and map regions of sub-shot-noise squeezing as a function of detection phase and frequency. Operating in the resolved sideband and strong coupling regime where mechanical modes hybridize with the optical mode, we observe consistent squeezing in the band 70-95 kHz with a lowest variance of 0.98 (2$\%$ below
vacuum fluctuations). We thus demonstrate optical squeezing mediated by multiple mechanical oscillators in their
quantum ground state, bridging mechanical
quantum control with non-classical light and establishing levitated optomechanics as a platform for multimode
quantum interactions.
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All authors A. Pontin, Q. Deplano, F. Marino, F. Marin
Quantum Physics · Cross-listed from cs.DC
A Semantic Quantum Circuit Cache for Scalable and Distributed Quantum-Classical Workflows
Mar Tejedor, Javier Conejero and Rosa M. Badia
Hybrid
quantum–classical workflows often execute large ensembles of
circuits that differ syntactically but implement identical operations, leading to substantial redundant computation. To address this, we introduce the
Quantum Circuit Cache, a content-addressable system that detects semantic equivalence and reuses previously computed results across executions, backends, and workflow stages. Our approach combines ZX-calculus reduction with isomorphism-invariant Weisfeiler–Leman graph hashing to generate deterministic
circuit identifiers, enabling constant-time lookup in distributed caches supporting both lightweight LMDB and scalable Redis deployments. The system integrates transparently into hybrid HPC workflows and remains backend-agnostic across CPU, GPU, and QPU environments. We evaluate the system on MareNostrum 5 with two representative workloads: distributed wire cutting and Differential
Evolution-based QAOA optimization. For wire cutting, caching eliminates up to 91.98% of redundant subcircuit simulations, yielding speedups up to 7.0 times on a single node and maintaining advantages at scale, with Redis-based caching achieving up to 1.6 times speedups under high parallelism. Validation on a 35-
qubit superconducting QPU confirms these benefits, achieving an 11.2 times speedup on real hardware. In distributed QAOA optimization, equivalence-aware caching avoids up to 27.6% of
circuit evaluations and consistently reduces execution cost without altering the optimization
algorithm. In both cases, reuse grows with concurrency and
circuit structure, highlighting redundancy as a major systems bottleneck and demonstrating the effectiveness of our
Quantum Circuit Cache.
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Quantum Physics · Cross-listed from physics.atom-ph
Fast, powerful, low-noise optical pumping of an atomic vapor with semiconductor optical amplifiers
Diana Méndez-Avalos, Théo Louzada Meireles, Morgan W. Mitchell et al. (+1 more)
We use a $^{87}\text{Rb}$ atomic vapor, suitable for an optically-pumped magnetometer (OPM) in Earth-field conditions, to study the noise properties of three strategies for generating pulsed optical pumping. We compare a frequency-modulated (FM)
laser, amplitude modulation (AM) via an acousto-optic modulator (AOM), and amplitude modulation via a
semiconductor optical amplifier (SOA). Pumping the ensemble to operate as a Bell-Bloom OPM, and with an equal degree of
spin polarization, the three methods give nearly identical sensitivity, showing that the SOA, despite being an active device, can introduce negligible additional noise. Pumping the ensemble to operate as a free-induction-decay OPM, we observe longer unpumped coherence times with the SOA-AM method than with the FM method. Finally, using the higher power available from the SOA, we demonstrate an environment-limited sensitivity of $80\text{fT}/\sqrt{\text{Hz}}$ at $600\text{Hz}$ and 200fT$200\text{fT}/\sqrt{\text{Hz}}$ at $4\text{kHz}$, one to two orders of magnitude beyond what was achievable with the other pumping methods.
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All authors Diana Méndez-Avalos, Théo Louzada Meireles, Morgan W. Mitchell, Aleksandra Sierant