Quantum Physics
Optimal Quantum State Testing Even with Limited Entanglement
Chirag Wadhwa and Sitan Chen
In this work, we consider the fundamental task of
quantum state certification: given copies of an unknown
quantum state $\rho$, test whether it matches some target state $\sigma$ or is $\epsilon$-far from it. For certifying $d$-dimensional states, $\Theta(d/\epsilon^2)$ copies of $\rho$ are known to be necessary and sufficient. However, the
algorithm achieving this complexity makes fully entangled measurements over all $O(d/\epsilon^2)$ copies of $\rho$. Often, one is interested in certifying states to a high precision; this makes such joint measurements intractable even for low-dimensional states. Thus, we study whether one can obtain optimal rates for
quantum state certification and related testing problems while only performing measurements on $t$ copies at once, for some $1 < t \ll d/\epsilon^2$. While it is well-understood how to use intermediate
entanglement to achieve optimal
quantum state learning, the only protocol known to achieve optimal testing is the one using fully entangled measurements. Our main result is a smooth copy complexity upper bound for state certification as a function of $t$, which achieves a near-optimal rate at $t = d^2$. In the high-precision regime, i.e., for $\epsilon < \frac{1}{\sqrt{d}}$, this is a strict improvement over the
entanglement used by the aforementioned optimal protocol. We also extend our techniques to develop new
algorithms for the related tasks of mixedness testing and purity estimation, and show tradeoffs achieving the optimal rates for these problems at $t = d^2$ as well. Our
algorithms are based on novel reductions from testing to learning and leverage recent advances in
quantum state
tomography in a non-black-box fashion. We complement our testing upper bounds with a (nearly) matching lower bound that holds in the high-precision regime, implying that one must take $t \geq \Omega(d^2)$ to achieve the optimal rate for certification.
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Quantum Physics
Quantum Simulation of Collective Neutrino Oscillations using Dicke States
Katarina Bleau, Nikolina Ilic, Joachim Kopp et al. (+2 more)
In dense
neutrino gases, which exist for instance in
supernovae, the flavour states of different
neutrinos may become entangled with one another. The theoretical description of such systems may therefore call for simulations on a
quantum computer. Existing
quantum simulations of simple toy systems are not optimal in the sense that they do not fully exploit the
symmetries of the system. Here, we propose a new class of
qubit-efficient
algorithms based on Dicke states and the $su(2)$
spin algebra. We demonstrate the excellent performance of these
algorithms both on classical and on
quantum hardware.
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All authors Katarina Bleau, Nikolina Ilic, Joachim Kopp, Ushak Rahaman, Xin Yue Yu
Quantum Physics
Operational criteria for quantum advantage in latency-constrained nonlocal games
Changhao Li, Seigo Kikura, Akihisa Goban et al. (+2 more)
Remote
entanglement enables coordinated decision making without communication and produces correlations beyond those achievable by any classical strategy, representing a practical
quantum advantage in time-critical distributed decision-making problems. However, existing analyses of
quantum-classical gaps in such latency-constrained tacit coordination (LCTC) have focused on idealized models that neglect the finite stationary window of the LCTC, finite operation times, and limited
entanglement generation rates, leaving fundamental constraints unaccounted for. In this work, we develop a comprehensive framework to quantitatively analyze
quantum advantage in LCTC that explicitly incorporates finite-duration and finite-rate operations, as well as generalized utility structures with a limited stationary window. These advances are made possible by adapting statistical certification methods for nonlocal games to the decision-making scenarios of LCTC, identifying operational criteria that must be satisfied by the hardware implementations to realize
quantum advantage with sufficient statistical significance. To meet the stringent criteria, we propose time-multiplexed, event-ready operations of cavity-assisted trapped-atom
quantum network nodes that provide a continuous stream of entangled
qubit pairs, with decision latencies of a microsecond and decision rates of $8\times 10^3~\text{s}^{-1}$ per channel for a representative metropolitan-scale $50$-km fiber network to keep up with the fast-changing environment, such as financial
markets and electric grid networks. These results bridge the gap between the theoretical notions of the
quantum-classical gap in nonlocal games and concrete implementations that meet the stringent operational criteria for achieving robust
quantum advantage in realistic coordination tasks.
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All authors Changhao Li, Seigo Kikura, Akihisa Goban, Hayata Yamasaki, Shinichi Sunami
Quantum Physics
When is randomization advantageous in quantum simulation?
Francesco Paganelli, Michele Grossi, Andrea Giachero et al. (+2 more)
We study the regimes in which
Hamiltonian simulation benefits from randomization. We introduce a sparse-QSVT construction based on composite
stochastic decompositions, where dominant terms are treated deterministically and smaller contributions are sampled stochastically. Crucially, we analyze how
stochastic and approximation errors propagate through block-encoding and QSVT procedures. To
benchmark this approach, we construct ensembles of random
Hamiltonians with controlled coefficient dispersion, locality, and number of terms, designed to favor randomization, and therefore providing an upper bound on its practical advantage. For
Hamiltonians with many terms and highly inhomogeneous coefficient distributions, randomized methods reduce gate counts by up to an order of magnitude. However, this advantage is confined to moderate-precision regimes: as the target error decreases, deterministic methods become more efficient, with a crossover near $\varepsilon \sim 10^{-3}$. Although this regime partially overlaps with
quantum chemistry
Hamiltonians, realistic systems exhibit additional structure, such as commutation patterns, not captured by our model, which are expected to further favor deterministic approaches.
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All authors Francesco Paganelli, Michele Grossi, Andrea Giachero, Thomas E. O'Brien, Oriel Kiss
Quantum Physics
Locked Subharmonic Oscillations in the Entanglement Spectrum of a Periodically Driven Topological Chain
Rishabh Jha
Periodically driven
quantum systems can exhibit subharmonic response, usually characterized through physical observables and often discussed in interacting settings. Here we show that a sharp subharmonic signature already appears in the
entanglement spectrum of a number-conserving free-
fermion system. We study a two-step driven Su-Schrieffer-Heeger chain whose Floquet operator supports
symmetry-protected edge modes at quasienergies $0$ and $\pi$. When the initial state is a coherent superposition of these two edge sectors, we show that the subsystem correlation
matrix alternates between two stroboscopic structures, and the
entanglement spectrum is period-doubled as a set, while an overlap-tracked
entanglement level shows a robust period-doubling response with
Fourier weight concentrated at half the drive frequency. By contrast, diagonal edge densities remain flat by sublattice
symmetry, while an off-diagonal edge-bond observable provides the corresponding linear one-body comparator. The effect disappears both when the initial state is replaced by a stroboscopically stationary Floquet eigenstate built from the same topological mode content, and when the system is placed in the topologically trivial phase where no edge modes exist. Altogether, these establish zero-$\pi$ Floquet
topology as a necessary condition and coherent nonequilibrium preparation as the additional sufficient ingredient. Our results identify
entanglement spectroscopy as a sharp subsystem-resolved probe of Floquet topological coherence.
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Quantum Physics
Ten-Second Electron-Spin Coherence in Isotopically Engineered Diamond
Takashi Yamamoto, H. Benjamin van Ommen, Kai-Niklas Schymik et al. (+7 more)
Solid-state
spin defects are a promising platform for
quantum networks. A key requirement is to combine long ground-state
spin-coherence times with a coherent optical transition for
spin-
photon entanglement. Here, we investigate the
spin and optical coherence of single nitrogen-vacancy (NV) centres in (111)-grown isotopically engineered diamond. Our diamond-growth process yields a precisely controlled $^{13}\mathrm{C}$ concentration and low-ppb nitrogen concentrations. Combined with the mitigation of 50 Hz noise using a real-time feedforward scheme and tailored decoupling sequences, this enables record defect-electron-
spin coherence times of $T_2 = 6.8(1)$ ms for a Hahn echo and of $T_2^{DD} = 11.2(8)$ s under dynamical decoupling. In addition, we observe coherent optical transitions with a near-lifetime-limited homogeneous linewidth of 16.9(4) MHz and characterize the spectral
diffusion dynamics. These results provide new avenues to investigate the incorporation of
impurities in diamond and new opportunities for improved
spin-
qubit control for
quantum networks and other
quantum technologies.
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All authors Takashi Yamamoto, H. Benjamin van Ommen, Kai-Niklas Schymik, Beer de Zoeten, Shinobu Onoda, Seiichi Saiki, Takeshi Ohshima, Hadi Arjmandi-Tash, René Vollmer, Tim H. Taminiau
Quantum Physics
Observation of genuine $2+1$D string dynamics in a U$(1)$ lattice gauge theory with a tunable plaquette term on a trapped-ion quantum computer
Rohan Joshi, Yizhuo Tian, Kevin Hemery et al. (+5 more)
Quantum simulations of high-energy physics in $2+1$D can probe dynamical phenomena nonexistent in one spatial dimension and access regimes that are challenging for existing classical simulation methods. For string dynamics – relevant to hadronization – a plaquette term is required to realize genuine $2+1$D behavior, as it endows the gauge field with dynamics and enables the propagation of
photon-like excitations. Here, we realize a U$(1)$
quantum link model of
quantum electrodynamics in two spatial dimensions with a tunable plaquette term on a Quantinuum System Model H2
quantum computer. We implement, to our knowledge, the largest
quantum simulation of string-breaking dynamics reported to date, on a $5 \times 4$ matter-site square
lattice using $51$
qubits. The simulation uses a shallow
circuit design with a two-
qubit gate depth of $28$ per Trotter step and up to $1540$ entangling gates. Starting from far-from-
equilibrium string configurations, we measure the probability for the string to propagate within the
lattice plane and find signatures of genuine $2+1$D dynamics only when the plaquette term is present. In a resonant regime, we observe the annihilation of string segments accompanied by the production of electron–positron pairs that screen them. We further find that, only with a nonzero plaquette term, matter creation extends across the
lattice plane rather than remaining confined to the initial string path. These results experimentally realize string breaking and demonstrate the emergence of dynamical gauge fields in two spatial dimensions, establishing a route to
photon-like propagation in programmable
quantum simulators of gauge theories.
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All authors Rohan Joshi, Yizhuo Tian, Kevin Hemery, N. S. Srivatsa, Jesse J. Osborne, Henrik Dreyer, Enrico Rinaldi, Jad C. Halimeh
Quantum Physics · Cross-listed from hep-lat
Observation of glueball excitations and string breaking in a $2+1$D $\mathbb{Z}_2$ lattice gauge theory on a trapped-ion quantum computer
Kaidi Xu, Umberto Borla, Kevin Hemery et al. (+4 more)
A major goal of the
quantum simulation of high-energy physics (HEP) is to probe real-time nonperturbative far-from-
equilibrium quantum processes underlying phenomena such as hadronization in
quantum chromodynamics (QCD). The
quantum simulation of the dynamics of confining strings and glueballs, both essential aspects of
quark confinement, in a controllable first-principles way is an important step towards this goal. Here, we realize a $\mathbb{Z}_2$
lattice gauge theory in $2+1$D with a tunable plaquette term on a Quantinuum System Model H2 trapped-ion
quantum computer. We implement a shallow depth-6 Trotter
circuit on a $6 \times 5$ matter-site square
lattice utilizing all $56$ available
qubits to execute over $1000$ entangling gates. We prepare far-from-
equilibrium initial string configurations that we quench across a range of parameters to observe rich dynamical phenomena, such as the formation of gauge-invariant closed-loop excitations reminiscent of glueballs in QCD and multi-order string breaking accompanied by spontaneous matter creation. We further demonstrate experimentally that the system displays genuine $2+1$D dynamics, as evidenced by string snapshots over time that cannot be trivially mapped to $1+1$D physics. Our results demonstrate digital
quantum simulations of nonequilibrium dynamics in a higher-dimensional
lattice gauge theory and provide an experimentally accessible setting for phenomena related to confinement physics.
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All authors Kaidi Xu, Umberto Borla, Kevin Hemery, Rohan Joshi, Henrik Dreyer, Enrico Rinaldi, Jad C. Halimeh
Quantum Physics
Comment on "Quantum theory based on real numbers cannot be experimentally falsified": On the compatibility of physical principles with information theory for fermions
Fatemeh Moradi Kalarde, Xiangling Xu and Marc-Olivier Renou
The manuscript [arXiv:2603.19208] proposes a physically motivated postulate to select the appropriate formulation of
quantum theory over real
Hilbert spaces, ruling out the theory considered in [Nature 600, 625-629 (2021)] in favour of an alternative theory which reproduces the predictions of standard complex
quantum information theory. Here, we argue that, to qualify as a general physical principle, this postulate should also hold in fermionic
quantum information theory, the standard framework describing information encoded in the presence or absence of identical
fermions. We show, however, that it fails in this framework. More broadly, our result highlights the importance of confronting proposed foundational principles with fermionic
quantum information theory, a point that also deserves further examination in recent related works such as [arXiv:2503.17307] and [arXiv:2504.02808].
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Quantum Physics
Borns Rule from Reversible Evolution and Irreversible Outcomes
Oskar Axelsson
We show that the quadratic measure associated with the Born rule need not be introduced as an independent probabilistic postulate, but follows, under the stated structural and regularity assumptions, from the compatibility between reversible linear
evolution and the formation of stable records. Physical processes alternate between reversible dynamics and episodes of record formation. Quantities that remain well defined throughout such processes must therefore admit a representation that remains consistent across both regimes. Prior to record formation, alternatives combine additively at the level of complex amplitudes, whereas successive record-forming distinctions admit a multiplicative representation through refinement. Requiring compatibility between these compositional structures constrains the admissible weight assignment to the family of weights proportional to the pth power of the amplitude magnitude. The finite-dimensional specialization of Lamperti's isometry
theorem then singles out p = 2 as the unique value compatible with continuous reversible linear mixing while preserving total weight. The Born rule therefore emerges as the unique weight assignment compatible with reversible linear composition, multiplicative record refinement, phase invariance, and continuous reversible invariance, without invoking probabilistic assumptions or a specific interpretation of
quantum measurement.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Superradiance enhances and suppresses fermionic pairing based on universal critical scaling in two order parameters systems
Yilun Xu
Distinguished from the system with one order parameter, systems described by two or more order parameters will manifest more complex and much richer phase diagram and critical phenomena. In systems of two order parameters, the phase transition of one order parameter may influence the strength of another. Focus on the Landau's theory of continuous phase transitions, we give a general physcial quantity to decide the changing rate of the two order parameters based on a general formula of free energy. Taking two-mode Rabi model and the 1D
Fermi Dicke model as the examples, we verify our analytical results and show how the superradiant phase transition manipulates the two-
spin pairing strength and the
superconductor band gap. Our work proposes the new paradigm to study the complex systems with two or more order parameters and provides novel avenue to enhancing or suppressing the desired physical effect by such interplay.
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Quantum Physics
Interaction-Mediated Non-Reciprocal Dynamics in Open Quantum Systems: From an Exactly Solvable Model to Generic Behavior
Pietro Borchia, Johannes Knolle and Andreas Nunnenkamp
Reservoir engineering has emerged as a powerful paradigm to realize non-reciprocal dynamics in open
quantum many-body systems. Here, we show that density-density interactions can transfer bath-induced non-reciprocity between different degrees of freedom. Specifically, we investigate a one-dimensional
lattice of
spin-$\frac{1}{2}$
fermions with all-to-all Hatsugai-Kohmoto interactions in the presence of an engineered reservoir. We establish the exact solvability of the Lindbladian dynamics and show that the interplay between non-reciprocity and interactions qualitatively reshapes the dynamics of excitations. Remarkably, interactions induce directional drift even in
spin sectors that are not directly coupled to the reservoir. By analyzing a driven-dissipative
Fermi-Hubbard chain, we show that the same mechanism persists for local interactions. The Hatsugai-Kohmoto model thus emerges as a minimal, exactly solvable platform for interaction-mediated non-reciprocal many-body dynamics.
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Quantum Physics
Transition Dipole Rotation Beyond the Condon Approximation in Single hBN Quantum Emitters
Serkan Paçal, Chanaprom Cholsuk, Mouli Hazra et al. (+4 more)
The design of polarization-encoded
quantum interfaces relies on the assumption that solid-state emitters possess static transition dipoles defined by the host
lattice symmetry. Here, we demonstrate that the transition dipole moment of single hexagonal boron nitride
quantum emitters is not a static property but rotates as a function of
photon energy. Through high-resolution energy-resolved spectroscopy, we reveal a continuous rotation of the emission dipole orientation reaching up to $40^{\circ}$ across the vibronic
manifold at room temperature, driven by coupling to the
phonon bath. This spectral rotation is effectively suppressed at cryogenic temperatures (6 K), where the acoustic
phonon population is negligible, identifying thermally activated
lattice vibrations as the primary driver of the reorientation. First-principles calculations on two representative defects spanning weak and strong electron-
phonon coupling regimes confirm that
phonon-displaced geometries produce a systematic deviation of the transition dipole orientation from the zero-
phonon line, with the magnitude scaling with vibronic coupling strength. The experimental observations and calculations demonstrate that single
quantum emitters can operate beyond the Condon approximation, with the transition dipole acquiring a dependence on the instantaneous
nuclear configuration. Our results identify a fundamental limit for polarization fidelity in solid-state
quantum networks and connect solid-state single-emitter physics to a class of effects previously accessible only in ensemble measurements in molecular and biological spectroscopy.
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All authors Serkan Paçal, Chanaprom Cholsuk, Mouli Hazra, Çağlar Samaner, Özgür Çak{\i}r, Tobias Vogl, Serkan Ateş
Quantum Physics · Cross-listed from hep-th
Groenewold-Moyal twists, integrable spin-chains and AdS/CFT
Riccardo Borsato and Miguel García Fernández
We take the first steps to address via integrability the spectral problem of AdS/CFT dual pairs deformed by Groenewold-Moyal twists. In particular, we start by considering a twisted
spin-chain that couples, through a Groenewold-Moyal twist deformation, two $\mathfrak{sl}(2)$-invariant
spin-chains. We interpret this deformed
spin-chain as a deformation of a subsector of the $AdS_3/CFT_2$
spin-chain, but the construction shares qualitative features also with the corresponding deformation of the $AdS_5/CFT_4$
spin-chain, for example. As in similar types of deformations, we show that there exists a certain basis in which the
spin-chain
Hamiltonian takes a Jordan-block form. At the same time, by working in the basis of eigenstates of the generators used to construct the Groenewold-Moyal twist, the
Hamiltonian appears to be diagonalisable and with a deformed spectrum. Employing the method of the Baxter equation, we write down the energy of the ground state and of excited states in a perturbation of the deformation parameter. We then consider the string-theory side of the duality, where the twist is realised as a deformation of AdS of the type of Maldacena-Russo-Hashimoto-Itzhaki. We construct a deformation of the usual BMN classical solution, and in the large-$J$ limit we match the leading $\mathcal O(J^{-3})$ term of the energy of the
spin-chain groundstate with a conserved charge of the string classical solution. Differently from the undeformed setup as well as similar kinds of deformations, we find that the general expression of this charge of the string sigma-model is non-local, and that it does not correspond to a standard isometry. Nevertheless, it can be computed from the monodromy
matrix and it is part of the tower of conserved charges provided by integrability.
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Quantum Physics
Physics-Informed Discrete-Event Simulation of Polarization-Encoded Quantum Networks
Abderrahim Amlou, Amar Abane, Cory M. Nunn et al. (+4 more)
We extend the SeQUeNCe discrete-event simulator with physics-based models for polarization-encoded photonic
quantum networks. Our framework integrates Jones-calculus optical components, including an SPDC Bell-state source, wave plates, and polarizing beam splitters, together with a multi-section fiber model capturing polarization mode dispersion, chromatic dispersion, and Raman noise from coexisting classical traffic. We validate the simulator by reproducing experimentally reported spectra, polarization correlations,
quantum state
tomography, and dispersion- and Raman-induced noise. The resulting platform enables hardware-parameterized prediction of
entanglement distribution performance under realistic deployment conditions.
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All authors Abderrahim Amlou, Amar Abane, Cory M. Nunn, M. V. Jabir, Van Sy Mai, Abdella Battou, Ahmed Lbath
Quantum Physics · Cross-listed from physics.atom-ph
Two-dimensional shelving spectroscopy of ultraviolet ground state transitions in dysprosium
Kevin S. H. Ng, Paul Uerlings, Fiona Hellstern et al. (+5 more)
The open inner-shell electronic structure of lanthanides with large
magnetic moments gives rise to a rich spectrum of transitions available for
laser cooling, trapping, and coherent control. Despite this, the large number of ultraviolet (UV) transitions below 400nm have so far been rarely utilized in dipolar atom experiments. Here, we investigate multiple UV ground state transitions in dysprosium. Several of these UV excited states have the largest decay strengths to the ultralong-lived, low-lying first excited state which are comparable to the most commonly used strongest transitions found in dipolar atoms. Using two-dimensional shelving spectroscopy which improves detection sensitivity and provides a straightforward way to determine the hyperfine-isotope structure and excited state total angular momentum $J$, we measure isotope shifts, hyperfine coefficients, and create King plots to determine their electronic nature. Such knowledge of these UV transitions which analogously exist in other
magnetic atoms is important for optically populating the first excited state and can be used towards creating an optical clock, high resolution imaging in
quantum gas microscopy, and probing lanthanide
nuclei with enhanced Schiff moments in search of physics beyond the standard model.
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All authors Kevin S. H. Ng, Paul Uerlings, Fiona Hellstern, Jens Hertkorn, Luis Weiß, Stephan Welte, Tilman Pfau, Ralf Klemt
Quantum Physics
Quantifying and detecting quantum-state texture
Xiangyu Chen and Qiang Lei
Quantum-state texture is a recently proposed
quantum resource that characterizes the inhomogeneity of a
quantum state's
matrix element distribution in the computational basis, enriching our understanding of
quantum state structure. To expand its quantification toolkit and establish detection methods, in this article, we investigate the resource theory of texture from both quantitative and detection perspectives. First, we construct a texture measure $\mathcal{T}^{\text{GR}}_{\alpha,z}(\rho)$ based on the $\alpha$-$z$ Rényi relative
entropy and present some of its inherent properties. Second, we analyze the mathematical relationships between several existing texture measures, revealing connections among different quantifiers. Finally, drawing on the witness concept from other resource theories, we systematically introduce texture witnesses into the texture theory and provide examples of texture witnesses with special properties.
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Quantum Physics
Analytical Fock-State Generation and SWAP using a Rabi-Driven Transmon
Natan Karaev, Eliya Blumenthal and Shay Hacohen-Gourgy
Deterministic Fock-state generation and inter-mode SWAP are foundational primitives for bosonic
quantum computing, yet most implementations rely on numerically optimized pulses, per-state calibration, strong dispersive coupling, or higher transmon levels, each adding control overhead that grows with system size. We present an analytical, calibration-light protocol operating entirely within the two-level g-e
manifold of a weakly dispersively coupled transmon. A Rabi drive on the
qubit, combined with a single sideband tone per mode, synthesizes an on-demand Jaynes-Cummings interaction whose entire family of pulse times follows the closed-form scaling $\tau_n=\tau_1/\sqrt{n}$. Once the single base time $\tau_1$ is set, every higher-$n$ operation is fixed analytically, with no per-state retuning, shelving, or numerical optimization. On a superconducting flute cavity with two high-Q modes, we deterministically prepare Fock states through $|n{=}5\rangle$, realize an inter-mode SWAP characterized on
vacuum, single-
photon, and coherent-state inputs, and generate and coherently swap the dual-rail Bell state $(|1,0\rangle+|0,1\rangle)/\sqrt{2}$, confirming that the operation preserves inter-mode coherence. Because the pulses are constant-amplitude and free of per-state optimization, the achievable fidelity is set directly by ancilla coherence and drive-ramp duration; a master-equation analysis isolates these hardware factors and shows that the analytical scaling itself imposes no obstacle to high-fidelity operation at high $n$. Requiring only one sideband line per mode and a single Rabi drive, the protocol is well suited to weakly coupled, high-$Q$ 3D architectures where calibration economy and analytical pulse design are at a premium.
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Quantum Physics · Cross-listed from cs.ET
Improving Feasibility in Quantum Approximate Optimization Algorithm for Vehicle Routing via Constraint-Aware Initialization and Hybrid XY-X Mixing
Yuan-Zheng Lei, Yaobang Gong, Xianfeng Terry Yang et al. (+1 more)
The
Quantum Approximate Optimization
Algorithm (QAOA) is a leading framework for
quantum combinatorial optimization. The Vehicle Routing Problem (VRP), a core problem in logistics and transportation, is a natural application target, but it poses a major feasibility challenge for standard QAOA because feasible solutions occupy only a tiny fraction of the search space, and the conventional Pauli-$X$ mixer can disrupt partial solution structures that satisfy key local constraints. To address this issue, we propose a constraint-aware QAOA framework with two complementary components. First, we design a lightweight initialization strategy that encodes a selected subset of simple yet informative local one-hot constraints into the initial state, thereby reducing the initial superposition space and increasing the probability mass on states with important local structure. Second, we introduce a hybrid XY-$X$ mixer that preserves the constraint structure imposed at initialization while retaining exploratory flexibility over the remaining unconstrained degrees of freedom during QAOA
evolution. We evaluate the proposed framework against standard QAOA under three progressively more realistic regimes: ideal statevector simulation, finite-shot sampling, and noisy finite-shot sampling. Across all regimes, the proposed method consistently achieves lower average energy and higher feasible-solution ratios than standard QAOA, indicating more effective guidance toward structurally valid, lower-cost VRP solutions. However, the performance gap narrows in the noisy regime. Because this setting adopts a hardware-inspired error model based on near-best-reported laboratory-level
qubit gate and readout fidelities, the observed attenuation suggests that the practical advantage of the more structured mixer is likely to grow as
quantum hardware improves and error rates decline.
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All authors Yuan-Zheng Lei, Yaobang Gong, Xianfeng Terry Yang, Nii Attoh-Okine
Quantum Physics
Quantum Gibbs sampling through the detectability lemma
Di Fang, Jianfeng Lu, Yu Tong et al. (+1 more)
Gibbs state preparation is an important subroutine in
quantum computing. In this work we use the detectability lemma to improve Gibbs state preparation. Specifically, we design new Gibbs state preparation methods that do not rely on simulating Lindbladian
evolution, thus avoiding the overhead from it. For local Lindbladians consisting of $M$ terms, this approach reduces the cost by a factor of $O(M)$. We also combine the detectability lemma operator and
quantum singular value transformation to implement ground state projection operators of frustration-free
Hamiltonians, resulting in a quadratic speedup in the spectral gap dependence. Applying this method to Lindbladians for the Gibbs state of local commuting
Hamiltonians, we achieve quadratically better dependence on the Lindbladian spectral gap.
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All authors Di Fang, Jianfeng Lu, Yu Tong, Chu Zhao
Quantum Physics · Cross-listed from physics.atom-ph
Defect-free arrays at the thousand-atom scale in a 4-K cryogenic environment
Desiree Lim, Hadriel Mamann, Grégoire Pichard et al. (+18 more)
We report on a cryogenic platform at 4 K incorporating high numerical aperture optics for the generation of large-scale tweezers arrays, and compatible with Rydberg-state manipulation. We achieve trapping lifetimes of around 5000 s, significantly extending the available experimental time for the preparation of large-scale arrays. By combining two trapping
lasers at different wavelengths and by minimizing other atom losses during the rearrangement and imaging processes, we demonstrate the preparation of defect-free arrays with up to 1024 atoms. Our cryogenic design opens exciting prospects for analog and digital
quantum computing.
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All authors Desiree Lim, Hadriel Mamann, Grégoire Pichard, Lilian Bourachot, Arvid Lindberg, Clotilde Hamot, Hugo Le Bars, Florian Fasola, Siddhy Tan, Gwennolé Cournez, Sylvain Dutartre, Thierry Cartry, Sylvain Lemettre, Richard Hostein, Julien Paris, Franck Ferreyrol, Andréa Collardey, Adrien Signoles, Thierry Lahaye, Corentin Monmeyran, Bruno Ximenez
Quantum Physics
On the Computational Power of Geometrically Local QAC circuits
Yangjing Dong, Fengning Ou and Penghui Yao
The computational power of $\mathsf{QAC}$, which are constant-depth
quantum circuit families consisting of arbitrary single-
qubit unitaries and n-
qubit generalized Toffoli gates, has gained tremendous focus recently. In this work, we initiate the study of the computational complexity of geometrically local $\mathsf{QAC}$
circuits, where the
qubits are arranged in a high-dimensional grid, and all the generalized Toffoli gates act on contiguous
qubits along one direction. We prove that $\mathsf{2D\text{-}QAC}$
circuits are universal: any $\mathsf{QAC}$
circuit can be exactly simulated by a two-dimensional geometrically local $\mathsf{QAC}$
circuit. Therefore, we focus on the computational power of $\mathsf{2D\text{-}QAC}$
circuits with grid width $w$. We prove that $\mathsf{PARITY}$ computation has a tight width–depth tradeoff of $w=n^{\Theta(1/d)}$ in the constant-depth regime if the
circuit allows arbitrary ancilla. Moreover, for
polynomial-size
circuits, a width lower bound of $n^{\omega(1/d)}$ for $\mathsf{PARITY}$ would imply $\mathsf{PARITY}\notin\mathsf{QAC}^0$. We also establish
Fourier tail and sensitivity bounds, together with tight width lower bounds and tight depth lower bounds in $\mathsf{1D\text{-}QAC}$
circuits for $\mathsf{MAJORITY}$ and $\mathsf{MOD}$ computation. Our main tool is local measurement variance, the maximum variance of normalized sums of observables on separated regions. We use a hybrid argument to show that each layer of a $\mathsf{2D\text{-}QAC}^0$
circuit increases this variance by at most a factor depending only on its width, yielding a variance bound for every $\mathsf{2D\text{-}QAC}^0$ state. The high local measurement variance of cat and Dicke states then gives the width lower bounds.
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Quantum Physics
Robust and High-Fidelity Controlled Two-Qubit Gates via Asymmetric Parallel Resonant Excitation
Licheng Lin, Jize Han, Peng Zhu et al. (+4 more)
Implementing high-fidelity controlled two-
qubit gates in dipole-dipole interacting systems, such as rare-earth-ion
crystals, in hindered by spectral inhomogeneity and weak coupling. Existing method often rely on detuned pulses, making them susceptible to frequency errors and AC Stark shifts. We propose a robust resonant scheme for arbitrary controlled two-
qubit gates that utilizes asymmetric excitation and pulse engineering to achieve decoupled, parallel
qubit control. Simulations on rare-earth-ion ensemble
qubits demonstrate gate fidelities exceeding 99% within a 170 kHz detuning range with off-resonant excitation below 0.2%. This approach offers a robust, scalable route for
quantum computing in spectrally crowded systems.
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All authors Licheng Lin, Jize Han, Peng Zhu, Ziyu Wang, Ying Yan, Jie Lu, Zhiguo Huang
Quantum Physics
Overlapped groupings for quantum energy estimation: Maximal variance reduction and deterministic algorithms for reducing variance
Jeremiah Rowland, Rahul Sarkar, Nicolas PD Sawaya et al. (+2 more)
Grouping-based measurement strategies are widely used to reduce measurement complexity in near-term
quantum algorithms. While these schemes have typically produced disjoint groups, recently this has been relaxed in what is known as overlapped grouping or coefficient splitting where operators may appear in more than one compatible group. In recent work, it has been numerically shown that this strategy can reduce the variance of energy estimates on small
benchmark problems, motivating both the application and further analysis of the method. Here we prove that overlapped grouping for energy estimation can lead to a maximal variance reduction that is linear in the number of
Hamiltonian terms. We introduce a new
algorithm which we call repacking to transform existing groups into overlapped groups, and we show this repacking procedure iteratively reduces variance under mild assumptions. We also perform numerical simulations with
Hamiltonians up to $44$
qubits and $575 \cdot 10^{3}$ terms, assessing overlapped grouping at scale on problems of practical importance. Our numerics show that the variance reduction relative to state-of-the-art (disjoint) grouping increases linearly with the problem size, suggesting that overlapped grouping methods can be a powerful strategy for
quantum energy estimation at the scale of Megaquop computers and beyond.
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All authors Jeremiah Rowland, Rahul Sarkar, Nicolas PD Sawaya, Norm M. Tubman, Ryan LaRose
Quantum Physics
Continuous-variable two-dimensional cluster states in the microwave domain
Fabio Lingua, Michele Cortinovis, J. C. Rivera Hernández et al. (+1 more)
We demonstrate the experimental realization of two-dimensional, continuous variable (CV) cluster states between 191 microwave frequency modes. This result is obtained by exposing
vacuum fluctuations to the input of a Josephson Parametric Amplifier, parametrically pumped by a sum of coherent tones around twice its resonant frequency. By carefully tuning pump frequencies, amplitudes, and phases we engineer the interference between mixing products and realize honeycomb and square
lattice CV cluster states with three and four pump tones respectively. We prove the presence of the cluster states with a suitable nullifier test, reaching up to $-1.2$ dB of squeezing of the cluster state's nullifiers. We study hidden
entanglement (HE) and show no hidden
entanglement up to $\sim -1$ dB of squeezing and negligible HE at optimal squeezing.
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All authors Fabio Lingua, Michele Cortinovis, J. C. Rivera Hernández, David B. Haviland
Quantum Physics
Quantum coherent transceivers toward Holevo-limited communications
Volkan Gurses, Suraj Samaga, Elianna Kondylis et al. (+1 more)
The Holevo limit bounds the channel capacity of a communication channel in which information is encoded in
quantum states in a
Hilbert space at the transmitter and decoded using
quantum measurements at the receiver. Saturating the Holevo limit requires
quantum-limited transceivers that either generate
quantum states of light or employ
quantum-limited measurements. Here, we demonstrate an integrated photonic-electronic
quantum-limited coherent receiver (QRX) achieving 14.0 dB shot noise clearance (SNC), 520 $\mu$W knee power, 2.57 GHz 3-dB bandwidth, 3.50 GHz shot-noise-limited bandwidth, and 90.2 dB common-mode rejection ratio ($\mathrm{CMRR}$). We scale this design to a 32-channel QRX array with median 26.6 dB $\mathrm{SNC}$, and automatic $\mathrm{CMRR}$ correction yielding a median 76.8 dB $\mathrm{CMRR}$ at minimum. Using the integrated QRX and fiber-optic transmitter, we measure $0.15\pm0.01$ dB of squeezing below the shot noise limit, limited by off-chip losses. We propose a squeezed light communication scheme that can surpass the Shannon limit, with a path toward the Holevo limit.
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All authors Volkan Gurses, Suraj Samaga, Elianna Kondylis, Ali Hajimiri
Quantum Physics
Postquantum steering in scenarios with multiple characterised parties
Ana Belen Sainz
The study of stronger-than-
quantum phenomena (i.e., postquantum) has enabled a deeper understanding of the scope of
quantum theory. Much is known about the case of correlations in Bell scenarios, where the device-independent framework allowed us to explore its possibilities independently of the formalism of
quantum theory. However, less is known about the phenomenon of Einstein-Podolsky-Rosen steering. Here, the `characterised parties' are assumed to describe their systems locally through the
quantum formalism, which inconveniences a theory-independent description. In addition, a
theorem by Gisin and Hughston, Josza and Wootters further hindered the discovery of the phenomenon. The study of postquantum steering, initiated about a decade ago, has been quite fruitful, including: the development of mathematical formalisms that frame the effect, resource theories that quantify it as a resource, and activation protocols that relate it to Bell correlations. However, all these results have a limitation in common: they apply to scenarios with only one
quantum party. Here we articulate the concept of postquantum steering for scenarios with multiple
quantum parties, bringing in the missing piece to the puzzle. We provide an
algorithm to certify postquantumness, which in some cases also certifies quantumness. We also define a hierarchy of semidefinite programs that bounds the set of
quantum assemblages from the outside. Moreover, we show that the study of postquantum steering is fundamentally relevant since it is not just a mere mathematical curiosity allowed by the no-signalling principle, but it may arise within compositional theories beyond
quantum theory. Our work further discovers a peculiarity of steering: its theory-independent description fundamentally prevents a direct connection with Bell nonlocality – e.g., nonclassical Bell correlations do not imply nonclassical steering.
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Quantum Physics
Complete coherent control of spin qubits in self-assembled InAs quantum dots under oblique magnetic fields
I. Samaras, K. Barr, C. Schneider et al. (+2 more)
We demonstrate complete coherent control of a single
spin qubit confined in a self-assembled InAs negatively charged
quantum dot subjected to an Oblique
magnetic field, and directly compare this regime with the conventional Voigt geometry. In the Oblique-field configuration, the groundstate
spin eigenstates are found to be unequal superpositions of the bare electron
spin, with their composition tunable via the orientation of the applied field. This tunable
spin mixing provides an additional degree of freedom to engineer the
spin basis and associated optical couplings in the charged
quantum dot system. Although this geometry has a distinct structure with important implications, it provides a regime in which we can fully and coherently control the tailored
spin qubit. We observe Rabi oscillations and Ramsey fringes, and demonstrate arbitrary single-
qubit rotations, enabling a direct comparison with the Voigt case. Our results establish that
spin-
qubit control does not necessarily require a pure Voigt geometry and can instead be achieved under Oblique
magnetic fields. This relaxes constraints on device and field
alignment and offers a versatile route to design and optimize
quantum information processing architectures in
semiconductor quantum dots.
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All authors I. Samaras, K. Barr, C. Schneider, S. Höfling, K.G. Lagoudakis
Quantum Physics
QNAS: A Neural Architecture Search Framework for Accurate and Efficient Quantum Neural Networks
Kooshan Maleki, Alberto Marchisio and Muhammad Shafique
Designing
quantum neural networks (QNNs) that are both accurate and deployable on NISQ hardware is challenging. Handcrafted ansatze must balance expressivity, trainability, and resource use, while limited
qubits often necessitate
circuit cutting. Existing
quantum architecture search methods primarily optimize accuracy while only heuristically controlling
quantum and mostly ignore the exponential overhead of
circuit cutting. We introduce QNAS, a neural architecture search framework that unifies hardware aware evaluation, multi objective optimization, and cutting overhead awareness for hybrid
quantum classical neural networks (HQNNs). QNAS trains a shared parameter SuperCircuit and uses NSGA-II to optimize three objectives jointly: (i) validation error, (ii) a runtime cost proxy measuring wall clock evaluation time, and (iii) the estimated number of subcircuits under a target
qubit budget. QNAS evaluates candidate HQNNs under a few epochs of training and discovers clear Pareto fronts that reveal tradeoffs between accuracy, efficiency, and cutting overhead. Across MNIST, Fashion-MNIST, and Iris
benchmarks, we observe that
embedding type and CNOT mode selection significantly impact both accuracy and efficiency, with angle-y
embedding and sparse entangling patterns outperforming other configurations on image
datasets, and amplitude
embedding excelling on tabular data (Iris). On MNIST, the best architecture achieves 97.16% test accuracy with a compact 8
qubit, 2 layer
circuit; on the more challenging Fashion-MNIST, 87.38% with a 5
qubit, 2 layer
circuit; and on Iris, 100% validation accuracy with a 4
qubit, 2 layer
circuit. QNAS surfaces these design insights automatically during search, guiding practitioners toward architectures that balance accuracy, resource efficiency, and practical deployability on current hardware.
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Quantum Physics · Cross-listed from physics.optics
A Simple and Robust Balanced Homodyne Detector for High-Repetition-Rate Pulsed Sources
Samuele Altilia, Edoardo Suerra, Pietro Puppi et al. (+3 more)
We design and experimentally characterize a balanced homodyne detector optimized for high-repetition-rate (100 MHz) pulsed optical sources. Unlike conventional transimpedance-amplifier architectures, which suffer from nonlinearities and dynamic instabilities with ultrashort pulses, our approach allows to directly amplify the photocurrent extracted at the common photodiode node without feedback loops. A theoretical model describing the detector response, noise, and pulse-to-pulse correlations is developed, providing quantitative predictions for the signal variance, signal-to-noise ratio (SNR), and inter-pulse correlations. Implemented with two matched InGaAs photodiodes illuminated by a 1030 nm mode-locked
laser at 100 MHz, the detector exhibits excellent linearity and shot-noise-limited scaling of the signal variance with optical power. Optimizing the temporal integration window yields a maximum SNR of about 14 dB, while correlation measurements confirm negligible inter-pulse correlations. These results demonstrate that the proposed architecture offers a robust and simple solution for high-speed pulsed homodyne detection, suitable for
quantum optics and continuous-variable
quantum information applications.
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All authors Samuele Altilia, Edoardo Suerra, Pietro Puppi, Sebastiano Corli, Enrico Prati, Simone Cialdi