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Thursday 30 October

Quantum Physics

Optimising physical parameters of a quantum network based on a loss-jitter trade-off

Marcus J. Clark and Siddarth K. Joshi

As quantum communication systems and networks are becoming a commercial reality, clarity on their future infrastructure is increasingly important. Based on the inevitable presence of some amount of loss, chromatic dispersion, and timing jitter, we present simulations to show that certain wavelengths and bandwidths have clear advantages.
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Quantum Physics · Cross-listed from cond-mat.quant-gas

Entropy transport in closed quantum many-body systems far from equilibrium

J. Marijan, H. Strobel, M. K. Oberthaler et al. (+1 more)

We investigate entropy transport for universal scaling phenomena in closed quantum many-body systems far from equilibrium. From spatially resolved experimental data of a spinor Bose gas, we demonstrate that entropy decreases on long-distance scales while it increases at short distances. A dynamical separation of scales leads to macrophysics with long-range order, which is insensitive to the highly entropic microphysical processes. Since the total von Neumann entropy is conserved on a fundamental level for the quantum system, our analysis reveals a reciprocal connection between the emergence of macroscopic structure and microscopic disorder. To illustrate the scope of this connection, we exemplify the universal phenomenon also in a relativistic quantum field theory calculation from first principles, which is relevant for particle physics and early-universe cosmology.
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All authors J. Marijan, H. Strobel, M. K. Oberthaler, J. Berges

Quantum Physics

Single-Photon-Level Atomic Frequency Comb Storage in Room Temperature Alkali Vapour

Zakary Schofield, Vanderli Laurindo Jr, Ori Ezrah Mor et al. (+1 more)

We have demonstrated the coherent storage and retrieval of single-photon-level light using the atomic frequency comb protocol in a room temperature rubidium vapour. Velocity-selective optical pumping is used to prepare the comb within the $F=2$ hyperfine ground state of rubidium, with the spacing between peaks coinciding with half the $F = 2 - F =3$ hyperfine splitting of the $5^2$P$_{3/2}$ excited state. Weak coherent states of average photon number $\mu_\mathrm{in} = 0.083(5)$ are stored with pre-programmed recall time of $7.5\,$ns with an efficiency of $\eta_{\textrm{AFC}} = 6.59(5)\,\%$, while two temporally distinct modes have been stored and recalled with $\eta_{\textrm{AFC}} = 2.6(1)\,\%$, allowing for time-bin qubit storage. Finally, the efficiency is observed to be independent of the input pulse polarisation, paving the way for polarisation qubit storage.
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All authors Zakary Schofield, Vanderli Laurindo Jr, Ori Ezrah Mor, Patrick M. Ledingham

Quantum Physics

A Non-Variational Quantum Approach to the Job Shop Scheduling Problem

Miguel Angel Lopez-Ruiz, Emily L. Tucker, Emma M. Arnold et al. (+3 more)

Quantum heuristics offer a potential advantage for combinatorial optimization but are constrained by near-term hardware limitations. We introduce Iterative-QAOA, a variant of QAOA designed to mitigate these constraints. The algorithm combines a non-variational, shallow-depth circuit approach using fixed-parameter schedules with an iterative warm-starting process. We benchmark the algorithm on Just-in-Time Job Shop Scheduling Problem (JIT-JSSP) instances on IonQ Forte Generation QPUs, representing some of the largest such problems ever executed on quantum hardware. We compare the performance of the algorithm against both the Variational Quantum Imaginary Time Evolution (VarQITE) algorithm and the non-variational Linear Ramp (LR) QAOA algorithm. We find that Iterative-QAOA robustly converges to find optimal solutions as well as high-quality, near-optimal solutions for all problem instances evaluated. We evaluate the algorithm on larger problem instances up to 97 qubits using tensor network simulations. The scaling behavior of the algorithm indicates potential for solving industrial-scale problems on fault-tolerant quantum computers.
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All authors Miguel Angel Lopez-Ruiz, Emily L. Tucker, Emma M. Arnold, Evgeny Epifanovsky, Ananth Kaushik, Martin Roetteler

Quantum Physics

The Particle in a Box in Koopman–von Neumann Mechanics: A Hilbert Space representation of Classical Mechanics

Abhijit Sen and Lev Kaplan

This paper revisits the textbook 'particle in a box', but from the point of view of Koopman-von Neumann (KvN) mechanics. KvN mechanics is a way to describe classical dynamics in a Hilbert space. That simple fact changes the usual expectation: hard walls do not force energy quantization here. We show, in a clear and physical way, why a KvN particle confined between two ideal walls still has a continuous range of energies. With the correct wall condition, one that captures ordinary elastic reflection rather than 'vanishing at the boundary,' the KvN description naturally produces spatial confinement without discrete energy levels. Beyond establishing this result, we also clean up common misunderstandings: for example, treating the KvN wavefunction like a quantum probability amplitude in position alone leads to the wrong boundary picture and, with it, the wrong conclusion about quantization.
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Quantum Physics

Programmable digital quantum simulation of 2D Fermi-Hubbard dynamics using 72 superconducting qubits

Faisal Alam, Jan Lukas Bosse, Ieva Čepaitė et al. (+41 more)

Simulating the time-dynamics of quantum many-body systems was the original use of quantum computers proposed by Feynman, motivated by the critical role of quantum interactions between electrons in the properties of materials and molecules. Accurately simulating such systems remains one of the most promising applications of general-purpose digital quantum computers, in which all the parameters of the model can be programmed and any desired physical quantity output. However, performing such simulations on today's quantum computers at a scale beyond the reach of classical methods requires advances in the efficiency of simulation algorithms and error mitigation techniques. Here we demonstrate programmable digital quantum simulation of the dynamics of the 2D Fermi-Hubbard model – one of the best-known simplified models of electrons in crystalline solids – at a scale beyond exact classical state-vector simulation. We implement simulations of this model on lattice sizes up to ${6\times 6}$ using 72 qubits on Google's Willow quantum processor, across a range of physical parameters, including different on-site electron-electron interaction strengths and magnetic flux values, and study phenomena including formation of magnetic polarons (charge carriers surrounded by local magnetic polarisation), dynamical symmetry-breaking in stripe-ordered states, attraction of charge carriers on an entangled background state known as a valence bond solid, and the approach to equilibrium through thermalisation. We validate our results against exact calculations in parameter regimes where these are feasible, and compare them to approximate classical simulations performed using tensor network and operator propagation methods. Our results demonstrate that meaningful programmable digital quantum simulation of many-body interacting electron models is now feasible on state-of-the-art quantum hardware.
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All authors Faisal Alam, Jan Lukas Bosse, Ieva Čepaitė, Adrian Chapman, Laura Clinton, Marcos Crichigno, Elizabeth Crosson, Toby Cubitt, Charles Derby, Oliver Dowinton, Paul K. Faehrmann, Steve Flammia, Brian Flynn, Filippo Maria Gambetta, Raúl García-Patrón, Max Hunter-Gordon, Glenn Jones, Abhishek Khedkar, Joel Klassen, Michael Kreshchuk, Edward Harry McMullan, Lana Mineh, Ashley Montanaro, Caterina Mora, John J. L. Morton, Dhrumil Patel, Pete Rolph, Raul A. Santos, James R. Seddon, Evan Sheridan, Wilfrid Somogyi, Marika Svensson, Niam Vaishnav, Sabrina Yue Wang, Gethin Wright, Dmitry Abanin, Mohammed Alghadeer, Jaehong Choi, Tyler Cochran, Gaurav Gyawali, Shashwat Kumar, Ricky Oliver, Eliott Rosenberg, Pedram Roushan

Quantum Physics

Cavity-assisted single-shot $\textit{T}$-center spin readout

Yu-En Wong and Songtao Chen

High-fidelity spin readout is a crucial component for quantum information processing with optically interfaced solid-state spins. Here, we propose and investigate two theoretical protocols for fast single-shot readout of cavity-coupled single $\textit{T}$ center electronic spins. For fluorescence-based readout, we selectively couple one of the $\textit{T}$ center spin-conserving transitions to a single-mode photonic cavity, exploiting the enhancement of the fluorescence emission and cyclicity. For reflection-based readout, we leverage the spin-dependent cavity reflection contrast to generate the qubit readout signal. We show that the cavity reflection approach enables high-fidelity spin readout even when the $\textit{T}$ center only has a modest cyclicity. With realistic system parameters, such as cavity quality factor $Q = 2\times10^5$ and $\textit{T}$-center optical linewidth $\Gamma/2\pi = 100$ MHz, we calculate a single-shot readout fidelity exceeding 99% within 10 $\mu$s for both spin readout protocols.
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Quantum Physics

Entanglement-assisted circuit knitting: Distributed quantum computing using limited entanglement resources

Shao-Hua Hu, Po-Sung Liu and Jun-Yi Wu

Distributed quantum computing (DQC) provides a promising route toward scalable quantum computation, where entanglement-assisted LOCC and circuit knitting represent two complementary approaches. The former deterministically realizes nonlocal operations but demands extensive entanglement resources, whereas the latter requires no entanglement yet suffers from exponential sampling overhead. Here, we propose a hybrid framework called entanglement-assisted circuit knitting that integrates these two paradigms by performing circuit knitting assisted with a limited amount of entanglement. We establish a general theoretical framework for entanglement-assisted circuit knitting. Optimal sampling overhead is achieved for Choi-stretchable unitaries with general entanglement resources, while for general unitaries we derive both lower and upper bounds for one-Bell-pair-assisted circuit knitting. We further extend the framework to the black-box setting, which can be treated as a class of quantum combs. This extension releases the need for explicit knowledge of the global unitary of a whole quantum circuit, enables a more flexible embedding structure, and broadens its applicability. Within this framework, we develop constructive protocols utilizing different resources, including entanglement, local operations, and classical communication. We derive the optimal mixed configuration among these protocols and provide an algorithm for its determination. Under dynamically probabilistic entanglement distribution, we reveal a trade-off between sampling overhead and entanglement cost in entanglement-assisted circuit knitting. This hybrid approach can thus be viewed as a form of hybrid classical-quantum computation, balancing sampling and entanglement efficiency, and enabling more resource-efficient implementations of distributed quantum computing.
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Quantum Physics · Cross-listed from hep-th

Perfect Particle Transmission through Duality Defects

Atsushi Ueda, Vic Vander Linden, Laurens Lootens et al. (+3 more)

We study wavepackets that propagate across (a) topological interfaces in quantum spin systems exhibiting non-invertible symmetries and (b) duality defects coupling dual theories. We demonstrate that the transmission is always perfect, and that a particle traversing the interface is converted into a nonlocal string-like excitation. We give a systematic way of constructing such a defect by identifying its Hilbert space with the virtual bond dimension of the matrix product operator representing defect lines. Our work both gives an operational meaning to topological interfaces, and provides a lattice analogue of recent results solving the monopole paradox in quantum field theory.
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All authors Atsushi Ueda, Vic Vander Linden, Laurens Lootens, Jutho Haegeman, Paul Fendley, Frank Verstraete

Quantum Physics

Witnessing genuine multipartite entanglement in phase space with controlled Gaussian unitaries

Lin Htoo Zaw, Jiajie Guo, Qiongyi He et al. (+2 more)

Many existing genuine multipartite entanglement (GME) witnesses for continuous-variable (CV) quantum systems typically rely on quadrature measurements, which are challenging to implement in platforms where the CV degrees of freedom can be indirectly accessed only through qubit readouts. In this work, we propose methods to implement GME witnesses through phase-space measurements in state-of-the-art experimental platforms, leveraging controlled Gaussian unitaries readily available in qubit-CV architectures. Based on two theoretical results showing that sufficient Wigner negativity can certify GME, we present five concrete implementation schemes using controlled parity, displacement, and beam-splitter operations. Our witnesses can detect paradigmatic GME states like the Dicke and multipartite $N00N$ states, which include the $W$ states as a special case, and GHZ-type entangled cat states. We analyze the performance of these witnesses under realistic noise conditions and finite measurement resolution, showing their robustness to experimental imperfections. Crucially, our implementations require exponentially fewer measurement settings than full tomography, with one scheme requiring only a single measurement on auxiliary modes. The methods are readily applicable to circuit/cavity quantum electrodynamics, circuit quantum acoustodynamics, as well as trapped ions and atomic systems, where such dichotomic phase-space measurements are already routinely performed as native readouts.
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All authors Lin Htoo Zaw, Jiajie Guo, Qiongyi He, Shuheng Liu, Matteo Fadel

Quantum Physics

Sufficient Wigner Negativity Implies Genuine Multipartite Entanglement

Lin Htoo Zaw, Jiajie Guo, Qiongyi He et al. (+2 more)

Wigner negativity and genuine multipartite entanglement (GME) are key nonclassical resources that enable computational advantages and broader quantum-information tasks. In this work, we prove two theorems for multimode continuous-variable systems that relate these nonclassical resources. Both theorems show that sufficient Wigner negativity-either a sufficiently-large Wigner negativity volume along a suitably-chosen two-dimensional slice, or a sufficiently-large nonclassicality depth of the center-of-mass mode of a system-certifies the presence of GME. Moreover, violations of the latter inequality provide lower bounds of the trace distance to the set of non-GME states. Our results also provide sufficient conditions for generating GME by interfering a state with the vacuum through a multiport interferometer, complementing long-known necessary conditions. Beyond these fundamental connections, our methods have practical advantages for systems with native phase-space measurements: they require only measuring the Wigner function over a finite region, or measuring a finite number of characteristic function points. Such measurements are frequently performed with readouts common in circuit and cavity quantum electrodynamic systems, trapped ions and atoms, and circuit quantum acoustodynamic systems. As such, our GME criteria are readily implementable in these platforms.
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All authors Lin Htoo Zaw, Jiajie Guo, Qiongyi He, Matteo Fadel, Shuheng Liu

Quantum Physics

Detection of non-Gaussian quantum correlations through measurement-after-interaction protocols

Jiajie Guo, Feng-Xiao Sun, Matteo Fadel et al. (+1 more)

Additional state evolutions performed before measurement, also called measurement-after-interactions (MAI) protocols, have shown a great potential for increasing the sensitivity of metrological scenarios. Here, we go beyond this result and show that MAI techniques can significantly enhance the detection capability of witnesses for quantum correlations. In particular, we show the possibility of detecting Einstein-Podolsky-Rosen steering and mode entanglement of non-Gaussian states from linear measurements only. Moreover, we show that such approach allows for a significantly higher noise robustness.
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All authors Jiajie Guo, Feng-Xiao Sun, Matteo Fadel, Qiongyi He

Quantum Physics

Approximate quantum error correction, eigenstate thermalization and the chaos bound

Shozab Qasim and Jason Pollack

Quantum error correction, thermalization, and quantum chaos are fundamental aspects of quantum many-body physics that have each developed largely independently, despite their deep conceptual overlap. In this work, we establish a precise link between all three in systems that satisfy the eigenstate thermalization hypothesis (ETH) and exhibit a well-defined hierarchy of time scales between dissipation and scrambling. Building on the ETH matrix ansatz and the structure of the out-of-time-order correlator (OTOC), we show that the chaos bound directly constrains the error of an approximate quantum error-correcting code. This establishes a quantitative relation between information scrambling, thermalization, and correctability. Furthermore, we derive bounds on dynamical fluctuations around the infinite-time average and on fluctuation-dissipation relations, expressed in terms of both the code error and the Lyapunov exponent. Our results reveal how the limits of quantum chaos constrain information preservation in thermalizing quantum systems.
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Quantum Physics

Quantum Enhanced Dark-Matter Search with Entangled Fock States in High-Quality Cavities

Benjamin Freiman, Xinyuan You, Andy C. Y. Li et al. (+5 more)

We present a quantum-enhanced protocol for detecting wave-like dark matter using an array of $N$ entangled superconducting cavities initialized in an $m$-photon Fock state. By distributing and recollecting the quantum state with an entanglement-distribution operation, the scan rate scales as $N^2(m+1)$ while thermal excitation is the dominant background, significantly outperforming classical single-cavity methods under matched conditions. We evaluate the robustness of our scheme against additional noise sources, including decoherence and beamsplitter infidelity, through theoretical analysis and numerical simulations. In practice, the key requirements, namely high-Q superconducting radio-frequency cavities that support long integration times, high-fidelity microwave beamsplitters, and universal cavity control, are already available on current experimental platforms, making the protocol experimentally feasible.
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All authors Benjamin Freiman, Xinyuan You, Andy C. Y. Li, Raphael Cervantes, Taeyoon Kim, Anna Grasselino, Roni Harnik, Yao Lu

Quantum Physics

Tunable frequency conversion and comb generation with a superconducting artificial atom

Fahad Aziz, Zhengqi Niu, Tzu-Yen Hsieh et al. (+12 more)

We investigate the power spectral density emitted by a superconducting artificial atom coupled to the end of a semi-infinite transmission line and driven by two continuous radio-frequency fields. In this setup, we observe the generation of multiple frequency peaks and the formation of frequency combs with equal detuning between those peaks. The frequency peaks originate from wave mixing of the drive fields, mediated by the artificial atom, highlighting the potential of this system as both a frequency converter and a frequency-comb generator. We demonstrate precise control and tunability in generating these frequency features, aligning well with theoretical predictions, across a relatively wide frequency range (tens of MHz, exceeding the linewidth of the artificial atom). The extensive and simple tunability of this frequency converter and comb generator, combined with its small physical footprint, makes it promising for quantum optics on chips and other applications in quantum technology.
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All authors Fahad Aziz, Zhengqi Niu, Tzu-Yen Hsieh, Kuan Ting Lin, Yu-Huan Huang, Yen-Hsiang Lin, Ching-Yeh Chen, Yu-Ting Cheng, Kai-Min Hsieh, Jeng-Chung Chen, Anton Frisk Kockum, Guin-Dar Lin, Zhi-Rong Lin, Ping-Yi Wen, Io-Chun Hoi

Quantum Physics · Cross-listed from physics.atom-ph

Wavefront Curvature and Transverse Atomic Motion in Time-Resolved Atom Interferometry: Impact and Mitigation

Noam Mouelle, Jeremiah Mitchell, Valerie Gibson et al. (+1 more)

Time-resolved atom interferometry, as employed in applications such as gravitational wave detection and searches for ultra-light dark matter, requires precise control over systematic effects. In this work, we investigate phase noise arising from shot-to-shot fluctuations in the atoms' transverse motion in the presence of the wavefront curvature of the interferometer beam, and analyse its dependence on the laser-beam geometry in long-baseline, large-momentum-transfer atom interferometers. We use a semi-classical framework to derive analytical expressions for the effective phase perturbation in position-averaged measurements and validate them using Monte Carlo simulations. Applied to 100-m and 1-km atom gradiometers representative of next-generation experiments, the model shows that configurations maximizing pulse efficiency also amplify curvature-induced phase noise, requiring micron-level control of the atom cloud's centre-of-mass position and sub-micron-per-second control of its centre-of-mass velocity to achieve sub-$10^{-5}$ rad phase stability. Alternative beam geometries can suppress this noise by up to two orders of magnitude, but at the cost of reduced pulse efficiency. To address this limitation, we propose a mitigation strategy based on position-resolved phase-shift readout, which empirically learns and corrects the wavefront-induced bias from measurable quantities such as the phase-shift gradient and final cloud position. This approach restores high-sensitivity operation in the maximum-pulse-efficiency configuration without detailed beam characterisation, providing a practical route towards next-generation, time-resolved atom interferometers operating at the $10^{-5}$ rad noise level.
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All authors Noam Mouelle, Jeremiah Mitchell, Valerie Gibson, Ulrich Schneider

Quantum Physics

Digitized Counterdiabatic Quantum Sampling

Narendra N. Hegade, Nachiket L. Kortikar, Balaganchi A. Bhargava et al. (+9 more)

We propose digitized counterdiabatic quantum sampling (DCQS), a hybrid quantum-classical algorithm for efficient sampling from energy-based models, such as low-temperature Boltzmann distributions. The method utilizes counterdiabatic protocols, which suppress non-adiabatic transitions, with an iterative bias-field procedure that progressively steers the sampling toward low-energy regions. We observe that the samples obtained at each iteration correspond to approximate Boltzmann distributions at effective temperatures. By aggregating these samples and applying classical reweighting, the method reconstructs the Boltzmann distribution at a desired temperature. We define a scalable performance metric, based on the Kullback-Leibler divergence and the total variation distance, to quantify convergence toward the exact Boltzmann distribution. DCQS is validated on one-dimensional Ising models with random couplings up to 124 qubits, where exact results are available through transfer-matrix methods. We then apply it to a higher-order spin-glass Hamiltonian with 156 qubits executed on IBM quantum processors. We show that classical sampling algorithms, including Metropolis-Hastings and the state-of-the-art low-temperature technique parallel tempering, require up to three orders of magnitude more samples to match the quality of DCQS, corresponding to an approximately 2x runtime advantage. Boltzmann sampling underlies applications ranging from statistical physics to machine learning, yet classical algorithms exhibit exponentially slow convergence at low temperatures. Our results thus demonstrate a robust route toward scalable and efficient Boltzmann sampling on current quantum processors.
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All authors Narendra N. Hegade, Nachiket L. Kortikar, Balaganchi A. Bhargava, Juan F. R. Hernández, Alejandro Gomez Cadavid, Pranav Chandarana, Sebastián V. Romero, Shubham Kumar, Anton Simen, Anne-Maria Visuri, Enrique Solano, Paolo A. Erdman

Quantum Physics

Graph theoretic quantum contextuality and unextendible Product Bases

Gurvir Singh and Arvind

Unextendible product bases(UPBs) are central to the study of local distinguishability of orthogonal product states. While their connection to quantum nonlocality via Bell inequalities is well established, their link to quantum contextuality remains largely unexplored. We establish a graph theoretic connection between contextuality and UPBs. First, an equivalence between Klyachko-Can-Binicioğlu-Shumovsky (KCBS) vectors and the Pyramid UPB is shown and then by constructing a one parameter family of UPB vectors, a quantitative connection between `contextuality strength' and bound entanglement of states associated with the corresponding UPB is demonstrated. This equivalence is extended to generalized KCBS vectors and the GenPyramid UPB. A new class of minimal UPBs in $\mathbb{C}^3 \otimes \mathbb{C}^n$ is constructed using Lovász-optimal orthogonal representations (LOORs) of cycle graphs and their complements which we term the GenContextual UPB. Any minimal UPB in this dimension is shown to be graph-equivalent to the GenContextual UPB. We briefly discuss the distinguishability properties of GenContextual UPB. In the reverse direction, we observe that the constituent vectors of the QuadRes UPB are LOORs of Paley graphs. The structural properties of these graphs make them suitable candidates for constructing noncontextuality inequalities, thereby establishing a bidirectional connection between quantum contextuality and UPBs.
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Quantum Physics

Witnessing Short- and Long-Range Nonstabilizerness via the Information Lattice

Yuliya Bilinskaya, Miguel F. Martínez, Soumi Ghosh et al. (+3 more)

We study nonstabilizerness on the information lattice and demonstrate that noninteger local information directly indicates nonstabilizerness. For states with a clear separation of short- and large-scale information, a noninteger total information at large scales $\Gamma$ serves as a witness of long-range nonstabilizerness. We propose a folding procedure to separate the global and edge-to-edge contributions to $\Gamma$. As an example we show that the ferromagnetic ground state of the spin-1/2 three-state Potts model has long-range nonstabilizerness originating from global correlations, while the paramagnetic ground state has at most short-range nonstabilizerness.
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All authors Yuliya Bilinskaya, Miguel F. Martínez, Soumi Ghosh, Thomas Klein Kvorning, Claudia Artiaco, Jens H. Bardarson

Quantum Physics

FlowQ-Net: A Generative Framework for Automated Quantum Circuit Design

Jun Dai, Michael Rizvi-Martel and Guillaume Rabusseau

Designing efficient quantum circuits is a central bottleneck to exploring the potential of quantum computing, particularly for noisy intermediate-scale quantum (NISQ) devices, where circuit efficiency and resilience to errors are paramount. The search space of gate sequences grows combinatorially, and handcrafted templates often waste scarce qubit and depth budgets. We introduce FlowQ-Net (Flow-based Quantum design Network), a generative framework for automated quantum circuit synthesis based on Generative Flow Networks (GFlowNets). This framework learns a stochastic policy to construct circuits sequentially, sampling them in proportion to a flexible, user-defined reward function that can encode multiple design objectives such as performance, depth, and gate count. This approach uniquely enables the generation of a diverse ensemble of high-quality circuits, moving beyond single-solution optimization. We demonstrate the efficacy of FlowQ-Net through an extensive set of simulations. We apply our method to Variational Quantum Algorithm (VQA) ansatz design for molecular ground state estimation, Max-Cut, and image classification, key challenges in near-term quantum computing. Circuits designed by FlowQ-Net achieve significant improvements, yielding circuits that are 10$\times$-30$\times$ more compact in terms of parameters, gates, and depth compared to commonly used unitary baselines, without compromising accuracy. This trend holds even when subjected to error profiles from real-world quantum devices. Our results underline the potential of generative models as a general-purpose methodology for automated quantum circuit design, offering a promising path towards more efficient quantum algorithms and accelerating scientific discovery in the quantum domain.
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Quantum Physics

Accelerated calculation of impurity Green's functions exploiting the extreme Mpemba effect

David J. Strachan, Archak Purkayastha and Stephen R. Clark

Simulating the dynamics of quantum impurity models remains a fundamental challenge due to the complex memory effects that arise from system-environment interactions. Of particular interest are two-time correlation functions of an impurity, which are central to the characterization of these many-body systems, and are a cornerstone of the description of correlated materials in dynamical mean field theory (DMFT). In this work, we extend our previous work on the extrapolation of single-time observables to demonstrate an efficient scheme for computing two-time impurity correlation functions, by combining the non-Markovian quantum Mpemba effect (NMQMpE) with a dynamical map-based framework for open quantum systems. Our method is benchmarked against exact and known accurate results in prototypical impurity models for both fermionic and bosonic environments, demonstrating significant computational savings compared to state-of-the-art methods.
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Quantum Physics

Discrete time quantum walk of locally interacting walkers

Vikash Mittal and Tomasz Sowiński

In this work, we introduce a general form of a two-parameter family of local interactions between quantum walkers conditioned on the internal state of their coins. By choosing their particular case, we systematically study the impact of these interactions on the dynamics of two initially localized and noncorrelated walkers. Our general interaction framework, which reduces to several previously studied models as special cases, provides a versatile platform for engineering quantum correlations with applications in quantum simulation, state preparation, and sensing protocols. It also opens up the possibility of analyzing many-body interactions for larger numbers of walkers.
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Quantum Physics

Diamond quantum sensing at record high pressure up to 240 GPa

Qingtao Hao, Ze-Xu He, Na Zuo et al. (+10 more)

Quantum sensing utilizing nitrogen-vacancy (NV) centers in diamond has emerged as a transformative technology for probing magnetic phase transition1-4, evidencing Meissner effect of superconductors1,5-9, and visualizing stress distribution3,9 under extreme conditions. Recent development in NV configurations and hydrostatic environments have raised the operational pressures of NV centers to 140 GPa2,6,10,11, but substantial challenges remain in extending sensing capabilities into multi-megabar range, critical for research in hydrogen-rich superconductors like La-Sc-H ($T_{\text{c}}$ of 271-298 K at 195-266 GPa)12 and evolution of minerals near Earth's core13. Here we report the fabrication of shallow NV centers through ion implantation followed by high-pressure and high-temperature (HPHT) annealing, leading to increased density, improved coherence, and mitigated internal stresses, a pre-requisite for reducing their degradation under compression. This NV magnetometry enable breakthrough of pressure capabilities exceeding 240 GPa, constrained by structural integrity of the 50 um diamond anvils, suggesting that the untapped pressure limit may enable further advancements with smaller cutlets or more robust diamonds. We present compelling evidence of the Meissner effect and trapped flux at record-high pressure of 180 GPa for superconducting transition in elemental titanium (Ti) as benchmark, establishing a solid foundation for high-pressure magnetometry in exploring complex quantum phenomena at previously unreachable pressures.
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All authors Qingtao Hao, Ze-Xu He, Na Zuo, Yang Chen, Xiangzhuo Xing, Xiaoran Zhang, Xinyu Zhuang, Zhixiang Shi, Xin Chen, Jian-Gang Guo, Gang-Qin Liu, Xiaobing Liu, Yanming Ma

Quantum Physics

Higher-order discrete time crystals and enhanced sensing in a quantum kicked top

Subhashis Das, Vishal Khan and Atanu Rajak

We characterize various dynamical phases of the simplest version of the quantum kicked top model, a paradigmatic system for studying quantum chaos, which exhibits both regular and chaotic behavior depending on the kick strength. In a previous study, the existence of higher-order discrete time crystals (DTCs) was observed in an infinite-range interacting $p$-spin model, where it was proposed that the order of the DTC satisfies the relation $q\le p$. Within this framework, the $p=2$ model is expected to host only a $2$-DTC phase. However, interestingly, we demonstrate here the existence of a robust $4$-DTC phase in the quantum kicked top, which effectively corresponds to a $p=2$ model with infinite-range interactions. We also show that the system hosts robust $2$-DTC and dynamical freezing (DF) phases around alternating rotationally symmetric points. We explain the emergence of higher-order DTC phases through the classical phase portraits of the system, connected with spin coherent states (SCSs), by identifying special islands that arise within a specific parametric regime. Unlike the $2$-DTC phase, the $4$-DTC phase appears only for certain initial states, as demonstrated through exact calculations. The robustness of the $4$-DTC phase is further investigated through the dynamics of the linear entropy as a function of the angular momentum. We also find an emergent conservation law for both the $2$-DTC and DF phases, while no dynamical conservation arises periodically for the $4$-DTC phase. By investigating the quantum Fisher information, we also demonstrate enhanced metrological sensitivity at the boundaries between different dynamical phases for the estimation of system parameters.
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Quantum Physics · Cross-listed from physics.chem-ph

Ring-polymer instanton theory for tunneling between asymmetric wells

Marit R. Fiechter, Gabriel Laude and Jeremy O. Richardson

Instanton theory has arisen as a practical tool for calculating tunneling splittings in molecular systems. Unfortunately, the original formulation of instanton theory fundamentally breaks down when trying to calculate the level splitting in asymmetric double wells, as there is no imaginary-time periodic orbit connecting the two non-degenerate minima. We have therefore developed a new formulation of instanton theory based on a projected flux correlation function that is applicable to these asymmetric systems. Comparison with exact quantum-mechanical results in one- and two-dimensional models demonstrates that it has a reasonably high accuracy, similar to that reported for instanton theory in the symmetric case. The theory is then applied to study tunneling between non-degenerate minima in the biomolecule $\alpha$-fenchol, for which we find good agreement with experiment. Finally, we use the connection to instanton rate theory, which is also derived from flux correlation functions, to discuss the often misunderstood relationship between tunneling splittings and reaction rate constants.
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Quantum Physics

Tackling the Challenges of Adding Pulse-level Support to a Heterogeneous HPCQC Software Stack: MQSS Pulse

Jorge Echavarria, Muhammad Nufail Farooqi, Amit Devra et al. (+22 more)

We study the problem of adding native pulse-level control to heterogeneous High Performance Computing-Quantum Computing (HPCQC) software stacks, using the Munich Quantum Software Stack (MQSS) as a case study. The goal is to expand the capabilities of HPCQC environments by offering the ability for low-level access and control, currently typically not foreseen for such hybrid systems. For this, we need to establish new interfaces that integrate such pulse-level control into the lower layers of the software stack, including the need for proper representation. Pulse-level quantum programs can be fully described with only three low-level abstractions: ports (input/output channels), frames (reference signals), and waveforms (pulse envelopes). We identify four key challenges to represent those pulse abstractions at: the user-interface level, at the compiler level (including the Intermediate Representation (IR)), and at the backend-interface level (including the appropriate exchange format). For each challenge, we propose concrete solutions in the context of MQSS. These include introducing a compiled (C/C++) pulse Application Programming Interface (API) to overcome Python runtime overhead, extending its LLVM support to include pulse-related instructions, using its C-based backend interface to query relevant hardware constraints, and designing a portable exchange format for pulse sequences. Our integrated approach provides an end-to-end path for pulse-aware compilation and runtime execution in HPCQC environments. This work lays out the architectural blueprint for extending HPCQC integration to support pulse-level quantum operations without disrupting state-of-the-art classical workflows.
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All authors Jorge Echavarria, Muhammad Nufail Farooqi, Amit Devra, Santana Lujan, Léo Van Damme, Hossam Ahmed, Martín Letras, Ercüment Kaya, Adrian Vetter, Max Werninghaus, Martin Knudsen, Felix Rohde, Albert Frisch, Eric Mansfield, Rakhim Davletkaliyev, Vladimir Kukushkin, Noora Färkkilä, Janne Mäntylä, Nikolas Pomplun, Andreas Spörl, Lukas Burgholzer, Yannick Stade, Robert Wille, Laura B. Schulz, Martin Schulz

Quantum Physics

Limits of Absoluteness of Observed Events in Timelike Scenarios: A No-Go Theorem

Sumit Mukherjee and Jonte R. Hance

Wigner's Friend-type paradoxes challenge the assumption that events are absolute – that when we measure a system, we obtain a single result, which is not relative to anything or anyone else. These paradoxes highlight the tension between quantum theory and our intuitions about reality being observer-independent. Building on a recent result that developed these paradoxes into a no-go theorem, namely the Local Friendliness Theorem, we introduce the Causal Friendliness Paradox, a time-ordered analogue of it. In this framework, we replace the usual locality assumption with Axiological Time Symmetry (ATS), and show that, when combined with the assumptions of Absoluteness of Observed Events (AOE), No Retrocausality (NRC), and Screening via Pseudo Events (SPE), we obtain a causal inequality. We then show that quantum mechanics violates this inequality and is therefore incompatible with at least one of these assumptions. To probe which assumption might be incompatible, we then examine whether AOE in its entirety is essential for this no-go result. We propose a weaker, operational form of AOE that still leads to inequalities that quantum mechanics violates. This result shows that even under relaxed assumptions, quantum theory resists reconciliation with classical notions of absolute events, reinforcing the foundational significance of Wigner's Friend-type paradoxes in timelike scenarios.
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Quantum Physics

Achieving Utility-Scale Applications through Full Stack Co-Design of Fault Tolerant Quantum Computers

Katerina Gratsea and Matthew Otten

Quantum computing promises revolutionary advances in modeling materials and molecules. However, the up-to-date runtime estimates for utility-scale applications on certain quantum hardware systems are in the order of years rendering quantum computations impractical. Our work incorporates state-of-the-art innovations in all key aspects of the fault-tolerant quantum computing (FTQC) stack to show how quantum computers could realistically and practically tackle CO$_2$ utilization for green energy production. We bring down the quantum computation runtime from 22 years to just 1 day, achieving a significant 7.9e03 reduction from previous state-of-the-art work. This reduction renders the quantum computation feasible, challenges state-of-the-art classical methods and results to a predicted run-time quantum advantage. We provide a rigorous analysis of how different innovations across the stack combine to provide such reductions. Our work provides strong evidence that all layers of FTQC are crucial in the quest for quantum advantage. Our analysis can be applied to related problems on FTQC and for any type of quantum architecture. Our methodology connects quantum algorithms to applications of positive real-world impact and leads to compelling evidence of achievable quantum advantage.
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Quantum Physics

An introduction to Markovian open quantum systems

Shovan Dutta

This is a concise, pedagogical introduction to the dynamic field of open quantum systems governed by Markovian master equations. We focus on the mathematical and physical origins of the widely used Lindblad equation, its unraveling in terms of stochastic pure-state trajectories and the corresponding continuous measurement protocols, the structure of steady states with emphasis on the role of symmetry and conservation laws, and a sampling of the novel physical phenomena that arise from nonunitary dynamics (dissipation and measurements). This is far from a comprehensive summary of the field. Rather, the objective is to provide a conceptual foundation and physically illuminating examples that are useful to graduate students and researchers entering this subject. There are exercise problems and references for further reading throughout the notes.
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Quantum Physics

Quantum Variational Methods for Supersymmetric Quantum Mechanics

John Kerfoot, Emanuele Mendicelli and David Schaich

We employ quantum variational methods to investigate a single-site interacting fermion-boson system – an example of a minimal supersymmetric model that can exhibit spontaneous supersymmetry breaking. Our study addresses the challenges inherent in calculating mixed fermion-boson systems and explores the potential of quantum computing to advance their analysis. By using adaptive variational techniques, we identify optimal ansätze that scale efficiently, allowing for reliable identification of spontaneous supersymmetry breaking. This work lays a foundation for future quantum computing investigations of more complex and physically rich fermion-boson quantum field theories in higher dimensions.
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