Pub Date : 2026-03-24DOI: 10.22331/q-2026-03-24-2046
Andrea Di Biagio, Richard Howl, Časlav Brukner, Carlo Rovelli, Marios Christodoulou
Locality is a central notion in modern physics, but different disciplines understand it in different ways. Quantum field theory focuses on relativistic locality, based on spacetime regions, while quantum information theory focuses circuit locality, based on the notion of subsystems. Here, we investigate how spacetime and subsystem locality are related in the context of systems getting entangled while interacting via a scalar field. We show how, when the systems are put in a quantum-controlled superposition of localised states, relativistic locality (in the form of microcausality) gives rise to a specific kind of circuit. The relation between these forms of locality is relevant for understanding whether it is possible to formulate quantum field theory in quantum circuit language, and has bearing on the recent discussions on low-energy tests of quantum gravity.
{"title":"Circuit locality from relativistic locality in scalar field mediated entanglement","authors":"Andrea Di Biagio, Richard Howl, Časlav Brukner, Carlo Rovelli, Marios Christodoulou","doi":"10.22331/q-2026-03-24-2046","DOIUrl":"https://doi.org/10.22331/q-2026-03-24-2046","url":null,"abstract":"Locality is a central notion in modern physics, but different disciplines understand it in different ways. Quantum field theory focuses on relativistic locality, based on spacetime regions, while quantum information theory focuses circuit locality, based on the notion of subsystems. Here, we investigate how spacetime and subsystem locality are related in the context of systems getting entangled while interacting via a scalar field. We show how, when the systems are put in a quantum-controlled superposition of localised states, relativistic locality (in the form of microcausality) gives rise to a specific kind of circuit. The relation between these forms of locality is relevant for understanding whether it is possible to formulate quantum field theory in quantum circuit language, and has bearing on the recent discussions on low-energy tests of quantum gravity.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"270 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147506876","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-03-24DOI: 10.22331/q-2026-03-24-2045
Gina Warttmann, Florian Meinert, Hans Peter Büchler, Sebastian Weber
We present a method to suppress crosstalk from implementing controlled-Z gates via local addressing in neutral atom quantum computers. In these systems, a fraction of the laser light that is applied locally to implement gates typically leaks to other atoms. We analyze the resulting crosstalk in a setup of two gate atoms and one neighboring third atom. We then perturbatively derive a spin-echo-inspired gate protocol that suppresses the leading order of the amplitude error, which dominates the crosstalk. Numerical simulations demonstrate that our gate protocol improves the fidelity by two orders of magnitude across a broad range of experimentally relevant parameters. To further reduce the infidelity, we develop a circuit to cancel remaining phase errors. Our results pave the way for using local addressing for high-fidelity quantum gates on Rydberg-based quantum computers.
{"title":"Suppressing crosstalk for Rydberg quantum gates","authors":"Gina Warttmann, Florian Meinert, Hans Peter Büchler, Sebastian Weber","doi":"10.22331/q-2026-03-24-2045","DOIUrl":"https://doi.org/10.22331/q-2026-03-24-2045","url":null,"abstract":"We present a method to suppress crosstalk from implementing controlled-Z gates via local addressing in neutral atom quantum computers. In these systems, a fraction of the laser light that is applied locally to implement gates typically leaks to other atoms. We analyze the resulting crosstalk in a setup of two gate atoms and one neighboring third atom. We then perturbatively derive a spin-echo-inspired gate protocol that suppresses the leading order of the amplitude error, which dominates the crosstalk. Numerical simulations demonstrate that our gate protocol improves the fidelity by two orders of magnitude across a broad range of experimentally relevant parameters. To further reduce the infidelity, we develop a circuit to cancel remaining phase errors. Our results pave the way for using local addressing for high-fidelity quantum gates on Rydberg-based quantum computers.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"14 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147506875","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Security analyses in quantum key distribution (QKD) and other adversarial quantum tasks often assume perfect device models. However, real-world implementations often deviate from these models. Thus, it is important to develop security proofs that account for such deviations from ideality. In this work, we extend the idea of squashing maps to develop a general framework for analysing imperfect threshold detectors, treating uncharacterised device parameters such as dark counts and detection efficiencies as adversarially controlled within some ranges. This approach enables a rigorous worst-case analysis with exactly characterised devices, ensuring security proofs remain valid under realistic conditions. Our results strengthen the connection between theoretical security and practical implementations by introducing a flexible framework for integrating detector imperfections into adversarial quantum protocols.
{"title":"Imperfect detectors for adversarial tasks with applications to quantum key distribution","authors":"Shlok Nahar, Devashish Tupkary, Norbert Lütkenhaus","doi":"10.22331/q-2026-03-24-2044","DOIUrl":"https://doi.org/10.22331/q-2026-03-24-2044","url":null,"abstract":"Security analyses in quantum key distribution (QKD) and other adversarial quantum tasks often assume perfect device models. However, real-world implementations often deviate from these models. Thus, it is important to develop security proofs that account for such deviations from ideality. In this work, we extend the idea of squashing maps to develop a general framework for analysing imperfect threshold detectors, treating uncharacterised device parameters such as dark counts and detection efficiencies as adversarially controlled within some ranges. This approach enables a rigorous worst-case analysis with exactly characterised devices, ensuring security proofs remain valid under realistic conditions. Our results strengthen the connection between theoretical security and practical implementations by introducing a flexible framework for integrating detector imperfections into adversarial quantum protocols.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"14 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147506877","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-03-24DOI: 10.22331/q-2026-03-24-2042
Trinidad B. Lantaño, Luciano Petruzziello, Susana F. Huelga, Martin B. Plenio
Current proposals to probe the quantum nature of gravity in the low-energy regime predominantly focus on the Newtonian interaction term. In this work, we present a theoretical exploration of gravitationally mediated entanglement arising from a genuinely general relativistic effect: frame dragging. This interaction gives rise to an effective dipolar coupling between the angular momenta of two rotating, spherically symmetric masses, allowing entanglement generation between angular momentum degrees of freedom. We represent the quantum states by angular momentum eigenstates and show that, while the maximal entangling rate is achieved for highly delocalized initial states, non-negligible quantum correlations can still emerge even when the initial states are not prepared in superposition. We then analyze the robustness of the resulting entanglement in the presence of common noise sources, explicitly acknowledging the challenges associated with a potential implementation. We also note that, for spherically symmetric masses, angular momentum degrees of freedom are intrinsically insensitive to Casimir and Coulomb interactions, thereby mitigating key decoherence channels present in existing proposals. Finally, we discuss possible state preparation and detection strategies while framing our results within the broader landscape of gravitationally mediated entanglement schemes, emphasizing the role of this framework as a conceptual avenue for exploring genuinely relativistic quantum gravitational effects.
{"title":"Angular Momentum Entanglement Mediated By General Relativistic Frame Dragging","authors":"Trinidad B. Lantaño, Luciano Petruzziello, Susana F. Huelga, Martin B. Plenio","doi":"10.22331/q-2026-03-24-2042","DOIUrl":"https://doi.org/10.22331/q-2026-03-24-2042","url":null,"abstract":"Current proposals to probe the quantum nature of gravity in the low-energy regime predominantly focus on the Newtonian interaction term. In this work, we present a theoretical exploration of gravitationally mediated entanglement arising from a genuinely general relativistic effect: frame dragging. This interaction gives rise to an effective dipolar coupling between the angular momenta of two rotating, spherically symmetric masses, allowing entanglement generation between angular momentum degrees of freedom. We represent the quantum states by angular momentum eigenstates and show that, while the maximal entangling rate is achieved for highly delocalized initial states, non-negligible quantum correlations can still emerge even when the initial states are not prepared in superposition. We then analyze the robustness of the resulting entanglement in the presence of common noise sources, explicitly acknowledging the challenges associated with a potential implementation. We also note that, for spherically symmetric masses, angular momentum degrees of freedom are intrinsically insensitive to Casimir and Coulomb interactions, thereby mitigating key decoherence channels present in existing proposals. Finally, we discuss possible state preparation and detection strategies while framing our results within the broader landscape of gravitationally mediated entanglement schemes, emphasizing the role of this framework as a conceptual avenue for exploring genuinely relativistic quantum gravitational effects.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"20 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147506881","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-03-24DOI: 10.22331/q-2026-03-24-2043
Tathagata Karmakar, Andrew N. Jordan
The Chantasri-Dressel-Jordan (CDJ) stochastic path integral formalism (Chantasri et al. 2013 and 2015) characterizes the statistics of the readouts and the most likely conditional evolution of continuously monitored quantum systems involving a few qubits or quantum harmonic oscillators in Gaussian states. In our work, we generalize the CDJ formalism to arbitrary continuously monitored systems by introducing a costate operator. We then prescribe a generalized Pontryagin's maximum principle for quantum systems undergoing arbitrary evolution and find conditions on optimal control protocols. We show that the CDJ formalism's most likely path can be cast as a quantum Pontryagin's maximum principle, where the cost function is the readout probabilities along a quantum trajectory. This insight allows us to derive general optimal control equations for arbitrary control parameters. We apply our results to a monitored oscillator in the presence of a parametric quadratic potential and variable quadrature measurements. We find the optimal potential strength and quadrature angle for fixed-end point problems. The optimal parametric potential is analytically shown to have a "bang-bang" form. We apply our protocol to three quantum oscillator examples relevant to Bosonic quantum computing. The first example considers a binomial codeword preparation from an error word, the second example looks into cooling to the ground state from an even cat state, and the third example investigates a cat state to cat state evolution. We compare the statistics of the fidelities of the final state with respect to the target state for trajectories generated under the optimal control with those generated under a sample control. Compared to the latter case, we see a 40-196% increase in the number of trajectories reaching more than 95% fidelities under the optimal control. Our work provides a systematic prescription for finding quantum optimal control for continuously monitored systems.
Chantasri- dressel - jordan （CDJ）随机路径积分形式（Chantasri et al. 2013和2015）描述了在高斯状态下涉及几个量子位或量子谐振子的连续监测量子系统的读出统计和最有可能的条件演化。在我们的工作中，我们通过引入协态算子将CDJ形式推广到任意连续监控系统。然后，我们对任意演化的量子系统给出了广义庞特里亚金极大原理，并找到了最优控制协议的条件。我们证明CDJ形式的最可能路径可以被转换为量子庞特里亚金最大原理，其中成本函数是沿着量子轨迹的读出概率。这种见解使我们能够推导出任意控制参数的一般最优控制方程。我们将我们的结果应用于一个存在参数二次势和变量正交测量的监测振荡器。我们找到了固定端点问题的最优位势强度和正交角。最优参数势解析显示为“bang-bang”形式。我们将我们的协议应用于三个与玻色子量子计算相关的量子振荡器实例。第一个示例考虑了从错误词到二项码字的准备，第二个示例研究了从偶猫状态到基态的冷却，第三个示例研究了从猫状态到猫状态的演化。我们比较了在最优控制和样本控制下生成的轨迹的最终状态相对于目标状态的保真度的统计数据。与后一种情况相比，我们看到在最优控制下，达到95%保真度以上的轨迹数量增加了40-196%。我们的工作为寻找连续监测系统的量子最优控制提供了一个系统的处方。
{"title":"CDJ-Pontryagin Optimal Control for General Continuously Monitored Quantum Systems","authors":"Tathagata Karmakar, Andrew N. Jordan","doi":"10.22331/q-2026-03-24-2043","DOIUrl":"https://doi.org/10.22331/q-2026-03-24-2043","url":null,"abstract":"The Chantasri-Dressel-Jordan (CDJ) stochastic path integral formalism (Chantasri et al. 2013 and 2015) characterizes the statistics of the readouts and the most likely conditional evolution of continuously monitored quantum systems involving a few qubits or quantum harmonic oscillators in Gaussian states. In our work, we generalize the CDJ formalism to arbitrary continuously monitored systems by introducing a costate operator. We then prescribe a generalized Pontryagin's maximum principle for quantum systems undergoing arbitrary evolution and find conditions on optimal control protocols. We show that the CDJ formalism's most likely path can be cast as a quantum Pontryagin's maximum principle, where the cost function is the readout probabilities along a quantum trajectory. This insight allows us to derive general optimal control equations for arbitrary control parameters. We apply our results to a monitored oscillator in the presence of a parametric quadratic potential and variable quadrature measurements. We find the optimal potential strength and quadrature angle for fixed-end point problems. The optimal parametric potential is analytically shown to have a \"bang-bang\" form. We apply our protocol to three quantum oscillator examples relevant to Bosonic quantum computing. The first example considers a binomial codeword preparation from an error word, the second example looks into cooling to the ground state from an even cat state, and the third example investigates a cat state to cat state evolution. We compare the statistics of the fidelities of the final state with respect to the target state for trajectories generated under the optimal control with those generated under a sample control. Compared to the latter case, we see a 40-196% increase in the number of trajectories reaching more than 95% fidelities under the optimal control. Our work provides a systematic prescription for finding quantum optimal control for continuously monitored systems.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"15 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147506879","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-03-23DOI: 10.22331/q-2026-03-23-2039
Aleksandrs Belovs, Stacey Jeffery
Given an algorithm that outputs the correct answer with bounded error, say $1/3$, it is sometimes desirable to reduce this error to some arbitrarily small $varepsilon$ – e.g., if one wants to call the algorithm many times as a subroutine. The usual method, for both quantum and randomized algorithms, is majority voting, which incurs a multiplicative overhead of $O(logfrac{1}{varepsilon})$ from calling the algorithm this many times. Transducers are a recently introduced model of quantum computation, and it is possible to reduce the “error'' of a transducer arbitrarily with only constant overhead, using a construction analogous to majority voting called purification. Even error-free transducers map to bounded-error quantum algorithms, so this does not let you reduce algorithmic error for free, but it does allow bounded-error quantum algorithms to be composed without incurring log factors. In this paper, we present a new highly simplified purifier, that can be understood as a weighted walk on a line similar to a random walk interpretation of majority voting. Our purifier has much smaller space and time complexity than the previous one. Indeed, it only uses one additional counter, and only performs two increment and two decrement operations on each iteration. It also has quadratically better dependence on the soundness-completeness gap of the algorithm being purified. We prove that our purifier has optimal query complexity, even down to the constant, which matters when one composes quantum algorithms to super-constant depth. Purifiers can be seen as a way of turning a “Monte Carlo'' quantum algorithm into a “Las Vegas'' quantum algorithm – a process for which there is no classical analogue. Our simplified construction sheds light on this strange quantum phenomenon, and could have implications for the complexity of composed quantum algorithms.
{"title":"Space-Efficient Quantum Error Reduction without log Factors","authors":"Aleksandrs Belovs, Stacey Jeffery","doi":"10.22331/q-2026-03-23-2039","DOIUrl":"https://doi.org/10.22331/q-2026-03-23-2039","url":null,"abstract":"Given an algorithm that outputs the correct answer with bounded error, say $1/3$, it is sometimes desirable to reduce this error to some arbitrarily small $varepsilon$ – e.g., if one wants to call the algorithm many times as a subroutine. The usual method, for both quantum and randomized algorithms, is majority voting, which incurs a multiplicative overhead of $O(logfrac{1}{varepsilon})$ from calling the algorithm this many times.<br/> Transducers are a recently introduced model of quantum computation, and it is possible to reduce the “error'' of a transducer arbitrarily with only constant overhead, using a construction analogous to majority voting called purification. Even error-free transducers map to bounded-error quantum algorithms, so this does not let you reduce algorithmic error for free, but it does allow bounded-error quantum algorithms to be composed without incurring log factors.<br/> In this paper, we present a new highly simplified purifier, that can be understood as a weighted walk on a line similar to a random walk interpretation of majority voting. Our purifier has much smaller space and time complexity than the previous one. Indeed, it only uses one additional counter, and only performs two increment and two decrement operations on each iteration. It also has quadratically better dependence on the soundness-completeness gap of the algorithm being purified. We prove that our purifier has optimal query complexity, even down to the constant, which matters when one composes quantum algorithms to super-constant depth.<br/> Purifiers can be seen as a way of turning a “Monte Carlo'' quantum algorithm into a “Las Vegas'' quantum algorithm – a process for which there is no classical analogue. Our simplified construction sheds light on this strange quantum phenomenon, and could have implications for the complexity of composed quantum algorithms.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"49 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147495317","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-03-23DOI: 10.22331/q-2026-03-23-2041
Guang Hao Low, Yuan Su
Quantum algorithms for linear systems produce the solution state $A^{-1}|brangle$ by querying two oracles: $O_A$ that block encodes the coefficient matrix and $O_b$ that prepares the initial state. We present a quantum linear system algorithm making $mathbf{Theta}left(1/sqrt{p}right)$ queries to $O_b$, which is optimal in the success probability, and $mathbf{O}left(kappalogleft(1/pright)left(loglogleft(1/pright)+logleft({1}/{epsilon}right)right)right)$ queries to $O_A$, nearly optimal in all parameters including the condition number and accuracy. Notably, our complexity scaling of initial state preparation holds even when $p$ is not known $textit{a priori}$. This contrasts with recent results achieving $mathbf{O}left(kappalogleft({1}/{epsilon}right)right)$ complexity to both oracles, which, while optimal in $O_A$, is highly suboptimal in $O_b$ as $kappa$ can be arbitrarily larger than $1/sqrt{p}$. In various applications such as solving differential equations, preparing ground states of operators with real spectra, and estimating and transforming eigenvalues of non-normal matrices, we can further improve the dependence on $p$ using a block preconditioning scheme to nearly match or outperform best previous results based on other methods, which also furnishes an extremely simple quantum linear system algorithm with an optimal query complexity to $O_A$. Underlying our results is a new Variable Time Amplitude Amplification algorithm with Tunable thresholds (Tunable VTAA), which fully characterizes generic nested amplitude amplifications, improves the $ell_1$-norm input cost scaling of Ambainis to an $ell_{frac{2}{3}}$-quasinorm scaling, and admits a deterministic amplification schedule for the quantum linear system problem.
{"title":"Quantum linear system algorithm with optimal queries to initial state preparation","authors":"Guang Hao Low, Yuan Su","doi":"10.22331/q-2026-03-23-2041","DOIUrl":"https://doi.org/10.22331/q-2026-03-23-2041","url":null,"abstract":"Quantum algorithms for linear systems produce the solution state $A^{-1}|brangle$ by querying two oracles: $O_A$ that block encodes the coefficient matrix and $O_b$ that prepares the initial state. We present a quantum linear system algorithm making $mathbf{Theta}left(1/sqrt{p}right)$ queries to $O_b$, which is optimal in the success probability, and $mathbf{O}left(kappalogleft(1/pright)left(loglogleft(1/pright)+logleft({1}/{epsilon}right)right)right)$ queries to $O_A$, nearly optimal in all parameters including the condition number and accuracy. Notably, our complexity scaling of initial state preparation holds even when $p$ is not known $textit{a priori}$. This contrasts with recent results achieving $mathbf{O}left(kappalogleft({1}/{epsilon}right)right)$ complexity to both oracles, which, while optimal in $O_A$, is highly suboptimal in $O_b$ as $kappa$ can be arbitrarily larger than $1/sqrt{p}$. In various applications such as solving differential equations, preparing ground states of operators with real spectra, and estimating and transforming eigenvalues of non-normal matrices, we can further improve the dependence on $p$ using a block preconditioning scheme to nearly match or outperform best previous results based on other methods, which also furnishes an extremely simple quantum linear system algorithm with an optimal query complexity to $O_A$. Underlying our results is a new Variable Time Amplitude Amplification algorithm with Tunable thresholds (Tunable VTAA), which fully characterizes generic nested amplitude amplifications, improves the $ell_1$-norm input cost scaling of Ambainis to an $ell_{frac{2}{3}}$-quasinorm scaling, and admits a deterministic amplification schedule for the quantum linear system problem.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"79 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147495318","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-03-23DOI: 10.22331/q-2026-03-23-2038
Luna Lima Keller, Daniel Jost Brod
Quantum walks in general graphs, or more specifically scattering on graphs, encompass enough complexity to perform universal quantum computation. Any given quantum circuit can be broken down into single- and two-qubit gates, which can then be translated into subgraphs – gadgets – that implement such unitaries on the logical qubits, simulated by particles traveling along a sparse graph. In this work, we start to develop a full theory of multi-particle scattering on graphs and give initial applications to build multi-particle gadgets with different properties.
{"title":"Two-particle scattering on general graphs","authors":"Luna Lima Keller, Daniel Jost Brod","doi":"10.22331/q-2026-03-23-2038","DOIUrl":"https://doi.org/10.22331/q-2026-03-23-2038","url":null,"abstract":"Quantum walks in general graphs, or more specifically scattering on graphs, encompass enough complexity to perform universal quantum computation. Any given quantum circuit can be broken down into single- and two-qubit gates, which can then be translated into subgraphs – gadgets – that implement such unitaries on the logical qubits, simulated by particles traveling along a sparse graph. In this work, we start to develop a full theory of multi-particle scattering on graphs and give initial applications to build multi-particle gadgets with different properties.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"82 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147495316","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-03-23DOI: 10.22331/q-2026-03-23-2036
Mario Collura, Jacopo De Nardis, Vincenzo Alba, Guglielmo Lami
We introduce an efficient method to quantify nonstabilizerness in fermionic Gaussian states, overcoming the long-standing challenge posed by their extensive entanglement. Using a perfect sampling scheme based on an underlying determinantal point process, we compute the Stabilizer Renyi Entropies (SREs) for systems with hundreds of qubits. Benchmarking on random Gaussian states with and without particle conservation, we reveal an extensive leading behavior equal to that of Haar random states, with logarithmic subleading corrections. We support these findings with analytical calculations for a set of related quantities, the participation entropies in the computational (or Fock) basis, for which we derive an exact formula. We also investigate the time evolution of non-stabilizerness in a random unitary circuit with Gaussian gates, observing that it converges in a time that scales logarithmically with the system size. Applying the sampling algorithm to a two-dimensional free-fermionic topological model, we uncover a sharp transition in non-stabilizerness at the phase boundaries, highlighting the power of our approach in exploring different phases of quantum many-body systems, even in higher dimensions.
{"title":"The non-stabilizerness of fermionic Gaussian states","authors":"Mario Collura, Jacopo De Nardis, Vincenzo Alba, Guglielmo Lami","doi":"10.22331/q-2026-03-23-2036","DOIUrl":"https://doi.org/10.22331/q-2026-03-23-2036","url":null,"abstract":"We introduce an efficient method to quantify nonstabilizerness in fermionic Gaussian states, overcoming the long-standing challenge posed by their extensive entanglement. Using a perfect sampling scheme based on an underlying determinantal point process, we compute the Stabilizer Renyi Entropies (SREs) for systems with hundreds of qubits. Benchmarking on random Gaussian states with and without particle conservation, we reveal an extensive leading behavior equal to that of Haar random states, with logarithmic subleading corrections. We support these findings with analytical calculations for a set of related quantities, the participation entropies in the computational (or Fock) basis, for which we derive an exact formula. We also investigate the time evolution of non-stabilizerness in a random unitary circuit with Gaussian gates, observing that it converges in a time that scales logarithmically with the system size. Applying the sampling algorithm to a two-dimensional free-fermionic topological model, we uncover a sharp transition in non-stabilizerness at the phase boundaries, highlighting the power of our approach in exploring different phases of quantum many-body systems, even in higher dimensions.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"17 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147495314","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In recent years, quantum key distribution (QKD) has evolved from a scientific research field to a commercially available security solution, supported by mathematically formulated security proofs. However, since the knowledge required for a full understanding of a security proof is scattered across numerous publications, it has proven difficult to gain a comprehensive understanding of all steps involved in the process and their limitations without considerable effort and attention to detail. Our paper aims to address this issue by providing a rigorous and comprehensive security proof for the finite-size 1-decoy and 2-decoy BB84 protocols against coherent attacks within Renner's entropic uncertainty relation framework. We resolve important technical flaws found in previous works regarding the fixed-length treatment of protocols and the careful handling of acceptance testing. To this end, we provide various technical arguments, including an analysis accounting for the important distinction of the 1-decoy protocol where statistics are computed after error correction, along with a slight improvement of the secure-key length. We also explicitly clarify the aspect of conditioning on events, addressing a technical detail often overlooked and essential for rigorous proofs. We extensively consolidate and unify concepts from many works, thoroughly discussing the underlying assumptions and resolving technical inconsistencies. Therefore, our contribution represents a significant advancement towards a broader and deeper understanding of QKD security proofs.
{"title":"A consolidated and accessible security proof for finite-size decoy-state quantum key distribution","authors":"Jerome Wiesemann, Jan Krause, Devashish Tupkary, Norbert Lütkenhaus, Davide Rusca, Nino Walenta","doi":"10.22331/q-2026-03-23-2037","DOIUrl":"https://doi.org/10.22331/q-2026-03-23-2037","url":null,"abstract":"In recent years, quantum key distribution (QKD) has evolved from a scientific research field to a commercially available security solution, supported by mathematically formulated security proofs. However, since the knowledge required for a full understanding of a security proof is scattered across numerous publications, it has proven difficult to gain a comprehensive understanding of all steps involved in the process and their limitations without considerable effort and attention to detail. Our paper aims to address this issue by providing a rigorous and comprehensive security proof for the finite-size 1-decoy and 2-decoy BB84 protocols against coherent attacks within Renner's entropic uncertainty relation framework. We resolve important technical flaws found in previous works regarding the fixed-length treatment of protocols and the careful handling of acceptance testing. To this end, we provide various technical arguments, including an analysis accounting for the important distinction of the 1-decoy protocol where statistics are computed after error correction, along with a slight improvement of the secure-key length. We also explicitly clarify the aspect of conditioning on events, addressing a technical detail often overlooked and essential for rigorous proofs. We extensively consolidate and unify concepts from many works, thoroughly discussing the underlying assumptions and resolving technical inconsistencies. Therefore, our contribution represents a significant advancement towards a broader and deeper understanding of QKD security proofs.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"15 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2026-03-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147495315","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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