IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-20 DOI: 10.22331/q-2025-05-20-1750

Satoshi Yoshida, Akihito Soeda, Mio Murao

{"title":"Universal adjointation of isometry operations using conversion of quantum supermaps","authors":"Satoshi Yoshida, Akihito Soeda, Mio Murao","doi":"10.22331/q-2025-05-20-1750","DOIUrl":"https://doi.org/10.22331/q-2025-05-20-1750","url":null,"abstract":"Identification of possible transformations of quantum objects including quantum states and quantum operations is indispensable in developing quantum algorithms. Universal transformations, defined as input-independent transformations, appear in various quantum applications. Such is the case for universal transformations of unitary operations. However, extending these transformations to non-unitary operations is nontrivial and largely unresolved. Addressing this, we introduce $textit{isometry adjointation}$ protocols that transform an input isometry operation into its adjoint operation, which include both unitary operation and quantum state transformations. The paper details the construction of parallel and sequential isometry adjointation protocols, derived from unitary inversion protocols using quantum combs and the (dual) Clebsch-Gordan transforms, and achieving optimal approximation error. This error is shown to be independent of the output dimension of the isometry operation. In particular, we explicitly obtain an asymptotically optimal parallel protocol achieving an approximation error $epsilon = Theta(d^2/n)$, where $d$ is the input dimension of the isometry operation and $n$ is the number of calls of the isometry operation. The research also extends to isometry inversion and universal error detection, employing semidefinite programming to assess optimal performances. The findings suggest that the optimal performance of general protocols in isometry adjointation and universal error detection is not dependent on the output dimension, and that indefinite causal order protocols offer advantages over sequential ones in isometry inversion and universal error detection.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"35 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2025-05-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144104734","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}

引用次数: 0

The measurement postulates of quantum mechanics are not redundant 量子力学的测量假设不是多余的

IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-20 DOI: 10.22331/q-2025-05-20-1749

Adrian Kent

{"title":"The measurement postulates of quantum mechanics are not redundant","authors":"Adrian Kent","doi":"10.22331/q-2025-05-20-1749","DOIUrl":"https://doi.org/10.22331/q-2025-05-20-1749","url":null,"abstract":"Masanes, Galley and Müller [39] argue that the measurement postulates of non-relativistic quantum mechanics follow from the structural postulates together with an assumption they call the \"possibility of state estimation\". Their argument also relies on what they term a \"theory-independent characterization of measurements for single and multipartite systems\". We refute their conclusion, giving explicit examples of non-quantum measurement and state update rules that satisfy all their assumptions. We also show that their \"possibility of state estimation\" assumption is neither necessary nor sufficient to ensure a sensible notion of state estimation within a theory whose states are described by the quantum formalism. We further show their purportedly \"theory-independent\" characterization assumes several properties of quantum measurements that exclude plausible alternative types of measurement. We illustrate all these points with specific alternative measurement postulates and post-measurement state update rules. We conclude that, contrary to some folklore, quantum mechanics is by no means an island in theory-space. It can consistently be extended by rules for obtaining information about quantum states other than via POVMs. Whether such rules are realised in nature, for example in linking quantum theory and gravity, is an empirical question that cannot be resolved by theoretical analysis alone.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"2 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2025-05-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144097664","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}

引用次数: 0

Trainability and Expressivity of Hamming-Weight Preserving Quantum Circuits for Machine Learning 用于机器学习的哈明-保权量子电路的可训练性和表达性

IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-15 DOI: 10.22331/q-2025-05-15-1745

Léo Monbroussou, Eliott Z. Mamon, Jonas Landman, Alex B. Grilo, Romain Kukla, Elham Kashefi

{"title":"Trainability and Expressivity of Hamming-Weight Preserving Quantum Circuits for Machine Learning","authors":"Léo Monbroussou, Eliott Z. Mamon, Jonas Landman, Alex B. Grilo, Romain Kukla, Elham Kashefi","doi":"10.22331/q-2025-05-15-1745","DOIUrl":"https://doi.org/10.22331/q-2025-05-15-1745","url":null,"abstract":"Quantum machine learning (QML) has become a promising area for real world applications of quantum computers, but near-term methods and their scalability are still important research topics. In this context, we analyze the trainability and controllability of specific Hamming weight preserving variational quantum circuits (VQCs). These circuits use qubit gates that preserve subspaces of the Hilbert space, spanned by basis states with fixed Hamming weight $k$. In this work, we first design and prove the feasibility of new heuristic data loaders, performing quantum amplitude encoding of $binom{n}{k}$-dimensional vectors by training an $n$-qubit quantum circuit. These data loaders are obtained using controllability arguments, by checking the Quantum Fisher Information Matrix (QFIM)'s rank. Second, we provide a theoretical justification for the fact that the rank of the QFIM of any VQC state is almost-everywhere constant, which is of separate interest. Lastly, we analyze the trainability of Hamming weight preserving circuits, and show that the variance of the $l_2$ cost function gradient is bounded according to the dimension $binom{n}{k}$ of the subspace. This proves conditions of existence/lack of Barren Plateaus for these circuits, and highlights a setting where a recent conjecture on the link between controllability and trainability of variational quantum circuits does not apply.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"96 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2025-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144066455","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}

引用次数: 0

On noise in swap ASAP repeater chains: exact analytics, distributions and tight approximations 交换ASAP中继器链中的噪声：精确分析，分布和紧密近似

IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-15 DOI: 10.22331/q-2025-05-15-1744

Kenneth Goodenough, Tim Coopmans, Don Towsley

{"title":"On noise in swap ASAP repeater chains: exact analytics, distributions and tight approximations","authors":"Kenneth Goodenough, Tim Coopmans, Don Towsley","doi":"10.22331/q-2025-05-15-1744","DOIUrl":"https://doi.org/10.22331/q-2025-05-15-1744","url":null,"abstract":"Losses are one of the main bottlenecks for the distribution of entanglement in quantum networks, which can be overcome by the implementation of quantum repeaters. The most basic form of a quantum repeater chain is the swap ASAP repeater chain. In such a repeater chain, elementary links are probabilistically generated and deterministically swapped as soon as two adjacent links have been generated. As each entangled state is waiting to be swapped, decoherence is experienced, turning the fidelity of the entangled state between the end nodes of the chain into a random variable. Fully characterizing the (average) fidelity as the repeater chain grows is still an open problem. Here, we analytically investigate the case of equally-spaced repeaters, where we find exact analytic formulae for all moments of the fidelity up to 25 segments. We obtain these formulae by providing a general solution in terms of a $textit{generating function}$; a function whose n'th term in its Maclaurin series yields the moments of the fidelity for n segments. We generalize this approach as well to a $textit{global cut-off}$ policy – a method for increasing fidelity at the cost of longer entanglement delivery times – allowing for fast optimization of the cut-off parameter by eliminating the need for Monte Carlo simulation. We furthermore find simple approximations of the average fidelity that are exponentially tight, and, for up to 10 segments, the full distribution of the delivered fidelity. We use this to analytically calculate the secret-key rate, both with and without binning methods.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"13 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2025-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143979906","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}

引用次数: 0

Block encoding bosons by signal processing 用信号处理对玻色子进行分组编码

IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-15 DOI: 10.22331/q-2025-05-15-1747

Christopher F. Kane, Siddharth Hariprakash, Neel S. Modi, Michael Kreshchuk, Christian W Bauer

{"title":"Block encoding bosons by signal processing","authors":"Christopher F. Kane, Siddharth Hariprakash, Neel S. Modi, Michael Kreshchuk, Christian W Bauer","doi":"10.22331/q-2025-05-15-1747","DOIUrl":"https://doi.org/10.22331/q-2025-05-15-1747","url":null,"abstract":"Block Encoding (BE) is a crucial subroutine in many modern quantum algorithms, including those with near-optimal scaling for simulating quantum many-body systems, which often rely on Quantum Signal Processing (QSP). Currently, the primary methods for constructing BEs are the Linear Combination of Unitaries (LCU) and the sparse oracle approach. In this work, we demonstrate that QSP-based techniques, such as Quantum Singular Value Transformation (QSVT) and Quantum Eigenvalue Transformation for Unitary Matrices (QETU), can themselves be efficiently utilized for BE implementation. Specifically, we present several examples of using QSVT and QETU algorithms, along with their combinations, to block encode Hamiltonians for lattice bosons, an essential ingredient in simulations of high-energy physics. We also introduce a straightforward approach to BE based on the exact implementation of Linear Operators Via Exponentiation and LCU (LOVE-LCU). We find that, while using QSVT for BE results in the best asymptotic gate count scaling with the number of qubits per site, LOVE-LCU outperforms all other methods for operators acting on up to $lesssim11$ qubits, highlighting the importance of concrete circuit constructions over mere comparisons of asymptotic scalings. Using LOVE-LCU to implement the BE, we simulate the time evolution of single-site and two-site systems in the lattice $varphi^4$ theory using the Generalized QSP algorithm and compare the gate counts to those required for Trotter simulation.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"29 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2025-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144066457","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}

引用次数: 0

Quantum detailed balance via elementary transitions 通过基本跃迁的量子详细平衡

IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-15 DOI: 10.22331/q-2025-05-15-1743

Rocco Duvenhage, Kyle Oerder, Keagan van den Heuvel

引用次数: 0

Spontaneous symmetry emergence in a Hermitian system of coupled oscillators without symmetry 无对称耦合振子厄米系统中的自发对称性出现

IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-15 DOI: 10.22331/q-2025-05-15-1746

T. T. Sergeev, E. S. Andrianov, A. A. Zyablovsky

引用次数: 0

Stochastic resetting in discrete-time quantum dynamics: steady states and correlations in few-qubit systems 离散时间量子动力学中的随机重置：少量子位系统中的稳态和相关性

IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-13 DOI: 10.22331/q-2025-05-13-1742

Sascha Wald, Louie Hong Yao, Thierry Platini, Chris Hooley, Federico Carollo

{"title":"Stochastic resetting in discrete-time quantum dynamics: steady states and correlations in few-qubit systems","authors":"Sascha Wald, Louie Hong Yao, Thierry Platini, Chris Hooley, Federico Carollo","doi":"10.22331/q-2025-05-13-1742","DOIUrl":"https://doi.org/10.22331/q-2025-05-13-1742","url":null,"abstract":"Time evolution in several classes of quantum devices is generated through the application of quantum gates. Resetting is a critical technological feature in these systems allowing for mid-circuit measurement and complete or partial qubit reset. The possibility of realizing discrete-time reset dynamics on quantum computers makes it important to investigate the steady-state properties of such dynamics. Here, we explore the behavior of generic discrete-time unitary dynamics interspersed by random reset events. For Poissonian resets, we compute the stationary state of the process and demonstrate, by taking a weak-reset limit, the existence of \"resonances\" in the quantum gates, allowing for the emergence of steady state density matrices which are not diagonal in the eigenbasis of the generator of the unitary gate. Such resonances are a genuine discrete-time feature and impact on quantum and classical correlations even beyond the weak-reset limit. Furthermore, we consider non-Poissonian reset processes and explore conditions for the existence of a steady state. We show that, when the reset probability vanishes sufficiently rapidly with time, the system does not approach a steady state. Our results highlight key differences between continuous-time and discrete-time stochastic resetting and may be useful to engineer states with controllable correlations on existing devices.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"3 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2025-05-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143940279","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}

引用次数: 0

Quantum and classical algorithms for nonlinear unitary dynamics 非线性酉动力学的量子和经典算法

IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-13 DOI: 10.22331/q-2025-05-13-1741

Noah Brustle, Nathan Wiebe

{"title":"Quantum and classical algorithms for nonlinear unitary dynamics","authors":"Noah Brustle, Nathan Wiebe","doi":"10.22331/q-2025-05-13-1741","DOIUrl":"https://doi.org/10.22331/q-2025-05-13-1741","url":null,"abstract":"Quantum algorithms for Hamiltonian simulation and linear differential equations more generally have provided promising exponential speed-ups over classical computers on a set of problems with high real-world interest. However, extending this to a nonlinear problem has proven challenging, with exponential lower bounds having been demonstrated for the time scaling. We provide a quantum algorithm matching these bounds. Specifically, we find that for a non-linear differential equation of the form $frac{d|urangle}{dt} = A|urangle + B|urangle^{otimes2}$ for evolution of time $T$, error tolerance $epsilon$ and $c$ dependent on the strength of the nonlinearity, the number of queries to the differential operators that approaches the scaling of the quantum lower bound of $e^{o(T|B|)}$ queries in the limit of strong non-linearity. Finally, we introduce a classical algorithm based on the Euler method allowing comparably scaling to the quantum algorithm in a restricted case, as well as a randomized classical algorithm based on path integration that acts as a true analogue to the quantum algorithm in that it scales comparably to the quantum algorithm in cases where sign problems are absent.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"51 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2025-05-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143940278","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}

引用次数: 0

Non-onsite symmetries and quantum teleportation in split-index matrix product states 分度矩阵积态中的非现场对称性和量子隐形传态

IF 6.4 2区物理与天体物理

Quantum Pub Date : 2025-05-12 DOI: 10.22331/q-2025-05-12-1738

David T. Stephen

{"title":"Non-onsite symmetries and quantum teleportation in split-index matrix product states","authors":"David T. Stephen","doi":"10.22331/q-2025-05-12-1738","DOIUrl":"https://doi.org/10.22331/q-2025-05-12-1738","url":null,"abstract":"We describe a class of spin chains with new physical and computational properties. On the physical side, the spin chains give examples of symmetry-protected topological phases that are defined by non-onsite symmetries, i.e., symmetries that are not a tensor product of single-site operators. These phases can be detected by string-order parameters, but notably do not exhibit entanglement spectrum degeneracy. On the computational side, the spin chains represent a new class of states that can be used to deterministically teleport information across long distances, with the novel property that the necessary classical side processing is a non-linear function of the measurement outcomes. We also give examples of states that can serve as universal resources for measurement-based quantum computation, providing the first examples of such resources without entanglement spectrum degeneracy. The key tool in our analysis is a new kind of tensor network representation which we call split-index matrix product states (SIMPS). We develop the basic formalism of SIMPS, compare them to matrix product states, show how they are better equipped to describe certain kinds of non-onsite symmetries including anomalous symmetries, and discuss how they are also well-suited to describing quantum teleportation and constrained spin chains.","PeriodicalId":20807,"journal":{"name":"Quantum","volume":"2 1","pages":""},"PeriodicalIF":6.4,"publicationDate":"2025-05-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143933466","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}

引用次数: 0

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