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Magic states and magic gates are crucial for achieving universal computation, but some important questions about how magic resources should be implemented to attain quantum advantage have remained unexplored, for instance, in the context of Measurement-based Quantum Computation (MQC) with only single-qubit measurements. This work bridges the gap between MQC and the resource theory of magic by introducing the concept of ``invested'' and ``potential" magic resources. The former quantifies the magic cost associated with the MQC framework, serving both as a witness of magic resources and an upper bound for the realization of a desired unitary transformation. Potential magic resources represent the maximum achievable magic resource in a given graph structure defining the MQC. We utilize these concepts to analyze the magic resource requirements of the Quantum Fourier Transform (QFT) and provide a fresh perspective on the universality of MQC of different resource states, highlighting the crucial role of non-Pauli measurements for injecting magic. We demonstrate experimentally our theoretical predictions in a high-fidelity four-photon setup and demonstrate the efficiency of MQC in generating magic states, surpassing the limitations of conventional magic state injection methods. Our findings pave the way for future research exploring magic resource optimization and novel distillation schemes within the MQC framework, contributing to the advancement of fault-tolerant universal quantum computation., Comment: 25 pages, 11 figures

We employ the Stabilizer Renyi Entropy (SRE) to characterize a quantum phase transition that has so far eluded any standard description and can thus now be explained in terms of the interplay between its non-stabilizer properties and entanglement. The transition under consideration separates a region with a unique ground state from one with a degenerate ground state manifold spanned by states with finite and opposite (intensive) momenta. We show that SRE has a jump at the crossing points, while the entanglement entropy remains continuous. Moreover, by leveraging on a Clifford circuit mapping, we connect the observed jump in SRE to that occurring between standard and generalized $W$-states with finite momenta. This mapping allows us to quantify the SRE discontinuity analytically., Comment: 8 pages including supplementary material, 3 figures

Geometrical methods in quantum information are very promising for both providing technical tools and intuition into difficult control or optimization problems. Moreover, they are of fundamental importance in connecting pure geometrical theories, like GR, to quantum mechanics, like in the AdS/CFT correspondence. In this paper, we first make a survey of the most important settings in which geometrical methods have proven useful to quantum information theory. Then, we lay down a general framework for an action principle for quantum resources like entanglement, coherence, and anti-flatness. We discuss the case of a two-qubit system., Comment: Slighted expanded version with respect to the article that appeared in the International Journal of Geometric Methods in Modern Physics (https://doi.org/10.1142/s0219887824400103)

We study the interplay between magic and entanglement in quantum many-body systems. We show that non-local magic, which is supported by the quantum correlations is lower bounded by the non-flatness of entanglement spectrum and upper bounded by the amount of entanglement in the system. We then argue that a smoothed version of non-local magic bounds the hardness of classical simulations for incompressible states. In conformal field theories, we conjecture that the non-local magic should scale linearly with entanglement entropy but sublinearly when an approximation of the state is allowed. We support the conjectures using both analytical arguments based on unitary distillation and numerical data from an Ising CFT. If the CFT has a holographic dual, then we prove that the non-local magic vanishes if and only if there is no gravitational back-reaction. Furthermore, we show that non-local magic is approximately equal to the rate of change of the minimal surface area in response to the change of cosmic brane tension in the bulk., Comment: 62 pages, 20 figures; title changed, Theorem 1 and 2 refined, references added

How complex is the structure of quantum geometry? In several approaches, the spacetime atoms are obtained by the SU(2) intertwiner called quantum tetrahedron. The complexity of this construction has a concrete consequence in recent efforts to simulate such models and toward experimental demonstrations of quantum gravity effects. There are, therefore, both a computational and an experimental complexity inherent to this class of models. In this paper, we study this complexity under the lens of stabilizer entropy (SE). We calculate the SE of the gauge-invariant basis states and its average in the SU(2) gauge invariant subspace. We find that the states of definite volume are singled out by the (near) maximal SE and give precise bounds to the verification protocols for experimental demonstrations on available quantum computers., Comment: 8 pages, 3 figures, 9 pages supplementary material

Traditionally, the production efficiency of a factory floor is evaluated using non-real time objective functions. These are based on scheduling punctuality criteria such as 'earliness (a measure of finishing operations ahead of schedule)' and 'tardiness (a measure of delay in executing certain operations)'. With process automation becoming more and more inevitable due to the emergence of Industry 4.0 / Industry 5.0, real-time objective functions are also gaining popularity. One such criterion is 'synchronization' or 'timing'. In this article, an IoT enabled real-time synchronization approach is presented in the context of Integrated Industrial Internet of Things (IIoT) from a service provider's perspective. A new business and operational model, termed as "Timing-As-A-Service (TAAS)", is introduced. In this model, the role of a service provider is to "supply" synchronization from the mobile networks to the IIoT domain., Comment: 7 pages

We investigate a system of two coupled qubits interacting with an Ohmic bath as a physical model for the implementation of one logical qubit. In this model, the interaction with the other qubit represents unitary noise while the Ohmic bath is responsible for finite temperature. In the presence of a one-dimensional decoherence-free subspace (DFS), we show that, while this is not sufficient to protect a qubit from decoherence, it can be exploited to encode one logical qubit with greater performance than the physical one. We show different possible strategies for the optimal encoding of a logical qubit through a numerical analysis based on matrix product states. This method reproduces faithfully the results of perturbative calculations, but it can be extended to cases of crucial interest for physical implementations, e.g., in the case of strong coupling with the bath. As a result, a logical qubit encoded in the subspace which is the direct sum of the antiferromagnetic states in Bell basis, the DFS and the one in the triplet, is the optimally robust one, as it takes advantage of both the anchoring to the DFS and the protection from the antiferromagnetic interaction. These authors contributed equally to this work, and their names are listed in alphabetical order., Comment: 13 pages, 12 figures

In this paper, we present a learning algorithm aimed at learning states obtained from computational basis states by Clifford circuits doped with a finite number $t$ of $T$-gates. The algorithm learns an exact tomographic description of $t$-doped stabilizer states in terms of Pauli observables. This is possible because such states are countable and form a discrete set. To tackle the problem, we introduce a novel algebraic framework for $t$-doped stabilizer states, which extends beyond $T$-gates and includes doping with any kind of local non-Clifford gate. The algorithm requires resources of complexity $\text{poly}(n,2^t)$ and exhibits an exponentially small probability of failure., Comment: L.L. and S.O. contributed equally to this work

Effective light cones may emerge in non-relativistic local quantum systems from the Lieb-Robinson bounds, resulting in exponentially decaying commutator norms of two space-time separated operators in the Heisenberg picture. Here, we derive a mechanism for the emergence and consequences of a logarithmic light cone (LLC). As a possible way, the LLC can emerge from a phenomenological model of many-body-localization. We show that the information scrambling is logarithmically slow in the regime of the LLC. We prove that the bipartite entanglement entropy grows logarithmically with time for arbitrary finite space dimensions and arbitrary initial pure states. As an application in quantum information processing, the LLC supports long-lived quantum memory after unitary time evolution: a quantum code with macroscopic code distance and exponentially long lifetime., Comment: 7+3 pages, 2 figures.In version 2, typos were fixed; the proof of theorem 1 was slightly modified

Stabilizer entropies (SE) measure deviations from stabilizer resources and as such are a fundamental ingredient for quantum advantage. In particular, the interplay of SE and entanglement is at the root of the complexity of classically simulating quantum many-body systems. In this paper, we study the dynamics of SE in a quantum many-body system away from the equilibrium after a quantum quench in an integrable system. We obtain two main results: (i) we show that SE, despite being an L-extensive quantity, equilibrates in a time that scales at most linearly with the subsystem size; and (ii) we show that there is a SE length increasing linearly in time, akin to correlations and entanglement spreading., Comment: 13 pages, 9 figures

Black holes are a recently observed theoretical prediction of General Relativity, characterized by event horizons, from which information cannot escape. Examined through the lenses of quantum mechanics, they can radiate at a definite temperature inverse to their mass and horizon radius. Hawking radiation, whose spectrum was calculated considering particles scattering off black holes, is connected to the paradox of the loss of information falling into them. Information can become non-fungible, due to scrambling. We demonstrate this feature not to be restricted to curved space-times: soft radiation scattering in a flat space-time does scramble information as well. To this end, we compute the scrambling of information through the tripartite mutual information in a scattering process off a black hole and compare it with the flat space-time analog. We show that the scrambling power of the gravitational field of a black hole is negligible with respect to the scrambling power of flat space-time., Comment: 90 pages

Observable scattering processes entail emission-absorption of soft photons. As these degrees of freedom go undetected, some information is lost. Whether some of this information can be recovered in the observation of the hard photons, depends of the actual pattern of the scrambling of information. We compute the information scrambling of photon scattering by the tripartite mutual information in terms of the 2-Renyi entropy, and find a finite amount of scrambling is present. The developed procedure thus sheds novel light on the black hole information loss paradox, showing that scrambling is a byproduct of decoherence achieved by the scattering system in its interaction with the environment, due to the emission-absorption of soft photons in fully unitary processes., Comment: 23 pages, 4 figures

Non-stabilizerness - also colloquially referred to as magic - is the a resource for advantage in quantum computing and lies in the access to non-Clifford operations. Developing a comprehensive understanding of how non-stabilizerness can be quantified and how it relates other quantum resources is crucial for studying and characterizing the origin of quantum complexity. In this work, we establish a direct connection between non-stabilizerness and entanglement spectrum flatness for a pure quantum state. We show that this connection can be exploited to efficiently probe non-stabilizerness even in presence of noise. Our results reveal a direct connection between non-stabilizerness and entanglement response, and define a clear experimental protocol to probe non-stabilizerness in cold atom and solid-state platforms.

Stabilizer Entropy (SE) quantifies the spread of a state in the basis of Pauli operators. It is a computationally tractable measure of non-stabilizerness and thus a useful resource for quantum computation. SE can be moved around a quantum system, effectively purifying a subsystem from its complex features. We show that there is a phase transition in the residual subsystem SE as a function of the density of non-Clifford resources. This phase transition has important operational consequences: it marks the onset of a subsystem purity estimation protocol that requires $poly(n)exp(t)$ many queries to a circuit containing $t$ non-Clifford gates that prepares the state from a stabilizer state. Then, for $t=O(\log_2 n)$, it estimates the purity with polynomial resources and, for highly entangled states, attains an exponential speed-up over the known state-of-the-art algorithms.

Scrambling of quantum information is an important feature at the root of randomization and benchmarking protocols, the onset of quantum chaos, and black-hole physics. Unscrambling this information is possible given perfect knowledge of the scrambler [arXiv:1710.03363.]. We show that one can retrieve the scrambled information even without any previous knowledge of the scrambler, by a learning algorithm that allows the building of an efficient decoder. Remarkably, the decoder is classical in the sense that it can be efficiently represented on a classical computer as a Clifford operator. It is striking that a classical decoder can retrieve with fidelity one all the information scrambled by a random unitary that cannot be efficiently simulated on a classical computer, as long as there is no full-fledged quantum chaos. This result shows that one can learn the salient properties of quantum unitaries in a classical form, and sheds a new light on the meaning of quantum chaos. Furthermore, we obtain results concerning the algebraic structure of $t$-doped Clifford circuits, i.e., Clifford circuits containing t non-Clifford gates, their gate complexity, and learnability that are of independent interest. In particular, we show that a $t$-doped Clifford circuit $U_t$ can be decomposed into two Clifford circuits $U_{0},U^{\prime}_0$ that sandwich a local unitary operator $u_t$, i.e., $U_t=U_{0} u_{t}U_{0}^{\prime}$. The local unitary operator $u_t$ contains $t$ non-Clifford gates and acts nontrivially on at most $t$ qubits. As simple corollaries, the gate complexity of the $t$-doped Clifford circuit $U_t$ is $O(n^2+t^3)$, and it admits a efficient process tomography using $\mathrm{poly}(n,2^t)$ resources., Comment: Corrected the typos and emphasized several results on learning Clifford circuits that were previously overlooked in the previous version

Quantum information scrambling is a unitary process that destroys local correlations and spreads information throughout the system, effectively hiding it in nonlocal degrees of freedom. In principle, unscrambling this information is possible with perfect knowledge of the unitary dynamics [B. Yoshida and A. Kitaev, arXiv:1710.03363.]. However, this Letter demonstrates that even without previous knowledge of the internal dynamics, information can be efficiently decoded from an unknown scrambler by monitoring the outgoing information of a local subsystem. Surprisingly, we show that scramblers with unknown internal dynamics, which are rapidly mixing but not fully chaotic, can be decoded using Clifford decoders. The essential properties of a scrambling unitary can be efficiently recovered, even if the process is exponentially complex. Specifically, we establish that a unitary operator composed of $t$ non-Clifford gates admits a Clifford decoder up to $t\le n$.

In this paper we study the transitions of entanglement complexity in an exemplary family of states - the Rokhsar-Kivelson-sign wavefunctions - whose degree of entanglement is controlled by a single parameter. This family of states is known to feature a transition between a phase exhibiting volume-law scaling of entanglement entropy and a phase with sub-extensive scaling of entanglement, reminiscent of the many-body-localization transition of disordered quantum Hamiltonians [Physical Review B 92, 214204 (2015)]. We study the singularities of the Rokhsar-Kivelson-sign wavefunctions and their entanglement complexity across the transition using several tools from quantum information theory: fidelity metric; entanglement spectrum statistics; entanglement entropy fluctuations; stabilizer R\'enyi Entropy; and the performance of a disentangling algorithm. Across the whole volume-law phase the states feature universal entanglement spectrum statistics. Yet a "super-universal" regime appears for small values of the control parameter in which all metrics become independent of the parameter itself; the entanglement entropy as well as the stabilizer R\'enyi entropy appear to approach their theoretical maximum; the entanglement fluctuations scale to zero as in output states of random universal circuits, and the disentangling algorithm has essentially null efficiency. All these indicators consistently reveal a complex pattern of entanglement. In the sub-volume-law phase, on the other hand, the entanglement spectrum statistics is no longer universal, entanglement fluctuations are larger and exhibiting a non-universal scaling; and the efficiency of the disentangling algorithm becomes finite. Our results, based on model wavefunctions, suggest that a similar combination of entanglement scaling properties and of entanglement complexity features may be found in high-energy Hamiltonian eigenstates., Comment: Journal version. Added 1 paragraph and 1 image

We advance the characterization of complexity in quantum many-body systems by examining $W$-states embedded in a spin chain. Such states show an amount of non-stabilizerness or "magic" (measured as the Stabilizer R\'enyi Entropy -SRE-) that grows logarithmic with the number of qubits/spins. We focus on systems whose Hamiltonian admits a classical point with an extensive degeneracy. Near these points, a Clifford circuit can convert the ground state into a $W$-state, while in the rest of the phase to which the classic point belongs, it is dressed with local quantum correlations. Topological frustrated quantum spin-chains host phases with the desired phenomenology, and we show that their ground state's SRE is the sum of that of the $W$-states plus an extensive local contribution. Our work reveals that $W$-states/frustrated ground states display a non-local degree of complexity that can be harvested as a quantum resource and has no counterpart in GHZ states/non-frustrated systems., Comment: 8 pages, 3 figures

The ground state degeneracy of topologically ordered gapped Hamiltonians is the bedrock for a quantum error-correcting code with macroscopic distance, which is unfortunately not stable away from equilibrium. In this work, we show that the presence of a bounded lightcone preserves topological order. As a quantum code, the initial ground space will keep its macroscopic distance during unitary time evolution. We also show how a bounded lightcone can emerge through suitable perturbations in the two-dimensional toric code. Our results suggest that topological quantum memory can be dynamically robust at zero temperature., Comment: 6+3 pages, 4 figures, the Supplemental Material is updated. arXiv admin note: substantial text overlap with arXiv:2102.03046

In a seminal paper[JHEP09(2007)120], Hayden and Preskill showed that information can be retrieved from a black hole that is sufficiently scrambling, assuming that the retriever has perfect control of the emitted Hawking radiation and perfect knowledge of the internal dynamics of the black hole. In this paper, we show that for $t-$doped Clifford black holes - that is, black holes modeled by random Clifford circuits doped with an amount $t$ of non-Clifford resources - an information retrieval decoder can be learned with fidelity scaling as $\exp(-\alpha t)$ using quantum machine learning while having access only to out-of-time-order correlation functions. We show that the crossover between learnability and non-learnability is driven by the amount of non-stabilizerness present in the black hole and sketch a different approach to quantum complexity.

Ground states of quantum many-body systems are both entangled and possess a kind of quantum complexity as their preparation requires universal resources that go beyond the Clifford group and stabilizer states. These resources - sometimes described as magic - are also the crucial ingredient for quantum advantage. We study the behavior of the stabilizer R\'enyi entropy in the integrable transverse field Ising spin chain. We show that the locality of interactions results in a localized stabilizer R\'enyi entropy in the gapped phase thus making this quantity computable in terms of local quantities in the gapped phase, while measurements involving $L$ spins are necessary at the critical point to obtain an error scaling with $O(L^{-1})$.

In conditional automation, a response from the driver is expected when a take over request is issued due to unexpected events, emergencies, or reaching the operational design domain boundaries. Cooperation between the automated driving system and the driver can help to guarantee a safe and pleasant transfer if the driver is guided through a haptic guidance system that applies a slight counter-steering force to the steering wheel. We examine in this work the impact of haptic guidance systems on driving performance after a take over request was triggered to avoid sudden obstacles on the road. We studied different driver conditions that involved Non-Driving Related Tasks (NRDT). Results showed that haptic guidance systems increased road safety by reducing the lateral error, the distance and reaction time to a sudden obstacle and the number of collisions.

In this work, we show how the resource theory of nonstabilizerness quantifies the hardness of direct fidelity estimation protocols. In particular, the resources needed for a direct fidelity estimation conducted on generic states, such as Pauli fidelity estimation and shadow fidelity estimation protocols, grow exponentially with the stabilizer R\'enyi entropy. Remarkably, these protocols are shown to be feasible only for those states that are useless to attain any quantum speedup or advantage. This result suggests the impossibility of estimating efficiently fidelity for generic states and, at the same time, leaves the window open to those protocols specialized at directly estimating the fidelity of particular states. We then extend our results to quantum evolutions, showing that the resources needed to certify the quality of the implementation of a given unitary $U$ are governed by the nonstabilizerness in the Choi state associated with $U$, which is shown to possess a profound connection with out-of-time order correlators.

Magic states are the resource that allows quantum computers to attain an advantage over classical computers. This resource consists in the deviation from a property called stabilizerness which in turn implies that stabilizer circuits can be efficiently simulated on a classical computer. Without magic, no quantum computer can do anything that a classical computer cannot do. Given the importance of magic for quantum computation, it would be useful to have a method for measuring the amount of magic in a quantum state. In this work, we propose and experimentally demonstrate a protocol for measuring magic based on randomized measurements. Our experiments are carried out on two IBM Quantum Falcon processors. This protocol can provide a characterization of the effectiveness of a quantum hardware in producing states that cannot be effectively simulated on a classical computer. We show how from these measurements one can construct realistic noise models affecting the hardware., Comment: Salvatore F.E. Oliviero and Lorenzo Leone contributed equally to this paper

Entanglement is the defining characteristic of quantum mechanics. Bipartite entanglement is characterized by the von Neumann entropy. Entanglement is not just described by a number, however; it is also characterized by its level of complexity. The complexity of entanglement is at the root of the onset of quantum chaos, universal distribution of entanglement spectrum statistics, hardness of a disentangling algorithm and of the quantum machine learning of an unknown random circuit, and universal temporal entanglement fluctuations. In this paper, we numerically show how a crossover from a simple pattern of entanglement to a universal, complex pattern can be driven by doping a random Clifford circuit with $T$ gates. This work shows that quantum complexity and complex entanglement stem from the conjunction of entanglement and non-stabilizer resources, also known as magic.

Local Convertibility refers to the possibility of transforming a given state into a target one, just by means of LOCC with respect to a given bipartition of the system and it is possible if and only if all the Renyi-entropies of the initial state are smaller than those of the target state. We apply this concept to adiabatic evolutions and ask whether they can be rendered through LOCC in the sense above. We argue that a lack of differential local convertibility (dLC) signals a higher computational power of the system's quantum phase, which is also usually connected with the existence of long-range entanglement, topological order, or edge-states. Remarkably, dLC can detect these global properties already by considering small subsystems. Moreover, we connect dLC to spontaneous symmetry breaking by arguing that states with finite order parameters must be the most classical ones and thus be locally convertible., Comment: Chapter for the Springer Volume "Entanglement in Spin Chains - Theory and Quantum Technology Applications", 33 pages, many figures, typos corrected. arXiv admin note: text overlap with arXiv:1604.06403, arXiv:1306.6685

Random quantum states and operations are of fundamental and practical interests. In this work, we investigate the entanglement properties of random hypergraph states, which generalize the notion of graph states by applying generalized controlled-phase gates on an initial reference product state. In particular, we study the two ensembles generated by random Controlled-Z(CZ) and Controlled-Controlled-Z(CCZ) gates, respectively. By applying tensor network representation and combinational counting, we analytically show that the average subsystem purity and entanglement entropy for the two ensembles feature the same volume law, but greatly differ in typicality, namely the purity fluctuation is small and universal for the CCZ ensemble while it is large for the CZ ensemble. We discuss the implications of these results for the onset of entanglement complexity and quantum chaos., Comment: 14+8 pages,9 figures,published version

To validate the safety of automated vehicles (AV), scenario-based testing aims to systematically describe driving scenarios an AV might encounter. In this process, continuous inputs such as velocities result in an infinite number of possible variations of a scenario. Thus, metamodels are used to perform analyses or to select specific variations for examination. However, despite the safety criticality of AV testing, metamodels are usually seen as a part of an overall approach, and their predictions are not questioned. This paper analyzes the predictive performance of Gaussian processes (GP), deep Gaussian processes, extra-trees, and Bayesian neural networks (BNN), considering four scenarios with 5 to 20 inputs. Building on this, an iterative approach is introduced and evaluated, which allows to efficiently select test cases for common analysis tasks. The results show that regarding predictive performance, the appropriate selection of test cases is more important than the choice of metamodels. However, the choice of metamodels remains crucial: Their great flexibility allows BNNs to benefit from large amounts of data and to model even the most complex scenarios. In contrast, less flexible models like GPs convince with higher reliability. Hence, relevant test cases are best explored using scalable virtual test setups and flexible models. Subsequently, more realistic test setups and more reliable models can be used for targeted testing and validation., Comment: 10 pages, 14 figures, 1 table, associated dataset at https://github.com/wnklmx/DSIOD