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Laser-assisted Fano resonance: attosecond quantum control and dynamical imaging. (arXiv:2302.04090v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 29 sec ago

A Fano resonance arises from the pathway interference between discrete and continuum states, playing a fundamental role in many branches of physics, chemistry and material science. Here, we introduce the concept of a laser-assisted Fano resonance, created from two interferometric pathways that are coupled together by an additional laser field, which introduces a controllable phase delay between them and results in a generalized Fano lineshape that can be actively controlled on the {\it attosecond} time scale. Based on our experimental results of unprecedented resolution, we dynamically image a resonant electron wave packet during its evolution directly in the time domain, extracting both the amplitude and the phase, which allows for the measurement of the {\it resonant} photoionization time delay. Ab-initio calculations and simulations employing a physically transparent two-level model agree with our experimental results, laying the groundwork for extending our concepts into attosecond quantum control of complex systems.

Categories: Journals, Physics

Generation of Pseudo-Random Quantum States on Actual Quantum Processors. (arXiv:2302.04101v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 29 sec ago

The generation of a large amount of entanglement is a necessary condition for a quantum computer to achieve quantum advantage. In this paper, we propose a method to efficiently generate pseudo-random quantum states, for which the degree of multipartite entanglement is nearly maximal. We argue that the method is optimal, and use it to benchmark actual superconducting (IBM's ibm_lagos) and ion trap (IonQ's Harmony) quantum processors. Despite the fact that ibm_lagos has lower single-qubit and two-qubit error rates, the overall performance of Harmony is better thanks to low error rate in state preparation and measurement and to the all-to-all connectivity of qubits. Our result highlights the relevance of the qubits network architecture to generate highly entangled state.

Categories: Journals, Physics

High-Harmonic Generation with a twist: all-optical characterization of magic-angle twisted bilayer graphene. (arXiv:2302.04127v1 [cond-mat.mes-hall])

arXiv.org: Quantum Physics - 22 min 29 sec ago

If we stack up two layers of graphene while changing their respective orientation by some twisting angle, we end up with a system that has striking differences when compared to single-layer graphene. For a very specific value of this twist angle, known as magic angle, twisted bilayer graphene displays a unique phase diagram that cannot be found in other systems. Recently, high harmonic generation spectroscopy has been successfully applied to elucidate the electronic properties of quantum materials. The purpose of the present work is to exploit the nonlinear optical response of magic-angle twisted bilayer graphene to unveil its electronic properties. We show that the band structure of magic-angle twisted bilayer graphene is imprinted onto its high-harmonic spectrum. Specifically, we observe a drastic decrease of harmonic signal as we approach the magic angle. Our results show that high harmonic generation can be used as a spectroscopy tool for measuring the twist angle and also the electronic properties of twisted bilayer graphene, paving the way for an all-optical characterization of moir\'e materials.

Categories: Journals, Physics

Separation of Wigner and Continuum-continuum Delays by Mirror-symmetry-broken Attosecond Interferometry. (arXiv:2302.04137v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 29 sec ago

Photoionization of matter is one of the fastest electronic processes in nature. Experimental measurements of photoionization dynamics have become possible through attosecond metrology. However, all experiments reported to date contain a so-far unavoidable measurement-induced contribution, known as continuum-continuum (CC) or Coulomb-laser-coupling delay. Exploiting the recently characterized circularly polarized attosecond pulse trains, we introduce the concept of mirror-symmetry-broken attosecond interferometry, which enables the direct and separate measurement of both the native one-photon ionization delays as well as the continuum-continuum delays. Our technique solves the longstanding challenge of experimentally isolating both the native one-photon-ionization (or Wigner) delays and the measurement-induced (CC) delays. This advance opens the door to a new generation of precision measurements that is likely to drive major progress in experimental and theoretical attosecond science with implications for benchmarking the accuracy of electronic-structure and electron-dynamics methods.

Categories: Journals, Physics

A simple coin for a $2d$ entangled walk. (arXiv:2302.04140v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 29 sec ago

We analyze the effect of a simple coin operator, built out of Bell pairs, in a $2d$ Discrete Quantum Random Walk (DQRW) problem. The specific form of the coin enables us to find analytical and closed form solutions to the recursion relations of the DQRW. The coin induces entanglement between the spin and position degrees of freedom, which oscillates with time and reaches a constant value asymptotically. We probe the entangling properties of the coin operator further, by two different measures. First, by integrating over the space of initial tensor product states, we determine the {\it Entangling Power} of the coin operator. Secondly, we compute the {\it Generalized Relative R\'{e}nyi Entropy} between the corresponding density matrices for the entangled state and the initial pure unentangled state. Both the {\it Entangling Power} and {\it Generalized Relative R\'{e}nyi Entropy} behaves similar to the entanglement with time. Finally, in the continuum limit, the specific coin operator reduces the $2d$ DQRW into two $1d$ massive fermions coupled to synthetic gauge fields, where both the mass term and the gauge fields are built out of the coin parameters.

Categories: Journals, Physics

An Energy Estimation Benchmark for Quantum Computers. (arXiv:2302.04144v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 29 sec ago

Certifying the performance of quantum computers requires standardized tests. We propose a simple energy estimation benchmark that is motivated from quantum chemistry. With this benchmark we statistically characterize the noisy outcome of the IBM Quantum System One in Ehningen, Germany. We find that the benchmark results hardly correlate with the gate errors and readout errors reported for the device. In a time-resolved analysis, we monitor the device over several hours and find two-hour oscillations of the benchmark results, as well as outliers, which we cannot explain from the reported device status. We then show that the implemented measurement error mitigation techniques cannot resolve these oscillations, which suggests that deviations from the theoretical benchmark outcome statistics do not stem solely from the measurement noise of the device.

Categories: Journals, Physics

Mean-field dynamics of open quantum systems with collective operator-valued rates: validity and application. (arXiv:2302.04155v1 [cond-mat.stat-mech])

arXiv.org: Quantum Physics - 22 min 29 sec ago

We consider a class of open quantum many-body Lindblad dynamics characterized by an all-to-all coupling Hamiltonian and by dissipation featuring collective ``state-dependent" rates. The latter encodes local incoherent transitions that depend on average properties of the system. This type of open quantum dynamics can be seen as a generalization of classical (mean-field) stochastic Markov dynamics, in which transitions depend on the instantaneous configuration of the system, to the quantum domain. We study the time evolution in the limit of infinitely large systems, and we demonstrate the exactness of the mean-field equations for the dynamics of average operators. We further derive the effective dynamical generator governing the time evolution of (quasi-)local operators. Our results allow for a rigorous and systematic investigation of the impact of quantum effects on paradigmatic classical models, such as quantum generalized Hopfield associative memories or (mean-field) kinetically-constrained models.

Categories: Journals, Physics

Optimal Sufficient Requirements on the Embedded Ising Problem in Polynomial Time. (arXiv:2302.04162v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 29 sec ago

One of the central applications for quantum annealers is to find the solutions of Ising problems. Suitable Ising problems, however, need to be formulated such that they, on the one hand, respect the specific restrictions of the hardware and, on the other hand, represent the original problems which shall actually be solved. We evaluate sufficient requirements on such an embedded Ising problem analytically and transform them into a linear optimization problem. With an objective function aiming to minimize the maximal absolute problem parameter, the precision issues of the annealers are addressed. Due to the redundancy of several constraints, we can show that the formally exponentially large optimization problem can be reduced and finally solved in polynomial time for the standard embedding setting where the embedded vertices induce trees. This allows to formulate provably equivalent embedded Ising problems in a practical setup.

Categories: Journals, Physics

Dynamical-Corrected Nonadiabatic Geometric Quantum Computation. (arXiv:2302.04167v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 29 sec ago

Recently, nonadiabatic geometric quantum computation has been received great attentions, due to its fast operation and intrinsic error resilience. However, compared with the corresponding dynamical gates, the robustness of implemented nonadiabatic geometric gates based on the conventional single-loop scheme still has the same order of magnitude due to the requirement of strict multi-segment geometric controls, and the inherent geometric fault-tolerance characteristic is not fully explored. Here, we present an effective geometric scheme combined with a general dynamical-corrected technique, with which the super-robust nonadiabatic geometric quantum gates can be constructed over the conventional single-loop and two-loop composite-pulse strategies, in terms of resisting the systematic error, i.e., $\sigma_x$ error. In addition, combined with the decoherence-free subspace (DFS) coding, the resulting geometric gates can also effectively suppress the $\sigma_z$ error caused by the collective dephasing. Notably, our protocol is a general one with simple experimental setups, which can be potentially implemented in different quantum systems, such as Rydberg atoms, trapped ions and superconducting qubits. These results indicate that our scheme represents a promising way to explore large-scale fault-tolerant quantum computation.

Categories: Journals, Physics

Generalizing Neural Wave Functions. (arXiv:2302.04168v1 [cs.LG])

arXiv.org: Quantum Physics - 22 min 30 sec ago

Recent neural network-based wave functions have achieved state-of-the-art accuracies in modeling ab-initio ground-state potential energy surface. However, these networks can only solve different spatial arrangements of the same set of atoms. To overcome this limitation, we present Graph-learned Orbital Embeddings (Globe), a neural network-based reparametrization method that can adapt neural wave functions to different molecules. We achieve this by combining a localization method for molecular orbitals with spatial message-passing networks. Further, we propose a locality-driven wave function, the Molecular Oribtal Network (Moon), tailored to solving Schr\"odinger equations of different molecules jointly. In our experiments, we find Moon requiring 8 times fewer steps to converge to similar accuracies as previous methods when trained on different molecules jointly while Globe enabling the transfer from smaller to larger molecules. Further, our analysis shows that Moon converges similarly to recent transformer-based wave functions on larger molecules. In both the computational chemistry and machine learning literature, we are the first to demonstrate that a single wave function can solve the Schr\"odinger equation of molecules with different atoms jointly.

Categories: Journals, Physics

On the algebraic approach to GUP in anisotropic space. (arXiv:2302.04170v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 30 sec ago

Motivated by current searches for signals of Lorentz symmetry violation in nature and recent investigations on generalized uncertainty principle (GUP) models in anisotropic space, in this paper we identify GUP models satisfying two criteria: (i) invariance of commutators under canonical transformations, and (ii) physical independence of position and momentum on the ordering of auxiliary operators in their definitions. Compliance of these criteria is fundamental if one wishes to unambiguously describe GUP using an algebraic approach but, surprisingly, neither is trivially satisfied when GUP is assumed within anisotropic space. As a consequence, we use these criteria to place important restrictions on what or how GUP models may be approached algebraically.

Categories: Journals, Physics

Multiobjective variational quantum optimization for constrained problems: an application to Cash Management. (arXiv:2302.04196v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 30 sec ago

Combinatorial optimization problems are ubiquitous in industry. In addition to finding a solution with minimum cost, problems of high relevance involve a number of constraints that the solution must satisfy. Variational quantum algorithms have emerged as promising candidates for solving these problems in the noisy intermediate-scale quantum stage. However, the constraints are often complex enough to make their efficient mapping to quantum hardware difficult or even infeasible. An alternative standard approach is to transform the optimization problem to include these constraints as penalty terms, but this method involves additional hyperparameters and does not ensure that the constraints are satisfied due to the existence of local minima. In this paper, we introduce a new method for solving combinatorial optimization problems with challenging constraints using variational quantum algorithms. We propose the Multi-Objective Variational Constrained Optimizer (MOVCO) to classically update the variational parameters by a multiobjective optimization performed by a genetic algorithm. This optimization allows the algorithm to progressively sample only states within the in-constraints space, while optimizing the energy of these states. We test our proposal on a real-world problem with great relevance in finance: the Cash Management problem. We introduce a novel mathematical formulation for this problem, and compare the performance of MOVCO versus a penalty based optimization. Our empirical results show a significant improvement in terms of the cost of the achieved solutions, but especially in the avoidance of local minima that do not satisfy any of the mandatory constraints.

Categories: Journals, Physics

Complete Graphical Language for Hermiticity-Preserving Superoperators. (arXiv:2302.04212v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 30 sec ago

Universal and complete graphical languages have been successfully designed for pure state quantum mechanics, corresponding to linear maps between Hilbert spaces, and mixed states quantum mechanics, corresponding to completely positive superoperators. In this paper, we go one step further and present a universal and complete graphical language for Hermiticity-preserving superoperators. Such a language opens the possibility of diagrammatic compositional investigations of antilinear transformations featured in various physical situations, such as the Choi-Jamio{\l}kowski isomorphism, spin-flip, or entanglement witnesses. Our construction relies on an extension of the ZW-calculus exhibiting a normal form for Hermitian matrices.

Categories: Journals, Physics

Incoherent amplification cannot compensate for nonclassicality loss in PT-symmetric systems. (arXiv:2302.04235v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 30 sec ago

We show that the loss of nonclassicality (including quantum entanglement) cannot be compensated by the (incoherent) amplification of PT-symmetric systems. We address this problem by manipulating the quantum fluctuating forces in the Heisenberg-Langevin approach. Specifically, we analyze the dynamics of two nonlinearly coupled oscillator modes in a PT-symmetric system. An analytical solution allows us to separate the contribution of reservoir fluctuations from the evolution of quantum statistical properties of the modes. In general, as reservoir fluctuations act constantly, the complete loss of nonclassicality and entanglement is observed for long times. To elucidate the role of reservoir fluctuations in a long-time evolution of nonclassicality and entanglement, we consider and compare the predictions from two alternative models in which no fatal long-time detrimental effects on the nonclassicality and entanglement are observed. This is so as, in the first semiclassical model, no reservoir fluctuations are considered at all. This, however, violates the fluctuation-dissipation theorem. The second, more elaborated, model obeys the fluctuation-dissipation relations as it partly involves reservoir fluctuations. However, to prevent from the above long-time detrimental effects, the reservoir fluctuations have to be endowed with the nonphysical properties of a sink model. In both models, additional incorporation of the omitted reservoir fluctuations results in their physically consistent behavior. This behavior, however, predicts the gradual loss of the nonclassicality and entanglement. Thus the effects of reservoir fluctuations related to damping cannot be compensated by those related to amplification.

Categories: Journals, Physics

Error-tolerant oblivious transfer in the noisy-storage model. (arXiv:2302.04236v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 30 sec ago

The noisy-storage model of quantum cryptography allows for information-theoretically secure two-party computation based on the assumption that a cheating user has at most access to an imperfect, noisy quantum memory, whereas the honest users do not need a quantum memory at all. This can be achieved through primitives such as Oblivious Transfer (OT). In general, the more noisy the quantum memory of the cheating user, the more secure the implementation of OT. For experimental implementations, one has to consider that also the devices held by the honest users are lossy and noisy, and error correction needs to be applied to correct these trusted errors. In general, this reduces the security of the protocol, since a cheating user may hide themselves in the trusted noise. Here we leverage known bounds on the security of OT to derive a tighter trade-off between trusted and untrusted noise.

Categories: Journals, Physics

Considerations of the duality in generalized coherent states formalism. (arXiv:2302.04245v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 30 sec ago

In the paper, a pair of dual operators is introduced with which a dual pair of generalized displacement operators is constructed. With these entities it is shown that the Barut - Girardello and Klauder - Perelomov generalized coherent states are dual states. The characteristics of these coherent states are constructed, separately and comparatively.

Categories: Journals, Physics

Quantum sensing of magnetic fields with molecular color centers. (arXiv:2302.04248v1 [cond-mat.mtrl-sci])

arXiv.org: Quantum Physics - 22 min 30 sec ago

Molecular color centers, such as $S=1$ Cr($o$-tolyl)$_{4}$, show promise as an adaptable platform for magnetic quantum sensing. Their intrinsically small size, i.e., 1-2 nm, enables them to sense fields at short distances and in various geometries. This feature, in conjunction with tunable optical read-out of spin information, offers the potential for molecular color centers to be a paradigm shifting materials class beyond diamond-NV centers by accessing a distance scale opaque to NVs. This capability could, for example, address ambiguity in the reported magnetic fields arising from two-dimensional magnets by allowing for a single sensing technique to be used over a wider range of distances. Yet, so far, these abilities have only been hypothesized with theoretical validation absent. We show through simulation that Cr($o$-tolyl)$_{4}$ can spatially resolve proximity-exchange versus direct magnetic field effects from monolayer CrI$_{3}$ by quantifying how these interactions impact the excited states of the molecule. At short distances, proximity exchange dominates through molecule-substrate interactions, but at further distances the molecule behaves as a typical magnetic sensor, with magnetostatic effects dominating changes to the energy of the excited state. Our models effectively demonstrate how a molecular color center could be used to measure the magnetic field of a 2D magnet and the role different distance-dependent interactions contribute to the measured field.

Categories: Journals, Physics

Scale-free localization and PT symmetry breaking from local non-Hermiticity. (arXiv:2302.04256v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 30 sec ago

We show that a local non-Hermitian perturbation in a Hermitian lattice system generically induces scale-free localization for the continuous-spectrum eigenstates. Furthermore, when the local non-Hermitian perturbation enjoys parity-time (PT) symmetry, the PT symmetry breaking of continuous spectrum is always accompanied by the emergence of scale-free localization. This type of PT symmetry breaking is highly sensitive to boundary conditions: The continuous spectrum of a periodic system undergoes a PT symmetry breaking as long as the non-Hermitian perturbation is strong enough; however, the counterpart under open boundary condition allows PT symmetry breaking only when the band structure satisfies certain condition that we unveil here. We also obtain the precise energy window in which the PT symmetry breaking is possible. Our results uncover a generic boundary-induced non-Hermitian phenomenon, which has unexpected interplay with PT symmetry.

Categories: Journals, Physics

Engineering Arbitrary Hamiltonians in Phase Space. (arXiv:2302.04257v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 30 sec ago

We introduce a general method to engineer arbitrary Hamiltonians in the Floquet phase space of a periodically driven oscillator, based on the non-commutative Fourier transformation (NcFT) technique. We establish the relationship between an arbitrary target Floquet Hamiltonian in phase space and the periodic driving potential in real space. We obtain analytical expressions for the driving potentials in real space that can generate novel Hamiltonians in phase space, e.g., rotational lattices and sharp-boundary well. Our protocol can be realised in a range of experimental platforms for nonclassical states generation and bosonic quantum computation.

Categories: Journals, Physics

Quantum Computation of Frequency-Domain Molecular Response Properties Using a Three-Qubit iToffoli Gate. (arXiv:2302.04271v1 [quant-ph])

arXiv.org: Quantum Physics - 22 min 30 sec ago

The quantum computation of molecular response properties on near-term quantum hardware is a topic of significant interest. While computing time-domain response properties is in principle straightforward due to the natural ability of quantum computers to simulate unitary time evolution, circuit depth limitations restrict the maximum time that can be simulated and hence the extraction of frequency-domain properties. Computing properties directly in the frequency domain is therefore desirable, but the circuits require large depth when the typical hardware gate set consisting of single- and two-qubit gates is used. Here, we report the experimental quantum computation of the response properties of diatomic molecules directly in the frequency domain using a three-qubit iToffoli gate, enabling a reduction in circuit depth by a factor of two. We show that the molecular properties obtained with the iToffoli gate exhibit comparable or better agreement with theory than those obtained with the native CZ gates. Our work is among the first demonstrations of the practical usage of a native multi-qubit gate in quantum simulation, with diverse potential applications to the simulation of quantum many-body systems on near-term digital quantum computers.

Categories: Journals, Physics
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