In this work, we show that, by exploiting continuous quantum nondemolition measurement, it is possible to preserve quantum advantage in a frequency estimation (or magnetometry) measurement scheme even in the presence of independent dephasing noise, usually the most detrimental type of noise. We thus verify that such enhancement is preserved thanks to non-classical correlations, namely spin...
We address the properties of continuous-time quantum walks with Hamiltonians of the form $H= L + \lambda L^2$, being $L$ the Laplacian matrix of the underlying graph and being the perturbation $\lambda L^2$ motivated by its potential use to introduce next-nearest-neighbor hopping. We consider cycle, complete, and star graphs because paradigmatic models with low/high connectivity and/or...
We provide a rigorous quantitative analysis of super-resolution imaging techniques which exploit temporal fluctuations of luminosity of the sources in order to beat the Rayleigh limit. We define an operationally justified resolution gain figure of merit, that allows us to connect the estimation theory concepts with the ones typically used in the imaging community, and derive fundamental...
Quantum Metrology is one of the most important quantum technologies where quantum resources are exploited to enhance the estimation of unknown parameters [1]. In this context, since realistic scenarios generally involve more than one parameter, quantum multiparameter estimation is a central and very active research area. Nevertheless, in such relatively new field, several open questions are...
Bayesian estimation is a powerful theoretical paradigm for the operation of quantum sensors. However, the Bayesian method for statistical inference generally suffers from demanding calibration requirements, that have so far restricted its use to systems that can be explicitly modelled. In this theoretical study, we formulate parameter estimation as a classification task and use artificial...
Quantum (multi-)parameter estimation provides the central ingredient for many quantum technological tasks like, e.g., quantum computation or precision measurements. Previous work focussed mainly on single phase estimation at the fundamental limit, the Heisenberg limit, or on multiphase estimation at an optimal point. Here, we propose a quantum algorithm to measure d completely unknown phases...
Physical systems close to a quantum phase transition exhibit a divergent susceptibility, suggesting that an arbitrarily high precision may be achieved by exploiting quantum critical systems as probes to estimate a physical parameter. However, such an improvement in sensitivity is counterbalanced by the closing of the energy gap, which implies a critical slowing down and an inevitable growth of...
A central tenant in the classification of phases is that boundary conditions cannot affect the bulk properties of a system. In our works, we show striking, yet puzzling, evidence of a clear violation of this assumption. We study some exactly solvable spin chains, mappable to free fermions, in a ring geometry with an odd number of sites. In such a setting, even at finite sizes, we are able to...
The topology of one-dimensional chiral systems is captured by the winding number of the Hamiltonian eigenstates. We proved that this invariant can be read-out by measuring the Mean Chiral Displacement of a single-particle wavefunction that is connected to a fully localized one via a unitary and translation-invariant map. Remarkably, this implies that the Mean Chiral Displacement can detect the...
We identify a class of dressed atom-photon states forming at the same energy of the atom at any coupling strength. As a hallmark, their photonic component is an eigenstate of the bare photonic bath with a vacancy in place of the atom. The picture allows to formalize and re-interpret all quantum optics phenomena where atoms behave as perfect mirrors, connecting in particular dressed bound...
We have studied the complete spectrum of spin-1/2 XXZ chain at root of unity, i.e. a paradigmatic model of quantum integrability. Making use of transfer matrix fusion relation, we constructed a family of 2-parameter transfer matrices, which help us obtain all the eigenstates in terms of Bethe roots. This elucidates a long-standing problem dated from the debate between McCoy and Baxter. We also...
A tool capable to efficiently generate realistic structural models of disordered systems has been a goal of material science for many years. We show the feasibility of quantum annealing in exploring the energy landscape of materials that deviate from the ideal crystalline phase, specifically vacancy defects in graphene and disordered silicon. By mapping the competing interactions onto...
Quantum annealers have grown in complexity to the point that devices with few thousand qubits are approaching capacities to tackle material science problems. Starting from a representation of crystal structures in terms of networks, we develop models of order-disorder phase transitions for two prototypical classes of materials (entropy stabilized alloys and perovskites) that are directly...
This talk introduces an open-source package for error-mitigation in quantum computation using zero-noise extrapolation. Error-mitigation techniques improve computational performance (with respect to noise) using minimal overhead in quantum resources by relying on a mixture of quantum sampling and classical post-processing techniques. Our error-mitigation package interfaces with multiple...
We give a converging semidefinite programming hierarchy of outer approximations for the set of quantum correlations of fixed dimension. Starting from the Navascués-Pironio-Acín (NPA) hierarchy for general quantum correlations, we identify additional semidefinite constraints for any fixed dimension, leading to analytical bounds on the convergence speed of the resulting hierarchy. Additionally,...
Initialization of composite quantum systems into highly entangled states is usually a must to enable their use for quantum technologies. However, unavoidable noise in the preparation stage makes the system state mixed, hindering this goal. Here, we address this problem in the context of identical particle systems within the operational framework of spatially localized operations and classical...
Entanglement is a well defined and useful notion for distinguishable particles. It provides a framework of locality and can be used as a resource in quantum information and communication protocols. However, for identical particles, no universal accepted definition exists. The symmetrization principle makes identical particle states look entangled when written in first quantization notation. In...
Variational hybrid quantum-classical optimization represents one of the most promising avenue to show the advantage of nowadays noisy intermediate-scale quantum computers in solving hard problems, such as finding the minimum-energy state of a Hamiltonian or solving some machine-learning tasks. In these devices noise is unavoidable and impossible to error-correct, yet its role in the...
The capability to control and manipulate high dimensional quantum states has become relevant in several fields ranging from the probing of fundamentals of quantum mechanics to the development of safer encryption algorithms. Various engineering techniques of high dimensional quantum states have been proposed, but they strongly depend on the experimental platform and do not provide a general...
We propose a generalization of the Wasserstein distance of order 1 to the quantum states of n qudits. Our proposal recovers the classical Wasserstein distance for quantum states diagonal in the canonical basis, hence the distance between vectors of the canonical basis coincides with the Hamming distance. Our distance is invariant with respect to permutations of the qudits and unitary...
The class of incoherent operations induces a pre-order on the set of quantum pure states. We study the maximal success probability of incoherent conversion between pairs of n-dimensional random pure states chosen independently, and find an explicit formula for its large-n asymptotic distribution. Our analysis shows that the statistics of the maximal conversion probability can be determined by...
The compatibility-hypergraph approach to contextuality (CA) and the contextuality-by-default approach (CbD) are usually seen as products of entirely different views on how physical measurements and measurement contexts should be understood: the latter is based on the idea that a physical measurement has to be seen as a collection of random variables, one for each context containing that...
