Enhancing qubit readout with Bayesian Learning

We introduce an efficient and accurate readout measurement scheme for single and multi-qubit states. Our method uses Bayesian inference to build an assignment probability distribution for each qubit state based on a reference characterization of the detector response functions. This allows us to account for system imperfections and thermal noise within the assignment of the computational basis. We benchmark our protocol on a quantum device with five superconducting qubits, testing initial state preparation for single and two-qubits states and an application of the Bernstein-Vazirani algorithm executed on five qubits. Our method shows a substantial reduction of the readout error and promises advantages for near-term and future quantum devices.Comment: 7 pages, 4 figure

Thermometry of ultracold atoms via non-equilibrium work distributions

Estimating the temperature of a cold quantum system is difficult. Usually, one measures a well-understood thermal state and uses that prior knowledge to infer its temperature. In contrast, we introduce a method of thermometry that assumes minimal knowledge of the state of a system and is potentially non-destructive. Our method uses a universal temperature-dependence of the quench dynamics of an initially thermal system coupled to a qubit probe that follows from the Tasaki-Crooks theorem for non-equilibrium work distributions. We provide examples for a cold-atom system, in which our thermometry protocol may retain accuracy and precision at subnanokelvin temperatures.Comment: Updated to published version. 6 pages plus 11 pages of supplemental material, and some numerical dat

Momentum-resolved and correlation spectroscopy using quantum probes

We address some key conditions under which many-body lattice models, intended mainly as simulated condensed-matter systems, can be investigated via immersed, fully controllable quantum objects, namely quantum probes. First, we present a protocol that, for a certain class of many-body systems, allows for full momentum-resolved spectroscopy using one single probe. Furthermore, we demonstrate how one can extract the two-point correlations using two entangled probes. We apply our theoretical proposal to two well-known exactly solvable lattice models, a one-dimensional (1D) Kitaev chain and 2D superfluid Bose-Hubbard model, and show its accuracy as well as its robustness against external noise

Sensor placement for combined sewer system monitoring in the Besòs river basin

In this paper, a sensor placement methodology for sewer systems monitoring in order to measure direct discharge to the river during intense rainfall events is presented. During these events, Combined Sewer Overflows (CSOs) may occur, causing serious problems of contamination of the corresponding receiving waters. The current national regulation compels sewer systems’ managers to monitor and quantify direct discharge to these receiving waters, in order to track these events. Hence, the selection of the appropriate sensor set in order to monitor the critical outlets of the network is of paramount importance to adequately monitor CSOs and minimize their effect by using the information gathered from these measurements. Here, a methodology considering relevant characteristics of each potential monitoring point —e.g. number of discharges, volume discharged or percentage of polluted mass— is defined to select the final sensor set. The presented methodology is applied to three different combined sewer systems in the Besòs river basin nearby Barcelona city area in Catalonia (Spain), i.e. Granollers, La Llagosta and Montornès systems.Peer ReviewedPostprint (published version

Emergence of anomalous dynamics from the underlying singular continuous spectrum in interacting many-body systems

We investigate the dynamical properties of an interacting many-body system with a nontrivial energy potential landscape that may induce a singular continuous single-particle energy spectrum. Focusing on the Aubry-Andre model, whose anomalous transport properties in the presence of interaction was recently demonstrated experimentally in an ultracold-gas setup, we discuss the anomalous slowing down of the dynamics it exhibits and show that it emerges from the singular-continuous nature of the single-particle excitation spectrum. Our study demonstrates that singular-continuous spectra can be found in interacting systems, unlike previously conjectured by treating the interactions in the mean-field approximation. This, in turns, also highlights the importance of the many-body correlations in giving rise to anomalous dynamics, which, in many-body systems, can result from a nontrivial interplay between geometry and interactions

Nonequilibrium quantum thermodynamics in Coulomb crystals

We present an in-depth study of the nonequilibrium statistics of the irreversible work produced during sudden quenches in proximity to the structural linear-zigzag transition of ion Coulomb crystals in 1+1 dimensions. By employing both an analytical approach based on a harmonic expansion and numerical simulations, we show the divergence of the average irreversible work in proximity to the transition. We show that the nonanalytic behavior of the work fluctuations can be characterized in terms of the critical exponents of the quantum Ising chain. Due to the technological advancements in trapped-ion experiments, our results can be readily verified

Statistics of orthogonality catastrophe events in localised disordered lattices

We address the phenomenon of statistical orthogonality catastrophe in insulating disordered systems. In more detail, we analyse the response of a system of non-interacting fermions to a local perturbation induced by an impurity. By inspecting the overlap between the pre- and post-quench many-body ground states we fully characterise the emergent statistics of orthogonality events as a function of both the impurity position and the coupling strength. We consider two well-known one-dimensional models, namely the Anderson and Aubry-Andre insulators, highlighting the arising differences. Particularly, in the Aubry-Andre model the highly correlated nature of the quasi-periodic potential produces unexpected features in how the orthogonality catastrophe occurs. We provide a quantitative explanation of such features via a simple, effective model. We further discuss the incommensurate ratio approximation and suggest a viable experimental verification in terms of charge transfer statistics and interferometric experiments using quantum probes

Reservoir engineering using quantum optimal control for qubit reset

We determine how to optimally reset a superconducting qubit which interacts with a thermal environment in such a way that the coupling strength is tunable. Describing the system in terms of a time-local master equation with time-dependent decay rates and using quantum optimal control theory, we identify temporal shapes of tunable level splittings which maximize the efficiency of the reset protocol in terms of duration and error. Time-dependent level splittings imply a modification of the system-environment coupling, varying the decay rates as well as the Lindblad operators. Our approach thus demonstrates efficient reservoir engineering employing quantum optimal control. We find the optimized reset strategy to consist in maximizing the decay rate from one state and driving non-adiabatic population transfer into this strongly decaying state

Tinnitus and equilibrium disorders in COVID-19 patients: preliminary results

Purpose: Tinnitus and equilibrium disorders such as dizziness and vertigo have been reported by patients with COVID-19; however, they have been rarely investigated. The aim of this study was to study the prevalence of subjective tinnitus and dizziness in a sample of COVID-19 patients using an online 10-item close-ended questionnaire. Methods: A multicentric study that included 15 Italian hospitals in different regions was conducted using an online 10-item close-ended questionnaire developed to identify the presence of tinnitus and balance disorders in patients with COVID-19 between May 5 and June 10, 2020. The questionnaire was administered to 185 patients in a period of > 30 – < 60 days after diagnosis of COVID-19; responses were recorded in an online Excel spreadsheet. The questionnaire was composed of three sections: (1) demographic information; (2) presence and characteristics of tinnitus and dizziness after COVID-19 diagnosis; (3) possible association with migraine. Results: Thirty-four patients (18.4%) reported equilibrium disorders after COVID-19 diagnosis. Of these, 32 patients reported dizziness (94.1%) and 2 (5.9%) reported acute vertigo attacks. Forty-three patients (23.2%) reported tinnitus; 14 (7.6%) reported both tinnitus and equilibrium disorders. Conclusion: This study suggests that the presence of subjective otoneurological symptoms such as tinnitus and balance disorders can affect COVID-19 patients; further studies are necessary to investigate the prevalence and pathophysiological mechanisms underlying these subjective symptoms in COVID-19 patients