Abstract: Systems and methods for estimating crosstalk in a quantum system are provided. The quantum system comprises a plurality of qudits. A plurality of interacting systems within the plurality of qudits during a quantum operation are identified. Each such interacting system comprises a subset of the plurality of qudits. A set of components is identified from the plurality of interacting systems. Each given component in the set of components is evolved along with the respective components in the set of components that interact with the given component in the quantum operation, thereby forming a plurality of maps for the set of components. For each respective component in the set of components, a corresponding marginal distribution is calculated using the corresponding map for the respective component, thereby computing a plurality of marginal distributions. An estimate of the Pauli error distribution is constructed for the quantum operation from the plurality of marginal distributions.

Abstract: For respective positive integer value n, performing an outer and inner procedures, for states sufficiently representing coherent-error in a quantum system. Inner procedure creates copies state entangling n-qudits using randomized compiling, and obtains measurements of n-qudits in a basis corresponding to f-states. An outcome bit string forms for the state from measurement. The outer procedure and the forming string repeats, obtaining outcome bit strings for each state. Error-rate is determined for each state using the outcome bit strings for the respective value n. The outer procedure through determining error-rate repeats for different n drawn from positive-integers, determines error-rate for each state for respective values n. For state-S, error-rate fits for each respective value-n of the state to a corresponding quadratic function, the error-rate as dependent variable and n as independent variable.

Abstract: Systems and methods for estimating a property of an error in a circuit implemented on an n-qubit quantum system are provided, where the circuit comprises a gate set that comprises a first subset () and a second subset () of elementary gates. The first subset comprises a third subset () of elementary gates each of which consists of an n-fold tensor product of a plurality of single qubit gates. A first procedure is executed that comprises preparing the system in a state ? and then applying D1=T1 to the system. The procedure further comprises, for each respective clock cycle t in clock cycles t?{2, . . . , m+1}, (a) applying H to the system, where H is an elementary gate in the second subset, and then (b) applying a gate Dt=TtGHTt?1†H† to the system, where Dt is an element of the first subset. The procedure further comprises performing a measurement readout R.

Abstract: Computer systems and methods for constructing a model of the noise afflicting a quantum computer comprising a plurality of qubits are provided. A graph G that describes a conditional independence structure of the noise is obtained. The graph G includes a node for each qubit in the plurality of qubits. The noise afflicting the quantum computer is logically reduced to Pauli noise. The graph G is broken into a plurality of sets. Each respective set Cj in the plurality of sets (i) corresponds a respective qubit j in the plurality of qubits and (ii) comprises a representation of the respective qubit j and the parent qubits ?+j in the graph G. For each respective set Cj in the plurality of sets, a corresponding local conditional probability distribution Pr(ej|e?+j) is characterized in which ej?j is a Pauli error on the jth qubit.