Reducing resources for verification of quantum computations

We present two verification protocols where the correctness of a "target" computation is checked by means of "trap" computations that can be efficiently simulated on a classical computer. Our protocols rely on a minimal set of noise-free operations (preparation of eight single-qubit states or measurement of four observables, both on a single plane of the Bloch sphere) and achieve linear overhead. To the best of our knowledge, our protocols are the least demanding techniques able to achieve linear overhead. They represent a step towards further reducing the quantum requirements for verification.Comment: Accepted versio

Devising accreditation protocols for near-term quantum computing devices

In theory, perfect quantum computers can solve certain problems that are con- sidered intractable with classical computers. In practice, quantum computers are imperfect, since their internal components are afflicted by noise. The aim of this thesis is to devise protocols to verify the correctness of the outputs of quantum computations implemented on the Noisy Intermediate-Scale Quantum (NISQ) com- puting devices currently being built. We begin by optimizing some of the existing protocols based on interactive proof systems. Moving beyond these protocols (which are impractical for NISQ devices due to their overhead in qubits and gates), we then present a protocol (that we name \accreditation protocol") that encompasses all the main limitations of NISQ devices, including the limited availability of qubits and noisy gates. The accreditation protocol returns an upper-bound on the variation distance between ideal and noisy probability distributions of the outputs of an arbitrary quantum computation. Relying on the accuracy of single-qubit gates (which are the least noisy components in all currently available NISQ devices), the accreditation protocol can detect all types of noise in state-preparation, measurements and two-qubit gates. To conclude our work, we present a modified version of the accreditation protocol that relies on more assumptions on the noise (motivated by empirical evidence) and provides tighter bounds on the variation distance. Our accreditation protocols are scalable, unlike the protocols based on classi- cal simulations of quantum circuits. They are practical for implementation on NISQ devices, unlike the protocols based on interactive proof systems. Moreover, they can detect noise that may be missed by protocols based on quantum process tomogra- phy and randomized benchmarking. Thus, they represent the state-of-the-art of circuit characterization, and we expect them to be widely used in future quantum computations

Experimental accreditation of outputs of noisy quantum computers

We provide and experimentally demonstrate an accreditation protocol that upper-bounds the variation distance between noisy and noiseless probability distributions of the outputs of arbitrary quantum computations. We accredit the outputs of twenty-four quantum circuits executed on programmable superconducting hardware, ranging from depth nine circuits on ten qubits to depth twenty-one circuits on four qubits. Our protocol requires implementing the "target" quantum circuit along with a number of random Clifford circuits and subsequently post-processing the outputs of these Clifford circuits. Importantly, the number of Clifford circuits is chosen to obtain the bound with the desired confidence and accuracy, and is independent of the size and nature of the target circuit. We thus demonstrate a practical and scalable method of ascertaining the correctness of the outputs of arbitrary-sized noisy quantum computers--the ultimate arbiter of the utility of the computer itself.Comment: Accepted versio

Study of the thermal distribution for YBCO based Transition Edge Bolometers working above 77 K

ransition Edge Bolometers (TEB) are among the simplest radiation detectors. The straightforward operation mode provides good results only if it is combined with a careful thermal optimization.In a TEB, the strong dependence of the electrical resistivity on the temperature in its transition zone enables the detection of a variation of the local temperature which can reach tens of µK. For this reason, it is essential to study the thermal profile of the superconducting active part of the detector, hence its substrate, to make it as homogeneous as possible.Irradiated YBa 2 Cu 3 O 7-x (YBCO) films can be used for position sensitive detection of infrared radiation. A TEB with a double meander pattern, one of which with a reduced critical temperature due to irradiation with high-energy heavy ions, was designed to work in a portable cryostat at a temperature above the liquid nitrogen (LN 2 ) point.In this work, we present a series of Finite Element Method simulations (using COMSOL Multiphysics ® ) aimed at the optimization of the thermal distribution above the YBCO film. Once the optimal working point for the device is found, various materials for the bolometer hosting are tested to identify the combination that provides the most homogeneous temperature distribution. The optimal configurations are then analyzed in response to a sudden change in the PID current to determine the one which presents the best behavior in a transient situation

Accrediting outputs of noisy intermediate-scale quantum computing devices

We present an accreditation protocol for the outputs of noisy intermediate-scale quantum devices. By testing entire circuits rather than individual gates, our accreditation protocol can provide an upper-bound on the variation distance between noisy and noiseless probability distribution of the outputs of the target circuit of interest. Our accreditation protocol requires implementation of quantum circuits no larger than the target circuit, therefore it is practical in the near term and scalable in the long term. Inspired by trap-based protocols for the verification of quantum computations, our accreditation protocol assumes that noise in single-qubit gates is bounded (but potentially gate-dependent) in diamond norm. We allow for arbitrary spatial and temporal correlations in the noise affecting state preparation, measurements and two-qubit gates. We describe how to implement our protocol on real-world devices, and we also present a novel cryptographic protocol (which we call `mesothetic' protocol) inspired by our accreditation protocol.Comment: Accepted versio

Superconducting and hybrid magnetic shields: comparison between 3D modeling and experiment

International audienceThe use of superconducting (SC) materials is crucial for shielding quasi-static magnetic fields. However, a successful approach requires the availability of a modeling procedure that can be exploited to guide the shielding devices’ design. In this work, we applied a 3D numerical modeling method based on the vector-potential formulation to predict the shielding properties of a short SC hollow cylinder with and without the superimposition of a ferromagnetic tube in both axial- and transverse-field configuration. Calculation outcomes were then compared with experimental data obtained on the same shielding arrangements. The agreement between computed and experimental results validates the simulation outputs and opens to the exploitation of this modeling approach for designing more efficient shielding solutio

Superconducting and hybrid magnetic shields: comparison between 3D modeling and experiment

International audienceThe use of superconducting (SC) materials is crucial for shielding quasi-static magnetic fields. However, a successful approach requires the availability of a modeling procedure that can be exploited to guide the shielding devices’ design. In this work, we applied a 3D numerical modeling method based on the vector-potential formulation to predict the shielding properties of a short SC hollow cylinder with and without the superimposition of a ferromagnetic tube in both axial- and transverse-field configuration. Calculation outcomes were then compared with experimental data obtained on the same shielding arrangements. The agreement between computed and experimental results validates the simulation outputs and opens to the exploitation of this modeling approach for designing more efficient shielding solutio