Distinguishing unitary gates on the IBM quantum processor

An unknown unitary gates, which is secretly chosen from several known ones, can always be distinguished perfectly. In this paper, we implement such a task on IBM's quantum processor. More precisely, we experimentally demonstrate the discrimination of two qubit unitary gates, the identity gate and the 23π-phase shift gate, using two discrimination schemes -- the parallel scheme and the sequential scheme. We program these two schemes on the \emph{ibmqx4}, a 5-qubit superconducting quantum processor via IBM cloud, with the help of the QSI modules [S. Liu et al.,~arXiv:1710.09500, 2017]. We report that both discrimination schemes achieve success probabilities at least 85%

Machine learning on quantum experimental data toward solving quantum many-body problems

Advancements in the implementation of quantum hardware have enabled the acquisition of data that are intractable for emulation with classical computers. The integration of classical machine learning (ML) algorithms with these data holds potential for unveiling obscure patterns. Although this hybrid approach extends the class of efficiently solvable problems compared to using only classical computers, this approach has been realized for solving restricted problems because of the prevalence of noise in current quantum computers. Here, we extend the applicability of the hybrid approach to problems of interest in many-body physics, such as predicting the properties of the ground state of a given Hamiltonian and classifying quantum phases. By performing experiments with various error-reducing procedures on superconducting quantum hardware with 127 qubits, we managed to acquire refined data from the quantum computer. This enabled us to demonstrate the successful implementation of classical ML algorithms for systems with up to 44 qubits. Our results verify the scalability and effectiveness of the classical ML algorithms for processing quantum experimental data.Comment: 25 pages, 5 figures; supplementary information 35 pages, 17 figures, 1 tabl

Quantum leakage detection using a model-independent dimension witness

Users of quantum computers must be able to confirm they are indeed functioning as intended, even when the devices are remotely accessed. In particular, if the Hilbert space dimension of the components are not as advertised -- for instance if the qubits suffer leakage -- errors can ensue and protocols may be rendered insecure. We refine the method of delayed vectors, adapted from classical chaos theory to quantum systems, and apply it remotely on the IBMQ platform -- a quantum computer composed of transmon qubits. The method witnesses, in a model-independent fashion, dynamical signatures of higher-dimensional processes. We present evidence, under mild assumptions, that the IBMQ transmons suffer state leakage, with a $p$ value no larger than $5{\times}10^{-4}$ under a single qubit operation. We also estimate the number of shots necessary for revealing leakage in a two-qubit system.Comment: 11 pages, 5 figure

QPCF: higher order languages and quantum circuits

qPCF is a paradigmatic quantum programming language that ex- tends PCF with quantum circuits and a quantum co-processor. Quantum circuits are treated as classical data that can be duplicated and manipulated in flexible ways by means of a dependent type system. The co-processor is essentially a standard QRAM device, albeit we avoid to store permanently quantum states in between two co-processor's calls. Despite its quantum features, qPCF retains the classic programming approach of PCF. We introduce qPCF syntax, typing rules, and its operational semantics. We prove fundamental properties of the system, such as Preservation and Progress Theorems. Moreover, we provide some higher-order examples of circuit encoding

Quantum computing on the IBM processor

We analyze the basic aspects of the quantum information theory, namely the quantum version of the theory that is behind all the classical implementations like computers and communications. We focus on quantum computation, defining its fundamental unit: the qubit, namely a quantum two-level system that will be described in detail in this thesis. We then concentrate on the possible transformations gates that can be applied to qubits, both unitary and non-unitary ones. The challenges in the construction of qubits are impressive: in particular, it is difficult to isolate the system from the environment. This interaction is modeled by non-unitary transformations. The characteristic times connected to these processes are the relaxation time, the time in which a state decays in another state, and the decoherence time, the time in which quantum coherence is lost. These times are fundamental in quantum computing since they quantify how many operations can be performed on qubits and still obtain reliable results. We measure these characteristic times on the IBM quantum processor ’ibmq_16_melbourne’ performing three different experiments: one for the relaxation time and two for the decoherence time, namely Ramsey and Echo experiments. Finally, we implement a quantum algorithm, an algorithm that uses the resources of quantum mechanics, like entanglement. We focus on finding a protocol to define the quantum version of the elementary cellular automata, the quantum elementary cellular automata, and run it on the IBM processor. They are dynamical systems defined on a lattice in which the evolution is defined using simple local update rules. Comparing the physical results with theoretical expectations and noise-affected simulations we show the performance of the IBM processor