Quantum circuit

Summary

In quantum information theory, a quantum circuit is a model for quantum computation, similar to classical circuits, in which a computation is a sequence of quantum gates, measurements, initializations of qubits to known values, and possibly other actions. The minimum set of actions that a circuit needs to be able to perform on the qubits to enable quantum computation is known as DiVincenzo's criteria. Circuits are written such that the horizontal axis is time, starting at the left hand side and ending at the right. Horizontal lines are qubits, doubled lines represent classical bits. The items that are connected by these lines are operations performed on the qubits, such as measurements or gates. These lines define the sequence of events, and are usually not physical cables. The graphical depiction of quantum circuit elements is described using a variant of the Penrose graphical notation. Richard Feynman used an early version of the quantum circuit notation in 1986. Reversible

Official source

https://en.wikipedia.org/wiki/Quantum_circuit

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Practical Compilation of Quantum Programs

Bruno Schmitt Antunes

It's been a little more than 40 years since researchers first suggested exploiting quantum physics to build more powerful computers. During this time, we have seen the development of many quantum algorithms and significant technological advances to make these devices. As a result, at this point, large-scale quantum computers, capable of providing valuable solutions to complex problems, seem like a certainty, even if still distant.Nonetheless, there is a great gap between the communities of quantum algorithms and quantum devices researchers. On the one hand, algorithm designers most often describe algorithms in a high level of abstraction, using a mixture of natural language, pseudocode, and mathematical formulas---a form deriving asymptotic complexity estimates while shielding researchers from the low-level complexities and restrictions. On the other hand, physical devices only understand algorithms implemented using the primitive low-level abstractions they support.Most programming systems available for quantum computing are intertwined with the quantum circuit model, so developers must implement algorithms in terms of low-level unitary operators. Not surprisingly, the implementation of quantum algorithms on such a low level of abstraction is very time-consuming, error-prone, and results in non-portable programs---given the technological diversity of quantum devices. In this thesis, I study problems related to the compilation of quantum programs, seeking forms of augmenting the expressive power of current frameworks and narrowing the gap between algorithmic research and concrete implementations. I focus on scalability and practicality---in particular, together with theoretical investigations, I developed concrete algorithms that are performant and scalable. The embodiment of my research contribution is a compiler companion library for the synthesis and compilation of quantum circuits called tweedledum.

EPFL2022

Experimental QND measurements of complementarity on two-qubit states with IonQ and IBM Q quantum computers

Christophe Marcel Georges Galland, Nicolas Schwaller, Valeria Vento

We report the experimental nondemolition measurement of coherence, predictability and concurrence on a system of two qubits. The quantum circuits proposed by De Melo et al. (Phys Rev Lett 98(25):250501, 2007) are implemented on IBM Q (superconducting circuit) and IonQ (trapped ion) quantum computers. Three criteria are used to compare the performance of the different machines on this task: measurement accuracy, nondemolition of the observable, and quantum state preparation. We find that the IonQ quantum computer provides constant state fidelity through the nondemolition process, outperforming IBM Q systems on which the fidelity consequently drops after the measurement. Our study compares the current performance of these two technologies at different stages of the nondemolition measurement of bipartite complementarity.

SPRINGER2022

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We introduce a novel hybrid algorithm to simulate the real-time evolution of quantum systems using parameterized quantum circuits. The method, named "projected - Variational Quantum Dynamics" (p-VQD) realizes an iterative, global projection of the exact time evolution onto the parameterized manifold. In the small time step limit, this is equivalent to the McLachlan's variational principle. Our approach is efficient in the sense that it exhibits an optimal linear scaling with the total number of variational parameters. Furthermore, it is global in the sense that it uses the variational principle to optimize all parameters at once. The global nature of our approach then significantly extends the scope of existing efficient variational methods, that instead typically rely on the iterative optimisation of a restricted subset of variational parameters. Through numerical experiments, we also show that our approach is particularly advantageous over existing global optimisation algorithms based on the time-dependent variational principle that, due to a demanding quadratic scaling with parameter numbers, are unsuitable for large parameterized quantum circuits.

VEREIN FORDERUNG OPEN ACCESS PUBLIZIERENS QUANTENWISSENSCHAF2021

Practical Compilation of Quantum Programs

Bruno Schmitt Antunes

EPFL2022