83 research outputs found
An Overview of Approaches to Modernize Quantum Annealing Using Local Searches
I describe how real quantum annealers may be used to perform local (in state
space) searches around specified states, rather than the global searches
traditionally implemented in the quantum annealing algorithm. The quantum
annealing algorithm is an analogue of simulated annealing, a classical
numerical technique which is now obsolete. Hence, I explore strategies to use
an annealer in a way which takes advantage of modern classical optimization
algorithms, and additionally should be less sensitive to problem
mis-specification then the traditional quantum annealing algorithm.Comment: In Proceedings PC 2016, arXiv:1606.06513. An extended version of this
contribution will appear on arXiv soon which will describe more detailed
algorithms, comment more on robustness to problem mis-specification, comment
on thermal sampling applications, and discuss applications on real device
Graphical Structures for Design and Verification of Quantum Error Correction
We introduce a high-level graphical framework for designing and analysing
quantum error correcting codes, centred on what we term the coherent parity
check (CPC). The graphical formulation is based on the diagrammatic tools of
the zx-calculus of quantum observables. The resulting framework leads to a
construction for stabilizer codes that allows us to design and verify a broad
range of quantum codes based on classical ones, and that gives a means of
discovering large classes of codes using both analytical and numerical methods.
We focus in particular on the smaller codes that will be the first used by
near-term devices. We show how CSS codes form a subset of CPC codes and, more
generally, how to compute stabilizers for a CPC code. As an explicit example of
this framework, we give a method for turning almost any pair of classical
[n,k,3] codes into a [[2n - k + 2, k, 3]] CPC code. Further, we give a simple
technique for machine search which yields thousands of potential codes, and
demonstrate its operation for distance 3 and 5 codes. Finally, we use the
graphical tools to demonstrate how Clifford computation can be performed within
CPC codes. As our framework gives a new tool for constructing small- to
medium-sized codes with relatively high code rates, it provides a new source
for codes that could be suitable for emerging devices, while its zx-calculus
foundations enable natural integration of error correction with graphical
compiler toolchains. It also provides a powerful framework for reasoning about
all stabilizer quantum error correction codes of any size.Comment: Computer code associated with this paper may be found at
https://doi.org/10.15128/r1bn999672
Quantum computing for quantum tunneling
We demonstrate how quantum field theory problems can be practically encoded by using a discretization of the field theory problem into a general Ising model, with the continuous field values being encoded into Ising spin chains. To illustrate the method, and as a simple proof of principle, we use a (hybrid) quantum annealer to recover the correct profile of the thin-wall tunnelling solution. This method is applicable to many nonperturbative problems
Cycle discrete-time quantum walks on a noisy quantum computer
The rapid development of quantum computing has led to increasing interest in
quantum algorithms for a variety of different applications. Quantum walks have
also experienced a surge in interest due to their potential use in quantum
algorithms. Using the qiskit software package, we test how accurately the
current generation of quantum computers provided by IBM can simulate a cycle
discrete-time quantum walk. Implementing an 8-node, 8-step walk and a simpler
4-node, 4-step discrete-time quantum walk on an IBM quantum device known as
ibmq_quito, the results for each step of the respective walks are presented. A
custom noise model is developed in order to estimate that noise levels in the
ibmq_santiago quantum device would need to be reduced by at least 94% in order
to execute a 16-node, 16-step cycle discrete-time quantum walk to a reasonable
level of fidelity
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