156 research outputs found
Model checking quantum protocols
This thesis describes model checking techniques for protocols arising in quantum information
theory and quantum cryptography. We discuss the theory and implementation of a practical
model checker, QMC, for quantum protocols. In our framework, we assume that the quantum
operations performed in a protocol are restricted to those within the stabilizer formalism; while
this particular set of operations is not universal for quantum computation, it allows us to develop
models of several useful protocols as well as of systems involving both classical and quantum
information processing. We detail the syntax, semantics and type system of QMC’s modelling
language, the logic QCTL which is used for verification, and the verification algorithms that have
been implemented in the tool. We demonstrate our techniques with applications to a number of
case studies
Minimally complex ion traps as modules for quantum communication and computing
Optically linked ion traps are promising as components of network-based
quantum technologies, including communication systems and modular computers.
Experimental results achieved to date indicate that the fidelity of operations
within each ion trap module will be far higher than the fidelity of operations
involving the links; fortunately internal storage and processing can
effectively upgrade the links through the process of purification. Here we
perform the most detailed analysis to date on this purification task, using a
protocol which is balanced to maximise fidelity while minimising the device
complexity and the time cost of the process. Moreover we 'compile down' the
quantum circuit to device-level operations including cooling and shutting
events. We find that a linear trap with only five ions (two of one species,
three of another) can support our protocol while incorporating desirable
features such as 'global control', i.e. laser control pulses need only target
an entire zone rather than differentiating one ion from its neighbour. To
evaluate the capabilities of such a module we consider its use both as a
universal communications node for quantum key distribution, and as the basic
repeating unit of a quantum computer. For the latter case we evaluate the
threshold for fault tolerant quantum computing using the surface code, finding
acceptable fidelities for the 'raw' entangling link as low as 83% (or under 75%
if an additional ion is available).Comment: 15 pages, 8 figure
Advances in Bosonic Quantum Error Correction with Gottesman-Kitaev-Preskill Codes: Theory, Engineering and Applications
Encoding quantum information into a set of harmonic oscillators is considered
a hardware efficient approach to mitigate noise for reliable quantum
information processing. Various codes have been proposed to encode a qubit into
an oscillator -- including cat codes, binomial codes and
Gottesman-Kitaev-Preskill (GKP) codes. These bosonic codes are among the first
to reach a break-even point for quantum error correction. Furthermore, GKP
states not only enable close-to-optimal quantum communication rates in bosonic
channels, but also allow for error correction of an oscillator into many
oscillators. This review focuses on the basic working mechanism, performance
characterization, and the many applications of GKP codes, with emphasis on
recent experimental progress in superconducting circuit architectures and
theoretical progress in multimode GKP qubit codes and
oscillators-to-oscillators (O2O) codes. We begin with a preliminary
continuous-variable formalism needed for bosonic codes. We then proceed to the
quantum engineering involved to physically realize GKP states. We take a deep
dive into GKP stabilization and preparation in superconducting architectures
and examine proposals for realizing GKP states in the optical domain (along
with a concise review of GKP realization in trapped-ion platforms). Finally, we
present multimode GKP qubits and GKP-O2O codes, examine code performance and
discuss applications of GKP codes in quantum information processing tasks such
as computing, communication, and sensing.Comment: 77+5 pages, 31 figures. Minor bugs fixed in v2. comments are welcome
Topological Code Architectures for Quantum Computation
This dissertation is concerned with quantum computation using many-body quantum systems encoded in topological codes. The interest in these topological systems has increased in recent years as devices in the lab begin to reach the fidelities required for performing arbitrarily long quantum algorithms. The most well-studied system, Kitaev\u27s toric code, provides both a physical substrate for performing universal fault-tolerant quantum computations and a useful pedagogical tool for explaining the way other topological codes work. In this dissertation, I first review the necessary formalism for quantum information and quantum stabilizer codes, and then I introduce two families of topological codes: Kitaev\u27s toric code and Bombin\u27s color codes. I then present three chapters of original work. First, I explore the distinctness of encoding schemes in the color codes. Second, I introduce a model of quantum computation based on the toric code that uses adiabatic interpolations between static Hamiltonians with gaps constant in the system size. Lastly, I describe novel state distillation protocols that are naturally suited for topological architectures and show that they provide resource savings in terms of the number of required ancilla states when compared to more traditional approaches to quantum gate approximation
Mapping quantum circuits to shallow-depth measurement patterns based on graph states
The paradigm of measurement-based quantum computing (MBQC) starts from a
highly entangled resource state on which unitary operations are executed
through adaptive measurements and corrections ensuring determinism. This is set
in contrast to the more common quantum circuit model, in which unitary
operations are directly implemented through quantum gates prior to final
measurements. In this work, we incorporate concepts from MBQC into the circuit
model to create a hybrid simulation technique, permitting us to split any
quantum circuit into a classically efficiently simulatable Clifford-part and a
second part consisting of a stabilizer state and local (adaptive) measurement
instructions, a so-called standard form, which is executed on a quantum
computer. We further process the stabilizer state with the graph state
formalism, thus enabling a significant decrease in circuit depth for certain
applications. We show that groups of fully commuting operators can be
implemented using fully-parallel, i.e., non-adaptive, measurements within our
protocol. In addition, we discuss how such circuits can be implemented in
constant quantum depths by employing quantum teleportation. Finally, we
demonstrate the utility of our technique on two examples of high practical
relevance: the Quantum Approximate Optimization Algorithm (QAOA) and the
Variational Quantum Eigensolver (VQE)
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