201 research outputs found
Ancilla-Driven Universal Quantum Computation
We propose a method of manipulating a quantum register remotely with the help
of a single ancilla that steers the evolution of the register. The fully
controlled ancilla qubit is coupled to the computational register solely via a
fixed unitary two-qubit interaction, E, and then measured in suitable bases. We
characterize all interactions E that induce a unitary, step-wise deterministic
measurement back-action on the register sufficient to implement any arbitrary
quantum channel. Our scheme offers significant experimental advantages for
implementing computations, preparing states and performing generalized
measurements as no direct control of the register is required.Comment: 4 pages, 3 figure
After the rain - learning the lessons from flood recovery in Hull. Final project report for 'Flood, Vulnerability and Urban Resilience: a real-time study of local recovery following the floods of June 2007 in Hull'.
The ongoing experience of recovery for households in Hull – response to the Pitt Review Interim Report Learning the lessons from the 2007 floods, Chapter 9 of the Pitt Review Interim Report
Generalized Flow and Determinism in Measurement-based Quantum Computation
We extend the notion of quantum information flow defined by Danos and Kashefi
for the one-way model and present a necessary and sufficient condition for the
deterministic computation in this model. The generalized flow also applied in
the extended model with measurements in the X-Y, X-Z and Y-Z planes. We apply
both measurement calculus and the stabiliser formalism to derive our main
theorem which for the first time gives a full characterization of the
deterministic computation in the one-way model. We present several examples to
show how our result improves over the traditional notion of flow, such as
geometries (entanglement graph with input and output) with no flow but having
generalized flow and we discuss how they lead to an optimal implementation of
the unitaries. More importantly one can also obtain a better quantum
computation depth with the generalized flow rather than with flow. We believe
our characterization result is particularly essential for the study of the
algorithms and complexity in the one-way model.Comment: 16 pages, 10 figure
Generalized Flow and Determinism in Measurement-based Quantum Computation
We extend the notion of quantum information flow defined by Danos and Kashefi
for the one-way model and present a necessary and sufficient condition for the
deterministic computation in this model. The generalized flow also applied in
the extended model with measurements in the X-Y, X-Z and Y-Z planes. We apply
both measurement calculus and the stabiliser formalism to derive our main
theorem which for the first time gives a full characterization of the
deterministic computation in the one-way model. We present several examples to
show how our result improves over the traditional notion of flow, such as
geometries (entanglement graph with input and output) with no flow but having
generalized flow and we discuss how they lead to an optimal implementation of
the unitaries. More importantly one can also obtain a better quantum
computation depth with the generalized flow rather than with flow. We believe
our characterization result is particularly essential for the study of the
algorithms and complexity in the one-way model.Comment: 16 pages, 10 figure
Delocalization power of global unitary operations on quantum information
We investigate how originally localized two pieces of quantum information
represented by a tensor product of two unknown qudit states are delocalized by
performing two-qudit global unitary operations. To characterize the
delocalization power of global unitary operations on quantum information, we
analyze the necessary and sufficient condition to deterministically relocalize
one of the two pieces of quantum information to its original Hilbert space by
using only LOCC. We prove that this LOCC one-piece relocalization is possible
if and only if the global unitary operation is local unitary equivalent to a
controlled-unitary operation. The delocalization power and the entangling power
characterize different non-local properties of global unitary operations.Comment: 14 pages, 1 figur
Information Theoretically Secure Hypothesis Test for Temporally Unstructured Quantum Computation (Extended Abstract)
The efficient certification of classically intractable quantum devices has been a central research question for some time. However, to observe a "quantum advantage", it is believed that one does not need to build a large scale universal quantum computer, a task which has proven extremely challenging. Intermediate quantum models that are easier to implement, but which also exhibit this quantum advantage over classical computers, have been proposed. In this work, we present a certification technique for such a sub-universal quantum server which only performs commuting gates and requires very limited quantum memory. By allowing a verifying client to manipulate single qubits, we exploit properties of measurement based blind quantum computing to give them the tools to test the "quantum superiority" of the server
Unconditionally verifiable blind computation
Blind Quantum Computing (BQC) allows a client to have a server carry out a
quantum computation for them such that the client's input, output and
computation remain private. A desirable property for any BQC protocol is
verification, whereby the client can verify with high probability whether the
server has followed the instructions of the protocol, or if there has been some
deviation resulting in a corrupted output state. A verifiable BQC protocol can
be viewed as an interactive proof system leading to consequences for complexity
theory. The authors, together with Broadbent, previously proposed a universal
and unconditionally secure BQC scheme where the client only needs to be able to
prepare single qubits in separable states randomly chosen from a finite set and
send them to the server, who has the balance of the required quantum
computational resources. In this paper we extend that protocol with new
functionality allowing blind computational basis measurements, which we use to
construct a new verifiable BQC protocol based on a new class of resource
states. We rigorously prove that the probability of failing to detect an
incorrect output is exponentially small in a security parameter, while resource
overhead remains polynomial in this parameter. The new resource state allows
entangling gates to be performed between arbitrary pairs of logical qubits with
only constant overhead. This is a significant improvement on the original
scheme, which required that all computations to be performed must first be put
into a nearest neighbour form, incurring linear overhead in the number of
qubits. Such an improvement has important consequences for efficiency and
fault-tolerance thresholds.Comment: 46 pages, 10 figures. Additional protocol added which allows
arbitrary circuits to be verified with polynomial securit
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