21 research outputs found
Towards practical linear optical quantum computing
Quantum computing promises a new paradigm of computation where information is processed in a way that has no classical analogue. There are a number of physical platforms conducive to quantum computation, each with a number of advantages and challenges. Single photons, manipulated using integrated linear optics, constitute a promising platform for universal quantum computation. Their low decoherence rates make them particularly favourable, however the inability to perform deterministic two-qubit gates and the issue of photon loss are challenges that need to be overcome.
In this thesis we explore the construction of a linear optical quantum computer based on the cluster state model. We identify the different necessary stages: state preparation, cluster state construction and implementation of quantum error correcting codes, and address the challenges that arise in each of these stages. For the state preparation, we propose a series of linear optical circuits for the generation of small entangled states, assessing their performance under different scenarios. For the cluster state construction, we introduce a ballistic scheme which not only consumes an order of magnitude fewer resources than previously proposed schemes, but also benefits from a natural loss tolerance. Based on this scheme, we propose a full architectural blueprint with fixed physical depth. We make investigations into the resource efficiency of this architecture and propose a new multiplexing scheme which optimises the use of resources. Finally, we study the integration of quantum error-correcting codes in the linear optical scheme proposed and suggest three ways in which the linear optical scheme can be made fault-tolerant.Open Acces
Deterministic generation of large-scale entangled photonic cluster state from interacting solid state emitters
The ability to create large highly entangled `cluster' states is crucial for
measurement-based quantum computing. We show that deterministic multi-photon
entanglement can be created from coupled solid state quantum emitters without
the need for any two-qubit gates and regardless of whether the emitters are
identical. In particular, we present a general method for controlled
entanglement creation by making direct use of the always-on exchange
interaction, in combination with single-qubit operations. This is used to
provide a recipe for the generation of two-dimensional, cluster-state entangled
photons that can be carried out with existing experimental capabilities in
quantum dots
Physical-depth architectural requirements for generating universal photonic cluster states
Most leading proposals for linear-optical quantum computing (LOQC) use
cluster states, which act as a universal resource for measurement-based
(one-way) quantum computation (MBQC). In ballistic approaches to LOQC, cluster
states are generated passively from small entangled resource states using
so-called fusion operations. Results from percolation theory have previously
been used to argue that universal cluster states can be generated in the
ballistic approach using schemes which exceed the critical threshold for
percolation, but these results consider cluster states with unbounded size.
Here we consider how successful percolation can be maintained using a physical
architecture with fixed physical depth, assuming that the cluster state is
continuously generated and measured, and therefore that only a finite portion
of it is visible at any one point in time. We show that universal LOQC can be
implemented using a constant-size device with modest physical depth, and that
percolation can be exploited using simple pathfinding strategies without the
need for high-complexity algorithms.Comment: 18 pages, 10 figure
Fault-tolerance thresholds for the surface code with fabrication errors
The construction of topological error correction codes requires the ability
to fabricate a lattice of physical qubits embedded on a manifold with a
non-trivial topology such that the quantum information is encoded in the global
degrees of freedom (i.e. the topology) of the manifold. However, the
manufacturing of large-scale topological devices will undoubtedly suffer from
fabrication errors---permanent faulty components such as missing physical
qubits or failed entangling gates---introducing permanent defects into the
topology of the lattice and hence significantly reducing the distance of the
code and the quality of the encoded logical qubits. In this work we investigate
how fabrication errors affect the performance of topological codes, using the
surface code as the testbed. A known approach to mitigate defective lattices
involves the use of primitive SWAP gates in a long sequence of syndrome
extraction circuits. Instead, we show that in the presence of fabrication
errors the syndrome can be determined using the supercheck operator approach
and the outcome of the defective gauge stabilizer generators without any
additional computational overhead or the use of SWAP gates. We report numerical
fault-tolerance thresholds in the presence of both qubit fabrication and gate
fabrication errors using a circuit-based noise model and the minimum-weight
perfect matching decoder. Our numerical analysis is most applicable to 2D
chip-based technologies, but the techniques presented here can be readily
extended to other topological architectures. We find that in the presence of 8%
qubit fabrication errors, the surface code can still tolerate a computational
error rate of up to 0.1%.Comment: 10 pages, 15 figure
From three-photon GHZ states to ballistic universal quantum computation
Single photons, manipulated using integrated linear optics, constitute a
promising platform for universal quantum computation. A series of increasingly
efficient proposals have shown linear-optical quantum computing to be formally
scalable. However, existing schemes typically require extensive adaptive
switching, which is experimentally challenging and noisy, thousands of photon
sources per renormalized qubit, and/or large quantum memories for
repeat-until-success strategies. Our work overcomes all these problems. We
present a scheme to construct a cluster state universal for quantum
computation, which uses no adaptive switching, no large memories, and which is
at least an order of magnitude more resource-efficient than previous passive
schemes. Unlike previous proposals, it is constructed entirely from
loss-detecting gates and offers a robustness to photon loss. Even without the
use of an active loss-tolerant encoding, our scheme naturally tolerates a total
loss rate of in the photons detected in the gates. This scheme
uses only 3-GHZ states as a resource, together with a passive linear-optical
network. We fully describe and model the iterative process of cluster
generation, including photon loss and gate failure. This demonstrates that
building a linear optical quantum computer need be less challenging than
previously thought.Comment: Minor changes to match published version. 10 pages and 14 figures
including the supplementary materia
Loss-tolerant teleportation on large stabilizer states
We present a general method for finding loss-tolerant teleportation on large,
entangled stabilizer states using only single-qubit measurements, known as
\emph{stabilizer pathfinding} (SPF). For heralded loss, SPF is shown to
generate optimally loss-tolerant measurement patterns on any given stabilizer
state. Furthermore, SPF also provides highly loss-tolerant teleportation
strategies when qubit loss is unheralded. We provide a fast algorithm for SPF
that updates continuously as a state is generated and measured, which is
therefore suitable for real-time implementation on a quantum-computing device.
When compared to simulations of previous heuristics for loss-tolerant
teleportation on graph states, SPF provides considerable gains in tolerance to
both heralded and unheralded loss, achieving a near-perfect teleportation rate
() in the regime of low qubit loss () on various graph state
lattices. Using these results we also present evidence that points towards the
existence of loss-tolerant thresholds on such states, which in turn indicates
that the loss-tolerant behaviour we have found also applies as the number of
qubits tends to infinity. Our results represent a significant advance towards
the realistic implementation of teleportation in both large-scale and
near-future quantum architectures that are susceptible to qubit loss, such as
linear optical quantum computation and quantum communication networks.Comment: 29 pages, 12 figures. Quantum Science and Technology (2018
Relative multiplexing for minimizing switching in linear-optical quantum computing
Many existing schemes for linear-optical quantum computing (LOQC) depend on
multiplexing (MUX), which uses dynamic routing to enable near-deterministic
gates and sources to be constructed using heralded, probabilistic primitives.
MUXing accounts for the overwhelming majority of active switching demands in
current LOQC architectures. In this manuscript, we introduce relative
multiplexing (RMUX), a general-purpose optimization which can dramatically
reduce the active switching requirements for MUX in LOQC, and thereby reduce
hardware complexity and energy consumption, as well as relaxing demands on
performance for various photonic components. We discuss the application of RMUX
to the generation of entangled states from probabilistic single-photon sources,
and argue that an order of magnitude improvement in the rate of generation of
Bell states can be achieved. In addition, we apply RMUX to the proposal for
percolation of a 3D cluster state in [PRL 115, 020502 (2015)], and we find that
RMUX allows a 2.4x increase in loss tolerance for this architecture.Comment: Published version, New Journal of Physics, Volume 19, June 201
CIBERER : Spanish national network for research on rare diseases: A highly productive collaborative initiative
Altres ajuts: Instituto de Salud Carlos III (ISCIII); Ministerio de Ciencia e Innovación.CIBER (Center for Biomedical Network Research; Centro de Investigación Biomédica En Red) is a public national consortium created in 2006 under the umbrella of the Spanish National Institute of Health Carlos III (ISCIII). This innovative research structure comprises 11 different specific areas dedicated to the main public health priorities in the National Health System. CIBERER, the thematic area of CIBER focused on rare diseases (RDs) currently consists of 75 research groups belonging to universities, research centers, and hospitals of the entire country. CIBERER's mission is to be a center prioritizing and favoring collaboration and cooperation between biomedical and clinical research groups, with special emphasis on the aspects of genetic, molecular, biochemical, and cellular research of RDs. This research is the basis for providing new tools for the diagnosis and therapy of low-prevalence diseases, in line with the International Rare Diseases Research Consortium (IRDiRC) objectives, thus favoring translational research between the scientific environment of the laboratory and the clinical setting of health centers. In this article, we intend to review CIBERER's 15-year journey and summarize the main results obtained in terms of internationalization, scientific production, contributions toward the discovery of new therapies and novel genes associated to diseases, cooperation with patients' associations and many other topics related to RD research