1,651 research outputs found
Probabilistic growth of large entangled states with low error accumulation
The creation of complex entangled states, resources that enable quantum
computation, can be achieved via simple 'probabilistic' operations which are
individually likely to fail. However, typical proposals exploiting this idea
carry a severe overhead in terms of the accumulation of errors. Here we
describe an method that can rapidly generate large entangled states with an
error accumulation that depends only logarithmically on the failure
probability. We find that the approach may be practical for success rates in
the sub-10% range, while ultimately becoming unfeasible at lower rates. The
assumptions that we make, including parallelism and high connectivity, are
appropriate for real systems including measurement-induced entanglement. This
result therefore shows the feasibility for real devices based on such an
approach.Comment: 5 pages, 3 figure
Distributed quantum computation with arbitrarily poor photon detection
In a distributed quantum computer scalability is accomplished by networking
together many elementary nodes. Typically the network is optical and inter-node
entanglement involves photon detection. In complex networks the entanglement
fidelity may be degraded by the twin problems of photon loss and dark counts.
Here we describe an entanglement protocol which can achieve high fidelity even
when these issues are arbitrarily severe; indeed the method succeeds with
finite probability even if the detectors are entirely removed from the network.
An experimental demonstration should be possible with existing technologies.Comment: 5 pages, 4 fig
Freely Scalable Quantum Technologies using Cells of 5-to-50 Qubits with Very Lossy and Noisy Photonic Links
Exquisite quantum control has now been achieved in small ion traps, in
nitrogen-vacancy centres and in superconducting qubit clusters. We can regard
such a system as a universal cell with diverse technological uses from
communication to large-scale computing, provided that the cell is able to
network with others and overcome any noise in the interlinks. Here we show that
loss-tolerant entanglement purification makes quantum computing feasible with
the noisy and lossy links that are realistic today: With a modestly complex
cell design, and using a surface code protocol with a network noise threshold
of 13.3%, we find that interlinks which attempt entanglement at a rate of 2MHz
but suffer 98% photon loss can result in kilohertz computer clock speeds (i.e.
rate of high fidelity stabilizer measurements). Improved links would
dramatically increase the clock speed. Our simulations employed local gates of
a fidelity already achieved in ion trap devices.Comment: corrected typos, additional references, additional figur
Adaptive strategies for graph state growth in the presence of monitored errors
Graph states (or cluster states) are the entanglement resource that enables
one-way quantum computing. They can be grown by projective measurements on the
component qubits. Such measurements typically carry a significant failure
probability. Moreover, they may generate imperfect entanglement. Here we
describe strategies to adapt growth operations in order to cancel incurred
errors. Nascent states that initially deviate from the ideal graph states
evolve toward the desired high fidelity resource without impractical overheads.
Our analysis extends the diagrammatic language of graph states to include
characteristics such as tilted vertices, weighted edges, and partial fusion,
which arise from experimental imperfections. The strategies we present are
relevant to parity projection schemes such as optical `path erasure' with
distributed matter qubits.Comment: 4 pages, 4 figures. Typos corrected, nicer figures, neater notation
and better rea
Quantum Information Processing with Delocalized Qubits under Global Control
Any technology for quantum information processing (QIP) must embody within it
quantum bits (qubits) and maintain control of their key quantum properties of
superposition and entanglement. Typical QIP schemes envisage an array of
physical systems, such as electrons or nuclei, with each system representing a
given qubit. For adequate control, systems must be distinguishable either by
physical separation or unique frequencies, and their mutual interactions must
be individually manipulable. These difficult requirements exclude many
nanoscale technologies where systems are densely packed and continuously
interacting. Here we demonstrate a new paradigm: restricting ourselves to
global control pulses we permit systems to interact freely and continuously,
with the consequence that qubits can become delocalized over the entire device.
We realize this using NMR studies of three carbon-13 nuclei in alanine,
demonstrating all the key aspects including a quantum mirror, one- and
two-qubit gates, permutation of densely packed qubits and Deutsch algorithms.Comment: 4 pages, 5 figure
Child education and work choices in the presence of a conditional cash transfer programme in rural Colombia
This research is part of a large evaluation effort, undertaken by a consortium formed by IFS, Econometria and SEI, which has considered the effects of Familias en Accion on a variety of outcomes one year after its implementation. In early reports, we focussed on the effects of the programme on school enrolment. In this paper, we both expand those results, by carefully analysing anticipation effects along with other issues, and complement them with an analysis of child labour - both paid and unpaid (including domestic) work. The child labour analysis is made possible due to a rich time use module of the surveys that has not previously been analysed. We find that the programme increased the school participation rates of 14 to 17 year old children quite substantially, by between 5 and 7 percentage points, and had lower, but non-negligible effects on the enrolment of younger children of between 1.4 and 2.4 percentage points. In terms of work, the effects are generally largest for younger children whose participation in domestic work decreased by around 10 to 12 percentage points after the programme but whose participation in income-generating work remained largely unaffected by the programme. We also find evidence of school and work time not being fully substitutable, suggesting that some, but not all, of the increased time at school may be drawn from children's leisure time
Rapid and robust spin state amplification
Electron and nuclear spins have been employed in many of the early
demonstrations of quantum technology (QT). However applications in real world
QT are limited by the difficulty of measuring single spins. Here we show that
it is possible to rapidly and robustly amplify a spin state using a lattice of
ancillary spins. The model we employ corresponds to an extremely simple
experimental system: a homogenous Ising-coupled spin lattice in one, two or
three dimensions, driven by a continuous microwave field. We establish that the
process can operate at finite temperature (imperfect initial polarisation) and
under the effects of various forms of decoherence.Comment: 5 pages, 2 figure
Quantum metrology with molecular ensembles
This work was supported by the EPSRC through QIP IRC (Grants No. GR/S82176/01 and No. GR/S15808/01), the National Research Foundation and Ministry of Education, Singapore, the DAAD, and the Royal Society.The field of quantum metrology promisesmeasurement devices that are fundamentally superior to conventional technologies. Specifically, when quantum entanglement is harnessed, the precision achieved is supposed to scale more favorably with the resources employed, such as system size and time required. Here, we consider measurement of magnetic-field strength using an ensemble of spin-active molecules. We identify a third essential resource: the change in ensemble polarization (entropy increase) during the metrology experiment. We find that performance depends crucially on the form of decoherence present; for a plausible dephasing model, we describe a quantum strategy, which can indeed beat the standard strategy.Publisher PDFPeer reviewe
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