326 research outputs found
Overcoming non-Markovian dephasing in single photon sources through post-selection
We study the effects of realistic dephasing environments on a pair of
solid-state single-photon sources in the context of the Hong-Ou-Mandel dip. By
means of solutions for the Markovian or exact non-Markovian dephasing dynamics
of the sources, we show that the resulting loss of visibility depends crucially
on the timing of photon detection events. Our results demonstrate that the
effective visibility can be improved via temporal post-selection, and also that
time-resolved interference can be a useful probe of the interaction between the
emitter and its host environment.Comment: 5 pages, 2 figures, published version, title changed, references
update
Fault tolerant quantum computation with very high threshold for loss errors
Many proposals for fault tolerant quantum computation (FTQC) suffer
detectable loss processes. Here we show that topological FTQC schemes, which
are known to have high error thresholds, are also extremely robust against
losses. We demonstrate that these schemes tolerate loss rates up to 24.9%,
determined by bond percolation on a cubic lattice. Our numerical results show
that these schemes retain good performance when loss and computational errors
are simultaneously present.Comment: 4 pages, comments still very welcome. v2 is a reasonable
approximation to the published versio
Privatisation in Ireland
Public enterprises in Ireland were offshoots of political nationalism. They were part of a protectionist economic policy and in 1980 employed over 90.000 staff in a total national employment figure of 1.1m. Public opinion moved away from public enterprises because of perceived high costs to both consumers and taxpayers. In the Celtic Tiger era since 1987 the share of public enterprises in total employment has fallen by almost two-thirds to 2.7 percent. Ireland has experienced major increases in GNP per head and in employment by adopting open economy policies and securing large increases in exports and in foreign direct investment. No privatised enterprise has been re-nationalised. It is public policy to retain network infrastructure such as the electricity and gas grids in public ownership while selling state companies in areas such as food, banking, telecoms, and shipping.
Fully fault tolerant quantum computation with non-deterministic gates
In certain approaches to quantum computing the operations between qubits are
non-deterministic and likely to fail. For example, a distributed quantum
processor would achieve scalability by networking together many small
components; operations between components should assumed to be failure prone.
In the logical limit of this architecture each component contains only one
qubit. Here we derive thresholds for fault tolerant quantum computation under
such extreme paradigms. We find that computation is supported for remarkably
high failure rates (exceeding 90%) providing that failures are heralded,
meanwhile the rate of unknown errors should not exceed 2 in 10^4 operations.Comment: 5 pages, 3 fig
Quantum computation via measurements on the low-temperature state of a many-body system
We consider measurement-based quantum computation using the state of a
spin-lattice system in equilibrium with a thermal bath and free to evolve under
its own Hamiltonian. Any single qubit measurements disturb the system from
equilibrium and, with adaptive measurements performed at a finite rate, the
resulting dynamics reduces the fidelity of the computation. We show that it is
possible to describe the loss in fidelity by a single quantum operation on the
encoded quantum state that is independent of the measurement history. To
achieve this simple description, we choose a particular form of spin-boson
coupling to describe the interaction with the environment, and perform
measurements periodically at a natural rate determined by the energy gap of the
system. We found that an optimal cooling exists, which is a trade-off between
keeping the system cool enough that the resource state remains close to the
ground state, but also isolated enough that the cooling does not strongly
interfere with the dynamics of the computation. For a sufficiently low
temperature we obtain a fault-tolerant threshold for the couplings to the
environment.Comment: 9 pages, 3 figures; v2 published versio
Preparing multi-partite entanglement of photons and matter qubits
We show how to make event-ready multi-partite entanglement between qubits
which may be encoded on photons or matter systems. Entangled states of matter
systems, which can also act as single photon sources, can be generated using
the entangling operation presented in quant-ph/0408040. We show how to entangle
such sources with photon qubits, which may be encoded in the dual rail,
polarization or time-bin degrees of freedom. We subsequently demonstrate how
projective measurements of the matter qubits can be used to create entangled
states of the photons alone. The state of the matter qubits is inherited by the
generated photons. Since the entangling operation can be used to generate
cluster states of matter qubits for quantum computing, our procedure enables us
to create any (entangled) photonic quantum state that can be written as the
outcome of a quantum computer.Comment: 10 pages, 4 figures; to appear in Journal of Optics
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