364 research outputs found
A photon loss tolerant Zeno CSIGN gate
We model an optical implementation of a CSIGN gate that makes use of the
Quantum Zeno effect [1,2] in the presence of photon loss. The raw operation of
the gate is severely affected by this type of loss. However, we show that by
using the same photon loss codes that have been proposed for linear optical
quantum computation (LOQC), the performance is greatly enhanced and such gates
can outperform LOQC equivalents. The technique can be applied to other types of
nonlinearities, making the implementation of nonlinear optical gates much more
attractive
Computation with Coherent States via Teleportations to and from a Quantum Bus
In this paper we present results illustrating the power and flexibility of
one-bit teleportations in quantum bus computation. We first show a scheme to
perform a universal set of gates on continuous variable modes, which we call a
quantum bus or qubus, using controlled phase-space rotations, homodyne
detection, ancilla qubits and single qubit measurement. The resource usage for
this scheme is lower than any previous scheme to date. We then illustrate how
one-bit teleportations into a qubus can be used to encode qubit states into a
quantum repetition code, which in turn can be used as an efficient method for
producing GHZ states that can be used to create large cluster states. Each of
these schemes can be modified so that teleportation measurements are
post-selected to yield outputs with higher fidelity, without changing the
physical parameters of the system.Comment: 10 pages, 12 figure
Stabilizer Quantum Error Correction with Qubus Computation
In this paper we investigate stabilizer quantum error correction codes using
controlled phase rotations of strong coherent probe states. We explicitly
describe two methods to measure the Pauli operators which generate the
stabilizer group of a quantum code. First, we show how to measure a Pauli
operator acting on physical qubits using a single coherent state with large
average photon number, displacement operations, and photon detection. Second,
we show how to measure the stabilizer operators fault-tolerantly by the
deterministic preparation of coherent cat states along with one-bit
teleportations between a qubit-like encoding of coherent states and physical
qubits.Comment: 4 pages, 5 figure
Universally Sloppy Parameter Sensitivities in Systems Biology
Quantitative computational models play an increasingly important role in
modern biology. Such models typically involve many free parameters, and
assigning their values is often a substantial obstacle to model development.
Directly measuring \emph{in vivo} biochemical parameters is difficult, and
collectively fitting them to other data often yields large parameter
uncertainties. Nevertheless, in earlier work we showed in a
growth-factor-signaling model that collective fitting could yield
well-constrained predictions, even when it left individual parameters very
poorly constrained. We also showed that the model had a `sloppy' spectrum of
parameter sensitivities, with eigenvalues roughly evenly distributed over many
decades. Here we use a collection of models from the literature to test whether
such sloppy spectra are common in systems biology. Strikingly, we find that
every model we examine has a sloppy spectrum of sensitivities. We also test
several consequences of this sloppiness for building predictive models. In
particular, sloppiness suggests that collective fits to even large amounts of
ideal time-series data will often leave many parameters poorly constrained.
Tests over our model collection are consistent with this suggestion. This
difficulty with collective fits may seem to argue for direct parameter
measurements, but sloppiness also implies that such measurements must be
formidably precise and complete to usefully constrain many model predictions.
We confirm this implication in our signaling model. Our results suggest that
sloppy sensitivity spectra are universal in systems biology models. The
prevalence of sloppiness highlights the power of collective fits and suggests
that modelers should focus on predictions rather than on parameters.Comment: Submitted to PLoS Computational Biology. Supplementary Information
available in "Other Formats" bundle. Discussion slightly revised to add
historical contex
Experimental simulation of closed timelike curves
Closed timelike curves are among the most controversial features of modern physics. As legitimate solutions to Einstein's field equations, they allow for time travel, which instinctively seems paradoxical. However, in the quantum regime these paradoxes can be resolved, leaving closed timelike curves consistent with relativity. The study of these systems therefore provides valuable insight into nonlinearities and the emergence of causal structures in quantum mechanics-essential for any formulation of a quantum theory of gravity. Here we experimentally simulate the nonlinear behaviour of a qubit interacting unitarily with an older version of itself, addressing some of the fascinating effects that arise in systems traversing a closed timelike curve. These include perfect discrimination of non-orthogonal states and, most intriguingly, the ability to distinguish nominally equivalent ways of preparing pure quantum states. Finally, we examine the dependence of these effects on the initial qubit state, the form of the unitary interaction and the influence of decoherence
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