2,020 research outputs found
Counterfactual Computation
Suppose that we are given a quantum computer programmed ready to perform a
computation if it is switched on. Counterfactual computation is a process by
which the result of the computation may be learnt without actually running the
computer. Such processes are possible within quantum physics and to achieve
this effect, a computer embodying the possibility of running the computation
must be available, even though the computation is, in fact, not run. We study
the possibilities and limitations of general protocols for the counterfactual
computation of decision problems (where the result r is either 0 or 1). If p(r)
denotes the probability of learning the result r ``for free'' in a protocol
then one might hope to design a protocol which simultaneously has large p(0)
and p(1). However we prove that p(0)+p(1) never exceeds 1 in any protocol and
we derive further constraints on p(0) and p(1) in terms of N, the number of
times that the computer is not run. In particular we show that any protocol
with p(0)+p(1)=1-epsilon must have N tending to infinity as epsilon tends to 0.
These general results are illustrated with some explicit protocols for
counterfactual computation. We show that "interaction-free" measurements can be
regarded as counterfactual computations, and our results then imply that N must
be large if the probability of interaction is to be close to zero. Finally, we
consider some ways in which our formulation of counterfactual computation can
be generalised.Comment: 19 pages. LaTex, 2 figures. Revised version has some new sections and
expanded explanation
Transport enhancement from incoherent coupling between one-dimensional quantum conductors
We study the non-equilibrium transport properties of a highly anisotropic
two-dimensional lattice of spin-1/2 particles governed by a Heisenberg XXZ
Hamiltonian. The anisotropy of the lattice allows us to approximate the system
at finite temperature as an array of incoherently coupled one-dimensional
chains. We show that in the regime of strong intrachain interactions, the weak
interchain coupling considerably boosts spin transport in the driven system.
Interestingly, we show that this enhancement increases with the length of the
chains, which is related to superdiffusive spin transport. We describe the
mechanism behind this effect, compare it to a similar phenomenon in single
chains induced by dephasing, and explain why the former is much stronger
Protein-DNA computation by stochastic assembly cascade
The assembly of RecA on single-stranded DNA is measured and interpreted as a
stochastic finite-state machine that is able to discriminate fine differences
between sequences, a basic computational operation. RecA filaments efficiently
scan DNA sequence through a cascade of random nucleation and disassembly events
that is mechanistically similar to the dynamic instability of microtubules.
This iterative cascade is a multistage kinetic proofreading process that
amplifies minute differences, even a single base change. Our measurements
suggest that this stochastic Turing-like machine can compute certain integral
transforms.Comment: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC129313/
http://www.pnas.org/content/99/18/11589.abstrac
On Nonzero Kronecker Coefficients and their Consequences for Spectra
A triple of spectra (r^A, r^B, r^{AB}) is said to be admissible if there is a
density operator rho^{AB} with (Spec rho^A, Spec rho^B, Spec rho^{AB})=(r^A,
r^B, r^{AB}). How can we characterise such triples? It turns out that the
admissible spectral triples correspond to Young diagrams (mu, nu, lambda) with
nonzero Kronecker coefficient [M. Christandl and G. Mitchison, to appear in
Comm. Math. Phys., quant-ph/0409016; A. Klyachko, quant-ph/0409113]. This means
that the irreducible representation V_lambda is contained in the tensor product
of V_mu and V_nu. Here, we show that such triples form a finitely generated
semigroup, thereby resolving a conjecture of Klyachko. As a consequence we are
able to obtain stronger results than in [M. Ch. and G. M. op. cit.] and give a
complete information-theoretic proof of the correspondence between triples of
spectra and representations. Finally, we show that spectral triples form a
convex polytope.Comment: 13 page
The detection of airborne transmission of tuberculosis from HIV-infected patients, using an in vivo air sampling model
Background. Nosocomial transmission of tuberculosis remains an important public health problem. We created an in vivo air sampling model to study airborne transmission of tuberculosis from patients coinfected with human immunodeficiency virus (HIV) and to evaluate environmental control measures.
Methods. An animal facility was built above a mechanically ventilated HIV‐tuberculosis ward in Lima, Peru. A mean of 92 guinea pigs were continuously exposed to all ward exhaust air for 16 months. Animals had tuberculin skin tests performed at monthly intervals, and those with positive reactions were removed for autopsy and culture for tuberculosis.
Results. Over 505 consecutive days, there were 118 ward admissions by 97 patients with pulmonary tuberculosis, with a median duration of hospitalization of 11 days. All patients were infected with HIV and constituted a heterogeneous group with both new and existing diagnoses of tuberculosis. There was a wide variation in monthly rates of guinea pigs developing positive tuberculin test results (0%–53%). Of 292 animals exposed to ward air, 159 developed positive tuberculin skin test results, of which 129 had laboratory confirmation of tuberculosis. The HIV‐positive patients with pulmonary tuberculosis produced a mean of 8.2 infectious quanta per hour, compared with 1.25 for HIV‐negative patients with tuberculosis in similar studies from the 1950s. The mean monthly patient infectiousness varied greatly, from production of 0–44 infectious quanta per hour, as did the theoretical risk for a health care worker to acquire tuberculosis by breathing ward air.
Conclusions. HIV‐positive patients with tuberculosis varied greatly in their infectiousness, and some were highly infectious. Use of environmental control strategies for nosocomial tuberculosis is therefore a priority, especially in areas with a high prevalence of both tuberculosis and HIV infection
- …