7,848 research outputs found
Transport properties of partially equilibrated quantum wires
We study the effect of thermal equilibration on the transport properties of a
weakly interacting one-dimensional electron system. Although equilibration is
severely suppressed due to phase-space restrictions and conservation laws, it
can lead to intriguing signatures in partially equilibrated quantum wires. We
consider an ideal homogeneous quantum wire. We find a finite temperature
correction to the quantized conductance, which for a short wire scales with its
length, but saturates to a length-independent value once the wire becomes
exponentially long. We also discuss thermoelectric properties of long quantum
wires. We show that the uniform quantum wire is a perfect thermoelectric
refrigerator, approaching Carnot efficiency with increasing wire length.Comment: 20 pages, 6 figure
Bounds on Information Propagation in Disordered Quantum Spin Chains
We investigate the propagation of information through the disordered XY
model. We find, with a probability that increases with the size of the system,
that all correlations, both classical and quantum, are suppressed outside of an
effective lightcone whose radius grows at most polylogarithmically with |t|.Comment: 4 pages, pdflatex, 1 pdf figure. Corrected the bound for the
localised propagator and quantified the probability it bound occur
On the comparison of stable and unstable P-completion
In this note we show that a p-complete nilpotent space X has a p-complete suspension spectrum if and only if its homotopy groups pi X-* are bounded p-torsion. In contrast, if pi X-* is not all bounded p-torsion, we locate uncountable rational vector spaces in the integral homology and in the stable homotopy groups of X. To prove this, we establish a homological criterion for p-completeness of connective spectra. Moreover, we illustrate our results by studying the stable homotopy groups of K(Z(p), n) via Goodwillie calculus
On single-photon quantum key distribution in the presence of loss
We investigate two-way and one-way single-photon quantum key distribution
(QKD) protocols in the presence of loss introduced by the quantum channel. Our
analysis is based on a simple precondition for secure QKD in each case. In
particular, the legitimate users need to prove that there exists no separable
state (in the case of two-way QKD), or that there exists no quantum state
having a symmetric extension (one-way QKD), that is compatible with the
available measurements results. We show that both criteria can be formulated as
a convex optimisation problem known as a semidefinite program, which can be
efficiently solved. Moreover, we prove that the solution to the dual
optimisation corresponds to the evaluation of an optimal witness operator that
belongs to the minimal verification set of them for the given two-way (or
one-way) QKD protocol. A positive expectation value of this optimal witness
operator states that no secret key can be distilled from the available
measurements results. We apply such analysis to several well-known
single-photon QKD protocols under losses.Comment: 14 pages, 6 figure
Approximate locality for quantum systems on graphs
In this Letter we make progress on a longstanding open problem of Aaronson
and Ambainis [Theory of Computing 1, 47 (2005)]: we show that if A is the
adjacency matrix of a sufficiently sparse low-dimensional graph then the
unitary operator e^{itA} can be approximated by a unitary operator U(t) whose
sparsity pattern is exactly that of a low-dimensional graph which gets more
dense as |t| increases. Secondly, we show that if U is a sparse unitary
operator with a gap \Delta in its spectrum, then there exists an approximate
logarithm H of U which is also sparse. The sparsity pattern of H gets more
dense as 1/\Delta increases. These two results can be interpreted as a way to
convert between local continuous-time and local discrete-time processes. As an
example we show that the discrete-time coined quantum walk can be realised as
an approximately local continuous-time quantum walk. Finally, we use our
construction to provide a definition for a fractional quantum fourier
transform.Comment: 5 pages, 2 figures, corrected typ
Advances in Composite Manufacturing of Helicopter Parts
This study investigates and compares different methods for improving standard autoclave composite manufacturing in order to find suitable approaches to a more efficient composite production. The goal is not only a reduction in manufacturing times and costs but also quality enhancement. Improved part quality while decreasing costs enables a manufacturer of composite parts to expand its market share, especially in the helicopter market, which has been constantly shrinking over the last two years. Various approaches such as improved tooling technology, the use of automated systems for lamination as well as outsourcing are examined to provide an overview of possible advancements in the area of autoclave manufacturing. The results firstly suggest that a thermal tooling optimization provides significant improvements; secondly, the use of automated systems and outsourcing will improve efficiency only under the condition that the relationship of potential cost reduction and the required investment is beneficial
- …