2,828 research outputs found

### Metal alloy resistivity measurements at very low temperatures

High speed, automated system accurately measures to approximately one percent in three minutes. System identifies materials having constant thermal or electric conductivity, predicts new material properties, develops alloys in accordance with desired specifications, and develops nondestructive devices for measuring precipitation hardening

### The Viscosity and Thermal Conductivity Coefficients of Dilute Neon, Krypton, and Xenon

Viscosity and thermoconductivity coefficients of dilute neon, krypton, and xeno

### Distinguishing n Hamiltonians on C^n by a single measurement

If an experimentalist wants to decide which one of n possible Hamiltonians
acting on an n dimensional Hilbert space is present, he can conjugate the time
evolution by an appropriate sequence of known unitary transformations in such a
way that the different Hamiltonians result in mutual orthogonal final states.
We present a general scheme providing such a sequence.Comment: 4 pages, Revte

### Effects of Noise, Correlations and errors in the preparation of initial states in Quantum Simulations

In principle a quantum system could be used to simulate another quantum
system. The purpose of such a simulation would be to obtain information about
problems which cannot be simulated with a classical computer due to the
exponential increase of the Hilbert space with the size of the system and which
cannot be measured or controlled in an actual experiment. The system will
interact with the surrounding environment, with the other particles in the
system and be implemented using imperfect controls making it subject to noise.
It has been suggested that noise does not need to be controlled to the same
extent as it must be for general quantum computing. However the effects of
noise in quantum simulations and how to treat them are not completely
understood. In this paper we study an existing quantum algorithm for the
one-dimensional Fano-Anderson model to be simulated using a liquid-state NMR
device. We calculate the evolution of different initial states in the original
model, and then we add interacting spins to simulate a more realistic
situation. We find that states which are entangled with their environment, and
sometimes correlated but not necessarily entangled have an evolution which is
described by maps which are not completely positive. We discuss the conditions
for this to occur and also the implications.Comment: Revtex 4-1, 14 pages, 21 figures, version 2 has typos corrected and
acknowledgement adde

### Limit Theorem for Continuous-Time Quantum Walk on the Line

Concerning a discrete-time quantum walk X^{(d)}_t with a symmetric
distribution on the line, whose evolution is described by the Hadamard
transformation, it was proved by the author that the following weak limit
theorem holds: X^{(d)}_t /t \to dx / \pi (1-x^2) \sqrt{1 - 2 x^2} as t \to
\infty. The present paper shows that a similar type of weak limit theorems is
satisfied for a {\it continuous-time} quantum walk X^{(c)}_t on the line as
follows: X^{(c)}_t /t \to dx / \pi \sqrt{1 - x^2} as t \to \infty. These
results for quantum walks form a striking contrast to the central limit theorem
for symmetric discrete- and continuous-time classical random walks: Y_{t}/
\sqrt{t} \to e^{-x^2/2} dx / \sqrt{2 \pi} as t \to \infty. The work deals also
with issue of the relationship between discrete and continuous-time quantum
walks. This topic, subject of a long debate in the previous literature, is
treated within the formalism of matrix representation and the limit
distributions are exhaustively compared in the two cases.Comment: 15 pages, title correcte

### Exploring the Cosmic Evolution of Habitability with Galaxy Merger Trees

We combine inferred galaxy properties from a semi-analytic galaxy evolution
model incorporating dark matter halo merger trees with new estimates of
supernova and gamma ray burst rates as a function of metallicity from stellar
population synthesis models incorporating binary interactions. We use these to
explore the stellar mass fraction of galaxies irradiated by energetic
astrophysical transients and its evolution over cosmic time, and thus the
fraction which is potentially habitable by life like our own. We find that 18
per cent of the stellar mass in the Universe is likely to have been irradiated
within the last 260 Myr, with GRBs dominating that fraction. We do not see a
strong dependence of irradiated stellar mass fraction on stellar mass or
richness of the galaxy environment. We consider a representative merger tree as
a Local Group analogue, and find that there are galaxies at all masses which
have retained a high habitable fraction (>40 per cent) over the last 6 Gyr, but
also that there are galaxies at all masses where the merger history and
associated star formation have rendered galaxies effectively uninhabitable.
This illustrates the need to consider detailed merger trees when evaluating the
cosmic evolution of habitability.Comment: 11 page, 10 figures. MNRAS accepted 13th Dec 2017. Updated to match
accepted version, with additional discussion of metallicity effect

### Measuring Energy, Estimating Hamiltonians, and the Time-Energy Uncertainty Relation

Suppose that the Hamiltonian acting on a quantum system is unknown and one
wants to determine what is the Hamiltonian. We show that in general this
requires a time $\Delta t$ which obeys the uncertainty relation $\Delta t
\Delta H \gtrsim 1$ where $\Delta H$ is a measure of how accurately the unknown
Hamiltonian must be estimated. We then apply this result to the problem of
measuring the energy of an unknown quantum state. It has been previously shown
that if the Hamiltonian is known, then the energy can in principle be measured
in an arbitrarily short time. On the other hand we show that if the Hamiltonian
is not known then an energy measurement necessarily takes a minimum time
$\Delta t$ which obeys the uncertainty relation $\Delta t \Delta E \gtrsim 1$
where $\Delta E$ is the precision of the energy measurement. Several examples
are studied to address the question of whether it is possible to saturate these
uncertainty relations. Their interpretation is discussed in detail.Comment: 12pages, revised version with small correction

### Quantum algorithm for the Boolean hidden shift problem

The hidden shift problem is a natural place to look for new separations
between classical and quantum models of computation. One advantage of this
problem is its flexibility, since it can be defined for a whole range of
functions and a whole range of underlying groups. In a way, this distinguishes
it from the hidden subgroup problem where more stringent requirements about the
existence of a periodic subgroup have to be made. And yet, the hidden shift
problem proves to be rich enough to capture interesting features of problems of
algebraic, geometric, and combinatorial flavor. We present a quantum algorithm
to identify the hidden shift for any Boolean function. Using Fourier analysis
for Boolean functions we relate the time and query complexity of the algorithm
to an intrinsic property of the function, namely its minimum influence. We show
that for randomly chosen functions the time complexity of the algorithm is
polynomial. Based on this we show an average case exponential separation
between classical and quantum time complexity. A perhaps interesting aspect of
this work is that, while the extremal case of the Boolean hidden shift problem
over so-called bent functions can be reduced to a hidden subgroup problem over
an abelian group, the more general case studied here does not seem to allow
such a reduction.Comment: 10 pages, 1 figur

### Optimal estimation of quantum dynamics

We construct the optimal strategy for the estimation of an unknown unitary
transformation $U\in SU(d)$. This includes, in addition to a convenient
measurement on a probe system, finding which is the best initial state on which
$U$ is to act. When $U\in SU(2)$, such an optimal strategy can be applied to
estimate simultaneously both the direction and the strength of a magnetic
field, and shows how to use a spin 1/2 particle to transmit information about a
whole coordinate system instead of only a direction in space.Comment: 4 pages, REVTE

### One- and two-dimensional quantum walks in arrays of optical traps

We propose a novel implementation of discrete time quantum walks for a
neutral atom in an array of optical microtraps or an optical lattice. We
analyze a one-dimensional walk in position space, with the coin, the additional
qubit degree of freedom that controls the displacement of the quantum walker,
implemented as a spatially delocalized qubit, i.e., the coin is also encoded in
position space. We analyze the dependence of the quantum walk on temperature
and experimental imperfections as shaking in the trap positions. Finally,
combining a spatially delocalized qubit and a hyperfine qubit, we also give a
scheme to realize a quantum walk on a two-dimensional square lattice with the
possibility of implementing different coin operators.Comment: 10 pages, 8 figures; v2: some comments added and other minor change

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