3,550 research outputs found
Measurement cost of metric-aware variational quantum algorithms
Variational quantum algorithms are promising tools for near-term quantum
computers as their shallow circuits are robust to experimental imperfections.
Their practical applicability, however, strongly depends on how many times
their circuits need to be executed for sufficiently reducing shot-noise. We
consider metric-aware quantum algorithms: variational algorithms that use a
quantum computer to efficiently estimate both a matrix and a vector object. For
example, the recently introduced quantum natural gradient approach uses the
quantum Fisher information matrix as a metric tensor to correct the gradient
vector for the co-dependence of the circuit parameters. We rigorously
characterise and upper bound the number of measurements required to determine
an iteration step to a fixed precision, and propose a general approach for
optimally distributing samples between matrix and vector entries. Finally, we
establish that the number of circuit repetitions needed for estimating the
quantum Fisher information matrix is asymptotically negligible for an
increasing number of iterations and qubits.Comment: 17 pages, 3 figure
The economic pitfalls and barriers of the sustainable tourism concept in the case of national parks
financial management;sustainable development;tourism;national parks
Sustainable tourism and policy
tourism;sustainable development;government policy;rural development
RXTE observations of the dipping low-mass X-ray binary 4U 1624-49
We analyse ~ 360 ks of archival data from the Rossi X-Ray Timing Explorer
(RXTE) of the 21 hr orbital period dipping low-mass X-ray binary 4U 1624-49. We
find that outside the dips the tracks in the colour-colour and
hardness-intensity diagrams (CDs and HIDs) are reminiscent of those of atoll
sources in the middle and upper parts of the banana branch. The tracks show
secular shifts up to ~ 10%. We study the power spectrum of 4U 1624-49 as a
function of the position in the CD. This is the first time power spectra of
this source are presented. No quasi-periodic oscillations (QPOs) are found. The
power spectra are dominated by very low frequency noise (VLFN), characteristic
for atoll sources in the banana state, and band limited noise (BLN) which is
not reliably detected but may, uncharacteristically, strengthen and increase in
frequency with spectral hardness. The VLFN fits to a power law, which becomes
steeper when the source moves to the harder part of the CD. We conclude that 4U
1624-49 is an atoll source which in our observations is in the upper banana
branch. Combining this with the high (0.5-0.7 L_Edd) luminosity, the long-term
flux stability of the source as seen with the RXTE All-Sky Monitor (ASM), and
with the fact that it is an X-ray dip source, we conclude that 4U 1624-49 is
most likely a GX atoll source such as GX 3+1 and GX 9+9, but seen edge on.Comment: 8 pages, 5 figures, 2 tables, accepted for publication in A&A. This
version: a few typos correcte
The aperiodic timing behaviour of the accretion-driven millisecond pulsar SAX J1808.4-3658
We studied the aperiodic X-ray timing behaviour of the accreting millisecond
pulsar SAX J1808.4-3658. The source was recently found to be the first
accreting millisecond pulsar that shows the kilohertz quasi-periodic
oscillations (kilohertz QPOs) that are found in many other X-ray binaries with
accreting neutron stars. The high frequency of these signals reflects the short
dynamical time scales in the region near the compact object where they
originate. We find that in addition to the kilohertz QPOs SAX J1808.4-3658
shows several low frequency timing features, based on which the source can be
classified as a so-called atoll source. The frequencies of the variability
components of the atoll sources follow a universal scheme of correlations. The
correlations in SAX J1808.4-3658 are similar but show a shift in upper
kilohertz QPO frequency. This discrepancy is perhaps related to a stronger or
differently configured magnetic field.Comment: 4 pages, 3 figures. To appear in the proceedings of the "The Restless
High-Energy Universe" (Amsterdam, The Netherlands), 2003, eds. E.P.J. van den
Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijer
Identification of âwasteful commutingâ using search theory
In this paper, we employ search theory as a micro-economic foundation for the wasteful commuting hypothesis. In the empirical analysis, the extent of the âwasteful commutingâ is identified by comparing the commute of employees and self-employed individuals who do not work from home. It is argued that the commute of the self-employed is the result of a search process for vacant workplaces, whereas employees search for vacant jobs. Because the arrival rate of workplaces exceeds the arrival rate of jobs, the self-employed have a shorter commute. We find that 35% of the commuting time may be considered âwastefulâ and reject alternative hypotheses why the self-employed have a shorter commute.
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