11,742 research outputs found
Time complexity and gate complexity
We formulate and investigate the simplest version of time-optimal quantum
computation theory (t-QCT), where the computation time is defined by the
physical one and the Hamiltonian contains only one- and two-qubit interactions.
This version of t-QCT is also considered as optimality by sub-Riemannian
geodesic length. The work has two aims: one is to develop a t-QCT itself based
on physically natural concept of time, and the other is to pursue the
possibility of using t-QCT as a tool to estimate the complexity in conventional
gate-optimal quantum computation theory (g-QCT). In particular, we investigate
to what extent is true the statement: time complexity is polynomial in the
number of qubits if and only if so is gate complexity. In the analysis, we
relate t-QCT and optimal control theory (OCT) through fidelity-optimal
computation theory (f-QCT); f-QCT is equivalent to t-QCT in the limit of unit
optimal fidelity, while it is formally similar to OCT. We then develop an
efficient numerical scheme for f-QCT by modifying Krotov's method in OCT, which
has monotonic convergence property. We implemented the scheme and obtained
solutions of f-QCT and of t-QCT for the quantum Fourier transform and a unitary
operator that does not have an apparent symmetry. The former has a polynomial
gate complexity and the latter is expected to have exponential one because a
series of generic unitary operators has a exponential gate complexity. The time
complexity for the former is found to be linear in the number of qubits, which
is understood naturally by the existence of an upper bound. The time complexity
for the latter is exponential. Thus the both targets are examples satisfyng the
statement above. The typical characteristics of the optimal Hamiltonians are
symmetry under time-reversal and constancy of one-qubit operation, which are
mathematically shown to hold in fairly general situations.Comment: 11 pages, 6 figure
Entanglement vs. the quantum-to-classical transition
We analyze the quantum-to-classical transition (QCT) for coupled bipartite
quantum systems for which the position of one of the two subsystems is
continuously monitored. We obtain the surprising result that the QCT can emerge
concomitantly with the presence of highly entangled states in the bipartite
system. Furthermore the changing degree of entanglement is associated with the
back-action of the measurement on the system and is itself an indicator of the
QCT. Our analysis elucidates the role of entanglement in von Neumann's paradigm
of quantum measurements comprised of a system and a monitored measurement
apparatus
Conditions for the Quantum to Classical Transition: Trajectories vs. Phase Space Distributions
We contrast two sets of conditions that govern the transition in which
classical dynamics emerges from the evolution of a quantum system. The first
was derived by considering the trajectories seen by an observer (dubbed the
``strong'' transition) [Bhattacharya, et al., Phys. Rev. Lett. 85: 4852
(2000)], and the second by considering phase-space densities (the ``weak''
transition) [Greenbaum, et al., Chaos 15, 033302 (2005)]. On the face of it
these conditions appear rather different. We show, however, that in the
semiclassical regime, in which the action of the system is large compared to
, and the measurement noise is small, they both offer an essentially
equivalent local picture. Within this regime, the weak conditions dominate
while in the opposite regime where the action is not much larger than Planck's
constant, the strong conditions dominate.Comment: 8 pages, 2 eps figure
Machine Learning for Observables: Reactant to Product State Distributions for Atom-Diatom Collisions
Machine learning-based models to predict product state distributions from a
distribution of reactant conditions for atom-diatom collisions are presented
and quantitatively tested. The models are based on function-, kernel- and
grid-based representations of the reactant and product state distributions.
While all three methods predict final state distributions from explicit
quasi-classical trajectory simulations with R > 0.998, the grid-based
approach performs best. Although a function-based approach is found to be more
than two times better in computational performance, the kernel- and grid-based
approaches are preferred in terms of prediction accuracy, practicability and
generality. The function-based approach also suffers from lacking a general set
of model functions. Applications of the grid-based approach to nonequilibrium,
multi-temperature initial state distributions are presented, a situation common
to energy distributions in hypersonic flows. The role of such models in Direct
Simulation Monte Carlo and computational fluid dynamics simulations is also
discussed
On the influence of collisional rate coefficients on the water vapour excitation
Water is a key molecule in many astrophysical studies. Its high dipole moment
makes this molecule to be subthermally populated under the typical conditions
of most astrophysical objects. This motivated the calculation of various sets
of collisional rate coefficients (CRC) for HO (with He or H) which are
necessary to model its rotational excitation and line emission. We performed
accurate non--local non--LTE radiative transfer calculations using different
sets of CRC in order to predict the line intensities from transitions that
involve the lowest energy levels of HO (E 900 K). The results obtained
from the different CRC sets are then compared using line intensity ratio
statistics. For the whole range of physical conditions considered in this work,
we obtain that the intensities based on the quantum and QCT CRC are in good
agreement. However, at relatively low H volume density ((H)
10 cm) and low water abundance ((HO) 10), these
physical conditions being relevant to describe most molecular clouds, we find
differences in the predicted line intensities of up to a factor of 3 for
the bulk of the lines. Most of the recent studies interpreting early Herschel
Space Observatory spectra used the QCT CRC. Our results show that although the
global conclusions from those studies will not be drastically changed, each
case has to be considered individually, since depending on the physical
conditions, the use of the QCT CRC may lead to a mis--estimate of the water
vapour abundance of up to a factor of 3
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