708 research outputs found
Relative resilience to noise of standard and sequential approaches to measurement-based quantum computation
A possible alternative to the standard model of measurement-based quantum
computation (MBQC) is offered by the sequential model of MBQC -- a particular
class of quantum computation via ancillae. Although these two models are
equivalent under ideal conditions, their relative resilience to noise in
practical conditions is not yet known. We analyze this relationship for various
noise models in the ancilla preparation and in the entangling-gate
implementation. The comparison of the two models is performed utilizing both
the gate infidelity and the diamond distance as figures of merit. Our results
show that in the majority of instances the sequential model outperforms the
standard one in regard to a universal set of operations for quantum
computation. Further investigation is made into the performance of sequential
MBQC in experimental scenarios, thus setting benchmarks for possible cavity-QED
implementations.Comment: 11 pages, 11 figures; close to published versio
Spectral Methods and Running Scales in Causal Dynamical Triangulations
The spectrum of the Laplace-Beltrami operator, computed on the spatial slices
of Causal Dynamical Triangulations, is a powerful probe of the geometrical
properties of the configurations sampled in the various phases of the lattice
theory. We study the behavior of the lowest eigenvalues of the spectrum and
show that this can provide information about the running of length scales as a
function of the bare parameters of the theory, hence about the critical
behavior around possible second order transition points in the CDT phase
diagram, where a continuum limit could be defined.Comment: 5 pages, 6 figure
"Muy señor nuestro Alessandro Varaldo". La ricognizione del mondo spagnolo e portoghese per riscoprire un autore italiano di successo ma dimenticato
Who is Alessandro Varaldo (1876-1953)? This first analysis about what was said, adapted and translated of Varaldo’s work in the Iberian Peninsula and South America, maybe the only areas outside Italy where some traces of Italian author can be found, is a chance to go along his literary paths that overtook two temporal barriers (1900 and 1950), although it has not overtaken the oblivion’s threshold.Questa è una prima ricognizione di ciò che è stato detto, adattato e tradotto di Alessandro Varaldo (1876-1953) nella Penisola iberica e in Sud America, che si sospetta siano gli unici territori stranieri dove si riesca a ritracciare qualcosa dell’autore italiano, ed è un’occasione per ripercorrerne la traiettoria letteraria che ha superato due transenne temporali come il 1900 e il 1950 ma non quella della dimenticanza
Quantum correlations and thermodynamic performances of two-qubit engines with local and collective baths
We investigate heat engines whose working substance is made of two coupled
qubits performing a generalised Otto cycle by varying their applied magnetic
field or their interaction strength during the compression and expansion
strokes. During the heating and cooling strokes, the two qubits are coupled to
local and common environments that are not necessarily at equilibrium. We find
instances of quantum engines coupled to non equilibrium common environments
exhibiting non-trivial connections to quantum correlations as witnessed by a
monotonic dependence of the work produced on quantum discord and entanglement.Comment: Close to published versio
Arbitrary multimode Gaussian operations on mechanical cluster states
We consider opto- and electro-mechanical quantum systems composed of a driven
cavity mode interacting with a set of mechanical resonators. It has been
proposed that the latter can be initialized in arbitrary cluster states,
including universal resource states for Measurement Based Quantum Computation
(MBQC). We show that, despite the unavailability in this set-up of direct
measurements over the mechanical resonators, computation can still be performed
to a high degree of accuracy. In particular, it is possible to indirectly
implement the measurements necessary for arbitrary Gaussian MBQC by properly
coupling the mechanical resonators to the cavity field and continuously
monitoring the leakage of the latter. We provide a thorough theoretical
analysis of the performances obtained via indirect measurements, comparing them
with what is achievable when direct measurements are instead available. We show
that high levels of fidelity are attainable in parameter regimes within reach
of present experimental capabilities.Comment: 12 pages, 8 figure
Non-equilibrium thermodynamics of the spin Seebeck and spin Peltier effects
We study the problem of magnetization and heat currents and their associated
thermodynamic forces in a magnetic system by focusing on the magnetization
transport in ferromagnetic insulators like YIG. The resulting theory is applied
to the longitudinal spin Seebeck and the spin Peltier effects. By focusing on
the specific geometry with one YIG layer and one Pt layer, we obtain the
optimal conditions for generating large magnetization currents into Pt or large
temperature effects in YIG. The theoretical predictions are compared with
experiments from the literature permitting to derive the values of the
thermomagnetic coefficients of YIG: the magnetization diffusion length m and the absolute thermomagnetic power coefficient TK.Comment: accepted for publication on Physical Review
Non-equilibrium thermodynamics of the longitudinal spin Seebeck effect
In this paper we employ non equilibrium thermodynamics of fluxes and forces
to describe magnetization and heat transport. By the theory we are able to
identify the thermodynamic driving force of the magnetization current as the
gradient of the effective field . This definition permits to define
the spin Seebeck coefficient which relates and the
temperature gradient . By applying the theory to the geometry of the
longitudinal spin Seebeck effect we are able to obtain the optimal conditions
for generating large magnetization currents. Furthermore, by using the results
of recent experiments, we obtain an order of magnitude for the value of
TK for yttrium iron garnet
(YFeO).Comment: accepted for publication on Physics Procedi
Gapped Two-Body Hamiltonian for continuous-variable quantum computation
We introduce a family of Hamiltonian systems for measurement-based quantum
computation with continuous variables. The Hamiltonians (i) are quadratic, and
therefore two body, (ii) are of short range, (iii) are frustration-free, and
(iv) possess a constant energy gap proportional to the squared inverse of the
squeezing. Their ground states are the celebrated Gaussian graph states, which
are universal resources for quantum computation in the limit of infinite
squeezing. These Hamiltonians constitute the basic ingredient for the adiabatic
preparation of graph states and thus open new venues for the physical
realization of continuous-variable quantum computing beyond the standard
optical approaches. We characterize the correlations in these systems at
thermal equilibrium. In particular, we prove that the correlations across any
multipartition are contained exactly in its boundary, automatically yielding a
correlation area law.Comment: 4 pages, one figure. New version: typos corrected, one reference
added. To appear in PR
Unconditional measurement-based quantum computation with optomechanical continuous variables
Universal quantum computation encoded over continuous variables can be
achieved via Gaussian measurements acting on entangled non-Gaussian states.
However, due to the weakness of available nonlinearities, generally these
states can only be prepared conditionally, potentially with low probability.
Here we show how universal quantum computation could be implemented
unconditionally using an integrated platform able to sustain both linear and
quadratic optomechanical-like interactions. Specifically, considering cavity
opto- and electro-mechanical systems, we propose a realisation of a
driven-dissipative dynamics that deterministically prepares the required
non-Gaussian cluster states --- entangled squeezed states of multiple
mechanical oscillators suitably interspersed with cubic-phase states. We next
demonstrate how arbitrary Gaussian measurements on the cluster nodes can be
performed by continuously monitoring the output cavity field. Finally, the
feasibility requirements of this approach are analysed in detail, suggesting
that its building blocks are within reach of current technology.Comment: 5 pages + 9 pages supplementary materia
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