4,147 research outputs found
Quantum process tomography with unknown single-preparation input states
Quantum Process Tomography (QPT) methods aim at identifying, i.e. estimating,
a given quantum process. QPT is a major quantum information processing tool,
since it especially allows one to characterize the actual behavior of quantum
gates, which are the building blocks of quantum computers. However, usual QPT
procedures are complicated, since they set several constraints on the quantum
states used as inputs of the process to be characterized. In this paper, we
extend QPT so as to avoid two such constraints. On the one hand, usual QPT
methods requires one to know, hence to precisely control (i.e. prepare), the
specific quantum states used as inputs of the considered quantum process, which
is cumbersome. We therefore propose a Blind, or unsupervised, extension of QPT
(i.e. BQPT), which means that this approach uses input quantum states whose
values are unknown and arbitrary, except that they are requested to meet some
general known properties (and this approach exploits the output states of the
considered quantum process). On the other hand, usual QPT methods require one
to be able to prepare many copies of the same (known) input state, which is
constraining. On the contrary, we propose "single-preparation methods", i.e.
methods which can operate with only one instance of each considered input
state. These two new concepts are here illustrated with practical BQPT methods
which are numerically validated, in the case when: i) random pure states are
used as inputs and their required properties are especially related to the
statistical independence of the random variables that define them, ii) the
considered quantum process is based on cylindrical-symmetry Heisenberg spin
coupling. These concepts may be extended to a much wider class of processes and
to BQPT methods based on other input quantum state properties
Beyond the density operator and Tr(\rho A): Exploiting the higher-order statistics of random-coefficient pure states for quantum information processing
Two types of states are widely used in quantum mechanics, namely
(deterministic-coefficient) pure states and statistical mixtures. A density
operator can be associated with each of them. We here address a third type of
states, that we previously introduced in a more restricted framework. These
states generalize pure ones by replacing each of their deterministic ket
coefficients by a random variable. We therefore call them Random-Coefficient
Pure States, or RCPS. We analyze their properties and their relationships with
both types of usual states. We show that RCPS contain much richer information
than the density operator and mean of observables that we associate with them.
This occurs because the latter operator only exploits the second-order
statistics of the random state coefficients, whereas their higher-order
statistics contain additional information. That information can be accessed in
practice with the multiple-preparation procedure that we propose for RCPS, by
using second-order and higher-order statistics of associated random
probabilities of measurement outcomes. Exploiting these higher-order statistics
opens the way to a very general approach for performing advanced quantum
information processing tasks. We illustrate the relevance of this approach with
a generic example, dealing with the estimation of parameters of a quantum
process and thus related to quantum process tomography. This parameter
estimation is performed in the non-blind (i.e. supervised) or blind (i.e.
unsupervised) mode. We show that this problem cannot be solved by using only
the density operator \rho of an RCPS and the associated mean value Tr(\rho A)
of the operator A that corresponds to the considered physical quantity. We
succeed in solving this problem by exploiting a fourth-order statistical
parameter of state coefficients, in addition to second-order statistics.
Numerical tests validate this result.Comment: 1 figur
Single-preparation unsupervised quantum machine learning: concepts and applications
The term "machine learning" especially refers to algorithms that derive
mappings, i.e. intput/output transforms, by using numerical data that provide
information about considered transforms. These transforms appear in many
problems, related to classification/clustering, regression, system
identification, system inversion and input signal restoration/separation. We
here first analyze the connections between all these problems, in the classical
and quantum frameworks. We then focus on their most challenging versions,
involving quantum data and/or quantum processing means, and unsupervised, i.e.
blind, learning. Moreover, we propose the quite general concept of
SIngle-Preparation Quantum Information Processing (SIPQIP). The resulting
methods only require a single instance of each state, whereas usual methods
have to very accurately create many copies of each fixed state. We apply our
SIPQIP concept to various tasks, related to system identification (blind
quantum process tomography or BQPT, blind Hamiltonian parameter estimation or
BHPE, blind quantum channel identification/estimation, blind phase estimation),
system inversion and state estimation (blind quantum source separation or BQSS,
blind quantum entangled state restoration or BQSR, blind quantum channel
equalization) and classification. Numerical tests show that our framework
moreover yields much more accurate estimation than the standard
multiple-preparation approach. Our methods are especially useful in a quantum
computer, that we propose to more briefly call a "quamputer": BQPT and BHPE
simplify the characterization of the gates of quamputers; BQSS and BQSR allow
one to design quantum gates that may be used to compensate for the
non-idealities that alter states stored in quantum registers, and they open the
way to the much more general concept of self-adaptive quantum gates (see longer
version of abstract in paper).Comment: 7 figure
Use and misuse of variances for quantum systems in pure or mixed states
As a consequence of the place ascribed to measurements in the postulates of
quantum mechanics, if two differently prepared systems are described with the
same density operator \r{ho}, they are said to be in the same quantum state.
For more than fifty years, there has been a lack of consensus about this
postulate. In a 2011 paper, considering variances of spin components, Fratini
and Hayrapetyan tried to show that this postulate is unjustified. The aim of
the present paper is to discuss major points in this 2011 article, and in their
reply to a 2012 paper by Bodor and Diosi claiming that their analysis was
irrelevant. Facing some ambiguities or inconsistencies in the 2011 paper and in
the reply, we first try to guess their aim, then establish results useful in
this context, and finally discuss the use or misuse of several concepts implied
in this debate
Development of correction algorithm for pulsed terahertz computed tomography (THz-CT)
For last couple of decades, there has been a considerable improvement in Terahertz (THz) science, technology, and imaging. In particular, the technique of 3-D computed tomography has been adapted to the THz range. However, it has been widely recognized that a fundamental limitation to THz computed tomography imaging is the refractive effects of the sample under study. The finite refractive index of materials in the THz range can severally refract THz beams which probe the internal structure of a sample during the acquisition of tomography data. Refractive effects lead to anomalously high local absorption coefficients in the reconstructed image near the material’s boundaries. Three refractive effects are identified: (a) Fresnel reflection power losses at the boundaries, (b) an increase in path length of the probing THz radiation, and (c) steering of the THz beam by the sample such that the emerging THz radiation is no longer collected by the THz detector. In addition, the finite size of the THz beam dominates the measured THz transmission when the edges of the sample are probed using THz tomography. These boundary phenomena can dominate in the reconstructed THz-CT images making it difficult to distinguish any hidden finer structural defect(s) inside the material. In this dissertation, an algorithm has been developed to remove these refractive and finite beam size effects from THz-CT reconstructed images. The algorithm is successfully implemented on cylindrical shaped objects.
A longer term goal of the research is to study the internal structure of natural cork wine stoppers by pulsed Terahertz tomography (THz-CT). It has previously been shown that THz imaging can detect the internal structure of natural cork. Moreover, the internal structure of natural cork stoppers dominates the diffusion of gasses and liquids through the cork. By using THz computed tomography, one can recreate a 3D image of the sample’s internal structure which could then be used to predict non-destructively the diffusion properties of the cork. However, refractive and boundary effects which arise in the THz tomographic image masks the presence of the cork’s internal structure. Applying the correction algorithms which are developed in this dissertation to natural cork stoppers suppresses the refractive and boundary anomalies enabling better visualization of the cork’s internal structure
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