293 research outputs found
Progress toward scalable tomography of quantum maps using twirling-based methods and information hierarchies
We present in a unified manner the existing methods for scalable partial
quantum process tomography. We focus on two main approaches: the one presented
in Bendersky et al. [Phys. Rev. Lett. 100, 190403 (2008)], and the ones
described, respectively, in Emerson et al. [Science 317, 1893 (2007)] and
L\'{o}pez et al. [Phys. Rev. A 79, 042328 (2009)], which can be combined
together. The methods share an essential feature: They are based on the idea
that the tomography of a quantum map can be efficiently performed by studying
certain properties of a twirling of such a map. From this perspective, in this
paper we present extensions, improvements and comparative analyses of the
scalable methods for partial quantum process tomography. We also clarify the
significance of the extracted information, and we introduce interesting and
useful properties of the -matrix representation of quantum maps that can
be used to establish a clearer path toward achieving full tomography of quantum
processes in a scalable way.Comment: Replaced with published version (only minor changes respect to the
first version
Chiral-mediated entanglement in an Aharonov-Bohm ring
We study the orbital entanglement in a biased Aharonov-Bohm ring connected in
a four-terminal setup. We find that the concurrence achieves a maximum when the
magnetic flux B coincides with an integer number of a half flux quantum 0 /2.
We show that this behavior is a consequence of the existence of degenerate
states of the ring having opposite chirality. We also analyze the behavior of
the noise as a function of and discuss the reliability of this quantity as
evidence of entanglement.Comment: 7 pages, 5 figures; To appear in Phys. Rev.
A quantum gate array can be programmed to evaluate the expectation value of any operator
A programmable gate array is a circuit whose action is controlled by input
data. In this letter we describe a special--purpose quantum circuit that can be
programmed to evaluate the expectation value of any operator acting on a
space of states of dimensions. The circuit has a program register whose
state encodes the operator whose expectation value is to be
evaluated. The method requires knowledge of the expansion of in a basis of
the space of operators. We discuss some applications of this circuit and its
relation to known instances of quantum state tomography.Comment: 4 pages, 3 figures include
Quantum computation with phase drift errors
We present results of numerical simulations of the evolution of an ion trap
quantum computer made out of 18 ions which are subject to a sequence of nearly
15000 laser pulses in order to find the prime factors of N=15. We analyze the
effect of random and systematic phase drift errors arising from inaccuracies in
the laser pulses which induce over (under) rotation of the quantum state.
Simple analytic estimates of the tolerance for the quality of driving pulses
are presented. We examine the use of watchdog stabilization to partially
correct phase drift errors concluding that, in the regime investigated, it is
rather inefficient.Comment: 5 pages, RevTex, 2 figure
Testing integrability with a single bit of quantum information
We show that deterministic quantum computing with a single bit (DQC1) can
determine whether the classical limit of a quantum system is chaotic or
integrable using O(N) physical resources, where is the dimension of the
Hilbert space of the system under study. This is a square root improvement over
all known classical procedures. Our study relies strictly on the random matrix
conjecture. We also present numerical results for the nonlinear kicked top.Comment: Minor changes taking into account Howard Wiseman's comment:
quant-ph/0305153. Accepted for publication in Phys. Rev.
Chilean IPNV isolates: Robustness analysis of PCR detection
Background: The genomes of several infectious pancreatic necrosis
viruses (IPNVs) isolated in Chile were sequenced with a single
amplification approach for both segments A and B. The resulting
sequences were then used to determine the conservation of the
primer-binding regions used in polymerase chain reaction (PCR)-based
diagnostic methods proposed in the literature. Thus, the robustness of
each technique was studied, particularly the eventual effect of further
mutations within the primer-binding sites. Results: On analysis, most
methods currently used to detect Chilean IPNV varieties were deemed
adequate. However, the primers were designed to be genogroup specific,
implying that most detection methods pose some risk of detecting all
strains prevalent in the country, due to the coexistence of genogroups
1 and 5. Conclusions: Negative resultsmust be interpreted carefully
given the high genomic variability of IPNVs. Detection techniques
(quantitative reverse transcription (qRT)-PCR) based on degenerate
primers can be used to minimize the possibilities of false-negative
detections
A Method for Modeling Decoherence on a Quantum Information Processor
We develop and implement a method for modeling decoherence processes on an
N-dimensional quantum system that requires only an -dimensional quantum
environment and random classical fields. This model offers the advantage that
it may be implemented on small quantum information processors in order to
explore the intermediate regime between semiclassical and fully quantum models.
We consider in particular system-environment couplings which
induce coherence (phase) damping, though the model is directly extendable to
other coupling Hamiltonians. Effective, irreversible phase-damping of the
system is obtained by applying an additional stochastic Hamiltonian on the
environment alone, periodically redressing it and thereby irreversibliy
randomizing the system phase information that has leaked into the environment
as a result of the coupling. This model is exactly solvable in the case of
phase-damping, and we use this solution to describe the model's behavior in
some limiting cases. In the limit of small stochastic phase kicks the system's
coherence decays exponentially at a rate which increases linearly with the kick
frequency. In the case of strong kicks we observe an effective decoupling of
the system from the environment. We present a detailed implementation of the
method on an nuclear magnetic resonance quantum information processor.Comment: 12 pages, 9 figure
Factoring in a Dissipative Quantum Computer
We describe an array of quantum gates implementing Shor's algorithm for prime
factorization in a quantum computer. The array includes a circuit for modular
exponentiation with several subcomponents (such as controlled multipliers,
adders, etc) which are described in terms of elementary Toffoli gates. We
present a simple analysis of the impact of losses and decoherence on the
performance of this quantum factoring circuit. For that purpose, we simulate a
quantum computer which is running the program to factor N = 15 while
interacting with a dissipative environment. As a consequence of this
interaction randomly selected qubits may spontaneously decay. Using the results
of our numerical simulations we analyze the efficiency of some simple error
correction techniques.Comment: plain tex, 18 pages, 8 postscript figure
Decoherence and Initial Correlations in Quantum Brownian Motion
We analyze the evolution of a quantum Brownian particle starting from an
initial state that contains correlations between this system and its
environment. Using a path integral approach, we obtain a master equation for
the reduced density matrix of the system finding relatively simple expressions
for its time dependent coefficients. We examine the evolution of delocalized
initial states (Schr\"odinger's cats) investigating the effectiveness of the
decoherence process. Analytic results are obtained for an ohmic environment
(Drude's model) at zero temperature.Comment: 15 pages, RevTex, 5 figures included. Submitted to Phys. Rev.
Discrete Wigner functions and the phase space representation of quantum teleportation
We present a phase space description of the process of quantum teleportation
for a system with an dimensional space of states. For this purpose we
define a discrete Wigner function which is a minor variation of previously
existing ones. This function is useful to represent composite quantum system in
phase space and to analyze situations where entanglement between subsystems is
relevant (dimensionality of the space of states of each subsystem is
arbitrary). We also describe how a direct tomographic measurement of this
Wigner function can be performed.Comment: 8 pages, 1 figure, to appear in Phys Rev
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