356,466 research outputs found
Quantum information and precision measurement
We describe some applications of quantum information theory to the analysis
of quantum limits on measurement sensitivity. A measurement of a weak force
acting on a quantum system is a determination of a classical parameter
appearing in the master equation that governs the evolution of the system;
limitations on measurement accuracy arise because it is not possible to
distinguish perfectly among the different possible values of this parameter.
Tools developed in the study of quantum information and computation can be
exploited to improve the precision of physics experiments; examples include
superdense coding, fast database search, and the quantum Fourier transform.Comment: 13 pages, 1 figure, proof of conjecture adde
Quantum Coherence, Coherent Information and Information Gain in Quantum Measurement
A measurement is deemed successful, if one can maximize the information gain
by the measurement apparatus. Here, we ask if quantum coherence of the system
imposes a limitation on the information gain during quantum measurement. First,
we argue that the information gain in a quantum measurement is nothing but the
coherent information or the distinct quantum information that one can send from
the system to apparatus. We prove that the maximum information gain from a pure
state, using a mixed apparatus is upper bounded by the initial coherence of the
system. Further, we illustrate the measurement scenario in the presence of
environment. We argue that the information gain is upper bounded by the entropy
exchange between the system and the apparatus. Also, to maximize the
information gain, both the initial coherence of the apparatus, and the final
entanglement between the system and apparatus should be maximum. Moreover, we
find that for a fixed amount of coherence in the final apparatus state the more
robust apparatus is, the more will be the information gain.Comment: 6 Pages, Comments are welcom
Information and noise in quantum measurement
Even though measurement results obtained in the real world are generally both
noisy and continuous, quantum measurement theory tends to emphasize the ideal
limit of perfect precision and quantized measurement results. In this article,
a more general concept of noisy measurements is applied to investigate the role
of quantum noise in the measurement process. In particular, it is shown that
the effects of quantum noise can be separated from the effects of information
obtained in the measurement. However, quantum noise is required to ``cover up''
negative probabilities arising as the quantum limit is approached. These
negative probabilities represent fundamental quantum mechanical correlations
between the measured variable and the variables affected by quantum noise.Comment: 16 pages, short comment added in II.B., final version for publication
in Phys. Rev.
Residual and Destroyed Accessible Information after Measurements
When quantum states are used to send classical information, the receiver
performs a measurement on the signal states. The amount of information
extracted is often not optimal due to the receiver's measurement scheme and
experimental apparatus. For quantum non-demolition measurements, there is
potentially some residual information in the post-measurement state, while part
of the information has been extracted and the rest is destroyed. Here, we
propose a framework to characterize a quantum measurement by how much
information it extracts and destroys, and how much information it leaves in the
residual post-measurement state. The concept is illustrated for several
receivers discriminating coherent states.Comment: 5 pages, 1 figur
Exploiting the quantum Zeno effect to beat photon loss in linear optical quantum information processors
We devise a new technique to enhance transmission of quantum information
through linear optical quantum information processors. The idea is based on
applying the Quantum Zeno effect to the process of photon absorption. By
frequently monitoring the presence of the photon through a QND (quantum
non-demolition) measurement the absorption is suppressed. Quantum information
is encoded in the polarization degrees of freedom and is therefore not affected
by the measurement. Some implementations of the QND measurement are proposed.Comment: 4 pages, 1 figur
How much a Quantum Measurement is Informative?
The informational power of a quantum measurement is the maximum amount of
classical information that the measurement can extract from any ensemble of
quantum states. We discuss its main properties. Informational power is an
additive quantity, being equivalent to the classical capacity of a
quantum-classical channel. The informational power of a quantum measurement is
the maximum of the accessible information of a quantum ensemble that depends on
the measurement. We present some examples where the symmetry of the measurement
allows to analytically derive its informational power.Comment: 3 pages, 2 figures, published in the proceedings of the 11th Quantum
Communication, Measurement, and Computing (QCMC) conference, Vienna, Austria,
30 July-3 August, 201
Optimal signal states for quantum detectors
Quantum detectors provide information about quantum systems by establishing
correlations between certain properties of those systems and a set of
macroscopically distinct states of the corresponding measurement devices. A
natural question of fundamental significance is how much information a quantum
detector can extract from the quantum system it is applied to. In the present
paper we address this question within a precise framework: given a quantum
detector implementing a specific generalized quantum measurement, what is the
optimal performance achievable with it for a concrete information readout task,
and what is the optimal way to encode information in the quantum system in
order to achieve this performance? We consider some of the most common
information transmission tasks - the Bayes cost problem (of which minimal error
discrimination is a special case), unambiguous message discrimination, and the
maximal mutual information. We provide general solutions to the Bayesian and
unambiguous discrimination problems. We also show that the maximal mutual
information has an interpretation of a capacity of the measurement, and derive
various properties that it satisfies, including its relation to the accessible
information of an ensemble of states, and its form in the case of a
group-covariant measurement. We illustrate our results with the example of a
noisy two-level symmetric informationally complete measurement, for whose
capacity we give analytical proofs of optimality. The framework presented here
provides a natural way to characterize generalized quantum measurements in
terms of their information readout capabilities.Comment: 13 pages, 1 figure, example section extende
Conceptual Inadequacy of the Shannon Information in Quantum Measurements
In a classical measurement the Shannon information is a natural measure of
our ignorance about properties of a system. There, observation removes that
ignorance in revealing properties of the system which can be considered to
preexist prior to and independent of observation. Because of the completely
different root of a quantum measurement as compared to a classical measurement
conceptual difficulties arise when we try to define the information gain in a
quantum measurement using the notion of Shannon information. The reason is
that, in contrast to classical measurement, quantum measurement, with very few
exceptions, cannot be claimed to reveal a property of the individual quantum
system existing before the measurement is performed.Comment: 11 pages, 5 figures, important Ref. [6] is now cited in all
appropriate place
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