67 research outputs found
Quantum State Discrimination on Reconfigurable Noise-Robust Quantum Networks
A fundamental problem in Quantum Information Processing is the discrimination
amongst a set of quantum states of a system. In this paper, we address this
problem on an open quantum system described by a graph, whose evolution is
defined by a Quantum Stochastic Walk. In particular, the structure of the graph
mimics those of neural networks, with the quantum states to discriminate
encoded on input nodes and with the discrimination obtained on the output
nodes. We optimize the parameters of the network to obtain the highest
probability of correct discrimination. Numerical simulations show that after a
transient time the probability of correct decision approaches the theoretical
optimal quantum limit. These results are confirmed analytically for small
graphs. Finally, we analyze the robustness and reconfigurability of the network
for different set of quantum states, and show that this architecture can pave
the way to experimental realizations of our protocol as well as novel quantum
generalizations of deep learning
Receiver Design for Quantum Communication
Born about a century ago, Quantum Mechanics has revolutionized the description and the interpretation of Physics at sub-microscopic level.
In the last decades, due to the influence of mathematical and engineering research fields, Quantum Mechanics has given birth to related research areas like Quantum Computation, Quantum Information and Quantum Communication.
With the discovery of the laser, and later the development of fiber optics and satellite networks, Quantum Communication and Quantum Optics seems to have a natural field of application in Communication Systems.
Despite this, the interest in this technology and studies for communication purpose has been overshadowed by the great results in communication networks achieved in the last decades with classical paradigms. However, due to the increasing demand of communication data rate, system designers are now looking at Quantum Mechanics for new and more performanting solutions in communication purposes.
Early theoretical studies on Quantum Discrimination Theory and Quantum Information predict better performance for Communication Systems that take advantage of the quantum laws.
In addition, Quantum Mechanics provides the deepest description of the physical phenomena, and there are scenarios where a quantum model fits best, as in in deep space communications, where the received signal is really weak, or in a satellite networks, where we are interested in strongly reducing the power of transmitted signals, possibly without sacrificing performance significantly.
However, if on one side Quantum Communication Theory promises great gains in the performance of communication systems, on the other hand it fails to describe how to implement physical devices that reach these ultimate limits. So far, only a few architectures achieving these performances are known, and only for simple modulation formats.
We are interested in the scenario of optical communications, where the message transmitted is encoded in a sequence of coherent states.
Transmitter devices for coherent modulation are well known and consist in laser pulse generators. Instead, receiver implementations working at the quantum limit performance limit are yet to be found.
In this Thesis I deal with different topics in the quantum transmission scenario.
First, I review existing classical (suboptimal) and quantum (suboptimal and optimal) receiver schemes for the binary coherent modulation. I present a new formulation of the optimal scheme known as Dolinar Receiver with the multiple copies problem, focusing on the information gained during the measurement.
Second, I analyze the binary communication in a noisy environment, studying the error probability and the capacity of the binary channel induced. Given the description of the quantum channel, I optimize both the transmitted quantum states and the measurement operators employed in the communication.
Third, I consider the Pulse Position Modulation, that is particularly suitable for space and satellite communication due to its simplicity of implementation and high capacity. I review some known suboptimal receivers, and I propose a receiver scheme which approaches the limit performance predicted with quantum theory outperforming the existing schemes.
To sum up the results of this Thesis, in order to approach the limit performance predicted by Quantum Mechanics, an optimization is always necessary to exceed the classical effects and trigger the quantum phenomena. In particular, the information gained during the measurement plays an important role, for example in the definition of adaptive receivers. In this Thesis both these aspects have been deeply investigated
Role of the filter functions in noise spectroscopy
The success of quantum noise sensing methods depends on the optimal interplay
between properly designed control pulses and statistically informative
measurement data on a specific quantum-probe observable. To enhance the
information content of the data and reduce as much as possible the number of
measurements on the probe, the filter orthogonalization method has been
recently introduced. The latter is able to transform the control filter
functions on an orthogonal basis allowing for the optimal reconstruction of the
noise power spectral density. In this paper, we formalize this method within
the standard formalism of minimum mean squared error estimation and we show the
equivalence between the solutions of the two approaches. Then, we introduce a
non-negative least squares formulation that ensures the non-negativeness of the
estimated noise spectral density. Moreover, we also propose a novel protocol
for the design in the frequency domain of the set of filter functions. The
frequency-designed filter functions and the non-negative least squares
reconstruction are numerically tested on noise spectra with multiple components
and as a function of the estimation parameters
Characterization of entangling properties of quantum measurement via two-mode quantum detector tomography using coherent state probes
Entangled measurement is a crucial tool in quantum technology. We propose a
new entanglement measure of multi-mode detection, which estimates the amount of
entanglement that can be created in a measurement. To illustrate the proposed
measure, we perform quantum tomography of a two-mode detector that is comprised
of two superconducting nanowire single photon detectors. Our method utilizes
coherent states as probe states, which can be easily prepared with accuracy.
Our work shows that a separable state such as a coherent state is enough to
characterize a potentially entangled detector. We investigate the entangling
capability of the detector in various settings. Our proposed measure verifies
that the detector makes an entangled measurement under certain conditions, and
reveals the nature of the entangling properties of the detector. Since the
precise characterization of a detector is essential for applications in quantum
information technology, the experimental reconstruction of detector properties
along with the proposed measure will be key features in future quantum
information processing.Comment: 18 pages, 6 figure
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