6 research outputs found

### Two-walker discrete-time quantum walks on the line with percolation

One goal in the quantum-walk research is the exploitation of the intrinsic
quantum nature of multiple walkers, in order to achieve the full computational
power of the model. Here we study the behaviour of two non-interacting
particles performing a quantum walk on the line when the possibility of lattice
imperfections, in the form of missing links, is considered. We investigate two
regimes, statical and dynamical percolation, that correspond to different time
scales for the imperfections evolution with respect to the quantum-walk one. By
studying the qualitative behaviour of three two-particle quantities for
different probabilities of having missing bonds, we argue that the chosen
symmetry under particle-exchange of the input state strongly affects the output
of the walk, even in noisy and highly non-ideal regimes. We provide evidence
against the possibility of gathering information about the walkers
indistinguishability from the observation of bunching phenomena in the output
distribution, in all those situations that require a comparison between
averaged quantities. Although the spread of the walk is not substantially
changed by the addition of a second particle, we show that the presence of
multiple walkers can be beneficial for a procedure to estimate the probability
of having a broken link.Comment: 16 pages, 9 figure

### Nonclassicality detection and communication bounds in quantum networks

Quantum information investigates the possibility of enhancing our ability to process and transmit information by directly exploiting quantum mechanical laws. When searching for improvement opportunities, one typically starts by assessing the range of outcomes classically attainable, and then investigates to what extent control over the quantum features of the system could be helpful, as well as the best performance that could be achieved. In this thesis we provide examples of these aspects, in linear optics, quantum metrology, and quantum communication.
We start by providing a criterion able to certify whether the outcome of a linear optical evolution cannot be explained by the classical wave-like theory of light. We do so by identifying a tight lower bound on the amount of correlations that could be detected among output intensities, when classical electrodynamics theory is used to describe the fields.
Rather than simply detecting nonclassicality, we then focus on its quantification. In particular, we consider the characterisation of the amount of squeezing encoded on selected quantum probes by an unknown external device, without prior information on the direction of application. We identify the single-mode Gaussian probes leading to the largest average precision in noiseless and noisy conditions, and discuss the advantages arising from the use of correlated two-mode probes.
Finally, we improve current bounds on the ultimate performance attainable in a quantum communication scenario. Specifically, we bound the number of maximally entangled qubits, or private bits, shared by two parties after a communication protocol over a quantum network, without restrictions on their classical communication. As in previous investigations, our approach is based on the evaluation of the maximum amount of entanglement that could be generated by the channels in the network, but it includes the possibility of changing entanglement measure on a channel-by-channel basis. Examples where this is advantageous are discussed.Open Acces

### Gaussian Discriminating Strength

We present a quantifier of non-classical correlations for bipartite,
multi-mode Gaussian states. It is derived from the Discriminating Strength
measure, introduced for finite dimensional systems in A. Farace et al., New. J.
Phys. 16, 073010 (2014). As the latter the new measure exploits the Quantum
Chernoff Bound to gauge the susceptibility of the composite system with respect
to local perturbations induced by unitary gates extracted from a suitable set
of allowed transformations (the latter being identified by posing some general
requirements). Closed expressions are provided for the case of two-mode
Gaussian states obtained by squeezing or by linearly mixing via a beam-splitter
a factorized two-mode thermal state. For these density matrices, we study how
non-classical correlations are related with the entanglement present in the
system and with its total photon number.Comment: 11+6 pages, 4 figure

### NonClassicality Criteria in Multiport Interferometry

Interference lies at the heart of the behavior of classical and quantum
light. It is thus crucial to understand the boundaries between which
interference patterns can be explained by a classical electromagnetic
description of light and which, on the other hand, can only be understood with
a proper quantum mechanical approach. While the case of two-mode interference
has received a lot of attention, the multimode case has not yet been fully
explored. Here we study a general scenario of intensity interferometry: we
derive a bound on the average correlations between pairs of output intensities
for the classical wavelike model of light, and we show how it can be violated
in a quantum framework. As a consequence, this violation acts as a
nonclassicality witness, able to detect the presence of sources with
sub-Poissonian photon-number statistics. We also develop a criterion that can
certify the impossibility of dividing a given interferometer into two
independent subblocks.Comment: 5 + 3 pages, published versio

### Versatile Gaussian probes for squeezing estimation

We consider an instance of â€śblack-boxâ€ť quantum metrology in the Gaussian framework, where we aim to estimate the amount of squeezing applied on an input probe, without previous knowledge on the phase of the applied squeezing. By taking the quantum Fisher information (QFI) as the figure of merit, we evaluate its average and variance with respect to this phase in order to identify probe states that yield good precision for many different squeezing directions. We first consider the case of single-mode Gaussian probes with the same energy, and find that pure squeezed states maximize the average quantum Fisher information (AvQFI) at the cost of a performance that oscillates strongly as the squeezing direction is changed. Although the variance can be brought to zero by correlating the probing system with a reference mode, the maximum AvQFI cannot be increased in the same way. A different scenario opens if one takes into account the effects of photon losses: coherent states represent the optimal single-mode choice when losses exceed a certain threshold and, moreover, correlated probes can now yield larger AvQFI values than all single-mode states, on top of having zero variance

### Non-Classical Correlations in Quantum States

The presence of correlations which do not have a classical counterpart, shared among the subparts of a given system, is one of the best signatures of non-classicality for a quantum state. Although entanglement is the most remarkable among these correlations, even some separable (i.e. not entangled) mixed states can exhibit non-classical behaviours. After reviewing Quantum Discord, that has been the first correlation measure historically introduced, several other known quantifiers are presented.
Particular attention is dedicated to the geometric measure known as Trace Distance
Discord, that in this thesis has been evaluated in a new class of two-qubit states and
whose maximum value on the set of separable states has been found when the tested subsystem is a qubit and the other one is infinite-dimensional.
Starting from this scenario, a new measure of correlations is then introduced and characterized. It has a clear operational interpretation in the context of state discrimination: it quantifies the ability of a bipartite state to distinguish between the application or not of an unknown local unitary map acting on one subsystem, labelled with A. The dependence upon the spectrum of such unitary map is discussed, showing in particular that the harmonic choice is optimal at least for pure states with dA = 3 and whichever dB (here dA and dB being the dimensions of the two subsystems). It is proven that for a two dimensional subsystem A the maximum over the set of separable states is reached by classical-quantum ones: its value has been computed analytically when dB >= 3 and numerically when dB = 2.
A comparison among all the discussed measures is performed, plotting their value as a function of the parameter of the amplitude damping channel applied to the first subsystem. The result confirms how an appropriate local operation on the tested subsystem can enhance the amount of non-classical correlations.
Eventually, a possible generalization of the new measure to the set of bosonic Gaussian states for continuous variable systems is discussed, evaluating at last the proposed expression on two-mode squeezed thermal states