Quantum pattern recognition on a quantum channel discrimination experimental sensing

The use of quantum states of light, such as entanglement and squeezing, allows surpassing the limitation of conventional measurement increasing the amount of information extracted from an object under investigation.
Here I will present the realization of the experimental sensing protocols in the framework of quantum hypothesis testing and channel discrimination focusing in three different tasks: quantum reading, quantum conformance test and quantum pattern recognition. The quantum hypothesis test is considered in the case of the parameter under investigation as an optical loss determined by the transmission properties of the object. The quantum enhancement in the estimation of the loss parameter distributed in a 2-D object leads to full field sub-shot-noise imaging.
Here we have considered the general multi-cell scenario, where the information can be stored in complex patterns, rather in each single cell of a memory or individual pixel of an image. The quantum enhanced readout of the cells is expected to produce a more efficient classification of the patterns. In this experiment we have considered the problem of handwritten digit classification with supervised learning algorithms.

Spatial indistinguishability-based thermal machine enabling quantum entanglement and cooling processes

Indistinguishability has a remarkable role when it comes to the understanding of identical quantum entities. Usually, this genuinely quantum property arises from the unaddressability of particles of the same kind when wavefunctions become spatially overlapping. Indistinguishability of identical quantum subsystems is an exploitable resource for quantum information processing, including teleportation, quantum estimation, entanglement distribution between nodes of a quantum network Here we show how an equilibrium thermal state, composed of two identical qubits, can be manipulated by adjusting the spatial indistinguishability (SI) of the qubits. Via this fundamental mechanism, we develop a SI-based quantum machine which produces robust quantum resources, such as entanglement and coherence, at any temperature. We also demonstrate that this thermal machine can act as a refrigerator by harnessing SI. These results open new pathways for SI-fueled quantum thermodynamic processes.

Entanglement Witnessing for Lattice Gauge Theories

LGTs are at the core of fundamental physics and, recently, substantial theoretical and experimental efforts have gone into simulating LGTs using quantum technologies., In the quantum realm, entanglement plays a crucial role and its detection can be efficiently performed using entanglement witnesses., Yet, entanglement witnessing in LGTs is extremely challenging due to the gauge constraints, that severely limit the operators that can be employed to detect quantum correlations., In this work, we develop the theoretical framework of entanglement witnessing in lattice gauge theories and, by way of illustration, consider bipartite entanglement witnesses in a U(1) LGT (with and without fermionic matter)., Our framework, which avoids the costly measurements required, e.g., by full-tomography,opens the way to future theoretical and experimental studies of entanglement in an important class of many-body models.

Cascaded optomechanical systems

In optomechanical systems, light modes interact with massive mechanical oscillators, leading to  quantum technologies applications. Depending on the configuration of the system, the optomechanical interaction can be used to drive or cool the mechanical mode near to its ground state , generating squeezing or create entanglement between optical and mechanical modes. A natural extension consists into consider not only one optical and one mechanical, but more modes.  One way to do that is to consider a  scheme in which the cavity modes are coupled to a unidirectional waveguide, resulting, in this way, placed in a cascaded configuration. This induce a non-reciprocal interaction at first glance between the cavities and indirectly between the mechanical oscillators. In the weak coupling regime the cavity field modes can be adiabatically eliminated resulting in an effective coupling between te two mechanical oscillators. This framework can be used to investigate the dynamics of the system and the possibility of engineering a temperature gradient.

Characterization of Kinetic Degrees of Freedom as a Control for Implementing Time-Dependent Hamiltonians

In many situations, the kinetic degree of freedom of moving particles is used to implement time-dependent Hamiltonians on internal degrees of freedom. The supposedly implemented (time-dependent, i.e., non-autonomous) dynamics is exact only in the ideal case of an infinitely massive point-like particle. Here, we compute the correction to the dynamics of the internal degrees of freedom due to the small yet finite spatial extension of the moving particle by using a fully quantum description. Looking at the dynamics from a thermodynamics perspective and using a generalized definition of work, we define the efficiency of the energy transfer between kinetic and internal degrees of freedom and use it to quantify the quality of the time-dependent Hamiltonian implementation. The analytical expression of both the correction to the dynamics and the quality of the time-dependent Hamiltonian implementation turn out to be proportional to the square of the spatial extension of the moving particle wavepacket.

Indistinguishability-based direct measurement of the exchange phase of identical quantum particles

The symmetrization postulate in quantum mechanics, leads to the appearance of, an exchange phase dictating the symmetry of identical, particle global states under particle swapping. Many indirect measurements of such a, fundamental phase have been reported so far, while a direct observation has been only recently, carried out for photons. We introduce, a general scheme capable to directly measure, the exchange phase of any type of particles, (bosons, fermions, anyons), exploiting spatial indistinguishability within the operational framework of spatially localized operations, and classical communication. An experimental, implementation has been performed in an all-optical platform, providing a direct measurement of the real bosonic exchange, phase of photons and a proof-of-principle measurement of different simulated exchange phases., Our results confirm the symmetrization tenet and, provide a tool to explore it in various scenarios: with this regard, an experimental implementation in double quantum dots is being designed to achieve the first direct measurement of the fermionic exchange phase.

Quantum Fisher Information as tool for detecting topological phases

Quantum Fisher Information (QFI) is known to provide a valuable tool for measuring the Multipartite Entanglement (ME) in one-dimensional models, which can give valuable information about the existence of topological phases. In this work we consider two paradigmatic models: the Kitaev chain, a toy model of a topological superconductor, and the Bilinear-Biquadratic model, a general SU(2)-invariant spin-1 chain. The former is also generalized to include a long-range coupling, which decays as function of the distance between sites with a power law. We show that the scaling of the QFI of strictly non-local observables can be used for characterizing the phase diagrams and, in particular, for detecting topological phases, where it scales maximally. Numerical results obtained with the DMRG algorithm, are tested against known results of the Bilinear-Biquadratic model and a new analytical calculation of the QFI for the Kitaev chain, showing the emergence of a a new kind of topological phase in the strongly long- range regime.

Support Vector Machine Classification of Entangled States

Quantum entanglement is one of the main features that distinguish a classical from a quantum state. This distinction has real application, as quantum entanglement is the basic resource for quantum computation advantage. Although the importance of detecting an entangled state is clear to the community, we still miss a universal recipe for entanglement classification, with analytical results obtained for low dimensional system (2 qubits or 1 qubit and 1 qutrit) and some special cases of higher dimensional system. Classification tasks have been solved with high precision by machine learning algorithms. In particular, we are interested in the support vector machine (SVM), which separates two regions of the space by an optimal hyperplane/hypersurface. In this work we develop an algorithm to use SVM classifiers to divide separable and entangled states. We apply this technique to two-qubit and three-qubit system, showing the power of this protocol. Finally, we relate the separating hyperplane to an operational procedure we can implement on a quantum computer, making use of copies of the input state.

Efficient optimisation for the implementation of QAOA on NISQ devices: a Bayesian approach

Quantum Approximate Optimization Algorithm (QAOA) is a variational hybrid
quantum-classical algorithm often considered as a benchmark to test the validity of quantum computers.
It relies on the estimation of the energy on a variational state prepared via a quantum circuit,
depending on parameters fixed via classical optimisation techniques.
While many theoretical results prove the efficiency of this algorithm there are
two main problems to deal with: the presence of barren plateaus in the optimization landscape,
the need to take into consideration the limits of the NISQ era devices.
In this work we propose a Bayesian optimization, a global approach that makes
use of a probabilistic model to sample in an efficient way the evaluation of objective function,
so that to make predictions about the landscape of the energy we are trying to minimize.
We apply it to typical combinatorial problems on graph and show it converges to
a minimum with a very a limited number of calls to the circuit with respect to
standard global optimization routines. We are also able to prove that it is resistant against noise.

Noisy quantum batteries: optimizing the output ergotropy

Energy-storing devices which use quantum effects (quantum batteries) are expected to provide an advantage in terms of charging power with respect to their classical counterparts. However another crucial feature that needs to be assessed is the ability of quantum batteries to store energy through a period of time withstanding self-discharging and noise.
In this work we characterize the best way to store a total energy E in an array of n (two-level) noisy quantum batteries, with the aim of retrieving the maximum possible energy after the batteries have undergone some environmental noise. We consider several kinds of detrimental noise: energy decay and thermalization (generalized amplitude damping channels), loss of coherence (dephasing channels) and depolarization.
We consider both the case in which the allowed number of quantum batteries n is restrained to a fixed fraction of the initial energy E to store in the batteries, and the case in which we are allowed to use an unlimited number of quantum batteries (E/n tending to infinity). For some noise channels (most notably, the generalized amplitude damping channel) storing the energy in a large number of batteries is the best way to prevent the degradation of extractable work due to the use of quantum coherence in energy allocation. However, this is not the case for all the kinds of models: we find some quantum channels for which the ergotropy is best preserved by keeping a finite ratio E/n. This result shows that quantum resources, apart from providing an advantage in the charging power of quantum batteries, can also be helpful in preventing their degradation by environmental noise.

Non-equilibrium quantum thermodynamics of a particle trapped in a controllable time-varying potential

Many advanced quantum techniques feature non-Gaussian dynamics, and the ability to manipulate the system in that domain is the next-stage in many experiments. One example of meaningful non-Gaussian dynamics is that of a double-well potential. Here we study the dynamics of a levitated nanoparticle undergoing the transition from an harmonic potential to a double-well in a realistic setting, subjecting to both thermalisation and localisation. We characterise the dynamics of the nanoparticle from a thermodynamic point-of-view, investigating the dynamics with the Wehrl entropy production and its rates. Furthermore, we investigate coupling regimes where the the quantum effect and thermal effect are of the same magnitude, and look at suitable squeezing of the initial state that provides the maximum coherence. The effects and the competitions of the unitary and the dissipative parts onto the system are demonstrated. We quantify the requirements to relate our results to a bonafide experiment with the presence of the environment, and discuss the experimental interpretations of our results in the end.

Spin phase-space approach to the study of the effect of coherence on the entropy production rate

Recent studies have pointed out the intrinsic dependence of figures of merit of thermodynamic relevance \[Dash] such as work, heat and entropy production \[Dash] on the amount of quantum coherences that is made available to a system. However, whether coherences hinder or enhance the value taken by such quantifiers of thermodynamic performance is yet to be ascertained. We show that, when considering entropy production generated in a process taking a finite-size bipartite quantum system out of equilibrium through local non-unitary channels, no general
hierarchy exists between the entropy production and degree of quantum coherence in the state of the system. A direct correspondence between such quantities can be retrieved when considering specific forms of open-system dynamics applied to suitably chosen initial states. Our results call for a systematic study of the role of genuine quantum features in the non-equilibrium thermodynamics of quantum processes.