Analysis of spin-squeezing generation in cavity-coupled atomic ensembles with continuous measurements

Spin-squeezed states have metrological relevance, as their intrinsic quantum noise is reduced with respect to the standard quantum limit of spin-coherent states, thus enabling quantum-enhanced atomic clocks.
We study the generation of conditional spin squeezing in ensembles of three-level atoms coupled to a near-resonant cavity through continuous Quantum-Non Demolition measurements, starting with fully numerical simulations of the system dynamics. In particular, we consider the dependence of squeezing on various system parameters, including the coupling strength between atoms and cavity, photon decay and pumping rate, and number of atoms.
We then analyse a series of approximations that simplify the computation of the trajectories’ dynamics. In particular, we have determined the regime in which we can adiabatically remove the cavity, reducing the system to just the atomic degrees of freedom. Having characterised this “bad cavity regime”, we are able to determine the optimal super-classical scaling for the squeezing parameter with the total spin. We also discuss the different degrees of squeezing reachable with or without continuous feedback in a continuous measurement protocol.

Non-locality breaks the relations between measures of quantum objectivity

We show the existence of two different aspects of quantum objectivity,”redundancy” and “con-sensus”.Though used as synonyms in this context,we prove that they quantify different features
of the emergence of classicality from quantum mechanics.We show that the two main frameworks
to measure quantum objectivity, namely spectrum broadcast structure and quantum Darwinism,naturally emerge from these two notions.Furthermore, by analyzing explicit examples of non-local
states, we highlight the potentially stark difference between the degrees of redundancy and consensus.In particular, this causes a break in the hierarchical relations between spectrum broadcast structure
and quantum Darwinism.Our framework provides a new perspective to interpret known and future
results in the context of quantum objectivity, which paves the way for a deeper understanding of the
emergence of classicality from the quantum realm.

Probing the quantum Fisher information of passive states

The quantum Fisher information (QFI) is a measure of the metrological enhancement obtained from quantum correlations. As such, it provides a bridge between quantum metrology and the study of entanglement. Recent works have leveraged this connection to use the QFI as a scalable tool for certifying the presence of multipartite entanglement in quantum many-body systems. However, in general, it remains a challenge to calculate and measure the QFI for both theory and experiment and this limits its usage for entanglement detection. In this talk, we discuss a protocol for extracting the QFI of thermal states and how it can be extended to a much broader class of equilibrium states. More specifically, we study so-called passive states and how virtual temperatures can be used to obtain the QFI with the help of linear response theory. Numerical results showcase our generalized protocol for a specific model. Our work broadens the scope of applicability of the QFI and opens interesting questions regarding possible extensions to different entanglement measures.

Thermodynamics of Reduced State of the Field

Recent years have seen the flourishing of research devoted to quantum effects on mesoscopic and macroscopic scales. In this context, in Entropy 2019, 21, 705, a formalism aiming at describing macroscopic quantum fields, dubbed Reduced State of the Field (RSF), was envisaged. While, in the original work, a proper notion of entropy for macroscopic fields, together with their dynamical equations, was derived, here, we expand thermodynamic analysis of the RSF, discussing the notion of heat, solving dynamical equations in various regimes of interest, and showing the thermodynamic implications of these solutions.

Quantum Approximation Optimization Algorithm with Quantum Natural Gradient optimizer

Variational Quantum Algorithms (VQA) are powerful tools in the Noisy Intermediate-Scale Quantum (NISQ) era. They are based on the encoding the problem into a cost function depending on some trainable parameters. This is the case of the Quantum Approximation Optimization Algorithm (QAOA), a parameterized ansatz for a quantum state obtained by repeated applications of time evolution operators of two non-commuting Hamiltonians, complemented by a classical optimization procedure.

In this work we consider a Quantum Natural Gradient Descent method to optimize the parameters of the variational wave function prepared by the quantum circuit. This iterative procedure selects the best direction in the parameter space exploiting the quantum information coming from the real part of the Quantum Geometric Tensor (QGT), that is the well-known Fubini-Study metric tensor.

We calculate the latter on states evolved with QAOA ansatz and we show how this routine outperforms other classical methods finding the ground states with fewer steps. This method is applied to some interesting problems related to combinatorial optimization on graphs, simple molecules in quantum chemistry and spin systems.

Criticality and compatibility in multi-parameter quantum metrology.

Many-body systems near a quantum phase transition (QPT) exhibit several properties which makes them appealing for metrological purposes. Indeed, it is now well established that the divergences of the quantum Fisher information (QFI) observed near a QPT can be used to increase the precision in the estimation of a parameter. Meanwhile, when it comes to the simultaneous estimation of multiple parameters, the benefits of criticality are much harder to analyze due to possible incompatibilities arising from the Heisenberg uncertainty. This involves the use of quite convoluted quantities, as the Holevo-Cramer-Rao bound, which are generally difficult to evaluate. Here we study the quantumness (R), a scalar index, which provides an asymptotic bound on the compatibility of a metrological scheme. The advantage of this approach is that R can be easily evaluated once the QFI and the mean Uhlmann curvature are known. Moreover, a scaling analysis of R reveals that many-body criticalities generally improve the compatibility in a multi-parameter framework. We also evaluate R in different representative systems, such as Ising chain and XY chain, in which we find this positive criticality effects.

Time Dependent Markovian Master equation beyond the adiabatic limit

We develop a Master equation in the Born-Markov approximation for arbitrary driven systems coupled, to an environment. Our approach is formulated on a combination of coarse-graining in time and, weak coupling limit for the system-environment interaction and it also requires a secular approximation., We prove that our derivation is fundamentally based, on the adiabatic time-evolution operator, although it can efficiently describe strongly driven systems, and, thus it can be seen as a natural extension of the famous Adiabatic Markovian Master equation., Our result is applied to the paradigmatic cases of a qubit subject to linear and periodic driving, moreover, the solutions are benchmarked with numerical simulations by using Tensor Networks.

Measuring Fano quantum coherence in a hot Rubidium atomic vapor under incoherent light. An experimental proposal

Quantum interference is a quantum process whose effects range from changing optical properties of media to enhancing energy transport in light-harvesting complexes or information transfer in quantum networks. It can arise through the application of coherent as well as incoherent processes.
Solar cells could see improvements exploiting quantum interference between internal states. Previous works have theoretically demonstrated that a V-type three-level system driven by incoherent light, as a minimal model of a photocell, can experience quantum interference between the transitions from the excited states to a ground state. The phenomenon leads to a mitigation of radiative recombination and thus to an increase in photocurrent that can be extracted from the cell.
We propose an experiment realizing a V-type three-level system in the hyperfine structure of hot Rb atoms driven by an incoherent field. The aim is the observation of quantum beats in the fluorescence spectrum that proves the presence of interference and provides us new insight about quantum coherence terms originated by non-coherent excitation. This shall find application in future novel high-efficiency solar cells.

Direct measurements of multipartite spatial indistinguishability

The direct measurement of the spatial indistinguishability of identical particles as a quantum resource is of practical importance when complete information of the system state is unknown. We provide an experimentally-friendly scheme for measuring the amount of spatial indistinguishability in many-particle systems. Our scheme can be used to witness the structure of spatial overlap among the identical particles of the system without state tomography. In this sense, the measurement procedure is somewhat similar to a Bell non-locality test. The procedure is based on photon counting by using photon number resolving detectors (PNRD). Spatial indistinguishability is obtained as a function of photon-counting via combinations of local and nonlocal joint probabilities. We first develop our method for two and three identical particles, then generalizing it to the case of N identical particles in M sites, with 2 <= M <= N. We finally show how tuned multipartite spatial indistinguishability can be directly related to the generation of multipartite entanglement

Observable Signatures of Quantum Charge-Field Interactions

Different facets of the quantum nature of the electromagnetic field have been investigated starting from the observation of Lamb shift in 1947. Currently, the experimental realisation of sophisticated optomechanical systems has furnished a suitable platform for further investigations on the vacuum fluctuations of the electromagnetic field. We find that moments of appropriately chosen observables of one or two charged particles could potentially indicate the presence of a background quantum electromagnetic field in the vacuum state. We also document the changes in these moments due to the specific nature of the initial state. To further understand and identify clear signatures of the vacuum fluctuations, we also consider three different scenarios: a free charged particle, a single charged particle in a harmonic trap and two charged particles each in a harmonic trap coupled through the Coulomb force. Considering the fact that there has been recent interest in observable signatures of quantum nature of gravity, our investigation could provide a possible stepping stone for similar investigations in that context too.

Quantum-enhanced imaging for large baseline telescopes

In this work, we use quantum metrology tools to enhance the imaging resolution of large baseline telescopes. These apparatuses, through interferometric measurements, constitute systems for imaging with large aperture size, resulting in a high resolving power. A major challenge is represented by the dissipation encounter in the transmission, which increases with distances and significantly limits the baseline. The use of a quantum network can overcome some of the limits. The introduction of a higher number of photons to perform more interferometric measurements leads to a better imaging resolution. However, the more photons are used the more dissipation will affect the system. In this work we investigate the interplay between these two factors in an imaging apparatus consisting of two faraway telescopes and a set of entangled photons emitted from central sources. The coherent star photon will be interfered at both telescopes’ sites with the entangled photons and then measured. We study the resolution to find the optimal trade-off between the number of entangled photons and the loss effects to extend the baseline beyond the current limits.

Engineering nonlinear boson-boson interactions

The use of continuous-variable states has led to significant progress in the fields of quantum communication,quantum computing and quantum state engineering.In particular,encoding a qubit in a superposition of two opposite-phase coherent states has proved useful for quantum teleportation protocols,schemes for universal quantum computing and more.Difficulties arise,however,when it comes to generating entangled coherent states and there is a need to achieve effective interaction Hamiltonians which we can tune to produce nonclassical states of this form.In this work,we present a protocol to engineer nonlinear boson-boson interactions using mediating spin systems.We make use of the result that nonlinear spin-boson interactions can be simulated using linear spin-boson couplings by adding spin rotations and making a suitable transformation[1].Taking two spin-boson systems and allowing the spins to interact,we aim to manipulate the system’s dynamics in order to obtain certain interactions between the bosonic modes.We first focus on producing a cross-Kerr interaction which allows for the creation of entangled coherent states.We find that this is indeed possible by acting on both spins with local operations halfway through the evolution.Our simulations show that the entanglement dynamics of the bosons matches that of our target interaction as long as the spin-spin coupling is sufficiently strong.However,the amount of entanglement we can gain is restricted by the need to work in the Lamb-Dicke regime