Quantum optics using atomic arrays

Our conventional theories of the quantum interactions between light and atomic media tend to treat the atoms as a smooth macroscopic medium, ignoring the possibility that the dynamics might depend strongly on microscopic configura- tions and disorder. Within classical optics, however, it is well-known that the de- tails of spatial configurations of scatterers — and the associated multiple scat- tering and interference of light — can give rise to important new phenomena and control, ranging from Anderson localization to photonic crystals and phased array antennas. We discuss our recent efforts to advance a theory of quantum multiple scattering of light. We also show how multiple scattering can be har- nessed as a powerful resource, including the possibility to realize polynomially or exponentially better error scalings for applications such as quantum memories of light and photon-photon gates, as a function of system resources.

Exploiting the photonic nonlinearity of free-space subwavelength arrays of atoms

In this talk I will discuss subwavelength atomic arrays – order arrays of atoms where the interatomic distance is smaller than wavelength of the relevant atomic transition – as a novel platform for light matter interaction at the quantum level. Ordered ensembles of atoms exhibit distinctive features from their disordered counterpart. In particular, while collective modes in disordered ensembles effi- ciently couple to light but show a linear optical response, collective subradiant excitations of subwavelength arrays are endowed with an intrinsic nonlinearity. Such nonlinearity has both a coherent and a dissipative component: two exci- tations propagating in the array scatter off each other leading to formation of correlations and to emission into free-space modes. We show how to take ad- vantage of such nonlinearity to coherently prepare a single excitation in a sub- radiant (dark) collective state of a one-dimensional array as well as to perform an entangling operation on dark states of parallel arrays. We discuss the main source of errors represented by disorder introduced by atomic center-of-mass fluctuations, and we propose a practical way to mitigate its effects.

Finite-size and multimerization effects in an array of emitters coupled to a waveguide

We present the features of a system of two-level quantum emitters, coupled to a single transverse mode of a closed waveguide, in which photon wavenum- bers and frequencies are discretized, and characterize the states in which one excitation is steadily shared between the field and the emitters. We quantify finite-size effects in the field-emitter interactions and identify a family of dressed bound states that represent the forerunners of bound states in the continuum. For these states, we discuss possible applications in the field of quantum infor- mation. We conclude by showing that, in the limit of infinite-length waveguide, we find the occurrence of multimerized bound states for multi-emitter arrays.

Quantum optics of giant atoms in engineered photonics baths

Giant atoms are an emerging paradigm of quantum optics, which can exhibit unprecedented effects thanks to their multiple, non-local coupling to a photonic waveguide/ lattice. Here, their behavior is for the first time settled within a gen- eral theory based on the Green’s function. This encompasses within a compre- hensive framework effects such as decoherence-free Hamiltonians in a waveg- uide and emergence of atom-photon bound states (BSs) in structured lattices. As a relevant application, we predict that in the photonic SSH model, in con- trast to normal atoms (local coupling), occurrence of zero-mode bound states is generally not guaranteed. This is shown to depend on the positions and phases of the coupling points inside the photonic lattice unit cell. [1] L. Leonforte, D. Valenti, B. Spagnolo, A. Carollo, F. Ciccarello, Nanophotonics 10, 4251 (2021), [2] L Leonforte, D. Valenti, B. Spagnolo, A. Carollo, F. Ciccarello, to appear on arXiv (2022)

Photonic quantum implementations of causal structures

General physical scenarios can be studied under the light shed by the causal framework where cause-effect relationships between variables are encoded in constraints on the observable correlations. This framework is a powerful tool fruitfully employed in different scientific disciplines, from medicine to physics. However, when quantum systems are employed and connected according to a causal structure, the classical causal constraints imposed by the considered sce- nario can be violated, giving rise to the so-called quantum nonlocality. This is the strongest evidence of the departure between classical and quantum physics. We present several photonic quantum implementations of different causal struc- tures. We start from the standard bipartite Bell scenario where violations of causal inequalities are optimized without any assumption on the system and the measurement devices. Then, we present the experimental study of new forms of quantum causal influences in a relaxed scenario by means of interventions, a fundamental tool in the causality framework. Finally, we describe the nonlocal correlations obtained in complex networks with independent sources where new forms of nonlocality arise.

Structured light: a tool for quantum information and ultra-sensitive measurements

Vectorial modes of light, a type of structured light where the polarization varies across the beam profile, are a useful tool in quantum information since they pro- vide large alphabets, rich entanglement structures and enhanced resilience to noise. In quantum communication, for instance, vectorial modes enable rota- tional invariant protocols, therefore overcoming the requirement of a shared ref- erence frame between users. Moreover, structured light can be a resource for enhanced sensing purposes as for instance in the “photonic gears” technique. This quantum inspired scheme enables a boost of sensitivity in mechanical dis- placements measurement thanks to a bidirectional mapping between the polar- ization state and a properly tailored vectorial mode of a paraxial light beam. By exploiting this technique, we recently measured, in ordinary ambient conditions, the relative shift between two objects with a resolution of 400 pm. Thanks to a single-optical-path scheme, photonic gears are intrinsically stable and could be implemented as a compact sensor, using cost effective integrated optics.

High-speed integrated source-device-independent quantum random number generator for space applications

Random numbers are a fundamental building block for many different appli- cations.Classical generators,based on algorithms or classical systems,are pre- dictable since they are based on deterministic processes,while Quantum ran- dom number generators (QRNG) exploit the intrinsic randomness of quantum mechanics to generate genuine randomness.However,practical QRNG are usu- ally trusting their devices and their security can be compromised in case of im- perfections or malicious external actions.Moreover,typical QRNG are complex and bulky devices,which cannot be fitted in mobile devices or are not suitable for space payloads.In this work we address the problems of security,speed and compactness with a single integrated device.In particular,we describe a source- device-independent protocol based on generic POVM.This allows us to certify the randomness without any assumption on the source.We implement it on a custom-designed silicon photonics chip of 5.6 x 2.5 mm.The silicon PIC inte- grates a full shot-noise-limited heterodyne detector,which allows to implement the QRNG protocol.Thanks to the performances of the devices we were able to certify 10.075 bits of entropy for measurement,for a record-breaking generation rate of 20.150 Gbps,including finite size effects.

Spin-orbit photonics for optical simulations of quantum walks

Engineering synthetic quantum evolutions in artificial systems has proved a powerful resource in various applications. An interesting example is provided by quantum walks (QWs), introduced back in the 90’s to describe a peculiar discrete- time motion of quantum particles on a lattice. Variants of QWs have been widely used for quantum simulation/computation, to model transport phenomena and to engineer topological phases of matter. Here I will report on our approach to the experimental implementation of QWs based on spin-orbit photonics. After associating walker positions with optical modes carrying quantized transverse momentum, we emulate the unitary QW evolution by coupling these via the diffractive action of periodic spin-orbit metasurfaces. These are liquid-crystal based birefringent optical elements, engineered to have a space-dependent ori- entation of the optic axis. After illustrating the working principle of this platform, I will present the results of a recent experiments on the implementation of QWs in their long-time limit. Eventually, I will discuss some prospects of these research activities.

Non-Markovian dynamics and measurement induced entanglement criticality

In recent years,there has been growing interest towards phase transitions in- duced by the interaction of a quantum system with its environment.These phase transitions arise from the competition between the unitarity of the system Hamil- tonian and the decoherence action of the environment,and can occur at the level of symmetry breaking (such as paramagnetic to ferromagnetic transitions) or at the level of the scaling law of the entanglement of the system.The latter transition has been studied in many settings,in an effort to understand under what condi- tions the information stored in a quantum system is robust against the action of the environment.However,all of the studies so far have focused on Markovian en- vironments,thus neglecting the memory effects present in realistic baths.In this talk,I will present results on the study of entanglement transitions in comparison to symmetry breaking transitions driven by the same dissipative mechanism.I will also discuss recent studies on how to study the dynamics of non-Markovian sys- tems at the level of quantum trajectories and the effect that the interaction with a non-Markovian environment has on the entanglement transition.

Quantum Generalized Hydrodynamics

Understanding the nonequilibrium dynamics of many-body quantum systems is typically a very hard task, due the exponential increase of the Hilbert space di- mension with the number of the system’s components. Though, in the case of quantum integrable models, a large-scale description of the nonequilibrium dy- namics is attained by means of an Euler hydrodynamic theory characterized by the presence of infinitely many conservation laws, dubbed Generalized Hydrody- namics (GHD). However, although GHD is able to predict the outcome of some experimental measures with great accuracy, such hydrodynamic viewpoint on the dynamics leads to a loss of large-scale quantum fluctuations and, conse- quently, to vanishing equal-time correlations. In order to capture these missing quantum effects, we incorporate an effective field theory description of leading quantum fluctuations over the evolving semi-classical background established by GHD. The resulting theory, called Quantum Generalized Hydrodynamics, gives asymptotically exact results for the dynamics of entanglement and of equal-time correlations which are not accessible by other standard methods at the current state of the art.

Natural gauge defermionization of lattice models

Numerical and quantum simulations of fermionic lattice models require a qubit encoding. Here I show that, by simply upgrading the fermion parity to a gauge symmetry, we naturally obtain an encoding with non-scaling complexity in the system size.