Thursday 15 September 2022

Edificio 18

9:00 – 9:35


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.

9:35 – 10:00


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.

10:00 – 10:25


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.

10:25 – 10:50


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)

10:50 – 11:20

Coffee Break

11:20 – 11:45


We exploit the properties of chain mapping transformations of bosonic environ- ments to identify a finite collection of modes able to capture the characteristic features, or fingerprint, of the environment. Moreover we show that the count- able infinity of modes composing the (featureless) residual bath modes can be replaced by a universal Markovian closure, namely a small collection of damped modes undergoing a Lindblad-type dynamics whose parametrization is indepen- dent of the spectral density under consideration. We discuss the computational speed-up provided by such universal closure and present some relevant applica- tions.


Genuine multipartite entanglement as an identifier of exotic quantum phases

Quantum entanglement is a fascinating concept of quantum theory and is con- sidered to be one of the key features in recent developments in quantum tech- nologies. Apart from the fundamental importance, characterization of quantum entanglement in a, quantum many-body system often helps us assess the suit- ability of the considered model as a resource for quantum information processing tasks. However, quantification of quantum entanglement in complex quantum many-body systems is a challenging task. In particular, characterization of gen- uine multipartite entanglement requires full knowledge of entanglement distri- bution in all possible bipartitions of the system. In this work, we consider one such quantum many-body model, namely, the one-dimensional Bose-Hubbard model comprising both nearest-neighbor (tN ) and next-nearest neighbor hop- pings (TNN) and examine how the interplay of onsite interaction (U) and hop- ping results in different quantum phases in the model. The model is considered to be a prototype of the physical system consisting of ultracold bosonic atoms in optical lattices. We then analyze the, behavior of genuine multipartite entangle- ment in the system and make a comparative study with bipartite entanglement and other relevant order parameters. We observe genuine multipartite entan- glement has a very rich behavior throughout the considered parameter regime and helps us in identifying most of the phase boundaries. ,Reference:, In prepa- ration: Leon Carl, Sudipto Singha Roy, and Philipp Hauke, ”Genuine multipar- tite entanglement as an identifier of exotic quantum phases”. ,Funding: We acknowledge support by the ERC Starting Grant StrEnQTh (project ID 804305), Provincia Autonoma di Trento, and by Q@TN, the joint lab between University of Trento, FBK-Fondazione Bruno Kessler, INFN-National Institute for Nuclear Physics and CNR-National Research Council.

11:45 – 12:10


Statistical time-domain characterization of non-periodic optical clocks

Measuring time means counting the occurrence of periodic phenomena. Over the past centuries a major effort was put to make stable and precise oscillators to be used as clock regulators. In this work we consider a different class of clocks based on stochastic clicking processes. We provide a rigorous statistical frame- work to study the performances of such devices and apply our results to a single coherently driven two-level atom under photodetection as an extreme exam- ple of non-periodic clock. Quantum Jump MonteCarlo simulations and photon counting waiting time distribution will provide independent checks on the main results.


Unveiling Quantum Entanglement in Many-Body Systems from Partial Information

Quantum entanglement is commonly assumed to be a central resource for quan- tum computing and quantum simulation. Nonetheless, the capability to detect it in many-body systems is severely limited by the absence of sufficiently scalable and flexible certification tools. This issue is particularly critical in situations where the structure of entanglement is a priori unknown, and where one cannot rely on existing entanglement witnesses. I will present a scheme in which the knowledge of the mean value of arbitrary observables can be used to probe multipartite en- tanglement in a scalable, certified, and systematic manner. Specifically, we rely on positive semidefinite conditions, independent of partial-transposition-based criteria, necessarily obeyed if the data can be reproduced by a separable state. The violation of any of these conditions yields a specific entanglement witness, tailored to the data of interest, revealing the salient features of the data which are impossible to reproduce without entanglement. I will show how this approach allows to probe theoretical many-body states of several hundreds of qubits rel- evant to existing experiments: a single-particle quench in a one-dimensional XX chain; a many-body quench in a two-dimensional XX model with 1/r3 interac- tions; and thermal equilibrium states of Heisenberg and transverse-field Ising chains. In all cases, these investigations have led us to discover new entangle- ment witnesses, some of which could be characterized analytically, generalizing existing results in the literature. The presentation will be based on the results in [PRX Quantum 3 (1), 010342].

12:10 – 12:35


Gibbs state preparation on a quantum computer

An important task in quantum state preparation is the production of finite-temperate thermal states of a given Hamiltonian, on a quantum computer. The reason being that Gibbs states (also known as thermal states) can be used for quantum sim- ulation, quantum machine learning, quantum optimization , and studying open quantum systems. In particular, combinatorial optimization problems , semidef- inite programming , and training quantum Boltzmann machines, can be tackled
by sampling from well-prepared Gibbs states.


Quantum and classical complexity methods in quantum communication network optimization

Complex quantum communication infrastructures where a large number of users share entanglement and information offer a natural test-bed for quantum net- work theory. The rate and security of quantum communications between users placed at arbitrary points of a quantum communication network indeed depends on the structure of the network, on its extension and on the nature of the com- munication channels. In this talk we present a theoretical framework to under- stand how the properties of the underlying network influence the performances of quantum communications and we illustrate a potential strategy of network optimization. Our approach intertwines classical network approaches, complex- ity theory and quantum information. Specifically, by suitably defining a weight associated to each network’s link, we construct optimal quantum communica- tion channels through the network by balancing the quantum security and the quantum communication rate. The optimal network is then constructed as the network of the optimal channels and its performances are evaluated by study- ing the scaling of average properties as functions of the number of node and network’s extension.

12:35 – 13:00

Di Palma

Digital qubit readout with a flux-switchable superconducting circuit

Quantum computing platforms based on superconducting qubits have emerged as one of the most promising candidates in the race to build a large scale quan- tum computer [1]. Controllability, standard chips fabrication techniques com- bined with the possibility of exploring unconventional hybrid systems [2] are well established advantages of superconducting qubits architectures as quantum processors. However, while the performance of small superconducting quantum processors has advanced the threshold necessary for fault tolerance, the current technique to control and readout the qubit state imposes severe system scaling challenges [3]. Within this framework, digital control based on cryogenic energy- efficient superconducting Single Flux Quantum (SFQ) logic is being adapted to perform qubit control and readout for scalable quantum 3D-architectures [4]. This is leading to the development of innovative concepts for quantum proces- sor control and benchmarking in this integrated digital-quantum hybrid system.
Here, we propose an SFQ-compatible approach to accomplish diabatic read- out of superconducting qubits based on a Josephson Digital Phase Detector (JDPD). When properly excited by flux bias pulse, the JDPD is able to quickly switch from a single-minima to a double-minima potential and, consequently, relax in one of the two stable configurations discriminating between two phase values of a coherent input tone at GHz frequency. The basic concepts behind this new readout scheme have been experimentally verified with a preliminary version of the JDPD. The capability to work as a phase detector has been demon- strated up to 100kHz tone with a remarkable agreement between the experi- mental outcomes and simulations [5].
By choosing design parameters, the JDPD will be sensitive at frequency in the range of GHz, the typical frequency of superconducting qubits. These character- istics make the JDPD suitable for the implementation of a high speed platform integrated with superconducting digital electronics for both control and readout the qubit’s state directly at 20 mK, providing a solid solution for highly scalable superconducting quantum processors.
[1] J. M. C. J. M. Gambetta and M. Steffen, “Superconducting quantum bits”, NPJ Quantum Inf 2, 1 (2017). [2] H. G. Ahmad et al., ”Hybrid ferromagnetic trans- mon qubit: Circuit design, feasibility, and detection protocols for magnetic fluc- tuations” Phys. Rev. B, 2022 [3] O. Mukhanov et al., ”Scalable Quantum Com- puting Infrastructure Based on Superconducting Electronics” IEEE International Electron Devices Meeting (IEDM), 2019, [4] R. McDermott et al., “Quantum– classical interface based on single flux quantum digital logic”, Quantum Science and technology 3 (2018). [5] Di Palma et al., “Discriminating the phase of a weak coherent tone with a flux-switchable superconducting circuit”, in prep.


Underwater Quantum Communication with mesoscopic twin-beam states of light

Quantum resources can improve the security of information transmission be- tween two parties. So far, Quantum Communication protocols have been im- plemented at the single-photon level by means of entangled states. In contrast to this domain, in the mesoscopic one the optical pulses contain sizeable num- bers of photons, thus resulting more robust against any kind of external degra- dation. In a recent work of ours, we have demonstrated that the transmission of one arm of a twin-beam (TWB) state through a lossy and noisy channel does not prevent the observation of nonclassical correlations between the two parties. Based on these successful results, here we consider a more realistic scenario, in which a portion of TWB is sent through water-filled tubes, while the other one undergoes free-space propagation. We investigate the role played by the length of the tubes, the number of optical elements, and the divergence of the beams through the different media. We demonstrate that, by properly acting on the light beams, we can still observe nonclassical correlations at moderate distances. The experimental implementations involve two classes of commercial photon-number-resolving detectors.

13:00 – 15:00


15:00 – 15:25


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.

15:25 – 15:50


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.

15:50 – 16:15


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.

16:15 – 16:40


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.

16:40 – 17:10

Coffee Break

17:10 – 17:35


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.

17:35 – 18:00


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.

18:00 – 18:25


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.