Super and subradiance in cold atomic clouds

The study of cooperative effects in spontaneous radiation emission by atomic ensembles has attracted great attention since Dicke’s 1954 seminal article on superradiance.
The phenomenon of subradiance, on the other hand, is less understood, and has been observed less than 10 years ago (by Guerin, Araújo and Kaiser), who detected very slow decay, with lifetimes of the order of 100 times the natural lifetime of individual (independent) atoms. We study here, by using random matrices, the features of the spontaneous decay of a dilute cold atomic cloud, and analyze both super- and subradiant states.

Nonlinear quantum logic with colliding graphene plasmons

Graphene has emerged as a promising platform to bring nonlinear quantum op- tics to the nanoscale, where a large intrinsic optical nonlinearity enables long- lived and actively tunable plas- mon polaritons to strongly interact. Here we theoretically study the collision between two counter- propagating plasmons in a graphene nanoribbon, where transversal subwavelength confinement en- dows propagating plasmons with a flat band dispersion that enhances their interaction. This scenario presents interesting possibilities towards the implementation of multi-mode plasmon gates that cir- cumvent limitations imposed by the Shapiro no-go theorem for photonic gates in nonlinear optical fibers. As a paradigmatic example we demonstrate the feasibility of a high fidelity conditional π phase shift (CZ), where the gate performance is fundamentally limited only by the sin- gle plasmon lifetime. These results open new exciting avenues towards quantum information and many-body applications with strongly-interacting polaritons.

Detecting the symmetry breaking of the quantum vacuum in a circuit QED system

Hybrid quantum systems in the ultrastrong, and even more in the deep-strong, coupling regimes can exhibit exotic physical phenomena and are promising for new applications in quantum technologies. In these nonperturbative regimes, a qubit resonator system has an entangled quantum vacuum with a nonzero average photon number in the resonator, where the photons are however virtual and cannot be directly detected. We have shown that the vacuum field is able to induce the symmetry breaking of a dispersively coupled probe qubit. A very recent experiment has observed this Higgs-like quantum-vacuum symmetry breaking induced by the field of a superconducting electromagnetic resonator deep-strongly coupled with a flux qubit. This result opens a way to experimentally explore the novel quantum-vacuum effects emerging in the deep-strong coupling regime, as well to explore the superradiance phase transitions in these systems.

Coherent Magnonics in the Ultrastrong Coupling Regime

One of the objectives in the study of light-matter coupled systems is to push the coupling strength to the highest achievable values. In the ultrastrong coupling regime, which is reached when the ratio between the coupling strength (gc) and the frequency of the uncoupled transitions (w0) exceeds 0.1, the conventional perturbative treatment of QED does not hold and higher-order processes are expected to produce relevant effects.

Scalable quantum computer with superconducting circuits in the ultrastrong coupling regime

So far, superconducting quantum computers have certain constraints on qubit connectivity, such as nearest-neighbor couplings. To overcome this limitation, we propose a scalable architecture to simultaneously connect several pairs of distant qubits via a dispersively coupled quantum bus. The building block of the bus is composed of orthogonal coplanar waveguide resonators connected through ancillary flux qubits working in the ultrastrong coupling regime. This regime activates virtual processes that boost the effective qubit–qubit interac- tion, which results in quantum gates on the nanosecond timescale. The interac- tion is switchable and preserves the coherence of the qubits.

Detection of virtual photons in ultrastrongly coupled quantum systems

Light-matter interaction,and understanding of the fundamental physics behind it,is the scenario of emerging quantum technologies.Solid state devices may ex- plore new regimes where coupling strengths are”ultrastrong”,i.e.comparable to the energies of the subsystems.New exotic phenomena occur as the entangled vacuum contains virtual photons[1].Despite more than a decade of research,the detection of ground-state virtual photons still awaits demonstration.In this work,we provide a solution for this long-standing problem.We find a design of a super- conducting quantum circuit and a protocol of coherent amplification yielding
a highly efficient,faithful and selective conversion of virtual photons of a ”false”
ground state[2,3] to real ones enabling their detection with state-of-the-art quan-
tum hardware.Supplemented by advanced control,our multilevel design can be
exploited for further quantum tasks in the USC regime.[1] P.Forn-Diaz,et al.,RMP
91,025005 (2019);A.Kockum,et al.,Nat.Rev.Phys.1,19 (2019);[2] R.Stassi,et al.,PRL
110,243601 (2013);G.Falci,et al.,Fort.Phys.65,1600077 (2017);[3] G.Falci,et al.,Ssci.Rep.9,9249 (2019).

Critical Quantum Sensing

Quantum critical systems in proximity of phase transitions exhibit a divergent sus- ceptibility, suggesting that an arbitrarily-high precision may be achieved when they are used as probes to estimate a physical parameter. However, such an im- provement in sensitivity is counterbalanced by the critical slowing down, which implies an inevitable growth of the protocol duration time. Here, we present sensing protocols based on phase transitions observable in a broad class of quantum optical systems. We show that, in spite of the critical slowing down, critical quantum optical probes can achieve quantum advantage in sensing ap- plications. Then, by going beyond the asymptotic regime of parameters, we show that Heisenberg-limited precision can be achieved with current quantum technologies. Finally, we propose specific applications for quantum magnetom- etry and for superconducting-qubit readout.

Characterization of integrated multiphase sensors via Neural Networks

Integrated photonic circuits represent a handy solution for studying multiparam- eter estimation problems. Such a platform indeed can be easily scaled, and it allows to implement complex and stable transformations with reconfiguration ca- pabilities. However, when the system dimensions increase the characterization of its operation, necessary for most estimation protocols, becomes a particular hard task to solve. Usually, the calibration of the device is done through tomog- raphy, which is a resource and computational expensive procedure, requiring the capability of generating different classes of input states. Moreover, in a real scenario, the number of resources is always limited and not all the desired probe states can be prepared. Here, we start investigating Bayesian estimation bounds in a limited resources scenario and then we prove the effectiveness of machine learning algorithms, more specifically of neural networks, to overcome the need of retrieving an explicit model of the device functioning. We demonstrate that in such framework almost optimal estimation performances can be achieved in an actual noisy multiparameter estimation experiment performed at the single- photon level.

Quantum simulation of scattering processes

We present a systematic treatment of scattering processes for quantum systems whose time evolution is discrete. We prove some general properties of the scat- tering operator and we develop two perturbative techniques for the power se- ries expansion of scattering amplitudes. This formalism is used to assess the performance of discrete-time quantum simulators in recovering the scattering amplitudes of continuous-time models.

New frontiers in measurements in weak coupling regime

Weak coupling regime allows for new quantum measurement paradigms show- ing unprecedented measurement capability, with possible applications spanning from quantum information to quantum metrology. Furthermore, if proper post- selection is added, one can retrieve the weak value of an observable, a peculiar quantity with many exotic traits, such as being not bounded to the observable eigenvalue spectrum (taking even imaginary values). In this regime, we achieved several results, e.g. sequential weak measurements of non-commuting observ- ables and the first implementation of protective measurements. I will present our latest results in this field, namely Robust Weak Measurements (RWMs) [1] and One-Shot Bell Measurements (OSBM). RWM is an iterative measurement protocol in which, instead of averaging over multiple acquisitions, even a single reading of the measuring device provides a reliable estimate of a (anomalous) weak value. In OSBM, instead, we exploit weak couplings in sequence to sense the whole Bell parameter with each entangled photon pair produced, a paradigm shift with respect to traditional Bell inequality tests. [1] E. Rebufello et al. Light: Science & Applications (2021) 10:106

Generation of pseudo-random states on actual quantum hardware

An ideal quantum computer operating with more than fifty qubits could outper- form a classical computer, and the quantum advantage for specific problems has been recently claimed. However, quantum advantage can only be reached with a high enough quantum gate precision and through processes generating a large enough amount of entanglement., In this work, we compare the effectiveness of different algorithms to generate pseudo-random quantum states, for which the multipartite entanglement is almost maximal, as characterized by the proba- bility distribution of bipartite entanglement between all possible bipartitions of the system., The algorithms are finally tested in actual quantum hardware, based either on superconducting or on ion qubits.

Ground state preparation of lattice gauge theories with the quantum approximate optimization algorithm

The preparation of the ground state of a Lattice Gauge Theory is a challenging problem, due to the dimension of the Hilbert space and the presence of gauge constraints. We will discuss [1] how to prepare the ground state and study the quantum phase diagram of a two-dimensional Z2 lattice gauge theory by means of the hybrid Quantum Approximate Optimization Algorithm, which requires a small number of parameters to reach high fidelities and can be efficiently scaled up on larger systems. Despite the reduced size of the considered lattices (up to 5×5), a transition between confined and deconfined regimes can be detected by measuring the expectation values of Wilson loop operators or the topolog- ical entropy. Moreover, if periodic boundary conditions are implemented, the same optimal solution is transferable among all four different topological sec- tors, without any need for further optimization on the variational parameters. These results show that variational quantum algorithms provide a useful technique to be added in the growing toolbox for digital simulations of lattice gauge theories, suitable for near-term quantum computers.

Bell nonlocality in quantum-gravity induced minimal-length quantum mechanics

Different approaches to quantum gravity converge in predicting the existence of a minimal scale of length. This raises the fundamental question as to whether and how an intrinsic limit to spatial resolution can affect quantum mechanical observables associated to internal degrees of freedom. In this talk, the ques- tion is answered in general terms by showing that the spin operator acquires a momentum-dependent contribution in quantum mechanics equipped with a minimal length. Among other consequences, this modification induces a form of scale-dependent quantum nonlocality stronger than the one arising in ordi- nary quantum mechanics. In particular, one can show that violations of the Bell inequality can exceed the maximum value allowed in ordinary quantum mechan- ics by a positive multiplicative function of the momentum operator.