Book of Abstracts

Francesco Pepe, University of Bari

Directional emission and photon bunching from a qubit pair in waveguide

Waveguide quantum electrodynamics represents a powerful platform to generate entanglement and tailor photonic states. We consider a pair of identical qubits coupled to a parity invariant waveguide in the microwave domain. By working in the one- and two-excitation sectors, we provide a unified view of decay processes and we show the common origin of directional single photon emission and two photon directional bunching. Unveiling the quantum trajectories, we demonstrate that both phenomena are rooted in the selective coupling of orthogonal qubits Bell states with different photon propagation directions. We comment on how to use this mechanism to implement optimized post-selection of Bell states, heralded by the detection of photons on one qubits side.

Luca Tagliacozzo, CSIC-IFF  Madrid

On temporal entanglement and its use to detect integrability in many-body dynamics

I will review the recent developments of the concept of temporal entanglement and show an application of it in discerning the presence or lack of integrability in one dimensional quantum many body systems.

Giuseppe Calajo, INFN - Padua

Few and many interacting photons in 2D waveguide QED

One-dimensional confinement in waveguide quantum electrodynamics plays a crucial role in enhancing light-matter interactions and inducing a strong quantum nonlinear optical response. In two- or higher-dimensional settings, this response is reduced since photons can be emitted within a larger phase space. In this talk, I will demonstrate that this reduction occurs in a 2D square array of atoms coupled to light confined within a two-dimensional waveguide. Specifically, I will show the occurrence of long-lived two-photon repulsive and bound states with genuine 2D features. When more excitations in the system are considered, these interactions can give rise to more complex many-body states. In particular, I will show how, at half-filling, the most subradiant state is well described by a quantum dimer covering ansatz, and I will discuss possible strategies for preparing this state.

Marco Di Liberto, University of Padua

Emergent orbital physics and chiral phases in dimerized pi-flux lattices

We show that a viable route to generate strongly-interacting chiral phases can exploit the interplay between onsite interactions and flux frustration for bosons in dimerized lattices with pi-flux. By constructing an effective theory, we demonstrate how this setting favours the spontaneous breaking of time-reversal symmetry. This can lead to the realization of the long-sought chiral Mott insulator phases, which we characterize via DMRG and variational calculations. Furthermore, dynamical properties like the chiral motion of impurities is identified via spectroscopy and quenches. Protocols to perform state preparation and current measurements will also be discussed. Recent advances in photonics offer a viable opportunity to implement this scheme in arrays of superconducting circuits.

M. Di Liberto and N. Goldman, Phys. Rev. Research 5, 023064 (2023)

A. Stepanenko et al. (in preparation)

Peter Rabl, Technical University of Munich

Light-matter interactions in a photonic quantum fluid

In this talk, I will discuss the coupling of two-level emitters to a 1D photonic waveguide in the presence of strong interactions between the photons. Such systems can be realized, for example, in the context of circuit QED, where Josephson junctions give rise to strong Kerr nonlinearities between propagating microwave photons. Compared to regular cavity QED systems, this leads to a competition between the usual Jaynes-Cummings type binding of the photons to the emitters and their local repulsion. After a brief general introduction to this system, I will present some of our ongoing numerical work on characterizing multi-photon bound states under these conditions, as well as the ground state phases of a lattice of emitters immersed in a fluid of strongly interacting photons.

Dario Cilluffo, Ulm University

Bosonic operator-based MPS for optical circuits

The tensor network formalism for quantum states and evolution offers numerical speedup primarily due to two interconnected factors: the efficient representation of quantum operators via a modular structure and the ability, in many cases, to approximate these operators with bounded error, leading to significant computational resource savings. Especially the first is a key reason for the widespread application of this formalism to optical quantum circuits, including BosonSampling experiments [1]. In such scenarios, the input modes of interferometers are represented as a chain of quantum harmonic oscillators, the input state is encoded in a Matrix Product State (MPS) with maximum bond dimension $D$, and the evolution is carried out by applying tensor networks (Matrix Product Operators or MPOs) corresponding to the multi-mode gates that compose the interferometer. However, calculating photon counting amplitudes, which correspond to the scalar product of an evolved Fock state and a target Fock state with the same number of excitations, requires $\mathcal{O}(D^3)$ operations when using MPSs due to necessary compression steps. It has been demonstrated [2] that exactly representing a Fock state evolved through a lossless passive circuit requires a bond dimension of $D = 2^n$, resulting in a computational cost of $\mathcal{O}(2^{3n})$ for photon counting amplitudes. On the other hand, it is known [1] that these amplitudes are given by the permanents of n×n complex matrices, which require $\mathcal{O}(n^2 2^n)$ operations. In this talk, I will introduce an alternative tensor-network-based approach, called Bosonic Operator-Based MPS (BOMPS) [3], to fill this complexity gap. Unlike the traditional Hamiltonian formulation, our approach leverages the known input-output relations of the circuit. This formalism, particularly in the Heisenberg picture, shows promising potential for calculating statistical moments of radiation, especially with complex input states or a large number of photons.

[1] Aaronson, Scott, and Alex Arkhipov. "The computational complexity of linear optics." Proceedings of the forty-third annual ACM symposium on Theory of computing. 2011.

[2] D. Cilluffo, N. Lorenzoni, M. B. Plenio arXiv preprint arXiv:2305.11215

[3] D. Cilluffo, N. Lorenzoni, M. Kost, M. B. Plenio, in preparation.

Dario Gerace, University of Pavia

Integrated quantum technologies

I will briefly present two results, one aimed at increasing the portfolio of possible platforms for photonic quantum computing with integrated circuits in the optical domain, and the other for low-power information processing in microwave technologies. In the first part, I will show that deterministic quantum gates can be engineered in quantum nonlinear interferometers with dual rail single-photon qubit encoding [1]. Then, I will show that superconducting quantum circuits currently used for quantum computing may actually become a viable platform for efficient and ultra-low power single-photon switching at microwaves [2].

[1] F. Scala, D. Nigro, D. Gerace. Communications Physics 7, 118 (2024).

[2] D. Rinaldi, D. Nigro, D. Gerace. Arxiv:2407.20092.