## Q@TN-workshop details

## Schedule on Monday 13

Hours | Speaker | Talks |
---|---|---|

9:30-10:30 | De Bernardis | Tutorial 1 |

10:30-11:00 | - | Break |

11:00-11:30 | Mazzola | Talk 1 |

11:30-12:00 | Giacomelli | Talk 2 |

12:00-12:30 | Trillo | Talk 3 |

12:30-14:00 | - | Lunch |

14:00-14:30 | Balducci | Talk 4 |

14:30-15:00 | De Lazzari | Talk 5 |

15:00-15:30 | - | Break |

15:30-16:00 | Ludescher | Talk 6 |

16:00-16:30 | Aw & Prodhon (40mins) | Talk 7 |

## Schedule on Thursday 14

Hours | Speaker | Talks |
---|---|---|

9:30-10:30 | Scarani | Tutorial 2 |

10:30-11:00 | - | Break |

11:00-11:30 | Foletto | Talk 8 |

11:30-12:00 | Sighinolfi | Talk 9 |

12:00-12:30 | Zaw | Talk 10 |

12:30-14:00 | - | Lunch |

14:00-14:30 | Tarabunga | Talk 11 |

14:30-15:00 | Roccuzzo | Talk 12 |

15:00-15:30 | - | Break |

15:30-16:00 | Sidajaya | Talk 13 |

16:00-16:30 | Leone | Talk 14 |

## Talks

**1: **Giulia Mazzola, ETH Zurich

**Title:** On the Black Hole War: A quantum information view to reconcile Hawking’s prediction of information loss with unitarity of quantum theory

**Abstract:** A major discovery by Hawking was that the interplay between general relativity and quantum theory leads to the prediction that black holes must radiate. In Hawking’s original calculation however, the black hole radiation was found to be of thermal character thus leaving behind a mixed state describing the radiation as soon as the black hole has fully evaporated. This conclusion stands in apparent contradiction to the reversibility of time evolution in quantum theory which predicts a pure final state of the radiation, thereby giving rise to the famous black hole information puzzle. This discrepancy can be more concretely illustrated in terms of (von Neumann) entropy: Hawking’s prediction leads to a continuously increasing entropy during the black hole evaporation while quantum theory instead dictates that the entropy should decrease in the final stages of the evaporation. Recent calculations based on random unitary models or using the gravitational path-integral integral formalism have shed new light on this puzzle, supporting the latter behavior of the radiation entropy in favor of the unitary evolution picture. Hence, was Hawking’s conclusion wrong? And if so, what was wrong in his calculation? In analyzing this question, it turns out that information-theoretic tools such as the Quantum de Finetti theorem allow us to interpret the different behaviors of the black hole radiation entropy during evaporation and therefore might help in understanding the relations between recent results and Hawking’s conclusions. In this talk, I will mainly focus on the random unitary model for black hole evaporation and, as an outlook, present a recently suggested framework in which Hawking’s original predictions and the unitary picture of quantum theory may be reconciled. Main references: [1] P. Hayden, J. Preskill, Black holes as mirrors: quantum information in random subsystems (2007), arXiv:0708.4025 [2] R. Renner, J. Wang, The black hole information puzzle and the quantum de Finetti theorem (2021), arXiv:2110.14653

**2:** Luca Giacomelli, INO-CNR BEC Trento

**Title:** Confirmed coming, talk soon

**Abstract:**

**3:** David Trillo, IQOQI University of Vienna

**Title: **Time translations on uncontrolled quantum systems

**Abstract:** A time translation is an operation which takes an unperturbed quantum system from an initial state to some state lying on its evolution curve. We show that it is possible to effect many kinds of time translations on a system without the need for any control or information of any kind other than its dimension. In particular, up to some constraints which depend on dimension, one can probabilistically reset, fast-forward or rewind the natural evolution of a system with a heralded protocol, which works universally. For qubits, the probability of success of some of these protocols can be made as close to 1 as wanted.

**4:** Federico Balducci, SISSA Trieste

**Title:** Signatures of many-body localization in the dynamics of two-level systems in glasses

**Abstract: **In this talk, I will discuss some consequences of the interplay of disorder and quantum effects in structural glasses at low temperatures. To this end, I will briefly introduce the two-level systems (TLS) model of impurities coupled to phonons. By integrating out the phonons within the the framework of the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equation, interactions among TLS are generated, as well as dissipation terms. I will show how the unitary dynamics of the TLS bears clear signatures of many-body localization physics, and that also in presence of dissipation there is a long, experimentally accessible, localized transient. This in turn entails that assuming ergodicity when discussing TLS physics might not be justified for all kinds of experiments on low-temperature glasses. I substantiate the analytical picture with numerical results on the time behavior of the concurrence, which measures pairwise entanglement also in nonisolated systems. For a couple of TLS, the concurrence presents a power-law decay both in the absence and in the presence of dissipation, which is again a signature of localization.

**5:** Claudia De Lazzari, Q@TN, University of Trento

**Title:** Dimension of Tensor Network Varieties

**Abstract:** A Tensor Network variety is an algebraic variety of tensors associated to a graph and two sets of positive integer weights on its edges and on its vertices respectively, called bond and physical dimensions. In quantum many-body physics they are used as a variational ansatz class to describe strongly correlated quantum systems whose entanglement structure is encoded by the underlying graph. In the talk I will present an upper bound on the dimension of the Tensor Network variety. I will discuss a refined upper bound in cases relevant for applications such as Matrix Product States, and highlight some examples where the bound is not sharp. This is based on a joint work with Alessandra Bernardi and Fulvio Gesmundo.

**6:** Stefan Ludescher, IQOQI, University of Vienna

**Title:** Entanglement/Asymmetry correspondence for internal quantum reference frames

**Abstract:** In recent years, the notion of quantum reference frames was vividly discussed throughout different communities. I will give a brief introduction to the quantum foundational and especially, to the perspective neutral approach to quantum reference frames. I will introduce the notions of perfect and imperfect reference frames. For imperfect quantum reference frames, the question “what is a good quantum reference frame?”, arises. I will propose a solution to the problem, if the underlying symmetry group is a compact Lie group.

**7: **AW Cenxin Clive & Julien PRODHON, CQT, National University of Singapore

**Title:** Remembering to take a Bath: Retrodiction, Dilation & Memory

**Abstract:** It was recently noticed that classical and quantum fluctuation relations can naturally be derived from Bayesian retrodiction [Buscemi and Scarani, PRE 103, 052111 (2011)], and by the same token expanded beyond their usual domain of application in thermal processes. This approach is a new bridge connecting the foundations of information theory and thermodynamics, both classical and quantum. In this talk we (1) introduce this approach and (2) extend it to scenarios with encoded memory. This uncovers in-roads to speaking of non-Markovian retrodiction with arbitrary priors and illuminates the logical foundation behind the thermodynamic 2nd Law and its generalizations.

**8:** Giulio Foletto, University of Padova

**Title:** Weak measurements in quantum information protocols

**Abstract:** The quantum weak measurements, first introduced in 1988, have proven useful in a broad range of applications, from the amplification of feeble signals to the study of paradoxes, through quantum state reconstruction. Although they can only extract partial information from a system, they perturb it less than standard projections, sometimes allowing sequences of meaningful measurements on a single state. In turn, this has enabled protocols that extract a valuable quantum resource repeatedly. In this talk, some of these protocols will be discussed, also showing proof-of-concept experimental implementations with single photons. We will see how weak measurements enable sequential quantum random access codes, the sequential certification of entanglement and nonlocality, and the sequential generation of device-independent random numbers. In all these cases, weak measurements enrich or boost the performance of well-known quantum information protocols.

**9: **Matteo Sighinolfi, Q@TN, University of Trento

**Title:** Stochastic dynamics of impurities in a Fermi bath

**Abstract: **Dynamics of impurities embedded in an ultra-cold Fermi gas is investigated by using a Generalized Langevin equation. The latter — derived by means of influence functional theory — describes the stochastic classical dynamics of the impurities and the quantum nature of the fermionic bath manifests in the emergent interaction between the impurities and in the viscosity tensor. By focusing on the two-impurity case, existence of bound states, in different conditions of coupling and temperature, is predicted and their life-time is analytically estimated.

**10:** Zaw Lin Htoo, CQT, National University of Singapore

**Title:** Generating entangled states with identical bosons

**Abstract: **A symmetrised state is required to describe a system containing many identical bosons, which results in a state that appears to be highly entangled. However, as these particles cannot be distinguished from one another, this entanglement cannot be accessed directly. In this talk, the idea of extracting entanglement from identical bosons is explored. This idea was first introduced with ideal mode splitting, which maps the entanglement structure of the symmetrised state onto distinguishable modes, converting it into useful entanglement. As ideal mode splitting requires non-destructive particle-number measurements, the more practical scenario of destructive measurement is considered in the context of boson subtraction. Finally, an experimental proposal to generate entangled states with a trapped-ion phonon setup is presented to demonstrate how a boson subtraction scheme might be carried out in the lab.

**11:** Poetri Tarabunga, SISSA Trieste

**Title:** Quantum Correlations at Finite-Temperature Critical Points

**Abstract:** It is well-known that long-range correlations play an important role in phase transitions. It is then of vital interest to study the nonlocal quantum correlations involved in phase transitions of quantum systems. Entanglement has been the most prominent measure of quantum correlations, but even separable states can also exhibit nonclassical behavior. A more basic form of quantum correlation, the quantum discord, captures more basic aspects of a quantum system, namely that measuring a quantum system necessarily disturbs it. In recent decades, quantum discord has been shown to be able to identify and characterize quantum critical points. However, the fate of quantum correlations at finite-temperature critical points is considerably less understood. Finite-temperature phase transitions are typically governed by classical field theories, and are driven by thermal fluctuations instead of quantum fluctuations. Therefore, it is unclear whether quantum correlations play any role in such transitions. In our recent work, we show that the two-body quantum discord can display genuine signatures of critical behavior at thermal critical points, in contrast to entanglement which does not display any long-range critical behavior.

**12: **Santo Maria Roccuzzo, Q@TN, University of Trento

**Title:** Supersolidity in a dipolar Bose gas

**Abstract:** Supersolids are exotic materials combining the frictionless flow of a superfluid with the crystal-like periodic density modulation of a solid. The supersolid phase of matter was predicted 50 years ago for solid helium, for which, despite decades of investigation, has not yet been demonstrated. Quantum gases with spin–orbit coupling, cavity-mediated interactions and long-range dipolar interactions are emerging as interesting alternatives. In this talk, I will present recent results on the supersolid properties exhibited by dipolar quantum gases, focusing in particular on the appearance of additional mechanisms for sound propagation, the emergence of non-classical rotational inertia, and the occurrence of quantized vortices.

**13:** Peter Sidajaya, CQT, National University of Singapore

**Title: **Simulation of non-maximally entangled states using one bit of communication

**Abstract:** From Bell’s theorem we know that local hidden variables could not simulate the behaviour of an entangled state measured using projective measurements. However, it is known that by adding one bit of communication between the parties, it is possible to simulate the behaviour of a maximally entangled state (singlet). We examine the case for a non-maximally entangled state. We use an Artificial Neural Network (ANN) constrained by locality and supplemented by one bit of communication to generate a protocol mimicking the behaviour of a non-maximally entangled state as closely as possible. Our results suggest that it might be possible to simulate the behaviour using only one bit.

**14: **Nicolo Leone, Q@TN, University of Trento

**Title:** Certifying quantum randomness using Single-Photon Entanglement

**Abstract:** Single-Photon Entanglement(SPE) is a particular type of entanglement in which two degrees of freedom of the same photon are correlated. This type of entanglement does not require non-linear optics to be generated: it is possible to obtain entangled single photon states of momentum and polarization just employing an attenuated light source like a laser or an LED, and linear optical components as beam splitters, mirrors and half-wave plates. Here we report on a new semi-device-independent quantum random number generator (QRNG) based on SPE. Entanglement is indeed an important resource for random number generation since it can be used to guarantee the randomness. Our certification scheme is based on Bell inequality in the CHSH form: every time a violation of the latter is observed for a generated sequence of random numbers, it is possible to lower bound the min-entropy of the produced bits. The min-entropy is an important parameter for RNG, since it quantifies the effectiveness of the best strategy that an adversary can apply to guess the generated sequence. It is important to point out that, since SPE is a local phenomenon, no coincidence measurements are directly necessary to test Bell inequality as only single detection events are required. As a consequence of that, commercial single photon avalanche diodes are used to detect single photon events. The proposed protocol is semi-device independent since it requires a characterization of some non-idealities of the experimental setup, like the memory effects introduced by detectors (dead time, dark counts and afterpulsing) and the polarization dependence of the optical components (i.e. beam splitters and mirrors in the Rotation stage). Moreover two additional hypothesis has to be introduced: firstly, the stationarity of the input state and of the measurement conditions is assumed and, secondly, the provider of the devices is considered trusted. The aim of the proposed protocol is indeed not to fight against a malicious eavesdropper, but essentially to provide robustness against unwanted and undetectable flaws of the system. Under these hypothesis we were able to obtain a kHz-rate certified semi-device independent QRNG, based on commercial linear optical components. Joint work with Stefano Azzini, Sonia Mazzucchi, Valter Moretti, Lorenzo Pavesi.