**Quantum Optics INRIM**

**Foundations of Quantum Mechanics**

**Foundations of Quantum Mechanics**

The advent of the new millennium represented a crucial turn point for physics, which saw the evolution between the first and second quantum revolution materialize.

This sweeping change is now steering the research in Quantum Mechanics, the well-established mathematical clockwork whose strength and accuracy in predictions are currently experienced in worldwide experimental laboratories, towards new horizons, the most futuristic of which is the quantum computer.

This transition has been possible thanks to the proactive interlacing between great progress achieved by scientists towards the very deep understanding of the fundamental concepts of quantum physics and the consequent dawn and development of quantum technology and engineering.

Nowadays, the practical capability to measure, control and manipulate quantum systems at individual single-particle level is at our fingertips, representing a basic experimental resource to be exploited in all those fields of investigation as quantum metrology and sensing, quantum imaging, quantum control and quantum communication. It is on the route of development of such quantum technology tools that the scientific community investigates the possibility of expanding our ability to the manipulation of even larger numbers of quantum entangled states.

However, as it naturally should be, our human comprehension of reality remains partial and veiled. In fact, notwithstanding with its impressive mathematical capacity, a long debated argument is even revolving on the fascinating facets of the foundational axioms of quantum theory, the main matters of the dispute being the non-local aspects of entanglement and quantum correlations, the ontological nature of the wave function and its collapse, the state measurement in quantum mechanics, the macro-objectivation problem (i.e. the transition from a microscopic probabilistic world to a macroscopic deterministic world described by classical mechanics).

Being this the contemporary scenario, in which on one side quantum mechanics seems to have approached a level of complete maturity, while on the other fundamental questions are still open, in the arena of the answers that are yet have been found, quantum optics plays, as it always has, an avant-garde role. All these reasons pushed, from its very beginnings, the INRIM Quantum Optics Group to focus on the inspiring and productive research field represented by Quantum Mechanics Foundations.

## Highlights

### Non-Monogamy of Spatio-Temporal Correlations and the Black Hole Information Loss Paradox

Pseudo-density matrices are a generalisation of quantum states and do not obey monogamy of quantum correlations. Could this be the solution to the paradox of information loss during the evaporation of a black hole? In this paper we discuss this possibility, providing a theoretical proposal to extend quantum theory with these pseudo-states to describe the statistics arising in black-hole evaporation. We also provide an experimental demonstration of this theoretical proposal, using a simulation in optical regime, that tomographically reproduces the correlations of the pseudo-density matrix describing this physical phenomenon.

### Theoretical description and experimental simulation of quantum entanglement near open time-like curves via pseudo-density operators

C. Marletto et al., Nature Communications volume 10, Article number: 182 (2019)

Closed timelike curves are striking predictions of general relativity allowing for time-travel. They are afflicted by notorious causality issues (e.g. grandfather’s paradox). Quantum models where a qubit travels back in time solve these problems, at the cost of violating quantum theory’s linearity—leading e.g. to universal quantum cloning. Interestingly, linearity is violated even by open timelike curves (OTCs), where the qubit does not interact with its past copy, but is initially entangled with another qubit. Non-linear dynamics is needed to avoid violating entanglement monogamy. Here we propose an alternative approach to OTCs, allowing for monogamy violations. Specifically, we describe the qubit in the OTC via a pseudo-density operator—a unified descriptor of both temporal and spatial correlations. We also simulate the monogamy violation with polarization-entangled photons, providing a pseudo-density operator quantum tomography. Remarkably, our proposal applies to any space-time correlations violating entanglement monogamy, such as those arising in black holes.

### Emergence of Constructor-Based Irreversibility in Quantum Systems: Theory and Experiment

How irreversibility arises in a universe with time-reversal symmetric laws is a central problem in physics. In this Letter, we discuss a radically different take on the emergence of irreversibility, adopting the recently proposed constructor theory framework. Irreversibility is expressed as the requirement that a task is possible, while its inverse is not. We prove the compatibility of such irreversibility with quantum theory’s time-reversal symmetric laws, using a dynamical model based on the universal quantum homogenizer. We also test the physical realizability of this model by means of an experimental demonstration with high-quality single-photon qubits.

## Most relevant pubblications

C. Marletto et al. "Emergence of Constructor-Based Irreversibility in Quantum Systems: Theory and Experiment" Phys. Rev. Lett. 128, 080401 (2022).

C. Marletto et al. "Non-Monogamy of Spatio-Temporal Correlations and the Black Hole Information Loss Paradox" Entropy 22 (2020) 228

C. Marletto et al. "Theoretical description and experimental simulation of quantum entanglement near open time-like curves via pseudo-density operators" Nature Communications 10, 182 (2019)

S Virzì, E Rebufello, A Avella, F Piacentini, M Gramegna, I.Ruo-Berchera, I. P. Degiovanni & M. Genovese "Optimal estimation of entanglement and discord in two-qubit states" Scientific Reports 9 (1), 1-9 (2019).

E Moreva, M Gramegna, G Brida, L Maccone, M Genovese "Quantum time: Experimental multitime correlations" Physical Review D 96 (10), 102005 (2017)

E. Moreva, G. Brida, M. Gramegna, S. Bose, D. Home, and M. Genoves. "Bell measurements as a witness of a dualism in entanglement" Physical Review A 91 (6), 062117 (2015)

E Moreva, G Brida, M Gramegna, V Giovannetti, L Maccone, M Genovese "Time from quantum entanglement: an experimental illustration" Physical Review A 89 (5), 052122 (2014)

A. Avella, M. Gramegna, A. Shurupov, G. Brida, M. Chekhova, and M. Genovese. "Separable Schmidt modes of a nonseparable state" Physical Review A 89 (2), 023808 (2014)

C. Benedetti, A.P. Shurupov, M.G.A. Paris, G. Brida, M. Genovese. "Experimental estimation of quantum discord for a polarization qubit and the use of fidelity to assess quantum correlations" Physical Review A 87 (5), 052136 (2013)

Yu. I. Bogdanov, G. Brida, I. D. Bukeev, M. Genovese, K. S. Kravtsov, S. P. Kulik, E. V. Moreva, A. A. Soloviev, and A. P. Shurupov "Statistical estimation of the quality of quantum-tomography protocols" Physical Review A 84 (4), 042108 (2011)

M. Genovese "Interpretations of quantum mechanics and measurement problem" Advanced Science Letters 3 (3), 249-258 (2010)

G Brida, IP Degiovanni, A Florio, M Genovese, P Giorda, A Meda, Matteo G. A. Paris, and Alexander Shurupov. "Experimental estimation of entanglement at the quantum limit" Physical review letters 104 (10), 100501 (2010)

M. Roncaglia, A. Montorsi, M. Genovese "Bipartite entanglement of quantum states in a pair basis" Physical Review A 90 (6), 062303 (2010)