# Quantum Optics INRIM

## New measurement paradigms and foundations of quantum mechanics

Thanks to the recent advances in quantum systems generation, manipulation and measurement, the new millennium witnessed the passage from the first to the second quantum revolution. Today, fundamental aspects of quantum theory, such as entanglement and the superposition principle, are at the core of many technologies able to outperform their classical counterparts (e.g., quantum information, metrology and sensing).

However, our comprehension of reality remains partial and veiled; a long debate is still ongoing on the peculiar aspects of quantum theory foundational axioms, like the nonlocality of quantum correlations, the ontological nature of the wave function and its collapse, the measurement process, the macro-objectivation problem (i.e., the transition from a microscopic probabilistic world to a macroscopic deterministic one).

Hence, although quantum mechanics looks almost mature, fundamental questions are still open, with implications also on quantum technologies. We address this topic through advanced experiments in quantum optics, such as the study of the concept of time in quantum mechanics (for example through the simulation of quantum particles traveling in time), the reconciliation between macroscopic irreversibility and quantum laws of the microscopic world ( temporally reversible) or the study of phenomena such as the quantum Zeno effect and its use in noise sensing techniques.

Furthermore, we develop and experimentally realize new quantum measurement paradigms in various regimes, from strong (projective) to “weak” (sequential weak measurements, protective measurement) characterized by peculiar properties overcoming the current limits of quantum measurements, paving the way for new research on the foundations of physics and innovative applications in the field of quantum technologies.

## Recent highlights

### Entanglement-preserving measurement of the Bell parameter on a single entangled pair

Salvatore Virzì et al., Quantum Sci. Technol. 9 045027 (2024)

Bell inequalities represent one of the cornerstones of quantum foundations, and a fundamental tool for quantum technologies. Although a lot of effort was put in exploring and generalizing them, because of the wave function collapse it was deemed impossible to estimate the entire Bell parameter from one entangled pair, since this would involve measuring incompatible observables on the same quantum state. Conversely, here it is reported the first implementation of a new generation of Bell inequality tests, able to extract a Bell parameter value from each entangled pair and, at the same time, preserve the pair entanglement instead of destroying it. This is obtained by exploiting sequences of weak measurements, allowing incompatible observable measurements on a quantum state without collapsing its wave function. On the fundamental side, by removing the need to choose between different measurement bases our approach stretches the concept of counterfactual definiteness, since it allows measuring the entangled pair in all the bases needed for the Bell inequality test, intrinsically eliminating the issues connected with the otherwise not-chosen bases. On the practical side, after our Bell parameter measurement the entanglement within the pair remains (basically) unaltered, hence exploitable for other quantum-technology-related or foundational purposes.

### Anomalous weak values via a single photon detection

E. Rebufello et al., Light: Science & Applications volume 10, Article number: 106 (2021)

Is it possible that a measurement of a spin component of a spin-1/2 particle yields the value 100? In 1988 Aharonov, Albert and Vaidman argued that upon pre- and postselection of particular spin states, weakening the coupling of a standard measurement procedure ensures this paradoxical result1. This theoretical prediction, called weak value, was realised in numerous experiments, but its meaning remains very controversial, since its “anomalous” nature, i.e., the possibility to exceed the eigenvalue spectrum, as well as its “quantumness” are debated. We address these questions by presenting the first experiment measuring anomalous weak values with just a single click, without the need for statistical averaging. The measurement uncertainty is significantly smaller than the gap between the measured weak value and the nearest eigenvalue. Beyond clarifying the meaning of weak values, demonstrating their non-statistical, single-particle nature, this result represents a breakthrough in understanding the foundations of quantum measurement, showing unprecedented measurement capability for further applications of weak values to quantum photonics.

### Quantum Zeno and Anti-Zeno Probes of Noise Correlations in Photon Polarization

We experimentally demonstrate, for the first time, noise diagnostics by repeated quantum measurements, establishing the ability of a single photon subjected to random polarization noise to diagnose non-Markovian temporal correlations of such a noise process. Both the noise spectrum and temporal correlations are diagnosed by probing the photon with frequent (partially) selective polarization measurements. We show that noise with positive temporal correlations corresponds to our single photon undergoing a dynamical regime enabled by the quantum Zeno effect (QZE), whereas noise characterized by negative (anti) correlations corresponds to regimes associated with the anti-Zeno effect (AZE). This is the first step toward a novel noise spectroscopy based on QZE and AZE in single-photon state probing able to extract information on the noise while protecting the probe state, a conceptual paradigm shift with respect to traditional interferometric measurements.

## Most relevant pubblications

Salvatore Virzì et al., "Entanglement-preserving measurement of the Bell parameter on a single entangled pair" Quantum Sci. Technol. 9 045027 (2024)

Salvatore Virzì et al., "Quantum Zeno and Anti-Zeno Probes of Noise Correlations in Photon Polarization", Phys. Rev. Lett. 129, 030401 (2022)

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

E. Rebufello et al. "Anomalous weak values via a single photon detection" Light: Science & Applications 10, 106 (2021)

E. Rebufello et al. "Protective Measurement - a new quantum measurement paradigm: detailed description of the first realisation", Applied Sciences 11(9), 4260 (2021)

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

S. Virzì et al. "Optimal estimation of entanglement and discord in two-qubit states" Scientific Reports 9 (1), 1-9 (2019)

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)

Salvatore Virzì et al. "Optimal estimation of entanglement and discord in two-qubit states" Scientific Reports 9 (1), 1-9 (2019).

F. Piacentini et al. "Investigating the effects of the interaction intensity in a weak measurement" Scientific reports 8 (1), 1-7 (2018)

F. Piacentini et al. "Determining the quantum expectation value by measuring a single photon" Nature Physics 13 (12), 1191-1194 (2017)

A. Avella et al. "Anomalous weak values and the violation of a multiple-measurement leggett-garg inequality" Physical Review A 96 (5), 052123 (2017).

E Moreva et al. "Quantum time: Experimental multitime correlations" Physical Review D 96 (10), 102005 (2017)

F. Piacentini et al. "Measuring incompatible observables by exploiting sequential weak values" Physical review letters 117 (17), 170402 (2016).

F. Piacentini et al. "Experiment investigating the connection between weak values and contextuality" Physical Review Letters 116 (18), 180401 (2016).

E. Moreva et al.. "Bell measurements as a witness of a dualism in entanglement" Physical Review A 91 (6), 062117 (2015)

E Moreva et al. "Time from quantum entanglement: an experimental illustration" Physical Review A 89 (5), 052122 (2014)

A. Avella et al. "Separable Schmidt modes of a nonseparable state" Physical Review A 89 (2), 023808 (2014)

C. Benedetti et al."Experimental estimation of quantum discord for a polarization qubit and the use of fidelity to assess quantum correlations" Physical Review A 87 (5), (2013)

Yu. I. Bogdanov et al. "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 et al.. "Experimental estimation of entanglement at the quantum limit" Physical review letters 104 (10), 100501 (2010)

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