Quantum Optics INRIM
Measurement Paradigms in Quantum Mechanics
One description provides only probabilities for obtaining various eigenvalues of a quantum variable. The eigenvalues and the corresponding probabilities specify the expectation value of a physical observable, which is known to be a statistical property of an ensemble of quantum systems. In contrast to this paradigm, here we demonstrate a method for measuring the expectation value of a physical variable on a single particle, namely, the polarization of a single protected photon. This realization of quantum protective measurements could find applications in the foundations of quantum mechanics and quantum-enhanced measurements.
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.
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., "Quantum Zeno and Anti-Zeno Probes of Noise Correlations in Photon Polarization", Phys. Rev. Lett. 129, 030401 (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)
S. 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).
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).