Quantum Key Distribution (QKD) is a technology that allows sharing secret cryptographic keys between two distant users (Alice and Bob), whose intrinsic security is guaranteed by fundamental principles of quantum mechanics. QKD is a mature technology even if one of the main remaining challenges is the integration of different solutions in already deployed telecommunication fiber networks, in particular in long-haul segments. An approach able to cover long distances is the Twin-field QKD (TF-QKD) protocol; TF-QKD exploits interference of optical pulses in a central untrusted node (Charlie), allowing to double the communication distance with respect to the conventional prepare-and-measure solutions. Here we present a solution to one of the main issues of Twin-Field QKD, the phase stabilization within the optical path, demonstrating a strong advantage in performances of real word TF- QKD and testing our solution in a segment of the Italian Quantum Backbone. Furthermore, we analyze in detail the expected gain in terms of key rate exploiting our stabilization technique in the main TF-QKD-based protocols, even when they are declared insensitive to the phase noise.
Fundamental phenomena like Quantum Zeno effect (QZE) and Anti-Zeno Effect (AZE) have been recognized as relevant tools for quantum control. Along this line, here we present two experiments in which we demonstrate the capability to extract information on noise events by exploiting QZE and AZE. In the first experiment, we realize noise diagnostics by frequent measurement, showing how a single photon undergoing a noise process (e.g., random polarization fluctuations) can diagnose non-Markovian temporal correlations within such a noise. In the second one, instead, we show how, by protecting via QZE a photonic qubit in a noisy quantum channel, it is possible to estimate the statistical distribution of the microscopic noise (decoherence) events by using the qubit itself as a probe. These techniques can become indispensable under extremely faint illumination, when traditional interferometric methods are usually ineffective.
Bell tests serve as a fundamental tool in both quantum technologies and quantum foundations investigation. The traditional Bell test framework involves the use of projective measurements, which, because of the wavefunction collapse and the Heisenberg uncertainty principle, do not allow for the full estimation of the Bell parameter from each entangled pair. In this work, we propose a novel weak-measurement-based scheme enabling the complete estimation of the entire Bell parameter from each entangled pair. Moreover, this approach prevents the collapse of the quantum state wavefunction, thereby preserving the entanglement within it. Our results, showing a 6 standard deviations violation of the Bell inequality tested, are obtained while leaving the entanglement within the photon pair almost unaltered after the weak measurement scheme (as demonstrated by our quantum tomographic reconstructions), allowing to exploit it for further foundational or practical purposes.
Quantum key distribution (QKD) allows the sharing of secret cryptographic keys between two distant users, whose intrinsic security is guaranteed by the laws of nature. Nowadays, the most promising technique for the integration of QKD in already deployed long-haul telecommunication fiber networks is the Twin-field QKD (TF-QKD) protocol, but it requires that the communication channel is stable in terms of phase oscillations and it is free from background light, that reduces the transmission key-rate. Recently, we presented a solution to the phase stabilization problem, derived from atomic clocks comparison technology, demonstrating advantages in performances of real word TF-QKD. Here we quantify and characterize the background photons, analyzing in details their effects on the transmission and the practicalities to reduce their contribution to a negligible level.
Nowadays, a technological challenge is to integrate quantum key distribution (QKD) protocols in already present telecommunication fiber networks. Twin-field QKD is one of the most promising techniques on long distances, but requires stabilizing the optical length of the communication channels between parties. Adapting interferometry techniques derived from frequency metrology, we developed a solution for the simultaneous key sharing and channel length control, and we demonstrated it on a 206 km field-deployed fiber with 65 dB loss. Our method reduces the quantum-bit-error-rate contributed by channel length variations to <1%, representing an effective solution for real-world quantum communications.
Weak value measurements have been a real breakthrough in the quantum measurement framework. In particular, quantum measurements may take advantage by anomalous weak values, i.e. values out of the eigenvalues spectrum of the measured observable, both for implementing new measurement techniques and studying Quantum Mechanics foundations. In this report we show three experiments with single photons presenting anomalous weak values: the first one tests the incompatibility between quantum mechanics and noncontextual hidden variables theories, the second one is the first realization of a sequential weak value evaluation of two incompatible observables on the same photon, and the last one shows how sequential weak values can be used to test Leggett-Garg inequalities extended to multiple-measurements scenarios.
In quantum mechanics, the eigenvalues and their corresponding probabilities specify the expectation value of a physical observable, which is known to be a statistical property related to large ensembles of particles. In contrast to this paradigm, we demonstrate a unique method allowing to extract the expectation value of a single particle, namely, the polarisation of a single protected photon, with a single experiment. This is the first realisation of quantum protective measurements.
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