Twin-Field Quantum Key Distribution (TF-QKD) is a promising protocol to extend the secure communication range beyond the limits of conventional QKD, provided that phase noise along the optical channels is tightly controlled. In this work, we experimentally investigate the electro-mechanical phase noise affecting optical fibers in electrical grid environments, as a preliminary step toward deploying TF-QKD over Optical Ground Wire (OPGW) networks that run together with the electricity grids. Measurements were performed in a medium and low voltage smart grid test facility, which offered an accessible and controllable environment that shares some key features with OPGW installations. We analyze phase noise under varying electrical load conditions and infer the corresponding Quantum Bit Error Rate (QBER), providing a first experimental benchmark for future quantum communication over power grid fibers.

Quantum Key Distribution (QKD) is a technology that enables the sharing of secret cryptographic keys between two distant users (Alice and Bob), with intrinsic security guaranteed by the fundamental laws of nature. QKD has become a mature technology, and in Europe, all 27 member states are collaborating on a European Commission initiative (EuroQCI) to design, develop, and deploy a quantum communication infrastructure. In Italy, the QUID project is responsible for implementing the Italian segment of EuroQCI. QKD relies on single photons to secure the distribution of the keys and, to become a viable real-world solution, the metrological characterization of optical components and systems is fundamental. To obtain the appropriate security requirements, test and evaluation methods at single-photon level need to be developed; in particular, since the single-photon detectors represent the most vulnerable part of a QKD system, their characterization in terms of operating parameters (quantum efficiency, dead time, jitter, afterpulsing..) is of the utmost importance. We present the INRIM efforts in the quantum efficiency calibration of single-photon avalanche detectors (SPADs), focusing on QKD application. The detection efficiency is evaluated for a fibre-coupled InGaAs/InP-SPAD and for a free-space Si-SPAD. The calibration is performed using different experimental setups and reference standards with proper traceability chains at the wavelength of 1550 nm and 850 nm respectively. Dependence of detection efficiency on polarization in superconducting nanowire single-photon detectors (SNSPDs) is also reported. The work is fundamental to align the Italian deployment of QKD, in the framework of QUID, with validation needs, providing test services for the characterization, validation and certification for QKD.

High-Q silicon nitride microresonators are versatile sources for generating photon pairs via four-wave mixing. We investigate the spectral coherence of this process, tracking the transition from the spontaneous quantum regime to the onset of optical parametric oscillation. By combining time-correlation measurements with phase-sensitive measurements, we continuously monitor the emission linewidth as it evolves from a cavity-lifetime-limited linewidth toward the pump-linewidth scale. This characterization is essential for optimizing integrated sources for scalable quantum networks.

In this study, we demonstrate the possibility to protect, with Quantum Key Distribution (QKD), a critical infrastructure as the fiber-based one used for time and frequency (TF) dissemination service. The proposed technique allows to disseminate secure and precise TF signals between two fiber-optic-connected locations, on a critical infrastructure, using both QKD and White Rabbit technique. This secure exchange enables the secret sharing of time information between two parties, allowing the synchronization of distant clocks with a stability of [Formula: see text] at 1 s, traceable to the Italian time scale. When encrypted, the time signals provide no useful information to a third party regarding the synchronization status, resulting in a time stability degraded by two orders of magnitude.

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: in Europe, all the 27 EU member states are working in a European Commission initiative (EuroQCI) to design, develop and deploy a quantum communication infrastructure composed by several national networks. For example, in Italy, the Italian Quantum Backbone will be used as a quantum communication testbed within the project QUID, that is in charge of the realization of the Italian part of EuroQCI.

We investigate the impact of noise sources in real-world implementations of Twin-Field Quantum Key Distribution (TF-QKD) protocols, focusing on phase noise from photon sources and connecting fibers. Our work emphasizes the role of laser quality, network topology, fiber length, arm balance, and detector performance in determining key rates. Remarkably, it reveals that the leading TF-QKD protocols are similarly affected by phase noise despite different mechanisms. Our study demonstrates duty cycle improvements of over 2x through narrow-linewidth lasers and phase-control techniques, highlighting the potential synergy with high-precision time/frequency distribution services. Ultrastable lasers, evolving toward integration and miniaturization, offer promise for agile TF-QKD implementations on existing networks. Properly addressing phase noise and practical constraints allows for consistent key rate predictions, protocol selection, and layout design, crucial for establishing secure long-haul links for the Quantum Communication Infrastructures under development in several countries.

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.

Twin-field quantum key distribution on optical fibers, a promising approach to long-distance secure communication, can take advantage from atomic clocks technologies such as narrow linewidth lasers and phase-coherent distribution of optical pulses. We will describe the integration of these technologies in a TF-QKD setup implemented on an ex-tended metropolitan fiber network and report on the expected QKD performances.

Twin-Field Quantum Key Distribution (TF-QKD) is an innovative family of protocols characterized by a weaker dependence of the achievable secret key rate on the channel loss, with respect to conventional QKD solutions. In this work, we discuss several important aspects encountered in TF-QKD when transitioning from point-to-point links to a network configuration. 1) The effects of path length mismatch between the two arms of the link (A-C and B-C) is discussed in several configurations. 2) The noise contributions (stronger in in-field deployment) are meticulously analyzed, their effect on the final key rate is estimated and solutions to mitigate the problem are implemented. 3) The topic of building complex and large networks with TF-QKD is tackled to find advantageous configurations. Interconnected macro-star networks based on TF-QKD are simulated by means of the “qkdnetsim” package of the network simulator “ns3”. The upcoming deployment of national QKD networks requires dedicated studies in this direction to build efficient and long-range solutions, compatible with current telecom standards.

The integration of Quantum Key Distribution (QKD) protocols in metropolitan networks, exploiting already deployed telecommunication fibers, requires particular attention due to the non-ideal environment in which the protocol operates. At present, Twin-field QKD is one of the most promising techniques to reach the extension of QKD transmission but its real-word exploitation requires that the communication channel is stable in terms of optical length and is free from background light that increase the errors in the transmitted keys. Adapting interferometry techniques derived from frequency metrology, we recently proposed a solution [1] for a tight channel length control, and demonstrated it on a 206 km metropolitan fiber with 65 dB loss. Here we analyze in details the effects of background photons and the solutions that we applied to reduce their contribution to a negligible level.

Quantum mechanics allows distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is one of the most promising techniques for its implementation on long-distance fiber networks, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical stabilization frames. In this approach, longer duty cycles for the key streaming come at the cost of a looser control of channel length, and a successful key-transfer using this technique in real world remains a significant challenge. Using interferometry techniques derived from frequency metrology, we develop a solution for the simultaneous key streaming and channel length control, and demonstrate it on a 206 km field-deployed fiber with 65 dB loss. Our technique reduces the quantum-bit-error-rate contributed by channel length variations to <1%, representing an effective solution for real-world quantum communications.

Single-photon detectors are a pivotal component in photonic quantum technologies. A precise and comprehensive calibration of the intrinsic detection efficiency is of utmost importance to ensure the proper evaluation of the performance in view of the specific technological application of interest, such as the protection against security breaches in quantum cryptographic solutions. Here we report on a systematic study on and comprehensive analysis of the estimation of the intrinsic detection efficiency of two commercial single-photon detectors based on single-photon avalanche diodes (SPADs) for various mean photon numbers and at high laser pulse repetition rates using different techniques. We observed an unexpected and signifucant drop in intrinsic detection efficiency at detection rates of 10 % and higher relative to the maximum detection rate. It is demonstrated that for data analysis a statistical model for the detection rate conveniently can be used if no timestamped data are available. We conclude that the full characterization of single-photon detectors used in critical applications should include the sensitivity of their intrinsic detection efficiency to high event rates.

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.

This manuscript discusses the most relevant aspects of the practical implementation of a long-range Quantum Key Distribution (QKD) link with trusted nodes, achieving the highest possible secret key rate generation within the security and system-level constraints. To this purpose, it describes the implementation of an end-to-end QKD system, including implementation aspects from the physical transmission of photon states through a standard telecommunications grade optical fiber, to consideration of quantum metrology and information reconciliation protocols based on forward error correction codes. In addition, since there are circumstances when a fiber optical link may not be available, it examines the problems involved with the implementation of a Free Space Optics (FSO) QKD link. The manuscript also discusses the problem of information reconciliation in Continuous Variable (CV) QKD scenarios on FSO links, showing that in long distance links, since the sign of the received Gaussian samples contains the largest fraction of information, Unequal Error Protection (UEP) reverse reconciliation schemes can be designed. The presented results have been achieved within the NATO SPS project “Analysis, design and implementation of an end-to-end 400 km QKD link”.

A quantum model for Josephson-based metamaterials working in the Three-Wave Mixing (3WM) and Four-Wave Mixing (4WM) regimes at the single-photon level is presented. The transmission line taken into account, namely Traveling Wave Josephson Parametric Amplifier (TWJPA), is a bipole composed by a chain of rf-SQUIDs which can be biased by a DC current or a magnetic field in order to activate the 3WM or 4WM nonlinearities. The model exploits a Hamiltonian approach to analytically determine the time evolution of the system both in the Heisenberg and interaction pictures. The former returns the analytic form of the gain of the amplifier, while the latter allows recovering the probability distributions vs. time of the photonic populations, for multimodal Fock and coherent input states. The dependence of the metamaterial's nonlinearities is presented in terms of circuit parameters in a lumped model framework while evaluating the effects of the experimental conditions on the model validity.

In quantum communication systems, the precise estimation of the detector's response to the incoming light is necessary to avoid security breaches. The typical working regime uses a free-running single-photon avalanche diode in combination with attenuated laser pulses at telecom wavelength for encoding information. We demonstrate the validity of an analytical model for this regime which considers the effects of dark counts and dead time on the measured count rate. For the purpose of gaining a better understanding of these effects, the photon detections were separated from the dark counts via a software-induced gating mechanism. The model was verified by experimental data for mean photon numbers covering three orders of magnitude as well as for laser repetition frequencies below and above the inverse dead time. Consequently, our model would be of interest for predicting the detector response not only in the field of quantum communications, but also in any other quantum physics experiment where high detection rates are needed.

Quantum enhanced optical measurement protocols aim at reducing the uncertainty in the estimation of some physical quantities of a system below the shot-noise limit, classically unavoidable. In particular when small number of photons is used the shot noise can be the main source of uncertainty, in these cases the use of quantum light is of great interest. Note that there are several situations where the number of photons in the probe can not be increased arbitrarily, as when fragile biological samples are under investigation. Two different imaging protocols are discussed in the following.