Co-propagation of quantum signals with intense classical channels over the same optical fiber is a key requirement for practical quantum networks integrated into existing telecommunication infrastructure. In this work we experimentally study the coexistence of quantum channels with long-haul White Rabbit (WR) time-transfer services over standard single-mode fibers by characterizing spontaneous Raman scattering (SpRS) generated by narrowband classical C-and O-band channels emulating the wavelengths and launch powers of WR transceivers. The large optical power gap between milliwatt-level WR traffic and few-photon-level quantum signals makes even weak nonlinear inelastic scattering in the transmission fiber a significant broadband noise source in spectral regions where quantum channels would operate. In our parameter regime this noise is dominated by SpRS. We implement a single-photon-counting testbed that measures forward and backward SpRS spectra in the O-and C-bands over several ITU-T fiber types (G.652D, G.654C, G.655), using tunable narrowband pumps and a reconfigurable filtering chain based on tunable band-pass filters and O/C-band filter stacks. From these data, favorable operating regions in terms of wavelength placement and launch power for classical WR channels relative to a quantum channel are identified, and experimentally grounded order-of-magnitude estimates are provided that can be used as input to link budgets and simulation studies of quantum-classical coexistence in WR-based networks.

Quantum process tomography (QPT) allows for the estimation of the quantum superoperator by performing multiple measurements using different input states. In this work, we estimate the superoperator that characterizes free-space propagation through atmospheric turbulence. For that, we propose the use of a fiber-based projective measurement station and a heralded single photon source to implement a proof-of-concept QPT device. We present a theoretical analysis of the setup and perform an experimental QPT of the atmospheric turbulence quantum channel acting on the polarization state for 5 different turbulence conditions. Our tests conclude that turbulence has a negligible impact on the polarization state of the photons transmitted through the channel, obtaining a probability for the identity operator close to 95% in all the turbulence conditions measured.

Gigabit-Capable Passive Optical Network (GPON) reach extension using optical amplification is well established, yet validation of user-facing service performance at the Optical Network Terminal (ONT) Ethernet interface remains limited. This paper addresses this gap by experimentally validating a distributed, circulator-separated reach extender based on two Semiconductor Optical Amplifiers (SOAs) in a controlled laboratory testbed, comparing a passive 50 km baseline with a 60 km reach-extended link. Service performance was assessed at the ONT Ethernet interface using RFC 6349 TCP throughput testing and ITU-T Y.1564 Ethernet service activation testing, with values reported as medians from 10 independent runs. Internet Protocol Television (IPTV) quality was evaluated using estimated Mean Opinion Score—Video (MOS-V) and Estimated Peak Signal-to-Noise Ratio (EPSNR) for H.264 and MPEG-2 streams. The 60 km configuration preserved upstream TCP throughput while reducing downstream throughput from 765.650 to 721.900 Mbit/s. Median latency increased from 1.336 to 1.408 ms, jitter decreased from 0.076 to 0.057 ms, and video-profile frame loss increased from 0 to approximately 0.0064. MOS-V values remained predominantly in the good range. The results indicate feasible 60 km GPON operation with measurable downstream and loss-related trade-offs, providing repeatable service-level validation beyond optical-budget-only reporting rather than introducing a new optical reach-extender architecture.

We present implementation of precise time dissemination using the White Rabbit protocol over shared optical fibers within research backbones. It highlights advancements in synchronization accuracy, shared infrastructure efficiency, and implications for scientific and possible industrial applications.

In this letter, we present the latest findings in coexistence of two rather different classes of optical signals in one fiber - classical data signals and quantum signals. The most significant and notable difference between the two is the optical power, which is several orders (7 or more) of magnitude distinct from each other. There are several approaches to address this issue - in this letter, at a glance, the realistic and practical approach, which relies on the selection of high-grade optical filters, was chosen. Rigorous, however affordable, optical filtering is required to avoid any possible crosstalk, not only from the classical data signals but also from the spontaneous nonlinear Raman scattering effect. In our experimental verification, it has been demonstrated that the selection of high-grade transmission components is a crucial element in the success of multiplexing strong and weak optical signals in a single fiber. In real-world applications, this approach significantly reduces infrastructure costs by eliminating the need for additional fiber.

With the increasing demand for ultra-precise time synchronization and frequency dissemination across various scientific, industrial, and communication fields, the Czech Republic has developed an innovative, non-commercial fiber-based infrastructure. This infrastructure serves as a shared platform, utilizing optical fibers to enable high-precision timing, coherent frequency transfer, and a newly implemented vibrational sensing capability. The project also addresses challenges posed by classical communication noise-particularly from Raman scattering-on quantum channels, especially for Quantum Key Distribution (QKD). By strategically separating classical and quantum channels into distinct wavelength bands, such as the C-band and O-band, the infrastructure achieves minimal interference while enabling multiple concurrent applications over shared fiber lines.

Commercial sources of polarization entanglement at telecommunication wavelengths are already available on the market, but they lack proper certification or third-party testing. We aim to provide a comprehensive testing framework for photon counting and correlation measurements to characterize the parameters of these sources in a scalable and repeatable manner. The detection setup is included in our considerations, as the non-idealities of the components negatively affect the relevance of the measurement results. We discuss bounds for both true and false coincidences with rigorous probabilistic approach, as their ratio directly impacts the resolution of coincidence measurements and is reflected in Quantum Bit Error Rate (QBER) in the quantum telecommunication system. Quantum State Tomography (QST), polarization visibility measurements, temporal correlations measurements, and computations of other statistics are to be performed and compared at the state-of-the-art level for a three commercially available sources. Given that QST is demanding in terms of number of measurements and post-processing analysis, we discuss the relevance of determining the degree of polarization entanglement considering solely other statistics of direct measurement approach.

The demand for stable and tunable laser sources is steadily growing across a wide range of applications. Optical Frequency Combs (OFCs) have emerged as a powerful reference standard, offering stable frequency spacing between the comb tones or high optical frequency stability. To obtain low-cost tunable lasers with their carriers or relative frequencies locked, we phaselocked a commercial Telecom grade Integrable-Tunable-Laser-Assembly (ITLA) to an OFC. We demonstrate such phaselock within the telecom C-band (1527 nm - 1565 nm), using sub-mW OFC power corresponding to per-tone power down to nW regime. This enables phaselocking of large number of tunable lasers to the same OFC via passive splitting of the OFC power. We achieved short-term integrated phase noise of 10 mrad and long-term frequency stability measured over 10 hours below +/- 0.01 Hz.

Polarization-sensitive receivers for single photons are of crucial importance in various applications within the fields of quantum communication and quantum sensing, and are more commonly implemented in free-space optics rather than in optical fibers. This is primarily due to the unpredictable and varying birefringence in single-mode optical fibers. We present a method for birefringence compensation in an all-fiber detection setup that relies solely on coincidence measurements of a polarization-entangled state, or known correlations in a prepare-and-measure scenario. We define coincidence entropies as functions of the measured coincidence counts. These quantify the randomness of measurement outcomes, remain independent of the transmitted Bell state, and serve as indicators of the degree of entanglement. By leveraging coincidence entropy as a cost function in a gradient descent algorithm, we are able to align the polarization bases between two distinct polarization-sensitive receivers. Additionally, coincidence entropies can be employed to monitor the quality of entanglement transmission, thereby enhancing the system's ability to detect potential eavesdropping attempts, such as intercept-resend quantum attacks.

Coherent optical systems are essential for global data transmission, supporting links from data centers to metropolitan networks. These systems are expected to soon deliver speeds of 1.6 Tb/s per lambda, even as slower formats like 800 Gb/s and 400 Gb/s continue to influence practices. Data modulation at 10 Gb/s has traditionally been based on on-off keying (OOK); indeed, certain non-data applications still use OOK despite the shift to coherent transmission. Our study examines the coexistence of an OOK signal and a 400 Gb/s coherent signal up to 800km using wavelengths of 1550.12nm and 1550.92 nm. Our findings indicate minimal impact on performance of the data signals in such scenarios for national research and education networks.

This paper presents a comparison of the benefits of optically amplified time transfer using the White Rabbit (WR) protocol compared to regeneration-based methods utilizing White Rabbit Switches (WRS). We highlight the advantages in terms of simpler way of calibration, lower jitter, and cost efficiency compared to conventional regeneration methods.

Ultra-stable frequency transfer and highly accurate time transfer methods, using optical fibers, have made a significant progress in recent years, demonstrating exceptional performance. Optical fiber links (cables) are increasingly relevant for other modern and demanding applications and novel scientific research activities, such as metrology, relativistic geodesy, distributed fiber sensing, quantum key distribution (QKD), fundamental research (including high energy physics), redefinition of SI units, optical clocks comparison and other advanced topics and various industrial and societal applications (GPS, distant early warnings). Optical fibers can offer a valuable alternative to radio- and satellite-based methods (because of serious concerns related to failures of such services especially known for the last few years). While many countries worldwide (not limited to Europe) have been building and developing national time and frequency dissemination fiber networks and establishing international fiber connections, a generic pan-European fiber network connecting these national fiber networks, and research time and frequency facilities (optical fountains, frequency cavities) does not yet exist. In response to recognized need, the GEANT Association has launched an ambitious initiative to connect these nowadays isolated fiber components and create a fiber-based infrastructure for widespread distribution of time and frequency throughout Europe. This initiative, to be implemented in the next phase of GEANT's GN5-2 project, will enhance existing national fiber connections and support time and frequency transfers. The result will be a comprehensive, Europe-wide time and frequency fiber infrastructure for before mentioned fields.

The dissemination of precise time and coherent optical frequency over dedicated fibers or "dark" channels within Dense Wavelength Division Multiplexing (DWDM) networks is crucial for advanced scientific research, geophysical monitoring, and emerging industrial applications. By leveraging actively stabilized phase-coherent optical frequency transfer technique, we maintain ultra-low timing jitter and frequency stability over long-haul fiber links without disrupting of data channels. This contribution explores utilization of a shared dark spectrum within existing DWDM infrastructure to enable high-precision time transfer and stable optical frequency dissemination while simultaneously facilitating distributed vibration sensing. Furthermore, the integration of distributed fiber sensing within the same spectral allocation enables real-time detection of environmental disturbances such as seismic events and structural vibrations. Experimental results demonstrate the feasibility of this dual-purpose approach, showcasing its potential for enhancing geophysical monitoring, smart infrastructure, and next-generation metrology applications while optimizing the use of existing telecom networks.

Achieving optimal synchronization accuracy between two White Rabbit devices hinges on the proper selection of transceivers, which act as electro-optical converters connecting WR devices to the optical network infrastructure. The correct choice of transceivers can significantly improve resilience to changes in the time offset between WR devices due to temperature fluctuations in the connecting optical fiber. To compare the performance of BiDi WDM and DWDM transceivers, an experimental setup was established under laboratory conditions to simulate a real optical network used for distributing precise time and frequency between two remote locations. The optical connection was emulated by integrating a 20 km G.652.D optical fiber into a climatic chamber, which provided variable environmental conditions similar to those experienced in real applications. The study compared BiDi WDM 1310/1550 nm transceivers with DWDM Ch33/Ch34 transceivers. Results showed that DWDM transceivers exhibited nearly thirteen times less sensitivity to temperature-induced changes in the optical connection, leading to a smaller time offset. Therefore, for achieving the highest accuracy in synchronizing WR devices in practical applications, DWDM transceiver technology is essential.

This work addresses the calibration of asymmetry in optical transmission paths for precise time and frequency distribution. Specifically, we focus on calibrating White Rabbit technology, where local calibration is not possible due to the considerable distance between synchronized nodes. We developed an automatic calibration system using micro-electromechanical optical switches, which we verified under laboratory conditions. The verification process utilizes a basic calibration method, employing an auxiliary communication channel to transmit the 1PPS signal from a remote synchronized White Rabbit node for local comparison. The remote 1PPS signal’s transmission direction is time-multiplexed. Experimental results from a laboratory model of a real optical transmission system demonstrated the alignment between the automatic calibration system and our verification chain. The change in optical system asymmetry, simulated by adding additional optical fiber to the existing path, resulted in measured asymmetry changes of 101.5 ps by the automatic system and 102.6 ps by the verification chain. The overall difference between these two calibration methods was 1.1 ps. These findings confirm that the automatic system provides reliable results for calibrating asymmetry in optical transmission systems.