The precise synchronization of distant clocks is a fundamental requirement for a wide range of applications. Here, we experimentally demonstrate an approach of quantum clock synchronization by distributing entangled and correlated photon pairs from a telecom-wavelength quantum dot over a metropolitan fiber network in the Stockholm area. By leveraging the tight time correlation between the emitted photons, we achieve a synchronization accuracy of tens of picoseconds. We show that our synchronization scheme is secure against spoofing attacks by performing a remote quantum state tomography to verify the origin of the entangled photons. We measured a distributed maximum entanglement fidelity of 0.817 +/- 0.040 to the |Phi+> Bell state and a concurrence of 0.660 +/- 0.086. These results highlight the potential of quantum dot-generated entangled pairs as a shared resource for secure time synchronization and quantum key distribution in real-world quantum networks. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Entangled photon pairs at telecom wavelengths are essential for quantum communication, distributed computing, and quantum-enhanced sensing. The telecom O-band offers low chromatic dispersion and fiber loss, which is ideal for long-distance networks. Site-controlled nanowire quantum dots have emerged as a promising platform for generating single and entangled photons, offering high extraction efficiency and scalability. However, their operation has largely been restricted to the visible and first near-infrared (NIR-I) windows. Here, we demonstrate a bright source of entangled photon pairs in the telecom O-band based on site-controlled nanowire quantum dots. We measure a fine-structure splitting of 4.6 μeV, confirming suitability for high-fidelity polarization entanglement. Quantum-state tomography of the biexciton-exciton cascade reveals a maximum fidelity of 85.8 ± 1.1% to the Φ⁺ Bell state and a maximum concurrence of 75.1 ± 2.1%. This work establishes nanowire quantum dots as viable entangled photon sources at telecom, advancing scalable quantum technologies for fiber-based networks.

Quantum state tomography is used in quantum communication, computing, and cryptography to characterize quantum states enabling assessment and control of quantum systems. Rapidly scaling quantum technologies require fast, accurate, and simple techniques, but current tomography methods are bulky, hard to scale, and slow. To overcome these limitations, we experimentally demonstrate Fourier tomography [1], of single-photon and entangled photon pairs generated by a quantum dot. As shown in Fig. 1(a) for two-photon tomography, the setup consists of a continuously rotating quarter waveplate, a fixed linear polarizer and a single photon detector per qubit. As the waveplates rotates with different speed, we record the coincidences rate between the two detectors. This signal can be expressed as a Fourier series whose coefficients can be related to the polarization state of light. Fig. 1(b) shows the simulated coincidence rate as the waveplate rotates for input Bell states |ϕ⁺〉 and |ϕ⁻〉. By capturing the coincidences variations as a Fourier series through a single rotating element and fixed polarizer, we simplify the tomography process, providing an accurate, and simple approach suitable for scalable quantum systems.

We present the first experimental demonstration of Fourier state tomography of photonic polarization states. We measure entangled pairs from a quantum dot, achieving results comparable to projective tomography with fewer components, verifying the authenticity and scalability of Fourier tomography for characterizing quantum state.

On-demand single-photon sources operating at telecom wavelengths are crucial for quantum communication and photonic quantum technologies. In this work, we demonstrate high-purity, indistinguishable single-photon generation in the telecom O-band from an InAsP/ InP nanowire quantum dot. We measured a single-photon purity of g²ð0Þ ¼ 0:006 6 0:003 under above-band excitation. Furthermore, we characterize two-photon interference via Hong–Ou–Mandel measurements and achieve a photon indistinguishability of 81:1% 6 3:7% with a temporal postselection of a 300 ps time window and 5:6% 6 1:5% without temporal postselection. We estimate a first-lens source efficiency of ～28%. These results highlight the potential of nanowire quantum dots as a promising source of telecom single photons for photonic quantum applications, offering deterministic positioning, efficient photon extraction, and scalable production.

We report on the experimental investigation of potential high-performance cavity length stabilization using odd-indexed higher-order spatial modes. Schemes based on higher-order modes are particularly useful for micro-cavities that are used for enhanced fluorescence detection of a few emitters, which need to minimize photons leaking from a stabilization beam. We describe the design and construction of an assembly for a microcavity setup with tunable high passive stability. In addition, different types of active stabilization techniques based on higher-order modes are then implemented and characterized based on their performance. We achieved a stability of about 0.5 pm rms, while the error photons leaking from the continuous locking beam to a fluorescence detector are suppressed by more than 100-fold. We expect these results to be important for quantum technology implementations of various emitter-cavity setups, where these techniques provide a useful tool to meet the highly challenging demands.

We study second harmonic generation in LiNbO3 nanowaveguides, operated with sub-pJ pulses at 1424 nm in the picosecond regime generating 0.5 μW at 712 nm. Spectra recorded at both wavelengths provide evidence for enhanced χ(2) cascading effects.