Sub-picosecond inter-core skew characterization in multicore fibers via Hong–Ou–Mandel interference
Letícia Lira Tacca, Lucas Marques Fagundes, Matías Morales, Mariana Navarro Asan-Srain, I. Machuca, Santiago Gómez López, Gustavo H. dos Santos, Jaime Cariñe, Gabriel Saavedra, Esteban S. Gómez, Gustavo Lima, Stephen Patrick Walborn
Inter-core skew (ICS), the differential group delay between cores of a multicore fiber (MCF), is a critical parameter for both classical space-division multiplexed communications and quantum photonic networks. We present a high-precision measurement of ICS in commercially available four-core fibers using two-photon Hong–Ou–Mandel (HOM) interference in a fiber-integrated 4 × 4 multiport beam splitter, using both photon pairs from spontaneous parametric down-conversion (SPDC) and a weak-coherent-state source (a bimodal pulsed laser). By extracting the center position of HOM interference dips and peaks across all twelve core-pair combinations, we obtain individual ICS values whose precision is limited by the delay-stage positioning uncertainty. This first multi-length characterization of RMS ICS, spanning laboratory to field-deployed scales, finds results consistent with the expected L stochastic random-walk scaling, σ τ ( L )= κ L + c with intrinsic coefficient κ =(39.80±0.003) ps / km . This measurement was made possible by HOM’s immunity to first-order path fluctuations, which render classical interferometric methods impractical for long installed fibers. The demonstrated ±0.11 ps precision for the two-photon interference represents a ∼180-fold improvement over correlation optical time-domain reflectometry (C-OTDR), the standard method for long-fiber ICS characterization. Fisher information analysis establishes a fundamental Cramér–Rao precision limit in the few-femtosecond range, indicating further improvement is achievable with better delay control. The SPDC source provides this precision on laboratory-testbed fibers, while the weak-coherent-state source trades precision for loss tolerance, extending the measurement to a 1300 m field-deployed fiber with >10 dB losses. These results establish a practical platform for characterizing timing uniformity in MCF-based networks for both quantum and classical space-division multiplexed applications.
Inter-core skew (ICS), the differential group delay between cores of a multicore fiber (MCF), is a critical parameter for both classical space-division multiplexed communications and quantum photonic networks. We present a high-precision measurement of ICS in commercially available four-core fibers using two-photon Hong–Ou–Mandel (HOM) interference in a fiber-integrated 4 × 4 multiport beam splitter, using both photon pairs from spontaneous parametric down-conversion (SPDC) and a weak-coherent-state source (a bimodal pulsed laser). By extracting the center position of HOM interference dips and peaks across all twelve core-pair combinations, we obtain individual ICS values whose precision is limited by the delay-stage positioning uncertainty. This first multi-length characterization of RMS ICS, spanning laboratory to field-deployed scales, finds results consistent with the expected L stochastic random-walk scaling, σ τ ( L )= κ L + c with intrinsic coefficient κ =(39.80±0.003) ps / km . This measurement was made possible by HOM’s immunity to first-order path fluctuations, which render classical interferometric methods impractical for long installed fibers. The demonstrated ±0.11 ps precision for the two-photon interference represents a ∼180-fold improvement over correlation optical time-domain reflectometry (C-OTDR), the standard method for long-fiber ICS characterization. Fisher information analysis establishes a fundamental Cramér–Rao precision limit in the few-femtosecond range, indicating further improvement is achievable with better delay control. The SPDC source provides this precision on laboratory-testbed fibers, while the weak-coherent-state source trades precision for loss tolerance, extending the measurement to a 1300 m field-deployed fiber with >10 dB losses. These results establish a practical platform for characterizing timing uniformity in MCF-based networks for both quantum and classical space-division multiplexed applications.
Inter-core skew (ICS), the differential group delay between cores of a multicore fiber (MCF), is a critical parameter for both classical space-division multiplexed communications and quantum photonic networks. We present a high-precision measurement of ICS in a commercially available four-core fiber using two-photon Hong--Ou--Mandel (HOM) interference in a fiber-integrated 4×4 multiport beam splitter. By extracting the center position of HOM interference dips and peaks across all twelve core-pair combinations, we obtain individual ICS values with a demonstrated precision of ±0.11ps, limited by the delay-stage positioning uncertainty. The root-mean-square ICS grows as στ(L)=κL+c with κ=48.7±2.5ps/km and c=9.76±1.2ps, over fiber lengths from 7.7m to 1300m. This first direct validation of the stochastic random-walk scaling across a length range spanning laboratory to field-deployed scales was made possible by HOM's immunity to first-order path fluctuations, which renders classical interferometric methods impractical for long installed fibers. The demonstrated ±0.11ps precision represents a ∼180-fold improvement over correlation optical time-domain reflectometry (C-OTDR), the standard method for long-fiber ICS characterization. Fisher information analysis establishes a fundamental Cramér--Rao precision limit in the femtosecond range, indicating further improvement is achievable with better delay control. These results establish a practical platform for characterising timing uniformity in MCF-based networks for both quantum and classical space-division multiplexed applications.
Dissipative Kerr solitons in optical microresonators underpin chip-scale frequency combs with applications ranging from coherent telecommunications to precision spectroscopy. Yet the characterization of their intrinsic femtosecond temporal structure remains challenging, as the low pulse energy and broad spectral bandwidth necessitate optical amplification and careful dispersion compensation in conventional ultrafast diagnostics, both of which can significantly distort the waveform. Here we demonstrate a quantum-interference metrology of microcomb solitons based on Hong-Ou-Mandel interference. By attenuating the soliton stream to the single-photon level and measuring fourth-order interference, we directly retrieve near transform-limited pulse durations without amplification or dispersion management, remaining accurate even after propagation through 25 km of standard fiber. The same interferogram also provides direct access to the temporal separations in multi-soliton states by converting inter-soliton separations into additional interference dips at corresponding delays, enabling sub-picosecond characterization of their intracavity temporal structure. This quantum-inspired paradigm introduces a fundamentally new metrological approach that is immune to amplification and dispersion distortions, offering a powerful tool for the characterization of complex soliton physics.
Hong-Ou-Mandel interference between independent photon sources (HOMI-IPS) plays an important role in quantum information processing. A variety of HOMI-IPS experiments have been demonstrated at near-infrared wavelengths or telecom wavelengths [1]. In all the previous HOMI-IPS experiments, the HOM dips are obtained by recording the four-fold coincidence counts as a function of the temporal delay. However, the spectral correlation during the interference is omitted, partially due to technical difficulties, e.g., the acquisition time is too long for measuring a joint spectral distribution. Spectral information, as the conjugate of the temporal information in the Fourier transformation, is definitely of great importance and should be investigated. Very recently, a new technique of spectral correlation measurement in a HOM interference has been developed by Gerrits, et al [2]. This technique has the merits of high speed and high resolution, therefore, has been used as a powerful tool to analyse the spectral correlation in a HOM interference between signal and idler photons from one spontaneous parametric down conversion (SPDC) source. In this work, we measure the joint spectral distribution in HOMI-IPS using the new technique developed in [2]. The experimental setup is shown in Fig. 1 (Left). The two independent sources are based on SPDC from two 30-mm-long periodically poled KTiOPO4 (PPKTP) crystals pumped by picosecond laser pulses (76 MHz, 792 nm, temporal duration ~ 2 ps). In Fig. 1 (Left), the dispersion compensation fiber model (DCFMs), the superconducting nanowire single-photon detectors (SNSPDs) [3,4,5] and the time interval analyzer (TIA) constitute a fiber spectral analyzer [6]. Fig. 1(Right: (a)) is the measured four-fold HOM interference between signal1 and signal2, with idler1 and idler2 as heralders. Without subtraction of any background counts, the raw visibility of the HOM dip is 69.9±3.2%, which is consistent with our previous results in [1,7,8]. Fig. 1(Right: (b)-(g)) are the measured joint spectral intensities at different delay positions. The splitting phenomenon in the joint spectral distribution is clearly observed. The experimental results agree well with our theoretical simulations. This experiment not only can deepen our understanding of HOMI-IPS from the viewpoint of spectral domain, but also can be used to improve the visibility of HOMI-IPS by temporal filtering.
Hong-Ou-Mandel (HOM) interferometry enables delay estimation at the quantum precision limit but is traditionally constrained to path differences within the coherence time of the interfering photons. Here, we demonstrate single-measurement path-delay sensing at the measurement Cramer-Rao bound using spectrally resolved HOM interference, thereby removing the conventional dynamic-range limitation imposed by the photon coherence window, with no scanning required for calibration. By extracting delay information from the spectral interference fringes of spectrally entangled photon pairs, we retain near-optimal sensitivity over an operational range exceeding the photon coherence time by over two orders of magnitude. Using one million detected photon pairs, we achieve a time-delay precision of 20 attosecond (6 nm), while real-time operation (at 1 Hz) yields 330 attosecond (100 nm) precision. Because the estimator relies on fringe periodicity rather than absolute coincidence rates, the method is intrinsically robust to photon losses and variations in interference visibility, eliminating the need for recalibration. As a practical demonstration, we measure the thickness of a 300 um transmissive target with nanometer-scale precision. These results mark a significant step towards deploying quantum-limited measurements in real-world sensing applications using HOM interferometry.
Abstract Hong–Ou–Mandel (HOM) dip from a biphoton source in a two-photon interferometer provides a myriad of quantum tools for quantum communication and sensing. But the stringent requirements for spatial coherence between the photon pair makes it prohibitively difficult to observe high-fidelity HOM dip in long-distance free-space implementations, e.g., for the photon pairs involved in quantum communication need to match the two path lengths within a few 10 s of micron because of the short coherence width of the two-photon wave-packet. While many techniques for the pathlength balancing of two interferometric arms have been studied and applied extensively, such balancing is further complicated in presence of turbulence in long-distance free-space propagation. On a table-top testbed with each arm of a Mach–Zehnder interferometer (MZI) ∼10 ∼ 10 -meter long in our turbulent atmospheric simulator for qubit (TASQ), we have employed below-threshold lasers to achieve classical low-coherence interferometry and show that we could narrow down the interrogation window for the pathlength balancing to obtain interference peak at the center. This method allowed us to reduce the interrogation distance for searching the HOM dip by four orders of magnitude, i.e., from tens of centimeters to a few tens of micron. Such pathlength balancing allowed realizing the HOM dip within $$160\,\upmu \textrm{m}$$ 160 μ m distance from the peak of the classical interference fringe. This technique could be helpful in a wide variety of long-pathlength interference-based quantum sensing.
In this work we study the application of two-photon correlated quantum states for high-precision time estimation. To achieve this, photon pairs were fed into a Hong-Ou-Mandel polarization interferometer to generate N00N entangled states with N=2. Two- and four-detector schemes were considered, and the achievable time estimation precision of each was studied, with particular emphasis on the system's Fisher information in each case. Our results reveal that using a four-detector scheme, enabling the measurement of both coincidence and photon-bunching terms, achieves the maximum precision given by the quantum Fisher information, without the need of characterizing system losses. We also show that correlations between repetitions arise due to systematic errors, limiting the precision enhancement that can be obtained by increasing the number of repetitions. Taking these correlations into account, we show that the four-detector scheme outperforms the two-detector scheme in the precision of time-delay estimation. With the experimentally determined calibration uncertainties, the four-detector scheme improves the dispersion by a factor of 2 for the repetitions performed, reaching up to a factor of 4 as both schemes approach their systematic floors.