arXiv subjects

J. Cariñe

Publications and source records attributed to J. Cariñe.

9 recordsLinked to original sources

arXiv2026

High-Dimensional Quantum Key Distribution via full Core-mode Encoding over Deployed Multicore Fibers

Quantum key distribution (QKD) provides information-theoretic security rooted in quantum physics, while high-dimensional (HD) encoding increases both noise tolerance and secret-key yield. Multicore fibers (MCFs), a leading platform for next-generation telecom networks, are a natural substrate for HD-QKD. Field demonstrations over deployed MCFs have so far relied on a hybrid qudit encoding strategy that combines two path (core modes) with the time-bin photonic degree of freedom, rather than exploiting the full set of available core modes. Although practical, this approach incurs intrinsic efficiency penalties that grow with dimension. Here we implement a four-dimensional ($d=4$) QKD protocol that directly exploits the full set of core modes of a four-core MCF, operating over an installed MCF network across the Universidad de Concepción campus under continuous environmental perturbations. We further benchmark the scheme using superconducting nanowire detectors at $10\,$dB channel loss, achieving a composable finite-key rate of $R = 6.19\times 10^{-3}\,$bits/pulse, the highest per-pulse rate reported to date for HD-QKD at comparable loss. This result establishes core-mode encoding as a viable architecture for realistic, high-rate quantum-secure communications.

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arXiv2026

Sub-picosecond inter-core skew characterization in multicore fibers via Hong--Ou--Mandel interference

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 $4\times4$ 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 $\pm0.11\,$ps, limited by the delay-stage positioning uncertainty. The root-mean-square ICS grows as $σ_τ(L) = κ\sqrt{L}+c$ with $κ= 48.7 \pm 2.5\,\mathrm{ps}/\!\sqrt{\mathrm{km}}$ and $c = 9.76 \pm 1.2\,$ps, over fiber lengths from $7.7\,$m to $1300\,$m. 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 $\pm0.11\,$ps precision represents a $\sim\!180$-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.

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arXiv2024

Efficient Experimental Qudit State Estimation via Point Tomography

Point tomography is a new approach to the problem of state estimation, which is arguably the most efficient and simple method for modern high-precision quantum information experiments. In this scenario, the experimenter knows the target state that their device should prepare, except that intrinsic systematic errors will create small discrepancies in the state actually produced. By introducing a new kind of informationally complete measurement, dubbed Fisher-symmetric measurements, point tomography determines deviations from the expected state with optimal efficiency. In this method, the number of outcomes of a measurement saturating the Gill-Massar limit for reconstructing a $d$-dimensional quantum states can be reduced from $\sim 4d-3$ to only $2d-1$ outcomes. Thus, providing better scalability as the dimension increases. Here we demonstrate the experimental viability of point tomography. Using a modern photonic platform constructed with state-of-the-art multicore optical fiber technology, we generate 4-dimensional quantum states and implement seven-outcome Fisher-symmetric measurements. Our experimental results exhibit the main feature of point tomography, namely a precision close to the Gill-Massar limit with a single few-outcome measurement. Specifically, we achieved a precision of $3.8/N$ while the Gill-Massar limit for $d=4$ is $3/N$ ($N$ being the ensemble size).

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arXiv2021

Maximizing Post-selected Quantum Correlations from Classical Interference in a Multi-core Fiber Beamsplitter

Fourth-order interference is an information processing primitive for photonic quantum technologies. When used in conjunction with post-selection, it forms the basis of photonic controlled logic gates, entangling measurements, and can be used to produce quantum correlations. Here, using classical weak coherent states as inputs, we study fourth-order interference in novel $4 \times 4$ multi-port beam splitters built within multi-core optical fibers. Using two mutually incoherent weak laser pulses as inputs, we observe high-quality fourth order interference between photons from different cores, as well as self-interference of a two-photon wavepacket. In addition, we show that quantum correlations, in the form of quantum discord, can be maximized by controlling the intensity ratio between the two input weak coherent states. This should allow for the exploitation of quantum correlations in future telecommunication networks.

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arXiv2020

Polarization-independent single-photon switch based on a fiber-optical Sagnac interferometer for quantum communication networks

An essential component of future quantum networks is an optical switch capable of dynamically routing single-photons. Here we implement such a switch, based on a fiber-optical Sagnac interferometer design. The routing is implemented with a pair of fast electro-optical telecom phase modulators placed inside the Sagnac loop, such that each modulator acts on an orthogonal polarization component of the single-photons, in order to yield polarization-independent capability that is crucial for several applications. We obtain an average extinction ratio of more than 19 dB between both outputs of the switch. Our experiment is built exclusively with commercial off-the-shelf components, thus allowing direct compatibility with current optical communication systems.

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arXiv2020

Multi-core fiber integrated multi-port beamsplitters for quantum information processing

Multi-port beamsplitters are cornerstone devices for high-dimensional quantum information tasks, which can outperform the two-dimensional ones. Nonetheless, the fabrication of such devices has been proven to be challenging with progress only recently achieved with the advent of integrated photonics. Here, we report on the production of high-quality $N \times N$ (with $N=4,7$) multi-port beamsplitters based on a new scheme for manipulating multi-core optical fibers. By exploring their compatibility with optical fiber components, we create 4-dimensional quantum systems and implement the measurement-device-independent random number generation task with a programmable 4-arm interferometer operating at a 2 MHz repetition rate. Thanks to the high visibilities observed, we surpass the 1-bit limit of binary protocols and attain 1.23 bits of certified private randomness per experimental round. Our result demonstrates that fast switching, low-loss and high optical quality for high-dimensional quantum information can be simultaneously achieved with multi-core fiber technology.

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arXiv2018

Challenging local realism with human choices

A Bell test is a randomized trial that compares experimental observations against the philosophical worldview of local realism. A Bell test requires spatially distributed entanglement, fast and high-efficiency detection and unpredictable measurement settings. Although technology can satisfy the first two of these requirements, the use of physical devices to choose settings in a Bell test involves making assumptions about the physics that one aims to test. Bell himself noted this weakness in using physical setting choices and argued that human `free will' could be used rigorously to ensure unpredictability in Bell tests. Here we report a set of local-realism tests using human choices, which avoids assumptions about predictability in physics. We recruited about 100,000 human participants to play an online video game that incentivizes fast, sustained input of unpredictable selections and illustrates Bell-test methodology. The participants generated 97,347,490 binary choices, which were directed via a scalable web platform to 12 laboratories on five continents, where 13 experiments tested local realism using photons, single atoms, atomic ensembles, and superconducting devices. Over a 12-hour period on 30 November 2016, participants worldwide provided a sustained data flow of over 1,000 bits per second to the experiments, which used different human-generated data to choose each measurement setting. The observed correlations strongly contradict local realism and other realistic positions in bipartite and tripartite scenarios. Project outcomes include closing the `freedom-of-choice loophole' (the possibility that the setting choices are influenced by `hidden variables' to correlate with the particle properties), the utilization of video-game methods for rapid collection of human generated randomness, and the use of networking techniques for global participation in experimental science.

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arXiv2016

High-dimensional decoy-state quantum key distribution over 0.3 km of multicore telecommunication optical fibers

Multiplexing is a strategy to augment the transmission capacity of a communication system. It consists of combining multiple signals over the same data channel and it has been very successful in classical communications. However, the use of enhanced channels has only reached limited practicality in quantum communications (QC) as it requires the complex manipulation of quantum systems of higher dimensions. Considerable effort is being made towards QC using high-dimensional quantum systems encoded into the transverse momentum of single photons but, so far, no approach has been proven to be fully compatible with the existing telecommunication infrastructure. Here, we overcome such a technological challenge and demonstrate a stable and secure high-dimensional decoy-state quantum key distribution session over a 0.3 km long multicore optical fiber. The high-dimensional quantum states are defined in terms of the multiple core modes available for the photon transmission over the fiber, and the decoy-state analysis demonstrates that our technique enables a positive secret key generation rate up to 25 km of fiber propagation. Finally, we show how our results build up towards a high-dimensional quantum network composed of free-space and fiber based links

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arXiv2014

Long-distance distribution of genuine energy-time entanglement

Any practical realization of entanglement-based quantum communication must be intrinsically secure and able to span long distances avoiding the need of a straight line between the communicating parties. The violation of Bell's inequality offers a method for the certification of quantum links without knowing the inner workings of the devices. Energy-time entanglement quantum communication satisfies all these requirements. However, currently there is a fundamental obstacle with the standard configuration adopted: an intrinsic geometrical loophole that can be exploited to break the security of the communication, in addition to other loopholes. Here we show the first experimental Bell violation with energy-time entanglement distributed over 1 km of optical fibers that is free of this geometrical loophole. This is achieved by adopting a new experimental design, and by using an actively stabilized fiber-based long interferometer. Our results represent an important step towards long-distance secure quantum communication in optical fibers.

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