arXiv subjects

Stephen P. Walborn

Publications and source records attributed to Stephen P. Walborn.

At least 19 recordsLinked to original sources

arXiv2026

Poled-fibre phase modulator for efficient high-dimensional quantum measurements

Efficient detection of quantum states underpins advanced device-independent quantum-information protocols that provide the ultimate level of security for tasks including quantum random number generation and quantum key distribution (QKD). High-dimensional encoding is a natural route to boost the performance of such protocols, offering enhanced noise resilience and higher information capacity, yet their practical implementation remains challenging. A key experimental bottleneck in higher dimensions is the typical need of active modulators for basis selection, which incur substantial optical losses and polarization-sensitive operation. Poled optical fiber phase modulators (PFPMs) are a fiber-native electro-optic technology that naturally addresses these challenges, combining sub-dB insertion loss, intrinsic polarization independence, and direct compatibility with standard telecommunications fiber. Here we report the first use of a PFPM for active quantum-state measurements in a fully fiber-integrated platform. Basis selection in our receiver for four-dimensional qudits is achieved using a single PFPM, substantially simplifying the receiver architecture. As a benchmark, we perform a four-dimensional QKD session and obtain a finite secret-key rate per pulse that, to the best of our knowledge, surpasses all previously reported QKD demonstrations. Our results establish poled-fiber electro-optic modulation as a broadly applicable platform for high-efficiency detection in fiber-integrated quantum information processing.

quant-ph↗

arXiv2026

Optimal Interaction Free Localization with Multipath Interferometers

Interaction-free measurement (IFM) certifies the presence of an absorbing object without a photon ever being absorbed by it. When several candidate locations are available, existing protocols can also identify which one holds the absorber, but they do so by testing paths sequentially through two-path interferometers, resolving a binary presence question at each step. We propose a different approach: probing all candidate locations at once, with the photon prepared in a coherent superposition across every path before a single measurement resolves the outcome. We prove that a three-stage protocol built on a $d$-path interferometer attains the exact one-shot optimum for this task, and we extend it to $k$ absorbers among $d$ paths, where the no-absorption branch encodes the entire absorber subset coherently rather than revealing individual locations one by one. The dark port therefore ceases to be a mere witness of presence and becomes a location-resolving signal. We then move beyond single-pass strategies using the quantum-comb formalism, casting the problem as an exact optimization over all multi-pass strategies and showing that adaptive protocols surpass the one-shot ceiling. Enriching the interferometer geometry with one additional path guaranteed to be empty, we show that sequential scanning, bright-port recycling, and Zeno-type interrogation all become particular feasible strategies within this same optimization, rather than separate benchmarks to compare against. This unified formulation identifies the optimal interaction-free localization strategy for any given set of resources, opening a route toward loss-resilient quantum imaging protocols for the study of fragile, absorption-sensitive samples.

quant-ph↗

arXiv2025

Experimental Investigation of Twist Conservation in Nonlinear Optical Three-Wave Mixing

We conduct an experimental investigation into the conservation of the twist phase in Twisted Gaussian Schell Model (TGSM) beams during both up- and down-conversion three-wave mixing nonlinear processes. Independently generated TGSM beams, prepared with varying twist parameters, are used to pump and seed the nonlinear interactions. The resulting beams are then analyzed to determine their twist properties. Our findings demonstrate that the twists of the up- and down-converted beams depend on those of the pump and seed beams. Additionally, the results indicate that the twist phase is conserved throughout the process, in qualitative agreement with theoretical predictions. This study is motivated by the increasing potential applications of TGSM beams in various fields.

physics.optics↗

arXiv2024

A new architecture for high speed core-selective switch for multicore fibers

The use of multicore optical fibers is now recognized as one of the most promising methods to implement the space-division multiplexing techniques required to overcome the impending capacity limit of conventional single-mode optical fibers. Nonetheless, new devices for networking operations compatible with these fibers will be required in order to implement the next-generation high-capacity optical networks. In this work, we develop a new architecture to build a high-speed core-selective switch, critical for efficiently distributing signals over the network. The device relies on multicore interference, and can change among outputs in less than 0.7 us, while achieving less than -18 dB of average inter-core crosstalk, making it compatible with a wide range of network switching tasks. The functionality of the device was demonstrated by routing a 1GBs optical signal and by successfully switching signals over a field-installed multicore fiber network. Our results demonstrate for the first time the operation of a multicore optical fiber switch functioning under real-world conditions, with switching speeds that are three orders of magnitude faster than current commercial devices. This new optical switch design is also fully compatible with standard multiplexing techniques and, thus, represents an important achievement towards the integration of high-capacity multicore telecommunication networks.

quant-ph↗

arXiv2024

Non-Markovianity in High-Dimensional Open Quantum Systems using Next-generation Multicore Optical Fibers

With the advent of quantum technology, the interest in communication tasks assisted by quantum systems has increased both in academia and industry. Nonetheless, the transmission of a quantum state in real-world scenarios is bounded by environmental noise, so that the quantum channel is an open quantum system. In this work, we study a high-dimensional open quantum system in a multicore optical fiber by characterizing the environmental interaction as quantum operations corresponding to probabilistic phase-flips. The experimental platform is currently state-of-the-art for quantum information processing with multicore fibers. At a given evolution stage we observe a non-Markovian behaviour of the system, which is demonstrated through a proof-of-principle implementation of the Quantum Vault protocol. A better understanding of phase-noise in multicore fibers will improve several real-world communication protocols, since they are a prime candidate to be adopted in future telecom networks.

quant-ph↗

arXiv2023

Transmission of optical communication signals through ring core fiber using perfect vortex beams

Orbital angular momentum can be used to implement high capacity data transmission systems that can be applied for classical and quantum communications. Here we experimentally study the generation and transmission properties of the so-called perfect vortex beams and the Laguerre-Gaussian beams in ring-core optical fibers. Our results show that when using a single preparation stage, the perfect vortex beams present less ring-radius variation that allows coupling of higher optical power into a ring core fiber. These results lead to lower power requirements to establish fiber-based communications links using orbital angular momentum and set the stage for future implementations of high-dimensional quantum communication over space division multiplexing fibers.

physics.optics↗

arXiv2023

Practical computational advantage from the quantum switch on a generalized family of promise problems

The quantum switch is a quantum computational primitive that provides computational advantage by applying operations in a superposition of orders. In particular, it can reduce the number of gate queries required for solving promise problems where the goal is to discriminate between a set of properties of a given set of unitary gates. In this work, we use Complex Hadamard matrices to introduce more general promise problems, which reduce to the known Fourier and Hadamard promise problems as limiting cases. Our generalization loosens the restrictions on the size of the matrices, number of gates and dimension of the quantum systems, providing more parameters to explore. In addition, it leads to the conclusion that a continuous variable system is necessary to implement the most general promise problem. In the finite dimensional case, the family of matrices is restricted to the so-called Butson-Hadamard type, and the complexity of the matrix enters as a constraint. We introduce the ``query per gate'' parameter and use it to prove that the quantum switch provides computational advantage for both the continuous and discrete cases. Our results should inspire implementations of promise problems using the quantum switch where parameters and therefore experimental setups can be chosen much more freely.

quant-ph↗

arXiv2022

Experimental quantum state discrimination using the optimal fixed rate of inconclusive outcomes strategy

The problem of non-orthogonal state discrimination underlies crucial quantum information tasks, such as cryptography and computing protocols. Therefore, it is decisive to find optimal scenarios for discrimination among quantum states. We experimentally investigate the strategy for the optimal discrimination of two non-orthogonal states considering a fixed rate of inconclusive outcomes (FRIO). The main advantage of the FRIO strategy is to interpolate between unambiguous and minimum error discrimination by solely adjusting the rate of inconclusive outcomes. We present a versatile experimental scheme that performs the optimal FRIO measurement for any pair of generated non-orthogonal states with arbitrary a priori probabilities and for any fixed rate of inconclusive outcomes. Considering different values of the free parameters in the FRIO protocol, we implement it upon qubit states encoded in the polarization mode of single photons generated in the spontaneous parametric down-conversion process. Moreover, we resort to a newfangled double-path Sagnac interferometer to perform a three-outcome non-projective measurement required for the discrimination task, showing excellent agreement with the theoretical prediction. This experiment provides a practical toolbox for a wide range of quantum state discrimination strategies using the FRIO scheme, which can greatly benefit quantum information applications and fundamental studies in quantum theory.

quant-ph↗

arXiv2022

Certification of a Nonprojective Qudit Measurement using Multiport Beamsplitters

Generalised quantum measurements go beyond the textbook concept of a projection onto an orthonormal basis in Hilbert space. They are not only of fundamental relevance but have also an important role in quantum information tasks. However, it is highly demanding to certify that a generalised measurement is indeed required to explain the results of a quantum experiment in which only the degrees of freedom are assumed to be known. Here, we use state-of-the-art multicore optical fiber technology to build multiport beamsplitters and faithfully implement a seven-outcome generalised measurement in a four-dimensional Hilbert space with a fidelity of $99.7\%$. We apply it to perform an elementary quantum communication task and demonstrate a success rate that cannot be simulated in any conceivable quantum protocol based on standard projective measurements on quantum messages of the same dimension. Our approach, which is compatible with modern photonic platforms, showcases an avenue for faithful and high-quality implementation of genuinely nonprojective quantum measurements beyond qubit systems.

quant-ph↗

arXiv2021

Entropic uncertainty relations for mutually unbiased periodic coarse-grained observables resemble their discrete counterparts

One of the most important and useful entropic uncertainty relations concerns a $d$ dimensional system and two mutually unbiased measurements. In such a setting, the sum of two information entropies is lower bounded by $\ln d$. It has recently been shown that projective measurements subject to operational mutual unbiasedness can also be constructed in a continuous domain, with the help of periodic coarse graining. Here we consider the whole family of Rényi entropies applied to these discretized observables and prove that such a scheme does also admit the uncertainty relation mentioned above.

quant-ph↗

arXiv2021

Periodic discretized continuous observables are neither continuous nor discrete

Most of the fundamental characteristics of quantum mechanics, such as non-locality and contextuality, are manifest in discrete, finite-dimensional systems. However, many quantum information tasks that exploit these properties cannot be directly adapted to continuous-variable systems. To access these quantum features, continuous quantum variables can be made discrete by binning together their different values, resulting in observables with a finite number "$d$" of outcomes. While direct measurement indeed confirms their manifestly discrete character, here we employ a salient feature of quantum physics known as mutual unbiasedness to show that such coarse-grained observables are in a sense neither continuous nor discrete. Depending on $d$, the observables can reproduce either the discrete or the continuous behavior, or neither. To illustrate these results, we present an example for the construction of such measurements and employ it in an optical experiment confirming the existence of four mutually unbiased measurements with $d = 3$ outcomes in a continuous variable system, surpassing the number of mutually unbiased continuous variable observables.

quant-ph↗

arXiv2021

Computational advantage from quantum superposition of multiple temporal orders of photonic gates

Models for quantum computation with circuit connections subject to the quantum superposition principle have been recently proposed. There, a control quantum system can coherently determine the order in which a target quantum system undergoes $N$ gate operations. This process, known as the quantum $N$-switch, is a resource for several information-processing tasks. In particular, it provides a computational advantage -- over fixed-gate-order quantum circuits -- for phase-estimation problems involving $N$ unknown unitary gates. However, the corresponding algorithm requires an experimentally unfeasible target-system dimension (super)exponential in $N$. Here, we introduce a promise problem for which the quantum $N$-switch gives an equivalent computational speed-up with target-system dimension as small as 2 regardless of $N$. We use state-of-the-art multi-core optical-fiber technology to experimentally demonstrate the quantum $N$-switch with $N=4$ gates acting on a photonic-polarization qubit. This is the first observation of a quantum superposition of more than $N=2$ temporal orders, demonstrating its usefulness for efficient phase-estimation.

quant-ph↗

arXiv2020

Boosting entanglement generation in down-conversion with incoherent illumination

Entangled photons produced by spontaneous parametric down-conversion have been of paramount importance for our current understanding of quantum mechanics and advances in quantum information. In this process, the quantum correlations of the down-converted photons are governed by the optical properties of the pump beam illuminating the non-linear crystal. Extensively, the pump beam has been modeled by either coherent beams or by the well-know Gaussian-Schell model, which leads to the natural conclusion that a high degree of optical coherence is required for the generation of highly entangled states. Here, we show that when a novel class of partially coherent Gaussian pump beams is considered, a distinct type of quantum state can be generated for which the amount of entanglement increases inversely with the degree of coherence of the pump beam. This leads to highly incoherent yet highly entangled multi-photon states, which should have interesting consequences for photonic quantum information science.

quant-ph↗

arXiv2020

Detection of quantum non-Markovianity close to the Born-Markov approximation

We calculate in an exact way the conditional past-future correlation for the decay dynamics of a two-level system in a bosonic bath. Different measurement processes are considered. In contrast to quantum memory measures based solely on system propagator properties, here memory effects are related to a convolution structure involving two system propagators and the environment correlation. This structure allows to detect memory effects even close to the validity of the Born-Markov approximation. An alternative operational-based definition of environment-to-system backflow of information follows from this result. We provide experimental support to our results by implementing the dynamics and measurements in a photonic experiment.

quant-ph↗

arXiv2019

Optical simulation of the free Dirac equation

We present a classical optics simulation of the one-dimensional Dirac equation for a free particle. Positive and negative energy components are represented by orthogonal polarizations of a free propagating beam, while the spatial profile represents the spatial wave function of the particle. Using a completely tunable time parameter, we observed the oscillation of the average value of the Dirac position operator - known as zitterbewegung (ZB). We are also able to measure the so called mean-position operator which is a single-particle observable and presents no oscillations. Our work opens the way for optical simulation of interesting phenomenon of relativistic systems, as well as condensed-matter physics, without any requirement for specially engineered medium.

quant-ph↗

arXiv2018

Phase conjugation and mode conversion in stimulated parametric down-conversion with orbital angular momentum: a geometrical interpretation

We report on an experiment that investigates the spatial mode conversion in the process of parametric down-conversion seeded by a light beam in a superposition of orbital angular momentum modes. This process is interpreted in terms of a geometric representation of first-order spatial modes in a Poincaré sphere, providing an intuitive image of the phase conjugation and the topological charge conservation. We also make a comparison with the analogous phenomenon for optical parametric oscillators.

physics.optics↗

arXiv2018

Uncertainty Relations for coarse-grained measurements: an overview

Uncertainty relations involving complementary observables are one of the cornerstones of quantum mechanics. Aside from their fundamental significance, they play an important role in practical applications, such as detection of quantum correlations and security requirements in quantum cryptography. In continuous variable systems, the spectra of the relevant observables form a continnuum and this necessitates the coarse graining of measurements. However, these coarse-grained observables do not necessarily obey the same uncertainty relations as the original ones, a fact that can lead to false results when considering applications. That is, one cannot naively replace the original observables in the uncertainty relation for the coarse-grained observables and expect consistent results. As such, a number of uncertainty relations that are specifically designed for coarse-grained observables have been developed. In recognition of the 90$^{th}$ anniversary of the seminal Heisenberg uncertainty relation, celebrated last year, and all the subsequent work since then, here we give a review of the state of the art of coarse-grained uncertainty relations in continuous variable quantum systems, as well as their applications to fundamental quantum physics and quantum information tasks. Our review is meant to be balanced in its content, since both theoretical considerations and experimental perspectives are put on an equal footing.

quant-ph↗

arXiv2018

Testing for entanglement with periodic coarse-graining

Continuous variables systems find valuable applications in quantum information processing. To deal with an infinite-dimensional Hilbert space, one in general has to handle large numbers of discretized measurements in tasks such as entanglement detection. Here we employ the continuous transverse spatial variables of photon pairs to experimentally demonstrate novel entanglement criteria based on a periodic structure of coarse-grained measurements. The periodization of the measurements allows for an efficient evaluation of entanglement using spatial masks acting as mode analyzers over the entire transverse field distribution of the photons and without the need to reconstruct the probability densities of the conjugate continuous variables. Our experimental results demonstrate the utility of the derived criteria with a success rate in entanglement detection of $\sim60\%$ relative to $7344$ studied cases.

quant-ph↗