arXiv subjects

Jacquiline Romero

Publications and source records attributed to Jacquiline Romero.

At least 19 recordsLinked to original sources

arXiv2026

Broadband transverse mode directional couplers using partial Euler bends

Transverse modes beyond the fundamental mode are increasingly being explored to increase information capacity in both mode-division multiplexing and high-dimensional quantum information processing. The ability to combine and separate these modes at arbitrary ratios over a broad bandwidth is central to these applications. Motivated by the lower bending loss and intermodal crosstalk of partial Euler bends in multimode waveguides, we develop a geometric framework based on partial Euler bends for broadband power coupling between the first two transverse electric modes on a silicon-on-insulator platform. Our methodology uses a hybrid of eigenmode expansion and finite-difference time-domain methods. Our designs feature a mode (de)multiplexer, as well as unbalanced couplers (2/3 and 1/3 splitters). The simulated 1-dB bandwidths of our directional couplers range from $101$ nm to $134$ nm. The best-performing fabricated mode (de)multiplexer has a measured 1-dB bandwidth of $90$ nm, a conversion efficiency of $94.0\%$ and crosstalk below $-18$ dB at $1550$ nm. Our results validate the use of partial Euler bends for broadband transverse mode directional couplers and provide a general framework for their design.

physics.optics↗

arXiv2026

Observing relativistic trajectories of single photons

While the standard interpretation of quantum mechanics does not assign definite trajectories to particles, the Bohmian interpretation does. Only recently has an operational method for reconciling Bohmian mechanics with relativity been proposed. Here, we experimentally reconstruct relativistic Bohmian trajectories of a single photon in a Michelson-Sagnac interferometer, where counter-propagating probability amplitudes interfere head-on at the speed of light. As predicted by the relativistic Bohmian theory, we observe subluminal and superluminal features of the Bohmian trajectories of the photon traversing through the fringes. Our work provides experimental access to relativistic Bohmian mechanics and enables exploration of its unusual and counterintuitive properties.

quant-ph↗

arXiv2026

High-Dimensional Quantum Photonics: Roadmap

The field of high-dimensional quantum photonics involves the use of multimode photonic degrees-of-freedom such as the spatial, temporal, or spectral structure of light to encode multi-level quantum states. Recent years have seen rapid progress in the development of methods to generate, manipulate, and distribute such quantum states of light and their use in a range of quantum technology applications that offer practical advantages over conventional qubit-based approaches. High-dimensional quantum states of light encoded in photonic time-bins, frequency-bins, transverse-spatial modes, waveguide paths, and temporal modes have enabled noise-robust fundamental tests of quantum mechanics, error-resilient and high-capacity quantum communication protocols, andas well as efficient approaches for quantum information processing, to name just a few examples. However, research in this field has progressed fairly independently, with little exchange across different photonic degrees-of-freedom or between experiment and theory and no comprehensive comparison between degrees-of-freedom. This roadmap aims to bridge this gap by surveying progress in each area and identifying shared challenges and opportunities that cut across two or more photonic degrees-of-freedoms. We review early work and state-of-the-art experimental techniques under development for high-dimensional quantum states encoded in single and entangled photons, as well as theoretical tools for their measurement and certification. We outline the main outstanding challenges for theory and each experimental degree-of-freedom, identifying promising future directions of research that may enable these to be overcome. We end by discussing interconnections and shared challenges centered around their distribution, measurement, and manipulation, with a view towards their integration into next-generation quantum technology platforms and applications.

quant-ph↗

arXiv2026

Universal Quantum Computation via Superposed Orders of Single-Qubit Gates

Superposed orders of quantum channels have already been proved - both theoretically and experimentally - to enable unparalleled opportunities in the quantum communication domain. As a matter of fact, superposition of orders can be exploited within the quantum computing domain as well, by relaxing the (traditional) assumption underlying quantum computation about applying gates in a well-defined causal order. In this context, we address a fundamental question arising with quantum computing: whether superposed orders of single-qubit gates can enable universal quantum computation. As shown in this paper, the answer to this key question is a definitive "yes". Indeed, we prove that any two-qubit controlled quantum gate can be deterministically realized, including the so-called Barenco gate that alone enables universal quantum computation.

quant-ph↗

arXiv2025

Photonic Quantum Computing

Photonic quantum computation refers to quantum computation that uses photons as the physical system for doing the quantum computation. The field is largely divided between discrete-variable (DV) and continuous-variable (CV) photonic quantum computation. In the former, quantum information is represented by one or more modal properties (e.g. polarisation) that take on distinct values from a finite set. Quantum information is processed via operations on these modal properties (e.g. waveplates in the case of polarisation), and eventually measured using single-photon detectors. In CV photonic quantum computation, quantum information is represented by properties of the electromagnetic field that take on any value in an interval (e.g. position). Both CV and DV implementations have been realized experimentally; each has a unique set of challenges that need to be overcome to achieve scalable universal photonic quantum computation. It is possible to combine both DV and CV in a hybrid CV-DV fashion to overcome the limitations of either approach.

quant-ph↗

arXiv2025

Enhanced Photonic Chip Design via Interpretable Machine Learning Techniques

Photonic chip design has seen significant advancements with the adoption of inverse design methodologies, offering flexibility and efficiency in optimizing device performance. However, the black-box nature of the optimization approaches, such as those used in inverse design in order to minimize a loss function or maximize coupling efficiency, poses challenges in understanding the outputs. This challenge is prevalent in machine learning-based optimization methods, which can suffer from the same lack of transparency. To this end, interpretability techniques address the opacity of optimization models. In this work, we apply interpretability techniques from machine learning, with the aim of gaining understanding of inverse design optimization used in designing photonic components, specifically two-mode multiplexers. We base our methodology on the widespread interpretability technique known as local interpretable model-agnostic explanations, or LIME. As a result, LIME-informed insights point us to more effective initial conditions, directly improving device performance. This demonstrates that interpretability methods can do more than explain models -- they can actively guide and enhance the inverse-designed photonic components. Our results demonstrate the ability of interpretable techniques to reveal underlying patterns in the inverse design process, leading to the development of better-performing components.

physics.optics↗

arXiv2025

Near-unity quantum interference of transverse spatial modes in an ultra-compact inverse-designed photonic device

The transverse spatial mode of photons is an untapped resource for scaling up integrated photonic quantum computing. To be practically useful for improving scalability, reliable and high-visibility quantum interference between transverse spatial modes on-chip needs to be demonstrated. We show repeatable quantum interference using inverse-designed transverse mode beamsplitters that have an ultra-compact footprint of 3 $μm$ $\times$ 3 $μm$ -- the smallest transverse mode beamsplitters for 1550 nm photons to date. We measure a Hong-Ou-Mandel visibility of up to 99.56$\pm$0.64 % from a single device, with an average visibility across three identical devices of 99.38$\pm$0.41 %, indicating a high degree of reproducibility. Our work demonstrates that inverse-designed components are suitable for engineering quantum interference on-chip of multimode devices, paving the way for future compact integrated quantum photonic devices that exploit the transverse spatial mode of photons for high-dimensional quantum information.

quant-ph↗

arXiv2024

Self-guided tomography of time-frequency qudits

High-dimensional time-frequency encodings have the potential to significantly advance quantum information science; however, practical applications require precise knowledge of the encoded quantum states, which becomes increasingly challenging for larger Hilbert spaces. Self-guided tomography (SGT) has emerged as a practical and scalable technique for this purpose in the spatial domain. Here, we apply SGT to estimate time-frequency states using a multi-output quantum pulse gate. We achieve fidelities of more than 99\% for 3- and 5-dimensional states without the need for calibration or post-processing. We demonstrate the robustness of SGT against statistical and environmental noise, highlighting its efficacy in the photon-starved regime typical of quantum information applications.

quant-ph↗

arXiv2024

On chip high-dimensional entangled photon sources

High-dimensional quantum entanglement is an important resource for emerging quantum technologies such as quantum communication and quantum computation. The scalability of metres-long experimental setups limits high-dimensional entanglement in bulk optics. Advancements in quantum technology hinge on reproducible, and reconfigurable quantum devices -- including photon sources, which are challenging to achieve in a scalable manner using bulk optics. Advances in nanotechnology and CMOS-compatible integration techniques have enabled the generation of entangled photons on millimeter-scale chips, significantly enhancing scalability, stability, replicability, and miniaturization for real-world quantum applications. In recent years we have seen several chip-scale demonstrations with different degrees of freedom including path, frequency-bin, time-bin, and transverse modes, on many material platforms. A complete quantum photonic integrated circuit requires the generation, manipulation, and detection of qudits, involving various active and passive quantum photonic components which further increase the degree of complexity. Here, we review and introduce the nonlinear optical processes that facilitate on-chip high-dimensional entangled photon sources and the currently used material platforms. We discuss a range of current implementations of on-chip high-dimensional entangled photon sources and demonstrated applications. We comment on the current challenges due to the limitations of individual material platforms and present future opportunities in hybrid and heterogeneous integration strategies for the next generation of integrated quantum photonic chips.

quant-ph↗

arXiv2023

Randomness-free Test of Non-classicality: a Proof of Concept

Quantum correlations and non-projective measurements underlie a plethora of information-theoretic tasks, otherwise impossible in the classical world. Existing schemes to certify such non-classical resources in a device-independent manner require seed randomness, which is often costly and vulnerable to loopholes, for choosing the local measurements performed on different parts of a multipartite quantum system. In this letter, we propose and experimentally implement a semi-device independent certification technique for both quantum correlations and non-projective measurements without seed randomness. Our test is semi-device independent in the sense that it requires only prior knowledge of the dimensions of the parts. We experimentally show a novel quantum advantage in correlated coin tossing by producing specific correlated coins from pairs of photons entangled in their transverse spatial modes. We establish the advantage by showing that the correlated coin obtained from the entangled photons cannot be obtained from two 2-level classical correlated coins. The quantum advantage requires performing qubit trine positive operator-valued measures (POVMs) on each part of the entangled pair, thus also certifying such POVMs in a semi-device-independent manner. This proof of concept firmly establishes a new cost-effective certification technique for both generating non-classical shared randomness and implementing non-classical measurements, which will be important for future multi-party quantum communications.

quant-ph↗

arXiv2023

Approximate reconstructability of quantum states and noisy quantum secret sharing schemes

We introduce and analyse approximate quantum secret sharing in a formal cryptographic setting, wherein a dealer encodes and distributes a quantum secret to players such that authorized structures (sets of subsets of players) can approximately reconstruct the quantum secret and omnipotent adversarial agents controlling non-authorized subsets of players are approximately denied the quantum secret. In particular, viewing the map encoding the quantum secret to shares for players in an authorized structure as a quantum channel, we show that approximate reconstructability of the quantum secret by these players is possible if and only if the information leakage, given in terms of a certain entanglement-assisted capacity of the complementary quantum channel to the players outside the structure and the environment, is small.

quant-ph↗

arXiv2022

Efficient Quantum State Tracking in Noisy Environments

Quantum state tomography, which aims to find the best description of a quantum state -- the density matrix, is an essential building block in quantum computation and communication. Standard techniques for state tomography are incapable of tracking changing states and often perform poorly in the presence of environmental noise. Although there are different approaches to solve these problems theoretically, experimental demonstrations have so far been sparse. Our approach, matrix-exponentiated gradient tomography, is an online tomography method that allows for state tracking, updates the estimated density matrix dynamically from the very first measurements, is computationally efficient, and converges to a good estimate quickly even with noisy data. The algorithm is controlled via a single parameter, its learning rate, which determines the performance and can be tailored in simulations to the individual experiment. We present an experimental implementation of matrix-exponentiated gradient tomography on a qutrit system encoded in the transverse spatial mode of photons. We investigate the performance of our method on stationary and evolving states, as well as significant environmental noise, and find fidelities of around 95% in all cases.

quant-ph↗

arXiv2021

Roadmap on Integrated Quantum Photonics

Integrated photonics is at the heart of many classical technologies, from optical communications to biosensors, LIDAR, and data center fiber interconnects. There is strong evidence that these integrated technologies will play a key role in quantum systems as they grow from few-qubit prototypes to tens of thousands of qubits. The underlying laser and optical quantum technologies, with the required functionality and performance, can only be realized through the integration of these components onto quantum photonic integrated circuits (QPICs) with accompanying electronics. In the last decade, remarkable advances in quantum photonic integration and a dramatic reduction in optical losses have enabled benchtop experiments to be scaled down to prototype chips with improvements in efficiency, robustness, and key performance metrics. The reduction in size, weight, power, and improvement in stability that will be enabled by QPICs will play a key role in increasing the degree of complexity and scale in quantum demonstrations. In the next decade, with sustained research, development, and investment in the quantum photonic ecosystem (i.e. PIC-based platforms, devices and circuits, fabrication and integration processes, packaging, and testing and benchmarking), we will witness the transition from single- and few-function prototypes to the large-scale integration of multi-functional and reconfigurable QPICs that will define how information is processed, stored, transmitted, and utilized for quantum computing, communications, metrology, and sensing. This roadmap highlights the current progress in the field of integrated quantum photonics, future challenges, and advances in science and technology needed to meet these challenges.

quant-ph↗

arXiv2021

Robust and Efficient High-dimensional Quantum State Tomography

The exponential growth in Hilbert space with increasing size of a quantum system means that accurately characterising the system becomes significantly harder with system dimension d. We show that self-guided tomography is a practical, efficient, and robust technique of measuring higher-dimensional quantum states. The achieved fidelities are over 99.9% for qutrits (d=3) and ququints (d=5), and 99.1% for quvigints (d=20), the highest values ever realised for qudits. We demonstrate robustness against experimental sources of noise, both statistical and environmental. The technique is applicable to any higher-dimensional system, from a collection of qubits through to individual qudits, and any physical realisation, be it photonic, superconducting, ionic, or spin.

quant-ph↗

arXiv2019

Distribution of high-dimensional orbital angular momentum entanglement at telecom wavelength over 1km of optical fibre

High-dimensional entanglement has demonstrated potential for increasing channel capacity and resistance to noise in quantum information processing. However, its distribution is a challenging task, imposing a severe restriction on its application. Here we report the first distribution of three-dimensional orbital angular momentum (OAM) entanglement via a 1-km-long optical fibre. Using an actively-stabilizing phase pre-compensation technique, we successfully transport one photon of a three-dimensional OAM entangled photon pair through the fibre. The distributed OAM entangled state still shows a fidelity up to 71\% with respect to the three-dimensional maximal-entangled-state (MES). In addition, we certify that the high-dimensional quantum entanglement survives the transportation by violating a generalized Bell inequality, obtaining a violation of $\sim3$ standard deviations with $I_{3}=2.12\pm0.04$. The method we developed can be extended to higher OAM dimension and larger distances in principle. Our results make a significant step towards future OAM-based high-dimensional long-distance quantum communication.

quant-ph↗

arXiv2016

A new device-independent dimension witness and its experimental implementation

A dimension witness is a criterion that sets a lower bound on the dimension needed to reproduce the observed data. Three types of dimension witnesses can be found in the literature: device-dependent ones, in which the bound is obtained assuming some knowledge on the state and the measurements; device-independent prepare-and-measure ones, that can be applied to any system including classical ones; and device-independent Bell-based ones, that certify the minimal dimension of some entangled systems. Here we consider the Collins-Gisin-Linden-Massar-Popescu (CGLMP) Bell-type inequality for four outcomes. We show that a sufficiently high violation of this inequality witnesses $d \geq 4$ and present a proof-of-principle experimental observation of such a violation. This presents a first experimental violation of the third type of dimension witness beyond qutrits.

quant-ph↗

arXiv2016

Coherent absorption of N00N states

Recent results in deeply subwavelength thickness films demonstrated coherent control and logical gate operations with both classical and single photon light sources. However, quantum processing and devices typically involve more than one photon and non-trivial input quantum states. Here we experimentally investigate two-photon N00N state coherent absorption in a multilayer graphene film. Depending on the N00N state input phase, it is possible to selectively choose between single or two photon absorption of the input state in the graphene film. These results demonstrate that coherent absorption in the quantum regime exhibits unique features opening up applications in multiphoton spectroscopy and imaging.

physics.optics↗

arXiv2014

Photons that travel in free space slower than the speed of light

That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wavevectors resulting in a change to their phase and group velocities. We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam's transverse spatial structure. Using time-correlated photon pairs we show a reduction of the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several microns over a propagation distance of the order of 1 m. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.

physics.optics↗