arXiv subjects

Kaisa Laiho

Publications and source records attributed to Kaisa Laiho.

18 recordsLinked to original sources

arXiv2026

Validating the calibrated creation of heralded single photons

Coincidence-count discrimination is a practical and straightforward tool for the characterization of photon-pair processes and heralded single photons. Here, we implement a heralded single photon source based on parametric down-conversion in a periodically poled KTiOPO$_4$ waveguide in the telecom wavelength range involving a low number of optical modes. We extend the toolbox for the loss-tolerant state characterization by combining conventional figures-of-merit in order to access the heralded state's mean photon number and its photon-number parity. Our experiment demonstrates that an accurate determination of these characteristics is possible just through simple photon-correlation measurements. We believe that our results can find usage in the calibrated creation of heralded single photons and in determining the expectation values of observables that are crucial for attributing a single quantum.

quant-ph↗

arXiv2026

NovaMoon: A Strategic Lunar Reference Station for Positioning, Timing, and Largely Enhanced Science in the Earth-Moon System

The renewed interest in lunar exploration and the development of future lunar communication and navigation services highlight the need for a precise, stable, and interoperable geodetic and timing infrastructure on the Moon. NovaMoon, proposed as a scientific and navigation payload for ESA's Argonaut lander, is designed as a lunar-based local differential, geodetic, and timing station supporting both operational needs in the Moon's south polar region and a broad range of scientific investigations. The payload integrates a lunar laser retroreflector, a Very Long Baseline Interferometry transmitter, a receiver for navigation signals compatible with LunaNet standards, high-stability atomic clocks, and direct-to-Earth radio links -- making it the first lunar station to co-locate multiple ranging, tracking, and timing techniques. NovaMoon will enable sub-metre to decimetre positioning, provide local differential corrections for lunar users, and ensure an accurate and stable realisation of position and time. Preliminary simulation studies show that this multi-technique dataset improves the lunar reference frame, orientation and ephemerides, and estimates of interior parameters like tidal response and core properties. NovaMoon will also provide the first long-duration physical realisation of a lunar time reference. Beyond its primary goals, it supports improved cartography, precise surface geolocation, and higher-resolution topography, contributing to safer landings and operations. It also enables new tests of fundamental physics, including constraints on relativity and possible deviations from classical gravity.

astro-ph.EP↗

arXiv2025

Narrowband Frequency-Entangled Photon Source for Hong-Ou-Mandel Interferometry

Hong-Ou-Mandel (HOM) interferometry with entangled photons exhibits distinctive quantum features. By introducing frequency entanglement (discrete-color entangled states) into HOM interference, the characteristic HOM dip is modulated by sinusoidal fringes, which significantly enhance the sensitivity of HOM sensors. The frequency-entangled photon sources demonstrated to date rely on non-resonant parametric down-conversion (PDC), which limits the photon coherence length and, consequently, restricts the sensing dynamic range to the sub-millimeter scale. In this work, we demonstrate narrowband frequency-entangled photon source based on resonant PDC in a crystalline whispering gallery mode resonator. The MHz-level spectral bandwidth of photons enables a meter-scale dynamic range. With highly nondegenerate frequency-entangled photon pairs featuring a 96 THz frequency detuning, we observe high-contrast quantum beating with sub-picosecond resolution in the HOM experiment. Our WGMR-based frequency-entangled photon source has potential applications in quantum metrology and quantum information processing.

quant-ph↗

arXiv2025

Achieving precision in measuring birefringence characteristics of a periodically-poled Lithium Niobate waveguide

The refraction of light in an optical medium is not only a subject of fundamental interest, but the refractive index also plays a crucial role in applications involving integrated and non-linear optics. One such application is photon-pair generation in waveguides with second-order nonlinearity. The spectral properties of the generated photon pairs are governed by the effective group refractive indices of the interacting modes, which normally are calculated via Sellmeier's equations for bulk crystals. However, in integrated optics, the effective group refractive indices experienced by the propagating modes can differ from those in bulk materials. Therefore, we present an accurate, in-situ measurement technique for determining the birefringence characteristics of a structure with high reflective end facets by performing a Fourier transformation of the light transmission spectrum and apply this method to a periodically-poled Lithium Niobate waveguide resonator in the telecom wavelength range. We directly predict important spectral figures of merit of the photon-pair generation process, which depend on the optical path length difference that can be resolved with a high precision of more than 16 standard deviations.

physics.optics↗

arXiv2025

Advancing the heralded photon-number-state characterization by understanding the interplay of experimental settings

We theoretically explore the properties of heralded number states including up to three photons that are generated from single-mode twin beams. We investigate the effects of different parameters involved in the state preparation by using the fidelity, normalized second-order factorial moment of photon number for the heralded state $(g^{(2)}_h)$, and photon-number parity as figures of merit. Especially, the photon-number parity offers a practical and robust tool for inferring the target state quality by capturing the contamination of all undesired photon-number contributions. We focus on expressing our results in terms of experimentally easily accessible parameters such as the coincidences-to-accidentals ratio and the detection efficiencies. Our results identify the optimal parameter regions for generating high quality photon-number states by heralding and provide useful insights for advancing their use in quantum technologies.

quant-ph↗

arXiv2024

Indistinguishable MHz-narrow heralded photon pairs from a whispering gallery resonator

Hong-Ou-Mandel interference plays a vital role in many quantum optical applications where indistinguishability of two photons is important. Such photon pairs are commonly generated as the signal and idler in the frequency and polarization-degenerate spontaneous parametric down conversion~(SPDC). To scale this approach to a larger number of photons we demonstrate how two independent signal photons radiated into different spatial modes can be rendered conditionally indistinguishable by a heralding measurement performed on their respective idlers. We use the SPDC in a whispering gallery resonator, which is already proven to be versatile sources of quantum states. Its extreme conversion efficiency allowed us to perform our measurements with only \qty{50}{nW} of in-coupled pump power in each propagation direction. The Hong-Ou-Mandel interference of two counter-propagating signal photons manifested itself in the four-fold coincidence rate, where the two idler photons detection heralds a pair of signal photons with a desired temporal overlap. We achieved the Hong-Ou-Mandel dip contrast of $74\pm 5\%$. Importantly, the optical bandwidth of all involved photons is of the order of a MHz and is continuously tunable. This, on the one hand, makes it possible to achieve the necessary temporal measurements resolution with standard electronics, and on the other hand, creates a quantum states source compatible with other candidates for qubit implementation, such as optical transitions in solid-state or vaporous systems. We also discuss the possibility of generating photon pairs with similar temporal modes from two different whispering gallery resonators.

quant-ph↗

arXiv2023

Polarization-entangled photons from a whispering gallery resonator

Crystalline Whispering Gallery Mode Resonators (WGMRs) have been shown to facilitate versatile sources of quantum states that can efficiently interact with atomic systems. These features make WGMRs an efficient platform for quantum information processing. Here, we experimentally show that it is possible to generate polarization entanglement from WGMRs by using an interferometric scheme. Our scheme gives us the flexibility to control the phase of the generated entangled state by changing the relative phase of the interferometer. The S value of the Clauser-Horne-Shimony-Holt's inequality in the system is $2.45 \pm 0.07$, which violates the inequality by more than 6 standard deviations.

physics.optics↗

arXiv2020

Understanding photoluminescence in semiconductor Bragg-reflection waveguides: Towards an integrated, GHz-rate telecom photon pair source

Compared to traditional nonlinear optical crystals, like BaB$_2$O$_4$, KTiOPO$_4$ or LiNbO$_3$, semiconductor integrated sources of photon pairs may operate at pump wavelengths much closer to the bandgap of the materials. This is also the case for Bragg-reflection waveguides (BRW) targeting parametric down-conversion (PDC) to the telecom C-band. The large nonlinear coefficient of the AlGaAs alloy and the strong confinement of the light enable extremely bright integrated photon pair sources. However, under certain circumstances, a significant amount of detrimental broadband photoluminescence has been observed in BRWs. We show that this is mainly a result of linear absorption near the core and subsequent radiative recombination of electron-hole pairs at deep impurity levels in the semiconductor. For PDC with BRWs, we conclude that devices operating near the long wavelength end of the S-band or the short C-band require temporal filtering shorter than 1 ns. We predict that shifting the operating wavelengths to the L-band and making small adjustments in the material composition will reduce the amount of photoluminescence to negligible values. Such measures enable us to increase the average pump power and/or the repetition rate, which makes integrated photon pair sources with on-chip multi-gigahertz pair rates feasible.

quant-ph↗

arXiv2019

Thresholdless transition to coherent emission at telecom wavelengths from coaxial nanolasers

The ongoing miniaturization of semiconductor lasers has enabled ultra-low threshold devices and even provided a path to approach thresholdless lasing with linear input-output characteristics. Such nanoscale lasers have initiated a discourse on the origin of the physical mechanisms involved and their boundaries, such as the required photon number, the importance of optimized light confinement in a resonator and mode-density enhancement. Here, we investigate high-$β$ metal-clad coaxial nanolasers, which facilitate thresholdless lasing. We experimentally and theoretically investigate both the conventional lasing characteristics, as well as the photon statistics of the emitted light. While the former lacks adequate information to determine the threshold to coherent radiation, the latter reveals a finite threshold pump power. Our work clearly highlights an important and often misunderstood aspect of high-$β$ lasers, namely that a thresholdless laser does have a finite threshold pump power and must not be confused with a hypothetical zero threshold laser.

physics.app-ph↗

arXiv2018

High-concurrence time-bin entangled photon pairs from optimized Bragg-reflection waveguides

Semiconductor Bragg-reflection waveguides are well-established sources of correlated photon pairs as well as promising candidates for building up integrated quantum optics devices. Here, we use such a source with optimized non-linearity for preparing time-bin entangled photons in the telecommunication wavelength range. By taking advantage of pulsed state preparation and efficient free-running single-photon detection, we drive our source at low pump powers, which results in a strong photon-pair correlation. The tomographic reconstruction of the state's density matrix reveals that our source exhibits a high degree of entanglement. We extract a concurrence of $88.9\pm 1.8\%$ and a fidelity of $94.2 \pm 0.9\%$ with respect to a Bell state.

quant-ph↗

arXiv2014

Time-frequency quantum process tomography of parametric down-conversion

Parametric down-conversion (PDC) is the established standard for the practical generation of a multiplicity of quantum optical states. These include two-mode squeezed vacuum, heralded non-Gaussian states and entangled photon pairs. Detailed theoretical studies provide insight into the time-frequency (TF) structure of PDC, which are governed by the complex-valued joint spectral amplitude (JSA) function. However in experiments, the TF structure of PDC is mostly characterised by intensity measurementsthat forbid access to the important phase of the JSA. In this paper, we present an amplitude-sensitive quantum process tomography technique that combines methods from ultrafast optics and classical three-wave mixing. Our approach facilitates a direct and phase-sensitive time-frequency tomography of PDC with high spectral resolution and excellent signal-to-noise ratio. This is important for all quantum optical applications, which rely on engineered parametric processes and base on minute knowledge of the quantum wave-function for the generation of tailored photonic quantum states.

quant-ph↗

arXiv2013

Spatio-spectral characteristics of parametric down-conversion in waveguide arrays

High dimensional quantum states are of fundamental interest for quantum information processing. They give access to large Hilbert spaces and, in turn, enable the encoding of quantum information on multiple modes. One method to create such quantum states is parametric down-conversion (PDC) in waveguide arrays (WGAs) which allows for the creation of highly entangled photon-pairs in controlled, easily accessible spatial modes, with unique spectral properties. In this paper we examine both theoretically and experimentally the PDC process in a lithium niobate WGA. We measure the spatial and spectral properties of the emitted photon-pairs, revealing strong correlations between spectral and spatial degrees of freedom of the created photons. Our measurements show that, in contrast to prior theoretical approaches, spectrally dependent coupling effects have to be taken into account in the theory of PDC in WGAs. To interpret the results, we developed a theoretical model specifically taking into account spectrally dependent coupling effects, which further enables us to explore the capabilities and limitations for engineering the spatial correlations of the generated quantum states.

quant-ph↗

arXiv2012

Characteristics of displaced single photons attained via higher order factorial moments

We investigate up to the fourth order normalized factorial moments of free-propagating and pulsed single photons displaced in phase space in a phase-averaged manner. Due to their loss independence, these moments offer expedient methods for quantum optical state characterization. We examine quantum features of the prepared displaced states, retrieve information on their photon-number content and study the reliability of the state reconstruction method used.

quant-ph↗

arXiv2012

A 2D Quantum Walk Simulation of Two-Particle Dynamics

Multi-dimensional quantum walks can exhibit highly non-trivial topological structure, providing a powerful tool for simulating quantum information and transport systems. We present a flexible implementation of a 2D optical quantum walk on a lattice, demonstrating a scalable quantum walk on a non-trivial graph structure. We realized a coherent quantum walk over 12 steps and 169 positions using an optical fiber network. With our broad spectrum of quantum coins we were able to simulate the creation of entanglement in bipartite systems with conditioned interactions. Introducing dynamic control allowed for the investigation of effects such as strong non-linearities or two-particle scattering. Our results illustrate the potential of quantum walks as a route for simulating and understanding complex quantum systems.

quant-ph↗

arXiv2011

Probing multimode squeezing with correlation functions

Broadband multimode squeezers constitute a powerful quantum resource with promising potential for different applications in quantum information technologies such as information coding in quantum communication networks or quantum simulations in higher dimensional systems. However, the characterization of a large array of squeezers that coexist in a single spatial mode is challenging. In this paper we address this problem and propose a straightforward method to determine the number of squeezers and their respective squeezing strengths by using broadband multimode correlation function measurements. These measurements employ the large detection windows of state of the art avalanche photodiodes to simultaneously probe the full Hilbert space of the generated state, which enables us to benchmark the squeezed states. Moreover, due to the structure of correlation functions, our measurements are not affected by losses. This is a significant advantage, since detectors with low efficiencies are sufficient. Our approach is less costly than full state tomography methods relying on multimode homodyne detection which build on much more demanding measurement and analysis tools and appear to be impractical for large Hilbert spaces.

quant-ph↗

arXiv2010

Probing the negative Wigner function of a pulsed single photon point by point

We investigate quantum properties of pulsed light fields point by point in phase space. We probe the negative region of the Wigner function of a single photon generated by the means of waveguided parametric down-conversion. This capability is achieved by employing loss-tolerant photon-number resolving detection, allowing us to directly observe the oscillations of the photon statistics in dependence of applied displacements in phase space. Our scheme is highly mode sensitive and can reveal the single-mode character of the signal state.

quant-ph↗

arXiv2010

Accessing the purity of a single photon by the width of the Hong-Ou-Mandel interference

We demonstrate a method to determine the spectral purity of single photons. The technique is based on the Hong-Ou-Mandel (HOM) interference between a single photon state and a suitably prepared coherent field. We show that the temporal width of the HOM dip is not only related to reciprocal of the spectral width but also to the underlying quantum coherence. Therefore, by measuring the width of both the HOM dip and the spectrum one can directly quantify the degree of spectral purity. The distinct advantage of our proposal is that it obviates the need for perfect mode matching, since it does not rely on the visibility of the interference. Our method is particularly useful for characterizing the purity of heralded single photon states.

quant-ph↗

arXiv2009

Spatial modes in waveguided parametric down-conversion

The propagation of several spatial modes has a significant impact on the structure of the emission from parametric down-conversion in a nonlinear waveguide. This manifests itself not only in the spatial correlations of the photon pairs but also, due to new phase-matching conditions, in the output spectrum, radically altering the degree of entanglement within each pair. Here we investigate both theoretically and experimentally the results of higher-order spatial-mode propagation in nonlinear waveguides. We derive conditions for the creation of pairs in these modes and present observations of higher-order mode propagation in both the spatial and spectral domains. Furthermore, we observe correlations between the different degrees of freedom and finally we discuss strategies for mitigating any detrimental effects and optimizing pair production in the fundamental mode.

quant-ph↗