arXiv subjects

M. P. van Exter

Publications and source records attributed to M. P. van Exter.

At least 19 recordsLinked to original sources

arXiv2025

Markov chain Monte Carlo Detector Tomography applied to a NbTiN nanobridge

We demonstrate the use of a flexible and highly accurate Markov chain Monte Carlo Quantum Detector Tomography method as a minimization algorithm to best describe the response of an efficient $120 nm$ wide NbTiN superconducting nanobridge single photon detector. Separation of the internal quantum efficiency and external quantum efficiency is possible due to the difference in saturation behavior of an ideal 1-photon threshold detector as compared to a detector with non-unity 1-photon internal quantum efficiency. From a statistical analysis of our measurements (at $T=4.23\pm0.01K$, $I=29.4\pm0.1μA$, $I/I_\text{switch}=0.90\pm0.01$) we find an external quantum efficiency $η=(1.60\pm0.05)\cdot10^{-6}$, a 1-photon internal quantum efficiency $p_1=0.568\pm0.008$ and unity multi-photon (2 or more) internal quantum efficiency.

quant-ph↗

arXiv2024

Observation of the Talbot effect from a surface acoustic wave dynamic grating

We demonstrate the dynamical Talbot effect caused by optical diffraction from standing surface acoustic waves (SAWs). The Talbot effect is a wave interference phenomenon in the Fresnel regime, and we observe it with a fiber-based scanning optical interferometer on a SAW Fabry-Perot cavity. By studying the interferometric signal at 1 GHz, we first discover the existence of an amplitude-modulated term, that can exceed in magnitude the usual phase-modulated term, enabling a new way of imaging surface acoustic waves. Secondly, by displacing the acoustic device from the beam focus we reveal the optical Talbot effect, where despite the curved wavefronts of the optical field, the conventional Talbot length appears. As a consequence, the amplitude modulation vanishes at periodic positions of the acoustic wave relative to the beam focus.

physics.optics↗

arXiv2022

Fabry-Perot microcavity spectra have a fine structure

Optical cavities can support many transverse and longitudinal modes. A paraxial scalar theory predicts that the resonance frequencies of these modes cluster in different orders. A non-paraxial vector theory predicts that the frequency degeneracy within these clusters is lifted, such that each order acquires a spectral fine structure, comparable to the fine structure observed in atomic spectra. In this paper, we calculate this fine structure for microcavities and show how it originates from various non-paraxial effects and is co-determined by mirror aberrations. The presented theory, which applies perturbation theory to Maxwell's equations with boundary conditions, proves to be very powerful. It generalizes the effective 1-dimensional description of Fabry-Perot cavities to a 3-dimensional multi-transverse-mode description. It thereby provides new physical insights in several mode-shaping effects and a detailed prediction of the fine structure in Fabry-Perot spectra.

physics.optics↗

arXiv2022

Observation of microcavity fine structure

We observe fine structure in the resonance spectra of optical microcavities. We identify the polarization-resolved modes in the spectrum and find that resonance frequencies split in accordance with the theoretical prediction. The observed fine structure is dominantly caused by an optical spin-orbit coupling and non-paraxial propagation and reflection. Both effects are intrinsic, i.e. present in an ideal rotation-symmetric system, and scale inversely proportional to the mirror radius of curvature. For cavities with a small radius of curvature, the influence of fine structure on the resonance spectrum is important and unavoidable and should thus be taken into account.

physics.optics↗

arXiv2021

Observation of mode-mixing in the eigenmodes of an optical microcavity

We present a method to determine the complex coupling parameter of a two-coupled-modes system by directly measuring the coupled eigenmodes rather than their eigenvalues. This method is useful because mode-mixing can be observed even if frequency shifts can not be measured. It also allows to determine the complex coupling parameter, from which we conclude that the observed coupling is mainly conservative. We observe mode-mixing in an optical microcavity, where the modes couple primarily at the mirror surface, as confirmed by AFM measurements. The presented method is general and can be applied to other systems to measure mode coupling more accurately and to determine the nature of the coupling.

physics.optics↗

arXiv2021

Microcavity resonance condition, quality factor, and mode volume are determined by different penetration depths

The penetration depth in a Distributed Bragg Reflector (DBR) co-determines the resonance condition, quality factor, and mode volume of DBR-based microcavities. Recent studies have used an incomplete description of the penetration depth and incorrect equations. We present a complete analysis that involves three different penetration depths. We also present a series of experiments on microcavities to accurately determine the frequency and modal penetration depth of our DBRs and compare these results with theoretical predictions. The obtained results are relevant for anyone who models a DBR as an effective hard mirror if lengths of the order of the wavelength are relevant, as is the case for microcavities.

physics.optics↗

arXiv2020

Extended polarized semiclassical model for quantum-dot cavity QED and its application to single-photon sources

We present a simple extension of the semi-classical model for a two-level system in a cavity, in order to incorporate multiple polarized transitions, such as those appearing in neutral and charged quantum dots (QDs), and two nondegenerate linearly polarized cavity modes. We verify the model by exact quantum master equation calculations, and experimentally using a neutral QD in a polarization non-degenerate micro-cavity, in both cases we observe excellent agreement. Finally, the usefulness of this approach is demonstrated by optimizing a single-photon source based on polarization postselection, where we find an increase in the brightness for optimal polarization conditions as predicted by the model.

physics.optics↗

arXiv2018

Single photons and unconventional photon blockade in quantum dot cavity-QED

We observe the unconventional photon blockade effect in quantum dot cavity QED, which, in contrast to conventional photon blockade, operates in the weak coupling regime. A single quantum dot transition is simultaneously coupled to two orthogonally polarized optical cavity modes, and by careful tuning of the input and output state of polarization, the unconventional photon blockade effect is observed. We find a minimum second-order correlation $g^{(2)}(0)\approx0.37$ which corresponds to $g^{(2)}(0)\approx0.005$ when corrected for detector jitter, and observe the expected polarization dependency and photon bunching and anti-bunching very close-by in parameter space, which indicates the abrupt change from phase to amplitude squeezing.

quant-ph↗

arXiv2017

A fiber coupled cavity QED source of identical single photons

A high-fidelity source of identical single photons is essential for numerous quantum technologies such as quantum repeaters and optical quantum information processing. Hallmarks thereof are a near-unity single-photon purity, near-unity indistinguishability of consecutively emitted photons, and high brightness through a near-unity number of photons per time bin. In order to embed such sources in quantum networks, optical fiber integration is essential but complicated by cryogenic compatability and noise. Here we demonstrate a resonantly pumped, quantum dot (QD) based, transmission operated, single-mode fiber coupled single photon source with a purity of 97%, indistinguishability of 90%, and a brightness of 17%. This is achieved by deploying a unique micropillar cavity design in a closed-cycle cryostat, which is operated using a through-fiber cross-polarization technique to remove the pump laser light from the resonantly scattered single photons. These results pave the way for fully fiber integrated photonic quantum networks, as our technology is equally applicable for cavity-QED based photonic quantum gates.

quant-ph↗

arXiv2016

Probing the Hotspot Interaction Length in NbN Nanowire Superconducting Single-Photon Detectors

We measure the maximal distance at which two absorbed photons can jointly trigger a detection event in NbN nanowire superconducting single photon detector (SSPD) microbridges by comparing the one-photon and two-photon efficiency of bridges of different overall lengths, from 0 to 400 nm. We find a length of $23 \pm 2$ nm. This value is in good agreement with to size of the quasiparticle cloud at the time of the detection event.

cond-mat.supr-con↗

arXiv2016

Purification of a single photon nonlinearity

We show that the lifetime-reduced fidelity of a semiconductor quantum dot-cavity single photon nonlinearity can be restored by polarization pre- and postselection. This is realized with a polarization degenerate microcavity in the weak coupling regime, where an output polarizer enables quantum interference of the two orthogonally polarized transmission amplitudes. This allows us to transform incident coherent light into a stream of strongly correlated photons with a second-order correlation function of g2(0)~40, larger than previous experimental results even in the strong-coupling regime. This purification technique might also be useful to improve the fidelity of quantum dot based logic gates.

quant-ph↗

arXiv2014

The Role of Spatial Coherence and Orbital Angular Momentum of Light in Astronomy

The orbital angular momentum (OAM) of light is potentially interesting for astronomical study of rotating objects such as black holes, but the effect of reduced spatial coherence of astronomical light sources such as stars is largely unknown. In a lab-scale experiment, we find that the detected OAM spectrum depends strongly on the position of the light-twisting object along the line of sight. We develop a simple intuitive model to predict the influence of reduced spatial coherence in astronomical observations, and discuss line-of-sight and intensity issues.

physics.optics↗

arXiv2014

The effect of magnetic field on the intrinsic detection efficiency of superconducting single-photon detectors

We experimentally investigate the effect of a magnetic field on photon detection in superconducting single-photon detectors. At low fields, the effect of a magnetic field is through the direct modification of the quasiparticle density of states of the superconductor, and magnetic field and bias current are interchangable, as is expected for homogeneous dirty-limit superconductors. At the field where a first vortex enters the detector, the effect of the magnetic field is reduced, up until the point where the critical current of the detector starts to be determined by flux flow. From this field on, increasing the magnetic field does not alter the detection of photons anymore, whereas it does still change the rate of dark counts. This result points at an intrinsic difference in dark and light counts, and also shows that no enhancement of the intrinsic detection efficiency of a straight SSPD wire is achievable in a magnetic field.

cond-mat.supr-con↗

arXiv2011

Complete experimental characterization of a superconducting multiphoton nanodetector

We present a complete method to characterize multiphoton detectors with a small overall detection efficiency. We do this by separating the nonlinear action of the multiphoton detection event from linear losses in the detector. Such a characterization is a necessary step for quantum information protocols with single and multiphoton detectors and can provide quantitative information to understand the underlying physics of a given detector. This characterization is applied to a superconducting multiphoton nanodetector, consisting of an NbN nanowire with a bowtie-shaped subwavelength constriction. Depending on the bias current, this detector has regimes with single and multiphoton sensitivity. We present the first full experimental characterization of such a detector.

quant-ph↗

arXiv2009

Two-photon speckle as a probe of multi-dimensional entanglement

We calculate the statistical distribution P_2(I_2) of the speckle pattern produced by a photon pair current I_2 transmitted through a random medium, and compare with the single-photon speckle distribution P_1(I_1). We show that the purity Tr rho^2 of a two-photon density matrix rho can be directly extracted from the first two moments of P_1 and P_2. A one-to-one relationship is derived between P_1 and P_2 if the photon pair is in an M-dimensional entangled pure state. For M>>1 the single-photon speckle disappears, while the two-photon speckle acquires an exponential distribution. The exponential distribution transforms into a Gaussian if the quantum entanglement is degraded to a classical correlation of M>>1 two-photon states. Two-photon speckle can therefore discriminate between multi-dimensional quantum and classical correlations.

quant-ph↗

arXiv2008

Shannon dimensionality of quantum channels and its application to photon entanglement

We introduce the concept of Shannon dimensionality D as a new way to quantify bipartite entanglement as measured in an experiment. This is applied to orbital-angular-momentum entanglement of two photons, using two state analyzers composed of a rotatable angular-sector phase plate that is lens-coupled to a single-mode fiber. We can deduce the value of D directly from the observed two-photon coincidence fringe. In our experiment, D varies between 2 and 6, depending on the experimental conditions. We predict how the Shannon dimensionality evolves when the number of angular sectors imprinted in the phase plate is increased and anticipate that D = 50 is experimentally within reach.

quant-ph↗

arXiv2007

Observation of Goos-Hänchen shifts in metallic reflection

We report the first observation of the Goos-H$\rm \ddot{\textbf{a}}$nchen shift of a light beam incident on a metal surface. This phenomenon is particularly interesting because the Goos-H$\rm \ddot{\textbf{a}}$nchen shift for $p$ polarized light in metals is negative and much bigger than the positive shift for $s$ polarized light. The experimental result for the measured shifts as a function of the angle of incidence is in excellent agreement with theoretical predictions. In an energy-flux interpretation, our measurement shows the existence of a backward energy flow at the bare metal surface when this is excited by a $p$ polarized beam of light.

physics.optics↗

arXiv2004

Huygens description of resonance phenomena in subwavelength hole arrays

We develop a point-scattering approach to the plane-wave optical transmission of subwavelength metal hole arrays. We present a real space description instead of the more conventional reciprocal space description; this naturally produces interfering resonant features in the transmission spectra and makes explicit the tensorial properties of the transmission matrix. We give transmission spectra simulations for both square and hexagonal arrays; these can be evaluated at arbitrary angles and polarizations.

physics.optics↗