arXiv subjects

Erik Fitzke

Publications and source records attributed to Erik Fitzke.

6 recordsLinked to original sources

arXiv2024

Frequency-Resolved Simulations of Highly Entangled Biphoton States: Beyond the Single-Pair Approximation. I. Theory

We discuss an expansion of the detection probabilities of biphoton states in terms of increasing orders of the joint spectral amplitude. The expansion enables efficient time- or frequency-resolved numerical simulations involving quantum states exhibiting a high degree of spectral entanglement. Contrary to usual approaches based on one- or two-pair approximations, we expand the expressions in terms corresponding to the amount of correlations between different pairs. The lowest expansion order corresponds to the limit of infinitely entangled states, where different pairs are completely uncorrelated and the full multi-pair statistics are inferred from a single pair. We show that even this limiting case always yields more accurate results than the single-pair approximation. Higher expansion orders describe deviations from the infinitely entangled case and introduce correlations between the photons of different pairs.

quant-ph↗

arXiv2024

Frequency-Resolved Simulations of Highly Entangled Biphoton States: Beyond the Single-Pair Approximation. II. Application to Entanglement-based Quantum Key Distribution

We present time- and frequency-resolved simulations of quantum key distribution~(QKD) systems employing highly entangled biphoton quantum states. Our simulations are based on expansions of the covariance matrix and photon detection probabilities of biphoton states in terms of increasing orders of the joint spectral amplitude that were introduced in the first part of this series. Employing these expansions allows us to efficiently evaluate the impact of multi-pair events on the performance of the QKD systems while systematically taking into account effects from the photon spectra and many relevant imperfections of the setup. The results are shown to be in agreement with corresponding measurements of the key rates and quantum bit error rates.

quant-ph↗

arXiv2023

A flexible modular all-fiber based photon pair source for quantum key distribution in a network

Entanglement-based QKD protocols require robust and stable photon pair sources in terms of high heralding efficiencies or photon pair generation rates even under harsh environmental conditions, e.g. when operated in the field. In this paper, we report on a flexible, tunable, alignment-free, all-fiber coupled photon source based on spontaneous parametric down-conversion in periodically poled crystals. It can be operated in continuous-wave and pulsed modes, respectively. Its rack-compatible and modular setup allows a straight forward plug-and-play integration of coding-modules e.g. interferometers to enable various QKD protocols such as phase or phase-time coding. We demonstrate operation as a type-II and a type-0 SPDC stage proving the high flexibility of our source. Furthermore, we demonstrate simultaneous operation of SHG and SPDC in a double-pass configuration within the same nonlinear crystal further simplifying the hardware requirements of our source. To evaluate the conversion efficiencies of our modules, we employ data post-processing to remove artefacts from detector afterpulsing and deadtimes of the detectors. We investigate the source performance for various repetition rates.

quant-ph↗

arXiv2023

Reducing the number of single-photon detectors in quantum key distribution networks by time multiplexing

We demonstrate a method to reduce the number of single-photon detectors (SPDs) required in multi-party quantum key distribution (QKD) networks by a factor of two by using detector time multiplexing (DTM). We implement the DTM scheme for an entanglement-based time-bin protocol and compare QKD results with and without DTM in our QKD network with four users. When small efficiency losses are acceptable, DTM enables cost-effective, scalable implementations of multi-user QKD networks.

quant-ph↗

arXiv2023

Simulating the Photon Statistics of Multimode Gaussian States by Automatic Differentiation of Generating Functions

Advances in photonics require photon-number resolved simulations of quantum optical experiments with Gaussian states. We demonstrate a simple and versatile method to simulate the photon statistics of general multimode Gaussian states. The derived generating functions enable simulations of the photon number distribution, cumulative probabilities, moments, and factorial moments of the photon statistics of Gaussian states as well as of multimode photon-added and photon-subtracted Gaussian states. Numerical results are obtained by automatic differentiation of these generating functions by employing the software framework PyTorch. Our approach is particularly well suited for practical simulations of the photon statistics of quantum optical experiments in realistic scenarios with low photon numbers, in which various sources of imperfections have to be taken into account. As an example, we calculate the detection probabilities for a recent multipartite time-bin coding quantum key distribution setup and compare them with the corresponding experimental values.

quant-ph↗

arXiv2022

A scalable network for simultaneous pairwise quantum key distribution via entanglement-based time-bin coding

We present a scalable star-shaped quantum key distribution (QKD) optical fiber network. We use wavelength-division demultiplexing (WDM) of broadband photon pairs to establish key exchange between multiple pairs of participants simultaneously. Our QKD system is the first entanglement-based network of four participants using BBM92 time-bin coding and the first network achieving timing synchronization solely by clock recovery based on the photon arrival times. We demonstrate simultaneous bipartite key exchange between any possible combination of participants and show that the quantum bit error rate (QBER) itself can be used to stabilize the phase in the interferometers by small temperature adjustments. The key distribution is insensitive to polarization fluctuations in the network, enabling key distribution using deployed fibers even under challenging environmental conditions. We show that our network can be readily extended to 34 participants by using a standard arrayed-waveguide grating for WDM with 100 GHz channel spacing and that reconfigurable network connections are possible with a wavelength-selective switch. In a field test we demonstrate secure key rates of 6.3 bit/s with a QBER of 4.5% over a total fiber length of 108 km with 26.8 km of deployed fiber between two participants with high stability. Our system features a relatively simple design of the receiver modules and enables scaling QKD networks without a trusted nodes to distances up to more than 100 km and to more than 100 users. With such a network, a secure communication infrastructure on a metropolitan scale can be established.

quant-ph↗