Within the EU-funded consortium QuDice technologies and systems for space-based Entanglement distributions are developed, amongst them also a compact and space-suitable entangled photon source. The source’s architecture is based on a high-performant linear Mach-Zehnder Interferometer in a beam-displacer configuration, using a ppKTP nonlinear material for spontaneous-parametric down-conversion to the C-Band. The source will be tested by means of thermal vacuum and mechanical vibration cycling. Its performance on the laboratory level proved 800..900k counts/s/mW for Signal and Idler and 300k coincidences, an overall heralding efficiency of >33% and visibilities >98%. The form factor of the Engineering Qualification Model fits into a volume of ca. 100x80x50 mm3, excluding the pump laser and electronics for the nonlinear material temperature levelling.
Within the EU-funded consortium QuDice technologies and systems for space-based Entanglement distributions are developed, amongst them also a compact and space-suitable entangled photon source. The source’s architecture is based on a high-performant linear Mach-Zehnder Interferometer in a beam-displacer configuration, using a ppKTP nonlinear material for spontaneous-parametric down-conversion to the C-Band. The source will be tested by means of thermal vacuum and mechanical vibration cycling. Its performance on the laboratory level proved 800..900k counts/s/mW for Signal and Idler and 300k coincidences, an overall heralding efficiency of >33% and visibilities >98%. The form factor of the Engineering Qualification Model fits into a volume of ca. 100x80x50 mm3, excluding the pump laser and electronics for the nonlinear material temperature levelling.
Quantum Key Distribution (QKD) will be the first applications for quantum technologies, addressing information security needs by providing a tool for quantum encryption. An Entangled Photon Source EPS, the key building block for QKD, is presented that is based on a non-linear, Spontaneous Parametric Down Conversion SPDC process in a temperature controlled ppKTP crystal that is double-side pumped at 405 nm in a sagnac interferometer setup. The polarization entangled photons are emitted at 810 nm and at a rate of up to ca. 300k per second at 8 mW pumping power. Visibilities in the HV and the DA polarization base in the range of 97-99% have been measured. The EPS integration was realized in a ruggedized, minimized setup to prove space-suitability during thermal and mechanical environmental testing.
Our team has an on-going quantum satellite network project in Canada called Quantum Encryption and Science Satellite (QEYSSat). I would like to introduce our polarization-based entangled photon source suitable for the satellite QKD. The idea of satellite based QKD came up to overcome the technical limitation of using fibers. We are aiming for our quantum source to sufficiently overcome the current distance limits. In order to achieve, my quantum source will have to meet the following three criteria; high pair production rate, narrow bandwidth of wavelengths and high stability. We require the minimum pair production rate for the satellite communication to be above 100 MHz (meaning 100 million photon pairs per second). Meanwhile, most telecommunication through the ground these days uses optical fibers which transports signals with 1550nm or 1310nm wavelengths. We have specifically chosen 791nm signal wavelength which can have 1550nm idler wavelength as its pair. Another focus was bandwidths of the photon wavelengths. A broad photon wavelength will require a large bandwidth filter to be used to detect the signal. This will result in our filter allowing more photon backgrounds which will reduce the accuracy of the measurements. The condition may become even worse in the daylight operations. After having conducted an extensive study in how a crystal changes its shape from thermal expansions caused by the pumped beam, or interactions with its surroundings, we have chosen the periodically poled lithium niobate crystal (PPLN), and experimentally confirmed its performance in the lab. Efficiency tests for the candidate source will soon be run outside the laboratory during the daytime to verify whether it meets the requirement. An entangled photon source with high stability and robustness can be achieved by improving the photon interferometer that is necessary to make the correlated photon pairs. Beam splitters are one of the optical devices commonly used to make such interferometers. The disadvantage of beam splitters that it is difficult to separate the pair with that much of large separation in the wavelengths (which is the wavelength difference between 791nm and 1550nm). Our team has come up with an alternative interferometer design which uses two beam displacers instead of the beam splitters, and its performances have been verified. Ground-to-ground EB QKD has already been well established, and now we are about to make a next huge step by applying it to a satellite networking system.
To overcome the challenges of establishing a global Quantum Key Distribution network using optical fibers, several communication protocols using satellites have been proposed. A major milestone towards such a goal is the demonstration of a working source of entangled photons on a satellite. We are building a compact instrument capable of generating entangled photon pairs through spontaneous parametric down conversion. We intend to follow an iterative development cycle with the first mission to demonstrate that the control electronics can reliably operate an optical source of correlated photons in orbit. The instrument we are building is designed to be integrated into a nanosatellite and conforms to the size, weight and power requirements of a 1U CubeSat. The instrument contains a power stabilized optical pump, single photon detectors and inertial free polarization rotators to measure polarization correlations onboard the nanosatellite. The single photon detection unit utilizes a software based controlling mechanism to achieve temperature independent operation in a typical low earth orbit nanosatellite. The integrated system has been tested for thermal, vacuum and vibration environments. The latest test on a high altitude balloon demonstrates the technology readiness of the electronics payload for space missions.
Experimental realization of entanglement-based quantum key distribution (QKD) in a daylight-uplink (ground-to-satellite) communication channel is highly challenging due to the very low signal-to-noise ratio (SNR) of this configuration, and it has yet to be successfully implemented to date. While current research efforts focus on weak coherent pulse (WCP) sources for daylight QKD, we present a proof-of-principle laboratory demonstration emulating daylight-uplink QKD in a free-space-fiber hybrid configuration using polarization-entangled photon pairs generated via a spontaneous parametric down-conversion (SPDC) process. The emulated uplink channel attenuation reaches up to 50 dB, equivalent to low-earth-orbit (LEO) distances. Furthermore, the detected noise level (up to a few MHz) in the receiver telescope is representative of daylight conditions. The enhancement of SNR is achieved by implementing rigorous filtering on spatial, spectral, and temporal modes. Our ultra-bright source of entangled photons, characterized by a narrow spectral bandwidth of 0.54 nm FWHM, ensures minimal signal attenuation. Using a theoretical model, we estimate the conditional feasibility of entanglement-based QKD for daylight-uplink LEO satellite links up to approximately 400 km, subject to the experimentally motivated system assumptions adopted in this work, including efficient single-mode fiber (SMF) coupling performance.
Experimental realization of entanglement-based quantum key distribution (QKD) in a daylight uplink (ground-to-satellite) communication channel is highly challenging due to the very low signal-to-noise ratio (SNR) of this configuration and it has yet to be successfully implemented to date. While current research efforts focus on weak coherent pulse (WCP) sources for daylight QKD, we present a proof-of-principle demonstration of uplink-daylight QKD in a free-space-fiber hybrid configuration using polarization-entangled photon pairs generated via a spontaneous parametric down-conversion (SPDC) process. The simulated uplink channel attenuation reaches up to 50 dB, equivalent to low-earth-orbit (LEO) distances. Furthermore, the detected noise level (up to a few MHz) in the receiver telescope is representative of daylight conditions. The enhancement of SNR is achieved by implementing rigorous filtering on spatial, spectral, and temporal modes. Our ultra-bright source of entangled photons, characterized by a narrow spectral bandwidth of 0.54 nm FWHM, ensures minimal signal attenuation. We also propose a novel theoretical model of entanglement-based QKD link that fits our experimental data perfectly and is consistent with the prevailing models in the field. Our model considers a non-ideal entangled photon source and is applicable to both continuous wave (CW) and pulse-pumped sources. Using this model, we demonstrate that entanglement-based QKD is feasible under uplink daylight conditions in LEO satellites, but only up to a distance of 400 km, utilizing current state-of-the-art single-mode fiber (SMF) coupling technology.
Eagle-1 satellite which is currently under development, by a consortium of over 20 European (EU) companies, will be the first space-based EU Quantum Key Distribution (QKD) system providing symmetrical quantum keys via the implementation of Prepare & Measure (P&M)-QKD links and is expected to be launched in late 2025. In parallel to this Eagle-1 milestone for European satellite-QKD networks, the European space quantum engineering community should work in parallel on the development of space-QKD components offering in the short-term even higher Key Performance Indicators (KPIs) to meet the demand for high secure key rates, in the middle-term to introduce space-QKD components for system designs and architectures beyond the horizon of Eagle-1, and in the longer term to prepare for the future quantum space internet. LaiQa (Leap in Advancing of crItical Quantum key distribution-spAce components - http://www.laiqahorizon.eu/) project is a collective effort of 8 partners funded by Horizon Europe launched in January 2024, and it aims to come as a pragmatic innovation action addressing the satellite QKD landscape beyond the Eagle-1 milestone, bringinginnovations across different timescales for the EU roadmap of satellite quantum communications. LaiQa will bring to the space-QKD ecosystem 1550 nm space-qualified Decoy-State (DS)-BB84 source for polarization-encoded Prepare & Measure (P&M)-QKD sources in architectures based on trusted satellites. As illustrated in LaiQas architecture presented in Figure 1, the horizon of LaiQas development path goes beyond the standard P&M-based QKD components, since it will pursue the development of a space-deployable high-brightness 1550 nm entangled photon pair source (EPPS). Apart from the novel space implementations, LaiQa aims to accelerate the quantum readiness level of the ground terminals operated as trusted nodes by delivering advanced adaptive optics and fiber-coupling systems for 1550 nm photons, to fully exploit the available fiber-based terrestrial European Quantum Communication Infrastructure (Euro-QCI) segments. Moving towards satellite quantum information networks for applications beyond QKD, LaiQa will bring Photonic Integrated Circuit (PIC)-based implementations for generating photon-pairs which will be combined with novel quantum memory blocks enabling quantum repeater technology, and thus, long-distance entanglement distribution to quantum processing nodes. This concept paper describes the vision, architecture, and implementation aspects of the EU-funded LaiQa project. Preliminary KPIs of LaiQa quantum space technological blocks will be presented and preliminary feasibility analysis for the overall architecture will also be discussed. The demonstration aspects will be also discussed since LaiQa technologies are intended to be deployed over two advance test infrastructures including 2.5 km terrestrial Free Space Optical (FSO)-QKD links and real field deployments in Helmos Optical Ground Station (OGS) in Greece.