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.
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.
We plan to operate our current low brightness entangled photon source onboard a 3u CubeSat with dimensions of 30x10x10 cm3 and a total weight (including the payload, solar panels, on-board computer and attitude control systems) of less than 4 kg. The satellite will be launched from the International Space Station into a low-earth orbit where it will demonstrate the violation of Bell’s inequality. The payload itself consists of a critically phase-matched entangled photon source, polarization analysis elements and passively quenched single photon detectors. We use critical phase matching (in contrast to quasi phase matching) due to the orbital thermal fluctuations. Although quasi phase matching can in principle yield higher pair rates, guaranteeing the required thermal stability is not practical with the limited power budget and thermal isolation of small scale satellites. In critical phase matching on the other hand, the angle of the nonlinear crystals must be controlled with high precision. This can be achieved with relative ease by ensuring high thermomechanical stability or employing piezoelectric actuators. The source consists of two beta barium borate (BBO) crystals with parallel optical axes set for type-I phase matching. Within the crystals, the 405 nm pump beam spontaneously converts to two lower energy daughter photons at the non-degenerate wavelengths of 785 nm and 837 nm for signal and idler photons, respectively. The wavelengths are chosen to yield optimized detection efficiency when taking the atmospheric absorption losses and detector efficiencies into account. The polarization of the photon pairs lies in the horizontal plane (with respect to the optical unit) upon generation. In between the two crystals, a special half-wave plate rotates the polarization of pairs generated in the first crystal by 90 degrees. The phase difference between the pairs born in different crystals is compensated using an yttrium orthovanadate compensation crystal. After temporal compensation, the output state is one of the maximally entangled co-polarized Bell states. Finally, the photons are split based on their wavelength and their polarization state is analyzed by two liquid crystal polarization rotators. While experiments in the laboratory show that detected pair rates of more than 100 000 pairs per second per milliwatt of input power are achievable in this configuration, the current version of the payload will produce only a fraction of this. The main goal of the present mission is not to demonstrate a QKD-qualified entangled photon source, but to show that the crucial component, namely the phase stability of the entangled state can be maintained from alignment to in-orbit operation. The source is enclosed in a titanium housing and the crystals are placed on specifically designed flexure stages to ensure high thermomechanical stability. Pre-flight tests of the payload model show a violation of the Clauser-Horne-Shimony-Holt inequality of S = 2.67 (0.02) after vibrational and thermal testing with a detected rate of approximately 500 photon pairs per second for each polarization basis. The final step, operation of a QKD-qualified high brightness source, is straightforward and only requires loose focusing of the pump and the signal and idler waists. Here, we present detailed results ranging from the laboratory stage of the entangled photon source to the fully operational payload.
We report on our progress developing highly-miniaturised, polarisation-entangled, photon pair sources for CubeSats. We have a correlated photon-pair source in orbit in the NUS Galassia 2U CubeSat. We also have an entangled photon pair source in production and a high brightness (1Mcps) entangled photon-pair source in development for our upcoming satellite missions. All our sources are proof-of-principle demonstrations that the hardware necessary for entanglement-based QKD can be miniaturised and made sufficiently robust for operation in nano-satellites. The photon pairs they produce are measured with liquid crystal-based Bell state analysers and Geiger-mode avalanche photo-diodes within the source. These space missions allow our on-the-ground radiation, thermal and vibration tests to be validated and the real-world operation and aging of the source in space to be studied. A BBM92 QKD-capable design of the source has been used in a phase A study of a satellite-to-satellite QKD demonstration mission by the University of New South Wales, Canberra. This mission study uses two 6U CubeSats in LEO and aims to demonstrate QKD over separations of increasing distances as the two CubeSats drift apart.
During the last decade, quantum entanglement has paved the way out to of the lab modern applications such as quantum computation and communication. Today, small scale quantum networks exist already, but they are limited to a few 100 km distance, due to intrinsic fiber transmission losses and non perfect detectors. These networks are typically established using photon pair sources based on spontaneous parametric down conversion (SPDC). Widely used entanglement observables are time-bin and polarization, where the latter one is undoubtedly simpler to analyze due to lack of interferometric devices. It was shown recently, that introducing quantum memories (QM) between the communication parties could increase both communication speed and distance. Such memories already exist, but their narrow absorption bandwidth does not match the broadband emission spectrum of SPDC sources. Our report is twofold. First, we introduce a new polarization entanglement scheme based on a birefringent delay line (BDL). A pair of incoming photons is projected on a polarizing beam splitter (PBS), where vertically polarized photons travel along a polarization maintaining fiber loop accumulating a delay δ compared to horizontally polarized photons, which exit the BDL directly upon reflection at the PBS. Via post selection of only simultaneously arriving photon pair events, we create entangled states of the form α|H>_1|H>_2 + e^(iφ)β|V>_1 |V>_2. The probability amplitudes α and β are simply tuned by controlling the input pair's polarization and the phase φ can be varied by piezo fiber stretcher (PZT) in the delay line. Therefore, any superposition of the polarization entangled Bell states can be created. The scheme may also be used for entanglement distillation of non maximally entangled states, e.g. emitted by quantum dots. We will introduce all requirements needed to create entanglement from such a BDL. Second, we show how to apply this scheme to produce polarization entanglement from one of the most efficient photon pair generators, namely a periodically poled Lithium Niobate waveguide structure (PPLN/W), where the quasi phase matching is optimized for the type-0 SPDC interaction. The pair's wavelength is chosen in the standardized ITU-21 channel of the low loss telecom C-band and a fiber Bragg grating is applied to reduce the initial spectral bandwidth from 5 THz to 540 MHz, which is suitable for some solid-state quantum memories. Using the BDL, we create the maximally entangled states, with near perfect quality, which is proven by a violation of the Bell's inequalities by more than 29 standard deviations. We achieve a source spectral brightness of 211 narrowband polarization entangled pairs created per second, mW of pump power, and MHz of spectral bandwidth, coupled into a single mode fiber. This is on the same level as the best time-bin entanglement sources reported to date. The source is very practical due to the use of standard telecom fiber components. Moreover the choice for polarization instead of time-bin entanglement allows for simple state analysis using only rotating PBSs. This is a non negligible advantage, especially for narrowband photons with long coherence times, where time-bin analysis requires interferometer path length differences on the order of a few meters stabilized to a few tens of nanometers. Non unit visibilities are solely explained by the nonlinear character of our photon pair source, giving rise to double pair contributions. Applying this scheme to deterministic photon pair sources should result in perfect quality entanglement. We believe that our scheme should play an important role for future quantum networking applications.
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.
Quantum key distribution (QKD), the technology to generate distant secure keys for later usage in secure communication, has become a mature topic since the 1980s. While terrestrial QKD over fiber networks is suitable for short-distance urban networks, QKD via satellites promises to gap the distance requirement on a global scale that limits terrestrial QKD. Satellite QKD requires the exchange of quantum states between a satellite and multiple ground stations. The goal is then to facilitate shared secret keys between the ground stations, which can then be relayed to users near the ground stations. The satellite usually carries the quantum sources as its payload and sends them to the ground station via a downlink. The QKD protocol is heavily influenced by the choice of the quantum source, with typically one of two types: approximate single photon source (such as a weak laser) or an entangled source. Similarly, the ground station must be equipped to receive, measure and process the quantum signals sent by the satellite. Although there are also reverse schemes, where the satellite acts as the receiver and the ground station as the sender, these schemes are less explored due to higher noise in the uplink and less engineering experience. As with any system containing a high heterogeneity, many different implementation schemes lead to a variety of optimized use cases based on different criteria. Satellite QKD carries the further complication that it must be adapted to international standards. In this paper we present different satellite and ground station architectures that lean on the developments in the projects EAGLE-1, QUBE, QuNET, RoGloQuaN, addressing different protocols, sources, satellite sizes and telescope sizes. We also discuss the need for agility and configuration of components in a heterogeneous environment.
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.