Minimize SpooQy-1

SpooQy-1 CubeSat Mission with QKD (Quantum Key Distribution)

Launch    Mission Status     Payload    Ground Segment    References

The CQT (Center for Quantum Technologies) at NUS (National University of Singapore) is developing the SPEQS (Small Photon Entangling Quantum System) experiment, a greatly miniaturized device, based on the QKD (Quantum Key Distribution) technology, which conforms to the CubeSat standard. Plans call for a demonstration flight on a 3U CubeSat, referred to as SpooQy-1 (also spelled as SpooQySat). 1) 2) 3) 4) 5)

The main objective of SpooQySat, the SpooQy-1 CubeSat, is to demonstrate in-orbit a space-compatible quantum light source to increase the Technology Readiness Level (TRL) of future global Quantum key distribution (QKD) networks. QKD is a family of techniques used to generate private random encryption keys and share them between only two parties. Basic features of quantum mechanics are exploited in order to measure the privacy of the key during the sharing process with the effect that the absence of an eavesdropper can be verified, and keys that have potentially been intercepted can be discarded. Once a key is distributed securely, it can be used as a symmetric key to encrypt and decrypt messages. These messages can be shared publicly as they are provably secure against future hacks from advanced computers, provided the keys were based on genuinely random processes and provided they remain private. QKD schemes based on quantum entanglement can make strong guarantees with respect to such privacy and randomness.

Essentially, QKD requires the exchange of individual photons and so very low-loss optical links need to be established. Optical fibers are limited to about 100 km before losses become overwhelming and free-space optics in atmosphere have similar limitations. Performing QKD in space is attractive because atmospheric losses are much less at altitudes higher than 20 km above the Earth’s surface, enabling much longer distance optical links. Space-based QKD has been studied by various groups around the world. 6) It is only recently that significant progress has been demonstrated in space. In 2017, the 630 kg Chinese Micius satellite successfully demonstrated quantum key distribution experiments between itself and ground observatories. 7)

Our approach at CQT is to develop similar, but highly-miniaturized, QKD technologies for CubeSats. Our current focus is a quantum light source called the Small Photon Entangling Quantum System (SPEQS). It is designed with the Size, Weight and Power (SWaP) constraints of a CubeSat platform in mind, providing a fast and cost-effective way to iterate the technology to maturity. The end goal is to produce a SPEQS device that would be sufficiently powerful to enable a QKD link between satellites, or between a satellite and an optical ground station, when it is paired with the relevant free-space link technologies.

The first device in this series is SPEQS-CS, which is a device for producing pairs of polarization correlated photons. Once validated in a balloon test, a SPEQS-CS device was integrated in the GomX-2 satellite for ISS launch in 2014. This satellite did not make it to orbit due to a rocket explosion, but it was recovered from the wreckage and both the satellite and the SPEQS payload were found to be completely operational. 8) Another SPEQS-CS device was integrated into the Galassia CubeSat, a satellite designed by National University of Singapore (NUS) engineering students in 2015. This unit has been successfully deployed and demonstrated in space and is still operational. 9)

The next generation of SPEQS is an enlarged and upgraded version that can produce polarization entangled photon-pairs. The SpooQySat mission has the goal of demonstrating SPEQS producing and detecting entangled photons in-orbit, (i.e. photons will not be beamed outside of the spacecraft). Currently, we are qualification testing the satellite (engineering model shown in Figure 1). With the help of the Singapore Space and Technology Association (SSTA), SpooQySat will be launched via Japan Aerospace Exploration Agency (JAXA) and will be deployed from the International Space Station (ISS) sometime in 2019, with a 6-month expected life time before atmospheric re-entry.

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Figure 1: Partially integrated engineering model of SpooQySat. Removed solar panels reveal structural model SPEQS payload (image credit: NUS/CQT)

Satellite design

The satellite bus is mainly designed using commercial-off-the-shelf (COTS) products. In 2015 we considered many of the CubeSat subsystem suppliers then available in the market with flight heritage, robustness, size and performance in mind. Ultimately, rather than designing an optimized selection of subsystems built around our payload, we selected the 3U GomX CubeSat platform (from GomSpace ApS). The baseline design for the SpooQySat satellite is GomX-313, with modifications to accommodate the SPEQS payload, and unrequired subsystems removed (such as fine pointing ADCS and camera). Using a single-supplier satellite platform allows us to integrate a proven, reliable satellite bus with a shorter development cycle and lower risks. This means that to some extent the SpooQySat subsystems are driven by the capabilities of the GomSpace hardware than our top-down definition of a space mission, i.e. the SPEQS device and it’s mounting structure is designed to accommodate the SWaP requirements of a 3U GomX CubeSat platform. The layout of the SpooQySat satellite is illustrated in Figure 2.

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Figure 2: SpooQySat layout with interstage panels, side solar panels, harnesses removed (image credit: NUS/CQT) 10)

The SpooQySat subsystems chosen include a half-duplex UHF transceiver combined with deployable canted turnstile UHF antennas used for both uplink and downlink; a 32-bit AVR controller with a 64MB flash storage used as the on-board controller (OBC) for housekeeping and data handling; an attitude determination system (onboard the OBC) with 3 magnetorquers for 3-axis detumbling; and a 38 Whr (4 lithium-ion 18650 cells, 7.7 Whr maximum depth of discharge) battery pack with the electrical power management system (EPS). The 3.3 V power bus will supply subsystems including the OBC, ADCS (onboard the OBC) and the communication system, while the 5V (max. 2 A) is supplied to the SPEQS payload. For the SpooQySat mission, the peak system power consumption is rated at 3.85 W and the peak payload (SPEQS) power consumption is rated at 2.5 W and the duration for each experiment is approximately 35minutes. 10 pairs of space qualified triple junction solar cells can provide on average 4.5Whr energy gain each (ISS) orbit.

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Figure 3: Photo of the 3U SpooQy-1 CubeSat (image credit: NUS/CQT)


Launch: The SpooQy-1 CubeSat was launched as a secondary payload on 17 April 2019 (20:46 UTC) on the NG Cygnus CRS-11 resupply mission to the ISS (Antares 230 vehicle) from Wallops Flight Facility, VA. 11) 12)

Total weight of cargo: 3,436 kg, consisting of 3,162 kg in pressurized cargo and 239 kg in unpressurized cargo.

Orbit: Near-circular orbit, altitude of ~ 400 km, inclination of 51.6º.

Secondary payloads:

• AeroCube 10A (JimSat) and 10B (DougSat) by The Aerospace Corporation

• Aeternitas, Ceres, and Libertas, three 1U CubeSats in the Virginia CubeSat Constellation, launched as part of NASA's ELaNa-26 mission

• EntrySat, a 3U CubeSat developed by the ISAE-SUPAERO aeronautics and space institute in France with support from CNES, the French space agency.

• IOD-1 GEMS (In-Orbit Demonstration - Global Environmental Monitoring Satellite)

• KRAKsat, a CubeSat created by Polish students from AGH University of Science and Technology and Jagiellonian University

• SASSI (Student Aerothermal Spectrometer Satellite of Illinois and Indiana)

• Seeker, a free-flying NASA CubeSat for vehicle inspection

• SpooQy-1, a 3U CubeSat developed at the National University of Singapore.

• Swiatowid, a 2U CubeSat, a technology demonstration satellite developed by SatRevolution S.A., a Polish startup company.

• Uguisu, Raavana 1, and NepaliSat 1 are 1U CubeSats developed by student and research teams in Japan, Sri Lanka and Nepal under the auspices of the international Birds program.




Mission status

• July 1, 2020: An international research team led by the National University of Singapore (NUS) has generated and detected quantum entanglement onboard a CubeSat nanosatellite weighing less than 2.6 kg and orbiting Earth. CubeSats are low-resource, cost-effective satellites and are smaller than a shoebox. 13) 14)

- As a first step, the researchers needed to demonstrate that a miniaturized photon source for quantum entanglement could remain intact through the stress of launch and operate successfully in the harsh environment of space within a satellite that provides minimal energy. They examined every component of the photon-pair source that would be used to generate quantum entanglement to see if it could be made smaller or more rugged.

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Figure 4: Researchers developed a miniaturized source of quantum entanglement that measures only 20 by 10 cm [image credit: Center for Quantum Technologies, NUS (National University of Singapore)]

- “At each stage of development, we were actively conscious of the budgets for mass, size, and power," engineer Aitor Villar said. “By iterating the design through rapid prototyping and testing, we arrived at a robust, small-form factor package for all the off-shelf components needed for an entangled photon-pair source.”

- The new miniaturized photon-pair source consists of a blue laser diode that shines on nonlinear crystals to create pairs of photons. To achieve high-quality entanglement, the researchers had to completely redesign the mounts that currently align the nonlinear crystals with a high degree of precision and stability.

- To qualify the instrument for space, the researchers tested its ability to withstand the vibration and thermal changes experienced during a rocket launch and in-space operation. The photon-pair source maintained high-quality entanglement throughout the testing while preserving crystal alignment even after repeated temperature cycling from -10 to 40ºC.

- The researchers incorporated their new instrument into SpooQy-1, a CubeSat that was deployed into orbit from the International Space Station on June 17, 2019. The instrument successfully generated entangled photon-pairs over temperatures from 16 to 21.5ºC.

- “This demonstration showed that miniaturized entanglement technology can work well while consuming little power,” Villar said. “This is an important step toward a cost-effective approach to the deployment of satellite constellations that can serve global quantum networks.”

- The team is now working with RALSpace in England to design and build a quantum nanosatellite similar to SpooQy-1, but with the capabilities needed to beam entangled photons from space to a ground receiver. This is slated for demonstration aboard a 2022 mission. The researchers are also collaborating with other teams to improve the ability of CubeSats to support quantum networks.

- “In the future, our system could be part of a global quantum network transmitting quantum signals to receivers on Earth or on other spacecraft,” Villar said. “These signals could be used to implement any type of quantum communications application, from quantum key distribution for extremely secure data transmission to quantum teleportation, where information is transferred by replicating the state of a quantum system from a distance.”

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Figure 5: The SpooQy-1 CubeSat contains a miniaturized quantum instrument that creates pairs of photons with the quantum property of entanglement. The entanglement is detected in correlations of the photons’ polarizations (image credit: Center for Quantum Technologies, National University of Singapore, and NASA)

• December 16, 2019: SpeQtral has now accepted the operations of the SpooQy-1 smallsat on behalf of the CQT (Center for Quantum Technologies) at the National University of Singapore (NUS). 15)

- The primary objective of the SpooQy-1 mission is to produce and characterize entangled photon pairs in space such that they violate the CHSH (Clauser-Horne-Shimony-Holt) Bell’s inequality. This is a core capability for future quantum communication networks. The CQT team is analyzing scientific data from the mission and expects to publish results on the source’s performance in 2020.

- In the meantime, CQT and SpeQtral have signed an agreement allowing SpeQtral to manage ongoing operations. Formed as a spin-out company to commercialize quantum communications technologies developed at CQT, SpeQtral will monitor the long-term performance of the quantum payload for radiation damage and other degradation effects in the space environment. This information will help guide the development of long-lived quantum systems in space, necessary for the commercial deployment of space-based QKD systems.

- Artur Ekert, Director of CQT, said establishing a partnership for the SpooQy-1 mission plays to all of the firm's strengths: at the Center for Quantum Technologies, the organization will concentrate on scientific objectives, while SpeQtral focuses on commercial applications.

- Chune Yang Lum, Co-Founder and CEO of SpeQtral, added that SpooQy-1 is pioneering quantum technologies for space-based quantum key distribution (QKD) systems. Being involved in this mission gives SpeQtral know-how that serves the company;s goal of delivering next-generation secure communication networks.

• The SpooQy-1 3U CubeSat was deployed from the ISS on June 17, 2018. 16)

- On June 17, the SpooQy project team from NUS and staff of the Singapore Space and Technology Association (SSTA) visited the Tsukuba Space Center to observe the moment of deployment from the Mission Control Room (MCR). While they were waiting for the historic moment in the VIP room behind the MCR, Astronaut Kanai introduced an overview of the ISS and the JEM Small Satellite Orbital Deployer (J-SSOD).




Entangled Photon Source Payload

The entangled photon source on-board SpooQySat inherits a lot of its design from the source (SPEQS-CS) on the Galassia mission with an increased number of optical components and higher requirements in alignment precision and stability. This drives the requirements for the optical structure to be larger to accommodate the additional (and larger) components. The finalized payload design occupies 2U in volume (Figure 2) of the satellite with a mass of ~0.9 kg.

SPEQS Design and Assembly: The optical designs of SPEQS payloads has been discussed in our previous works. 17) While several designs have been tested and some built as functional models, the optical layout used in the SpooQySat engineering, qualification and flight model devices is shown in Figure 7. The optical path is designed to be straight from the pump source to the detectors, omitting the fold mirror found in the SPEQS-CS designs so reducing reflection losses. The achromatic half wave plate (component 5 in Figure 6) allows for a brighter, stabler and more compact design than those previously considered. 18)

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Figure 6: SPEQS optical layout. Note: The blue dots are additional components compared to SPEQS CS (image credit: NUS/CQT)

The optical unit and the baseplate are in contact with each other to minimize the thermal gradient. During operation, the laser diode at one end of the unit is a significant heat source and can cause a temperature gradient. A heater is designed to fit into a 0.4 mm deep cut-out in the middle section of the baseplate, which is used to minimize the temperature gradient and maintain the thermal and thermoelastic stability of the optical unit. A Viton seal is designed as to insulate the optical unit from the PCB (Printed Circuit Board). The enclosure is made from black anodized aluminum. The construction material for both the optical unit and the baseplate are titanium (specifically, Ti-6AL-4V, grade 5) for thermal expansion compatibility.

Close attention has been paid to thermal expansion and mechanical creep of the satellite structure, which may result in deformation of the titanium optical bench structure. An isostatic mount for the payload has been designed by our collaborators in the University of New South Wales (UNSW) Canberra, Australia. The mounting structure consists of three stainless-steel (SS301) blade-like mounts. Each of these mounts has two-degree of freedom to isolate the thermoelastic extension of the satellite structure as well as providing thermal isolation. The baseplate is mounted on top of the isostatic mount, which mounts to a specially designed side panel that replaces a section of the ISIS CubeSat chassis.

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Figure 7: SPEQS payload shown floating to reveal the UNSW Canberra designed isostatic mount beneath (image credit: NUS7CQT, UNSW)

Thermal Compensation Circuit: Another engineering challenge for SpooQySat is how to compensate the rapid change of the thermal environment in low earth orbit (LEO) when active thermal stabilization (e.g. thermoelectric cooler based thermal control) is not practical due to the SWaP constraints. From the Galassia mission, we have identified a temperature dependent response of the LCPR (Liquid Crystal Polarization Rotator) - an electrically controllable non-inertial polarization rotator used as part of the photon polarization detector. Such temperature dependent response is compensated by a novel, low power capacitance tracking based control system for the LCPR operating in the temperature range of 10-30 degrees Celsius. 19)

Random Number Generation: A by-product of the quantum entanglement experiment is we can use the detected photons to generate quantum random numbers. On board SpooQySat, we have developed a special experiment profile that can generate and publicly broadcast the numbers to amateur UHF ground stations using periodic beacons. On the ground side, we have developed a Quantum-Safe key expansion algorithm using a randomness beacon and a 256 bits HMAC as a pseudorandom function. This algorithm allows mobile devices that have limited or no QKD capacity, but access to such a public randomness beacon, to carry out high volume secure communication. The algorithm will be discussed in detail in a paper in preparation.

SPEQS/OBC Software Interface Design: The SPEQS payload has its dedicated controller to operate the desired experiment and can save the experiment data into its own memory before transferring it to the OBC. The satellite bus only needs to provide power, an initialization command, and then receive the data after the experiment is completed. Once SPEQS is switched on, it acts as an autonomic system until the end of the experiment to prevent unintended interruption during the science experiment.

The SPEQS device is designed to communicate with the OBC via a serial connection. 16 experiment profiles are designed for the mission with different purposes. Once SPEQS is turned on by the OBC, it will direct its internal program with the desired experiment profile based on the configuration bits embedded in the initiation command from the OBC. Each experiment lasts approximately 35minutes, generating 1024 kbit of scientific data along with the error handling data at the completion of a single experiment. This data is framed before sending to the OBC. The OBC will send a status frame to SPEQS after the data transfer is finished to acknowledge receipt and then switch off SPEQS.

Science Phase Concept of Operations: In the ISS orbit (400 km) with an estimated field of view (FOV) of 140 degrees, the average number of passes per day is 1.7 with a 4-minute downlink time window each pass. In practice, we round this down to 1 useful orbit per day for nominal operation, due to the likelihood of satellite passes happening at times when there are no operators at the ground stations. At this stage, we are only planning 1 experiment per day, although the on-board resources potentially allow more. If the first few experiments can be successfully downloaded, we can consider increasing the duty-cycle of the payload operation.

The scientific payload can be initiated when the ground operations team is confident that the satellite performs nominally after the in-orbit commissioning. The payload operation for SpooQySat is irrespective of the satellite’s attitude inclination, altitude etc. However, the performance of the payload is related to the environmental temperature. Therefore, the ground operators shall analyze the housekeeping data for the previous passes and plan the payload operation at the appropriate orbital time.




UHF Ground Station

All photon pairs produced in SpooQySat will only be detected in-situ within the SPEQS units, so no optical ground station is needed - only UHF ground stations will be used. The Singapore ground station is located on top of an eighteen-storage building in the NUS campus. A secondary UHF ground station is established in Switzerland to provide a backup system and to provide additional data download opportunities. The ground stations are built using the GomSpace UHF Ground Station and have identical setups (Figure 3). Both ground stations are equipped with a twinned Yagi antenna with a tracking mount. The rotor is controlled by a Linux based server computer (NanoCom MS100). The ground station radio (NanoCom GS100) is the ground counterpart (with a 25 W power amplifier) for the NanoCom AX100 radio on board SpooQySat, designed specifically as an integrated component to request/respond via the CSP (CubeSat Space Protocol) during operation.

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Figure 8: Left: Singapore UHF ground station on the roof top. Right: Switzerland UHF ground station (image credit: NUS/CQT) 20)



1) Tom Vergoossen, Robert Bedington, Aitor Villar,Alexander Lohrmann, Huai Ying Lim,Xueliang Bai, Alexander Ling, ”SpooQy-1, a CubeSat to demonstrate an entangled photon light source,” Proceedings of the 70th IAC (International Astronautical Congress), Washington DC, USA, 21-25 October 2019, paper: IAC-19,B4,2,12, URL: https://iafastro.directory/iac/proceedings/IAC-19
/IAC-19/B4/2/manuscripts/IAC-19,B4,2,12,x52890.pdf

2) Xueliang Bai, Robert Bedington, Karthik Ilangovan, Hong Nhung Nguyen,Rakhitha Chandrasekara,Alexander Lohrmann, Aitor Villar, Md. Tanvirul Islam, Alexander Ling, ”Validating an entangled photon light source in space with the SpooQy-1CubeSat,” Proceedings of the 32nd Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 4-9, 2018, paper: SSC18-II-03, URL: https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=4072&context=smallsat

3) Robert Bedington, Tang Zhongkan, Rakhitha Chandrasekara, Cliff Cheng, Tan Yue Chuan, Kadir Durak, Aitor Villa Zafra, Edward Truong-cao, Alexander Ling, ”Small Photon Entangling Quantum System (SPEQS) for Space-Based Quantum Key Distribution (QKD),” Proceedings of the 66th International Astronautical Congress (IAC 2015), Jerusalem, Israel, Oct.12-16, 2015, paper: IAC-15-B2.5.9

4) R. Bedington, E. Truong-Cao, Tan Y. C., C. Chenga, K. Durak, J. Grieve, J. Larsen, D. Oib an A. Ling, ”Deploying quantum light sources on nanosatellites II: lessons and perspectives on CubeSat spacecraft,” Proceedings of SPIE, Vol. 9648, 964849, 'Electro-Optical and Infrared Systems: Technology and Applications XII; and Quantum Information Science and Technology,' Toulouse, France, August 28, 2015, URL: http://arxiv.org/pdf/1508.07074.pdf

5) Robert Bedington, Alex Ling Group, ”Quantum Tech demos on CubeSat nanosatellites,” CQT, URL: http://www.qtspace.eu/sites/testqtspace.eu/files/presentations
/Bedington%20-%20CQT%20Singapore%20-%20Quantum%20CubeSats_1.pdf

6) Robert Bedington, Juan Miguel Arrazola, Alexander Ling, ”Progress in satellite quantum key distribution,” NPJ Quantum Information, Vol. 3, Article No 30, Published: 9 August 2017, https://doi.org
/10.1038/s41534-017-0031-5

7) Juan Yin, Yuan Cao, Yu-Huai Li, Ji-Gang Ren, Sheng-Kai Liao, Liang Zhang, Wen-Qi Cai, Wei-Yue Liu, Bo Li, Hui Dai, Ming Li, Yong-Mei Huang, Lei Deng, Li Li, Qiang Zhang, Nai-Le Liu, Yu-Ao Chen, Chao-Yang Lu, Rong Shu, Cheng-Zhi Peng, Jian-Yu Wang, and Jian-Wei Pan, ”Physical Review Letters, Volume 119, 200501 – Published 13 November 2017, https://doi.org/10.1103/PhysRevLett.119.200501

8) Zhongkan Tang, Rakhitha Chandrasekara, Yue Chuan Tan, Cliff Cheng, Kadir Durak & Alexander Ling, ”The photon pair source that survived a rocket explosion,” Scientific Reports, Vol. 6, Article number: 25603, Published: 10 May 2016, https://doi.org/10.1038/srep25603

9) Zhongkan Tang, Rakhitha Chandrasekara, Yue Chuan Tan, Cliff Cheng, Luo Sha, Goh Cher Hiang, Daniel Oi, Alexander Ling, ”Generation and analysis of correlated pairs of photons on board a nanosatellite,” Phys. Rev. Applied 5, 054022 (2016), URL: https://arxiv.org/pdf/1603.06659.pdf

10) James A. Grieve, Robert Bedington, Zhongkan Tang, Rakhitha C.M.R.B. Chandrasekara, Alexander Ling, ”SpooQySats: CubeSats to demonstrate quantum key distribution technologies,” Acta Astronautica, Volume 151, October 2018, Pages 103-106, https://doi.org/10.1016/j.actaastro.2018.06.005

11) Rob Garner, ”News Conference, Launch Blog Coverage Conclude,” NASA post-launch news conference for Northrop Grumman’s 11th NASA-contracted commercial resupply mission, 17 April 2019, URL: https://blogs.nasa.gov/northropgrumman/category/northrop-grumman/crs-11/

12) Stephen Clark, ”Cygnus supply ship delivers 3.8-ton cargo load to International Space Station ,” Spaceflight Now, 19 April 2019, URL: https://spaceflightnow.com/2019/04/19
/cygnus-supply-ship-delivers-3-8-ton-cargo-load-to-international-space-station/

13) ”Quantum Entanglement Demonstrated Aboard Orbiting CubeSat,” Photonics, 1 July 2020, URL: https://www.photonics.com/Articles/Quantum_Entanglement_Demonstrated_Aboard_Orbiting/a65910

14) Aitor Villar, Alexander Lohrmann, Xueliang Bai, Tom Vergoossen, Robert Bedington, Chithrabhanu Perumangatt, Huai Ying Lim, Tanvirul Islam, Ayesha Reezwana, Zhongkan Tang, Rakhitha Chandrasekara, Subash Sachidananda, Kadir Durak, Christoph F. Wildfeuer, Douglas Griffin, Daniel K. L. Oi, and Alexander Ling, ”Entanglement demonstration on board a nano-satellite,” Optica, Vol. 7, Issue 7, pp. 734-737, Published: 25 June 2020, https://doi.org/10.1364/OPTICA.387306, URL: https://www.osapublishing.org/DirectPDFAccess/8DB88EE0-F442-8337-B845D45FFD9BA764_432928/optica-7-7-734.pdf?da=1&id=432928&seq=0&mobile=no

15) ”SpooQy-1 Smallsat Now Operated by SpeQtral for the Center for Quantum Technologies,” Satnews Daily, 16 December 2019, URL: http://www.satnews.com/story.php?number=86658109

16) ”Successful Deployment of SpooQy-1 from Kibo!,” JAXA, 18 June 2019, URL: http://iss.jaxa.jp/en/kuoa/news/190618_SpooQy.html

17) Robert Bedington, Xueliang Bai, Edward Truong-Cao, Yue Chuan, TanKadir, Durak Aitor, Villar Zafra, James A Grieve, Daniel KL Oi, Alexander Ling, ”Nanosatellite experiments to enable future space-based QKD missions,” EPJ Quantum Technology, December 2016, https://doi.org/10.1140/epjqt/s40507-016-0051-7

18) Alexander Lohrmann, Aitor Villar, and Alexander Ling, ”Generating entangled photon pairs in a parallel crystalgeometry,” ArXiv:1805.11809v1, (2018), URL: https://arxiv.org/pdf/1805.11809.pdf

19) Rakhitha Chandrasekara, Kadir Durak, and Alexander Ling, ”Tracking capacitance of liquid crystal devices to improve polarization rotation accuracy,” Optics Express Vol. 25, Issue 17, pp. 20363-20368 (2017), https://doi.org/10.1364/OE.25.020363

20) X. Bai, et al. ”SpooQy-1, a CubeSat to demonstrate an entangled photon light source,” Advances in the Astronautical Sciences, Vol. 163, No. 2, Rome, Italy, (2017)


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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