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arXiv:1710.03907

Demonstrating Miniaturised, entangled photon-pair sources on board nano satellites to enable future QKD missions

Rakhitha Chandrasekara, Robert Bedington, Xueliang Bai, Karthik Ilangovan, Yau Yong Sean, Denis P. Naughton, Barraclough, Simon, Douglas K. Griffin, Russell Boyce, Alexander Ling

Published Oct 11, 2017• 0 citations
arXivHTMLPDF

Abstract

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.

DOI
10.48550/arxiv.1710.03907

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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.
Secured global quantum communication networks sharing an encryption key can be established with entangled photon sources onboard satellites in Earth orbit. Even though optical transceivers for ground-to-space communication exist, no space capable source of entangled photons has been demonstrated. A faster and cost-effective solution to reach orbit is by means of CubeSats. This demands significant miniaturisation work on the photon source to fit within the size, weight and power restrictions of CubeSats. The first milestone in our programme is to deploy a source of correlated photon pairs in space as a pathfinder experiment to demonstrate the principle and raise the technology readiness level. The first attempt was unsuccessful when the launch vehicle (CRS Orb-3) failed shortly after take-off, although our payload was successfully recovered intact and found to be fully operational. We are pleased to report that the second attempt with a newly built payload, has been successful and the first milestone has been accomplished. The source was launched on board the Galassia CubeSat (PSLV C29) to an orbit of approximately 550 km and 150 inclination. We observed inorbit generation of high quality photon pair correlations (with a contrast of 97 ± 2%). These results are compatible with baseline data collected prior to launch and show no degradation after spending 140 days in orbit. Data will be presented on the effects of temperature and radiation on the source performance. We will also present plans for upcoming and future missions.
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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.
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.
Global quantum networks for secure communication can be realized using large fleets of satellites distributing entangled photon pairs between ground-based nodes. Because the cost of a satellite depends on its size, the smallest satellites will be most cost-effective. This Letter describes a miniaturized, polarization entangled, photon-pair source operating on board a nano-satellite. The source violates Bell’s inequality with a Clauser–Horne–Shimony–Holt parameter of 2.60 ± 0.06 . This source can be combined with optical link technologies to enable future quantum communication nano-satellite missions.