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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

The next iteration of the small photon entangling quantum system (SPEQS-2.0)

Kadir Durak, Aitor Villar, Brigitta Septriani, Zhongkan Tang, Rakhitha Chandrasekara, Robert Bedington, Alexander Ling

Published Mar 15, 2016• 11 citations
Publisher

Abstract

The Small Photon Entangling Quantum System (SPEQS) is an integrated instrument where the pump, photon pair source and detectors are combined within a single optical tray and electronics package. This footprint enables the instrument to be placed onboard nanosatellites or the CubeLab facility within the International Space Station. The first mission to understand the different environmental conditions that may affect the operation of an entangled photon source in low Earth orbit (LEO) is underway. Here we present a work towards a violation of Bell's inequality with a brightness and visibility that can facilitate quantum key distribution (QKD) from space to ground.

DOI
10.1117/12.2212078

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The Small Photon Entangling Quantum System (SPEQS) is an integrated instrument where the pump, photon pair source and detectors are combined within a single optical tray and electronics package. This footprint enables the instrument to be placed onboard nanosatellites or the CubeLab facility within the International Space Station. The first mission to understand the different environmental conditions that may affect the operation of an entangled photon source in low Earth orbit (LEO) is underway. Here we present a work towards a violation of Bell's inequality with a brightness and visibility that can facilitate quantum key distribution (QKD) from space to ground.
The Small Photon Entangling Quantum System is an integrated instrument where the pump, photon pair source and detectors are combined within a single optical tray and electronics package that is no larger than 10cm×10cm×3cm. This footprint enables the instrument to be placed onboard nanosatellites or the CubeLab facility within the International Space Station. The first mission is to understand the different environmental conditions that may affect the operation of an entangled photon source in low Earth orbit. This understanding is crucial for the construction of cost-effective entanglement based experiments that utilize nanosatellite architecture. We will discuss the challenges and lessons we have learned over three years of development and testing of the integrated optical platform and review the perspectives for future advanced experiments.
To enable space-based quantum key distribution proposals the Centre for Quantum Technologies is developing a source of entangled photons ruggedized to survive deployment in space and greatly miniaturised so that it conforms to the strict form factor and power requirements of a 1U CubeSat. The Small Photon Entangling Quantum System is an integrated instrument where the pump, photon pair source and detectors are combined within a single optical tray and electronics package that is no larger than 10 cm x 10 cm x 3 cm. This footprint enables the instrument to be placed onboard nanosatellites or the CubeLab structure aboard the International Space Station. We will discuss the challenges and future prospects of CubeSat-based missions.
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.
The Secure Photon Entangled Quantum communication Technology Readiness Experiment (SPEQTRE) project is a technology demonstration mission aiming to show entanglement-based quantum key distribution (QKD) from a nanosatellite platform. The satellite will be a 12U CubeSat, measuring 20 x 20 x 30 cm. It is being developed in collaboration with SpeQtral, a Singaporean company who are providing the quantum source and receiver, and ISISpace, a company based in the Netherlands who are providing the satellite platform and its control systems. The project involves the design, build, test, launch and operation of a satellite payload as well as the development and operation of associated ground station facilities. RAL Space are developing multiple new systems for this mission, including an off-axis telescope for the satellite, a control system to stabilise the optical pointing and an optical ground station at Chilbolton to receive the entangled photons. This paper discussed the optical delivery system which has been designed to transmit an optical signal from LEO to an optical ground station.
Satellite nodes are an enabler of global quantum networks by overcoming the distance limitations of fiber and free-space links on ground. The design of quantum sources and receivers for satellites, however, is challenging in terms of size, weight, and power consumption, as well as mechanical and thermal stability. This is all the more true for cost-efficient nanosatellites such as the popular CubeSat platform standard. Here we report on the follow-up mission of SpooQy-1, a 3U CubeSat that successfully demonstrated the generation of polarization-entangled photons in orbit. The next iteration of the mission will showcase satellite-to-ground quantum key distribution based on a compact source of polarization-entangled photon-pairs, and we have recently completed the integration of a fully functional demonstrator as a milestone towards the flight model. We also briefly describe the design of the optical ground station that we are currently building in Singapore for receiving the quantum signal. We present the most important subsystems and illustrate the concept of operation.