Zhongkan Tang

Near-space flight of a correlated photon system

We report the successful test flight of a device for generating and monitoring correlated photon pairs under near-space conditions up to 35.5 km altitude. Data from ground based qualification tests and the high altitude experiment demonstrate that the device continues to operate even under harsh environmental conditions. The design of the rugged, compact and power-efficient photon pair system is presented. This design enables autonomous photon pair systems to be deployed on low-resource platforms such as nanosatellites hosting remote nodes of a quantum key distribution network. These results pave the way for tests of entangled photon technology in low earth orbit.

Generation and Analysis of Correlated Pairs of Photons aboard a Nanosatellite

Satellites carrying sources of entangled photons could establish a global quantum network, enabling private encryption keys between any two points on Earth. Despite numerous proposals, demonstration of space-based quantum systems has been limited due to the cost of traditional satellites. We are using very small spacecraft to accelerate progress. We report the in-orbit operation of a photon pair source aboard a 1.65 kg nanosatellite and demonstrate pair generation and polarization correlation under space conditions. The in-orbit photon correlations exhibit a contrast of 97+/-2%, matching ground-based tests. This pathfinding mission overcomes the challenge of demonstrating in-orbit performance for the components of future entangled photon experiments. Ongoing operation establishes the in-orbit lifetime of these critical components. More generally, this demonstrates the ability for nanosatellites to enable faster progress in space-based research.

Generation and analysis of correlated pairs of photons on board a nanosatellite

We report the operation of a photon pair source on board a nanosatellite. Pair generation and polarization correlation are demonstrated in low Earth orbit: an important milestone towards compact entangled photon pair sources for future space-based quantum communication.

The photon pair source that survived a rocket explosion.

We report on the performance of a compact photon pair source that was recovered intact from a failed space launch. The source had been embedded in a nanosatellite and was designed to perform pathfinder experiments leading to global quantum communication networks using spacecraft. Despite the launch vehicle explosion soon after takeoff, the nanosatellite was successfully retrieved from the accident site and the source within it was found to be fully operational. We describe the assembly technique for the rugged source. Post-recovery data is compared to baseline measurements collected before the launch attempt and no degradation in brightness or polarization correlation was observed. The survival of the source through an extreme environment provides strong evidence that it is possible to engineer rugged quantum optical systems.

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

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 Bells inequality with a brightness and visibility that can facilitate quantum key distribution (QKD) from space to ground.

Single photon counting for space based quantum experiments

We present a software based control system for Geiger-mode avalanche photodiodes (GM-APDs) that enables constant photon detection efficiency irrespective of the diodes junction temperature. Furthermore, we demonstrate that this control system enables passively quenched GM-APDs to double the rate of photon detection events before saturation compared to the standard control method that fixes the junction temperature and applied bias voltage. We present data demonstrating the robustness of the GM-APD control system when tested in near-space conditions using a correlated photon pair source carried by a weather balloon to an altitude of 35.5 km.

SpooQySats: CubeSats to demonstrate quantum key distribution technologies

Abstract Satellite-based quantum key distribution (QKD) offers the potential to share highly secure encryption keys between optical ground stations all over the planet. SpooQySats is a programme for establishing the space worthiness of highly-miniaturized, polarization entangled, photon pair sources using CubeSat nanosatellites. The sources are being developed iteratively with an early version in orbit already and improved versions soon to be launched. Once fully developed, the photon pair sources can be deployed on more advanced satellites that are equipped with optical links. These can allow for very secure uplinks and downlinks and can be used to establish a global space-based quantum key distribution network. This would enable highly secure symmetric encryption keys to be shared between optical ground stations all over the planet.

Robust photon-pair source survives rocket explosion

Quantum key distribution (QKD), i.e., using quantum signals to generate secure symmetric key material at distant sites, is of much interest for quantum communications because of its high level of privacy (underpinned by quantum mechanics). In particular, entanglement-based QKD1 is a powerful technique in which quantum correlations between photons are leveraged. In this process, the entangled photons can be distributed with the use of optical fibers or ground-level free-space links. Current QKD networks, however, suffer from a distance limit because of fiber losses2, 3 and the lack of quantum repeaters.4 There are several ongoing efforts to overcome this distance limit and to produce regional/global QKD networks.5–10 In these approaches, a source of entangled photons, on board a satellite, would be used to beam the photons down to widely separated receivers. A major milestone in achieving this endeavor would therefore be the successful demonstration of an entangled-photon pair source in low Earth orbit (LEO). It has previously been proposed that CubeSat nanosatellites are a costeffective way to realize this aim (because technology validation experiments can be performed in small, iterative steps).11 However, spontaneous parametric downconversion12—the ‘workhorse’ method for generating entangled-photon pairs—requires the use of precisely aligned bulk optics. This means that it can be challenging to design a photon source that has sufficiently low size, weight, and power requirements to be used on a nanosatellite. We have thus been working on the development of a compact device that is designed to perform pathfinder experiments in the pursuit of achieving global quantum communication networks. In particular, our device is designed to generate and measure photon pairs in space. In the first step toward validation in a space environment, we assembled a science package that comprised a correlated photon-pair source (with one spontaneous Figure 1. Diagram of the customized toolkit used to align the components of the optical unit in the photon pair source. Two separate knobs are used to adjust the yaw and pitch angles independently. The optical components are aligned within a pocket that is machined into the aluminum housing. After alignment, epoxy is applied to secure the component within the pocket and the adjuster is removed (to maintain the device’s compact form factor) once the epoxy has cured. KMS/M: Compact kinetic mirror mount from Thorlabs.

The compact photon pair source that survived a rocket explosion

We report on the performance of a compact photon pair source that was retrieved from a failed space launch. The source had been installed in a nanosatellite and was found to be completely operational upon recovery. Comparison of post-recovery and baseline data suggests that there is no degradation in brightness or polarization correlation between photon pairs. We describe the assembly technique for the robust source. Its survival provides strong evidence that it is possible to design rugged quantum optical systems.