Generation and Analysis of Correlated Pairs of Photons aboard a Nanosatellite

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 $97\%\pm 2\%$, matching ground-based tests. This pathfinding mission overcomes the challenge of demonstrating in-orbit performance for the components of future entangled-photon experiments.

Figure 1

(a) Schema of components in the photon-pair source (items 1 to 6) and the measurement section (7–14). A 405-nm pump diode (1) supplies the pump beam aligned by a pair of prisms (2). The pump is prepared by a band-pass filter (3) and polarization half-wave plate (4) to maximize spontaneous parametric downconversion (SPDC) production within the $\beta$-barium borate (BBO) crystal (5). An excess pump beam is removed from the optical path using filters (6). A gold mirror (7) folds the optical path. Signal and idler photons are separated by a dichroic mirror (8) before the photons are interrogated by polarization rotators (9 and 12) and polarizing beam splitters (10 and 13) in front of Geiger-mode avalanche photodiodes, APD1 (11) and APD2 (14). (b) The 3D model of the optics. The crystal is mounted on a steel flexure stage that enables the crystal tilt to be adjusted by screws. Also shown is the position of temperature sensors T1 and T2. A thin-film heater (not shown) for raising the payload temperature is located below APD2. The optical housing has sufficient volume for additional crystals that can be inserted to enable the generation of polarization-entangled-photon pairs.

Figure 2

(a) Payload temperatures recorded during operation in space (blue). For comparison, the temperature from the final test on Earth is provided (red). At activation ($t=0$), the payload is heated to the target temperature ($18°\mathrm{C}$). The detectors are then run for 3 min to observe their noise performance (gray region), following which polarization measurement commences. The T2 sensor initially records a higher reading due to its proximity to the heater. The Earth-based test starts at a higher temperature and does not involve heater operation. (b) Comparison of detector noise (dark counts) observed on Earth and in orbit. The in-orbit dark-count rate is higher than predicted by models that simulate radiation damage. The drop in the dark-count rate for APD2 is attributed to a fall in temperature after the heater (located beneath APD2) is turned off.

Figure 3

Polarization correlation (after background subtraction) recorded in space and on Earth (integration time of 30 s at each setting). These data are used to generate the polarization rotator performance (lines) using Malus’s law. The rotator achieves a polarization contrast of 97% $\pm 2\%$ in both cases and is sufficient for use in measuring the polarization entanglement between photon pairs.