2.1

Telescope

The front-end optical receiver for the QKD payload is an afocal 25 cm on-axis catadioptric telescope designed to operate at multiple wavelengths in order to reformat the received uplink signals to a 17 mm input beam for the QTRAC. The telescope features a wide field of view (FOV) of 0.3 degrees half-angle, in order to accommodate for potential mis-pointing of the spacecraft bus towards the ground station. A breadboard version of this telescope (see Figure 3) was recently produced by Honeywell for CSA, and its Quantum Bit Error Rater (QBER) performance has been measured at the Institute for Quantum Computing [5]. Despite the requirement for a wide field of view, the on-axis catadioptric design was preferred over an all-reflective three-mirror anastigmat as the on-axis design is lower mass and has inherently better performance in terms of polarization extinction ratio, which is the major driver of QBER.

Figure 3

The breadboard QKD terminal telescope (left) is an on-axis catadioptric design with a 25 cm primary mirror which has been demonstrated to maintain transmitted polarization states with average quantum bit error rate below 0.03% (right).

The opto-mechanical design, shown in Figure 2, is based on Honeywell’s aluminum diamond machining processes, with aluminum mirrors and structure. Updates from the breadboard include the replacement of the spider structure with a solid aluminum tube (for improved stray-light performance and mechanical stability), improvements to the primary mirror mounting interface to reduce mirror distortion, and the incorporation of an internal baffle near the field lens. An innovative titanium bipod flexure design (seen in yellow in Figure 2) has been implemented to provide compliance in the z direction (optical axis) and reduce distortions of the primary mirror from mounting to the spacecraft panel in order to meet the stringent wavefront error requirements of the system. Finally, a short baffle has been incorporated into the design in order to reduce the risk of sun-blinding during ground station acquisition, where there is a risk that the payload may be looking near the Earth’s limb.

2.2

QTRAC Assembly

Once the uplink beacon and quantum signal beams have been received by the telescope they are passed to the pointing and tracking assembly. The tracking assembly is a modified version of Honeywell’s commercially-oriented Optical Pointing and Tracking Relay Assembly for Communications (OPTRAC) which has been developed for LEO optical intersatellite links (OISL); both of these systems are shown in Figure 4. The QTRAC is based on a modified OPTRAC layout which has been adapted in order to encompass the larger field of view and larger collimated beam required for the QEYSSat mission. The de-lidded QTRAC shown in Figure 4 comprises a fold mirror, to accept the incoming beam from the telescope at nominal incidence, a fine-steering mirror (FSM), and various dichroic mirrors for separating and combining received and transmitted beacon and quantum signals.

Figure 4

the QTRAC is a modified pointing and tracking relay based on the commercially-oriented OPTRAC

As discussed previously, the QEYSSat mission uses the satellite attitude control system to perform coarse tracking of the ground station, hence the QKD terminal optics require a wide FOV to accommodate potential spacecraft mis-pointing. To perform acquisition and fine tracking functions over this wide FOV Honeywell is making use of a pixelated sensor, leveraging techniques developed on the CALTRAC star tracker and JWST FGS. The uplink beacon signal entering QTRAC is directed to an InGaAs imaging array, which centroids the uplink beacon at 10 kHz and permits tracking anywhere within the FOV at an accuracy of 1/15^th pixel (<0.3 arcsec, real-space).

The use of a pixelated array confers a number of advantages over quadrant photodiode sensors which are commonly used as tracking sensors in similar applications including previous experiments to develop the QEYSSat mission concept [3]. First, background and stray light sources as well as radiation hits over the full area of the device have a reduced impact, since the pixelated sensor readout is sub-windowed to a small area of interest. Second, the pixelated sensor can provide accurate centroid measurements anywhere within the sensor array, and therefore does not need to maintain a tight alignment to the quantum uplink (Rx) boresight so long as the boresight alignment can be re-calibrated. This can result in substantially looser alignment tolerances during assembly, as well as over the thermal operational range. Finally, the pixelated sensor offers greater tracking sensitivity, delivering a lower noise-equivalent angle than equivalent quadrant photodiodes.

Use of an InGaAs tracking sensor also allows for an NIR uplink beacon in order to maximize wavelength distance from the sensitive single-photon QKD detectors—which are silicon-based. Radiation testing up to a 10-year lifetime has been completed on the selected InGaAs sensor, showing acceptable performance at end of life. Since the QEYSSat mission has a nominal one-year duration, radiation susceptibility is not a concern. The pointing controller uses the position of the received beacon signal on the tracking sensor to drive a fine steering mirror to correct for both satellite pointing errors as well as platform jitter, thus maximizing the coupling of uplink photons into the quantum receiver.

Following the pointing and tracking system, the QKD uplink signal is directed to an optical relay, where it is re-formatted to match the 5 mm acceptance pupil for the polarization analyzer and narrowband spectral filter. Finally, the downlink beacon and QKD signal are passed to the QTRAC via polarization-maintaining fiber and are expanded to weakly diverging beams using fiber collimators. Note that quantum uplink and downlink will not occur simultaneously, so the QTRAC does not need to provide point ahead correction using a second mechanism. The point-ahead angle is the angular difference between the point where the beacon is seen, and the point to where the transmitter needs to aim to ensure photons hit the partner terminal. Point-ahead is required due to the motion of the satellite during the finite delay between photon transmission and reception. During QKD uplink, the uplink quantum and beacon signals originate from the same point, so point-ahead is not required at the satellite. For a quantum downlink, the point-ahead angle will be applied by tracking the uplink beacon signal to an off-center position on the pixelated ATS. In order to ensure that the ground station can always track the downlink satellite beacon, the beacon is defocused to provide a broadened beam with “passive” point-ahead.

Finally, the received quantum signal is directed to a polarization analyzer in order to determine the initial state of the uplinked photon. The polarization analyzer uses a non-polarizing 50/50 beam-splitter in order to sort incoming photons into either the horizontal-vertical (HV) or antidiagonal-diagonal (AD) bases with equal probability. Once the incoming photons have been sorted into a particular basis (e.g. horizontal-vertical) they are then separated by polarizing beam-splitters in order to measure the polarization state. Once the uplink has been concluded, spurious measurements (e.g. wherein the uplink photon was prepared in the HV basis but detected in the AD basis) are discarded and the remaining measurements are used to generate a secure encryption key.

The alignment and optical performance of the BB QTRAC has undergone performance testing at Honeywell which has shown that the QTRAC is meeting or exceeding its requirements. To maximize performance for the quantum uplink, each optical element within the QTRAC is designed in order to maintain the fidelity of the polarized uplink signal and minimize system QBER—similar to the telescope shown in Figure 3. Under testing, the polarization extinction ratio (a major driver of QBER) of the QTRAC was measured to be over 450:1 in all channels, and the optical throughput of the QTRAC was measured to be 81.6%.

2.3

Detector module

The QKD terminal uses a four-channel single-photon detection system based on silicon avalanche photodiodes (APDs) operating in Geiger mode. The silicon APDs are connected optically to the QKD terminal using multi-mode fibers, and the detectors themselves are housed on the backside of a radiative cold plate that will be used to passively cool the detectors. Previous studies have shown that APDs are highly susceptible to the space radiation environment, however the impacts of radiation damage can be mitigated by cooling the detectors in order to reduce dark counts [6,7]. The APDs in the QKD terminal will be cooled to below -90°C by a combination of passive cooling from the radiator panel, and active cooling from a two-stage Peltier module integrated into the APD packages (as shown in Figure 5).

Figure 5

the detector module (left) comprises a large radiator panel for passive cooling of the detector interface card (middle) which houses the single photon counting APDs (right). The detector module is thermally isolated from the rest of the payload in order to maximize the cooling effect of the radiator.

3.1

Quantum link

Detailed link calculations for the QKD signal uplink, and beacon uplink and downlinks have been developed to ensure that the QKD terminal is able to meet the user requirements for quantum key generation. The link budget for the quantum uplink is presented in

Table 1. The table presents a receiver sensitivity value of -122.7 dBm in order to demonstrate the level of link margin for different link conditions, however in reality the receiver sensitivity for the quantum link is a dynamic value which is determined not only by the loss in the channel, but also by the amount of background light in the channel—which may vary with link conditions. The values presented in the link budget are derived from the input power level at which the instantaneous QBER falls to 5% (the mission threshold). Integrating the instantaneous QBER, signal and decoy rates, channel loss, and background counts over the duration of a given satellite pass, and by calculating the lower bound on the secure key generation rate [8], we determine the minimum length of secure key that may be generated for a given pass. These minimum key lengths are presented for a variety of passes in Table 2.

Table 1

Quantum uplink (Rx) budget presented at 90 degree elevation (zenith) and 48.5 degree elevation

Table 2

Quantum keys generated for overhead passes at various maximum elevation angles and for various quantum sources.

The quantum link budget uses as its baseline a 12 cm diameter ground station telescope (or equivalent diverging beam), however as part of the mission goals there is interest to work with a variety of ground stations and the link budget does not preclude the use of smaller telescopes since the range loss factor due to long term beam wander (atmospheric dropout) dominates over intrinsic diffraction loss. The QEYSSat ground station is assumed to achieve diffraction-limited performance, resulting in a static transmitted wavefront error loss (Strehl) of -0.9 dB. The link budget assumes the use of a WCP source operating at a 400 MHz repetition rate, as well as an entangled source operating at 100 MHz repetition rate.

Channel loss through the atmosphere is modeled as a combination of molecular absorption, and beamspread due to long-term beam wandering and diffraction broadening. A rural atmosphere with 10 km visibility (a typical value near Ottawa) is used to estimate atmospheric absorption. Additionally, an accumulation of tilt errors introduced during propagation through the atmosphere results in a wandering beam in the far-field; in the long term, this effect produces greater average link loss, as-if the transmitter had been de-focused or had a substantially larger diffraction-limited beamspread. The beamspreading effect is calculated separately from scintillation (self-interference) effects and results in an effective beam radius of approximately 10 m at 600 km [9, 2].

The satellite receiver shown in Figure 2 is modeled using the combination of all reflection and transmission terms for each element in the optical chain. Wavefront error and pointing losses in the receiver are accounted for in a worst-case 80% throughput of the spatial filter (pinhole), which is achievable without the need for diffraction-limited performance. The link budget also considers background count contributions from detector dark current, as well as light contributions from reflected moonlight, and human nighttime activity. The magnitude of the background light is maintained at a low level via spatial filtering, which results in a ground receiver footprint of ≈22 m wide.

In order to ensure that the mission is compatible with a variety of quantum sources, the optical payload is designed to accept single-photon uplink signals between 780 nm and 795 nm with polarization-encoding in the horizontal, vertical, anti-diagonal and diagonal bases. The downlink source also uses polarization-encoding in linear states and is designed to operate at 850 nm.

3.2

Beacon sensitivity

As described previously, the QEYSSat mission requires the use of both uplink and downlink beacons in order to enable acquisition and tracking for both the transmitter and the receiver. The acquisition and tracking sensor may acquire the beacon signal with unmodulated input irradiance of 80 pW/m² at the telescope entrance and is able to establish its fine-tracking loop with an input irradiance at 8 nW/m². The primary QEYSSat downlink beacon is designed to deliver an unmodulated irradiance of >10 nW/m² at the ground station.

In the primary mission the satellite acquisition is initiated at a low angle of elevation, resulting in significant distortion of the beacon due to atmospheric scintillation and the increased propagation distance. These losses amount to an additional link loss of over 25 dB with respect to the values in

Table 1. Accordingly, the uplink beacon power and divergence are oversized with respect to the expected tilt-errors induced by the atmosphere, as well as the expected uncertainty on the spacecraft position.