Responsive Operations for Key Services ROKS A Modular Low SWaP Quantum Communications Payload Craig D. ColquhounHazel Jerey Steve Greenland Sonali Mohapatra Colin Aitken Mikulas Cebecauer Charlotte

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Responsive Operations for Key Services (ROKS): A Modular, Low SWaP Quantum

Communications Payload

Craig D. Colquhoun,∗Hazel Jeﬀrey, Steve Greenland, Sonali Mohapatra, Colin Aitken, Mikulas Cebecauer, Charlotte

Crawshaw, Kenny Jeﬀrey, Toby Jeﬀreys, Philippos Karagiannakis, Ahren McTaggart, Caitlin Stark, and Jack Wood

Craft Prospect Ltd, Suite 12 Fairﬁeld, 1048 Govan Road, Glasgow, UK, G51 4XS

Siddarth K. Joshi,†Jaya Sagar, Elliott Hastings, Peide Zhang, Milan Stefko, David Lowndes, and John G. Rarity

Quantum Engineering Technology Labs & Department of Electrical and Electronic Engineering, University of Bristol, UK

Jasminder S. Sidhu,‡Thomas Brougham, Duncan McArthur, Roberto G. Pousa, and Daniel K. L. Oi

SUPA Department of Physics, University of Strathclyde, Glasgow, G4 0NG, UK

Matthew Warden,§Eilidh Johnston, and John Leck

Fraunhofer Centre for Applied Photonics, Technology and Innovation Centre, 99 George Street, Glasgow, G1 1RD, UK

Quantum key distribution (QKD) is a theoretically proven future-proof secure encryption method

that inherits its security from fundamental physical principles. With a proof-of-concept QKD pay-

load having ﬂown on the Micius satellite since 2016, eﬀorts have intensiﬁed globally. Craft Prospect,

working with a number of UK organisations, has been focused on miniaturising the technologies that

enable QKD so that they may be used in smaller platforms including nanosatellites. The signiﬁcant

reduction of size, and therefore the cost of launching quantum communication technologies either

on a dedicated platform or hosted as part of a larger optical communications will improve potential

access to quantum encryption on a relatively quick timescale.

The Responsive Operations for Key Services (ROKS) mission seeks to be among the ﬁrst to send

a QKD payload on a CubeSat into low Earth orbit, demonstrating the capabilities of newly devel-

oped modular quantum technologies. The ROKS payload comprises a quantum source module that

supplies photons randomly in any of four linear polarisation states fed from a quantum random num-

ber generator; an acquisition, pointing, and tracking system to ﬁne-tune alignment of the quantum

source beam with an optical ground station; an imager that will detect cloud cover autonomously;

and an onboard computer that controls and monitors the other modules, which manages the payload

and assures the overall performance and security of the system. Each of these modules have been

developed with low Size, Weight and Power (SWaP) for CubeSats, but with interoperability in mind

for other satellite form factors.

We present each of the listed components, together with the initial test results from our test

bench and the performance of our protoﬂight models prior to initial integration with the 6U Cube-

Sat platform systems. The completed ROKS payload will be ready for ﬂight at the end of 2022,

with various modular components already being baselined for ﬂight and integrated into third party

communication missions.

I. INTRODUCTION

Responsive Operations for Key Services (ROKS) is a

UK Space Agency funded mission that aims to launch

a 6U CubeSat with quantum key distribution (QKD)

and cloud-detection capabilities [1] into Low-Earth Or-

bit (LEO). ROKS has supported the progression of QKD

instrumentation from apparatus spanning optical test

benches in university laboratories, to miniaturised, mod-

ular, space-ready subsystems.

Craft Prospect Ltd (CPL) and partners have designed,

manufactured, assembled, tested, and now integrated

several modules for this mission [2], though with recon-

∗Electronic address: craig.colquhoun@craftprospect.com

†Electronic address: sk.joshi@bristol.ac.uk

‡Electronic address: jasminder.sidhu@strath.ac.uk

§Electronic address: matthew.warden@fraunhofer.co.uk

ﬁgurability and interoperability in mind for future ﬂight

opportunities. This approach enables ROKS subsystems

to be supplied either individually or bundled for a range

of satellite form factors. The individual capabilities of

one of CPL’s modules will be demonstrated in the up-

coming Canadian space agency mission QEYSSat [3]-a

variant of the JADE quantum source produced for ROKS

will be driven by a QRNG board developed at Univer-

sity of Waterloo as a secondary payload on the satel-

lite, demonstrating its ability to interface with diﬀerent

driving electronics, and with diﬀerent QRNGs [4] from a

range of suppliers.

The remainder of the paper is laid out as follows: the

rest of this section gives the motivation for using orbital

CubeSats for QKD, Section II lists and describes each of

the modules and their purposes in the mission, Section III

presents some key module tests and results, Section IV

describes some of the ongoing Optical Ground Station

work at University of Bristol, some of the lessons learned

as part of the test and integration process are listed in

arXiv:2210.11285v1 [quant-ph] 20 Oct 2022

Section V, and Section VI concludes the paper.

A. Why QKD?

Encryption, the use of secret keys to encrypt or decrypt

information, is now ubiquitous on the internet, to the

point that most people use encryption on a daily basis.

Two of the most common types of encryption used to se-

cure data online are RSA [5] which relies on the multipli-

cation of two large prime numbers for its key generation,

and AES which involves performing several matrix oper-

ations to do the same. There are algorithms for quantum

computers that threaten the security of these methods -

Shor’s algorithm [6] signiﬁcantly reduces the complexity

of prime number factorisation when compared with clas-

sical computers, and Grover’s algorithm [7] eﬀectively

halves the bit length of keys for brute forcing attacks.

The former poses a signiﬁcant threat [8] to RSA en-

cryption of all currently used bit lengths, and the latter

greatly reduces the amount of time it would take to cor-

rectly guess an AES key assuming no vulnerabilities for

the method would be discovered.

With technological advances improving the scalability,

fault tolerance, and commercial availability of quantum

computers; it is only a matter of time until cyber crimi-

nals and state actors gain access to them, and until these

encryption methods are rendered insecure. Such nefar-

ious actors need not wait until capable quantum com-

puters are available to decrypt conﬁdential information,

they may already be storing encrypted data to decrypt

later, so quantum-secure encryption methods are critical

at present [8]. Quantum key distribution (QKD) is an

encryption method that relies on the quantum mechani-

cal properties of light for its key generation rather than

mathematical complexity, making it theoretically secure

against attacks from quantum computers [9,10].

While other QKD protocols use the polarisations of en-

tangled photons or time binning to distinguish between

photons, or qubits, that represent a 0 or 1, in ROKS a

symmetric BB84 [11] protocol is used. In the quantum

communications channel, weak coherent pulses (<1 pho-

ton per pulse average) are generated in one of four linear

polarisations at random: horizontal (H) ←→, vertical (V)

l, diagonal (D) .%, or anti-diagonal (A) &-. These po-

larisations are paired oﬀ into two polarisation bases with

their orthogonal counterparts - the H-V basis ←l→, and

the D-A basis .%&- - within which one polarisation corre-

sponds to a 1 and the other to a 0. All properties of the

photons other than their polarisations should be identi-

cal so the photons are otherwise indistinguishable from

one another.

After the bits have been sent a reconciliation pro-

cess occurs over classical communication channels, dur-

ing which only the basis in which each photon was sent

is transmitted so the receiver can determine if the pho-

ton was detected in the correct polarisation basis. If the

basis is correct, the qubit has almost certainly been cor-

rectly measured so it is kept; if the basis is incorrect, the

qubit will only be correctly measured 50% of the time,

so this bit is ignored and does not contribute to the ﬁnal

secret key. Because only single photons are transmitted,

any eavesdroppers can be detected by decreases in the

numbers of detected photons, which can also be stated

as an increase in the quantum bit error rate (QBER).

Eavesdroppers also cannot reliably reproduce photons

they pick oﬀ, because there is a 50% probability they

measure each photon in the incorrect polarisation basis.

As well as ‘signal states’ that are kept and contribute

to the ﬁnal secret key, ‘decoy states’ [12] are also used

for added complexity in case of eavesdroppers. Decoy

states contain all the same information as signal states,

just at a diﬀerent intensity. During the reconciliation

process over the classical communications channel, the

transmitter declares which pulses were signal and which

were decoy.

Some BB84 QKD solutions are commercially available,

either coupled into free-space or ﬁbre-coupled. The dis-

tances over which these solutions can be used are lim-

ited by atmospheric losses [13] and ﬁbre attenuation [14],

respectively. Using satellites from various Earth orbits

should overcome these limits [15–17] as this can oﬀer

global reach, and the atmosphere exists at relatively low

altitudes for some of which its density is reduced, limit-

ing atmospheric losses [18,19]. The ROKS mission aims

to demonstrate the feasibility of supplying quantum keys

from CubeSats in LEO [20], with the potential for con-

stellations in future to reduce lapses in coverage at com-

patible ground stations [21–23].

B. MISSION GOALS

ROKS is a demonstrator mission both for the devices de-

veloped for the platform, and the services that these de-

vices can provide. The main mission goals are to demon-

strate that QKD technology can be successfully imple-

mented on a CubeSat [16,24], and to demonstrate that

onboard intelligence can be used to deliver mission au-

tonomy and improve utility of a potential CubeSat QKD

service.

II. MISSION PAYLOAD

The ROKS payload consists of 5 modular hardware sub-

systems, described below. The quantum source (JADE)

and acquisition, pointing, and tracking (APATITE) com-

ponents were designed and manufactured in conjunction

with mission partners University of Bristol (UoB) and

University of Strathclyde (UoS). The optical telescope

(GARNET) was developed by Fraunhofer Centre for Ap-

plied Photonics (FhCAP), and licensed to CPL for com-

mercialisation. Each of these modules is shown in its

position in the CubeSat structure in Fig. 1.

JADE: The JADE quantum source combines opti-

FIG. 1: A Computer Render of the 6U Structure with Key

Components Highlighted

cal components that produce quantum signal pulses

(GNEISS), with the electronics used to drive them. The

JADE PCB includes a Zynq FPGA, a series of quan-

tum random number generators (QRNGs), several laser

diode driver chips intended for pulsed operation, sensors

to provide telemetry for the module, and interfaces to

other modules. Operational ﬂexibility is provided by the

ability to tune FPGA parameters during runtime - the

user may tune the pulse rate, individual pulse widths

and laser currents, or whether to pulse using numbers

form the QRNGs or to pulse arbitrary patterns for test

and calibration purposes. The speciﬁed pulse rate for the

ROKS mission is 100 MHz, with 1 ns FWHM pulse width,

generating signal states of 0.8 photons per pulse at the

telescope exit and two decoy states of 0.4 and 0 photons

per pulse. An image of the JADE module internals is

shown in Fig. 2(a).

GNEISS: The optical component of the JADE module

generates weak coherent pulses of 785 nm light in four

linear polarisations. Thermal control of each laser diode

is used to tweak the wavelengths for indistinguishabil-

ity between photon sources. There is also a photodiode

for diagnostic purposes, and an 830 nm alignment laser

source that is used in the APT subsystem. Crucially, all

the beams produced in the module are coupled into a

single optical ﬁbre so that they all share the same spatial

mode upon leaving the JADE module.

APATITE: The APATITE acquisition, pointing, and

tracking system uses a series of dichroic mirrors to sepa-

rate the alignment beam from the quantum source beams

generated by JADE, and combine the quantum source

light with a downlink beacon laser beam. The alignment

beam is directed onto a camera sensor as reference for

where the quantum source is pointing - the camera sen-

sor will also be used to detect uplink beacon light from

compatible optical ground stations (OGSs). The down-

link beacon beam serves three purposes: it provides an

alignment signal for OGSs; it serves as a polarisation ref-

erence so the OGS may calibrate its optics to optimise

detection of the quantum source polarisation states; and

it supplies timing and synchronisation information dur-

FIG. 2: Images of the Modules Contained in the Optical Pay-

load (OPAL) Segment of ROKS (a) JADE Quantum Source

Module; (b) APATITE Acquisition, Pointing, and Tracking

Module; (c) GARNET Optical Telescope; (d) All the OPAL

Modules Connected in a Test Bench Conﬁguration

ing the quantum key transmission phase. The alignment

of the downlink beacon, the quantum source, and the

uplink beacon beams is managed using a MEMS mirror.

The optomechanical parts, camera board, and APATITE

driver board can be seen inside the enclosure in Fig. 2(b).

Any light that leaves APATITE has to pass through the

satellite’s telescope.

GARNET: The optical telescope was developed by the

Fraunhofer Centre for Applied Photonics (FhCAP) and

licensed to CPL. This Schmidt-Cassegrain reﬂecting tele-

scope was designed speciﬁcally for ROKS but it can be

used for any optical communications CubeSat in princi-

ple. GARNET (shown in Fig. 2(c)) occupies a volume of

1.5 U, has a 90 mm (80 mm clear) aperture and provides

30x magniﬁcation to outgoing beams with low distortion

with a ±0.25◦ﬁeld of view. Its exit pupil is located ex-

ternally to the telescope to allow convenient interfacing

with a steering mirror - on ROKS it interfaces directly

with the front of APATITE. The telescope housing is de-

signed to minimise contact with the CubeSat platform to

reduce environmental stresses experienced by the optics.

OPAL: The optical payload (OPAL) encompasses JADE,

APATITE, and GARNET as the full optical subsystem,

as shown in Fig. 2(d). This is deﬁned as a bundle that

may be oﬀered for future commercial ﬂight opportunities

rather than its individual parts.

FLI-NT: The Forwards Looking Imager (FLI), CPL’s

ﬁrst commercially available product, is a camera sensor

instrument powered by a Zynq FPGA embedded with a

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ResponsiveOperationsforKeyServices(ROKS):AModular,LowSWaPQuantumCommunicationsPayloadCraigD.Colquhoun,HazelJerey,SteveGreenland,SonaliMohapatra,ColinAitken,MikulasCebecauer,CharlotteCrawshaw,KennyJerey,TobyJereys,PhilipposKaragiannakis,AhrenMcTaggart,CaitlinStark,andJackWoodCraftProspectLtd,Suit...

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Responsive Operations for Key Services ROKS A Modular Low SWaP Quantum Communications Payload Craig D. ColquhounHazel Jerey Steve Greenland Sonali Mohapatra Colin Aitken Mikulas Cebecauer Charlotte

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