Advances in entanglement-based QKD for space applications Sebastian Eckera Johannes Pseinerab Jorge Pirisb and Martin Bohmannc aInstitute for Quantum Optics and Quantum Information IQOQI Boltzmanngasse 3 1090

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Advances in entanglement-based QKD for space applications

Sebastian Eckera, Johannes Pseinera,b, Jorge Pirisb, and Martin Bohmannc

aInstitute for Quantum Optics and Quantum Information (IQOQI), Boltzmanngasse 3, 1090

Vienna, Austria

bEuropean Space Research and Technology Centre (ESTEC) - European Space Agency (ESA),

Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, Netherlands

cQuantum Technology Laboratories GmbH (qtlabs), Clemens-Holzmeisterstr-Str. 6/6, 1100

Vienna, Austria

ABSTRACT

Quantum key distribution (QKD) enables tap-proof exchange of cryptographic keys guaranteed by the very laws

of physics. One of the last remaining roadblocks on the way towards widespread deployment of QKD is the high

loss experienced during terrestrial distribution of photons, which limits the distance between the communicating

parties. A viable solution to this problem is to avoid the terrestrial distribution of photons via optical ﬁbers

altogether and instead transmit them via satellite links, where the loss is dominated by diﬀraction instead of

absorption and scattering. First dedicated satellite missions have demonstrated the feasibility of this approach,

albeit with relatively low secure key rates. In order for QKD to become commercially viable, the design of future

satellite missions must be focused on achieving higher key rates at lower system costs. Current satellite missions

are already operating at almost optimal system parameters, which leaves little room for enhancing the key rates

with currently deployed technology. Instead, fundamentally new techniques are required to drastically reduce

the costs per secret bit shared between two distant parties. Entanglement-based protocols provide the highest

level of security and oﬀer several pathways for increasing the key rate by exploiting the underlying quantum

correlations. In this contribution, we review the most relevant advances in entanglement-based QKD which are

implementable over free-space links and thus enable distribution of secure keys from orbit. The development of

satellite missions is notoriously lengthy. Possible candidates for a new generation of quantum payloads should

therefore be scrutinized as early as possible in order to advance the development of quantum technologies for

space applications.

Keywords: Quantum key distribution, QKD, Entanglement-based QKD, Satellite-based QKD, Entanglement

distribution, Quantum technology

1. INTRODUCTION

In our modern information-technology driven world, secure communication is getting ever more important. To

secure the privacy of communication channels, cryptographic tools are used. Encryption systems have been

constantly further developed, driven by the competition with code breakers, who always ﬁnd more elaborate

ways to challenge secure communication. Today, the computational power of supercomputers and even more

so the rise of quantum computers pose a substantial threat to the security and privacy of our communication

channels which has a dramatic impact on our private, political, and economical life. This threat stems from the

fact that modern encryption is based on mathematical problems which are hard to solve by classical computers,

but can be eﬃciently solved on a quantum computer.

One possibility of overcoming this quantum threat also lies in the quantum realm; namely quantum commu-

nication and more particular quantum key distribution (QKD). QKD is an approach to exchange a secure key

for encryption between two or more users guaranteeing security by the laws of quantum physics. An extensive

Further author information: (Send correspondence to S.E. or M.B.)

S.E.: E-mail: sebastian.ecker@oeaw.ac.at

M.B.: E-mail: martin.bohmann@qtlabs.at

arXiv:2210.02229v1 [quant-ph] 5 Oct 2022

introduction and overview of the ﬁeld can be found, for example, in the following review articles.1–4Here, we

will just brieﬂy recall the main ideas and concepts. Central to QKD is the so-called no-cloning theorem that

states that an arbitrary quantum state, and therefore the quantum information it is carrying, cannot be simply

copied, which is the key ingredient for the security of QKD. Due to their transmission properties and minimal

interaction with the environment, quantum states of light are usually used to encode and distribute quantum

information based on which it is possible to extract a secure key for encryption afterwards. In QKD, there

are two main branches, namely the continuous-variable (CV) and discrete-variable (DV) regime. In the former,

quantum information is encoded in the ﬁeld properties of an optical mode while in the latter one properties and

correlations of individual photons are used. In the DV regime the most common approaches are the prepare and

measure protocols based on so-called BB84 protocol5and entanglement-based approaches which exploit quantum

entanglement between two photons.6,7As the name already suggests, prepare and measure protocols need an

active encoding of the information onto the quantum state of a single photon or weak coherent states if one uses

the decoy-state protocol.8This active encoding demands that one communication partner (usually called Alice)

possesses the sending device, that the device is trusted, and that one has access to true random numbers for

the active encoding. On the other hand, entanglement-based schemes are based on the creation of two entan-

gled photons through a physical process and the subsequent distribution of these photons to the communication

partners Alice and Bob. This approach does not require that one of the communication partners possesses the

source of entangled photons nor is it necessary to trust the device as the quantum correlations between the two

photons measured by the communication partners cannot be emulated or faked by a malicious adversary. For

BB84 and entanglement-based protocols one has to choose the degree of freedom (physical property) in which one

wants to encode and measure the quantum information that can be, for example, polarization, time, or orbital

angular momentum. Note that there exist also other QKD schemes such as measurement-device independent or

twin-ﬁeld QKD; again, see Refs. 1–4for an overview.

Let us brieﬂy recall some further fundamentals of entangled-photon sources. In principle, there are several

physical processes that allow to generate entangled photons for QKD such as exciton decay in quantum-dots9or

nonlinear optical processes like spontaneous parametric down-conversion (SPDC) (see, e.g. Ref. 10) or four-wave

mixing.11 SPDC is the most advanced and most broadly used technique for implementing entanglement-based

QKD. Its basic working principle is the conversion of pump photons in a material with second-order nonlinearity

into two (down-converted) photons that are entangled. Depending on the design of the SPDC source, the

generated photons are entangled in diﬀerent degrees of freedom and can even feature simultaneous entanglement

– so-called hyperentanglement – in several degrees of freedom.12 Notably, high-dimensional degrees of freedom

provide a larger information content per sent photon13 and show better resistance against noise.14 An overview

on diﬀerent designs of entangled photon sources (EPS) can be found in Ref. 15. Among others, important

parameters of SPDC source are: the SPDC emission spectrum, the source’s heralding eﬃciency, the brightness

describing the average number of entangled photon pairs per second (and per pump power and spectral width),

and deviations from the ideal (maximally entangled) target state. A deﬁnition of these properties and their

inﬂuence on the performance of QKD protocols can be found in Ref. 16. It is also important to mention that

entangled photons cannot only be used for QKD applications but they will be an essential building block of a

future quantum internet.

The fundamental principle which provides security in QKD systems also limits the implementation, as quan-

tum information cannot be simply ampliﬁed resulting in the fact that losses (attenuation) play a crucial role

in QKD. Therefore, the implementation of QKD systems is practically limited to a few hundred kilometers in

optical ﬁbers,17,18 because the losses in ﬁber systems scale exponentially with the communication distance. A

possible way to overcome this problem is to use satellite-to-ground links, which do not scale exponentially with

the distance between the communication parties and therefore even allow for global quantum communication.

An overview on free-space satellite QKD implementations is provided in Section 2. The implementation of free-

space QKD links, however, needs to deal with the inﬂuence of the turbulent atmosphere19,20 that can have a

detrimental eﬀect on the transmitted quantum correlations.21–23 Additionally, the losses in satellite-based QKD

are still high which limits the attainable key rate. Therefore, it is important to develop strategies to optimize

and maximize the achievable secure key rate in satellite QKD systems.

In this paper, we present potential advances in future entanglement-based QKD systems involving satellites,

including their demands and challenges. The discussed approaches cover the exploitation of physical properties

as well as technological advances. We will elaborate on the development of high-performance and integrated

entangled photon sources. Furthermore, we will elaborate how wavelength-multiplexing and high-dimensional

QKD can be beneﬁcial. Finally, we also discuss the potential of adaptive optics and space-based quantum

repeaters for quantum communication.

This paper is organized as follows. In Section 2, we provide a motivation and the state of the art for

entanglement-based QKD from space. In Section 3, we list and discuss diﬀerent pathways for advances in

space-based QKD. We ﬁnish with a conclusion and discussion in Section 4.

2. MOTIVATION AND STATE OF THE ART

Prior to any satellite launch, the idea of using long-distance optical free-space links for QKD was successfully

put to the test over terrestrial links.24–26 These studies demonstrated the feasibility of satellite-based QKD by

comparing the atmospheric turbulences on a horizontal free-space link with the turbulences experienced over a

vertical free-space link.25 In 2016, the ﬁrst satellite with a dedicated quantum payload, named Micius, was suc-

cessfully launched. This satellite launch was part of the QUESS mission,27 spearheaded by the Chinese Academy

of Sciences, which demonstrated several quantum communication protocols via satellite links. Speciﬁcally, sev-

eral QKD conﬁgurations, such as satellite-relayed QKD,28 satellite-to-ground QKD29,30 and entanglement-based

QKD via a dual downlink,31 have been successfully demonstrated. Several other missions have deployed dedicated

satellite transmitter payloads which served as a driver for the development of robust quantum technologies.32–35

The preferred photonic property for quantum state encoding over free-space links is the polarization domain

due to its robustness in free-space propagation, the availability of high-quality polarization-encoded photon

sources and the availability of eﬃcient and compact receiving modules. Most advances discussed in the following

will therefore rely on the polarization domain for key generation.

Another important design choice concerns the type of QKD protocol. While the ﬁrst QKD protocol was of the

prepare-and-measure type,36 where Alice prepares the quantum states and Bob receives them, entanglement-

based protocols,6,7where both Alice and Bob receive photons from an entangled photon pair source, have

already been successfully demonstrated in space.31 Entanglement-based QKD protocols are advantageous for

space applications for two reasons. In contrast to prepare-and-measure protocols, the party owning the source of

entangled photon pairs can be malicious, since the communicating parties can infer any eavesdropping attempt

from the correlations between their measurement outcomes. This is particularly relevant for space-based quantum

cryptography, since the satellite operator hosting the entangled photon pair source can in principle be malicious,

which is essential, since trust in the cryptographic devices is not a good selling point in QKD implementations.

Apart from that, entanglement itself is a valuable resource in quantum information processing,37 and the long-

distance distribution of entanglement is therefore crucial for future quantum technological applications in space.

The secure key rate distributed via a dual downlink from Micius between two parties on ground separated by

1120 km was 0.12 bits/s.31 While this is impressive for a ﬁrst in-orbit demonstration, the commercial success of

QKD is directly linked to the costs per secret bit. Importantly, the hardware employed on the satellite and on the

ground stations is already operating close to the optimum,38 which leaves little room for further optimisation. In

order for QKD to be commercially viable, fundamentally new methods and techniques are required to increase

the secure key rate and lower the costs per secret bit. Fortunately, the quantum correlations which are inherent to

entanglement-based QKD protocols open up several pathways for increasing the secure key rate with reasonable

technological overhead.

3. ADVANCES IN SPACE-BASED QKD

In each of the following subsections, a method towards advancing satellite-based QKD is introduced. While all of

the methods are compatible with satellite links, the same methods can also be employed to increase the key rate

in ﬁber-based QKD. All of the presented advances have a technology readiness level of 4-7, which means they

have been demonstrated in laboratory environments and some even have seen demonstrations over terrestrial

free-space links. The introduced advances comply with the following key requirements:

•Increase the secure key rate

•Reduce size, weight, power (SWaP) and complexity of quantum payloads

•Decrease the costs per secret bit

In the Discussion section as well as in each of the following subsections we assess and rank these advances

according to their space suitability, their deployment timeline and their key rate potential.

3.1 High-performance entangled photon-pair sources

Entangled photon pair sources play a crucial role in the overall performance of a QKD system, and they have

been the bottleneck in the achievable key rate for some time. The performance of entangled photon-pair sources

for QKD can be captured by three parameters:

•Brightness: Photon pair rate per unit of pump power

•Fidelity: Closeness of the produced quantum state to a maximally entangled quantum state

•Heralding eﬃciency: Probability of a photon-pair detection conditioned on a single-photon detection

However, only one of the three parameters is relevant for state-of-the-art entanglement sources. While the

ﬁdelity of the source has a huge impact on the achievable secure key rate, most polarization-entangled photon

pair sources based on SPDC are characterized by ﬁdelities in excess of 99%,39 which means they emit almost

perfect quantum states. The potential of achieving slightly higher key rates by investing in this remaining 1%

for achieving 100% ﬁdelity is currently not worth the eﬀort. Optimizations on the heralding eﬃciency are also

insigniﬁcant, since slightly higher heralding eﬃciencies are overshadowed by high optical losses in satellite links.

The brightness of photon pair sources is the one parameter which can make a diﬀerence in secure key rate.

Naively, one might expect that an increase in brightness corresponds to a proportional increase in secure key rate.

Due to so-called accidental coincidences, this is however not always the case. Pairs of photons produced in SPDC

are highly correlated on a ps timescale. They are identiﬁed by correlating the detection times of Alice’s and

Bob’s detection events. Whenever a photon detection at Alice coincides with a photon detection at Bob within

a certain coincidence window, a photon pair is identiﬁed. With increasing source brightness and background

noise levels, detection events are mistakenly identiﬁed as a photon pair. These accidental coincidences cannot be

distinguished from genuine photon-pair detections, which results in a decreased ﬁdelity and therefore potentially

results in a decreased key rate (see Figure 1). These fake identiﬁcations become more frequent if the pair rate is

increased beyond the timing resolution of the detection system. This necessitates an adaptation of the pair rate to

the available timing resolution of the detection system and the chosen coincidence window. While single-photon

avalanche diodes (SPADs) have a typical timing resolution in the order of 100 ps, another widespread detector

technology, superconducting nanowire single-photon detectors (SNSPDs),40 have typical timing resolutions in

the order of 10 ps and therefore enable signiﬁcantly smaller coincidence windows. The design of QKD systems

therefore requires careful ﬁne-tuning of the transmitter and receiver speciﬁcations.

While this is true for static quantum links, dynamically changing quantum links require adaptive measures

for eﬃcient operation even more. For example, in LEO satellite links the optical loss varies drastically depending

on the elevation angle, weather conditions and cloudage. Therefore, adapting the photon pair rate via the

pump power of the SPDC process is essential for optimizing non-static satellite links. This has recently been

demonstrated over a 143-km-long terrestrial free-space link between two Canary islands, where one photon is

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Advancesinentanglement-basedQKDforspaceapplicationsSebastianEckera,JohannesPseinera,b,JorgePirisb,andMartinBohmanncaInstituteforQuantumOpticsandQuantumInformation(IQOQI),Boltzmanngasse3,1090Vienna,AustriabEuropeanSpaceResearchandTechnologyCentre(ESTEC)-EuropeanSpaceAgency(ESA),Keplerlaan1,Postbus299...

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