1 Long-Range QKD without Trusted Nodes is Not Possible with Current Technology

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Long-Range QKD without Trusted Nodes is

Not Possible with Current Technology

Authors:

Bruno Huttner, ID Quantique, Switzerland

†

;

Romain Alléaume, Telecom Paris - Institut Polytechnique de Paris, France;

Eleni Diamanti, Sorbonne University, CNRS - LIP6, France;

Florian Fröwis, ID Quantique Europe, Austria;

Philippe Grangier, Université Paris-Saclay, IOGS, CNRS, France;

Hannes Hübel, Austrian Institute of Technology, Austria;

Vicente Martin, Center for Computational Simulation / ETSIInf. Universidad Politécnica

de Madrid, Spain;

Andreas Poppe, Austrian Institute of Technology, Austria;

Joshua A. Slater, QuTech - Delft University of Technology, The Netherlands ;

Tim Spiller, University of York, UK;

Wolfgang Tittel,

QuTech and Kavli Institute of Nanoscience, Delft Technical University, The Netherlands;

Department of Applied Physics, University of Geneva, Switzerland; Schaffhausen

Institute of Technology in Geneva, Switzerland;

Benoit Tranier, ThalesAleniaSpace, France;

Adrian Wonfor, University of Cambridge, UK;

Hugo Zbinden, Department of Applied Physics, University of Geneva, Switzerland.

Abstract

A recently published patent[1] has claimed the development of a novel quantum key distribution

protocol purporting to achieve long-range quantum security without trusted nodes and without use

of quantum repeaters. Here we present a straightforward analysis of this claim, and reach the

conclusion that it is largely unfounded.

Introduction

Today, point-to-point quantum key distribution (QKD) is already a commercial reality. The range of

commercial QKD systems is typically one hundred kilometers over optical fibers. Academic systems

and new protocols can reach several hundreds of kilometers[2],[3]. Free-space QKD links to Low

Earth Orbit satellites have been demonstrated by the Chinese Micius satellite[4]. However, the range

of a single point-to-point link is still limited by the power loss of the link[5]. To expand real-world

applications of QKD it is necessary to extend the range up to global QKD and offer more complex

network topologies[6]. Pending new technologies, such as the quantum repeater, this expanded

versatility is achieved by so-called Trusted Nodes (TNs)[7]. In a TN, the quantum signal is measured

and converted to a classical signal. A new classical signal is generated, converted to quantum, and

sent to the next node. TNs can be used as relays, to provide long-distance QKD, and as switches, to

†

Corresponding author: bruno.huttner@idquantique.com

2 
 
provide  complex  topologies[6].  However,  since  the  TN  contains  a  classical  signal,  which  can  in 
principle  be  copied,  there  is  no  quantum  security  within  the  TN.  The  TN  must  be  trusted  and 
physically  protected,  to  avoid  any  leakage  of  the  data[6].  Therefore,  for  security  purposes,  TNs 
represent weak  points  in a  complete end-to-end QKD  transmission. In  this  paper, the  term  long-
distance QKD means global QKD, i.e. the  ability to  deploy  and implement QKD between any two 
points on Earth. 
Recently,  the  UK  Intellectual  Property  Office  granted  the  patent  number  GB2590064[1]  to  the 
company Arqit Ltd. In the following, we will refer to this patent as the ARQ19 patent. We will also 
refer to the protocol described in this patent as the ARQ19 protocol. This patent purports to offer 
long-distance QKD with no trusted nodes. According to the claims, global QKD  could be achieved 
now with untrusted satellites. This would represent a game-changer for QKD. Therefore, it is clearly 
important that these claims are investigated. Unfortunately, as far as we are aware, they have not 
been validated by an accompanying public disclosure in any scientific journal. Therefore, we base 
our analysis on the ARQ19 patent and on the 20-F annual report filed by Arqit at the Securities and 
Exchange Commission (SEC)[8]. This report will be referred to as the SEC filing. We present here our 
results.  We  believe  that  an  open  discussion  is  essential  to  the  provision  of  trustful  solutions  for 
future secure communications, quantum or otherwise. This is similar to the process currently being 
carried  out  by  the  National  Institute  of  Science  and  Technology  (NIST)  for  Post-Quantum 
Cryptography.  In  this  process,  the  NIST  has  solicited  researchers  worldwide  to  submit  quantum-
resistant  public-key  cryptographic  algorithms.  The  NIST  has  been  evaluating  them  and  has  now 
decided to standardize some of them. To improve trust in the choice, all the candidate algorithms 
were presented and discussed publicly[9]. 
Quick Reminder on QKD  
In the following, we follow the convention of the ARQ19 patent: the two end-users are named Bob 
and Carol. The TN, which acts as a broker for Bob and Carol is named Alice. The two partners, Bob 
and Carol, want to exchange confidential information. Quantum Key Distribution (QKD) provides a 
solution to establish a shared secret key with information-theoretic security[3],[10]. QKD guarantees 
that the key cannot be broken in the future, even by a computationally unbounded adversary. The 
secret key established by QKD can then be combined with an authenticated encryption scheme to 
perform  secure  communication  between  Bob  and  Carol,  with  a  security  gain  compared  to 
conventional key distribution methods[10]. In particular, secure communication with information-
theoretic security can be obtained by combining QKD with One-Time-Pad encryption. Alternatively, 
QKD can be combined with symmetric encryption such as AES to strengthen the resilience of secure 
communications. In this document, we adopt the terminology Quantum Security for the security that 
can be reached in the exchange of confidential information between Bob and Carol, using QKD or 
any similar quantum technology providing information-theoretic security for the key exchange. 
QKD relies on two elements: 
1. A  quantum  channel,  which  permits  the  exchange  of  quantum  objects,  typically  single 
photons, over an optical fiber (ground applications) or over free space (satellites). The main 
property of this quantum channel is that any attempt at eavesdropping will modify the state 
of the objects exchanged through the channel. This will be discovered during subsequent 
information reconciliation steps. 
2. A classical discussion or reconciliation channel, which allows the processing of the quantum 
exchange and the establishment of the secret key. This channel is authenticated (this means 

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1Long-RangeQKDwithoutTrustedNodesisNotPossiblewithCurrentTechnologyAuthors:BrunoHuttner,IDQuantique,Switzerland†;RomainAlléaume,TelecomParis-InstitutPolytechniquedeParis,France;EleniDiamanti,SorbonneUniversity,CNRS-LIP6,France;FlorianFröwis,IDQuantiqueEurope,Austria;PhilippeGrangier,UniversitéParis-...

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