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Journey towards the quantum internet
Koji Azuma Distinguished scientist, NTT Research Center for Theoretical Quantum Physics
Information is described by utilizing differencesof states of physical systems, while physicalsystems follow the laws of physics. Hence, theultimate potential for information processing
should be determined by such physics. This
concept, given by renowned IBM fellow Rolf
Landauer (1927-1999), is encapsulated in his
famous proverbial statement ‘Information isphysical’. Nowadays, since the most precise
and successful theory in physics is quantum
mechanics (although it provides predictions
far beyond our intuitions developed by ourdaily life or classical mechanics), the conceptof Landauer can be interpreted as that theultimate potential of information processing isdetermined by ‘quantum mechanics’ (beyondclassical mechanics). In fact, this concept has
triggered the finding of quantum computationand communication as our ultimate limit ofinformation processing and has shown that wecan accomplish information processing tasks
that are intractable by conventional means.
In this context a ‘quantum internet’ [1], aquantum version of the conventional Internet,
may be an ultimate form of our information
processing, given that the current internet isthe biggest computer network on the Earth.
Therefore it is important to understand, realize
and expand the potential of a future quantum
internet which would enable us to answer oneof the most fundamental questions: what kindof information processing can be allowed bynature?

Possibilities of quantum internet
If you imagine the structure of the current internet,
you would notice that it is composed of information
processing nodes (such as computers and smartphones) and communication channels (such as
optical fibres and free space) to connect themtogether. As a result, the internet enables arbitraryclients on Earth to communicate. Analogously, thequantum internet would be composed of ‘quantum’
information processing nodes (such as quantum

One of IBM’s supercomputer models.
computers) and ‘quantum’ channels (such as freespace and optical fibres, as will appear later),which should enable arbitrary clients, independentof their spatial distances, to perform ‘quantum’communication tasks in an efficient manner.
Such a global quantum internet would have many
applications [2]. For instance, the quantum internet
would enable arbitrary users to perform informationtheoretic secure communication using quantum key
distribution (QKD) [3], even if eavesdroppers in
the network are allowed to use universal quantum
computers freely. This highly secure communication
could be used for a referendum, a top-level meeting,a financial deal, an exchange of genetic/biologicalinformation and so on. Furthermore, the quantum
internet enables any client to teleport unknown
quantum states of their system to another [4], whichis the basis of distributed quantum computing [5],quantum cloud computing [6,7], and ultimately, largescale quantum computer networks.
Besides this, the quantum internet could be used forsynchronizing atomic clocks with unprecedented
stability and accuracy, in a completely securemanner [8]. It would also enable us to make baselines oftelescope arrays unprecedentedly longer, contributing
to the progress of astronomy [9]. Therefore, aworldwide race towards building up the quantum
internet with these fascinating applications has now
started. China has launched a satellite to achieve longdistance quantum communication and created a trusted
relay QKD network over thousands of kilometres,
the EU has started a project called the ‘quantum
internet alliance’, and the US has just announced theJanuari 2019 | De Physicus

THEMApreparation of a large budget (about 1.2 billion USdollars) for quantum technologies, called the ‘USnational quantum initiative’ that will include research
towards a future quantum internet.

Quantum internet in practiceHow can we build up such a quantum internet in practice?
The main role of the quantum internet is quite simple:
it just needs to distribute ‘quantum entanglement’for distributed clients in an efficient manner. The
entanglement is a very peculiar correlation which can beheld only by quantum systems and cannot be explainedin the framework of classical mechanics or conventional
probability/information theory. This entanglement wasoriginally introduced by Podolsky, Einstein and Rosen topoint out the (apparent) paradoxical nature of quantum
mechanics [10]; ironically, it exists indeed and it is now
known to be an extremely useful and even universal
resource for achieving not only quantum communication
but also quantum computation. Thus, we need to build
up the physical layer for the quantum internet. Thisshould be able to distribute such entanglement to clients,
according to their requests, in an efficient manner.
An optical fibre network is regarded as a plausible
candidate for the physical channel layer for the quantum
internet, because optical fibres can efficiently deliver
quantum information embedded into optical pulses
and they are already installed in the world. Indeed, an
optical fibre has already been used to perform pointto-point quantum communication in the form of thedirect fibre transmission of photons between a senderand a receiver. The communication distance of thisscheme has already reached about 400 km. However,
it is considered that this communication distancecannot be more than that. This is because any opticalfibre has photon loss which increases exponentiallywith its length. For instance, the success probabilitiesof transmission of a single photon, which is a typicalquantum information carrier, over 50 km, 100 km,

The global fiber optic network.

De Physicus | Januari 2019

150 km and 200 km optical fibres are approximately
10%, 1%, 0.1% and 0.01%, respectively. Therefore, forquantum communication over a 1000 km optical fibre,even with a GHz repetition rate, it would take over
100 years to obtain a unit of entanglement. This meansthat point-to-point direct quantum communication
has a limitation on the achievable distance, which
is not enough to achieve a global quantum internet.
So how can we realize a quantum internet over an
optical fibre network? The natural solution to thisis to replace conventional repeaters with ‘quantum’
repeaters [11,12,13].

Use of quantum repeaters
The repeaters are used even in conventional
communication networks to achieve long-distancecommunication against the photon loss of optical fibres.
Here, the role of each repeater is to amplify (or measureand resend) signals of optical pulses which have been
weakened by loss in the optical fibres. However, thismechanism cannot be used in quantum repeaters, because
quantum signals cannot be amplified, owing to the nocloning theorem [14] in the quantum world. Instead,‘quantum’ repeaters generate entanglement between
adjacent repeater nodes by exchanging photons throughoptical fibres. They transform, by their local operations,the established entanglement between adjacent repeater
nodes into entanglement between a sender and a receiveras the resource for their quantum communication. This
indirect establishment of entanglement with quantum
repeaters has an exponential improvement over thepoint-to-point quantum communication in terms ofcommunication efficiency, because quantum repeaters
merely use optical fibres connecting adjacent repeater
nodes, rather than ones connecting the sender and thereceiver directly. Therefore, it is important to develop
quantum repeaters.
The necessity and importance of quantum repeaters has
also been proven by our recent theoretical progress on
the understanding of the quantum/private capacity oflossy optical channel networks, where a lossy opticalchannel corresponds to the theoretical model of an
optical fibre. The quantum (private) capacity gives themaximum obtainable entanglement (secret key) per
use of given channels, by using arbitrary operations
allowed in quantum mechanics. Therefore, it represents
the true theoretical limit of the performance of quantum
communication with given channels. Recently,

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excellent upper bounds on quantum and private capacityof point-to-point quantum communication over lossy
optical channels (or optical fibres) have been derived[15,16]. Notably, these upper bounds have no scaling gap
with ‘existing’ point-to-point quantum communication
schemes, rendering the necessity of quantum repeaters
for long-distance quantum communication conclusive.
Subsequently, the upper bounds were generalized tohave upper bounds on the quantum/private capacity ofarbitrary lossy optical channel networks [17,18], beyond
point-to-point communication. Besides, it was shown thatthe performance of running quantum repeater schemes
between a sender and a receiver in a parallel manner,which is called aggregated quantum repeater scheme,
achieves these upper bounds [19]. This concludes thatthe aggregated quantum repeater scheme accomplishes
the quantum/private capacity of arbitrary lossy opticalchannel networks, irrespective of their topology, which
implies that quantum repeaters must be a fundamentalbuilding block for the quantum internet. Therefore,
even from a theoretical viewpoint, it is important for thequantum internet to develop quantum repeaters.

Realization of quantum repeaters
Now to realize quantum repeaters, we have twoapproaches [13]; one is to prepare quantum repeaters withmatter quantum memories to store quantum information
conveyed by photons; the other is to prepare quantum
repeaters which work without quantum memory, towards
a highest bandwidth quantum internet. In the first
approach, we need to develop matter quantum memories
which can efficiently entangle with photons and has along coherence time (i.e., memory time). As candidates
for this, atomic ensembles and a single qubit system(such as a nitrogen-vacancy (NV) centre in a diamond, an
ion trap, a single atom and a quantum dot) are extensively
studied both theoretically and experimentally. The
second approach, which is relatively new, tries todevelop information-processing matter qubits [20] or
to sophisticate existing optical devices [21] (such as
single-photon sources, linear optical elements, activefeedforward techniques and photon detectors), but it does
not need quantum memories at all. Both approaches havedifferent advantages, meaning that experimental progressalong both approaches are important.

will further be revealed and many experimental
breakthroughs will occur. This progress must be
because we are heading to the quantum internet as
the Holy Grail of quantum information technology,
consciously or even unconsciously.

I thank W. J. Munro for his helpful comments to writeup this article. I also acknowledge the support, in part,from PRESTO, JST JPMJPR1861.

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Koji Azuma received his B.E., M.E., andPh.D. degrees in physics from Osaka
University, University of Tokyo and Osaka
University, Japan, in 2005, 2007, and 2010
respectively. He joined NTT Basic ResearchLaboratories in 2010. His current interests
are quantum information theory, especiallyfor building the theory for a quantum
internet. He was appointed as Distinguished

Our journey towards the quantum internet has justbegun. In the near future, more fascinating applicationsof the quantum internet will be found, its potential

Scientist of NTT in 2018. He is currently a member of Theoretical
Quantum Physics Research Group. He is a member of the PhysicalSociety of Japan. E-mail: azuma.koji@lab.ntt.co.jp

Januari 2019 | De Physicus