Quantum internet: A vision for the road ahead

TLDR: What it will take to achieve this so-called quantum internet is reviewed and different stages of development that each correspond to increasingly powerful applications are defined, including a full-blown quantum internet with functional quantum computers as nodes connected through quantum communication channels.

Abstract: The internet-a vast network that enables simultaneous long-range classical communication-has had a revolutionary impact on our world. The vision of a quantum internet is to fundamentally enhance internet technology by enabling quantum communication between any two points on Earth. Such a quantum internet may operate in parallel to the internet that we have today and connect quantum processors in order to achieve capabilities that are provably impossible by using only classical means. Here, we propose stages of development toward a full-blown quantum internet and highlight experimental and theoretical progress needed to attain them.

Figures

Figure 3: A quantum internet consists of three essential quantum hardware elements. First, we need a physical connection (quantum channel) that supports the transmission of qubits. Examples are standard telecom fibers as they are presently used to communicate classical light. Second, we need a means to extend these short distances. Quantum channels are inherently lossy. For instance, the transmissivity of fiber optical channels scale exponentially with the distance. This scaling has strong implications for applications. For instance, both for entanglement and key distribution, the achievable rates can at most be proportional to the transmissivity (105, 106). Hence, in order to reach longer distances, intermediate nodes called quantum repeaters are necessary (see e.g. (96, 107–109), and for reviews (90, 91)). Such a repeater is placed at certain intervals along the optical fiber connection, in theory allowing qubits to be transmitted over arbitrarily long distances. In the future, powerful repeaters may also double as long-distance routers in a quantum network. The final element are the end nodes, that is, the quantum processors connected to the quantum internet. These may range from extremely simple nodes that can only prepare and measure single qubits, to large scale quantum computers. End nodes may also act as quantum repeaters themselves, although this is not a requirement. A quantum internet is not meant to replace classical communication, but rather to supplement it with quantum communication. We hence assume all nodes can communicate classically, for example over the classical internet, in order to exchange control information.

Figure 2: Quantum repeaters work fundamentally different from classical repeaters. In its simplest form, a quantum repeater works by first generating entanglement (dashed line) between the repeater (middle) and each of the end nodes (left and right) individually. Intuitively, this can be done because the distance of each end point to the repeater is still sufficiently small to allow direct entanglement generation by transmitting photons over telecom fiber. Subsequently, the repeater teleports one of the qubits entangled with node one onto node two. This procedure is known as entanglement swapping, and allows the creation of entanglement over distances where direct transmission is infeasible. After establishing long distance entanglement, a data qubit may now be sent using quantum teleportation.

Figure 7: Performance measures of a quantum internet. It is an interesting open question to devise integrated tests which are more refined then estimating the errors ε of the individual operations.

Figure 6: Examples of known protocols and their requirements. It is a challenge for quantum network programmers to find protocols for the same tasks, that can be realized with lower stages of a quantum internet. It is an interesting open question what minimum stage is required in order to realize a specific task.

Figure 8: Essential elements of a future quantum network stack.

Figure 1: One application of a quantum internet is to allow secure acccess to remote quantum computers in the cloud (2). Specifically, a simple quantum terminal capable of preparing and measuring only single qubits can use a quantum internet to access a remote quantum computer in such a way that the quantum computer can learn nothing about which computation it has performed. Almost all other applications of a quantum internet can be understood from two special features of quantum entanglement. First, if two qubits at different network nodes are entangled with each other, then such entanglement enables stronger than classical correlation and coordination. For example, for any measurement on qubit one, if we made the same measurement on qubit two, then we instantaneously obtain the same answer, even though that answer is random and was not determined ahead of time. Very roughly, it is this feature that makes entanglement so well suited for tasks that require coordination. Examples include clock synchronization (3), leader election and achieving consensus about data (52), or even using entanglement to help two online bridge players coordinate their actions (39). The second feature of quantum entanglement is that it cannot be shared. If two qubits are maximally entangled with each other, then it is impossible by the laws of quantum mechanics for a third qubit to be just as entangled with any of them. This makes entanglement inherently private, bringing great advantages to tasks that require security such as, for example, generating encryption keys (12) or secure identification (24, 25). .

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Delft University of Technology
Quantum internet
A vision for the road ahead
Wehner, Stephanie; Elkouss, David; Hanson, Ronald
DOI
10.1126/science.aam9288
Publication date
2018
Document Version
Accepted author manuscript
Published in
Science
Citation (APA)
Wehner, S., Elkouss, D., & Hanson, R. (2018). Quantum internet: A vision for the road ahead.
Science
,
362
(6412), 1-9. [eaam9288]. https://doi.org/10.1126/science.aam9288
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Quantum Internet: a vision for the road ahead
Stephanie Wehner,
1∗
, David Elkouss,
1
, Ronald Hanson,
1,2
1
QuTech, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
2
Kavli Institute of Nanoscience, Delft University of Technology,
PO Box 5046, 2600 GA Delft, The Netherlands
∗
To whom correspondence should be addressed; E-mail: s.d.c.wehner@tudelft.nl.
The internet - a vast network that enables simultaneous long-range classical
communication - has had a revolutionary impact on our world. The vision of a
quantum internet is to fundamentally enhance internet technology by enabling
quantum communication between any two points on earth. Such a quantum
internet may operate in parallel to the internet that we have today, and con-
nect quantum processors in order to achieve capabilities that are provably
impossible using only classical means. Here, we propose stages of develop-
ment towards a full-blown quantum internet, and highlight experimental and
theoretical progress to attain them.
Introduction
A quantum internet enables us to solve problems that are fundamentally out of reach for the
classical internet
1
. The most well-known application of a quantum internet is quantum key
distribution (QKD), which enables two remote network nodes to establish an encryption key
1
Some researchers still go one step further and believe all communication will eventually be done over quantum
channels (1).

whose security only relies on the laws of quantum mechanics. A quantum internet, however,
has many other applications ranging from secure access to remote quantum computers (2), clock
synchronization (3), and even extending the baselines of telescopes (4) (see Figure 1). More-
over, as quantum internet research expands, other useful applications will likely be discovered
in the next decade.
Central to all these applications is that a quantum internet enables us to send quantum bits
(qubits), which are fundamentally different than classical bits. While classical bits can take
only two values, ’0’ and ’1’, qubits can be in a superposition of being ’0’ and ’1’ at the same
time. Importantly, qubits cannot be copied, and in fact any attempt to do so can be detected.
It is this feature that makes qubits naturally well suited for security applications, but at the
same time makes transmitting qubits over long distances a truly formidable endeavour. Since
qubits canot be copied or amplified, repetition or signal amplification are ruled out as a means
to overcome imperfections, and a radically new technological development is needed in order
to build a quantum internet (5) (see Figure 2).
We are now at an exciting moment in time, akin to the eve of the classical internet. In late
1969, the first message was sent over the nascent four node network then still referred to as
ARPANET. Recent technological progress (6–9) now predicts that we may see the first small-
scale implementations of a quantum internet within the next five years.
At first glance, realizing a quantum internet (see Figure 3) may seem even more difficult than
building a large scale quantum computer. After all, we might imagine that in full analogy to the
classical internet, the ultimate version of a quantum internet consists of fully-fledged quantum
computers that can exchange an essentially arbitrary number of qubits. Thankfully, it turns out
that many quantum network protocols do not require large quantum computers to be realized: a
quantum device with a single qubit at the end point is already sufficient for many applications.
What’s more, errors in quantum internet protocols can often be dealt with using classical rather

than quantum error correction, imposing fewer demands on the control and quality of the qubits.
The reason why quantum internet protocols can outperform classical communication with such
relatively modest resources is due to the fact that their advantages rely solely on inherently
quantum properties such as quantum entanglement, which can be exploited already with very
few qubits. In contrast, a quantum computer must feature more qubits than can be simulated on
a classical computer in order to offer a computational advantage. Given the challenges posed
by the development of a quantum internet, it is useful to reflect on what capabilities are needed
to achieve specific quantum applications, and what technology is required to realize them.
Here, we identify stages of development towards a full-blown quantum internet. These
stages are functionality driven: central to their definition is not experimental difficulty itself, but
the essential question of what is needed to actually enable useful applications. Each stage is in-
teresting in its own right, and distinguished by a specific quantum functionality that is sufficient
to support a certain class of protocols. To illustrate, we give examples of known application
protocols in each stage where a quantum internet is already known to bring advantages.
Realizing a quantum internet demands significant development to realize quantum repeaters
as well as end nodes. It is clear that in the short term, one may optimize both repeaters and end
nodes relatively independently. That is, one can imagine a quantum internet using relatively
simple end nodes, while using repeaters powerful enough to cover larger distances. Similarly,
a near term quantum internet may be optimized for shorter - for example, pan European -
distances, while employing much more powerful end nodes capable of realizing a larger set
of protocols. Ideally, all these designs ensure forward compatibility to achieve the ultimate
goal of a full-blown world wide quantum internet. We note that while the intermediate repeater
nodes need to be able to support the functionality of each stage, an application centric view
makes no other statements regarding the capabilities of the repeater nodes of the network.
Finally, we discuss progress towards implementing a quantum internet, which poses signif-

icant challenges to physics, engineering and computer science.
Stages of functionality and applications
Let us formulate the functionality driven stages of quantum internet development. Each stage is
distinguished by an increasing amount of functionality, at the expense of increasing experimen-
tal difficulty. We say that an experimental implementation has reached a certain stage only if
the functionality of that stage and all previous stages (Figure 4) is available to all the end nodes
using the network.
Crucial to the distinction between the stages is that the subsequent stage offers a fundamen-
tally new functionality not available in the previous one, rather than simply improving parame-
ters or offering “more of the same” by increasing the number of qubits. For the sake of clarity,
the stages and tests below target systems that prepare and transmit qubits, but it is also possible
to phrase both in terms of qudits or continuous variables. For each stage, we describe some of
the application protocols that are already known and that can be realized with the functionality
provided in that stage. It is conceivable that a simpler protocol, or better theoretical analysis,
may be found in the future that solves the same task, but is less demanding in terms of function-
ality. In parallel to the daunting experimental challenges in making quantum internet a reality,
there is thus a challenge for quantum software developers to design protocols that can realize a
task in a stage that can be implemented more easily. We identify relevant parameters for each
stage to establish a common language between hardware and software developers. These pa-
rameters can be estimated using a series of simple tests, allowing us to certify the performance
of an experimental implementation in attaining a specific stage, as well as the performance of
protocols depending on these parameters. So far, most application protocols have only been an-
alyzed for perfect parameters. As such, the exact requirements of many application protocols on
these parameters have not yet been determined, and deserve much needed future investigation.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Quantum internet: a vision for the road ahead" ?

In this paper, the authors identify stages of development towards a full-blown quantum internet and discuss the essential question of what is needed to actually enable useful applications. 

Q2. What is the status of long-distance quantum networks?

The current status of long-distance quantum networks is at the lowest stage - trusted-repeater networks - with several commercial systems for quantum key distribution on the market. 

Q3. What is the striking example of a quantum advantage in distributed systems?

One of the most striking examples of a quantum advantage in distributed systems can be found for the task of byzantine agreement. 

Q4. What are the intrinsic coherence times of platforms?

The intrinsic coherence times of most above-mentioned platforms are very long (for instance, more than a second for ions and NV centers). 

Q5. What is the definition of fault tolerance?

Fault tolerance is not necessary for many known quantum internet protocols, but fault-tolerant operations available would allow the execution of local quantum computation of high circuit depth, as well as an (in theory) arbitrary extension of storage times to execute protocols with an arbitrary number of rounds of communication. 

Q6. What is the way to achieve provable security for all such tasks?

it is possible to achieve provable security for all such relevant tasks by sending more qubits than the adversary can store easily within a short time frame, known as the bounded (29), or more generally noisystorage model (30, 31). 

Q7. What is the reason why quantum communication can help solve these problems?

Very intuitively, the reason why quantum communication can help solve these problems is that entanglement allows coordination among distant processors that greatly surpasses what is possible classically. 

Q8. What is the main advantage of a radically different approach to quantum repeaters?

It is noteworthy that a radically different approach to quantum repeaters has emerged in recent years in which the quantum state of interest is encoded in multiple photons such that error correction performed at the repeater stations can erase errors due to photon loss and decoherence during transmission (96–99). 

Q9. What is the well-known application of a quantum internet?

The most well-known application of a quantum internet is quantum key distribution (QKD), which enables two remote network nodes to establish an encryption key1Some researchers still go one step further and believe all communication will eventually be done over quantum channels (1).whose security only relies on the laws of quantum mechanics.