Applications of an Entangled Quantum Internet
Rodney Van Meter
Faculty of Environment and Information Studies
Keio University
5322 Endo, Fujisawa, Kanagawa, 252-8520, Japan
Email: rdv@sfc.wide.ad.jp
Byung-Soo Choi
Department of Electronics Engineering
Ewha Womans University
11-1 Daehyun-dong, Seodaemun-gu, Seoul, 120-750, South Korea
Email: bschoi3@gmail.com

Abstract

that it is impossible to fully describe the state of only oneof the members; the state of the entire system is correlated.
Consider a pair of quanta with quantum spin as the stateof interest. An entangled state, for example, might be one
in which either both members of the pair are spinning with
their axes pointed “up”, or both members are spinning with
their axes pointed “down”, but the state is definitely not
one up and one down, and which state they are in is notknown. When one of the members of the pair is measured,
the other’s state then becomes determined, even if the pairis separated by a large distance. Einstein famously referredto this phenomenon as “spooky action at a distance,” but it
was later shown that entanglement cannot be used to violatecausality or to transmit information faster than the speed oflight.
We begin with a brief, qualitative description of the operation of a network of quantum repeaters, followed bysome possible uses of the entanglement. Most of the uses
presented here are highly speculative, and have not yet been
analyzed with any mathematical rigor; this paper should beviewed as a discussion of ideas rather than confirmed facts.
We conclude with a short discussion of future work.

Physicists and engineers are making technical progress
toward the creation of intercontinental quantum networks.
But if they succeed, what new applications will a quantum
Internet enable? This paper presents a series of potential
uses, some fairly well-established and some highly speculative. The entanglement generated by a quantum network
will be useful both as a digital computational resource,
and as a gyroscopic reference, providing both phase (time)and directional information. Computational applications
include the well-known quantum key distribution (QKD)
process and distributed leader election, as well as the traditional uses of networks to connect geographically distributed resources. The gyroscopic reference uses are morespeculative, but include the possibility of improving some“Big Science” projects by utilizing quantum entanglement
to beat single-system quantum limits on precision measurements, including the accuracy of clocks.

1 Introduction

2Quantum networks, if they can be built, may enable new
applications, including both distributed, digital computation and large-scale analog or semi-digital systems used as
quantum sensor networks. Quantum networks come in twoflavors, those that will operate by creating entanglement
between two (or more) distant quantum systems, and thosethat do not. In this paper, we focus on entangling quantum
networks, and in particular uses of a wide-area, many-node
network [15, 3].
Quantum systems can have a shared state of a form such

Quantum Repeaters

Large-scale quantum networks may be built using verysmall, special-purpose quantum computers that create andmaintain distributed quantum state. The network nodes arecalled “quantum repeaters”, but serve a role equivalent tothat of Internet routers, rather than analog signal repeaters.
They require only minimal quantum resources, and are notfully general quantum computers themselves, but involvea large classical component in executing the algorithms,1

of random numbers between two nodes, using the laws ofquantum mechanics to detect the presence of an eavesdropper. The shared stream of random numbers can then beused as a key, either for IPsec or as a one-time pad. QKD,
therefore, is likely to be deployed first highly secure network environments, such a intra-bank and intelligence networks. QKD is also being considered as a technology fore.g. digital cash; in that context, QKD all the way from acentralized server to a user’s home PC or to an ATM may
have value.
Experimentally, QKD has already been implemented,
and demonstration networks are in operation in the U.S.,
Europe, and Japan [7, 1, 16, 19]. Single-hop QKD necessarily faces an exponential decline in throughput as the link
lengthens. If multiple unentangled hops are used, the intermediate nodes must be trusted, which is a serious drawback
in a secure network architecture.
Although the existing networks and available QKD
products do not depend on long-lived, long-distance entanglement, it is known that it is possible to use entanglement
to execute QKD [6], meaning that are expected can be used
to extend the distance. Using entanglement and repeaters
allows the intermediate nodes to be untrusted. Thus, thefirst likely use of an entangled quantum network will be
QKD.
One distributed quantum algorithm that may serve as abuilding block for more complex uses is leader election inanonymous networks, which utilizes entanglement to deterministically choose one node from among a set of equals
in a deterministic number of rounds [20]. Quantum leader
election runs in a polynomial number of rounds, and always
chooses a leader deterministically, whereas no classical algorithm can elect a leader in deterministic time, given thesame set of constraints. Leader election was recently experimentally demonstrated [17]. Quantum leader election has
been shown to work in arbitrary network graphs, one of the
few quantum problems to have been addressed in such a realistic environment. This algorithm represents an important
theoretical advance, and may be a useful building blockfor larger quantum algorithms, but likely would not independently serve to spur development of quantum networks.
Classical networks are, in general, not completely anonymous, and a variety of algorithms for solving the problem
in non-anonymous networks are known, and are in use fora variety of problems.
Secret sharing is an important cryptographic primitive.
In quantum secret sharing, a secret, consisting of a numberof qubits, can be distributed to a number of participants,
so that only a subset of the participants can reconstruct theoriginal secret [11]. In a classical secret sharing scheme,
the number of collaborators k required to reconstruct thesecret can be any number, 1 ≤ k ≤ n, where n is the number of secret shares. In a quantum secret sharing scheme,to satisfy the no-cloning theorem, n/2 ≤ k ≤ n [10, 26].

the performance which is primarily limited by the speedof light.
Quantum repeaters will utilize three concepts to execute a distributed algorithm that creates entangled quantum states between nodes that are far apart: a basic entanglement mechanism, entanglement swapping and purification [2, 27]. Entanglement swapping extends the spanof the entanglement from single-hop distances to arbitrary
nodes throughout the network, while purification is a specialized form of error correction. An entangled pair ofqubits is referred to as a Bell pair.
Figure 1 presents the protocol stack for a quantum repeater network as defined by Van Meter et al. [22]. The
middle layers of the stack implement a distributed algorithm that turns short-distance, medium-fidelity entangled
Bell pairs into long-distance, high-fidelity Bell pairs. Thus,
they collectively achieve the reliable transport that is acommon characteristic of ISO Layer 4 implementations,but its distributed nature makes it questionable to simply
call the total process the “transport” layer. Unlike classicalnetworks, information does not simply propagate along thepath one hop at a time. All of the repeaters along the entire
path are repeatedly involved in creating each end-to-end
Bell pair.
Research on the physical mechanisms for transmitting
quantum states typically assumes transmission through afiber, but free-space optical links and even satellite links
can also be used, with repeater nodes at each end of thelink [25].

3 Applications of Entanglement
We can safely assert that, if one quantum computer isuseful, then a network of them is more useful; indeed,that has already been shown for quantum multicomputers [24, 23]. But why would we want the computers tobe geographically distributed? As in classical systems, it
may be the case that large quantum datasets, or large computational facilities, are in different locations for political,
economic, or simply historical reasons. The motivation
for creating a geographically distributed quantum network,
then, is the same as for a classical network: to connect resources that are not, or cannot be, colocated. Often, the“resources” are human beings. Because quantum entanglement is a physical resource, as well as a computational one,the consumers of the entanglement can be either quantum
computers or scientific instruments.

3.1

Digital Uses

The most well-known use of quantum networks to dateis quantum key distribution, or QKD. QKD uses a quantum channel and an authenticated, but not necessarily secret, public channel to create a “tamper-proof” shared set
2

End−to−end

Application

Final end−to−end

Purification
Control (PC)

Repeated ateach separate
distance

Distance=1
single hop only

Application Layer

Distributed computation
executed along entire pathfrom source to destination
implementing reliable
delivery (Transport Layer?)

Entanglement
Swapping Control (ESC)
Purification
Control (PC)
Entanglement
Control (EC)Classicalcommunication
onlyLink Layer

PhysicalEntanglement (PE)Physical Layer

Quantum
communication

Figure 1. Quantum repeater protocol stackstate could improve the sensitivity; it may be possible toconvert an entangled Bell pair to coordinated squeezed
states at both locations that can be used.
Distributed quantum algorithms have also been proposed that will synchronize clocks to better-than-atomicclock precision over a distance [5, 14, 12]. Is it possiblethat a worldwide quantum network could provide a clockreference that is better than GPS? What value would sucha network have?
GPS is known to provide synchronization to about 0.11.0 nanosecond (10−9 s to 10−10 s) [13], and further improvement is considered to be possible. Optical latticeclocks (a candidate to succeed atomic clocks, whose precision is nearly played out), operate with an accuracy of
one part in 1015 , and further improvements (possibly as far
as one part in 1018 ) are expected [18]. At that level of accuracy, the relativistic gravitational red shift of frequencies
will be measurable with an altitude difference of 1cm!
One obvious demanding application for clock synchronization is very long baseline interferometry (VLBI), aform of radio astronomy. However, the current operational
accuracy of ∼ 1 nsec is considered acceptable, and the GPSsystem can be improved by several orders of magnitude.
Interferometry does not require improvement of accuracy
linear in the wavelength being used, so the shift from performing interferometry on radio frequencies to optical frequencies may be feasible even with current technology.

Quantum secret sharing has been experimentally demonstrated [21]. As with leader election, secret sharing seems
likely to be a useful building block but not provide the incentive to actually build out large-scale quantum networks.

3.2

Gyroscopic Uses

Beyond the uses with the familiar feel of digital behavior (albeit with the complexity of quantum mechanics), wenote that high-fidelity entanglement can serve as a gyroscopic reference. That reference provides both time anddirectional synchronization. Are there ways in which thiseffect can be used?
The most obvious areas in which to pursue these kindsof uses would be in “Big Science” applications that might
desire long-distance clock synchronization or directional
information. Entangled quantum states are known to improve the precision of measuring a variety of phenomena,
though those algorithms have not been adapted to use theform of entanglement we will generate using a networkof quantum repeaters, and not all forms of entanglement
are easily transformable into each other. Giovannetti etal. described a variety of uses of entanglement in scientific applications, including improved interferometry, positional measurement, tests of relativity, and metrology [8].
Some of these are best suited to single-laboratory work,
others, such as the tests of relativity, are inherently distributed. We are most interested in those that use entanglement as gyroscopic information, including time reference
and directional reference (which Giovannetti called coordinate transfer).
The LIGO gravity wave observatory can use “squeezedlight”, which is a highly non-classical state of light, to improve its precision for measuring vibrational movement ofthe system within one location [9]. It is an open question
whether entanglement between Lousiana and Washington

4Future WorkTo date, repeater research has primarily focused on thephysical and mathematical tools for building repeaters,some of which have been experimentally demonstrated [4],but little direct attention has been given to the need for
network protocols to manage the information flow nor thenecessity of consistent decisions being made in a widely3

distributed system in a timely fashion. Routing and resource management in quantum networks are important
problems [22].
Finally, we wish to note that the term internet is appropriate in this context; quantum networks are highly likely
to involve different physical entanglement mechanisms, including different wavelengths of light and physical qubit
representations, purification algorithms, and routing algorithms, and likely will span multiple administrative domains. Bridging those differences will exercise many of
the same organizational and technological capabilities built
up in the Internet community over the last forty years.
Entangled quantum networks are on the technologicalhorizon, and it is time to focus more effort on finding uses
for the technology. Quantum key distribution is alreadycommercially available for single hops, and should reachdistances of 100km or so; extending that distance will require repeaters and the use of entanglement. Additional
new functionality, such as leader election and secret sharing, provide distributed building blocks for new computational uses, and entanglement is a promising avenue for improvement in various scientific experiments. We expect theresults to more than repay the R&D costs with new functionality, some of which has not yet been conceived.

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Acknowledgments
The authors thank T.D. Ladd, W.J. Munro, G. Milburn,K. Nemoto, C. Walker, and M. Yun for interesting conversations, though we alone bear the responsibility for thewildness of any included ideas.

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