Quantum Internet Research Group W. Kozlowski
Internet-Draft S. Wehner
Intended status: Informational QuTech
Expires: 18 June 2022 R. Van Meter
Keio University
B. Rijsman
Individual
A. S. Cacciapuoti
M. Caleffi
University of Naples Federico II
S. Nagayama
Mercari, Inc.
15 December 2021
Architectural Principles for a Quantum Internet
draft-irtf-qirg-principles-08
Abstract
The vision of a quantum internet is to fundamentally enhance Internet
technology by enabling quantum communication between any two points
on Earth. To achieve this goal, a quantum network stack should be
built from the ground up to account for the fundamentally new
properties of quantum entanglement. The first realisations of
quantum entanglement networks are imminent, but there is no practical
proposal for how to organise, utilise, and manage such networks. In
this memo, we attempt to lay down the framework and introduce some
basic architectural principles for a quantum internet. This is
intended for general guidance and general interest, but also to
provide a foundation for discussion between physicists and network
specialists. This document is a product of the Quantum Internet
Research Group (QIRG).
Status of This Memo
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provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 18 June 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Quantum information . . . . . . . . . . . . . . . . . . . . . 4
2.1. Qubit . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Multiple qubits . . . . . . . . . . . . . . . . . . . . . 5
3. Entanglement as the fundamental resource . . . . . . . . . . 7
4. Achieving quantum connectivity . . . . . . . . . . . . . . . 8
4.1. Challenges . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. The measurement problem . . . . . . . . . . . . . . . 8
4.1.2. No-cloning theorem . . . . . . . . . . . . . . . . . 9
4.1.3. Fidelity . . . . . . . . . . . . . . . . . . . . . . 9
4.1.4. Inadequacy of direct transmission . . . . . . . . . . 10
4.2. Bell pairs . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Teleportation . . . . . . . . . . . . . . . . . . . . . . 11
4.4. The life cycle of entanglement . . . . . . . . . . . . . 12
4.4.1. Elementary link generation . . . . . . . . . . . . . 12
4.4.2. Entanglement swapping . . . . . . . . . . . . . . . . 13
4.4.3. Error Management . . . . . . . . . . . . . . . . . . 14
4.4.4. Delivery . . . . . . . . . . . . . . . . . . . . . . 17
5. Architecture of a quantum internet . . . . . . . . . . . . . 17
5.1. Challenges . . . . . . . . . . . . . . . . . . . . . . . 18
5.2. Classical communication . . . . . . . . . . . . . . . . . 19
5.3. Abstract model of the network . . . . . . . . . . . . . . 20
5.3.1. The control and data planes . . . . . . . . . . . . . 20
5.3.2. Elements of a quantum network . . . . . . . . . . . . 21
5.3.3. Putting it all together . . . . . . . . . . . . . . . 22
5.4. Network boundaries . . . . . . . . . . . . . . . . . . . 23
5.4.1. Boundaries between different physical
architectures . . . . . . . . . . . . . . . . . . . . 23
5.4.2. Boundaries between different administrative
regions . . . . . . . . . . . . . . . . . . . . . . . 24
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5.4.3. Boundaries between different error management
schemes . . . . . . . . . . . . . . . . . . . . . . . 24
5.5. Physical constraints . . . . . . . . . . . . . . . . . . 24
5.5.1. Memory lifetimes . . . . . . . . . . . . . . . . . . 24
5.5.2. Rates . . . . . . . . . . . . . . . . . . . . . . . . 25
5.5.3. Communication qubits . . . . . . . . . . . . . . . . 25
5.5.4. Homogeneity . . . . . . . . . . . . . . . . . . . . . 25
6. Architectural principles . . . . . . . . . . . . . . . . . . 25
6.1. Goals of a quantum internet . . . . . . . . . . . . . . . 26
6.2. The principles of a quantum internet . . . . . . . . . . 29
7. A thought experiment inspired by classical networks . . . . . 31
8. Security Considerations . . . . . . . . . . . . . . . . . . . 33
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 33
11. Informative References . . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
Quantum networks are distributed systems of quantum devices that
utilise fundamental quantum mechanical phenomena such as
superposition, entanglement, and quantum measurement to achieve
capabilities beyond what is possible with non-quantum (classical)
networks [Kimble08]. Depending on the stage of a quantum network
[Wehner18] such devices may range from simple photonic devices
capable of preparing and measuring only one quantum bit (qubit) at a
time all the way to large-scale quantum computers of the future. A
quantum network is not meant to replace classical networks, but
rather form an overall hybrid classical-quantum network supporting
new capabilities which are otherwise impossible to realise
[VanMeterBook].
This new networking paradigm offers promise for a range of new
applications such as quantum cryptography [Bennett14] [Ekert91],
distributed quantum computation [Crepeau02], secure quantum computing
in the cloud [Fitzsimons17], quantum-enhanced measurement networks
[Giovanetti04], or higher-precision, long-baseline telescopes
[Gottesman12]. The field of quantum communication has been a subject
of active research for many years and the most well-known application
of quantum communication, quantum key distribution (QKD) for secure
communications, has already been deployed at short (roughly 100km)
distances [Elliott03] [Peev09] [Aguado19].
Fully quantum networks capable of transmitting and managing entangled
quantum states in order to send, receive, and manipulate distributed
quantum information are now imminent [Castelvecchi18] [Wehner18].
Whilst a lot of effort has gone into physically realising and
connecting such devices [Hensen15], and making improvements to their
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speed and error tolerance, there are no worked out proposals for how
to run these networks. To draw an analogy with a classical network,
we are at a stage where we can start to physically connect our
devices and send data, but all sending, receiving, buffer management,
connection synchronisation, and so on, must be managed by the
application itself at a level below conventional assembly language,
where no common interfaces yet exist. Furthermore, whilst physical
mechanisms for transmitting quantum states exist, there are no robust
protocols for managing such transmissions.
This document, the result of the Quantum Internet Research Group
(QIRG), introduces the subject matter for quantum networks and
presents general guidelines for the design and construction of such
networks. Overall, it is intended as an introduction to the subject
for network engineers and researchers. It should not be considered
as a conclusive statement on how quantum network should or will be
implemented. This document was discussed on the QIRG mailing list
and several IETF meetings and represents the consensus of the QIRG
members, both of experts in the subject matter (from the quantum as
well networking domain) as well as newcomers who are the target
audience.
2. Quantum information
In order to understand the framework for quantum networking, a basic
understanding of quantum information is necessary. The following
sections aim to introduce the bare minimum necessary to understand
the principles of operation of a quantum network. This exposition
was written with a classical networking audience in mind. It is
assumed that the reader has never before been exposed to any quantum
physics. We refer to e.g. [SutorBook] [NielsenChuang] for an in-
depth introduction to quantum information.
2.1. Qubit
The differences between quantum computation and classical computation
begin at the bit-level. A classical computer operates on the binary
alphabet { 0, 1 }. A quantum bit, called a qubit, exists over the
same binary space, but unlike the classical bit, it can exist in a
superposition of the two possibilities:
a |0> + b |1>,
where |X> is Dirac's ket notation for a quantum state, here the
binary 0 and 1, and the coefficients a and b are complex numbers
called probability amplitudes. Physically, such a state can be
realised using a variety of different technologies such as electron
spin, photon polarisation, atomic energy levels, and so on.
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Upon measurement, the qubit loses its superposition and irreversibly
collapses into one of the two basis states, either |0> or |1>. Which
of the two states it ends up in may not be deterministic, but can be
determined from the readout of the measurement. The measurement
result is a classical bit, 0 or 1, corresponding to |0> and |1>
respectively. The probability of measuring the state in the |0>
state is |a|^2 and similarly the probability of measuring the state
in the |1> state is |b|^2, where |a|^2 + |b|^2 = 1. This randomness
is not due to our ignorance of the underlying mechanisms, but rather
is a fundamental feature of a quantum mechanical system [Aspect81].
The superposition property plays an important role in fundamental
gate operations on qubits. Since a qubit can exist in a
superposition of its basis states, the elementary quantum gates are
able to act on all states of the superposition at the same time. For
example, consider the NOT gate:
NOT (a |0> + b |1>) -> a |1> + b |0>.
2.2. Multiple qubits
When multiple qubits are combined in a single quantum state the space
of possible states grows exponentially and all these states can
coexist in a superposition. For example, the general form of a two-
qubit register is
a |00> + b |01> + c |10> + d |11>
where the coefficients have the same probability amplitude
interpretation as for the single qubit state. Each state represents
a possible outcome of a measurement of the two-qubit register. For
example, |01> denotes a state in which the first qubit is in the
state |0> and the second is in the state |1>.
Performing single qubit gates affects the relevant qubit in each of
the superposition states. Similarly, two-qubit gates also act on all
the relevant superposition states, but their outcome is far more
interesting.
Consider a two-qubit register where the first qubit is in the
superposed state (|0> + |1>)/sqrt(2) and the other is in the
state |0>. This combined state can be written as:
(|0> + |1>)/sqrt(2) x |0> = (|00> + |10>)/sqrt(2),
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where x denotes a tensor product (the mathematical mechanism for
combining quantum states together). Let us now consider the two-
qubit controlled-NOT, or CNOT, gate. The CNOT gate takes as input
two qubits, a control and target, and applies the NOT gate to the
target if the control qubit is set. The truth table looks like
+====+=====+
| IN | OUT |
+====+=====+
| 00 | 00 |
+----+-----+
| 01 | 01 |
+----+-----+
| 10 | 11 |
+----+-----+
| 11 | 10 |
+----+-----+
Table 1
Now, consider performing a CNOT gate on the state with the first
qubit being the control. We apply a two-qubit gate on all the
superposition states:
CNOT (|00> + |10>)/sqrt(2) -> (|00> + |11>)/sqrt(2).
What is so interesting about this two-qubit gate operation? The
final state is *entangled*. There is no possible way of representing
that quantum state as a product of two individual qubits; they are no
longer independent and the behaviour of either qubit cannot be fully
described without accounting for the other qubit. The states of the
two individual qubits are now correlated beyond what is possible to
achieve classically. Neither qubit is in a definite |0> or |1>
state, but if we perform a measurement on either one, the outcome of
the partner qubit will *always* yield the exact same outcome. The
final state, whether it's |00> or |11>, is fundamentally random as
before, but the states of the two qubits following a measurement will
always be identical.
Once a measurement is performed, the two qubits are once again
independent. The final state is either |00> or |11> and both of
these states can be trivially decomposed into a product of two
individual qubits. The entanglement has been consumed and the
entangled state must be prepared again.
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3. Entanglement as the fundamental resource
Entanglement is the fundamental building block of quantum networks.
Consider the state from the previous section:
(|00> + |11>)/sqrt(2).
Neither of the two qubits is in a definite |0> or |1> state and we
need to know the state of the entire register to be able to fully
describe the behaviour of the two qubits.
Entangled qubits have interesting non-local properties. Consider
sending one of the qubits to another device. This device could in
principle be anywhere: on the other side of the room, in a different
country, or even on a different planet. Provided negligible noise
has been introduced, the two qubits will forever remain in the
entangled state until a measurement is performed. The physical
distance does not matter at all for entanglement.
This lies at the heart of quantum networking, because it is possible
to leverage the non-classical correlations provided by entanglement
in order to design completely new types of application protocols that
are not possible to achieve with just classical communication.
Examples of such applications are quantum cryptography [Bennett14]
[Ekert91], blind quantum computation [Fitzsimons17], or distributed
quantum computation [Crepeau02].
Entanglement has two very special features from which one can derive
some intuition about the types of applications enabled by a quantum
network.
The first stems from the fact that entanglement enables stronger than
classical correlations, leading to opportunities for tasks that
require coordination. As a trivial example, consider the problem of
consensus between two nodes who want to agree on the value of a
single bit. They can use the quantum network to prepare the state
(|00> + |11>)/sqrt(2) with each node holding one of the two qubits.
Once either of the two nodes performs a measurement, the state of the
two qubits collapses to either |00> or |11>, so whilst the outcome is
random and does not exist before measurement, the two nodes will
always measure the same value. We can also build the more general
multi-qubit state (|00...> + |11...>)/sqrt(2) and perform the same
algorithm between an arbitrary number of nodes. These stronger than
classical correlations generalise to more complicated measurement
schemes as well.
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The second feature of entanglement is that it cannot be shared, in
the sense that if two qubits are maximally entangled with each other,
then it is physically impossible for any other system to have any
share of this entanglement [Terhal04]. Hence, entanglement forms a
sort of private and inherently untappable connection between two
nodes once established.
Entanglement is created through local interactions between two qubits
or as a product of the way the qubits were created (e.g. entangled
photon pairs). To create a distributed entangled state, one can then
physically send one of the qubits to a remote node. It is also
possible to directly entangle qubits that are physically separated,
but this still requires local interactions between some other qubits
that the separated qubits are initially entangled with. Therefore,
it is the transmission of qubits that draws the line between a
genuine quantum network and a collection of quantum computers
connected over a classical network.
A quantum network is defined as a collection of nodes that is able to
exchange qubits and distribute entangled states amongst themselves.
A quantum node that is able only to communicate classically with
another quantum node is not a member of a quantum network.
More complex services and applications can be built on top of
entangled states distributed by the network, see e.g. [ZOO]
4. Achieving quantum connectivity
This section explains the meaning of quantum connectivity and the
necessary physical processes at an abstract level.
4.1. Challenges
A quantum network cannot be built by simply extrapolating all the
classical models to their quantum analogues. Sending qubits over a
wire like we send classical bits is simply not as easy to do. There
are several technological as well as fundamental challenges that make
classical approaches unsuitable in a quantum context.
4.1.1. The measurement problem
In classical computers and networks we can read out the bits stored
in memory at any time. This is helpful for a variety of purposes
such as copying, error detection and correction, and so on. This is
not possible with qubits.
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A measurement of a qubit's state will destroy its superposition and
with it any entanglement it may have been part of. Once a qubit is
being processed, it cannot be read out until a suitable point in the
computation, determined by the protocol handling the qubit, has been
reached. Therefore, we cannot use the same methods known from
classical computing for the purposes of error detection and
correction. Nevertheless, quantum error detection and correction
schemes exist that take this problem into account and how a network
chooses to manage errors will have an impact on its architecture.
4.1.2. No-cloning theorem
Since directly reading the state of a qubit is not possible, one
could ask if we can simply copy a qubit without looking at it.
Unfortunately, this is fundamentally not possible in quantum
mechanics [Park70] [Wootters82].
The no-cloning theorem states that it is impossible to create an
identical copy of an arbitrary, unknown quantum state. Therefore, it
is also impossible to use the same mechanisms that worked for
classical networks for signal amplification, retransmission, and so
on as they all rely on the ability to copy the underlying data.
Since any physical channel will always be lossy, connecting nodes
within a quantum network is a challenging endeavour and its
architecture must at its core address this very issue.
4.1.3. Fidelity
In general, it is expected that a classical packet arrives at its
destination without any errors introduced by hardware noise along the
way. This is verified at various levels through a variety of error
detection and correction mechanisms. Since we cannot read or copy a
quantum state error detection and correction is more involved.
To describe the quality of a quantum state, a physical quantity
called fidelity is used [NielsenChuang]. Fidelity takes a value
between 0 and 1 -- higher is better, and less than 0.5 means the
state is unusable. It measures how close a quantum state is to the
state we have tried to create. It expresses the probability that one
state will pass a test to identify as the other. Fidelity is an
important property of a quantum system that allows us to quantify how
much a particular state has been affected by noise from various
sources (gate errors, channel losses, environment noise).
Interestingly, quantum applications do not need perfect fidelity to
be able to execute -- as long as the fidelity is above some
application-specific threshold, they will simply operate at lower
rates. Therefore, rather than trying to ensure that we always
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deliver perfect states (a technologically challenging task)
applications will specify a minimum threshold for the fidelity and
the network will try its best to deliver it. A higher fidelity can
be achieved by either having hardware produce states of better
fidelity (sometimes one can sacrifice rate for higher fidelity) or by
employing quantum error detection and correction mechanisms.
4.1.4. Inadequacy of direct transmission
Conceptually, the most straightforward way to distribute an entangled
state is to simply transmit one of the qubits directly to the other
end across a series of nodes while performing sufficient forward
quantum error correction (Section 4.4.3.2) to bring losses down to an
acceptable level. Despite the no-cloning theorem and the inability
to directly measure a quantum state, error-correcting mechanisms for
quantum communication exist [Jiang09] [Fowler10] [Devitt13]
[Mural16]. However, quantum error correction makes very high demands
on both resources (physical qubits needed) and their initial
fidelity. Implementation is very challenging and quantum error
correction is not expected to be used until later generations of
quantum networks.
An alternative relies on the observation that we do not need to be
able to distribute any arbitrary entangled quantum state. We only
need to be able to distribute any one of what are known as the Bell
pair states [Briegel98].
4.2. Bell pairs
Bell pair states are the entangled two-qubit states:
|00> + |11>, |00> - |11>, |01> + |10>, |01> - |10>,
where the constant 1/sqrt(2) normalisation factor has been ignored
for clarity. Any of the four Bell pair states above will do, as it
is possible to transform any Bell pair into another Bell pair with
local operations performed on only one of the qubits. When each
qubit in a Bell pair is held by a separate node, either node can
apply a series of single qubit gates to their qubit alone in order to
transform the state between the different variants.
Distributing a Bell pair between two nodes is much easier than
transmitting an arbitrary quantum state over a network. Since the
state is known, handling errors becomes easier and small-scale error-
correction (such as entanglement distillation discussed in a later
section) combined with reattempts becomes a valid strategy.
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The reason for using Bell pairs specifically as opposed to any other
two-qubit state is that they are the maximally entangled two-qubit
set of basis states. Maximal entanglement means that these states
have the strongest non-classical correlations of all possible two-
qubit states. Furthermore, since single-qubit local operations can
never increase entanglement, less entangled states would impose some
constraints on distributed quantum algorithms. This makes Bell pairs
particularly useful as a generic building block for distributed
quantum applications.
4.3. Teleportation
The observation that we only need to be able to distribute Bell pairs
relies on the fact that this enables the distribution of any other
arbitrary entangled state. This can be achieved via quantum state
teleportation [Bennett93]. Quantum state teleportation consumes an
unknown qubit state that we want to transmit and recreates it at the
desired destination. This does not violate the no-cloning theorem as
the original state is destroyed in the process.
To achieve this, an entangled pair needs to be distributed between
the source and destination before teleportation commences. The
source then entangles the transmission qubit with its end of the pair
and performs a read out of the two qubits (the sum of these
operations is called a Bell state measurement). This consumes the
Bell pair's entanglement, turning the source and destination qubits
into independent states. The measurements yields two classical bits
which the source sends to the destination over a classical channel.
Based on the value of the received two classical bits, the
destination performs one of four possible corrections (called the
Pauli corrections) on its end of the pair, which turns it into the
unknown qubit state that we wanted to transmit. This requirement to
communicate the measurement read out over a classical channel
unfortunately means that entanglement cannot be used to transmit
information faster than the speed of light.
The unknown quantum state that was transmitted was never fed into the
network itself. Therefore, the network needs to only be able to
reliably produce Bell pairs between any two nodes in the network.
Thus, a key difference between a classical and quantum data planes is
that a classical one carries user data, but a quantum data plane
provides the resources for the user to transmit user data themselves
without further involvement of the network.
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4.4. The life cycle of entanglement
Reducing the problem of quantum connectivity to one of generating a
Bell pair has facilitated the problem, but it has not solved it. In
this section, we discuss how these entangled pairs are generated in
the first place, and how their two qubits are delivered to the end-
points.
4.4.1. Elementary link generation
In a quantum network, entanglement is always first generated locally
(at a node or an auxiliary element) followed by a movement of one or
both of the entangled qubits across the link through quantum
channels. In this context, photons (particles of light) are the
natural candidate for entanglement carriers, called flying qubits.
The rationale for this choice is related to the advantages provided
by photons such as moderate interaction with the environment leading
to moderate decoherence, convenient control with standard optical
components, and high-speed, low-loss transmissions. However, since
photons are hard to store, a transducer must transfer the flying
qubit's state to a qubit suitable for information processing and/or
storage (often referred to as a matter qubit).
Since this process may fail, in order to generate and store
entanglement efficiently, we must be able to distinguish successful
attempts from failures. Entanglement generation schemes that are
able to announce successful generation are called heralded
entanglement generation schemes.
There exist three basic schemes for heralded entanglement generation
on a link through coordinated action of the two nodes at the two ends
of the link [Cacciapuoti19]:
* "At mid-point": in this scheme an entangled photon pair source
sitting midway between the two nodes with matter qubits sends an
entangled photon through a quantum channel to each of the nodes.
There, transducers are invoked to transfer the entanglement from
the flying qubits to the matter qubits. In this scheme, the
transducers know if the transfers succeeded and are able to herald
successful entanglement generation via a message exchange over the
classical channel.
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* "At source": in this scheme one of the two nodes sends a flying
qubit that is entangled with one of its matter qubits. A
transducer at the other end of the link will transfer the
entanglement from the flying qubit to one of its matter qubits.
Just like in the previous scheme, the transducer knows if its
transfer succeeded and is able to herald successful entanglement
generation with a classical message sent to the other node.
* "At both end-points": in this scheme both nodes send a flying
qubit that is entangled with one of their matter qubits. A
detector somewhere in between the nodes performs a joint
measurement on the two qubits, which stochastically projects the
remote matter qubits into an entangled quantum state. The
detector knows if the entanglement succeeded and is able to herald
successful entanglement generation by sending a message to each
node over the classical channel.
The "mid-point source" scheme is more robust to photon loss, but in
the other schemes the nodes retain greater control over the entangled
pair generation.
Note that whilst photons travel in a particular direction through the
quantum channel the resulting entangled pair of qubits does not have
a direction associated with it. Physically, there is no upstream or
downstream end of the pair.
4.4.2. Entanglement swapping
The problem with generating entangled pairs directly across a link is
that efficiency decreases with channel length. Beyond a few 10s of
kilometres in optical fibre or 1000 kilometres in free space (via
satellite) the rate is effectively zero and due to the no-cloning
theorem we cannot simply amplify the signal. The solution is
entanglement swapping [Briegel98].
A Bell pair between any two nodes in the network can be constructed
by combining the pairs generated along each individual link on a path
between the two end-points. Each node along the path can consume the
two pairs on the two links that it is connected to in order to
produce a new entangled pair between the two remote ends. This
process is known as entanglement swapping. Pictorially it can be
represented as follows:
+---------+ +---------+ +---------+
| A | | B | | C |
| |------| |------| |
| X1~~~~~~~~~~X2 Y1~~~~~~~~~~Y2 |
+---------+ +---------+ +---------+
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where X1 and X2 are the qubits of the entangled pair X and Y1 and Y2
are the qubits of entangled pair Y. The entanglement is denoted with
~~. In the diagram above, nodes A and B share the pair X and nodes B
and C share the pair Y, but we want entanglement between A and C.
To achieve this goal, we simply teleport the qubit X2 using the pair
Y. This requires node B to perform a Bell state measurement on the
qubits X2 and Y1 which result in the destruction of the entanglement
between Y1 and Y2. However, X2 is recreated in Y2's place, carrying
with it its entanglement with X1. The end-result is shown below:
+---------+ +---------+ +---------+
| A | | B | | C |
| |------| |------| |
| X1~~~~~~~~~~~~~~~~~~~~~~~~~~~X2 |
+---------+ +---------+ +---------+
Depending on the needs of the network and/or application, a final
Pauli correction at the recipient node may not be necessary since the
result of this operation is also a Bell pair. However, the two
classical bits that form the read out from the measurement at node B
must still be communicated, because they carry information about
which of the four Bell pairs was actually produced. If a correction
is not performed, the recipient must be informed which Bell pair was
received.
This process of teleporting Bell pairs using other entangled pairs is
called entanglement swapping. Quantum nodes that create long-
distance entangled pairs via entanglement swapping are called quantum
repeaters in academic literature [Briegel98] and we will use the same
terminology in this memo.
4.4.3. Error Management
4.4.3.1. Distillation
Neither the generation of Bell pairs nor the swapping operations are
noiseless operations. Therefore, with each link and each swap the
fidelity of the state degrades. However, it is possible to create
higher fidelity Bell pair states from two or more lower fidelity
pairs through a process called distillation (sometimes also referred
to as purification) [Dur07].
To distil a quantum state, a second (and sometimes third) quantum
state is used as a "test tool" to test a proposition about the first
state, e.g., "the parity of the two qubits in the first state is
even." When the test succeeds, confidence in the state is improved,
and thus the fidelity is improved. The test tool states are
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destroyed in the process, so resource demands increase substantially
when distillation is used. When the test fails, the tested state
must also be discarded. Distillation makes low demands on fidelity
and resources compared to quantum error correction, but distributed
protocols incur round-trip delays due to classical communication
[Bennett96].
4.4.3.2. Quantum Error Correction
Just like classical error correction, quantum error correction (QEC)
encodes logical qubits using several physical (raw) qubits to protect
them from errors described in Section 4.1.3 [Jiang09] [Fowler10]
[Devitt13] [Mural16]. Furthermore, similarly to its classical
counterpart, QEC can not only correct state errors but also account
for lost qubits. Additionally, if all physical qubits which encode a
logical qubit are located at the same node, the correction procedure
can be executed locally, even if the logical qubit is entangled with
remote qubits.
Although QEC was originally a scheme proposed to protect a qubit from
noise, QEC can also be applied to entanglement distillation. Such
QEC-applied distillation is cost-effective but requires a higher base
fidelity.
4.4.3.3. Error management schemes
Quantum networks have been categorized into three "generations" based
on the error management scheme they employ [Mural16]. Note that
these "generations" are more like categories; they do not necessarily
imply a time progression and do not obsolete each other, though the
later generations do require more advanced technologies. Which
generation is used depends on the hardware platform and network
design choices.
Table 2 summarises the generations.
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+===========+=================+=======================+============+
| | First | Second generation | Third |
| | generation | | generation |
+===========+=================+=======================+============+
| Loss | Heralded | Heralded entanglement | Quantum |
| tolerance | entanglement | generation (bi- | Error |
| | generation (bi- | directional classical | Correction |
| | directional | signaling) | (no |
| | classical | | classical |
| | signaling) | | signaling) |
+-----------+-----------------+-----------------------+------------+
+-----------+-----------------+-----------------------+------------+
| Error | Entanglement | Entanglement | Quantum |
| tolerance | distillation | distillation (uni- | Error |
| | (bi-directional | directional classical | Correction |
| | classical | signaling) or Quantum | (no |
| | signaling) | Error Correction (no | classical |
| | | classical signaling) | signaling) |
+-----------+-----------------+-----------------------+------------+
Table 2: Classical signaling and generations
Generations are defined by the directions of classical signalling
required in their distributed protocols for loss tolerance and error
tolerance. Classical signalling carries the classical bits and
incurs round-trip delays described in Section 4.4.3.1, hence they
affect the performance of quantum networks, especially as the
distance between the communicating nodes increases.
Loss tolerance is about tolerating qubit transmission losses between
nodes. Heralded entanglement generation, as described in
Section 4.4.1, confirms the receipt of an entangled qubit using a
heralding signal. A pair of directly connected quantum nodes
repeatedly attempt to generate an entangled pair until the a
heralding signal is received. As described in Section 4.4.3.2, QEC
can be applied to complement lost qubits eliminating the need for re-
attempts. Furthermore, since the correction procedure is composed of
local operations, it does not require a heralding signal. However,
it is possible only when the photon loss rate from transmission to
measurement is less than 50%.
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Error tolerance is about tolerating quantum state errors.
Entanglement distillation is the easiest mechanism for improved error
tolerance to implement, but it incurs round-trip delays due the
requirement for bi-directional classical signalling. The
alternative, QEC, is able to correct state errors locally so that it
does not need any classical signalling between the quantum nodes. In
between these two extremes, there is also QEC-applied distillation,
which requires uni-directional classical signalling.
The three "generations" summarised:
1. First generation quantum networks use heralding for loss
tolerance and entanglement distillation for error tolerance.
These networks can be implemented even with a limited set of
available quantum gates.
2. Second generation quantum networks improve upon the first
generation with QEC codes for error tolerance (but not loss
tolerance). At first, QEC will be applied to entanglement
distillation only which requires uni-directional classical
signalling. Later, QEC codes will be used to create logical Bell
pairs which no longer require any classical signalling for the
purposes of error tolerance. Heralding is still used to
compensate for transmission losses.
3. Third generation quantum networks directly transmit QEC encoded
qubits to adjacent nodes, as discussed in Section 4.1.4.
Elementary link Bell pairs can now be created without heralding
or any other classical signalling. Furthermore, this also
enables direct transmission architectures in which qubits are
forwarded end-to-end like classical packets rather than relying
on Bell pairs and entanglement swapping.
4.4.4. Delivery
Eventually, the Bell pairs must be delivered to an application (or
higher layer protocol) at the two end-nodes. A detailed list of such
requirements is beyond the scope of this memo. At minimum, the end-
nodes require information to map a particular Bell pair to the qubit
in their local memory that is part of this entangled pair.
5. Architecture of a quantum internet
It is evident from the previous sections that the fundamental service
provided by a quantum network significantly differs from that of a
classical network. Therefore, it is not surprising that the
architecture of a quantum internet will itself be very different from
that of the classical Internet.
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5.1. Challenges
This subsection covers the major fundamental challenges building
quantum networks. Here, we only describe the fundamental
differences. Technological limitations are described later.
1. Bell pairs are not equivalent to payload carrying packets.
In most classical networks, including Ethernet, Internet Protocol
(IP), and Multi-Protocol Label Switching (MPLS) networks, user
data is grouped into packets. In addition to the user data, each
packet also contains a series of headers which contain the
control information that lets routers and switches forward it
towards its destination. Packets are the fundamental unit in a
classical network.
In a quantum network, the entangled pairs of qubits are the basic
unit of networking. These qubits themselves do not carry any
headers. Therefore, quantum networks will have to send all
control information via separate classical channels which the
repeaters will have to correlate with the qubits stored in their
memory.
2. "Store and forward" vs "store and swap" quantum networks.
As described in Section 4.4.1, quantum links provide Bell pairs
that are undirected network resources, in contrast to directed
frames of classical networks. This phenomenological distinction
leads to architectural differences between quantum networks and
classical networks. Quantum networks combine multiple elementary
link Bell pairs together to create one an end-to-end Bell pair,
whereas classical networks deliver messages from one end to the
other end hop by hop.
Classical networks receive data on one interface, store it in
local buffers, then forward the data to another appropriate
interface. Quantum networks store Bell pairs and then execute
entanglement swapping instead of forwarding in the data plane.
Such quantum networks are "store and swap" networks. In "store
and swap" networks, we do not need to care about the order in
which the Bell pairs were generated since they are undirected.
This distinction makes control algorithms and optimisation of
quantum networks different from classical ones. Note that third
generation quantum networks, as described in Section 4.4.1, will
be able to support a "store and forward" architecture in addition
to "store and swap".
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3. An entangled pair is only useful if the locations of both qubits
are known.
A classical network packet logically exists only at one location
at any point in time. If a packet is modified in some way,
whether headers or payload, this information does not need to be
conveyed to anybody else in the network. The packet can be
simply forwarded as before.
In contrast, entanglement is a phenomenon in which two or more
qubits exist in a physically distributed state. Operations on
one of the qubits change the mutual state of the pair. Since the
owner of a particular qubit cannot just read out its state, it
must coordinate all its actions with the owner of the pair's
other qubit. Therefore, the owner of any qubit that is part of
an entangled pair must know the location of its counterpart.
Location, in this context, need not be the explicit spatial
location. A relevant pair identifier, a means of communication
between the pair owners, and an association between the pair ID
and the individual qubits is sufficient.
4. Generating entanglement requires temporary state.
Packet forwarding in a classical network is largely a stateless
operation. When a packet is received, the router does a lookup
in its forwarding table and sends the packet out of the
appropriate output. There is no need to keep any memory of the
packet any more.
A quantum node must be able to make decisions about qubits that
it receives and is holding in its memory. Since qubits do not
carry headers, the receipt of an entangled pair conveys no
control information based on which the repeater can make a
decision. The relevant control information will arrive
separately over a classical channel. This implies that a
repeater must store temporary state as the control information
and the qubit it pertains to will, in general, not arrive at the
same time.
5.2. Classical communication
In this memo we have already covered two different roles that
classical communication must perform:
* communicate classical bits of information as part of distributed
protocols such as entanglement swapping and teleportation,
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* communicate control information within a network, including both
background protocols such as routing as well as signalling
protocols to set up end-to-end entanglement generation.
Classical communication is a crucial building block of any quantum
network. All nodes in a quantum network are assumed to have
classical connectivity with each other (within typical administrative
domain limts). Therefore, quantum nodes will need to manage two data
planes in parallel, a classical one and a quantum one. Additionally,
a node must be able to correlate information between the two planes
so that the control information received on a classical channel can
be applied to the qubits managed by the quantum data plane.
5.3. Abstract model of the network
5.3.1. The control and data planes
Control plane protocols for quantum networks will have many
responsibilities similar to their classical counterparts, namely
drawing the network topology, resource management, populating data
plane tables, etc. Most of these protocols do not require the
manipulation of quantum data and can operate simply by exchanging
classical messages only. There may also be some control plane
functionality that does require the handling of quantum data, e.g. a
quantum ping [I-D.irtf-qirg-quantum-internet-use-cases]. As it is
not clear if there is much benefit in defining a separate quantum
control plane given the significant overlap in responsibilities with
its classical counterpart, the question of whether there should be a
separate quantum control plane is beyond the scope of this document.
However, the data plane separation is much more distinct and there
will be two data planes: a classical data plane and a quantum data
plane. The classical data plane processes and forwards classical
packets. The quantum data plane processes and swaps entangled pairs.
Third generation quantum networks may also forward qubits in addition
to swapping Bell pairs.
In addition to control plane messages, there will also be control
information messages that operate at the granularity of individual
entangled pairs, such as heralding messages used for elementary link
generation (Section 4.4.1). In terms of functionality, these
messages are closer to classical packet headers than control plane
messages and thus we consider them to be part of the quantum data
plane. Therefore, a quantum data plane also includes the exchange of
classical control information at the granularity of individual qubits
and entangled pairs.
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5.3.2. Elements of a quantum network
We have identified quantum repeaters as the core building block of a
quantum network. However, a quantum repeater will have to do more
than just entanglement swapping in a functional quantum network. Its
key responsibilities will include:
1. Creating link-local entanglement between neighbouring nodes.
2. Extending entanglement from link-local pairs to long-range pairs
through entanglement swapping.
3. Performing distillation to manage the fidelity of the produced
pairs.
4. Participating in the management of the network (routing, etc.).
Not all quantum repeaters in the network will be the same; here we
break them down further:
* Quantum routers (controllable quantum nodes) - A quantum router is
a quantum repeater with a control plane that participates in the
management of the network and will make decisions about which
qubits to swap to generate the requested end-to-end pairs.
* Automated quantum nodes - An automated quantum node is a data
plane only quantum repeater that does not participate in the
network control plane. Since the no-cloning theorem precludes the
use of amplification, long-range links will be established by
chaining multiple such automated nodes together.
* End-nodes - End-nodes in a quantum network must be able to receive
and handle an entangled pair, but they do not need to be able to
perform an entanglement swap (and thus are not necessarily quantum
repeaters). End-nodes are also not required to have any quantum
memory as certain quantum applications can be realised by having
the end-node measure its qubit as soon as it is received.
* Non-quantum nodes - Not all nodes in a quantum network need to
have a quantum data plane. A non-quantum node is any device that
can handle classical network traffic.
Additionally, we need to identify two kinds of links that will be
used in a quantum network:
* Quantum links - A quantum link is a link which can be used to
generate an entangled pair between two directly connected quantum
repeaters. This may include additional mid-point elements
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described in Section 4.4.1. It may also include a dedicated
classical channel that is to be used solely for the purpose of
coordinating the entanglement generation on this quantum link.
* Classical links - A classical link is a link between any node in
the network that is capable of carrying classical network traffic.
Note that passive elements, such as optical switches, do not destroy
the quantum state. Therefore, it is possible to connect multiple
quantum nodes with each other over an optical network and perform
optical switching rather than routing via entanglement swapping at
quantum routers. This does require coordination with the elementary
link entanglement generation process and it still requires repeaters
to overcome the short-distance limitations. However, this is a
potentially feasible architecture for local area networks.
5.3.3. Putting it all together
A two-hop path in a generic quantum network can be represented as:
| App |-------------------CC-------------------| App |
|| ||
------ ------ ------
| EN |----QL & CC----| QR |----QL & CC----| EN |
------ ------ ------
App - user-level application
QR - quantum repeater
EN - end-node
QL - quantum link
CC - classical channel (can consist of many classical links)
An application running on two end-nodes attached to a network will at
some point need the network to generate entangled pairs for its use.
This may require negotiation between the end-nodes (possibly ahead of
time), because they must both open a communication end-point which
the network can use to identify the two ends of the connection. The
two end-nodes use the classical connectivity available in the network
to achieve this goal.
When the network receives a request to generate end-to-end entangled
pairs it uses the classical communication channels to coordinate and
claim the resources necessary to fulfill this request. This may be
some combination of prior control information (e.g. routing tables)
and signalling protocols, but the details of how this is achieved are
an active research question and thus beyond the scope of this memo.
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During or after the distribution of control information, the network
performs the necessary quantum operations such as generating
entanglement over individual links, performing entanglement swaps,
and further signalling to transmit the swap outcomes and other
control information. Since Bell pairs do not carry any user data,
some of these operations can be performed before the request is
received in anticipation of the demand.
The entangled pair is delivered to the application once it is ready,
together with the relevant pair identifier. However, being ready
does not necessarily mean that all link pairs and entanglement swaps
are complete, as some applications can start executing on an
incomplete pair. In this case the remaining entanglement swaps will
propagate the actions across the network to the other end, sometimes
necessitating fixup operations at the end node.
5.4. Network boundaries
Just like classical networks, various boundaries will exist in
quantum networks.
5.4.1. Boundaries between different physical architectures
There are many different physical architectures for implementing
quantum repeater technology. The different technologies differ in
how they store and manipulate qubits in memory and how they generate
entanglement across a link with their neighbours. Different
architectures come with different trade-offs and thus a functional
network will likely consist of a mixture of different types of
quantum repeaters.
For example, architectures based on optical elements and atomic
ensembles [Sangouard11] are very efficient at generating
entanglement, but provide little control over the qubits once the
pair is generated. On the other hand, nitrogen-vacancy architectures
[Hensen15] offer a much greater degree of control over qubits, but
have a harder time generating the entanglement across a link.
It is an open research question where exactly the boundary will lie.
It could be that a single quantum repeater node provides some
backplane connection between the architectures, but it also could be
that special quantum links delineate the boundary.
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5.4.2. Boundaries between different administrative regions
Just like in classical networks, multiple quantum networks will
connect into a global quantum internet. This necessarily implies the
existence of borders between different administrative regions. How
these boundaries will be handled is also an open question and thus
beyond the scope of this memo.
5.4.3. Boundaries between different error management schemes
Not only are there physical differences and administrative
boundaries, but there are important distinctions in how errors will
be managed, as described in Section 4.4.3.3, which affect the content
and semantics of messages that must cross those boundaries -- both
for connection setup and real-time operation [Nagayama16]. How to
interconnect those schemes is also an open research question.
5.5. Physical constraints
The model above has effectively abstracted away the particulars of
the hardware implementation. However, certain physical constraints
need to be considered in order to build a practical network. Some of
these are fundamental constraints and no matter how much the
technology improves, they will always need to be addressed. Others
are artefacts of the early stages of a new technology. Here, we
consider a highly abstract scenario and refer to [Wehner18] for
pointers to the physics literature.
5.5.1. Memory lifetimes
In addition to discrete operations being imperfect, storing a qubit
in memory is also highly non-trivial. The main difficulty in
achieving persistent storage is that it is extremely challenging to
isolate a quantum system from the environment. The environment
introduces an uncontrollable source of noise into the system which
affects the fidelity of the state. This process is known as
decoherence. Eventually, the state has to be discarded once its
fidelity degrades too much.
The memory lifetime depends on the particular physical setup, but the
highest achievable values in quantum network hardware currently are
on the order of seconds [Abobeih18] although a lifetime of a minute
has also been demonstrated for qubits not connected to a quantum
network [Bradley19] (as of 2020). These values have increased
tremendously over the lifetime of the different technologies and are
bound to keep increasing. However, if quantum networks are to be
realised in the near future, they need to be able to handle short
memory lifetimes, for example by reducing latency on critical paths.
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5.5.2. Rates
Entanglement generation on a link between two connected nodes is not
a very efficient process and it requires many attempts to succeed
[Hensen15] [Dahlberg19]. Currently, the highest achievable rates of
success between nodes capable of storing the resulting qubits are on
the order of 10 Hz. Combined with short memory lifetimes this leads
to very tight timing windows to build up network-wide connectivity.
5.5.3. Communication qubits
Most physical architectures capable of storing qubits are only able
to generate entanglement using only a subset of available qubits
called communication qubits [Dahlberg19]. Once a Bell pair has been
generated using a communication qubit, its state can be transferred
into memory. This may impose additional limitations on the network.
In particular, if a given node has only one communication qubit it
cannot simultaneously generate Bell pairs over two links. It must
generate entanglement over the links one at a time.
5.5.4. Homogeneity
Currently all hardware implementations are homogeneous and they do
not interface with each other. In general, it is very challenging to
combine different quantum information processing technologies at
present. Coupling different technologies with each other is of great
interest as it may help overcome the weaknesses of the different
implementations, but this may take a long time to be realised with
high reliability and thus is not a near-term goal.
6. Architectural principles
Given that the most practical way of realising quantum network
connectivity is using Bell pair and entanglement swapping repeater
technology, what sort of principles should guide us in assembling
such networks such that they are functional, robust, efficient, and
most importantly, they work? Furthermore, how do we design networks
so that they work under the constraints imposed by the hardware
available today, but do not impose unnecessary burdens on future
technology?
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As quantum networking is a completely new technology that is likely
to see many iterations over its lifetime, this memo must not serve as
a definitive set of rules, but merely as a general set of recommended
guidelines for the first generations of quantum networks based on
principles and observations made by the community. The benefit of
having a community built document at this early stage is that
expertise in both quantum information and network architecture is
needed in order to successfully build a quantum internet.
6.1. Goals of a quantum internet
When outlining any set of principles we must ask ourselves what goals
do we want to achieve as inevitably trade-offs must be made. So what
sort of goals should drive a quantum network architecture? The
following list has been inspired by the history of computer
networking and thus it is inevitably very similar to one that could
be produced for the classical Internet [Clark88]. However, whilst
the goals may be similar the challenges involved are often
fundamentally different. The list will also most likely evolve with
time and the needs of its users.
1. Support distributed quantum applications
This goal seems trivially obvious, but makes a subtle, but
important point which highlights a key difference between quantum
and classical networks. Ultimately, quantum data transmission is
not the goal of a quantum network - it is only one possible
component of more advanced quantum application protocols
[Wehner18]. Whilst transmission certainly could be used as a
building block for all quantum applications, it is not the most
basic one possible. For example, entanglement-based QKD, the
most well known quantum application protocol, only relies on the
stronger-than-classical correlations and inherent secrecy of
entangled Bell pairs and does not have to transmit arbitrary
quantum states [Ekert91].
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The primary purpose of a quantum internet is to support
distributed quantum application protocols and it is of utmost
importance that they can run well and efficiently. Thus, it is
important to develop performance metrics meaningful to
application to drive the development of quantum network
protocols. For example, the Bell pair generation rate is
meaningless if one does not also consider their fidelity. It is
generally much easier to generate pairs of lower fidelity, but
quantum applications may have to make multiple re-attempts or
even abort if the fidelity is too low. A review of the
requirements for different known quantum applications can be
found in [Wehner18] and an overview of use-cases can be found in
[I-D.irtf-qirg-quantum-internet-use-cases].
2. Support tomorrow's distributed quantum applications
The only principle of the Internet that should survive
indefinitely is the principle of constant change [RFC1958].
Technical change is continuous and the size and capabilities of
the quantum internet will change by orders of magnitude.
Therefore, it is an explicit goal that a quantum internet
architecture be able to embrace this change. We have the benefit
of having been witness to the evolution of the classical Internet
over several decades and seen what worked and what did not. It
is vital for a quantum internet to avoid the need for flag days
(e.g. NCP to TCP/IP) or upgrades that take decades to roll out
(e.g. IPv4 to IPv6).
Therefore, it is important that any proposed architecture for
general purpose quantum repeater networks can integrate new
devices and solutions as they become available. The architecture
should not be constrained due to considerations for early-stage
hardware and applications. For example, it is already possible
to run QKD efficiently on metropolitan scales and such networks
are already commercially available. However, they are not based
on quantum repeaters and thus will not be able to easily
transition to more sophisticated applications.
3. Support heterogeneity
There are multiple proposals for realising practical quantum
repeater hardware and they all have their advantages and
disadvantages. Some may offer higher Bell pair generation rates
on individual links at the cost of more difficult entanglement
swap operations. Other platforms may be good all around, but are
more difficult to build.
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In addition to physical boundaries, there may be distinctions in
how errors are managed (Section 4.4.3.3). These difference will
affect the content and semantics of messages that cross these
boundaries -- both for connection setup and real-time operation.
The optimal network configuration will likely leverage the
advantages of multiple platforms to optimise the provided
service. Therefore, it is an explicit goal to incorporate varied
hardware and technology support from the beginning.
4. Ensure security at the network level
The question of security in quantum networks is just as critical
as it is in the classical Internet, especially since enhanced
security offered by quantum entanglement is one of the key
driving factors.
It turns out that as long as the underlying implementation
corresponds to (or sufficiently approximates) theoretical models
of quantum cryptography, quantum cryptographic protocols do not
need the network to provide any guarantees about the
confidentiality or integrity of the transmitted qubits or the
generated entanglement. Instead, applications, such as QKD,
establish such guarantees in an end-to-end fashion using the
classical network in conjunction with the quantum one.
Nevertheless, whilst applications can ensure their own secure
operation, network protocols themselves should be security aware
in order to protect the network itself and limit disruption.
Whilst the applications remain secure they are not necessarily
operational or as efficient in the presence of an attacker.
Security concerns in quantum networks are described in more
detail in [Satoh17] [Satoh20].
5. Make them easy to monitor
In order to manage, evaluate the performance of, or debug a
network it is necessary to have the ability to monitor the
network while ensuring there will be mechanisms in place to
protect the confidentiality and integrity of the devices
connected to it. Quantum networks bring new challenges in this
area so it should be a goal of a quantum network architecture to
make this task easy.
The fundamental unit of quantum information, the qubit, cannot be
actively monitored as any readout irreversibly destroys its
contents. One of the implications of this fact is that measuring
an individual pair's fidelity is impossible. Fidelity is
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meaningful only as a statistical quantity which requires the
constant monitoring and the sacrifice of generated Bell pairs for
tomography or other methods.
Furthermore, given one end of an entangled pair, it is impossible
to tell where the other qubit is without any additional classical
metadata. It is impossible to extract this information from the
qubits themselves. This implies that tracking entangled pairs
necessitates some exchange of classical information. This
information might include (i) a reference to the entangled pair
that allows distributed applications to coordinate actions on
qubits of the same pair, (ii) the two bits from each entanglement
swap necessary to identify the final state of the Bell pair
(Section 4.4.2).
6. Ensure availability and resilience
Any practical and usable network, classical or quantum, must be
able to continue to operate despite losses and failures, and be
robust to malicious actors trying to disable connectivity. What
differs in quantum networks as compared to classical networks in
this regard is that we now have two data planes and two types of
channels to worry about: a quantum and a classical one.
Therefore, availability and resilience will most likely require a
more advanced treatment than they do in classical networks.
6.2. The principles of a quantum internet
The principles support the goals, but are not goals themselves. The
goals define what we want to build and the principles provide a
guideline in how we might achieve this. The goals will also be the
foundation for defining any metric of success for a network
architecture, whereas the principles in themselves do not distinguish
between success and failure. For more information about design
considerations for quantum networks see [VanMeter13.1] [Dahlberg19].
1. Entanglement is the fundamental service
The key service that a quantum network provides is the
distribution of entanglement between the nodes in a network. All
distributed quantum applications are built on top of this key
resource. Bell pairs are the minimal entanglement building block
that is sufficient to develop these applications. However, a
quantum network may also distribute multipartite entangled states
(entangled states of three or more qubits) [Meignant19] as this
may be more efficient under certain circumstances.
2. Bell Pairs are indistinguishable
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Any two Bell Pairs between the same two nodes are
indistinguishable for the purposes of an application provided
they both satisfy its required fidelity threshold. This
observation is likely to be key in enabling a more optimal
allocation of resources in a network, e.g. for the purposes of
provisioning resources to meet application demand. However, the
qubits that make up the pair themselves are not indistinguishable
and the two nodes operating on a pair must coordinate to make
sure they are operating on qubits that belong to the same Bell
pair.
3. Fidelity is part of the service
In addition to being able to deliver Bell pairs to the
communication end-points, the Bell Pairs must be of sufficient
fidelity. Unlike in classical networks where errors are
effectively eliminated before reaching the application, many
quantum applications only need imperfect entanglement to
function. However, quantum applications will generally have a
threshold for Bell pair fidelity below which they are no longer
able to operate. Different applications will have different
requirements for what fidelity they can work with. It is the
network's responsibility to balance the resource usage with
respect to the applications' requirements. It may be that it is
cheaper for the network to provide lower fidelity pairs that are
just above the threshold required by the application than it is
to guarantee high fidelity pairs to all applications regardless
of their requirements.
4. Time is an expensive resource
Time is not the only resource that is in short supply (memory,
and communication qubits are as well), but ultimately it is the
lifetime of quantum memories that imposes some of the most
difficult conditions for operating an extended network of quantum
nodes. Current hardware has low rates of Bell pair generation,
short memory lifetimes, and access to a limited number of
communication qubits. All these factors combined mean that even
a short waiting queue at some node could be enough for a Bell
pair to decohere or result in an end-to-end pair below an
application's fidelity threshold. Therefore, managing the idle
time of qubits holding live quantum states should be done
carefully. Ideally by minimising the idle time, but potentially
also by moving the quantum state for temporary storage to a
quantum memory with a longer lifetime.
5. Be flexible with regards to capabilities and limitations
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This goal encompasses two important points. First, the
architecture should be able to function under the physical
constraints imposed by the current generation hardware. Near-
future hardware will have low entanglement generation rates,
quantum memories able to hold a handful of qubits at best, and
decoherence rates that will render many generated pairs unusable.
Second, the architecture should not make it difficult to run the
network over any hardware that may come along in the future. The
physical capabilities of repeaters will improve and redeploying a
technology is extremely challenging.
7. A thought experiment inspired by classical networks
To conclude, we discuss a plausible quantum network architecture
inspired by MPLS. This is not an architecture proposal, but rather a
thought experiment to give the reader an idea of what components are
necessary for a functional quantum network. We use classical MPLS as
a basis as it is well known and understood in the networking
community.
Creating end-to-end Bell pairs between remote end-points is a
stateful distributed task that requires a lot of a-priori
coordination. Therefore, a connection-oriented approach seems the
most natural for quantum networks. In connection-oriented quantum
networks, when two quantum application end-points wish to start
creating end-to-end Bell pairs, they must first create a quantum
virtual circuit (QVC). As an analogy, in MPLS networks end-points
must establish a label switched path (LSP) before exchanging traffic.
Connection-oriented quantum networks may also support virtual
circuits with multiple end-points for creating multipartite
entanglement. As an analogy, MPLS networks have the concept of
multi-point LSPs for multicast.
When a quantum application creates a quantum virtual circuit, it can
indicate quality of service (QoS) parameters such as the required
capacity in end-to-end Bell pairs per second (BPPS) and the required
fidelity of the Bell pairs. As an analogy, in MPLS networks
applications specify the required bandwidth in bits per second (BPS)
and other constraints when they create a new LSP.
Quantum networks need a routing function to compute the optimal path
(i.e. the best sequence of routers and links) for each new quantum
virtual circuit. The routing function may be centralized or
distributed. In the latter case, the quantum network needs a
distributed routing protocol. As an analogy, classical networks use
routing protocols such as open shortest path first (OSPF) and
intermediate-system to intermediate system (IS-IS). However, note
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that the definition of "shortest-path"/"least-cost" may be different
in a quantum network to account for its non-classical features, such
as fidelity [VanMeter13.2].
Given the very scarce availability of resources in early quantum
networks, a traffic engineering function is likely to be beneficial.
Without traffic engineering, quantum virtual circuits always use the
shortest path. In this case, the quantum network cannot guarantee
that each quantum end-point will get its Bell pairs at the required
rate or fidelity. This is analogous to "best effort" service in
classical networks.
With traffic engineering, quantum virtual circuits choose a path that
is guaranteed to have the requested resources (e.g. bandwidth in
BPPS) available, taking into account the capacity of the routers and
links and taking into account the resources already consumed by other
virtual circuits. As an analogy, both OSPF and IS-IS have traffic
engineering (TE) extensions to keep track of used and available
resources, and can use constrained shortest path first (CSPF) to take
resource availability and other constraints into account when
computing the optimal path.
The use of traffic engineering implies the use of call admission
control (CAC): the network denies any virtual circuits for which it
cannot guarantee the requested quality of service a-priori. Or
alternatively, the network pre-empts lower priority circuits to make
room for the new one.
Quantum networks need a signaling function: once the path for a
quantum virtual circuit has been computed, signaling is used to
install the "forwarding rules" into the data plane of each quantum
router on the path. The signaling may be distributed, analogous to
the resource reservation protocol (RSVP) in MPLS. Or the signaling
may be centralized, similar to OpenFlow.
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Quantum networks need an abstraction of the hardware for specifying
the forwarding rules. This allows us to de-couple the control plane
(routing and signaling) from the data plane (actual creation of Bell
pairs). The forwarding rules are specified using abstract building
blocks such as "creating local Bell pairs", "swapping Bell pairs",
"distillation of Bell pairs". As an analogy, classical networks use
abstractions that are based on match conditions (e.g. looking up
header fields in tables) and actions (e.g. modifying fields or
forwarding a packet to a specific interface). The data-plane
abstractions in quantum networks will be very different from those in
classical networks due to the fundamental differences in technology
and the stateful nature of quantum networks. In fact, choosing the
right abstractions will be one of the biggest challenges when
designing interoperable quantum network protocols.
In quantum networks, control plane traffic (routing and signaling
messages) is exchanged over a classical channel, whereas data plane
traffic (the actual Bell pair qubits) is exchanged over a separate
quantum channel. This is in contrast to most classical networks,
where control plane traffic and data plane traffic share the same
channel and where a single packet contains both user fields and
header fields. There is, however, a classical analogy to the way
quantum networks work. Generalized MPLS (GMPLS) networks use
separate channels for control plane traffic and data plane traffic.
Furthermore, GMPLS networks support data planes where there is no
such thing as data plane headers (e.g. DWDM or TDM networks).
8. Security Considerations
Security is listed as an explicit goal for the architecture and this
issue is addressed in the section on goals. However, as this is an
informational memo it does not propose any concrete mechanisms to
achieve these goals.
9. IANA Considerations
This memo includes no request to IANA.
10. Acknowledgements
The authors want to thank Carlo Delle Donne, Matthew Skrzypczyk, Axel
Dahlberg, Mathias van den Bossche, Patrick Gelard, Chonggang Wang,
Scott Fluhrer, Joey Salazar, Joseph Touch, and the rest of the QIRG
community as a whole for their very useful reviews and comments to
the document.
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Authors' Addresses
Wojciech Kozlowski
QuTech
Building 22
Lorentzweg 1
2628 CJ Delft
Netherlands
Email: w.kozlowski@tudelft.nl
Stephanie Wehner
QuTech
Building 22
Lorentzweg 1
2628 CJ Delft
Netherlands
Email: s.d.c.wehner@tudelft.nl
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Rodney Van Meter
Keio University
5322 Endo, Kanagawa
252-0882
Japan
Email: rdv@sfc.wide.ad.jp
Bruno Rijsman
Individual
Email: brunorijsman@gmail.com
Angela Sara Cacciapuoti
University of Naples Federico II
Department of Electrical Engineering and Information Technologies
Claudio 21
80125 Naples
Italy
Email: angelasara.cacciapuoti@unina.it
Marcello Caleffi
University of Naples Federico II
Department of Electrical Engineering and Information Technologies
Claudio 21
80125 Naples
Italy
Email: marcello.caleffi@unina.it
Shota Nagayama
Mercari, Inc.
Roppongi Hills Mori Tower 18F
6-10-1 Roppongi, Minato-ku,
106-6118
Japan
Email: shota.nagayama@mercari.com
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