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Architectural Principles
for a Quantum InternetQuTechBuilding 22Lorentzweg 1Delft2628 CJNetherlands+31 (0)15 2787077w.kozlowski@tudelft.nl
General
Quantum Internet Research GroupQuantum InternetArchitectureRepeaterBell PairEPR PairThe 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 must be built from
the ground up as the physical nature of the communication is fundamentally
different. The first realisations of quantum networks are imminent, but
there is no practical proposal for how to organise, utilise, and manage
such networks. In this memo, we attempt 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.Quantum networks are distributed systems of quantum computers that
utilise fundamental quantum mechanical phenomena such as superposition,
entanglement, and quantum measurement to achieve capabilities beyond what
is possible with classical networks. This new networking paradigm offers
promise for a range of new applications such as tamper-proof
communications , distributed quantum
computation , and quantum sensor networks
. The field of quantum communication has
been a subject of active research for many years and the most well-known
application of quantum computers that has already been deployed, quantum
key distribution (QKD), is a protocol used for secure communications.Fully quantum networks capable of transmitting and managing entangled
states in order to send, receive, and manipulate distributed quantum
states are now imminent . Whilst a lot of effort has gone into
physically connecting the devices and bringing down the error rates there
are no concrete proposals for how to run these networks. To draw an
analogy with a classical network, we are at a stage where we can
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 what is essentially assembly level.
Furthermore, whilst physical mechanisms for forwarding quantum states
exist, there are no protocols for managing it.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 be 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.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, a qubit, exists over the same binary
space, but unlike the classical bit, it can exist in a so-called
superposition of the two possibilities:a |0> + b |1>,where |X> denotes 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.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 is not deterministic. 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. This
randomness is not due to our ignorance of the underlying mechanisms, but
rather it is a fundamental feature of a quantum mechanical system .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>.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 isa |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),where x denotes a tensor product (the mathematical mechanism for
combining quantum states together). Let us now consider the two-qubit
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 likeINOUT0000010110111110Now, consider performing a CNOT gate on the ensemble 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 their behaviour 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 if the same measurement
is to be repeated, the entangled state must be prepared again.Entanglement is the fundamental building block of quantum networks. To
see this, consider the final 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.Now consider sending one of the qubits to another device. This device
can 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 these non-classical correlations in order to design completely
new types of algorithms that are not possible to achieve with just
classical communication. Examples of such applications are quantum
cryptography, blind quantum computation, or distributed quantum
computation.As a trivial example consider the problem of reaching consensus between
two nodes. The two nodes want to agree on the value of a single bit. In
a quantum network they can simply request the network to generate the
state (|00> + |11>)/sqrt(2) for them and that is essentially all that
needs to be done. Once any 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, 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.However, it is impossible to entangle two qubits without ever having
them directly interact with each other (e.g. by performing a local
two-qubit gate, such as the CNOT). A local interaction is necessary to
create entanglement and thus such states cannot be created between two
quantum computers that cannot transmit quantum states to each other.
Therefore, it is the entanglement property of multi-qubit states 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
distribute entangled states amongst themselves. A quantum computer that
is able to communicate classically with another quantum computer is not a
member of a quantum network.This is a crucial difference between classical and quantum networks.
Classical applications transmit data over the network to synchronise
distributed state. Quantum network applications obtain distributed
states, synchronised at the physical level via entanglement, from the
network to perform quantum algorithms.More complex services and applications can be built on top of entangled
states distributed by the network.To build a network we must first physically connect all the nodes
with quantum channels that enable them to distribute the entanglement.
Unfortunately, our ability to transfer quantum states is complicated by
the no-cloning theorem.The no-cloning theorem states that it is impossible to create an
identical copy of an arbitrary unknown quantum state. Since performing
a measurement on a quantum state destroys its superposition, there is no
practical way of learning the exact state of a qubit in an unknown
state. Therefore, it is impossible to use the same mechanisms that
worked for classical networks for error-correction, 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 a quantum network is a challenging endeavour and its
architecture must at its core address this very issue.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 error correction 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 . However, even in the most optimistic
scenarios the hardware requirements to fault-tolerantly transmit a
single qubit are beyond near-term capabilities. Nevertheless, due to
the promise of fault-tolerance and its favourable poly-logarithmic
scaling with distance, this may eventually become a desirable method for
entanglement distribution.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. 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 state 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. That is, either of
the nodes that hold the two qubits of the Bell Pair can apply a series
of single qubit gates to just their qubit in order to transform the
ensemble 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 error-correction is easier and error-detection combined
with reattempts becomes a valid strategy.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.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. Quantum state teleportation consumes an unknown
quantum state that we want to transmit and recreates it at the desired
destination.To achieve this, a Bell Pair needs to be distributed between the
source and destination. The source then entangles the transmission
qubit with its end of the Bell Pair and performs a measurement. This
consumes the Bell Pair's entanglement turning the source and
destination qubits into independent states. However, this process
transforms the Bell Pair's qubit at the destination into the
transmission qubit's original state. Note he process requires the
source to also communicate its two-bit measurement result so that the
destination can correct for the randomness of the outcome.The unknown quantum state that was transmitted never entered the
network itself. Therefore, the network needs to only be able to
reliably produce Bell Pairs between any two nodes in the network.Reducing the problem to one of generating a Bell Pair state has
facilitated the problem, but it has not solved it.The technology to generate a Bell Pair between two directly
connected quantum nodes already exists and has been demonstrated in
laboratory conditions . Interestingly,
neither of the two qubits of the pair need to be transmitted any
further.A Bell Pair between any two nodes in the network can be constructed
from Bell Pairs generated along each individual link on the path
between the two end-points. Each node along the path can consume the
two Bell Pairs on the two links that it is connected to in order to
produce a new Bell Pair between the two far ends. This process is
known as entanglement swapping. Pictorially it can be represented as
follows:where x~~x denotes a Bell Pair with individual qubits represented
by x, -- denotes a quantum link, and [ ] denotes a node. The diagram
above represents the situation after the middle node has generated a
Bell Pair with two of its directly connected neighbours. Now, the
middle node performs an entanglement swap operation (the exact details
of the mechanism are beyond the scope of this memo). This operation
consumes the two Bell Pairs and produces a new Bell Pair between the
two far ends of this three-node network as follows:The outcome is guaranteed to be a Bell Pair between the two end
nodes, but which of the four possible Bell Pairs is produced is not
deterministic. However, the middle node will know which one was
produced as the entanglement swap is a measurement operation that
yields two classical bits. The final state can be inferred from this
two-bit readout. Therefore, the middle node needs only to communicate
the outcome over a classical channel to one or both ends who can apply
a correction to transform the pair into any of its other forms (if so
desired).Neither the Bell Pair or the swapping operations are lossless
operations. Therefore, with each link and each swap the quality of
the state degrades. However, it is possible to create higher quality
Bell Pair states from two or more lower quality Bell Pair states.
Therefore, once the quality loss over a given distance become
prohibitive, additional redundancy may be used to restore the state
quality.Direct state transmission whilst simpler conceptually is much more
demanding to implement reliably in practice which means that any
near-term practical realisation is more likely to succeed if it is based
on the Bell Pair and entanglement swapping architecture. This is the
architecture that we will focus on in the rest of this memo for
practical reasons.Nevertheless, we are not entirely discarding the direct transmission
proposal. Whilst it does enable the fault-tolerant transmission of
unknown quantum states, it might still be more beneficial to use it to
distribute Bell Pairs instead. Distributing Bell Pairs via direct
transmission means that one can leverage the advantages of entanglement
swapping which allows for paralellisation as the Bell Pairs can be built
up from both ends simultaneously. Furthermore, the generic nature of
the Bell Pair means that a network may provision resources better before
it receives any request.A generic quantum network of three nodes could be represented asWhere "App" is some application running over a quantum network,
--CC-- denote classical communication links (e.g. over the public
Internet or a private LAN), and "QNet" is a generic network stack.
Architectures for the network stack have been proposed already , but their discussion is beyond the scope of
this memo. However, they all map onto this generic diagram. Nodes
within a quantum network that are capable of performing the entanglement
swap operation are often referred to as quantum repeaters and we shall
adopt this terminology from this point on. End-hosts connecting at the
edge of the network are not necessarily repeaters themselves.The key message here is that a network stack relies on the hardware
being able to provide two services: Bell Pair generation across a link,
and swap operation. In any network model it is assumed that the
physical device is capable of providing both of these services and
offers a suitable interface for their usage.Strictly speaking quantum memories are not needed for a functional
quantum network as long as the network is able to generate the Bell
Pairs, swap the entanglement, and deliver the final Bell Pair to the
application in a usable form. However, in general, to be able to
provide the two services above, the hardware will also need to be able
to store the qubits in memory which is highly non-trivial.Furthermore, it is also assumed that the applications are able to
communicate classically, and that the nodes themselves are also
connected over some classical channel. The classical links between the
nodes need not always have an associated quantum link, but it is assumed
that any quantum link has a classical link running in parallel.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.The quality of a quantum state is described by a physical quantity
called fidelity. Fidelity is the measure of how close a quantum state
is to the quantum state we desire it to be in. 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 stems
from the fact that no physical operation is perfect. Furthermore,
applications will in general require the fidelity of a quantum state
to be above some minimum threshold in order to guarantee the
correctness of their algorithm and it is the responsibility of the
network to provide such a state.Additionally, entanglement swap operations, even if perfect, lead
to a further reduction in the fidelity of the final state. Two
imperfect Bell Pairs when combined will produce a slightly worse Bell
Pair. Whilst distillation is one of the available mechanisms to
correct for these errors it requires additional Bell Pairs to be
produced. There will be a trade-off between how much distillation is
to be done versus what fidelity is acceptable.This is a fundamental constraint as perfect noiseless operations
and lossless communication channels are unachievable. Therefore, no
Bell Pair will be generated with perfect fidelity and the network must
account for this.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's 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 currently are on the order of hundreds
of milliseconds. 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.
An architecture that handles short lifetimes may also be more
cost-efficient in the future.Entanglement generation on a link between two connected nodes is
not a very efficient process and it requires many attempts to succeed.
A fast repetition rate for Bell Pair generation is achievable, but
only one in a few thousands will succeed. Currently, the highest
achievable rates of success are of the order of 10 Hz. Combined with
short memory lifetimes this leads to very tight timing windows to
build up network-wide connectivity. Achievable rates are likely to
increase with time, but just like with quantum memories, it may be
more cost-efficient in the future to provide low-rate links in some
parts of the network.Some physical architectures are not able to generate entanglement
using any memory qubit that they have access to. In these systems,
entanglement is generated using a communication qubit and once a Bell
Pair has been generated, the qubit state is 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.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 due to
their sensitivity to losses. Coupling different technologies with
each other is of great interest as it may help overcome the weaknesses
of the different implementations, but this is not a near-term
goal.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 burden on future technology. Redeploying
network technology is a non-trivial process.As this 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 guide 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.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 the classical
Internet, but it will inevitably evolve with time and the needs of its
users. The goals are listed in order of priority which in itself may
also evolve as the community learns more about the technology.Support distributed quantum applications
The primary purpose of a quantum internet is to run distributed
quantum algorithms and it is of utmost importance that they can
run well and efficiently. Therefore, the needs of quantum
applications should always be considered first.
If a network is able to distribute entanglement it is officially
quantum. However, if it is unable to distribute these states with
a sufficiently high fidelity at a reasonable rate for a majority
of potential applications it is not practical.Support tomorrow's distributed quantum applications
There are many applications already proposed to run over a quantum
internet. However, more algorithms will be invented as the
community grows as well as the robustness and the reliability of
the technology. Any proposed architecture should not constrain
the capabilities of the network for short-term benefit.Hardware heterogeneity
There are multiple proposals for realising practical quantum
repeaters and they all have their advantages and disadvantages. It
is also very likely that the most optimal technologies in the
future will be hybrid combinations of the many different solutions
currently under development. It should be an explicit goal of the
architecture to allow for a large variety of hardware
implementations.Be flexible with regards to hardware capabilities and
limitations
This goal encompasses two important points. First, the
architecture should be able to function under the physical
constraints imposed by the current generation hardware. Second,
it 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.Security, availability, and resilience
Whilst the priority for the first quantum networks should be to
simply work, we cannot forget that ultimately they have to also be
secure. There are three key security considerations at the
network level, confidentiality, integrity, and authenticity.
Confidentiality and integrity - it is vital that the network can
provide a reasonable guarantee of the minimum fidelity of a
delivered Bell Pair as the application's own security mechanisms
rely on this. Uncertainty about the fidelity of a Bell Pair may
potentially expose its data to an eavesdropper.
Authenticity - it is important that any application can have
confidence that the other end of the Bell Pair has been delivered
to the desired partner.
Additionally a practical and usable network is able to continue to
operate despite losses and failures, and will be robust to
malicious actors trying to disable connectivity. These may be
simply considered different aspects of security, but it is
worthwhile to address them explicitly at the architectural level
already.Easy to manage and monitor
Quantum networks rely on complex physical phenomena and require
hardware that is challenging to build. Furthermore, the quantum
resources will at first be very scarce and potentially very
expensive. This entails a need for a robust management solution.
It is important that a good management solution needs to come with
adequate monitoring capabilities.
Good management solutions may also be key to optimising the
networks which in turn may be crucial in making them economically
feasible. Unlike user data that is transmitted over classical
networks, quantum networks only need to generate generic Bell
Pairs. This leaves a lot of room for pre-allocating resources in
an efficient manner.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.Bell Pairs are the fundamental building block
The key service that a quantum network provides is the
distribution of entanglement between the nodes in a network. This
point additionally specifies that the entanglement is primarily
distributed in the form of the entangled Bell Pair states which
should be used as a building block in providing other services,
including more complex entangled states.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. However, 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 application's 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.Bell Pairs are indistinguishable
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 point is
crucial in enabling the reuse of resources of a network and for
the purposes of provisioning resources to meet application
demand.Time as an expensive resource
With the current technology, time is the most expensive resource.
It 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 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 the Bell Pairs to decohere.
However, time is only expensive once quantum operations are
underway. If no quantum operations are currently being processed
then the network can use this time to prepare and provision
resources.
As hardware improves, the need for carefully timing quantum
operations may become smaller. It is currently unknown what the
cost of these improvements will be, but it is conceivable that
there is value in having relatively cheap and undemanding links
connected at the edges of a network which will have very short
memory lifetimes and low rates of Bell Pair generation.Limit classical communication
This point offers a practical guideline to the issue of timing.
A bottleneck in many quantum networked algorithms is the classical
communication needed between quantum operations to synchronise
state.
For example, some quantum protocols may need to perform a correct
for the random outcome of a quantum measurement. For this, they
will block the state from further operations until a classical
message is received with the information necessary to perform the
correction. The time during which the quantum state is blocked is
effectively wasted. It reduces the time available for subsequent
operations possibly rendering the state useless for an
application.
Trade-offs that allow a protocol to limit the number of blocking
classical communication rounds once quantum operations have
commenced will in general be worth considering.Parallelise quantum operations
A further point to address the issue of timing constraints in the
network. The Bell Pairs on the individual links need not be
generated one after another along the path between the
communication end-points. The order does not matter at all.
Furthermore, the order of the swap operations is flexible as long
as they don't reduce the fidelity too much. Parallelising these
operations is key to optimising quantum protocols.Avoid time-based coordination when possible
A solution to timing constraints is to synchronise clocks and
agree on the timing of events. However, such solutions have
several downsides. Whilst network clock synchronisation may be
accurate enough for certain purposes it introduces an additional
element of complexity, especially when multiple nodes in different
networks must be synchronised. Furthermore, clock synchronisation
will never be perfect and it is conceivable that hardware
capabilities advance so much that time-based mechanisms
under-utilise resources in the more efficient parts of the
network.
Nevertheless, it may not be possible to avoid clocks, but such
solutions should be adequately justified.Pre-allocate resources
Regardless of what application is running over the network it will
have the same needs as any other application: a number of Bell
Pairs of sufficient fidelity. Whilst the fidelity is a variable
number, the indistinguishability of Bell Pairs means that there is
lots of flexibility in how a network may provision resources to
meet demand. The additional timing constraints mean that
pre-allocation of resources will be central to a usable quantum
network. Even though no user data enters a quantum network security is
explicitly listed as a goal in this memo. However, as this is an
informational memo it does not propose any concrete mechanisms to achieve
these goals.In summary:Confidentiality and integrity in the quantum context is the
network's guarantee on the minimum fidelity of the delivered Bell Pair
states. Uncertainty about the fidelity of a Bell Pair may potentially
expose an application to an eavesdropper.Authenticity in a quantum network is the guarantee that the other
end of the Bell Pair is with the requested partner and not any other
third party.This memo includes no request to IANA.The author would like to acknowledge funding received the Quantum
Internet Alliance.The author would further like to acknowledge Stephanie Wehner, Carlo
Delle Donne, Matthew Skrzypczyk, and Axel Dahlberg for useful discussions
on this topic prior to the submission of this memo.Quantum cryptography: Public key distribution and coin
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