]>
Architectural Principles
for a Quantum InternetQuTechBuilding 22Lorentzweg 12628 CJDelftNetherlandsw.kozlowski@tudelft.nlQuTechBuilding 22Lorentzweg 12628 CJDelftNetherlandsS.D.C.Wehner@tudelft.nlKeio Univeristy5322 EndoFujisawaKanagawa252-0882Japanrdv@sfc.wide.ad.jpIndividualbrunorijsman@gmail.com
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 should 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 devices that
utilise fundamental quantum mechanical phenomena such as superposition,
entanglement, and quantum measurement to achieve capabilities beyond what
is possible with classical networks. Depending on the stage of a quantum
network such devices may be 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.
This new networking paradigm offers promise for a range of new
applications such as secure communications , distributed quantum computation , or 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 communication, quantum key distribution (QKD) for
secure communications, has already been deployed at short (roughly 100km)
distances.Fully quantum networks capable of transmitting and managing entangled
quantum states in order to send, receive, and manipulate distributed
quantum information are now imminent . Whilst a lot of effort has gone into physically
realising and connecting such devices, and making improvements to their
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 what is even lower
than assembly level where no common interfaces yet exist. Furthermore,
whilst physical mechanisms for forwarding quantum states exist, there are
no robust protocols for managing such transmissions.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. for an in-depth introduction to quantum
information.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, but it can be
determined from the readout of the measurement, a classical bit, 0 or 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 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 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, blind quantum computation, or
distributed quantum computation.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 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 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.The second feature of entanglement is that it cannot be shared, in the
sense that if two qubits are maximally entangled with each other, than it
is physically impossible for any other system to have any share of this
entanglement. Hence, entanglement forms a sort of private and inherently
untappable connection between two nodes once established.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 - or mediated - interaction is necessary
to create entanglement and thus such states cannot be created between two
quantum nodes that cannot transmit quantum states to each other.
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. >This section explains the meaning of quantum connectivity and the
necessary physical processes at an abstract level.A quantum network cannot be built by simply extrapolating all the
classical models to their quantum analogues. One cannot just send qubits
like one can send bits over a wire. There are several technological as
well as fundamental challenges that make classical approaches unsuitable
in a quantum context.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.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.Since directly reading the state of a qubit is not possible, one could
ask the question if we can simply copy a qubit without looking at it.
Unfortunately, this is fundamentally not possible in quantum
mechanics.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.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
checksums. Since we cannot read or copy a quantum state a similar
approach is out of question for quantum networks.To describe the quality of a quantum state a physical quantity called
fidelity is used. 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 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 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 it is above some application-specific
threshold, they will simply operate at lower rates. Therefore, rather
than trying to ensure that we always 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.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 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, 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.
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 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.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. 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 on 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 quantum state that we wanted to transmit.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 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 its two qubits are delivered to the end-points.[waiting for contrib]The problem with generating entangled pairs directly across a link is
that its efficiency decreases with its length. Beyond a few 10s of kms
the rate is effectively zero and due to the no-cloning theorem we
cannot simply amplify the signal. The solution is entanglement
swapping.A Bell pair between any two nodes in the network can be constructed by
combining the pairs generated along each individual link on the 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: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 performs 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 transmitted and recreated in Y2's
place carrying with it its entanglement with X1. The end-result is
shown below: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.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 or purification.To purify 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 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 destroyed in the process, so
resource demands increase substantially when distillation is used.
When the test fails, the tested state must also be discarded.
Purification makes low demands on fidelity and resources, but
distributed protocols incur round-trip delays .The bare minimum requirements of an application for every Bell pair
delivered to the two end-nodes are:Information about which of the four Bell pairs was delivered.
The network may choose to not perform Pauli corrections at all
and simply notify the application of which state the delivered
pair is in or it may perform the Pauli corrections and always
deliver the same state.An identifier that allows the applicatqion to unambiguously
determine which qubits at the two end-points belong to which
entangled pair.An estimate of the fidelity of the delivered pair. This should
be above the minimum threshold determined by the application.
However, this will only be an estimate and not a guarantee. This
has security implications for applications which will be
discussed in the section on security.There are several other features an application might want to be
able to request (e.g. multiple pairs delivered together close in
time, but doesn't matter when they are delivered), but they are
beyond the scope of this memo.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. All
near-term experimental implementations of quantum repeaters are based
on this approach. Therefore, this is the architecture that we will
focus on in the rest of this memo.Nevertheless, the direct transmission proposal may be relevant in the
future as it has better fault-tolerance properties and much better
scaling with transmission distance. It might even be beneficial to
utilise a hybrid approach that combines the fault-tolerance of direct
transmission with the generic nature of Bell pairs which lends itself
to paralellisation and resource provisioning. That is, we still use
Bell pairs for transmission of user data, but direct transmission may
be used for some of hops for the purposes of Bell pair generation
rather than just relying solely on entanglement swapping.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.This subsection covers the major fundamental challenges building quantum
networks. Here, we only describe the fundamental differences,
technological limitations are described later.There is no quantum equivalent of a payload carrying packet.
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 pairs are handled individually -- they are
not grouped into packets and they 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.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, 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.Generating entanglement requires temporary state.
Packet forwarding in a classical network is largely a stateless
operation. When a packet is received, the router looks up 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 repeater 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.Generating end-to-end entanglement is a parallelisable operation.
Classical packets carry user data from source destination by
performing a series of hops across the network. This process is
necessarily sequential -- it is impossible to forward a packet ahead
of time as the user data it carries cannot be known in advance.
A quantum network does not carry any user data. It is only
responsible for generating entangled pairs in any of the generic
Bell states. The process of creating an end-to-end Bell pair is by
its nature parallelisable -- all of the individual link pairs can be
generated independently of one another. Furthermore, there is no
ordering requirement on the entanglement swapping operations either,
they can happen in any order as long as the network can keep track
of which pairs were swapped so that it can correctly identify the
two ends of the final Bell pair. This parallelism must be exploited
to make the most efficient use of the quantum network's
resources.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,communicate control information within a network - this includes
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 routers will need to manage two data planes
in parallel, a classical one and a quantum one. Additionally, it must be
able to correlate information between them so that the control
information received on a classical channel can be applied to the qubits
managed by the quantum data plane.Collecting all the pieces described so far, a quantum network will
consist of the following elements:Quantum repeaters - A quantum repeater is a node in the network
that is capable of generating entangled pairs with its directly
connected neighbours and performing entanglement swap operations
on them.Quantum routers - A quantum router is a quantum repeater that is
connected to more than two quantum repeaters as neighbours. This
distinguishes it from quantum repeaters composed into a linear
chain to connect two quantum routers (since no-cloning prohibits
quantum signal amplification).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.Quantum links - A quantum link is a link which can be used to
generate an entangled pair between two directly connected quantum
repeaters. It may 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.A two-hop path in a generic quantum network can be represented
as: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 will require negotiation between the end-nodes, because they must
both open a communication end-point (a quantum socket) 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 fulfil 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.During or after the control information is distributed the network
performs the necessary quantum operations such as generating entangled
over individual links, performing entanglement swaps, and further
signalling to transmit the swap outcomes and other control
information. Since none of the entangled pairs 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 once 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.Just like classical network, there will various boundaries will exist in
quantum networks.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 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 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.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.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. We here consider a highly abstract
scenario and refer to for pointers to the
physics literature.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 currently are on the order of seconds.
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 between nodes capable of storing the
resulting qubits 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.Most physical architectures capable of storing qubits are only able
to generate entanglement using only a subset of its available qubits
called communication qubits. 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.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.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 set of recommended guidelines
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 protocols and it is of utmost importance that they can run
well and efficiently. Therefore, the needs of quantum applications
should always be considered first. The requirements for different
applications can be found in .
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
Whilst the priority for the first quantum networks should be to
simply work, we cannot forget that ultimately they have to also be
secure. This has implications for the physical realisations (do
they satisfy the idealised theoretical models) and also the design
of the control stack.
It is actually difficult to guarantee security at the network
level and even if the network did provide such guarantees, the
application would still need to perform its own verification
similarly to how one ensures end-to-end security in classical
networks.
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 authenticity,
confidentiality, or integrity of the transmitted qubits or the
generated entanglement. Instead, applications such as QKD
establish such guarantees using the classical network in
conjunction with he quantum one. This is much easier than
demanding that the network deliver secure entanglement, which
indeed is not needed for quantum applications.
Nevertheless, control protocols themselves should be security
aware in order to protect the operation of the network itself and
limit disruption.Availability and resilience
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. For more information about design
considerations for quantum networks see .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. Unlike in classical networks where errors should
essentially be eliminated for most application protocols, many
quantum applications only need imperfect entanglement to function.
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.
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.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. Ideally, classical control mechanisms that require
increased memory lifetimes should be avoided.
For example, some quantum protocols may need to perform a
correction 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 listed as
an explicit goal for the architecture and this issue is addressed in the
section on goals. Even though user data doesn't enter the network, it is
still possible to attack the control protocols and violate the
authenticity, confidentiality, and integrity of communication. However, as
this is an informational memo it does not propose any concrete mechanisms
to achieve these goals.In summary: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 authenticity, confidentiality, or integrity of the
transmitted qubits or the generated entanglement. Instead, applications
such as QKD establish such guarantees using the classical network in
conjunction with he quantum one. This is much easier than demanding that
the network deliver secure entanglement.This memo includes no request to IANA.The authors of this memo acknowledge funding received from the EU
Flagship on Quantum Technologies through Quantum Internet Alliance
project.The authors would further like to acknowledge 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
tossingSecure multi-party quantum computation. Proceedings of
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limitThe Quantum Internet has arrived (and it hasn't)Quantum internet: A vision for the road aheadExperimental Tests of Realistic Local Theories via Bell's
TheoremUltrafast and Fault-Tolerant Quantum Communication across Long
DistancesDesigning quantum repeater networksA Link Layer Protocol for Quantum NetworksQuantum Computation and Quantum InformationMixed State Entanglement and Quantum Error Correction