Applications and Use Cases for the Quantum Internet

4.1. Overview

The Quantum Internet is expected to be extremely beneficial for a subset of existing and new applications. The expected applications using Quantum Internet are still being developed as we are in the formative stages of the Quantum Internet [Castelvecchi] [Wehner]. However, an initial (and non-exhaustive) list of the applications to be supported on the Quantum Internet can be identified and classified using two different schemes.¶

4.2. Classification by Application Usage

Applications may also be grouped by the usage that they serve into a tripartite classification. Specifically, applications may be classified according to the following usages:¶

• Quantum cryptography applications - Refers to the use of quantum information technology to ensure secure communications (e.g., QKD).¶
• Quantum sensors applications - Refers to the use of quantum information technology for supporting distributed sensors or Internet of Things (IoT) devices (e.g., clock synchronization).¶
• Quantum computing applications - Refers to the use of quantum information technology for supporting remote quantum computing facilities (e.g., distributed quantum computing).¶

This is a useful classification scheme as it can be easily understood by both a technical and non-technical audience. Following are some more details.¶

4.3. Control vs Data Plane Classification

The majority of routers currently used in the Classical Internet separate control plane functionality and data plane functionality for, amongst other reasons, stability, capacity and security. In order to classify applications for the Quantum Internet, a somewhat similar distinction can be made. Specifically some applications can be classified as being responsible for initiating sessions and performing other control plane functionality. Other applications carry application or user data and can be classified as data plane functionality.¶

Some examples of what may be called control plane applications in the Classical Internet are Domain Name Server (DNS), Session Information Protocol (SIP), and Internet Control Message Protocol (ICMP). Furthermore, examples of data plane applications are E-mail, web browsing, and video streaming. Note that some applications may require both control plane and data plane functionality. For example, a Voice over IP (VoIP) application may use SIP to set up the call and then transmit the VoIP user packets over the data plane to the other party.¶

Similarly, nodes in the Quantum Internet applications may also use the classification paradigm of control plane functionality versus data plane functionality where:¶

• Control Plane - Network functions and processes that operate on (1) control bits/packets or qubits (e.g., to setup up end-user encryption); or (2) management bits/packets or qubits (e.g., to configure nodes). For example, a quantum ping could be implemented as a control plane application to test and verify if there is a quantum connection between two quantum nodes. Another example is quantum superdense encoding (which is used to transmit two classical bits by sending only one qubit). This approach does not need classical channels. Quantum superdense coding can be leveraged to implement a secret sharing application to share secrets between two parties. This secret sharing application based on quantum superdense encoding can be classified as control plane functionality.¶
• Data Plane - Network functions and processes that operate on end-user application bits/packets or qubits (e.g., voice, video, data). Sometimes also referred to as the user plane. For example, a data plane application can be video conferencing, which uses QKD-based secure communication setup (which is a control plane function) to share a secret key for encrypting and decrypting video frames.¶

As shown in the table in Figure 1, control and data plane applications vary for different types of networks. For a standalone Quantum Subnetwork (i.e., that is not integrated into the Internet), entangled qubits are its "data" and thus entanglement distribution can be regarded as its data plane application, while the signalling for controlling entanglement distribution be considered as control plane. But looking at Quantum Internet, QKD-based secure communication setup, which may be based on and leverage entanglement distribution, is in fact a control plane application, while video conference using QKD-based secure communication setup is a data plane application.¶

+----------+-----------+----------------+----------------------+
|          |           |   Standalone   |                      |
|          | Classical |    Quantum     |       Quantum        |
|          | Internet  |   Subnetwork   |       Internet       |
|          | Examples  |    Examples    |       Examples       |
+----------+-----------+----------------+----------------------+
|  Control | ICMP,     | Signalling for | QKD-based secure     |
|  Plane   | DNS       | controlling    | communication        |
|          |           | entanglement   | setup; Quantum ping  |
|          |           | distribution   |                      |
---------------------------------------------------------------|
|  Data    | Web       | Entanglement   | Video conference     |
|  Plane   | Browsing  | distribution   | using QKD-based      |
|          |           |                | secure communication |
|          |           |                | setup                |
+--------------------------------------------------------------+

5.1. Secure Communication Setup

In this scenario, two banks (i.e., Bank #1 and Bank #2) need to have secure communications for transmitting important financial transaction records (see Figure 2). For this purpose, they first need to securely exchange a classic secret cryptographic key (i.e., a sequence of classical bits), which is triggered by an end-user banker at Bank #1. This results in a source quantum node A at Bank #1 to securely send a classic secret key to a destination quantum node B at Bank #2. This is referred to as a secure communication setup. Note that the quantum node A and B may be either a bare-bone quantum end-node or a full-fledged quantum computer. This use case shows that the Quantum Internet can be leveraged to improve the security of Classical Internet applications of which the financial application shown in Figure 2 is an example.¶

One requirement for this secure communication setup process is that it should not be vulnerable to any classical or quantum computing attack. This can be realized using QKD [ETSI-QKD-Interfaces]. QKD can securely establish a secret key between two quantum nodes, without physically transmitting it through the network and thus achieving the required security. QKD is the most mature feature of the quantum information technology, and has been commercially deployed in small-scale and short-distance deployments. More QKD use cases are described in ETSI documents [ETSI-QKD-UseCases].¶

In general, QKD (e.g., [BB84]) without using entanglement works as follows:¶

1. The source quantum node A (e.g. Alice) transforms the secret key to qubits. Basically, for each classical bit in the secret key, the source quantum node A randomly selects one quantum computational basis and uses it to prepare/generate a qubit for the classical bit.¶
2. The source quantum node A sends qubits to the destination quantum node B (e.g. Bob) via quantum channel.¶
3. The destination quantum node receives qubits and measures them based on its random quantum basis.¶
4. The destination node informs the source node of its random quantum basis.¶
5. The source node informs the destination node which random quantum basis is correct.¶
6. Both nodes discard any measurement bit under different quantum basis and store all remaining bits as the secret key.¶

It is worth noting that:¶

1. There are some entanglement-based QKD protocols such as [Treiber], which work differently than above steps. The entanglement-based schemes, where entangled states are prepared externally to Alice and Bob, are not normally considered "prepare-and-measure" as defined in [Wehner]; other entanglement-based schemes, where entanglement is generated within Alice can still be considered "prepare-and-measure"; send-and-return schemes can still be "prepare-and-measure", if the information content, from which keys will be derived, is prepared within Alice before being sent to Bob for measurement.¶
2. There are many enhanced QKD protocols based on [BB84]. For example, a series of loopholes have been identified due to the imperfections of measurement devices; there are several solutions to take into account these attacks such as measurement-device-independent QKD [ZhangPeiyu]. These enhanced QKD protocol can work differently than the steps of BB84 protocol [BB84].¶
3. For large-scale QKD, QKD Networks (QKDN) are required, which can be regarded as a subset of a Quantum Internet. A QKDN may consist of a QKD application layer, a QKD network layer, and a QKD link layer [QinHao]. One or multiple trusted QKD relays [ZhangQiang] may exist between the source quantum node A and the destination quantum node B, which are connected by a QKDN. Alternatively, a QKDN may rely on entanglement distribution and entanglement-based QKD protocols; as a result, quantum-repeaters/routers instead of trusted QKD relays are needed for large-scale QKD.¶
4. In general, there are three types of QKD solutions: 1) Basic QKD: In this case, QKD only works for two directly connected quantum nodes within a short distance or a network segment. The end-to-end security relies on some trusted nodes, which however could be attacked; 2) E2E QKD: In this case, QKD works for two faraway quantum nodes to provide the end-to-end security without relying on trusted nodes; and 3) Advanced E2E QKD: In this case, QKD leverages entanglement distribution to achieve the end-to-end security.¶

As a result, the Quantum Internet in Figure 2 contains quantum channels. And in order to support secure communication setup especially in large-scale deployment, it also requires entanglement generation and entanglement distribution [I-D.van-meter-qirg-quantum-connection-setup], quantum repeaters/routers, and/or trusted QKD relays.¶

+---------------+
|   End User    |
|(e.g., Banking |
|  Application) |
+---------------+
      ^
      | User Interface
      | (e.g., GUI)
      V
+-----------------+     /--------\     +-----------------+
|                 |--->( Quantum  )--->|                 |
|     Source      |    ( Internet )    |  Destination    |
|     Quantum     |     \--------/     |    Quantum      |
|     Node A      |                    |     Node B      |
| (e.g., Bank #1) |     /--------\     | (e.g., Bank #2) |
|                 |    ( Classical)    |                 |
|                 |<-->( Internet )<-->|                 |
+-----------------+     \--------/     +-----------------+

5.2. Secure Quantum Computing with Privacy Preservation

Secure computation with privacy preservation refers to the following scenario:¶

1. A client node with source data delegates the computation of the source data to a remote computation node.¶
2. Furthermore, the client node does not want to disclose any source data to the remote computation node and thus preserve the source data privacy.¶
3. Note that there is no assumption or guarantee that the remote computation node is a trusted entity from the source data privacy perspective.¶

As an example illustrated in Figure 3, the client node could be a virtual voice-controlled home assistant device like Amazon's Alexa product. The remote computation node could be a quantum computer in the cloud. A resident as an end-user uses voice to control the home device. The home device captures voice-based commands from the end-user. Then, the home device interfaces to a home quantum terminal node (e.g., a home gateway), which interacts with the remote computation node to perform computation over the captured voice-based commands. The home quantum terminal could be either a bare-bone quantum end-node or a full-fledged quantum computer.¶

In this particular case, there is no privacy concern since the source data (i.e., captured voice-based commands) will not be sent to the remote computation node which could be compromised. Protocols [Fitzsimons] for delegated quantum computing or blind quantum computation can be leveraged to realize secure delegated computation and guarantee privacy preservation simultaneously. Using delegated quantum computing protocols, the client node does not need send the source data but qubits with some measurement instructions to the remote computation node (e.g., a quantum computer).¶

After receiving qubits and measurement instructions, the remote computation node performs the following actions:¶

1. It first performs certain quantum operations on received qubits and measure them according to received measurement instructions to generate computation results (in classic bits).¶
2. Then it sends the computation results back to the client node via classical channel.¶
3. In this process, the source data is not disclosed to the remote computation node and the privacy is preserved.¶

In Figure 3, the Quantum Internet contains quantum channels and quantum repeaters/routers for long-distance qubits transmission [I-D.irtf-qirg-principles].¶

+----------------+
|   End-User     |
|(e.g., Resident)|
+----------------+
        ^
        | User Interface
        | (e.g., voice commands)
        V
+----------------+
|   Home Device  |
+----------------+
        ^
        | Classic
        | Channel
        V
+----------------+     /--------\     +----------------+
|                |--->( Quantum  )--->|                |
|   Quantum      |    ( Internet )    |   Remote       |
|   Terminal     |     \--------/     |   Computation  |
|   Node         |                    |   Node         |
|  (e.g., Home   |     /--------\     |   (e.g., QC    |
|   Gateway)     |    ( Classical)    |   in Cloud)    |
|                |<-->( Internet )<-->|                |
+----------------+     \--------/     +----------------+

5.3. Distributed Quantum Computing

In this scenario, Noisy Intermediate-Scale Quantum (NISQ) computers distributed in different locations are available for sharing. According to the definition in [Preskill], a NISQ computer can only realize a small number of qubits and has limited quantum error correction. In order to gain higher computation power before fully-fledged quantum computers become available, NISQ computers can be connected via classic and quantum channels. This scenario is referred to as distributed quantum computing [Caleffi] [Cacciapuoti01] [Cacciapuoti02]. This use case reflects the vastly increased computing power which quantum computers as a part of the Quantum Internet can bring, in contrast to classical computers in the Classical Internet.¶

As an example, scientists can leverage these connected NISQ computer to solve highly complex scientific computation problems such as analysis of chemical interactions for medical drug development (see Figure 4). In this case, qubits will be transmitted among connected quantum computers via quantum channels, while classic control messages will be transmitted among them via classical channels for coordination and control purpose. Qubits from one NISQ computer to another NISQ computer are very sensitive and cannot be lost. For this purpose, quantum teleportation can be leveraged to teleport sensitive data qubits from one quantum computer A to another quantum computer B. Note that Figure 4 does not cover measurement-based distributed quantum computing, where quantum teleportation may not be required.¶

Specifically, the following steps happen between A and B. In fact, LOCC [Chitambar] operations are conducted at the quantum computer A and B in order to achieve quantum teleportation as illustrated in Figure 4.¶

1. The quantum computer A locally generates some sensitive data qubits to be teleported to the quantum computer B.¶
2. A shared entanglement is established between the quantum computer A and the quantum computer B (i.e., there are two entangled qubits: |q1> at A and |q2> at B).¶
3. Then, the quantum computer A performs a Bell measurement of the entangled qubit |q1> and the sensitive data qubit.¶
4. The result from this Bell measurement will be encoded in two classical bits, which will be physically transmitted via a classical channel to the quantum computer B.¶
5. Based on the received two classical bits, the quantum computer B modifies the state of the entangled qubit |q2> in the way to generate a new qubit identical to the sensitive data qubit at the quantum computer A.¶

In Figure 4, the Quantum Internet contains quantum channels and quantum repeaters/routers [I-D.irtf-qirg-principles]. This use case needs to support entanglement generation in order to enable quantum teleportation, entanglement distribution or quantum connection setup [I-D.van-meter-qirg-quantum-connection-setup] in order to support long-distance quantum teleportation.¶

                  +-----------------+
                  |     End-User    |
                  |(e.g., Scientist)|
                  +-----------------+
                           ^
                           |User Interface (e.g. GUI)
                           |
        +------------------+-------------------+
        |                                      |
        |                                      |
        V                                      V
+----------------+     /--------\     +----------------+
|                |--->( Quantum  )--->|                |
|                |    ( Internet )    |                |
|   Quantum      |     \--------/     |   Quantum      |
|   Computer A   |                    |   Computer B   |
| (e.g., Site #1)|     /--------\     | (e.g., Site #2)|
|                |    ( Classical)    |                |
|                |<-->( Internet )<-->|                |
+----------------+     \--------/     +----------------+

6.1. Background

Quantum technologies are steadily evolving and improving. Therefore, it is hard to predict the timeline and future milestones of quantum technologies as pointed out in [Grumbling] for quantum computing. Currently, a NISQ computer can achieve fifty to hundreds of qubits with some given error rate. In fact, the error rates of two-qubit quantum gates have decreased nearly in half every 1.5 years (for trapped ion gates) to 2 years (for superconducting gates). The error rate also increases as the number of qubits increases. For example, a current 20-qubit machine has a total error rate which is close to the total error rate of a 7 year old two-qubit machine [Grumbling].¶

On the network level, six stages of Quantum Internet development are described in [Wehner] as follows:¶

1. Trusted repeater networks (Stage-1)¶
2. Prepare and measure networks (Stage-2)¶
3. Entanglement distribution networks (Stage-3)¶
4. Quantum memory networks (Stage-4)¶
5. Fault-tolerant few qubit networks (Stage-5)¶
6. Quantum computing networks (Stage-6)¶

The first stage are simple trusted repeater networks, while the final stage are quantum computing (sub)networks where the full-blown Quantum Internet will be achieved. Each intermediate stage brings with it new functionality, new applications, and new characteristics. Figure 5 illustrates Quantum Internet use cases as described in this document mapped to the Quantum Internet stages described in [Wehner]. For example, secure communication setup can be supported in Stage-1, Stage-2, or Stage-3, but with different QKD solutions. More specifically:¶

In Stage-1, basic QKD is possible and can be leveraged to support secure communication setup but trusted nodes are required to provide end-to-end security.¶

In Stage-2, E2E QKD without relying on trusted nodes is possible to support secure communication setup too and the primary requirement is long-distance qubit transmission.¶

In Stage-3, Advanced E2E QKD can be enabled based on quantum repeater and entanglement distribution to support the same secure communication setup application.¶

In Stage-4, Secure quantum computing with privacy-preservation can be enabled since it needs quantum memory for multiple rounds of quantum computation.¶

Finally, in Stage-6, distributed quantum computing relaying more qubits can be supported.¶

+---------+---------------------------+------------------------+
| Quantum |     Example Quantum       |                        |
| Internet|      Internet Use         |   Characteristic       |
| Stage   |         Cases             |                        |
+---------+---------------------------+------------------------+
| Stage-1 | Secure Comm Setup         |  Trusted Nodes         |
|         | with Basic QKD            |                        |
|--------------------------------------------------------------|
| Stage-2 | Secure Comm Setup         |  Long-distance qubit   |
|         | with E2E QKD              |  transmission          |
|--------------------------------------------------------------|
| Stage-3 | Secure Comm Setup         |  Entanglement          |
|         | with Advanced E2E QKD     |  distribution          |
|--------------------------------------------------------------|
| Stage-4 | Secure/Blind Quantum      |  Quantum memory        |
|         | Computing                 |                        |
|--------------------------------------------------------------|
| Stage-5 | Higher-accuracy clock     |  Fault tolerance       |
|         | synchronization           |                        |
|--------------------------------------------------------------|
| Stage-6 | Distributed quantum       |  More qubits           |
|         | computing                 |                        |
+--------------------------------------------------------------+

6.2. Requirements

Some general and functional requirements on the Quantum Internet from the networking perspective, based on the above applications and use cases, are identified as follows:¶

1. Methods for facilitating quantum applications to interact efficiently with entanglement qubits are necessary in order for them to trigger distribution of designated entangled qubits to potentially any other quantum node residing in the Quantum Internet. To accomplish this specific operations must be performed on entangled qubits (e.g., entanglement swapping, entanglement distillation). Quantum nodes may be quantum end-nodes, quantum repeaters/routers, and/or quantum computers.¶
2. Quantum repeaters/routers should support robust and efficient entanglement distribution in order to extend and establish entanglement connection between two quantum nodes. For achieving this, it is required to first generate an entangled pair on each hop of the path between these two nodes.¶
3. Quantum end-nodes must send additional information on classical channels to aid in transmission of qubits across quantum repeaters/receivers. This is because qubits are transmitted individually and do not have any associated packet overhead which can help in transmission of the qubit. Any extra information to aid in routing, identification, etc., of the qubit(s) must be sent via classical channels.¶
4. Methods for managing and controlling the Quantum Internet including quantum nodes and their quantum resources are neccessary. The resources of a quantum node may include quantum memory, quantum channels, qubits, established quantum connecctions, etc. Such management methods can be used to monitor network status of the Quantum Internet, diagnose and identify potential issues (e.g. quantum connections), and configure quantum nodes with new actions and/or policies (e.g. to perform a new entanglement swapping operation). New management information model for the Quantum Internet may need to be developed.¶