What are entanglement-based quantum networks?
Entanglement-based quantum networks are real and being used today for great benefit. For example, the EPB quantum network launched in 2023 and is the first commercial quantum network in North America.
Entanglement-based quantum networks can connect a variety of quantum hardware devices by distributing entanglement between these quantum devices and enabling a wide range of use cases
- Quantum Secure Communication is the family of security solutions that are enabled by entanglement-based quantum networks. Entanglement-based key generation and teleportation create additional security for communicating sensitive information. Entanglement-based quantum networks can also be used to directly communicate classical and/or quantum information in a provably secure manner and to detect eavesdroppers.
- Distributed quantum computing is the interconnecting of multiple quantum computers. In entanglement-based quantum networks, these computers are connected by shared entanglement. These computers can be in the same room, or located across far distances. By connecting the computers together, it’s possible to achieve much greater computing power than individual quantum computers are able to achieve.
- Distributed quantum sensing is the interconnecting of multiple quantum sensors. Quantum sensors are far more sensitive than their classical counterparts, making it possible to make observations at a higher resolution than ever before. In entanglement-based networks, these sensors are connected by shared entanglement, making it possible to achieve far greater accuracy, sensitivity, and precision than individual quantum sensors can achieve.
- The Quantum Internet is a revolutionary advancement in communication technology, allowing for the interconnecting of all these various quantum technologies across the globe. Just as the classical Internet fundamentally changed the world across entire industries, the Quantum Internet will also have a dramatic impact on a wide variety of public and private sectors.
Elements of quantum networks: Qubits
Quantum bits – or qubits – are the quantum analog of classical bits. Just like classical bits are the basic unit of information that classical devices like your phone, your computer, and the Internet are built upon, quantum bits are the basic unit of quantum information that quantum computers, quantum sensors, and quantum networks are built on.
While classical bits and quantum bits are analogous in this way, they are actually quite different in behavior. Quantum bits have many quantum properties that classical bits do not. It is these properties of qubits that enable the quantum technologies that use them to outperform their classical counterparts.
Elements of quantum networks: Entanglement
Qubits can be coupled together in such a way that measuring one qubit will affect the state of the other qubit (or even many other qubits). This type of coupling is known as entanglement and we say that coupled qubits are entangled.
Measuring one qubit will instantaneously affect the state of a qubit (or the many qubits) it is entangled to – even if these qubits are many light-years apart. This is exactly the ‘spooky action at a distance’ that upset Einstein and many other physicists in the early 20th century.
While measuring one qubit will affect the state of a qubit that it is entangled with faster than light can travel between them, it is important to note that this does not mean we can use entanglement for faster-than-speed of light communication.That being said, use of entanglement in a network setting still has many incredible benefits.
Entanglement is said to be distributed between two users on a quantum network if each user receives a qubit from an entangled pair. The distributed entanglement can be utilized by users to achieve things like faster computing, more precise sensing, bolstered cybersecurity, etc.
A simple quantum network example
This example of a simple quantum network has two parties, Alice and Bob, who need to communicate top secret information. There are many types of information that need to remain secure, such as information related to defense and military, intellectual property, financial information, medical information, etc. Keeping information secure can be a matter of personal, organizational, and even national security.
Alice and Bob will use an entanglement-based quantum network to communicate their top secret information, as this method of security is provably secure against attacks from classical and quantum computers. There are many entanglement-based applications beyond key generation that can be used to help secure communications. The family of all such applications are known as quantum secure communications or QSC.
Alice and Bob are located 200 km apart. This means they cannot use a single point-to -point connection for their quantum channel, as point-to-point connections typically support links of up to 50-100 kms. However, through use of three intermediary nodes (in the form of quantum repeaters) evenly spaced between them, Alice and Bob will be able to utilize their quantum network and secure their communications! In the image above, the blue line represents the quantum channel between Alice and Bob, and the red line represents Alice and Bob’s classical channel.
Each of the five nodes in this quantum network contains:
- Entangled photon sources to produce entanglement on each individual node
- Quantum memories to store quantum information on the nodes
- CNOT gates for an entanglement purification protocol, which we will discuss later.
- Bell-state measurement stations for elementary entanglement generation and entanglement swapping protocols
Alice and Bob’s nodes will also use:
- Quantum Random Number Generators to provide true randomness for their quantum key generation protocol
- Classical communication channels will be used for heralding results, timing and synchronization, etc.
Hardware used by quantum networks can differ in type of components used - the makes, models, and vendors of the components may all be varied. Components will continue to evolve, and requirements for each network will differ based on use cases and limitations such as environmental factors and budget restrictions. There may never be a completely uniform set of components used for quantum networks, but the components will still operate under the principles laid out in this article.
The quantum network stack:
The quantum network stack is the implementation of protocols needed to accomplish our two big quantum networking goals of distributing entanglement and then utilizing the distributed entanglement.
The stack is typically broken into 5 layers:
- The physical layer is used to attempt entanglement generation
- The link layer is used for robust direct entanglement generation
- The network layer is used for end-to-end entanglement distribution
- The transport layer is used for qubit transmission
- Once the tasks of the first four layers are completed, end-user applications, the top layer of the stack, can be carried out
While specifics of protocols used in the stack could differ, the image above is still a helpful representation of the quantum network stack.
Quantum network requirements
For a quantum network to be able to meet the needs of its users and run their desired applications, it must distribute entanglement and meet the following requirements:
- The distributed entanglement must have high quality. Quality of entanglement is usually measured in terms of fidelity. The distributed entanglement needs to meet a certain level of fidelity in order to run a desired end-user application, such as quantum secure communication.
- The network needs high enough throughput, or message delivery rate, for entanglement distribution to be successful. Applications will often require many entangled pairs, and a network must be able to distribute enough entangled pairs to run the desired applications.
- It will sometimes be necessary to connect distant quantum devices. For example, provably secure communication between two distant nodes.
It is important to note that while large-scale quantum networks are desirable, small-scale entanglement-based quantum networks are also incredibly useful. For example, clustered quantum computing will typically take place in a single warehouse. There are also times when QSC between two nearby users will be needed.
There are many many factors to consider when designing a quantum network. In order to meet the quantum networking needs listed above, it’s necessary to choose the appropriate hardware, software, protocols, and topology. It’s also necessary to make logistical decisions and to integrate the quantum network with existing classical systems. While quantum networks share the same general goals of distributing entanglement and then utilizing the distributed entanglement for end user applications, each user will have unique requirements for their use case, and could face different constraints, such as budget restrictions, logistical challenges, etc.
- Use of a quantum network simulator, with enough accuracy and capabilities to model, verify, and validate network designs, is vital for making the design process efficient and effective.
- A quantum network simulator will be useful for upgrading and scaling an existing quantum network as well.
There are three main processes involved to meet our goals for distributing entanglement:
- Elementary entanglement generation (EEG), which is used to distribute entanglement between nearby devices. Note that for local area quantum networks we do not need repeaters, or other scaling technologies. We could solely use EEG for entanglement distribution.
- Entanglement purification, sometimes known as entanglement distillation, which is used to ensure that the quality or fidelity of the entanglement stays high enough throughout the entanglement distribution process so that we can use it for end-user applications.
- Entanglement swapping, which is used to extend the distance of the entanglement distributed by EEG. Using swapping and EEG together will allow us to distribute entanglement across far distances.
The quantum hardware and protocols discussed above, even in a small network, requires control and orchestration software software to operate optimally.
- Software is needed to configure, manage, run, and actually use the networks. Even for very simple networks with relatively few components it is nearly impossible to manually configure and manage hardware for a singular experiment, yet alone for actual end-user applications.
- Software is needed to handle other important distribution related tasks, such as retransmission and routing of information. With things like noise, loss, and probabilistic protocols, we will almost certainly need to retransmit information at multiple layers of the stack.
- Quantum networks will vary greatly in terms of hardware employed, and thus it is vital for the software to be hardware agnostic, since we don’t want to create a whole software stack for each quantum network or every time a hardware component is upgraded/changed.
Entanglement-based key generation
Entanglement-based key generation utilizes distributed entanglement to generate a shared secret key between end-users. This method has information-theoretic security, which means it cannot be broken, regardless of the computational power an adversary uses – whether classical or quantum.
Beyond information-theoretic security, entanglement-based key generation is also secure in implementation, in part because the key is never revealed anywhere on the network. The shared secret key can be used in many ways by classical networks. We will give several such examples in a later slide, but in our example, Alice and Bob will use their shared secret key via a symmetric encryption algorithm, such as AES 256, to encrypt and decrypt messages and thus secure their communication.
In the image above, Alice and Bob are connected by classical and quantum channels. The key is generated using the quantum channel with support from the classical channel. The key is then used by the classical channel to secure Alice and Bob’s classical communications.
Is this the same as QKD (quantum key distribution)?
QKD typically refers to the prepare-and-measure QKD systems and networks that have been around for decades. The more recently available/deployed entanglement-based networks differ from QKD networks in some important ways:
- QKD networks are single-purpose, and can only be used for key distribution. Entanglement-based quantum networks can be used for distributed quantum computing, distributed quantum sensing, and quantum secure communication (which extends beyond just key generation).
- QKD networks use a different methodology (they are prepare-and-measure-based instead of entanglement-based). QKD networks employ trusted relay nodes when extending the distance of quantum information transmission. These are not relay nodes we can trust, but rather relay nodes we are forced to trust. If they become compromised, so too will our information. Entanglement-based quantum networks use repeaters instead of relay nodes, which don’t have that same security vulnerability: the information being communicated is never present on the network and so even if a repeater becomes compromised, the information being transmitted will not be compromised.
Quantum networks beyond the simple example
The quantum network in the above example had a linear topology (i.e. there was only one path between end-users), was 200 km long, and employed hardware, software, and protocols to carry out QSC - specifically quantum key generation.
This example does not encompass all quantum networks. Quantum networks can vary in many important ways: topology, size, components, protocols, use-cases, applications, etc.
Part of the reason quantum networks vary so much is that they can be used for so many use cases. That is not to say the same entanglement-based network cannot be used for multiple use-cases simultaneously - it definitely can and this is one of the defining features of entanglement-based quantum networks! However, a local area quantum network that connects together quantum computers in a warehouse versus a quantum network that connects a vast variety quantum devices together across the globe for a multitude of use-cases will most likely differ in several ways.
Common misconceptions about quantum networks
A few common misunderstandings continue to plague organizations that would benefit from entanglement-based quantum networks:
Misconception #1: Quantum networks completely replace classical networks, and major infrastructure overhauls will be necessary. This is completely untrue. Quantum networks will be used in tandem with classical networks. Implementation of quantum networks will not require a massive overhaul/ obsoletion of classical systems.
Misconception #2: Quantum networks are a technology for the distant future. This is false! Underestimating the Technology Readiness Level is extremely common. Quantum networks are ready for deployment today, and the sooner you begin your quantum networking journey, the better you’ll be positioned to leverage it.
Misconception #3: Overestimating Level of Effort/Timeline. Aliro has seen client projects go from zero to running a Local Area Quantum Network equipped with Quantum Secure Communication applications in under a year. This is a technology that can be implemented swiftly.
Misconception #4: All quantum networks are single-purpose networks. This is not true for entanglement-based quantum networks. Any entanglement-based quantum network can be used for multiple purposes, including but not limited to: Quantum Secure Communication, distributed quantum sensing, and distributed quantum computing.
Entanglement-based quantum networks are not a futuristic concept, but a tangible reality shaping the landscape of secure communication and computational power. The quantum future is not a distant dream but an unfolding reality. Understanding entanglement-based quantum networks is a critical step in harnessing this revolutionary technology for your organization.
Our on-demand webinar, Quantum Networking 101, is an in-depth introduction to quantum networking designed to equip you with the foundational knowledge you need to navigate the quantum revolution and harness its immense potential. You can find it at this link on BrightTalks.
