Aliro, Quantum Networking 101

Entanglement-based Quantum Networking 101

Entanglement-based Quantum Networks leverage entanglement for secure communication while simultaneously supporting a wide variety of applications.

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Overview and background information

Throughout this white paper, “quantum network” will refer to entanglement-based quantum networks. Protocols, hardware and software components employed by quantum networks, and the use cases and applications that are enabled by these networks are explained through the example of a 5-node quantum network. While this example will show what quantum networks could actually look like at each layer of the quantum networking stack, it will not and cannot capture what every single quantum network will look like. This is due to the fact that there are a wide range of possibilities for quantum networks that vary in architecture, design, and use cases, and with that variety comes incredible versatility. Entanglement-based quantum networks can interconnect quantum computers, quantum sensors, and other quantum devices. By distributing entanglement between these devices, the distributed entanglement can be used to run powerful end user applications such as Quantum Secure Communications, or QSC.

Entanglement-based quantum networks do not exist purely in science fiction or in the distant future: there are already entanglement-based quantum networks up and running today.

The Fundamentals of Quantum Networking

Quantum networks operate in fundamentally different ways from classical networks, leveraging these properties of quantum mechanics. It is these unique properties that enable superior communications security, enhanced computational speed and capability, and increased capacity across a variety of technologies such as atomic clocks, magnetometers, and gravity gradiometers. The foundations of quantum mechanics with particular relevance to quantum networking include qubits, entanglement, superposition, teleportation, and the no-cloning theorem.

Qubits

Quantum bits, or qubits, are the quantum analog of classical bits. Just like classical bits are the basic unit of information by 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 entanglement-based networks are built on.

There are many different ways of creating qubits. A few of the most common types of qubits are:

  1. Superconducting qubits. These qubits are based on superconducting circuits that exhibit quantum properties. They operate at extremely low temperatures, close to absolute zero. Superconducting qubits are used in quantum processors for computation. Companies such as IBM, Google, and Rigetti use this type of qubit in their quantum computers. These are stationary qubits: they aren’t used for quantum communication.

  2. Trapped ion qubits. In this approach, individual charged atoms are trapped with electromagnetic fields in a vacuum chamber. The quantum state of the qubit is defined by the internal energy states of the ion or the spin states of the electrons. Trapped ion qubits are highly accurate for quantum operations. They have relatively long coherence times (the time a qubit can maintain its quantum state), making them ideal candidates for complex quantum computing algorithms, such as Shor’s algorithm and Grover’s algorithm. These are stationary qubits: they generally aren’t used for quantum communication.

  3. Photonic qubits. At a high level, photonic qubits are individual photons: units of light. These qubits are particularly useful in entanglement-based communication, like Quantum Secure Communications. Photonic qubits are ideal for Quantum Secure Networks because classical networking infrastructure supports their use, they travel at the speed of light, and are less susceptible to decoherence. A photon becomes a qubit when it is encoded with quantum information. Encoding quantum information in photons typically involves defining certain properties of photons such as their polarization, phase, frequency, or path. Special devices are used to accomplish this.

Entanglement

Qubits can be coupled together in such a way that measuring one qubit will affect the state of another qubit or qubits. This type of coupling is known as entanglement. Qubits are considered to be entangled when measuring one qubit will instantaneously affect the state of the qubit (or qubits) it is entangled with, 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. Measuring one qubit will affect the state of a qubit it is entangled with faster than light can travel between them; however, this does not mean entanglement can be used for faster-than-speed-of-light communication.

Entanglement is said to be distributed between users of a Quantum Network. For example, if two users receive a qubit from an entangled pair, entanglement has been distributed between them. This distributed entanglement can then be consumed by the users to achieve faster and more powerful computation, more precise sensing, bolstered cybersecurity, and other entanglement-powered use cases.

Superposition

Quantum superposition refers to the ability of a quantum system, like a qubit, to be in multiple states simultaneously.

A classical bit can only have one of two values: 0 or 1. Qubits can have the value zero, one, or some combination of these values - and there are infinite such combinations of these values. When qubits are in a combination of values, we say that it is in a superposition of these values. This might sound abstract, so let’s use an analogy that mirrors this phenomenon, as depicted in the illustrative example of a skateboarder provided by the National Institute of Standards and Technology (NIST).

Imagine a skateboarder on a halfpipe ramp. In classical terms, the skateboarder could be either at the bottom (location zero) or at the top (location one) of the ramp. In quantum mechanics, however, the skateboarder (akin to a qubit) can exist simultaneously at both locations and everywhere in between. This superposition state is a combination of potential positions.

Teleportation

Quantum teleportation can be thought of as a way of moving quantum information from one place to another, but instead of moving a physical qubit, its state is transferred. The process of quantum teleportation uses distributed entanglement in order to make this transfer without physically sending that information through the network’s infrastructure.

No-Cloning Theorem

The no-cloning theorem states that it is impossible to create an exact copy of an arbitrary unknown quantum state. This is a rule that emerges directly from the principles of quantum mechanics (the way quantum information behaves) and has profound implications for how information is handled in quantum systems. Quantum information is delicate and complex.

How Quantum Networks Distribute Entanglement

Entanglement-based quantum networks are multipurpose networks that can support a wide variety of applications simultaneously. This is possible through distributing entanglement across the network. At a high level, this is accomplished by generating pairs of entangled photons, which are encoded with quantum information through their quantum state, and distributing them to different network nodes. These entangled qubits can then be used to transmit quantum information with a high level of security due to the quantum properties previously discussed. Quantum repeaters can be used to extend the range of quantum networks across far distances.

Quantum Networking Hardware

Each of the five nodes in this network contains:

The Bell state measurement stations are used for the elementary entanglement generation protocol and the entanglement swapping protocol. In addition to the above hardware, Alice’s and Bob’s nodes will also use quantum random number generators (QRNGs) to provide true randomness for carrying out BBM92, one of the canonical entanglement-based Quantum Secure Communications protocols.

Quantum Networking Software

There are many factors to consider when designing a quantum network. Use of an entanglement-based quantum network simulator with enough accuracy and the capabilities to model, verify, and validate network designs is vital for an efficient and effective entanglement-based quantum network design process.

The Quantum Network Stack

The quantum network stack is the implementation of protocols needed to accomplish the primary objectives of entanglement-based quantum networking: distributing entanglement, and then utilizing that distributed entanglement for a specific use case. The stack is typically broken into five layers, starting from the bottom:

Protocols for Entanglement Distribution

There are three main processes/protocols involved in distributing entanglement for end user applications:

Elementary Entanglement Generation

In Elementary Entanglement Generation (EEG), entanglement is distributed between two nearby quantum nodes. There are many protocols, and even families of protocols, for performing EEG. The example shown in the graphic below depicts a meet-in-the-middle scheme.

Entanglement Purification

In entanglement purification, multiple entangled states with lower fidelity are used to create a single entangled state with higher fidelity.

Example of Protocols Being Used for Entanglement Distribution

The following example demonstrates entanglement distribution on Alice and Bob's network. For brevity, this example assumes that all of the probabilistic processes work on the first attempt.

How Entanglement-based Quantum Networks Perform Quantum Secure Communication

While entanglement-based networks have vast capabilities that can be simultaneously enabled or executed, the focus in this example is on Quantum Secure Communication. There are many types of information that must remain private, 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.

Entanglement-based key generation

Using BBM92 begins with the distribution of entanglement between Alice and Bob, in a process like the previous example. This process is performed many times, in order to distribute a sufficient amount of high-fidelity entanglement between Alice and Bob. Once there is sufficient entanglement between Alice and Bob, the key generation process can begin.

The entanglement-based key generation process consists of multiple steps including basis selection, measurement, conversion to bits, sifting, calculating qubit error rate, and generating the final key.

Quantum Repeaters

A question that frequently comes up in discussing entanglement-based quantum networks is why classical repeaters and routers can’t be used for entanglement-based communication.

Common misconceptions along the Quantum Networking journey

There are several common misconceptions that organizations often have about deploying entanglement-based quantum networks:

  1. Entanglement-based networks will completely replace classical networks.
  2. Underestimating the technology readiness level of entanglement-based quantum networks.
  3. Overestimating the level of effort and the timeline of deployment for an entanglement-based quantum network.
  4. These networks are single-purpose networks and can only perform one application.

Closing

Entanglement-based quantum networks are being built today by a variety of organizations for a variety of use cases – benefiting organizations internally, as well as providing great value to an organization’s customers. Telecommunications companies, national research labs, intelligence organizations, and systems integrators are just a few examples of the organizations Aliro is helping to leverage the capabilities of quantum networking.

References

[NIST] Fortier, Tara. “Demystifying Quantum: It's Here, There and Everywhere.” National Institute of Standards and Technology https://www.nist.gov/blogs/taking-measure/demystifying-quantum-its-here-there-and-everywhere.

[NETWORK-STACK] Pompili, M. & Donne, C. & Raa, I. & Vecht, B. & Skrzypczyk, M. & Ferreira, G. & Kluijver, L. & Stolk, A. & Hermans, S. & Pawełczak, P. & Kozlowski, W. & Hanson, R. & Wehner, S.. (2022). Experimental demonstration of entanglement delivery using a quantum network stack. npj Quantum Information. 8. 10.1038/s41534-022-00631-2. https://www.researchgate.net/publication/364542247 theorizing_quantum_networking