Quantum Repeaters

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CONTENTS

Introduction 
Entanglement-based Quantum Networks 
Scaling Quantum Networks
Introduction to Quantum Repeaters 
How Quantum Repeaters Work 
Example 1: What happens inside a quantum repeater (basic) 
Example 2: Entanglement swapping in a chain of repeaters 
The Evolution of Quantum Repeaters 
1G quantum repeaters 
2G quantum repeaters
3G quantum repeaters
Building Entanglement-based Quantum Networks Today 
References

 

Introduction

Entanglement-based quantum networks are capable of transmitting and manipulating quantum states across far distances. These networks hold the promise of revolutionizing secure communication, enhancing computation power, and enabling breakthroughs in distributed quantum sensing across many industries. However, the inherent fragility of quantum states poses a significant challenge when it comes to transmitting quantum information over long distances. While other methods exist for extending the distance of quantum communication channels, quantum repeaters will help pave the way to the realization of large-scale wide area quantum networks.

This white paper addresses the role of quantum repeaters in extending the reach and reliability of Quantum Secure Communication, enabled by entanglement-based quantum networks. By exploring the fundamental principles and operational mechanisms of these devices, this paper aims to foster a deeper understanding of their operation, as well as set expectations as quantum repeaters advance in development and as they are deployed in entanglement-based quantum networking topologies.

 

Entanglement-based quantum networks

Entanglement-based quantum networks offer a wide range of use cases by harnessing the unique properties of quantum entanglement to provide unparalleled levels of security and communication capabilities. The primary goal of quantum networks is to distribute the entanglement between members of the network.^{[ENTANGLEMENT]}  Aliro quantum networks support local area networks, campus area networks, metro area networks, and wide area networks with a secure connection that is maintained end-to-end with direct fiber connections, free space connections, and, as they become commercially available, quantum repeaters or routers.

Over time, this same quantum networking that reliably maintains real-time quantum secure communications can be scaled to carry traffic beyond establishing encryption keys from site to site.  There are a variety of use cases enabled by entanglement-based quantum networks, including:

• Secure communication channels. Quantum networks can establish highly secure communication channels between distant parties. By exploiting the phenomenon of entanglement, these networks offer tamper-proof transmission of information, providing end-to-end encryption and protecting against unauthorized access.
• Quantum secure direct communication. Quantum networks enable direct and secure communication between two parties without the need for transmitting encryption keys. By establishing a shared entangled state, information can be transmitted securely and directly, mitigating potential information leakage.
• Distributed quantum sensing. Quantum networks offer significant advantages in sensor networks, enhancing their sensitivity and precision. By entangling the states of multiple sensors, the network can facilitate collective measurement and correlation of physical properties with improved accuracy, enabling applications such as precise metrology, environmental monitoring, and detecting minute changes or anomalies.
• Distributed and blind quantum computing. Quantum networks play a crucial role in secure quantum computing. By establishing entanglement between cloud servers and remote sites, data privacy and security are ensured during the computation process, protecting sensitive information from potential attacks or unauthorized access.

Quantum networks offer a new paradigm of secure information exchange, revolutionizing industries that demand uncompromising security and privacy, while simultaneously enabling other applications - including the creation of the Quantum Internet.

 

Scaling quantum networks

The need for quantum networks is clear. However, progress is hindered by the ability to scale these networks beyond LANs and MANs to WANs. To scale geographically, entanglement-based wide area networks must use a variety of technologies.^[ROADMAP] In order of maturity, the following technologies can be used to extend the distance between nodes:

• Free space / Terrestrial satellites
  • Satellite based intercontinental quantum networks. 
  • Air-based mobile platforms (e.g. airplanes, drones) connected to the quantum networking using free-space lasers.
  • Free space laser telescope communications between stationary endpoints (e.g. buildings without fiber connectivity).
• Ground station to satellite
  • Land-based mobile platforms (e.g. trucks, tanks) connected to the quantum networking using free-space lasers.
• Ground deployment (via fiber optic cable)
  • Fiber-based secure communications between locations in a state or country, enabled by quantum repeaters and routers

Satellites and lasers provide the best quantum communication channels today. However, to truly scale up to a global quantum networking, quantum repeaters will be required to mitigate the detrimental effects of quantum noise and signal degradation that arise during long-distance ground deployments via fiber optic cable. By leveraging the principles of quantum entanglement and entanglement swapping, quantum repeaters enable the efficient distribution of entanglement across network nodes, facilitating long-range quantum communication with unprecedented fidelity and efficiency.

 

Introduction to Quantum Repeaters

Before exploring the role of quantum repeaters in the Quantum Internet, let’s consider a comparable device: the “classical” repeater.

The classical Internet transfers information in the form of bits along conduits such as fiber optic cables. Some of these cables travel long distances, such as the SEA-ME-WE 3 undersea cable that reaches from Germany to Japan. However, as light passes through these fibers, it suffers from loss: the photons can disappear, or they can get lost as they're traversing the fiber. To account for this, a repeater is inserted between nodes. Repeaters measure the signal coming in, copy it, then retransmit it at higher power. As a result, the Internet is able to transmit information reliably over very long distances.

Loss is a problem in entanglement-based quantum networking as well. However, the same technique of measuring, copying, and retransmitting doesn’t work in quantum communications. This is due to a fundamental aspect of quantum information: it cannot be copied. This fact is known as the no-cloning theorem.

It turns out that quantum states cannot be measured on their way from point A to point B without destroying them. This actually enables secure communication, but also means the same idea for mitigating loss in classical repeaters won’t work for entanglement-based quantum networks.

In addition to loss, entanglement-based quantum networks are very sensitive to noise. Noise comes in the form of physical vibrations, temperature, and other environmental factors.

How do we overcome transmission losses and the introduction of noise in an entanglement-based quantum networking? 

Quantum repeaters play a key role in generating reliable, end-to-end entanglement by fulfilling three central roles: establishing link-layer entanglement, building entanglement sessions from elementary links, and detecting and managing errors caused by noise and loss.^[VANMETER] 

 

How Quantum Repeaters Work

 Despite their name, quantum repeaters use a very different strategy than classical repeaters to handle the problem of loss. Quantum repeaters mitigate this problem of loss by segmenting long links into much shorter links, and using entanglement swapping to extend the range of the entanglement and distribute it across the network. 

This is achieved through:

Elementary Entanglement Generation. Entanglement generation starts with a request from two end nodes to generate entanglement. From there, entanglement must first be established on each link. Later we will see how elementary entanglement can be stitched together into end-to-end entanglement.

Entanglement Swapping. Quantum entanglement networks use quantum repeaters to create end-to-end entanglement indirectly. Quantum repeaters consume elementary link-layer entanglement at each hop to produce end-to-end entanglement. This is achieved through a process called entanglement swapping.^[SWAPPING]

Entanglement Purification. As entangled states are propagated across the network, they accumulate noise, which reduces the quality of entanglement. Purification combats this issue by producing one high-quality entangled pair from a collection of low-quality entangled pairs.

Teleportation. Once reliable high-quality entanglement is established between endpoints, qubits can be transported using quantum teleportation. This is the service provided by the quantum transport layer. Teleportation consumes an entangled pair provided by the network in order to transmit a user qubit from one endpoint to another. This process is inherently secure, since no quantum data is transmitted across the network. A classical message, which does not include any state-related information, is required to correct the output state.

Entanglement distribution unlocks all kinds of applications, including teleporting qubits.

 

Example 1: What happens inside a quantum repeater (basic)

There is entanglement between Alice and a memory module in the repeater. Similarly, Bob is entangled with another memory module inside the same repeater. 

 

Once those entanglements are established, the entangled photons can be emitted to a Bell state measurement station to perform the entanglement swapping operation. The entanglement swapping operation essentially creates a direct entanglement between Alice and Bob in this example.

Example 2: Entanglement swapping in a chain of repeaters

This is a five node network in a chain topology. Alice and Bob are too far apart to make a direct link between each other in this entanglement-based quantum networking. There are three repeaters between Alice and Bob.

The goal of this small network, the service it is providing, is entanglement between these 2 distant end nodes, Alice and Bob, at a high rate of high fidelity. Photons are used in this network to generate and distribute entanglement. However, single photons are fragile and can be lost in the fiber along the way. The probability that photons actually are lost increases exponentially with distance, which is why direct links to every node aren’t possible. Quantum repeaters will need to extend the distance of this entanglement. 

The first step in entangling Alice to Bob is elementary entanglement generation, or point-to-point entanglement. This is short distance entanglement between neighboring nodes.

 

This begins with point-to-point links, and then these point-to-point links are stitched together with entanglement swapping. Entanglement swapping is a clever way to resolve the problem of loss without violating the no-cloning theorem. A quantum repeater uses entanglement swapping to create long distance entanglement between nodes. In this case, repeater one does the first entanglement swap, resulting in a slightly longer distance entanglement between Alice and repeater two in the middle. Bob’s node undergoes the same process as repeater three does a swap.

 

At this point, there are two entanglement segments spanning the distance between Alice and Bob. 

The final entanglement swap in the middle will stitch these two entanglements together, creating a long range entanglement between Alice and Bob. In this way, a first generation quantum repeater distributes entanglement.

 

Another process that early quantum repeaters will enable is entanglement purification. The quality of the entanglement, also known as fidelity, may not be high enough for the intended purpose. If this is the case, an operation called distillation, or purification, can be performed. This involves taking multiple entangled qubit pairs that are weakly entangled and combining them into a single pair that is strong, with high enough fidelity for your purposes. By performing multiple rounds of swapping and distillation, the elementary entanglements produced in the first step are converted into a stream of high-fidelity end-to-end entanglements.

 

The Evolution of Quantum Repeaters 

A step-by-step evolution of quantum repeater technology has emerged, separating repeaters into three categories: 1st generation, 2nd generation, and 3rd generation.^[REPEATERS] These generations do not necessarily make the previous generation obsolete, but they show how networks can expand to support increasingly powerful applications as the technology improves. Each new generation is better at meeting the challenges of the two biggest hurdles to long distance quantum networking: loss and noise.

 

1G quantum repeaters

First generation quantum repeaters can conduct entanglement swapping operations in a heralded manner. The swapping operation is confirmed as successful or not successful. 

Quantum repeaters rely on quantum processors to accomplish their task. However, today’s quantum processors are error-prone. To make up for this, first generation repeaters will use a process called entanglement distillation. Entanglement distillation “distills” a high quality entanglement from many copies of low quality entanglement. While a network with 1G quantum repeaters will enable groundbreaking applications, its communication rate is limited by the process of distillation.

 

2G quantum repeaters

2G quantum repeaters will increase bandwidth, as these repeaters are better at generating entanglement at a higher rate and higher fidelity. As error rates improve, quantum repeaters will transition from relying on entanglement distillation to using quantum error correction to fix operation errors. 2G quantum repeaters have quantum processing capabilities to perform quantum operations on these quantum states at the actual repeater, and can perform quantum error correction to detect and correct for errors that may have occurred on the quantum state.

Quantum error correction corrects errors by encoding information into blocks of qubits, where errors can be more easily mitigated. This allows networks to transfer information at much higher speeds and enable further applications.

 

3G quantum repeaters

As quantum devices improve, quantum error correction will be used to handle both loss and operation errors. Essentially, this allows nodes to trust that their information will travel safely to other nodes, without having to listen and hear from each repeater that entanglement was established. 

In 3G quantum repeaters, there is an important paradigm shift. In first and second generation repeaters, each link in the path from node to node will need to establish link-wise entanglement independently, and these are subsequently stitched together via entanglement swapping to generate the end-to-end entanglement between distant nodes. Third generation quantum repeaters do not use entanglement swapping and behave more like a transport network, similar to the classical Internet. Third generation quantum repeaters will enable similar performance in quantum networks. At each hop in the path, a 3G quantum repeater will use error correction to correct for any errors that the logical quantum state may have incurred while traveling from the previous node. This is similar to classical networks with feed-forward error correction. For example, in the case of entanglement distribution from node A to node Z, the entangled state can just be physically transported from A to Z via 3G repeaters. These 3G repeaters operate at high rates, enabling even more complex applications on the entanglement-based quantum network.

 

Building Quantum Networks Today

It is evident that the landscape of security, especially in the context of Quantum Secure Communication advancements, is rapidly evolving. We stand on the brink of a significant transformation, where classical cryptographic techniques are giving way to the more robust protocols enabled by quantum networks. Looking forward, quantum networking promises not only enhanced protection but also a vast array of new applications.  Preparing your cybersecurity infrastructure for the future involves proactive adaptation and thoughtful integration of these technologies into your existing frameworks to create a secure, resilient, and efficient networked world.

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 entanglement-based quantum networking. 

Building entanglement-based quantum networks is no easy task. It requires: 

• Emerging hardware components necessary to build the entanglement-based network.
• The software necessary to design, simulate, run, and manage the entanglement-based quantum networking.
• A team with expertise in quantum physics and classical networking.
• Years of hard work and development.

This may seem overwhelming, but Aliro is uniquely positioned to help you build your quantum networking. The steps you can take to ensure your organization is meeting the challenges and leveraging the benefits of the quantum networking revolution are part of a clear, unified solution already at work in networks like the EPB Quantum Network℠ powered by Qubitekk in Chattanooga, Tennessee.

AliroNet™, the world’s first full-stack entanglement-based quantum networking solution, consists of the software and services necessary to ensure customers will fully meet their secure networking goals. Each component within AliroNet™ is built from the ground up to be compatible with entanglement-based quantum networks of any scale and architecture. AliroNet™ is used to simulate, design, run, and manage entanglement-based networks as well as test, verify, and optimize quantum hardware for network performance. AliroNet™ leverages the expertise of Aliro personnel in order to ensure that customers get the most value out of the software and their investment.

Depending on where customers are in their quantum networking journeys, AliroNet™  is available in three modes that create a clear path toward building full-scale entanglement-based quantum networks: (1) Emulation Mode, for emulating, designing, and validating quantum networks, (2) Pilot Mode for implementing a small-scale quantum networking testbed, and (3) Deployment Mode for scaling quantum networks and integrating end-to-end applications. AliroNet™ has been developed by a team of world-class experts in quantum physics and classical networking. 

 

 

To get started (or continue on your quantum networking journey), reach out to the Aliro team for additional information on how AliroNet™ can enable your quantum networking. info@alirotech.com

 

References

[ENTANGLEMENT] Briegel, H.J., Cirac, J.I., Dür, W., Giedke, G., Zoller, P. Quantum Repeaters for Quantum Communication. In: Greenberger, D., Reiter, W.L., Zeilinger, A. (eds) Epistemological and Experimental Perspectives on Quantum Physics. Vienna Circle Institute Yearbook [1999], vol 7. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-1454-9_11

[REPEATERS] Muralidharan, S., Li, L., Kim, J. et al. Optimal architectures for long distance quantum communication. Sci Rep 6, 20463 (2016). https://doi.org/10.1038/srep20463

[ROADMAP] Antonio Acín et al. The quantum technologies roadmap: a European community view.  New J. Phys. 20 080201 (2018). https://doi.org/10.1088/1367-2630/aad1ea

[SWAPPING] Duan, LM., Lukin, M., Cirac, J. et al. Long-distance quantum communication with atomic ensembles and linear optics. Nature 414, 413–418 (2001). https://doi.org/10.1038/35106500

[VANMETER] Van Meter, Rodney. Quantum Networking. John Wiley & Sons, Ltd. 2014