Quantum Networking Ready For Prime Time
Will networking be the first killer application of Quantum Technologies? Odds are YES!
In this post we dive into Quantum Networking and some of its key technologies and directions. One notable characteristic of Quantum Networking is that it will leverage and integrate with existing fiber optic/light based systems and satellite technologies which will accelerate actual real world stable implementations. Labs and test beds are already demonstrating successful deployments spanning thousands of kilometers.
Quantum networking leverages the unique properties in quantum mechanics of superposition and entanglement to transmit quantum information (qubits) between devices. Quantum networks hold the potential to enable fundamentally new capabilities, including ultra-secure communication with unbreakable encryption, distributed quantum computing, and hold out the promise of creating a “Quantum Internet”.
Secure communications
Today’s communications networks are vulnerable and subject to hacking and intrusion. Quantum networks can enable completely new forms of security that relies only on the laws of physics and is unbreakable and completely hack proof. Quantum networks establish connections using entanglement, rather than passing data directly through the network. In addition, because of the properties of quantum states, it is possible to detect with certainty if communication has been intercepted.
How it works
The basic structure of a quantum network and more generally a quantum internet is analogous to a classical network. In very simple terms, there are end points, communication links/infrastructure with repeaters to overcome limitations due to distance, and degraded performance caused by noise and other degradations.
First, we have end nodes/end points on which applications are ultimately run. These end nodes are quantum processors of at least one qubit. Some applications of a quantum internet require quantum processors of several qubits as well as quantum memory at the end nodes.
Quantum networks leverage qubits and because they can exist in superpositions of 0 and 1, so they make quantum transmissions more complex than classical bits. Also, because qubits can be entangled, where the state of one instantly affects the other, even across long distances, quantum networks leverage entanglement for secure communication.
In quantum networks Qubits are typically encoded into photons. Photons enable transmission and can leverage existing fiber optic infrastructure – a HUGE positive! Entangled photon pairs are generated at specialized sources, where two photons become quantum mechanically correlated. Entangled qubits have no predetermined states until measured. When one photon’s quantum state is observed, it instantly determines the corresponding state of its entangled partner.
There are a variety of photon encoding technologies including:
Polarization Encoding: The polarization state of a photon (such as horizontal vs. vertical or diagonal orientations) serves as the logical 0 and 1 for the qubit.
Path (or Dual-Rail) Encoding: This scheme uses two distinct optical paths for a single photon. The photon’s presence in one path is a logical 0, and the other is a logical 1.
Time-Bin Encoding: Qubit states are encoded based on the time a photon arrives at a detector. Typically, one time slot is 0, and another delayed slot encodes 1; superpositions involve the photon being in a quantum mixture of both times.
Frequency Encoding: Different frequencies (colors) of photons are used to represent different qubit states, with superpositions involving photon states across different frequencies.
Additional more complex encoding schemes use squeezed light, “cat” states or Gottesman-Kitaev-Preskill (GKP) states or amplitude and phase quadratures of electromagnetic fields. These schemes are typically used to protect quantum states in unique ways to develop resilience to certain errors.
Deployment over fiber optic networks. To transport qubits from one node to another, we need communication links. For the purpose of quantum communication, standard telecom optical fibers can be used. Optical networks have the advantage of reduced chances of decoherence. Typically, photons (particles of light) serve as carriers of qubits, sent through optical fibers or free-space links (see below for more). As the photons travel through fiber, new specialized technologies are required to preserve qubit states and entanglement.
Quantum repeaters. In fiber-optic networks transmission loss is compensated for by introducing amplifiers, which boost weak signals by producing many copies of the input photons. However, due to the no cloning theorem, an optic amplifier cannot perfectly copy a quantum state. The noise produced in the process of amplification destroys the entanglement necessary for quantum communications.
To overcome distance limitations and keep entanglement, quantum repeaters are used. Repeaters appear in between end nodes. Since qubits cannot be copied (no-cloning theorem), classical signal amplification is not possible. By necessity, a quantum repeater operates in a fundamentally different way than a classical repeater. These devices extend quantum communication by relaying entangled states over multiple short hops, enabling connections beyond the range of direct transmission.
As noted above, quantum repeaters are needed to overcome distance limitations in a quantum network. Repeaters perform entanglement swapping to reliably extend the distance between which two devices can become entangled. Quantum repeaters correct for photon loss without disrupting the quantum state of the communicated information by catching and storing (rather than measuring) the quantum bits encoded in photons.
Quantum networking integrations into the fiber-optic network infrastructure is important for ensuring privacy in optical communications. Multi-core fibers (MCFs), the likely building blocks of future high-capacity optical networks, offer new opportunities for such integration.
Other deployment methods
Direct free-space distribution. Entangled photon pairs are sent directly between endpoints on the Earth’s surface or between satellites and ground stations. Ground based experiments have demonstrated successful transfer of entangled photons over several kilometers in open-air, even though turbulent and noisy atmospheric conditions.
Satellite based systems. By equipping satellites with entangled photon sources and high-precision pointing, it is possible to distribute entanglement across long distances (1,200 km) between ground stations. Entangled photons are generated on a satellite and transmitted directly to two separate ground stations, or vice versa. These systems implement narrow-beam divergence and advanced pointing and tracking techniques to maximize efficiency and overcome losses from diffraction and atmospheric absorption. Systems involving multiple satellites and ground stations are under development to further extend the range of these techniques. Satellite based systems are important in that they make it possible to establish secure quantum links globally, beyond fiber’s reach, and may well form the backbone of the futuristic “Quantum Internet”.
Hybrid satellite + fiber networks. Research is also making progress on systems that combine satellite-based distribution for long-haul connections with ground based fiber and repeater networks for regional links. Optimization algorithms are used to determine which nodes and links should actively relay or receive entangled photons.
Quantum key distribution (QKD)
Quantum networks can enable Quantum Key Distribution protocols, that provide encryption keys that are physically impossible for an eavesdropper to copy or intercept without detection. This is made possible due to the fundamental properties of quantum mechanics, including quantum superposition, entanglement, and where the process of measuring a quantum system disturbs the system which is easily detected. This unique property ensures that the distributed keys remain secure, as any attempt at interception will be immediately apparent and will invalidate the exchanged key.
This domain is seeing a lot of research and commercialization work. The development of QKD systems has reached a high level of technical maturity, with multiple commercial vendors producing products tailored for various applications. New protocols, integration with machine learning, and practical deployment over fiber and satellite networks characterize recent progress. Further, researchers are working on real-world implementation, multi-user systems, and addressing scalability, noise resistance, and security vulnerabilities.
Core QKD protocols. There are several different methodologies for quantum key distribution. The BB84 protocol is a seminal QKD method first introduced in 1984.
BB84 is the most established and is a simple QKD protocol, relying on single photons with polarization encoding, and each photon representing a bit of data (zero or one). It achieves security by detecting eavesdropping via disturbances produced during measurement of non-orthogonal states.
E91 uniquely leverages quantum entanglement and Bell inequality violations, giving it device-independent security not reliant on trust in devices, but it has more implementation complexity and lower key rates.
B92 simplifies state choices to reduce implementation overhead, and continuous variable protocols encode information in quadrature amplitudes for compatibility with standard telecom technologies.
QKD protocols is a deep dark complex topic, and it deserves a dedicated post. More on this in the future. 😊
Research and testing implementations
Development in the quantum networking space is undergoing hyper acceleration on a global scale. There are way too many research, testing and commercialization efforts underway to list here. Suffice to say, quantum network test beds worldwide are transitioning from laboratory prototypes to integrated, field-ready systems supporting distributed quantum computing applications spanning university campuses, metropolitan areas, regional networks, and satellite links.
It is also not surprising that many of most advanced quantum networking research and commercialization activities combine university-led experiments, national test beds, and aggressive commercialization moves by industry players. Here are a few notable projects:
United States
Purdue University Quantum Network Testbed. Connects three laboratories via optical fiber underground, distributing photonic entanglement. Enables quantum key distribution (QKD), quantum process tomography, and microwave photonics over fiber.
Oak Ridge National Laboratory (ORNL) Quantum Communications and Networking Group. Maintains a metropolitan-scale test bed on a “dark fiber” network over 300+ km. Supports 400-gigabit conventional data transfer plus dedicated quantum channels.
The MIT Lincoln Laboratory Quantum Network Test Bed is a collaborative research initiative focused on building a scalable, high-rate quantum network for real-world testing of quantum networking applications. The test bed consists initially of two 43-kilometer optical fiber links connecting the Laboratory with MIT facilities.
Energy and Telecommunications Organization: EPB Quantum Network (Chattanooga, Tennessee). Commercial-scale quantum network where paying subscribers get access to an entanglement-based network with over 10 quantum nodes, equipment hubs, and operational control centers utilizing existing fiber infrastructure.
The U.S. Army Research Laboratory (ARL) and Air Force Research Laboratory (AFRL) have multi-node entanglement distribution and terrestrial/space-based test beds. These projects focus on both open research and applied tests with fiber and satellite links.
The U.S. Department of Energy (DOE) sponsors Quantum-in-Space collaborations, working with commercial vendors IonQ and Honeywell to leverage microgravity and advance quantum-secure communications, sensing, and scalable quantum networking.
Additional programs and test beds are operating at CalTech, Los Alamos National Labs, Argonne National Lab, Fermilab, and Brookhaven National Labs to name a few.
European Test beds
European test beds, around 15 have been reported across all major countries in Europe are often supported through the EU Quantum Flagship program. The European Commission fostered the creation of the Quantum Internet Alliance (QIA) with €24 million in funding to build “a global quantum internet made in Europe”.
Asia Test beds
China leads in quantum satellite communications, linking satellite QKD with terrestrial fiber networks. A team of researchers in China has demonstrated an integrated space-to-ground quantum communication network that combines a fiber network of more than 700 fiber links and two high-speed satellite-to-ground free-space links. Projects are also underway in Japan, India, Singapore, and South Korea.
Wrapping Up
Challenges remain in the march toward a fully functioning quantum network. Reliable quantum memories are one. Another important missing piece is the ability to extend the reach of a quantum link to arbitrarily long distances, using quantum repeaters. Quantum states cannot be simply copied and regurgitated, as is done with classical information. Quantum nodes will need sophisticated quantum logic gates to ensure that entanglement is preserved in the face of losses from interaction with the environment. Commercial projects and technologies are beginning to be introduced into the market. We can expect quantum networking to become mainstream quickly.
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