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Quantum Internet Protocol Stack: a Comprehensive Survey

Jessica Illiano, Marcello Caleffi, Antonio Manzalini, Angela Sara Cacciapuoti

arXiv:2202.10894v2cs.NIquant-ph

TL;DR

Classical Internet protocol stacks rely on layered separation of concerns and information replication, but quantum measurement, no-cloning, and entanglement challenge those assumptions. The survey reviews proposed Quantum Internet stack models and open problems, concluding that effective design requires further joint effort across quantum physics, computing, and telecommunications.

  • Problem

    Quantum mechanics prevents safely reading or copying quantum information without alteration, conflicting with classical protocol mechanisms that rely on replication and challenging direct adaptation of the classical stack.

  • Method

    The paper surveys proposed Quantum Internet protocol-stack models and synthesizes their open design problems and research directions.

  • Results

    The analysis concludes that the Quantum Internet requires a major protocol-stack paradigm shift because its design is governed by quantum information and entanglement.

  • Takeaways & Limitations

    Building an effective Quantum Internet protocol stack requires further joint effort across quantum physics, computer engineering, and telecommunications engineering.

Abstract

from arXiv · show

Classical Internet evolved exceptionally during the last five decades, from a network comprising a few static nodes in the early days to a leviathan interconnecting billions of devices. This has been possible by the separation of concern principle, for which the network functionalities are organized as a stack of layers, each providing some communication functionalities through specific network protocols. In this survey, we aim at highlighting the impossibility of adapting the classical Internet protocol stack to the Quantum Internet, due to the marvels of quantum mechanics. Indeed, the design of the Quantum Internet requires a major paradigm shift of the whole protocol stack for harnessing the peculiarities of quantum entanglement and quantum information. In this context, we first overview the relevant literature about Quantum Internet protocol stack. Then, stemming from this, we sheds the light on the open problems and required efforts toward the design of an effective and complete Quantum Internet protocol stack. To the best of authors' knowledge, a survey of this type is the first of its own. What emerges from this analysis is that the Quantum Internet, though still in its infancy, is a disruptive technology whose design requires an inter-disciplinary effort at the border between quantum physics, computer and telecommunications engineering.

1. Introduction

Classical Internet design uses layered protocol stacks and separation of concerns, but quantum mechanics invalidates key assumptions behind classical communication protocols. Quantum entanglement further changes network connectivity and motivates a major protocol-stack redesign.

  • Classical Stack Background: Layered protocol stacks organize network functionalities into modular services exchanged through specific protocols.This separation of concerns enabled the classical Internet to evolve from a small network into one interconnecting billions of devices.
  • Quest for a Major Paradigm Shift: These quantum-mechanical properties require a major paradigm shift rather than a direct adaptation of the classical protocol stack.The paper frames this shift as a response to constraints imposed by quantum information and entanglement.
  • Quest for a Major Paradigm Shift: Quantum measurement and no-cloning make safely reading or copying quantum information impossible without altering it.Classical mechanisms such as ARQ and caching rely on information being readable and replicable.
  • Quest for a Major Paradigm Shift: Quantum entanglement provides nonclassical correlations that can support classical and quantum information transmission and applications unavailable in classical networks.The cited examples include secure communications, blind computing, and distributed quantum computing.

V. Beyond Physical Connectivity

The survey examines Quantum Internet protocol-stack designs and the open problems that remain for an effective stack. Its structure spans state-of-the-art models, connectivity concepts, and research directions.

  • Open Issues and Research Directions: The paper organizes later discussion around Quantum Internet protocol-stack models, connectivity, and open research issues.Listed topics include virtual, augmented, and on-demand connectivity, latency, synchronization, medium access, broadcasting, and quantum addressing.
  • Quantum Internet Protocol Stack: State-of-the-Art: The survey first reviews state-of-the-art efforts toward designing the Quantum Internet protocol stack.This review is followed by analysis of unresolved problems affecting effective stack design.

Qubit in a nutshell

A qubit can occupy a superposition of basis states and is represented by a normalized state vector with complex amplitudes. Measurement collapses the state, while environmental interaction can irreversibly affect it.

  • Qubit in a nutshell: A qubit can simultaneously be in a superposition of the basis states |0⟩ and |1⟩.Its state is expressed as a linear combination of those basis states.
  • Qubit in a nutshell: The amplitudes α0 and α1 are complex numbers whose squared magnitudes give standard-basis measurement probabilities.They satisfy the normalization condition |α0|^2 + |α1|^2 = 1.
  • Qubit in a nutshell: Measurement irreversibly alters the original quantum state by collapsing a superposition into a basis state associated with the measurement device.Quantum states are also described as fragile because environmental interactions can irreversibly affect them.
  • Qubit in a nutshell: An n-qubit system can be in a superposition of all 2^n basis states, unlike n classical bits, which occupy one of those states at a time.The amplitudes of the n-qubit state obey a normalization condition.
  • Qubit in a nutshell: The paper uses Dirac, or bra-ket, notation to represent quantum states with ket and bra vectors.A pure state can be described by a ket vector in state-vector form.

2. Preliminaries

Classical network architecture uses layered abstractions, with OSI emphasizing detailed layer descriptions and TCP/IP emphasizing heterogeneous-network interconnection. The survey introduces these models as a basis for examining why Quantum Internet design requires different abstractions.

  • 2. Preliminaries: Abstract models simplify complex communication networks by standardizing functionalities while abstracting from underlying technologies.The OSI and TCP/IP models provide the classical architectural basis discussed in the preliminaries.
  • 2. Preliminaries: Each classical layer provides services upward, uses services below, and communicates with the corresponding layer in another network entity through protocols.A protocol defines the rules and messages for same-layer interactions and service performance.
  • 2.1. Classical Stack Background: The classical core layers cover raw-bit transmission, reliable intra-network packet delivery, path discovery, forwarding, and transport services.TCP/IP groups intra-network functions into its lowest network-access layer and uses the internet layer for forwarding across networks.
  • 2.1. Classical Stack Background: OSI has seven layers with detailed service descriptions, whereas TCP/IP has four broader layers designed to interconnect heterogeneous networks.The current Internet roughly combines the first two OSI layers with the last three TCP/IP layers.
  • 2.1. Classical Stack Background: Separation of concerns is effective but sub-optimal because it restricts cross-layer interaction.TCP may misinterpret wireless packet losses as congestion, illustrating why lower-layer information can matter to higher-layer decisions.

Entanglement in a nutshell

Entanglement is a shared quantum property whose behavior depends on how a composite system is divided into subsystems. The section introduces bipartite and multipartite entanglement and its implications for quantum-network design.

  • The Bell state |Φ+⟩ is a maximally entangled two-qubit state shared by distant parties such as Alice and Bob.Each party’s independent measurement is random, while compared outcomes coincide.
  • Three-qubit systems include unentangled, biseparable, and genuinely tripartite-entangled configurations.The section notes that larger systems have broader entanglement classifications.
  • Entanglement is not absolute: a state may be entangled under one subsystem decomposition and unentangled under another.The decomposition must therefore be specified in context.
  • A qubit can occupy a superposition, but measurement irreversibly collapses its state into one of two orthogonal basis states.The measurement postulate therefore changes the original quantum state.

Quantum Teleportation

Quantum teleportation transfers an unknown qubit without physically moving its encoding particle. It combines shared entanglement, local operations, and classical communication between source and destination.

  • Quantum teleportation requires an EPR pair, local quantum operations at both endpoints, and two classical bits sent from source to destination.
  • Teleportation avoids physical transfer of the particle encoding an unknown qubit, while relying on entanglement and a classical channel.
  • Unknown qubits cannot be copied or safely recovered through measurement after loss or decoherence, preventing direct reuse of classical communication techniques.
  • The source performs a Bell state measurement using a CNOT gate, a Hadamard gate, and measurements of two qubits.The information qubit is the CNOT control and the entangled qubit is the target.
  • The destination uses the received measurement results to choose a unitary post-processing operation on its entangled qubit.

Entanglement Swapping

Entanglement swapping extends quantum communication beyond directly connected links by converting short-range entanglement into end-to-end entanglement. Its operation is constrained by decoherence and, here, the discussion focuses on first-generation repeaters.

  • Entanglement distribution over long distances is limited because decoherence affects entangled pairs during distribution.
  • Entanglement swapping distributes entanglement between remote nodes that are not directly connected by a quantum link.
  • Bell state measurements at repeaters consume sub-link entanglement and generate end-to-end entanglement between the source and destination.The original entanglement is destroyed during the operation.
  • Multiple repeaters can extend the procedure across source–repeater, repeater–repeater, and repeater–destination sub-links.
  • Swapping operations may occur in any order, but decoherence imposes a time constraint on the entangled pairs.
  • The discussion limits its attention to first-generation quantum repeaters based on entanglement swapping.

3. Classical Information vs Quantum Information vs Quantum Entanglement

Quantum bits and entanglement differ fundamentally from classical bits in storage, measurement, copying, and network state requirements. These differences affect the entire protocol stack rather than only isolated layers.

  • The Quantum Internet therefore requires a paradigm shift rather than a one-to-one replacement of classical protocol layers.
  • Quantum information irreversibly degrades through decoherence, imposing hard temporal constraints unlike the long-lived storage of classical information.
  • Communicating quantum information differs from distributing entanglement as a resource.
  • Unlike stateless bits, entanglement requires nodes storing correlated qubits to cooperate and retain information about one another.
  • Entanglement is a heterogeneous resource with different classes of states, properties, and enabled applications.

4. Entanglement: a Deeper Look

Entanglement is the central communication resource of the Quantum Internet, with different multipartite states offering distinct connectivity, persistence, noise robustness, and application properties. The survey contrasts GHZ and W states and relates their conversion and distribution to network design.

  • Entanglement basics: Bipartite entanglement distinguishes separable states from entangled states, with Bell states or EPR pairs providing maximal nonclassical correlation.Perfect deterministic quantum teleportation is achievable only with maximally entangled pairs.
  • Entanglement classes: Tripartite systems include fully separable, biseparable, and genuinely tripartite-entangled states, while larger systems have increasingly complex classifications that remain incompletely understood.The survey notes that infinitely many SLOCC classes exist for larger systems, including cluster states.
  • Entanglement classes: GHZ and W states are inequivalent SLOCC classes with different network properties: GHZ states are maximally connected, whereas W-state EPR extraction is probabilistic.A 3-qubit GHZ deterministically yields an EPR pair after a Hadamard gate and computational-basis measurement on the residual qubit; a 3-qubit W yields one with probability 2/3.
  • Connectivity and persistency: For an n-qubit W state, EPR-pair extraction succeeds with probability 2/n, while accidental measurement produces an unentangled state with probability 1/n.Thus, extraction probability decreases linearly with n, and W states offer robustness against losses or accidental measurement.
  • Applications and robustness: GHZ and W states support different applications and noise responses: GHZ symmetry suits consensus or synchronization, while W states are more persistent and noise advantages depend on the noise type.GHZ states are more robust to X-noise, W states to Y-noise, and Z-noise robustness depends on the acting noise.
  • Entanglement transformations: Multipartite states can be converted into EPR pairs through local operations and classical communication, while multiple EPR pairs can be fused into states such as GHZ.When extracting an EPR pair from GHZ, the classical measurement output determines whether the final state is |Φ+⟩ or |Φ−⟩.

5. Beyond Physical Connectivity

Quantum entanglement introduces virtual, augmented, and on-demand forms of connectivity that differ fundamentally from classical physical connectivity. These dynamic connectivity concepts reshape how Quantum Internet protocol functionalities must be designed.

  • 5.1. Virtual Connectivity: Entanglement enables a virtual quantum link between nodes, allowing qubit transmission without using the underlying quantum link at transmission time.Shared entanglement creates virtual connectivity, but teleportation consumes the entangled resource and requires later regeneration.
  • 5.1. Virtual Connectivity: Virtual connectivity is active after successful entanglement distribution, persists despite physical-channel variability, and ends when entanglement is consumed or destroyed.Decoherence also affects the shared entangled resource, so virtual connectivity must be restored through new entanglement distribution.
  • 5.2. Augmented Connectivity: Entanglement swapping extends virtual connectivity across multiple hops, creating augmented links between remote nodes that lack direct physical quantum connections.A repeater performs measurements on locally held qubits, while the measurement outcome must be communicated to recover the resulting entangled state.
  • 5.2. Augmented Connectivity: Augmented connectivity redefines network neighborhood because physically remote nodes can become direct neighbors in the augmented graph.This neighborhood concept has no classical-network counterpart and affects protocol-stack design.
  • 5.3. On-Demand Connectivity: Multipartite entanglement supports on-demand connectivity by allowing EPR pairs and virtual links to be selected according to communication needs.Multiple unicast channels can be formed between disjoint node pairs, while the connected identities remain dynamically selectable.
  • 5.3. On-Demand Connectivity: Entanglement-driven connectivity is highly dynamic, with link number, link characteristics, and connected-node identities changing through operations such as swapping and measurement.This dynamism has no classical counterpart and must be incorporated into Quantum Internet network-function design.

6. Quantum Internet Protocol Stack: State-of-the-Art

The survey reviews three major Quantum Internet protocol-stack proposals, emphasizing their layered mechanisms for entanglement generation, control, purification, swapping, and hardware interaction. The models differ notably in their treatment of layer boundaries and entanglement structure.

  • The survey focuses on three comprehensive Quantum Internet protocol-stack proposals developed by Van Meter et al., Wehner et al., and Dür et al.The overview is intended to clarify open problems and research efforts toward an effective and complete stack.
  • Van Meter et al.: Van Meter’s repeater model separates single-hop entanglement generation and control from multi-hop purification and swapping control.Generation attempts and ACK/NACK signaling recur until an EPR pair is shared, while purification and swapping recur across multi-hop routes.
  • Van Meter et al.: The Van Meter architecture introduces quantum sockets to manage application access to network-provided entanglement and quantum services.Applications may retain entangled states or consume entangled qubits through immediate measurement.
  • Wehner et al.: Wehner et al.’s five-layer model uses a hardware-informed physical layer and a link layer that supports robust, parameterized, and hardware-independent entanglement generation.The QEGP accepts requests containing parameters such as remote node, pair count, minimum fidelity, request type, and measurement basis; a hardware abstraction sub-layer supports portability.
  • Dür et al.: Dür et al.’s proposal explicitly exploits multipartite entanglement, while further research is needed to analyze its impacts and trade-offs.This distinguishes it from the Van Meter and Wehner architectures, which are described as EPR-based models.
  • Model comparison: Among the EPR-based models, a clear boundary separates layers implementing quantum communications from layers using only local operations and classical communications.This boundary is explicit in Van Meter’s and Wehner’s models but cannot be explicitly drawn in Dür et al.’s model.

7. Open Issues and Research Directions

The survey identifies unresolved protocol-stack issues involving synchronization, signaling, performance metrics, entanglement access, and broadcasting. These issues arise from quantum-specific temporal constraints, resource coordination requirements, and restrictions such as the no-broadcasting theorem.

  • Effective Quantum Internet stack design still requires substantial research because pivotal open issues remain unresolved.The survey presents these issues alongside research directions and opportunities for contributions from multiple areas of expertise.
  • Latency and synchronization: Entanglement generation can require nanosecond-scale temporal matching, and failures are irreversible when synchronization constraints are unmet.The synchronization requirements may exceed current Internet performance and also affect quantum-repeater operations involving entanglement swapping.
  • Signaling: Quantum-network signaling is mainly envisioned through classical messages, while possible advantages of quantum signaling remain unexplored.The issue is illustrated by signaling among entities involved in atom-based optical-cavity generation schemes.
  • Metrics: Quantum-network performance metrics must account for the interplay between classical signaling throughput and stochastic entanglement-generation phenomena.The survey calls for interdisciplinary work to identify parameters that characterize performance at each protocol-stack layer.
  • Resources: The number of communication qubits available at network nodes is an additional performance dimension without a classical-network counterpart.At least one qubit at each processor must be reserved for generating an entangled state.
  • Metrics: No univocal metric definitions currently support a fair quantitative comparison among proposed Quantum Internet protocol-stack models.The survey identifies this comparison as an open problem, made more complex by the field’s early stage.
  • Medium access: Entangled resources require coordinated access because an uncoordinated operation can irreversibly corrupt them.An Entanglement Access Control protocol is needed for EPR-pair conflicts, and coordination overhead may increase with multiparty entanglement.
  • Broadcasting: The no-broadcasting theorem prevents broadcasting an unknown quantum state to multiple receivers, leaving the necessity of such functionality unresolved.This contrasts with classical uses such as ARP and DHCP, which rely on simultaneous transmission to nodes in a physical network portion.

7.5. Networking

Quantum networking must support both physical and entanglement-based virtual connectivity, while coordinating quantum operations through classical signaling. Existing layered abstractions remain incomplete because quantum functionalities span layers and require broader cross-layer interaction.

  • Networking functionalities: Quantum networking requires path discovery, forwarding, and routing designed around entanglement as a communication resource.
  • Connectivity: Physical neighbor discovery may use classical protocols, but virtual neighbor discovery remains unresolved because entanglement changes connectivity and neighborhood semantics.Multipartite entanglement requires identifying all entangled nodes rather than answering only whether one node is a neighbor.
  • Routing: Routing must combine a physical quantum path with a classical path for exchanging signaling between quantum nodes.
  • Entanglement generation: The interplay between proactive or reactive entanglement generation and routing remains unresolved, with hardware coherence time creating competing preferences.Short coherence times may favor reactive generation but can make reactive routing less attractive because on-demand path discovery adds delay.
  • Classical-quantum interplay: Quantum networks are unlikely to be autonomous because both physical and virtual connectivity require tight coordination through classical signaling.
  • Cross-layer design: Quantum communication functionalities are distributed across layers, making adjacent-layer separation of concern inadequate and motivating unified cross-layer interaction.Possible approaches include classical signaling through the classical Internet or explicit cross-layer interactions within the quantum stack.
  • Classical-quantum interface: The classical-quantum interface must support bidirectional, multi-layer interactions rather than a single fixed service boundary.

7.7. Quantum Addressing and Quantum Path

Quantum addressing and quantum paths extend quantum networking beyond encrypted content toward private identities and nonclassical channel placement. These possibilities introduce open architectural choices, including how quantum hardware should be abstracted and whether the network should use packet or circuit switching.

  • Quantum Addressing: Quantum addressing could make both message content and source-destination identities private through quantum states.The paper presents this as the basis for Quantum virtual Private Networks and identifies quantum addressing as largely unexplored.
  • Quantum Addressing: Existing quantum-address proposals targeted superposition of paths and tasks, unlike the proposed quantum equivalent of IP addressing.
  • Quantum Path: Quantum paths allow channel placement, as well as information carriers, to be treated quantum mechanically and can involve simultaneous propagation across multiple space-time trajectories.The survey reports theoretical and experimental verification and argues that these effects influence the entire protocol stack.
  • Hardware and software: Quantum hardware diversity creates a need for abstractions and interfaces that decouple underlying technologies from upper software layers.The survey notes that industry has not consolidated around one hardware technology and connects this need with quantum compilers and software.
  • Network philosophy: Choosing packet switching or circuit switching is a fundamental Quantum Internet design decision with systemwide cascade effects.The choice concerns decentralized best-effort infrastructure versus centrally managed network optimization and management.

8. Conclusions

The Quantum Internet is expected to support secure communications, distributed quantum computing, and new scientific applications through successive developmental stages. The survey emphasizes that realizing this vision requires further joint effort rather than definitive answers to its open design problems.

  • The Quantum Internet could enable ultra-secure communications, distributed quantum computing, and new scientific applications.
  • Quantum networks are expected to progress from trusted repeaters and entanglement distribution toward memory, fault-tolerant qubit networks, and integration with the current Internet.
  • The survey frames its contribution as highlighting the need for further joint effort to build the Quantum Internet rather than resolving its central open issues.
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