Session Date/Time: 07 Sep 2022 12:00

QIRG

Summary

This session featured a presentation by Stephen Diadamo on the concept of packet switching in quantum networks as a pathway toward a scalable quantum internet. The presentation covered existing quantum network architectures (QKD-specific and entanglement-based), highlighted their limitations, and proposed a vision for universal, transparent, and scalable packet-switched quantum networks, including an intermediate burst-switching approach and a hybrid network model. The discussion included technical challenges and simulation results for key quantum applications.

Key Discussion Points

Introduction to Quantum Networks:
- A quantum network connects quantum-capable devices, possibly alongside classical nodes.
- Channels typically use free space or fiber optics, with photons as the transport medium.
- An OSI-like layered model is commonly proposed for quantum network stacks.
- Networks can operate at local (data center), metropolitan (city-wide QKD), or wide-area (interconnecting quantum networks with quantum repeaters or satellites) scales.
Applications of Quantum Networks:
- Quantum Key Distribution (QKD): Near-term, well-established application for secure key exchange.
- Blind Quantum Computing: Performing quantum computations on a remote quantum computer without revealing the algorithm.
- Distributed Quantum Computing: Connecting quantum computers to perform larger-scale algorithms.
- Networked Quantum Sensors: Enhancing sensitivity for measuring time, magnetic fields, gravity, etc., by connecting quantum sensors.
Speaker's Group Focus (Cisco Quantum Research Group):
- Focused on lower layers of the network stack: protocols for quantum state transport, quantum network simulation, control, security, and hardware (optical routers/switches, quantum repeaters, quantum memory).
Existing Quantum Network Architectures:
- Quantum Key Distribution (QKD) Networks:
  - Application-specific (only for QKD), often deployed side-by-side with classical networks.
  - Utilizes "trusted repeaters" (trusted relays) to overcome distance limitations due to lack of true quantum repeaters.
- Entanglement-Based Quantum Networks:
  - Relies on establishing entanglement between nodes, then using quantum teleportation to transport quantum states.
  - General purpose (can send any quantum state), but technologically challenging (robust entanglement distribution and storage, potentially slow rates).
  - Often envisioned as working alongside the classical internet.
Limitations of Existing Designs:
- Scalability: Rely on circuit switching or static switching models, which historically don't scale well for many users.
- Application Specificity: QKD networks are often single-purpose; entanglement-based might not be optimal for prepare-and-measure QKD.
- Integrability: Limited integration with existing classical internet infrastructure and protocols.
Vision for Future Quantum Networks: Packet Switching
- Three Pillars: Universality (support all quantum applications), Transparency (integrate with classical technology, e.g., shared fibers/hardware), Scalability (scalable protocols and state transport).
- Packet Switching Concept:
  - Contrast with circuit switching (dynamic vs. reserved routes, ordered vs. out-of-order arrival, fair use).
  - Proposed Hybrid Data Frame: Classical Header + Quantum Payload + Classical Trailer.
  - Allows dynamic routing of quantum information.
- Multiplexing Schemes: Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM) for header, payload, and trailer.
- Node Architecture: Classical control unit triggers C-Q transmissions. At intermediate nodes, demultiplexing occurs, the classical header is processed, the quantum payload is stored in a quantum memory or delay line, and then recombined and re-multiplexed.
- Long-Term Vision: Quantum Reconfigurable Optical Add/Drop Multiplexers (ROADM) incorporating quantum memories, error correction, and frequency conversion.
Challenges for Packet Switching in Quantum Networks:
- No Quantum Amplification: Requires extremely low-loss optical hardware.
- Distance Limitations: Switches may need to be closely spaced (e.g., single-digit kilometers).
- Crosstalk: Minimizing Raman noise between strong classical and weak quantum signals sharing a fiber.
- Hardware Constraints: Integration of classical and quantum switching in devices, potentially requiring cryogenic temperatures.
- Networking Specifics: Robust quantum memories, quantum error correction, choice between quantum relays (dumb forwarders) and repeaters (complex processing), network security.
Intermediate Compromise: Burst Switching:
- Sends classical header ahead of time to prepare the switch, minimizing the need for quantum memory storage at intermediate nodes for the quantum payload.
- Involves precise classical-quantum transmission scheduling and channel reservation (less stringent than circuit switching, more than pure packet switching).
- Can mitigate crosstalk effects.
- Challenges include determining optimal burst time (guard time) based on network parameters and number of hops.
Hybrid Network Model:
- Combine packet switching for short-distance segments with entanglement-based "teleportation channels" (first-generation quantum repeaters) for longer distances.
Simulation Results:
- QKD (BB84) over a line network: Simple model shows positive secret key rates are achievable over multiple hops, even with some loss (optimistic scenario).
- Entanglement Distribution (Bell states): NetSquid simulations show achievable fidelity with noisy quantum memories and channels, particularly at shorter distances. Millisecond-regime memory storage appears necessary. Processing time for packets at nodes also critical.
Open Problems and Future Outlook:
- Crosstalk mitigation (especially Raman noise) in network and protocol design.
- Identifying scenarios where packet switching outperforms other designs and defining relevant metrics.
- Efficient quantum frame generation and guard time optimization for burst switching.
- Reducing classical header processing time and defining essential header information.
- Emphasized the importance of considering user scalability and hybrid approaches now, based on lessons from classical internet history favoring packet switching.
Q&A Discussion:
- Classical Header/Quantum Payload Separation and Loss: Separated by time or wavelength multiplexing. Loss management depends on the application; quantum payloads lost if header is lost. Retransmission may be possible for QKD, but complex for quantum computing.
- Quantum Error Correction (QEC): QEC involves encoding logical qubits onto multiple photons (a "set of photons") that would travel together as part of the quantum payload. This is a general challenge for quantum repeaters, not specific to packet switching.
- Routing Schemes: The proposed packet-switching model can accommodate various dynamic routing approaches, including source routing or label switching, not just hop-by-hop.
- Wavelengths for Classical/Quantum Coexistence: Discussion on using 1310nm for quantum and 1550nm for classical traffic to reduce crosstalk, even with good multiplexing. Hollow-core fiber was mentioned as a potential solution for mixing signals with less crosstalk, though it currently has higher loss.
- Standardized Icons: A request was made for Stephen Diadamo to consider adopting a set of standardized icons for quantum network node types for consistency across research groups.

Decisions and Action Items

Stephen Diadamo was requested to share a link to the proposed standardized quantum network icons for consideration.
Discussions on the presented material are encouraged to continue on the QIRG mailing list.

Next Steps

The QIRG plans to continue hosting seminars approximately twice a year, with announcements posted on the mailing list.
Participants were encouraged to attend the upcoming IETF meeting in Tokyo in March.