About this Video

• Video Title: Theoretical Tutorial: Quantum communications
• Channel: CQC2T | Centre for Quantum Computation and Communication Technology
• Speakers: (Speaker name not explicitly mentioned in the transcript)
• Duration: 00:39:58

Introduction

This video is a theoretical tutorial on quantum communications. The speaker explains the concept, explores the challenges of long-distance quantum communication due to signal loss, and discusses different approaches to overcome these challenges using quantum repeaters. The tutorial also compares discrete and continuous variable approaches to quantum communication.

Key Takeaways

1. Quantum Communication Defined: Moving quantum information encoded in a quantum state from one location to another, requiring the physical transfer of a quantum particle or system while maintaining entanglement.
2. Applications of Quantum Communication: Secure communication (distributing random keys), quantum computing (connecting distant qubits), and enhanced sensing.
3. Challenges of Long-Distance Quantum Communication: Signal loss due to factors like scattering, absorption, temperature gradients, and beam spreading. Different encoding methods (single photon qubits, coherent/squeezed states, cat/GKP states, field qubits) experience different types of errors from this loss.
4. Quantum Repeaters: Methods to extend the range of quantum communication by breaking the channel into segments, distributing entanglement between segments, and using purification or error correction to maintain entanglement over long distances. Three generations of quantum repeater proposals are discussed, each with its own advantages and disadvantages.
5. Discrete vs. Continuous Variables: A comparison of using single-photon qubits (discrete) versus coherent/squeezed states (continuous) for encoding quantum information. Discrete variables offer advantages in post-selection, while continuous variables offer deterministic sources and may be more versatile. The tutorial also discusses the challenges of integrating quantum communication into existing telecom infrastructure.

The transcript highlights several advantages and disadvantages of discrete versus continuous variables for quantum communication:

Discrete Variables (e.g., single-photon qubits):

• Advantage: Post-selection allows effective screening of loss; if a photon reaches the receiver, it's known not to have been lost to the environment. This preserves entanglement, though at the cost of reduced rate.

• Disadvantages: Lack of deterministic high-quality sources (quantum dots are improving but still not fully deterministic). Incompatibility with existing telecom infrastructure; single-photon sources and detectors are not currently part of the telecom network.

Continuous Variables (e.g., coherent states):

• Advantages: Deterministic high-quality sources (lasers readily produce coherent states). Compatibility with existing telecom infrastructure (coherent states and homodyne detection are used in telecom systems). CV teleportation offers the potential to move any quantum state, not just those matched to the teleporter.

• Disadvantages: Classical correlations leak information even with no entanglement to the environment, requiring more effort to remove in QKD protocols. Repeaters are inherently incompatible with existing telecom infrastructure. While potentially easier to manipulate many photons in a single mode compared to discrete variables, the transcript does not give specific details on this.

The choice between discrete and continuous variables depends on the specific application and priorities, such as the need for high rates, long distances, or compatibility with existing technology.