Overview

Quantum communication leverages the principles of quantum mechanics to transmit information securely and efficiently. Unlike classical communication, which uses bits (0 or 1), quantum communication uses quantum bits (qubits) that can exist in superpositions, enabling unique protocols for security and data transfer.

Historical Context

  • 1970s: Quantum key distribution (QKD) was first proposed, notably by Stephen Wiesner and later Charles Bennett and Gilles Brassard (BB84 protocol, 1984).
  • 1990s: Experimental demonstrations of QKD and quantum teleportation.
  • 2000s-Present: Development of quantum networks, satellite-based quantum communication, and integration with classical infrastructure.

Core Principles

1. Superposition

A qubit can be in a state |0⟩, |1⟩, or any linear combination:
|ψ⟩ = α|0⟩ + β|1⟩, where |α|² + |β|² = 1

2. Entanglement

Two qubits can be entangled, meaning their states are correlated regardless of distance.
Example: Bell state
|Φ⁺⟩ = (|00⟩ + |11⟩)/√2

3. No-Cloning Theorem

It is impossible to create an exact copy of an arbitrary unknown quantum state.
Equation:
If |ψ⟩ is unknown, there is no unitary operation U such that U(|ψ⟩|0⟩) = |ψ⟩|ψ⟩.

Quantum Key Distribution (QKD)

QKD allows two parties to share a secret key using quantum states.
BB84 Protocol Steps:

  1. Sender (Alice) encodes bits using two bases (rectilinear and diagonal).
  2. Receiver (Bob) measures using random bases.
  3. Alice and Bob compare bases over a public channel.
  4. Matching bases yield a shared key; mismatches are discarded.

Security:
Any eavesdropping introduces detectable errors due to quantum measurement disturbance.

Quantum Teleportation

Quantum teleportation transmits a qubit’s state using entanglement and classical communication.

Steps:

  1. Alice and Bob share an entangled pair.
  2. Alice performs a Bell measurement with her qubit and the unknown state.
  3. Alice sends classical bits to Bob.
  4. Bob applies a quantum operation to recover the original state.

Key Equation: If Alice wants to teleport |ψ⟩ = α|0⟩ + β|1⟩, after measurement and communication, Bob’s qubit becomes |ψ⟩.

Quantum Repeaters

Quantum signals degrade over distance due to loss and decoherence. Quantum repeaters extend range by:

  • Entanglement swapping
  • Error correction
  • Purification of entangled states

Latest Discoveries & Research

Quantum Satellite Communication

  • Micius satellite (China): Demonstrated QKD over 1200 km (Nature, 2017).
  • Quantum Internet: Ongoing efforts to link quantum devices globally.

Integrated Quantum Networks

  • 2021: Researchers at TU Delft achieved entanglement between nodes in a quantum network over metropolitan distances (Nature, 2021).
  • 2022: Quantum communication protocols tested with photonic chips, paving the way for scalable quantum internet (ScienceDaily, 2022).

Quantum Memory and Error Correction

  • Advances in quantum memory stability and quantum error correction codes are enabling longer-distance communication and robust networks.

Key Equations

1. Qubit State

|ψ⟩ = α|0⟩ + β|1⟩
Normalization: |α|² + |β|² = 1

2. Entangled State

|Φ⁺⟩ = (|00⟩ + |11⟩)/√2

3. Fidelity

Fidelity measures the accuracy of quantum state transfer:
F(|ψ⟩, |φ⟩) = |⟨ψ|φ⟩|²

Diagrams

Quantum Key Distribution

QKD Diagram

Quantum Teleportation

Quantum Teleportation

Quantum Network

Quantum Network

Surprising Facts

  1. Quantum communication is not limited by distance: Satellite-based QKD has already achieved secure communication over thousands of kilometers.
  2. Quantum signals are fundamentally immune to undetected eavesdropping: Any interception changes the quantum state, alerting users.
  3. Quantum networks can connect quantum computers and sensors: Creating a future “quantum internet” with applications in cryptography, distributed computing, and ultra-precise sensing.

Applications

  • Secure communication: Banking, military, government.
  • Quantum internet: Linking quantum computers for distributed processing.
  • Quantum sensor networks: Enhanced precision in scientific measurements.

Summary Table

Concept Description Key Equation/Principle
Qubit Quantum bit, superposition of 0 and 1
Entanglement Correlated quantum states
QKD Secure key distribution using quantum states No-cloning theorem
Quantum Teleportation State transfer via entanglement and classical bits Fidelity
Quantum Repeater Extends communication range Entanglement swapping

References

Bonus Fact

The human brain has more connections (synapses) than there are stars in the Milky Way, highlighting the complexity of biological networks compared to even the most advanced quantum networks.