Quantum Teleportation in Optical Fibers

January 3, 2025 15 min read

Rodrigo Arce Computer Engineer

Research Overview

A groundbreaking study by researchers at Northwestern University demonstrates quantum teleportation coexisting with classical communications in optical fiber networks. This achievement represents a significant step toward practical quantum networks.

Authors: Jordan M. Thomas, Fei I. Yeh, Jim Hao Chen, et al.
Institution: Northwestern University
Key Achievement: First demonstration of quantum teleportation over fibers carrying conventional telecommunications traffic

Introduction

I'm not a quantum physicist, but i'm a computer engineer and i'm very interested in quantum mechanics. This is a very interesting experiment that shows that we can achieve quantum teleportation coexisting with classical communications in optical fiber networks. Quantum mechanics is always a fun field to explore until you REALLY study it. It is always really hard to understand, so i made the job for you: i read the paper and i'll explain it to you in a way that is easy to understand.

The integration of quantum networks with existing classical fiber infrastructure represents one of the most significant challenges in quantum communication. This groundbreaking experiment demonstrates, for the first time, quantum teleportation coexisting with high-speed classical data transmission in the same optical fiber - a crucial milestone for practical quantum networks.

While previous experiments have shown quantum state transmission alongside classical signals, quantum teleportation presents unique challenges due to its reliance on quantum entanglement and the need for precise Bell state measurements. The success of this experiment opens new possibilities for building quantum networks using existing telecommunications infrastructure.

Significance

This experiment demonstrates several key breakthroughs:

First demonstration of quantum teleportation in fibers carrying live telecommunications traffic
Achieved high fidelity state transfer while maintaining classical data rates
Proved the feasibility of quantum-classical network integration
Established practical noise mitigation techniques for hybrid networks

Technical Implementation

The photon generation system employs a sophisticated configuration utilizing Type-0 cascaded second harmonic generation (SHG) combined with spontaneous parametric down-conversion (SPDC). At the heart of this setup are PPLN (Periodically Poled Lithium Niobate) waveguides that are specifically phase-matched for 650 nm second harmonic generation. The system is driven by a 1300-nm continuous-wave distributed feedback laser serving as the pump source. This laser operates with precise temporal characteristics - a 65-picosecond temporal full-width at half maximum and a pulse repetition rate of 500 MHz. These carefully chosen parameters ensure optimal photon pair generation while maintaining the coherence necessary for quantum teleportation.

To understand this in simpler terms, imagine a game of pool where you need to hit two balls perfectly. The laser is like your pool cue - it needs to strike with just the right force and timing. The PPLN waveguides are like specially designed rails that guide the balls (photons) exactly where they need to go. Just as a pool player needs perfect timing and precision to make a complex shot, this system needs precise timing (65 picoseconds) and repetition (500 MHz) to create and guide the photon pairs needed for quantum teleportation.

Noise Mitigation Strategy

One of the most significant challenges in quantum-classical coexistence is managing noise from spontaneous Raman scattering (SpRS). The experiment implements several innovative approaches to address this:

Multi-Layer Noise Suppression

The system achieves exceptional noise suppression through a combination of techniques:

Wavelength Engineering:
By utilizing O-band (1290 nm) quantum channels, the system minimizes Raman scattering from the C-band classical signals. This wavelength choice provides nearly optimal anti-Stokes frequency detuning, reducing noise by approximately an order of magnitude compared to conventional 1310-nm channels.
Advanced Filtering:
The implementation of narrow-band spectro-temporal filtering through 60-pm bandwidth Fiber Bragg Gratings provides exceptional isolation from classical channel noise. The system achieves >190 dB rejection of C-band light through cascaded filtering stages.
Temporal Correlation:
Multi-photon coincidence detection within a precise 500-ps window significantly reduces the impact of uncorrelated noise photons. This temporal filtering is particularly effective when combined with the inherent timing correlations of the photon-pair sources.

Performance Analysis

The system demonstrates remarkable stability and high fidelity across various quantum states. Particularly noteworthy is the maintenance of quantum state fidelity even with classical power levels 150 times higher than the minimum required for 400-Gbps communication. This headroom suggests the potential for supporting multi-terabit classical communication alongside quantum teleportation in future implementations.

The achieved average fidelity of 89.9 ± 3.1% significantly exceeds the classical limit of 2/3, providing clear evidence of quantum teleportation. The system maintains high fidelity across both the computational basis states (H/V) and superposition states (D/A), demonstrating robust quantum state preservation across the complete Bloch sphere.

Technical Overview

Key Innovation

First demonstration of quantum teleportation over optical fibers while simultaneously carrying conventional telecommunications traffic. The experiment achieved:

Distance: 30.2 km fiber link
Classical Data: 400-Gbps C-band traffic
Quantum Channel: O-band (1290 nm)
High Fidelity: Average fidelity of 89.9 ± 3.1%

System Architecture

Three-Node Configuration

Alice (Sender):
- Prepares quantum state |𝜓⟩𝐴 for teleportation
- Uses heralded single-photon source
- Encodes qubits in photon polarization
Charlie (Midpoint):
- Performs Bell State Measurement (BSM)
- Uses Hong-Ou-Mandel interference
- Employs 50:50 beam splitter configuration
Bob (Receiver):
- Generates entangled photon pairs
- Maintains one photon locally
- Sends other photon to Charlie

Key Quantum Concepts:

Quantum State: A mathematical description of a quantum system's properties (like spin or polarization). Unlike classical states that are definite, quantum states can exist in multiple states simultaneously (superposition).
Qubit: The quantum equivalent of a classical bit. While a classical bit is either 0 or 1, a qubit can be in a superposition of both states simultaneously, enabling quantum computing's unique capabilities.
Bell State Measurement (BSM): A critical measurement in quantum teleportation that determines the quantum correlation between two particles. It's like checking how two quantum particles are "entangled" or connected.
Hong-Ou-Mandel Interference: A quantum optical effect where two identical photons entering a beam splitter from different ports will always exit together. This phenomenon is fundamental for many quantum operations including BSM.

Quantum teleportation nodes — Three nodes configuration.

Noise Mitigation Techniques

Key Strategies

Wavelength Selection:
- O-band quantum channels (1290 nm)
- Minimizes Raman scattering from C-band
- Anti-Stokes frequency detuning
Filtering:
- Narrow-band spectro-temporal filtering
- 60-pm bandwidth Fiber Bragg Gratings
- >190 dB rejection of C-band light
Detection:
- Multi-photon coincidence detection
- 500-ps coincidence window
- Superconducting nanowire detectors (>90% efficiency)

Experimental Setup

Fiber Network Configuration

Quantum Links:
- Alice to Charlie: 15.2 km spooled fiber
- Bob to Charlie: 15.0 km spooled fiber
- Total quantum distribution: 30.2 km
Classical Link:
- Total distance: 78.2 km (including deployed fiber)
- 24 km deployed fiber to Chicago campus
- 30.2 km experimental spool
- Additional 24 km deployed fiber

Experimental Results

Key Measurements

1. Entanglement Distribution

Visibility:
- Vertical basis: 97.5 ± 0.1%
- Anti-diagonal basis: 95.3 ± 0.2%
- Well above 1/√2 bound for non-local entanglement

2. Hong-Ou-Mandel Interference

Visibility:
- Without classical signal: 82.9 ± 4.5%
- With 74 mW classical power: 80.3 ± 3.8%
- Exceeds classical bound of 50%

3. State Transfer Fidelity

H/V Basis:
- |H⟩: 97.5 ± 1.2%
- |V⟩: 95.8 ± 2.5%
D/A Basis:
- |D⟩: 87.5 ± 3.9%
- |A⟩: 85.5 ± 3.7%

Classical Communication Performance

Data Rate: 400 Gbps
Wavelength: 1547.32 nm (C-band)
Power Levels:
- Minimum required: 0.5 mW (-3 dBm)
- Maximum tested: 74 mW (18.7 dBm)
- 150x higher than minimum required power
Link Budget:
- Total loss: 22.8 dB over 78.2 km
- Quantum channel loss: 10.1 dB over 30.2 km
- BSM bypass insertion loss: 1.2 dB

Conclusion

Just imagine the possibilities of this. We can have quantum networks coexisting with classical networks. We can achieve millions of things with this. Quantum mechanics applications are always very messy or very hard to implement, but this experiment shows that we can achieve this with existing technology.

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