⚛️ quantum physics

High-Visibility Franson Interference Enabled by Passive Photonic Integrated Interferometers at Telecom Wavelengths

This paper demonstrates high-visibility (97.1%) Franson interference at telecom wavelengths using a compact, fiber-integrated platform that combines a PPLN waveguide photon-pair source with fully passive, thermally tuned photonic integrated Mach-Zehnder interferometers, eliminating the need for active phase stabilization.

Original authors: Ramin Emadi, Domenico Ribezzo, Giulia Guarda, Davide Bacco, Alessandro Zavatta

Published 2026-03-30

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ramin Emadi, Domenico Ribezzo, Giulia Guarda, Davide Bacco, Alessandro Zavatta

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to send a secret message using a pair of magical, perfectly synchronized dice. In the world of quantum physics, these "dice" are actually pairs of photons (particles of light) that are entangled. This means that no matter how far apart they are, if you roll one and get a "6," the other will instantly show a "6" as well, even if it's on the other side of the planet.

This paper is about building a super-reliable, compact machine to create these magical dice pairs and prove they are truly synchronized, all while using the same fiber-optic cables that carry your internet.

Here is the story of how they did it, broken down into simple concepts:

1. The Problem: The "Fickle" Light

To make these entangled pairs, scientists usually use a special crystal (like a magical prism). However, keeping the light perfectly synchronized is like trying to balance a pencil on its tip while standing on a boat in a storm.

The Storm: Tiny vibrations, temperature changes, and air currents can mess up the timing of the light.
The Old Solution: Usually, scientists have to use complex electronic systems to constantly "nudge" the light back into place (active stabilization). It's like having a robot constantly adjusting the pencil. This is bulky, expensive, and power-hungry.

2. The Solution: The "Passive" Boat

The team in this paper built a new kind of machine that doesn't need a robot to balance the pencil. Instead, they built a Photonic Integrated Circuit (PIC).

The Analogy: Imagine a tiny, solid glass chip where the light travels through microscopic tunnels. Because the whole chip is made of one solid piece of glass, if the temperature changes, everything changes together. The "boat" moves, but the "pencil" stays balanced because the whole system moves as one unit.
The Magic Trick: They didn't need electronic nudges. They just gently warmed up the whole chip (thermal tuning) to slowly shift the phase, like turning a dial on a radio to find the perfect station. It's slow, but incredibly stable.

3. The Engine: The "Double-Step" Factory

To create the entangled light, they used a clever two-step process involving a material called PPLN (a special crystal):

Step 1 (The Shrink): They take a standard laser beam (red light) and squeeze it to make it twice as energetic (blue light).
Step 2 (The Split): They shine that blue light into the crystal, and it splits into two new red photons.

Why do this? It's like using a high-quality, narrow-beam flashlight to paint a picture, rather than a flickering candle. This ensures the two new photons are born with perfect "sibling" characteristics, making them highly indistinguishable.

4. The Test: The "Franson" Dance

To prove the photons are entangled, they sent them through a Franson Interferometer.

The Setup: Imagine two runners (the photons) starting at the same time. Each runner has a choice: take a Short Path or a Long Path (a detour that adds a tiny delay).
The Rules:
- If Runner A takes the Short Path and Runner B takes the Long Path, they arrive at different times. We know who did what. No magic here.
- If both take the Short Path, they arrive together.
- If both take the Long Path, they also arrive together (even though they are late).
The Quantum Magic: Because the photons are entangled, nature "forgets" which path they took. The universe treats the "Both Short" and "Both Long" scenarios as the same event. When these two possibilities overlap, they create an interference pattern (like ripples in a pond meeting).
The Result: By measuring how often the photons arrive together, the scientists saw a beautiful wave pattern. The height of these waves (visibility) tells us how "quantum" the connection is.

5. The Results: A Record-Breaking Dance

The team achieved some impressive numbers:

97.1% Visibility: This is like a dance where the partners are so perfectly synchronized that they are almost indistinguishable from a perfect mirror image. In the world of quantum chips, this is a top-tier score.
Low Noise: They managed to keep "accidental" collisions (noise) very low, ensuring the signal was clean.
Compact & Ready: The whole system is small enough to fit in a box and can be plugged directly into standard telecom cables (the ones in your basement or street).

Why Does This Matter?

Think of the future "Quantum Internet." It needs to send secret keys over long distances using existing fiber-optic cables.

Old way: Big, fragile tables full of lasers and electronics that need constant tweaking. Hard to put in a city.
This paper's way: A small, sturdy chip that sits quietly in a box, needing no constant adjustment. It's robust, cheap to make, and ready to be deployed in the real world.

In a nutshell: They built a tiny, self-stabilizing factory that creates perfectly synchronized light twins and proved they are dancing in perfect step, all without needing a complex control room to keep them on beat. This is a huge step toward making quantum technology practical for everyday use.

1. Problem Statement

The distribution of entangled quantum states over long distances is a prerequisite for a future quantum internet. While energy-time entanglement is robust against polarization fluctuations and compatible with Dense Wavelength Division Multiplexing (DWDM) in standard telecom fibers, analyzing these states presents significant challenges:

Stability: Traditional Franson interferometers (unbalanced Mach-Zehnder interferometers) often require active phase stabilization (e.g., piezoelectric actuators or on-chip heaters) to maintain interference visibility, which adds complexity and power consumption.
Scalability: Fiber-based setups are bulky and difficult to integrate into compact, deployable networks.
Noise: Achieving high visibility while maintaining a high Coincidence-to-Accidental Ratio (CAR) at practical pump powers is difficult due to multi-pair emission and dark counts.
Compatibility: Many high-visibility demonstrations rely on post-selection or non-integrated components, limiting their utility in device-independent protocols or scalable network nodes.

The authors aim to develop a compact, fiber-integrated platform that generates high-quality energy-time entanglement and analyzes it using fully passive photonic integrated circuits (PICs) without active phase locking, while maintaining compatibility with standard telecom DWDM channels.

2. Methodology

The experimental setup combines a cascaded nonlinear source with monolithic passive interferometers:

Entanglement Source (Cascaded PPLN):
- Pump: A narrow-linewidth continuous-wave (CW) laser at 1560.48 nm (Telecom C-band).
- Architecture: A cascaded Second-Harmonic Generation (SHG) followed by Spontaneous Parametric Down-Conversion (SPDC) within two Periodically Poled Lithium Niobate (PPLN) waveguides.
- Process: The 1560 nm pump is converted to 780 nm (SHG), which then drives the SPDC process to generate signal and idler photon pairs around 1560 nm. This cascaded approach ensures a narrowband pump for the SPDC stage, enhancing spectral indistinguishability.
- Filtering: A 32-channel DWDM demultiplexer selects specific ITU grid channels (Channel 22 and 20, separated by ~200 GHz) to define the photon wavelengths and limit the bandwidth to ~100 GHz. This sets the single-photon coherence time ( $\tau_c \approx 3$ ps).
Analysis (Passive PIC Interferometers):
- Device: Two unbalanced Mach-Zehnder interferometers (uMZIs) fabricated on a thermally stable borosilicate glass photonic integrated circuit.
- Passive Operation: The interferometers contain no active phase shifters or heaters. Instead, the relative phase ( $\phi$ ) between the short and long arms is tuned globally by controlling the chip's temperature via the thermo-optic effect.
- Delay: The path length difference ( $\Delta t$ ) is $\approx 0.8$ ns, which is significantly larger than the single-photon coherence time ( $\tau_c$ ) but smaller than the pump coherence time ( $\tau_p$ ). This condition suppresses single-photon interference while allowing two-photon Franson interference.
- Detection: Photons are detected using Superconducting Nanowire Single-Photon Detectors (SNSPDs) with high efficiency and low jitter (~50 ps).
Data Analysis:
- Coincidence histograms are analyzed to separate the indistinguishable paths (Short-Short and Long-Long, $SS/LL$) from distinguishable paths (Short-Long and Long-Short, $SL/LS$).
- Metrics include Heralding Efficiency, Coincidence-to-Accidental Ratio (CAR), and Franson visibility (raw, background-corrected, and fit-derived).

3. Key Contributions

Fully Passive PIC Analyzer: Demonstrated high-visibility Franson interference using monolithic PICs that require no on-chip active phase control, relying solely on thermal tuning for phase scanning. This significantly reduces system complexity and power requirements.
Cascaded Source Integration: Successfully combined a cascaded SHG $\to$ SPDC PPLN source with DWDM filtering to generate highly indistinguishable energy-time entangled pairs at telecom wavelengths.
High Performance in Compact Form: Achieved state-of-the-art visibility metrics in a compact, fiber-pigtailed platform, proving that passive integrated interferometers can rival active fiber-based systems in stability and performance.
DWDM Compatibility: The system operates on standard ITU grid channels, making it directly deployable in existing fiber-optic telecommunications infrastructure.

4. Key Results

Interference Visibility:
- Raw Visibility: 95.2%
- Background-Corrected Visibility: 95.6%
- Fit-Derived Visibility: 97.1% (from sinusoidal fringe fitting).
- These values represent one of the highest visibilities reported for a PIC-based analyzer in the telecom band.
Noise Performance:
- Achieved a Coincidence-to-Accidental Ratio (CAR) exceeding $10^3$ at a pump power of only 1.7 mW.
- The CAR scales as $1/P^2$ (where $P$ is pump power), confirming that noise is dominated by multi-pair emission rather than detector dark counts.
Heralding Efficiency:
- Measured system heralding efficiencies of 4.8% (peak) for both signal and idler channels.
- The authors note this is limited by fiber coupling losses, DWDM insertion loss (~5.4 dB), and detector efficiency, rather than intrinsic source limitations.
Stability: The passive interferometers maintained stable phase relationships over hours of operation, with phase drifts being minimal due to the common-mode rejection of the monolithic design.

5. Significance

This work addresses a critical bottleneck in quantum networking: the need for scalable, robust, and low-complexity entanglement analyzers.

Practical Deployment: By eliminating active phase locking and using passive PICs, the system is more suitable for field deployment in quantum networks where power and maintenance are constraints.
Telecom Integration: The use of standard DWDM channels and fiber-pigtailed components ensures seamless integration with existing fiber infrastructure.
Benchmarking: The results (97.1% fit visibility) place this platform among the top-performing energy-time entanglement analyzers, comparable to complex fiber-based setups but with a significantly smaller footprint.
Future Outlook: While current heralding efficiency is moderate, the authors suggest that future improvements in on-chip demultiplexing and reduced coupling losses could push efficiencies higher, potentially enabling device-independent quantum protocols.

In summary, the paper demonstrates a viable pathway toward high-performance, field-deployable quantum entanglement distribution by leveraging the synergy between cascaded nonlinear sources and passive photonic integration.