⚛️ quantum physics

Long distance quantum illumination and ranging using polarization entangled photon pairs in a lossy environment

This paper demonstrates a robust scheme for long-distance quantum illumination and ranging by showing that polarization entanglement can be maintained and recovered even after probe photons undergo kilometer-scale propagation through a lossy free-space environment.

Original authors: Sujai Matta, Soumya Asokan, Sanchari Chakraborti, Mayank Joshi, Rahul Dalal, C. M. Chandrashekar

Published 2026-02-10

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CC BY 4.0

Original authors: Sujai Matta, Soumya Asokan, Sanchari Chakraborti, Mayank Joshi, Rahul Dalal, C. M. Chandrashekar

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

The Quantum Flashlight: Finding Objects in the Dark

Imagine you are standing in a pitch-black, foggy field at night. You are trying to find a small, dark object—like a black marble—lying on the ground.

If you use a regular flashlight, the light travels out, hits the marble, and bounces back. But in a thick fog, most of that light hits water droplets in the air and scatters away. By the time any light makes it back to your eyes, it’s so faint and messy that you can’t tell if you saw the marble or just a random glint of light from the fog.

This paper describes a "Quantum Flashlight" that uses a special trick to see through that mess.

The Secret Sauce: "The Buddy System" (Entanglement)

The researchers didn't just use regular light; they used entangled photons.

Think of entanglement like a pair of magical, synchronized dice. Usually, if you throw two dice, the results are random. But these magical dice are "entangled": if you roll one and it shows a 6, the other one instantly shows a 6, no matter how far apart they are. They are perfectly in sync.

In this experiment, the scientists create these "buddy" photons:

The Idler (The Homebody): One photon stays safely inside the laboratory. It’s like keeping one of the magical dice in your pocket.
The Probe (The Explorer): The other photon is sent out into the dark, foggy field to find the object.

The Challenge: The Great Disappearing Act

The "fog" in this experiment is the real world—atmospheric turbulence, dust, and distance. As the Explorer photon travels out to the object and tries to bounce back, it faces a nightmare scenario. Most of them get lost, scattered, or "killed" by the environment.

By the time the researchers get to a distance of 500 meters (a 1-kilometer round trip), they are receiving only a tiny, tiny trickle of photons—sometimes just a few dozen per second. It’s like trying to hear a whisper in the middle of a heavy metal concert.

The Solution: The Quantum Handshake

Here is where the magic happens. Even if 99.9% of the Explorer photons are lost, the researchers don't just look for any light coming back. They look for a "Quantum Handshake."

When a photon finally makes it back from the object, the scientists compare it to the Homebody photon waiting in the lab. Because they are entangled, they should still be perfectly "in sync."

If it’s just random noise (fog/background light): The light coming back will be random. It won't match the Homebody photon. It’s like a stranger trying to pretend they are your twin—they won't pass the test.
If it’s the real signal (the object): The photon will show that perfect, magical synchronization with the Homebody.

By checking for this specific "handshake" (which scientists call the CHSH value), they can confirm, "Yes, that tiny speck of light we just saw wasn't just noise; it was definitely the object!"

Why This Matters (The "Ranging" Part)

Not only did they prove they could detect the object, but they also proved they could measure how far away it is.

Because they know exactly when the Homebody photon was created, they can time how long it takes for the Explorer to go out, hit the object, and come back. It’s like a high-tech version of shouting into a canyon and timing how long it takes for the echo to return.

The Big Picture

Before this, doing this over long distances was incredibly difficult because entanglement is very "fragile"—it usually breaks the moment it hits a bit of dust or air.

This paper proves that polarization entanglement (the specific way the light waves are oriented) is incredibly tough. It can survive a kilometer-long journey through the messy, real world. This opens the door to "Quantum Radar" and advanced sensors that could see through clouds, smoke, or even deep underwater, using the power of quantum "twins" to find what is hidden.

Technical Summary: Long-Distance Quantum Illumination and Ranging Using Polarization Entangled Photon Pairs

1. Problem Statement

Quantum entanglement is a powerful resource for sensing, but it is notoriously fragile. In practical applications like Quantum Illumination (QI)—which aims to detect low-reflectivity objects in noisy, lossy environments—the entanglement between a signal photon (sent to the target) and an idler photon (retained at the receiver) is typically destroyed by environmental decoherence, atmospheric turbulence, and significant photon loss. The central challenge addressed in this paper is how to maintain a measurable quantum advantage for both object detection (presence/absence) and ranging (distance measurement) over kilometer-scale distances in realistic, lossy free-space channels.

2. Methodology

The researchers implemented a robust QI protocol using polarization-entangled photon pairs. The experimental architecture consists of several key components:

Entangled Photon Source: A high-flux, high-visibility polarization-entangled state $|\Psi^+\rangle = \frac{1}{\sqrt{2}}(|H\rangle_s|V\rangle_i + |V\rangle_s|H\rangle_i)$ is generated via Spontaneous Parametric Down-Conversion (SPDC) in a Type-II PPKTP crystal within a Sagnac interferometer configuration.
Three-Path Protocol: The signal photon is probabilistically routed into two distinct paths using a beam splitter:
1. Reference Path: A direct path to the measurement unit to monitor the source's entanglement integrity in real-time.
2. Probe Path: A path directed through a sending telescope toward a distant object, then collected by a receiving telescope.
Detection and Correlation: The system performs joint correlation measurements between the retained idler photons and the photons returning from the probe/reference paths. The degree of quantum correlation is quantified using the CHSH Bell parameter ( $S$ ).
Ranging Mechanism: Distance information is extracted from the temporal delay (time-of-flight) between the arrival of the idler photon and the reflected probe photon, recorded using a time-correlated single-photon counter.
Environmental Mitigation: To handle polarization scrambling caused by multi-mode fiber collection at the receiver, a polarizing beam splitter (PBS) is placed immediately before the receiving telescope to filter polarization-preserving photons.

3. Key Contributions

Robustness of Polarization Encoding: The study demonstrates that encoding photons in specific polarization bases before transmission allows for the recovery of entanglement even after significant scattering and attenuation.
Simultaneous Detection and Ranging: Unlike many QI schemes that focus solely on detection, this protocol successfully integrates temporal information to provide distance (ranging) capabilities.
Real-time Source Monitoring: By using a reference path, the researchers can distinguish between a loss of entanglement due to the environment and a degradation of the source itself, ensuring reliable data interpretation.

4. Results

High Initial Entanglement: The source achieved a high CHSH parameter of $S = 2.802 \pm 0.002$ and visibilities up to 99.5%.
Kilometer-Scale Performance: The protocol was validated over round-trip distances approaching 1 km (500 m one-way).
Strong Quantum Correlations: Remarkably, the researchers observed $S > 2.6$ even when the return signal was extremely weak (only a few tens of probe photons per second were detected).
Successful Ranging: Coincidence peaks were clearly identified at a time delay of approximately $3.37 \times 10^6$ ps, corresponding to a round-trip distance of $\sim 1000$ m, confirming accurate ranging.
Object Discrimination: The system successfully detected both a high-reflectivity mirror ( $\sim 96\%$ ) and a low-reflectivity scattering object, demonstrating its ability to operate under varying target conditions.

5. Significance

This work establishes a practical foundation for scalable quantum-assisted sensing. By proving that polarization entanglement can survive kilometer-scale atmospheric propagation and significant loss, the researchers have moved QI from theoretical models and short-range laboratory setups toward real-world field applications. The ability to perform simultaneous detection and ranging with a quantum advantage suggests potential future applications in quantum radar, remote sensing, and secure long-distance quantum communication networks.