⚛️ quantum physics

Entanglement Assisted Non-local Optical Interferometry in a Quantum Network

This paper demonstrates a proof-of-concept entanglement-assisted non-local optical interferometry using Silicon-vacancy centers in diamond across a 1.55 km fiber link, showing that remote quantum entanglement can significantly enhance the sensitivity of phase measurements for weak light in quantum networks.

Original authors: P. -J. Stas, Y. -C. Wei, M. Sirotin, Y. Q. Huan, U. Yazlar, F. Abdo Arias, E. Knyazev, G. Baranes, B. Machielse, S. Grandi, D. Riedel, J. Borregaard, H. Park, M. Lončar, A. Suleymanzade, M. D. Lukin

Published 2026-03-13

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: P. -J. Stas, Y. -C. Wei, M. Sirotin, Y. Q. Huan, U. Yazlar, F. Abdo Arias, E. Knyazev, G. Baranes, B. Machielse, S. Grandi, D. Riedel, J. Borregaard, H. Park, M. Lončar, A. Suleymanzade, M. D. Lukin

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 take a super-clear photo of a tiny, distant star. To get that level of detail, astronomers usually need a telescope the size of a city. But building a single mirror that big is impossible. So, they use many small telescopes spread out over a large distance and combine their light, acting like one giant "synthetic" eye.

However, there's a catch. When the star is very dim (which most are), the light is so weak that by the time it travels through the long cables connecting the telescopes, almost all of it disappears. It's like trying to hear a whisper across a football field while someone is shouting static in your ear. The signal gets lost in the noise.

This paper describes a breakthrough experiment that uses quantum magic to solve this problem. Here is how they did it, explained simply:

1. The Problem: The "Lost Whisper"

In a normal setup, to measure the tiny difference in the time light arrives at two telescopes (which tells us the star's position), you have to send the light from both telescopes to a central meeting point. But if the telescopes are far apart, the light fibers act like leaky buckets. The faint signal leaks out before it gets there.

2. The Solution: A Quantum "Teleportation" Trick

Instead of sending the fragile light all the way to the middle, the researchers used Quantum Entanglement. Think of entanglement as a "spooky" connection between two particles, like a pair of magic dice. If you roll one in New York and it lands on "6," the other one in London instantly becomes a "6," no matter the distance.

The team created a network of these "magic dice" (using special defects in diamond called Silicon-Vacancy centers) at two different stations. These stations were connected by a fiber optic cable 1.55 kilometers long (about 1 mile).

3. The Three-Step Quantum Dance

To measure the star's light without losing it, they performed a clever three-step dance:

Step 1: The "Arming" (Setting the Trap)
Before the starlight even arrived, they used their quantum network to "arm" the system. They created a shared entangled state between the two stations. This is like setting up a synchronized alarm system that knows when something happens, without needing to see it directly.
Step 2: The "Eraser" (Hiding the Clues)
When the faint starlight arrived, it hit the diamond sensors. Normally, if you detect a photon at the left station, you know it came from the left. But to measure the difference between the two stations, you need to know that a photon arrived somewhere, but not where.
They used a technique called Photon Mode Erasure. Imagine you have a secret note. You mix it with a bunch of other random papers (a "coherent state" of light) and shuffle them. Now, if you find the note, you know it's there, but you can't tell which hand put it in the pile. This hides the "which-path" information, preserving the delicate phase data needed for the measurement.
Step 3: The "Herald" (The Quantum Bell)
This is the most important part. In a normal system, you have to guess if a photon arrived or if it was just empty space (vacuum noise). This guesswork creates static.
Here, they used their entangled "magic dice" to act as a Herald. They measured the quantum state of the diamonds. If the measurement showed a specific result, it was a "herald" (a signal) saying, "Yes! A photon arrived!"
Crucially, this herald told them a photon was present without revealing which station it hit. This allowed them to filter out all the empty, noisy moments (vacuum fluctuations) and only keep the data when a real photon was actually there.

4. The Result: Seeing the Unseeable

By combining these steps, they successfully measured the phase of the light between the two stations over a 1.55 km distance.

Why it matters: Current giant telescope arrays are limited to about 330 meters. This experiment proved that using quantum networks, we can stretch that baseline to kilometers (and eventually much further) without losing the signal.
The Analogy: It's like being able to hear that whisper across the football field, not by shouting louder, but by using a secret code that tells you exactly when the whisper happened, ignoring all the background noise.

The Big Picture

This isn't just about better telescopes. It's a proof-of-concept for a Quantum Internet that can do things classical computers can't.

Future Applications: This technology could lead to telescopes that can see planets orbiting other stars (exoplanets) in incredible detail, or communication systems that are unhackable and work over vast distances in space.

In short, the researchers built a quantum bridge that lets us measure the universe with a sensitivity that was previously thought impossible, turning the "leaky bucket" of fiber optics into a "perfect pipe" using the strange rules of quantum mechanics.

1. Problem Statement

Optical interferometry is a cornerstone of high-resolution imaging, particularly in astronomy where combining light from physically separated telescopes (long-baseline interferometry) creates a synthetic aperture. However, two major limitations hinder the scalability of this technology:

Exponential Signal Loss: In conventional long-baseline setups, photons must be transmitted via optical fibers to a central location for interference. Signal loss in fibers scales exponentially with distance, making long baselines (e.g., >1 km) impractical for weak signals (typical of astronomical sources).
Vacuum Fluctuation Noise: Alternative local measurement schemes (mixing signal with a local oscillator) avoid transmission loss but suffer from a fundamental limitation: they cannot distinguish between the arrival of a signal photon and vacuum fluctuations. This results in a Signal-to-Noise Ratio (SNR) that scales linearly with the average photon number ( $\mu_{sig}$ ), rather than the optimal square-root scaling ( $\sqrt{\mu_{sig}}$ ) achievable with direct interference.

The paper addresses the challenge of performing non-local phase measurements on weak optical signals over long distances without incurring exponential photon loss or being degraded by vacuum noise.

2. Methodology

The authors propose and experimentally demonstrate a protocol using quantum memory-assisted non-local interferometry. The system utilizes a quantum network of Silicon-Vacancy (SiV) centers in diamond nanophotonic cavities.

Key Technical Components:

Quantum Nodes: Two stations separated by a fiber link (up to 1.55 km). Each node contains an SiV center acting as a two-qubit register: a communication qubit (electron spin) and a memory qubit ( $^{29}$ Si nuclear spin).
Entanglement Generation: The protocol begins by generating remote entanglement between the nuclear spins of the two stations. The authors implement a parallel entanglement scheme (using a Mach-Zehnder interferometer configuration) rather than serial schemes, achieving a 7.5x higher efficiency.
Signal Collection & Storage: Weak signal photons (simulating starlight) are reflected off the SiV-cavity systems. The interaction entangles the photonic state with the electron spins.
Photon Mode Erasure: To preserve the differential phase information ( $\phi$ ) while hiding "which-path" information (i.e., which station the photon arrived at), the signal photons are interfered with a coherent local oscillator (LO) on a beamsplitter. This erases the spatial mode information.
Non-Destructive Non-Local Heralding: This is the core innovation. By measuring the parity of the electron spins at both stations, the system heralds the arrival of a signal photon without revealing its location. This allows the system to filter out vacuum events (where no photon arrived) while preserving the phase coherence.
Phase Readout: Once a photon is successfully heralded, the phase information is transferred to the nuclear spins. A local measurement of the nuclear spin parity reveals the differential phase $\phi$ .

3. Key Contributions

Experimental Demonstration of Non-Local Interferometry: The first experimental realization of a quantum memory-assisted interferometer that performs non-local phase sensing.
Parallel Entanglement Scheme: Implementation of a parallel entanglement generation protocol between SiV centers, significantly increasing the entanglement rate compared to previous serial approaches.
Non-Destructive Photon Heralding: Demonstration of a method to detect the presence of a signal photon across a network without destroying the phase information or revealing the photon's path, effectively filtering out vacuum noise.
Long-Baseline Operation: Successful operation of the protocol over a fiber baseline of 1.55 km (3.1 km total fiber length in the interferometer), which is five times larger than the current state-of-the-art optical telescope array baseline (330 m).
SNR Scaling Improvement: The experiment demonstrates that the SNR scaling improves from linear ( $\sim \mu_{sig}$ ) to near-optimal ( $\sim \sqrt{\mu_{sig}}$ ) by filtering vacuum fluctuations, a feat impossible with classical local measurements.

4. Results

Entanglement Fidelity: The team generated entangled Bell states between electron spins with a fidelity of $F = 0.83(3)$ and between nuclear spins with $F = 0.73(4)$ .
Baseline Extension: The system maintained nuclear Bell state fidelity ( $F = 0.63(3)$ ) well above the classical limit even with a 1.55 km fiber separation.
Phase Sensing Visibility:
- Without non-local heralding, the visibility decreased significantly as the signal strength ( $\mu_{sig}$ ) dropped, dominated by vacuum noise.
- With non-local heralding, the visibility improved to 0.090(26) (averaged over $\mu_{sig}$ ), compared to 0.031(18) without heralding.
- At a 1.55 km baseline, the nuclear parity oscillation visibility was measured at 0.11(4).
Fisher Information: The results confirm that the protocol achieves a Fisher information scaling proportional to $\mu_{sig}$ (optimal for weak signals) rather than $\mu_{sig}^2$ , validating the theoretical advantage of quantum memory assistance.

5. Significance and Outlook

This work represents a major step toward quantum-enhanced imaging and long-baseline astronomy.

Overcoming Loss Limits: By using quantum memories and entanglement, the protocol circumvents the exponential photon loss that plagues traditional fiber-based interferometry, enabling baselines that were previously impossible.
Applications: The technology has transformative potential for:
- Exoplanet Detection: Improving the sensitivity required to detect faint signals from distant planets.
- Deep-Space Communication: Enhancing the efficiency of optical links over vast distances.
- Fundamental Physics: Enabling tests of quantum theory in curved spacetime and high-precision metrology.
Future Directions: The authors note that further improvements in entanglement rates (via quantum repeaters and multiplexing), increasing the number of qubits per station, and utilizing phase-based spin-photon gates (instead of amplitude-based) could further boost efficiency and SNR, paving the way for practical, large-scale quantum telescope arrays.

In summary, the paper successfully bridges the gap between theoretical quantum network proposals and practical optical interferometry, demonstrating a viable path to quantum-enhanced imaging that outperforms classical limits in the weak-signal regime.